CoRoT (French: ; English: Convection, Rotation and planetary Transits) was a
space telescope
A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launched ...
mission which operated from 2006 to 2013. The mission's two objectives were to search for
extrasolar planet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s with short orbital periods, particularly those of large
terrestrial size, and to perform
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
by measuring
solar-like oscillations Solar-like oscillations are oscillations in stars that are excited in the same way as those in the Sun, namely by turbulent convection in its outer layers. Stars that show solar-like oscillations are called solar-like oscillators. The oscillations a ...
in stars.
The mission was led by the
French Space Agency (CNES) in conjunction with the
European Space Agency (ESA) and other international partners.
Among the notable discoveries was
CoRoT-7b
CoRoT-7b (previously named CoRoT-Exo-7b) is an exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros (constellation), Monoceros, from Earth. It was first detected Methods of detecting extrasolar planets#Transit photometry, photome ...
, discovered in 2009 which became the first exoplanet shown to have a rock or metal-dominated composition.
CoRoT was launched at 14:28:00
UTC on 27 December 2006, atop a
Soyuz 2.1b rocket,
reporting
first light on 18 January 2007.
Subsequently, the probe started to collect science data on 2 February 2007. CoRoT was the first
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
dedicated to the detection of
transiting extrasolar planets
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty ...
, opening the way for more advanced probes such as
Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
and
TESS Tess or TESS may refer to:
Music
* Tess (band), a Spanish pop band active from 2000 to 2005
* TESS (musician), a UK musician
Film and theatre
* ''Tess'' (1979 film), a 1979 film adaptation of '' Tess of the d'Urbervilles''
* ''Tess'' (2016 film) ...
. It detected its first extrasolar planet,
CoRoT-1b
CoRoT-1b (previously named CoRoT-Exo-1b) is a transiting extrasolar planet approximately 2,630 light-years away in the constellation of Monoceros. The planet was discovered orbiting the yellow dwarf star CoRoT-1 in May 2007. The planet was t ...
, in May 2007,
just 3 months after the start of the observations. Mission flight operations were originally scheduled to end 2.5 years from launch
but operations were extended to 2013.
On 2 November 2012, CoRoT suffered a computer failure that made it impossible to retrieve any data from its telescope. Repair attempts were unsuccessful, so on 24 June 2013 it was announced that CoRoT has been retired and would be decommissioned; lowered in orbit to allow it to burn up in the atmosphere.
Overview
Spacecraft design
The CoRoT optical design minimized stray light coming from the Earth and provided a
field of view
The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.
Humans a ...
of 2.7° by 3.05°. The CoRoT optical path consisted of a diameter off-axis
afocal telescope housed in a two-stage
opaque baffle specifically designed to block sunlight reflected by the Earth and a camera consisting of a
dioptric
Dioptrics is the branch of optics dealing with refraction, similarly the branch dealing with mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of ...
objective
Objective may refer to:
* Objective (optics), an element in a camera or microscope
* ''The Objective'', a 2008 science fiction horror film
* Objective pronoun, a personal pronoun that is used as a grammatical object
* Objective Productions, a Brit ...
and a
focal box. Inside the focal box was an array of four
CCD detectors protected against
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
by aluminum
shielding 10mm thick. The
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
CCDs are defocused by 760μm toward the dioptric objective to avoid saturation of the brightest stars. A
prism
Prism usually refers to:
* Prism (optics), a transparent optical component with flat surfaces that refract light
* Prism (geometry), a kind of polyhedron
Prism may also refer to:
Science and mathematics
* Prism (geology), a type of sedimentary ...
in front of the planet detection CCDs gives a small
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
designed to disperse more strongly in the blue wavelengths.
![Corot Focal plane](https://upload.wikimedia.org/wikipedia/commons/5/52/Corot_Focal_plane.jpg)
The four
CCD detectors are model 4280 CCDs provided b
E2V Technologies These CCDs are frame-transfer, thinned, back-illuminated designs in a 2,048 by 2,048 pixel array. Each pixel is in size which corresponds to an angular pixel size of 2.32 arcsec. The CCDs are cooled to . These detectors are arranged in a square pattern with two each dedicated to the planetary detection and
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
. The data output stream from the CCDs are connected in two ''chains''. Each chain has one planetary detection CCD and one
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
CCD. The
field of view
The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.
Humans a ...
for planetary detection is 3.5°.
[
]
The satellite, built in the
Cannes Mandelieu Space Center
The Cannes Mandelieu Space Center is an industrial plant dedicated to spacecraft manufacturing, located in both the towns of Cannes and Mandelieu in France. After a long history in aircraft manufacturing, starting in 1929, the center became incre ...
, had a launch mass of 630 kg, was 4.10 m long, 1.984 m in diameter and was powered by two solar panels.
[
]
Mission design
The satellite observed perpendicular to its orbital plane, meaning there were no Earth
occultation
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
s, allowing up to 150 days of continuous observation. These observation sessions, called "Long Runs", allowed detection of smaller and long-period planets. During the remaining 30 days between the two main observation periods, CoRoT observed other patches of sky for a few weeks long "Short Runs", in order to analyze a larger number of stars for the asteroseismic program. After the loss of half the field of view due to
failure
Failure is the state or condition of not meeting a desirable or intended objective (goal), objective, and may be viewed as the opposite of Success (concept), success. The criteria for failure depends on context, and may be relative to a parti ...
of Data Processing Unit No. 1 in March 2009, the observation strategy changed to 3 months observing runs, in order to optimize the number of observed stars and detection efficiency.
In order to avoid the Sun entering in its field of view, during the northern summer CoRoT observed in an area around
Serpens Cauda
Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...
, toward the
Galactic Center
The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact rad ...
, and during the winter it observed in
Monoceros
Monoceros (Greek: Μονόκερως, "unicorn") is a faint constellation on the celestial equator. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, ...
, in the
Galactic anticenter
The galactic anticenter is a direction in space directly opposite to the Galactic Center, as viewed from Earth. This direction corresponds to a point on the celestial sphere.
From the perspective of an observer on Earth, the galactic anticenter ...
. Both these "eyes" of CoRoT have been studied in preliminary observations carried out between 1998 and 2005, allowing the creation of a
database
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases sp ...
, called CoRoTsky, with data about the stars located in these two patches of sky. This allowed selecting the best fields for observation: the exoplanet research program requires a large number of
dwarf stars
The Dwarf Stars Award is an annual award presented by the Science Fiction Poetry Association to the author of the best horror, fantasy, or science fiction poem of ten lines or fewer published in the previous year. The award was established in 20 ...
to be monitored, and to avoid
giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...
s, for which planetary
transits are too shallow to be detectable. The asteroseismic program required stars brighter than magnitude 9, and to cover as many different
types of stars as possible. In addition, in order to optimize the observations, the fields shouldn't be too sparse – fewer targets observed – or too crowded – too many stars overlapping.
Several fields have been observed during the mission:
*IRa01, from 18 January 2007 to 3 April 2007 – 9,879 stars observed;
*SRc01, from 3 April 2007 to 9 May 2007 – 6,975 stars observed;
*LRc01, from 9 May 2007 to 15 October 2007 – 11,408 stars observed;
*LRa01, from 15 October 2007 to 3 March 2008 – 11,408 stars observed;
*SRa01, from 3 March 2008 to 31 March 2008 – 8,150 stars observed;
*LRc02, from 31 March 2008 to 8 September 2008 – 11,408 stars observed;
*SRc02, from 8 September 2008 to 6 October 2008 – 11,408 stars observed;
*SRa02, from 6 October 2008 to 12 November 2008 – 10,265 stars observed;
*LRa02, from 12 November 2008 to 30 March 2009 – 11,408 stars observed;
*LRc03, from 30 March 2009 to 2 July 2009 – 5,661 stars observed;
*LRc04, from 2 July 2009 to 30 September 2009 – 5,716 stars observed;
*LRa03, from 30 September 2009 to 1 March 2010 – 5,289 stars observed;
*SRa03, from 1 March 2010 to 2 April 2010;
*LRc05, from 2 April 2010 to 5 July 2010;
*LRc06, from 5 July 2010 to 27 September 2010;
*LRa04, from 27 September 2010 to 16 December 2010;
*LRa05, from 16 December 2010 to 5 April 2011;
*LRc07, from 5 April 2011 to 30 June 2011;
*SRc03, from 1 July 2011 to 5 July 2011 – a run made to reobserve the transit of
CoRoT-9b;
*LRc08, from 6 July 2011 to 30 September 2011;
*SRa04, from 30 September 2011 to 28 November 2011;
*SRa05, from 29 November 2011 to 9 January 2012;
*LRa06, from 10 January 2012 to 29 March 2012 – a run dedicated to reobservation of
CoRoT-7b
CoRoT-7b (previously named CoRoT-Exo-7b) is an exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros (constellation), Monoceros, from Earth. It was first detected Methods of detecting extrasolar planets#Transit photometry, photome ...
;
*LRc09, from 10 April 2012 to 5 July 2012;
*LRc10, from 6 July 2012 to 1 November 2012 - interrupted by the fatal failure which ended the mission.
The spacecraft monitored the brightness of stars over time, searching for the slight dimming that happens in regular intervals when planets
transit
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1979 film), a 1979 Israeli film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world
* ''Transit'' (2006 film), a 2006 ...
their host star. In every field, CoRoT recorded the brightness of thousands stars in the
V-magnitude range from 11 to 16 for the extrasolar planet study. In fact, stellar targets brighter than 11 saturated the exoplanets CCD detectors, yielding inaccurate data, whilst stars dimmer than 16 don't deliver enough
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s to allow planetary detections. CoRoT was sensitive enough to detect
rocky planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the S ...
s with a radius two times larger than Earth, orbiting stars brighter than 14; it is also expected to discover new gas giants in the whole magnitude range.
[
]
CoRoT also studied
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
. It can detect
luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...
variations associated with acoustic pulsations of stars. This phenomenon allows calculation of a star's precise mass, age and chemical composition and will aid in comparisons between the sun and other stars. For this program, in each field of view there was one main target star for asteroseismology as well as up to nine other targets. The number of observed targets have dropped to half after the loss of Data Processing Unit No. 1.
The mission began on 27 December 2006 when a Russian
Soyuz 2-1b rocket lifted the satellite into a circular
polar orbit
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of about ...
with an altitude of 827 km . The first scientific observation campaign started on 3 February 2007.
Until March 2013, the mission's cost will amount to , of which 75% is paid by the French space agency CNES and 25% is contributed by Austria, Belgium, Germany, Spain, Brazil and the European Space Agency ESA.
Development
The primary contractor for the construction of the CoRoT vehicle was CNES,
to which individual components were delivered for vehicle assembly. The CoRoT equipment bay, which houses the data acquisition and pre-processing electronics, was constructed by the LESIA Laboratory at the
Paris Observatory
The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...
and took 60
person-years
A man-hour (sometimes referred to as person-hour) is the amount of work performed by the average worker in one hour. It is used for estimation of the total amount of uninterrupted labor required to perform a task. For example, researching and wr ...
to complete.
[ The design and building of the instruments were done by the Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA) de l' Observatoire de Paris, the Laboratoire d'Astrophysique de Marseille, the ]Institut d'Astrophysique Spatiale
The Institut d'astrophysique spatiale (IAS; English: Institute of Space Astrophysics) is a French research institute supporting advanced research in aerospace and astrophysics. It is located in Orsay, just south of Paris. It is a public resear ...
(IAS) from Orsay, the Centre spatial de Liège
Center or centre may refer to:
Mathematics
* Center (geometry), the middle of an object
* Center (algebra), used in various contexts
** Center (group theory)
** Center (ring theory)
* Graph center, the set of all vertices of minimum eccentrici ...
(CSL) in Belgium, the IWF in Austria, the DLR (Berlin) in Germany and the ESA Research and Science Support Department. The 30 cm afocal telescope Corotel has been realized by Alcatel Alenia Space
Thales Alenia Space () is a Franco-Italian aerospace manufacturer. A joint venture between the French technology corporation Thales Group (67%) and Italian defense conglomerate Leonardo (33%), the company is the largest satellite manufacturer ...
in the Centre spatial de Cannes Mandelieu.
Potential
Before the beginning of the mission, the team stated with caution that CoRoT would only be able to detect planets few times larger than Earth or greater, and that it was not specifically designed to detect habitable planet
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from a ...
s. According to the press release announcing the first results, CoRoT's instruments are performing with higher precision than had been predicted, and may be able to find planets down to the size of Earth with short orbits around small stars.
The transit method
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty o ...
requires the detection of at least two transits, hence the planets detected will mostly have an orbital period under 75-day. Candidates that show only one transit have been found, but uncertainty remains about their exact orbital period.
CoRoT should be assumed to detect a small percentage of planets within the observed star fields, due to the low percentage of exoplanets that would transit from the angle of observation of the Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. The chances of seeing a planet transiting its host star is inversely proportional to the diameter of the planet's orbit, thus close in planets detections will outnumber outer planets ones. The transit method
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty o ...
is also biased toward large planets, since their very depth transits are more easily detected than the shallows eclipses induced by terrestrial planets.
Failure of Data Processing Unit No. 1
On 8 March 2009 the satellite suffered a loss of communication with Data Processing Unit No. 1, processing data from one of the two photo-detector chains on the spacecraft. Science operations resumed early April with Data Processing Unit No. 1 offline while Data Processing Unit No. 2 operating normally. The loss of photo-detector chain number 1 results in the loss of one CCD dedicated to asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
and one CCD dedicated to planet detection. The field of view
The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.
Humans a ...
of the satellite is thus reduced by 50%, but without any degradation of the quality of the observations. The loss of channel 1 appears to be permanent.
Follow-up program
The rate of discoveries of transiting planets is dictated by the need of ground-based, follow-up observations, needed to verify the planetary nature of the transit candidates. Candidate detections have been obtained for about 2.3% of all CoRoT targets, but finding periodic transit events isn't enough to claim a planet discovery, since several configurations could mimic a transiting planet, such as stellar binaries, or an eclipsing fainter star very close to the target star, whose light, blended in the light curve, can reproduce transit-like events. A first screening is executed on the light curves, searching hints of secondary eclipses or a rather V-shaped transit, indicative of a stellar nature of the transits. For the brighter targets, the prism in front of the exoplanets CCDs provides photometry in 3 different colors, enabling to reject planet candidates that have different transit depths in the three channels, a behaviour typical of binary stars. These tests allow to discard 83% of the candidate detections, whilst the remaining 17% are screened with photometric and radial velocity follow-up from a network of telescopes around the world. Photometric observations, required to rule out a possible contamination by a diluted eclipsing binary in close vicinity of the target, is performed on several 1 m-class instruments, but also employs the 2 m Tautenburg telescope in Germany and the 3,6 m CFHT/Megacam in Hawaii. The radial velocity follow-up allows to discard binaries or even multiple star system
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speaki ...
and, given enough observations, provide the mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of the exoplanets found. Radial velocity follow-up is performed with high-precision spectrograph
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...
s, namely SOPHIE, HARPS
The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. H ...
and HIRES Hires may refer to:
* High Resolution Fly's Eye, ultra-high-energy cosmic ray observatory
* High Resolution Echelle Spectrometer or High Resolution Echelle Spectrograph, W. M. Keck Observatory's spectrometer
* Hires Big H, restaurant chain headquar ...
. Once the planetary nature of the candidate is established, high-resolution spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
is performed on the host star, in order to accurately determine the stellar parameters, from which further exoplanet characteristics can be derived. Such work is done with large aperture telescopes, as the UVES spectrograph or HIRES Hires may refer to:
* High Resolution Fly's Eye, ultra-high-energy cosmic ray observatory
* High Resolution Echelle Spectrometer or High Resolution Echelle Spectrograph, W. M. Keck Observatory's spectrometer
* Hires Big H, restaurant chain headquar ...
.
Interesting transiting planets could be further followed-up with the infrared Spitzer Space Telescope
The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...
, to give an independent confirmation at a different wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
and possibly detect reflected light from the planet or the atmospheric compositions. CoRoT-7b
CoRoT-7b (previously named CoRoT-Exo-7b) is an exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros (constellation), Monoceros, from Earth. It was first detected Methods of detecting extrasolar planets#Transit photometry, photome ...
and CoRoT-9b have already been observed by Spitzer
Spitzer is a surname. Notable people with the surname include:
* Andre Spitzer (1945–1972), Israeli fencing coach and victim of the Munich massacre
* Bernard Spitzer (1924–2014), American real estate developer and philanthropist, father of Eli ...
.
Papers presenting the results of follow-up operations of planetary candidates in the IRa01, LRc01, LRa01, SRc01 fields have been published.
In April 2019, a summary of the exoplanet search results have been published, with 37 planets and brown dwarves confirmed, and a further one hundred planet candidates still to be verified.
Sometimes the faintness of the target star or its characteristics, such as a high rotational velocity or strong stellar activity, do not allow to determine unambiguously the nature or the mass of the planetary candidate.
Discoveries
Asteroseismology and stellar physics
Stars vibrate according to many different pulsation modes in much the same way that musical instruments emit a variety of sounds. Listening to an air on the guitar does not leave any doubt as to the nature of the instrument, and an experienced musician can even deduce the cords' material and tension. Similarly, stellar pulsation modes are characteristic of global stellar properties and of the internal physical conditions. Analyzing these modes is thus a way of probing stellar interiors to infer stellar chemical composition, rotation profiles and internal physical properties such as temperatures and densities. Asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
is the science which studies the vibration modes of a star. Each of these modes can be mathematically represented by a spherical harmonic
In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields.
Since the spherical harmonics form a ...
of degree l and azimuthal order m. Some examples are presented here below with a color scheme in which blue (red) indicates contracting (expanding) material. The pulsation amplitudes are highly exaggerated.
When applied to the Sun, this science is called helioseismology
Helioseismology, a term coined by Douglas Gough, is the study of the structure and dynamics of the Sun through its oscillations. These are principally caused by sound waves that are continuously driven and damped by convection near the Sun's surfa ...
and has been ongoing for a few decades by now. The solar surface helium abundance was derived very accurately for the first time, which has definitely shown the importance of microscopic diffusion in the solar structure. Helioseismology analyses have also unveiled the solar internal rotational profile, the precise extent of the convective envelope and the location of the helium ionization zone. Despite enormous technical challenges, it was thus tempting to apply similar analyses to stars. From the ground this was only possible for stars close to the Sun such as α Centauri, Procyon, β Virginis... The goal is to detect extremely small light variations (down to 1 ppm) and to extract the frequencies responsible for these brightness fluctuations. This produces a frequency spectrum
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...
typical of the star under scrutiny. Oscillation periods vary from a few minutes to several hours depending on the type of star and its evolutionary state. To reach such performances, long observing times devoid of day/night alternations are required. Space is thus the ideal asteroseismic laboratory. By revealing their microvariability, measuring their oscillations at the ppm level, CoRoT has provided a new vision of stars, never reached before by any ground-based observation.
At the beginning of the mission, two out of four CCDs were assigned to asteroseismic observations of bright stars (apparent magnitude 6 to 9) in the so-called ''seismo field'' while the other CCDs were reserved for exoplanet hunting in the so-called ''exo field''. Albeit with a lower signal to noise ratio
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
, interesting science on stars was also obtained from the exoplanets channel data, where the probe records several thousands of light curves
In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...
from every observed field. Stellar activity, rotation periods, star spot
Starspots are stellar phenomena, so-named by analogy with sunspots.
Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in gene ...
evolution, star–planet interactions, multiple star systems
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...
are nice extras in addition to the main asteroseismic program. This exo field also turned out to be of incalculable richness in asteroseismic discoveries. During the first six years of its mission, CoRoT has observed about 150 bright stars in the ''seismo field'' and more than 150 000 weak stars in the ''exo field''. The figure shows where most of them are located in the Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...
together with some others observed from the ground.
Discoveries were numerous, including the first detection of solar-like oscillations Solar-like oscillations are oscillations in stars that are excited in the same way as those in the Sun, namely by turbulent convection in its outer layers. Stars that show solar-like oscillations are called solar-like oscillators. The oscillations a ...
in stars other than the Sun, the first detection of non-radial oscillations in red giant stars, the detection of solar-like oscillations in massive stars, the discovery of hundreds of frequencies in δ Scuti stars, the spectacular time evolution of the frequency spectrum of a Be (emission lines B) star during an outburst, the first detection of a deviation from a constant period spacing in gravity modes in an SPB (Slowly Pulsating B) star. Interpreting those results opened new horizons in humanity's vision of stars and galaxies. In October 2009 the CoRoT mission was the subject of a special issue of ''Astronomy and Astrophysics
''Astronomy & Astrophysics'' is a monthly peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. The journal is run by a Board of Directors representing 27 sponsoring countries plus a re ...
'', dedicated to the early results of the probe. Below are some examples of breakthrough contributions to stellar astrophysics, based on CoRoT's data:
Extension of the chemically mixed zone in main sequence stars
Above the convective core where mixing of chemicals is instantaneous and efficient, some layers can be affected by partial or total mixing during the main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
phase of evolution. The extent of this ''extra mixed zone'' as well as the mixing efficiency are, however, difficult to assess. This additional mixing has very important consequences since it involves longer time scales for nuclear burning phases and may in particular affect the value of the stellar mass at the transition between those stars which end up their life as white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
s and those which face a final supernova explosion. The impact on the chemical evolution of the galaxy is obvious. Physical reasons for this extra-mixing are various, either a mixing induced by internal rotation or a mixing resulting from convective bubbles crossing the convective core boundary to enter the radiative zone where they finally lose their identity (overshooting), or even some other poorly known processes.
# Solar-like stars: The solar-like star HD 49933 is illustrative of this extra-mixing problem. Its convective envelope is responsible for the presence of solar-like oscillations Solar-like oscillations are oscillations in stars that are excited in the same way as those in the Sun, namely by turbulent convection in its outer layers. Stars that show solar-like oscillations are called solar-like oscillators. The oscillations a ...
. Comparing the observed frequency spectrum with that obtained from theoretical models of 1.19 Mʘ computed with and without additional mixing clearly excludes a model without extra mixing.
# Sub-giant stars: Such an additional mixing also affects the structure of more evolved sub-giant stars since the mass extension of the helium core formed during core hydrogen burning is increased. The sub-giant star HD 49385 of 1.3 Mʘ was submitted to CoRoT scrutiny and although not fully conclusive, new constraints were brought to the modeling of such stars.
# SPB stars: More massive SPB (Slowly Pulsating B) stars show a frequency spectrum dominated by high order gravity modes excited by the κ mechanism at work in layers where ionizations of iron group elements produces an opacity peak. In such stars, the convective core is surrounded by a region of varying chemical composition, the so-called μ-gradient region, left by the progressive withdrawal of the convective core as hydrogen is transformed into helium. This region is rather thin and constitutes a sharp transition region, which induces a very subtle signature in the gravity modes frequency spectrum. Instead of a constant period spacing found in a homogeneous stellar model, periodic deviations from this constant value are expected in models affected by a sharp transition region. Moreover, the period of the deviations is directly related to the precise location of the sharp transition. This phenomenon has been detected in two hybrid B stars (showing at the same time acoustic β Cephei and gravity SPB modes): (1) HD 50230 where an extra-mixing with a somewhat smooth shape is clearly required in the modeling and (2) HD 43317.
Structure of the superficial stellar layers
# Transition layers in stellar envelopes: Transition layers such as the helium ionization region or the lower boundary of the convective envelope in low mass and red giant stars also affect frequency spectra. In a structure devoid of such discontinuities, high order acoustic modes obey some regularities in their frequency distribution (large frequency separation, second difference...). Transition zones introduce periodic deviations with respect to these regularities and the periods of the deviations are directly related to the precise location of the transition zones. These deviations were predicted by theory and were first observed in the Sun. Thanks to CoRoT they were also detected in the solar-like star HD 49933 and also in the red giant star HD 181907. In both cases the location of the helium ionization zone could be accurately derived.
# Amplitudes and line widths in solar-like oscillation spectra: One of the major successes of the CoRoT space mission has definitely been the detection of solar-like oscillations in stars slightly hotter than the Sun. As was previously done for the Sun, measurements of amplitudes and line widths in their frequency spectra resulted in new constraints in the modeling of stochastic excitations of acoustic modes by turbulent convection. The frequency spectrum of HD 49933 was confronted to the stochastic excitation model developed by Samadi et al. Except at high frequencies, a good agreement can be reached by adopting a metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
ten times smaller than the solar metallicity. With the solar value on the contrary, disagreements in amplitudes can reach a factor 2 at low frequencies.
# Granulation: The presence of granulation was detected in the frequency spectrum of HD 49933. Analyses have been done with 3D hydrodynamical model atmospheres computed at solar and ten times smaller than solar metallicities. Here again the model with the lowest metallicity shows up to be closer to the observations although significant disagreements still remain.
Red giants and chemical evolution of this galaxy
Following exhaustion of hydrogen in the core, the overall stellar structure drastically changes. Hydrogen burning now takes place in a narrow shell surrounding the newly processed helium core. While the helium core quickly contracts and heats up, the layers above the hydrogen-burning shell undergo important expansion and cooling. The star becomes a red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
whose radius and luminosity increase in time. These stars are now located on the so-called red giant branch
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...
of the Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...
; they are commonly named ''RGB stars''. Once their central temperature reaches 100 106 K, helium starts burning in the core. For stellar masses smaller than about 2 Mʘ, this new combustion takes place in a highly degenerate
Degeneracy, degenerate, or degeneration may refer to:
Arts and entertainment
* Degenerate (album), ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed
* Degenerate art, a term adopted in the 1920s by the Nazi Party i ...
matter and proceeds through a helium flash
A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is ...
. The readjustment following the flash brings the red giant to the so-called red clump
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondary ...
(RC) in the Hertzsprung-Russell diagram.
Whether RGB or RC, these stars all have an extended convective envelope favorable to the excitation of solar-like oscillations. A major success of CoRoT has been the discovery of radial and long-lived non-radial oscillations in thousands of red giants in the exo field. For each of them, the frequency at maximum power νmax in the frequency spectrum as well as the large frequency separation between consecutive modes Δν could be measured, defining a sort of individual seismic passport.
# Red giant population in this galaxy: Introducing these seismic signatures, together with an estimation of the effective temperature, in the scaling laws relating them to the global stellar properties, gravities (seismic gravities), masses and radii can be estimated and luminosities and distances immediately follow for those thousands of red giants. Histogram
A histogram is an approximate representation of the distribution of numerical data. The term was first introduced by Karl Pearson. To construct a histogram, the first step is to " bin" (or "bucket") the range of values—that is, divide the ent ...
s could then be drawn and a totally unexpected and spectacular result came out when comparing these CoRoT histograms with theoretical ones obtained from theoretical synthetic populations of red giants in this galaxy. Such theoretical populations were computed from stellar evolution models, with adopting various hypotheses to describe the successive generations of stars along the time evolution of this galaxy
Andrea Miglio
and collaborators noticed that both types of histograms were spitting images of one another, as can be seen in the histograms picture. Moreover, adding the knowledge of the distances of these thousands of stars to their galactic coordinates, a 3D map of this galaxy was drawn. This is illustrated in the figure where different colors relate to different CoRoT runs and to ''Kepler'' observations (green points).
# Age-metallicity relation in this galaxy: The age of a red giant is closely related to its former main sequence lifetime, which is in turn determined by its mass and metallicity. Knowing the mass of a red giant amounts to knowing its age. If the metallicity is known the uncertainty in age does not exceed 15%! Observational missions such a
APOGEE
''(Apache Point Observatoty Galactic Evolution Environment)'' whose goal is to measure metallicities for 100 000 red giants in this galaxy
GALAH
''(Galactic Archaeology with HERMES)'' and GAIA
In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parthenog ...
''(Global Astrometric Interferometer for Astrophysics)'' could of course widely benefit from these seismic gravities with the ultimate output of establishing the age-metallicity relation in this galaxy. Asteroseismology has crossed the doorstep of the structure and chemical evolution of this galaxy.
# Seismic signatures and extension of mixed zones during central hydrogen and helium burning: Increasing even further the scrutiny in analyzing the CoRoT and ''Kepler'' frequency spectra of red giants brought new important discoveries. Small and subtle differences in seismic signatures allow us to distinguish RGB from RC stars notwithstanding their similar luminosities. This is now theoretically confirmed thanks to elaborate red giant modeling. The period spacings of gravity-dominated modes are expected to be especially meaningful. Their detection for a large number of red giants could give us clues to establishing the extent of the extra-mixed region above the convective core during core hydrogen burning, but also the extent of the extra-mixed region during core helium burning, both mixing processes being ''a priori'' totally unrelated.
Massive stars
Massive variable main sequence stars have frequency spectra dominated by acoustic modes excited by the κ mechanism at work in layers where partial ionization of iron group elements produce a peak in opacity. In addition the most advanced of these stars present mixed modes i.e. modes with a g-character in deep layers and p-character in the envelope. Hydrogen burning takes place in a convective core surrounded by a region of varying chemical composition and an envelope mostly radiative except for tiny convective layers related to partial ionization of helium and/or iron group elements. As in lower mass stars the extent of the fully or partially mixed region located just above the convective core ''(extra-mixed zone)'' is one of the main uncertainties affecting theoretical modeling.
# β Cephei stars: Seismic analyses of β Cephei stars show that it is not obvious to derive a one-to-one extent of this extra-mixed zone. A rather large extent seems to be required to model θ Ophiuchi while a much smaller one is favored for HD 129929, for β Canis Majoris, for δ Ceti, and for 12 Lacertae. This extra-mixed zone could even be absent in the structure of V1449 Aquilae (HD 180642) and ν Eridani. It would be extremely interesting to establish a relation between the extent of this zone and the rotation velocity and/or the magnetic field of the star. Seismic analysis of V2052 Ophiuchi shows that this star although rapidly rotating, which would favor extra-mixing, could be devoid of such a region. The magnetic field detected in this star could be the reason of this lack of extra-mixing.
# Be stars: Late Be type stars HD 181231 and HD 175869 are very rapid rotators, about 20 times more rapid than the Sun. Their seismic analysis seems to require a centrally mixed zone about 20% larger than what is expected from convection only. Another Be star, HD 49330, had a very exciting surprise in store. Observed by CoRoT during an outburst of matter towards its circumstellar disk, which is typical of such stars, its frequency spectrum suffered drastic changes. Firstly dominated by acoustic modes the spectrum showed the appearance of gravity modes with amplitudes strictly in line with the outburst. Such a link between the nature of the excited modes and a dynamical phenomenon is of course a gold mine in the quest for the internal structure of Be stars.
# O stars: Many O stars have been observed by CoRoT. Among them HD 46150 and HD 46223 (members of the galactic cluster NGC 2264) and HD 46966 (member of the OB association Mon OB2) do not seem to pulsate, which is in agreement with stellar modeling of stars with similar global parameters. The frequency spectrum of the Plaskett's star HD 47129 on the contrary shows a peak with six harmonics in the frequency range expected from theoretical modeling.
Another unexpected CoRoT discovery was the presence of solar-like oscillations in massive stars. The small convective shell related to the opacity peak resulting from the ionization of iron group elements at about 200 000 K (iron opacity peak) could indeed be responsible for the stochastic excitation of acoustic modes like those observed in the Sun.
# V1449 Aquilae (HD 180642): This CoRoT target is a β Cephei star whose frequency spectrum reveals high frequency and very small amplitude acoustic modes. A careful analysis has shown that they were solar-like oscillations excited by turbulent bubbles origination from this convective iron opacity peak zone or even from the convective core. This is indeed a major discovery since it was the first time that pulsations excited by the κ mechanism acting in the iron opacity peak zone were present side by side in the same star with pulsations stochastically excited by this very same zone. This is the reason why Kevin Belkacem, main discoverer of these solar-like oscillations in V1449 Aquilae, added a new baptismal certificate to this β Cephei star and named it ''Chimera''. The figure illustrates the behavior of the frequency versus time for two modes in the frequency spectrum of Chimera, a solar-like mode (top) and a β Cephei mode (bottom). The stochastic nature of the solar-like mode reveals itself in the instability of its frequency as time goes on and in the spread in frequency on several μHz. The contrast with the stability in frequency and the narrow frequency range of the β Cephei mode is striking.
# HD 46149: Later on solar-like oscillations were even discovered in a more massive O star member of the binary system HD 46149. Constraints coming from the binary nature of the system coupled with seismic constraints led to the determination of the orbital parameters of the system as well as to the global properties of its members.
The open cluster NGC 2264
During a 23-day observing run in March 2008, CoRoT observed 636 members of the young open cluster NGC 2264
NGC 2264 is the designation number of the New General Catalogue that identifies two astronomical objects as a single object: the Cone Nebula, and the Christmas Tree Cluster. Two other objects are within this designation but not officially incl ...
. The so-called ''Christmas tree cluster
NGC 2264 is the designation number of the New General Catalogue that identifies two astronomical objects as a single object: the Cone Nebula, and the Christmas Tree Cluster. Two other objects are within this designation but not officially incl ...
'', is located in the constellation Monoceros
Monoceros (Greek: Μονόκερως, "unicorn") is a faint constellation on the celestial equator. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, ...
relatively close to us at a distance of about 1800 light years. Its age is estimated to be between 3 and 8 million years. At such a young age, the cluster is an ideal target to investigate many different scientific questions connected to the formation of stars and early stellar evolution. The CoRoT data of stars in NGC 2264 allow us to study the interaction of recently formed stars with their surrounding matter, the rotation and activity of cluster members as well as their distribution, the interiors of young stars by using asteroseismology, and planetary and stellar eclipses.
The stellar births and the stars' childhoods remain mostly hidden from us in the optical light because the early stars are deeply embedded in the dense molecular cloud from which they are born. Observations in the infrared or X-ray enable us to look deeper into the cloud, and learn more about these earliest phases in stellar evolution.
Therefore, in December 2011 and January 2012, CoRoT was part of a large international observing campaign involving four space telescopes and several ground-based observatories. All instruments observed about 4000 stars in the young cluster NGC 2264 simultaneously for about one month at different wavelengths. The Canadian space mission MOST
Most or Möst or ''variation'', may refer to:
Places
* Most, Kardzhali Province, a village in Bulgaria
* Most (city), a city in the Czech Republic
** Most District, a district surrounding the city
** Most Basin, a lowland named after the city
** A ...
targeted the brightest stars in the cluster in the optical light, while CoRoT observed the fainter members. MOST and CoRoT observed NGC 2264 continuously for 39 days. The NASA satellites Spitzer
Spitzer is a surname. Notable people with the surname include:
* Andre Spitzer (1945–1972), Israeli fencing coach and victim of the Munich massacre
* Bernard Spitzer (1924–2014), American real estate developer and philanthropist, father of Eli ...
and Chandra
Chandra ( sa, चन्द्र, Candra, shining' or 'moon), also known as Soma ( sa, सोम), is the Hindu god of the Moon, and is associated with the night, plants and vegetation. He is one of the Navagraha (nine planets of Hinduism) and ...
measured at the same time the stars in the infrared (for 30 days) and the X-ray domains (for 300 kiloseconds). Ground-based observations were taken also at the same time, for example, with the ESO Very Large Telescope
The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across, ...
in Chile, th
Canadian-French-Hawaiian Telescope
in Hawaii, the McDonald Observatory
McDonald Observatory is an astronomical observatory located near unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Locke in the Davis Mountains of West Texas, with additional facil ...
in Texas, or the Calar Alto Observatory
The Calar Alto Observatory (Centro Astronómico Hispano en Andalucía or "Spanish Astronomical Centre in Andalusia") is an astronomical observatory located in Almería province in Spain on Calar Alto, a mountain in the Sierra de Los Filabres ra ...
in Spain.
The CoRoT observations led to the discovery of about a dozen pulsating pre-main sequence (PMS) δ Scuti stars and the confirmation of the existence of γ Doradus pulsations in PMS stars. Also the presence of hybrid δ Scuti/γ Doradus pulsations was confirmed in members of NGC 2264. The CoRoT observations included also the well known pre-main sequence pulsators, V 588 Mon and V 589 Mon, which were the first discovered members of this group of stars. The precision attained in the CoRoT light curves also revealed the important role of granulation
Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation invo ...
in pre-main sequence stars.
The investigation of T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri, but it i ...
stars and their interaction with their circumstellar matter using CoRoT data revealed the existence of a new class, the AA Tauri
AA Tauri is a young variable star in the equatorial constellation of Taurus, located in the Taurus-Auriga star-forming region. It is too faint to view with the naked eye, having an apparent visual magnitude that varies from 12.2 down ...
type objects. Previously to the CoRoT observations, T Tauri stars were known to either show sinusoidal light variations that are caused by spots on the stellar surface, or completely irregular variability that is caused by the gas and dust disks surrounding the young stars. AA Tauri type objects show periodically occurring minima that are different in depth and width, hence are semi-regular variables. With the CoRoT observations this class of objects could be established. Exciting insights into the earliest phases of stellar evolution also come from the comparison of the variability present in the optical light to that in the infrared and the X-ray regime.
Binary systems
A large number of binary systems
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that ...
with non-radially pulsating members were observed by CoRoT. Some of them, which were eclipsing binaries
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
with members of γ Doradus type, were discovered during CoRoT runs. The eclipse phenomenon plays a key role since global parameters can immediately follow, bringing invaluable constraints, in addition to the seismic ones, to stellar modeling.
# AU Monocerotis: This semi-detached binary system contains a Be star
Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...
interacting with its G star companion. Its observation by CoRoT provided an extremely high quality lightcurve. Global parameters could then be improved and new ephemeris
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly vel ...
for the orbital motion as well as for another long term variation were derived. This long period variation seems to originate from a periodic light attenuation by circumstellar dust.
# HD 174884: Tidally induced pulsations have been detected in the high eccentricity (e=0.29) and short period binary system HD 174884 consisting of two B stars. The upper panel of the figure shows the full light curve of the system. In the second panel tiny secondary eclipses are seen with a depth of about 1% of the depth of the primary eclipse. Actually the system is formed of stars of similar mass, size and temperature. Were the orbit circular, the eclipses would be similar in depth. However the orbit is highly eccentric and its orientation in space with respect to us is such that the secondary eclipse occurs when the stars are at a larger distance than at primary eclipse. The third panel of the figure shows the projection on the plane of the sky (i.e. the system as we see it) at different orbital phases.
# CoRoT 102918586 (alias CoRoT Sol 1): The relatively bright eclipsing system CoRoT 102918586 is a double-lined spectroscopic binary, observed by CoRoT, which revealed clear evidence of γ Doradus type pulsations. In addition to CoRoT photometry, a spectroscopic follow-up was performed which yielded the radial velocity curves, the component effective temperatures, the metallicity, and the line-of-sight projected rotational velocities. The eclipsing binary light curve analysis, combined with the spectroscopic results, provided system physical parameters with 1–2% accuracy while the comparison with evolutionary models led to constraints on the age of the system. After subtracting the best–fitting eclipsing binary model, the residuals were analyzed to determine the pulsation properties. The primary star pulsates with typical γ Dor frequencies and shows a period spacing consistent with high order g-modes of degree l=1.
# HR 6902: The binary system ζ Aurigae HR 6902 containing a red giant and a B star was observed by CoRoT during two runs, which allowed us to fully cover the primary as well as the secondary eclipses. This system is presently being analyzed with the ultimate goal of bringing new constraints on the internal structure of the red giant in particular.
# A low mass binary: One of the binary system observed by CoRoT is of particular interest since the less massive component is a late M star of 0.23 M⊙ with an estimated effective temperature of about 3000 K. The primary component is a 1.5 M⊙ star MS star.
# A beaming effect in a binary: A binary system observed by CoRoT showed out of eclipses variations which were interpreted as a beaming effect (also called Doppler boosting). This effect results from the variation in brightness of source approaching or moving away from the observer, with an amplitude proportional to the radial velocity divided by the speed of light. The periodic variation in the velocity of an orbiting star will thus produce a periodic beaming variation in the light curve. Such an effect can confirm the binary nature of a system even without any detectable eclipses nor transits. One of the main advantages of the beaming effect is the possibility to determine the radial velocity directly from the light curve but very different luminosities of the binary components are required and a single radial velocity curve can only be obtained as in an SB1 binary system. The out of eclipse variations were modeled with the BEER (Beaming Ellipsoidal Reflection) algorithm.
Exoplanets
To find extra solar planets, CoRoT uses the method of transits detection. The primary transit is the occultation of a fraction of the light from a star when a celestial object, such as a planet, passes between the star and the observer. Its detection is made possible by the sensitivity of CCD to very small changes in light flux. Corot is capable of detecting changes in brightness of about 1/10,000. Scientists can thus hope finding planets with a size of approximately 2 times that of the Earth with this method, a class of planet called Super-Earth; detection of Corot-7b, whose radius is 1.7 times that of the Earth has shown that these predictions were correct. CoRoT takes an exposure of 32 seconds duration, each 32 seconds, but the image is not fully transmitted to Earth because the data flow would be too large. The onboard computer performs an important work of data reduction: the field around each target star, previously selected by the exoplanets team, is defined on a certain number of pixels described by a particular mask, the sum all pixels within the mask is then performed and several exposures are added (usually 16, which amounts to an integration time of about 8 minutes) before sending this information to the ground. For some stars, considered particularly of interest, data of each exposure is transmitted every 32 seconds. Such a sampling of 32s or 512s is well suited to the detection of a planetary transit that lasts from a little less than an hour to several hours.
A feature of this method is that it requires to detect at least three successive transits separated by two equal time intervals before one can consider a target as a serious candidate. A planet of orbital period ''T'' should at least be observed for a time interval between 2''T'' and 3''T'' to have a chance to detect three transits. The distance of the planet to the star ( which is characterized by ''a'' the semi-major axis of the elliptical orbit ) is linked to its orbital period by the second law of Kepler / Newton ''a''3 = ''T''2 Mstar, using respectively as units for ''a'', M and ''T'': the distance from the Earth to the Sun (150 million km), the mass of the Sun, the orbital period of the Earth (1 year); this implies that if the observing time is less a year, for example, the orbits of the detectable planets will be significantly smaller than that of the Earth.
So, for CoRoT, due to the maximum duration of 6 months of observation for each star field, only planets closer to their stars than 0.3 Astronomic Units (less than the distance between the Sun and Mercury) can be detected, therefore generally not in the so-called habitable zone. The Kepler mission (NASA) has continuously observed the same field for many years and thus had the ability to detect Earth sized planets located farther from their stars.
The moderate number of exoplanets discovered by CoRoT (34 during the 6 years of operation), is explained by the fact that a confirmation should absolutely be provided by ground-based telescopes, before any announcement is made. Indeed, in the vast majority of cases, the detection of several transits does not mean the detection of a planet, but rather that of a binary star system, either one that corresponds to a grazing occultation of a star by the other, or that the system is close enough to a bright star (the CoRoT target) and the effect of transit is diluted by the light of this star; in both cases the decrease in brightness is low enough to be compatible with that of a planet passing in front of the stellar disk. To eliminate these cases, one performs observations from the ground using two methods: radial velocity spectroscopy and imaging photometry with a CCD camera. In the first case, the mass of the binary stars is immediately detected and in the second case one can expect to identify in the field the binary system near the target star responsible for the alert: the relative decline of brightness will be greater than the one seen by CoRoT which adds all the light in the mask defining the field of measurement. In consequence, the CoRoT exoplanet science team has decided to publish confirmed and fully characterized planets only and not simple candidate lists. This strategy, different from the one pursued by the Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
mission, where the candidates are regularly updated and made available to the public, is quite lengthy. On the other hand, the approach also increases the scientific return of the mission, as the set of published CoRoT discoveries constitute some of the best exoplanetary studies carried out so far.
Timeline of planetary discoveries
CoRoT discovered its first two planets in 2007: the hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s CoRoT-1b
CoRoT-1b (previously named CoRoT-Exo-1b) is a transiting extrasolar planet approximately 2,630 light-years away in the constellation of Monoceros. The planet was discovered orbiting the yellow dwarf star CoRoT-1 in May 2007. The planet was t ...
and CoRoT-2b. Results on asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
were published in the same year.
In May 2008, two new exoplanets of Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
size, CoRoT-4b and CoRoT-5b, as well as an unknown massive celestial object, CoRoT-3b, were announced by ESA.
In February 2009, during the First CoRoT Symposium, the super-Earth CoRoT-7b
CoRoT-7b (previously named CoRoT-Exo-7b) is an exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros (constellation), Monoceros, from Earth. It was first detected Methods of detecting extrasolar planets#Transit photometry, photome ...
was announced, which at the time was the smallest exoplanet to have its diameter confirmed, at 1.58 Earth diameters. The discoveries of a second non-transiting planet in the same system, CoRoT-7c
CoRoT-7c is an extrasolar planet which orbits the G-type main sequence star CoRoT-7, located approximately 489 light years away in the constellation Monoceros. It's either a super-Earth or a Neptune-like planet, orbiting at 0.046 AU from th ...
, and of a new Hot Jupiter, CoRoT-6b, were also announced at the Symposium.
In March 2010 CoRoT-9b was announced. It's a long period planet (95.3 days) in an orbit close to that of Mercury.
In June 2010 the CoRoT team announced six new planets, CoRoT-8b, CoRoT-10b, CoRoT-11b, CoRoT-12b, CoRoT-13b, CoRoT-14b, and a brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
, CoRoT-15b. All the planets announced are Jupiter sized, except CoRoT-8b, which appears to be somewhat between Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
and Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
. The probe was also able to tentatively detect the reflected light at optical wavelengths of HD46375 b, a non-transiting planet.
In June 2011, during the Second CoRoT Symposium, the probe added ten new objects to the Exoplanet catalogue: CoRoT-16b, CoRoT-17b, CoRoT-18b, CoRoT-19b, CoRoT-20b, CoRoT-21b, CoRoT-22b, CoRoT-23b, CoRoT-24b, CoRoT-24c.
As of November 2011, around 600 additional candidate exoplanets are being screened for confirmation.
Main results
Among the exoplanets CoRoT detected, one can highlight a subset with the most original features :
* CoRot-1b, the first planet detected by CoRoT is a hot Jupiter. By further analysis, CoRoT-1b
CoRoT-1b (previously named CoRoT-Exo-1b) is a transiting extrasolar planet approximately 2,630 light-years away in the constellation of Monoceros. The planet was discovered orbiting the yellow dwarf star CoRoT-1 in May 2007. The planet was t ...
became the first exoplanets to have its secondary eclipse detected in the optical, thanks to the high precision lightcurve delivered by CoRoT.
* CoRoT-3b, with a mass of 22 MJup, it appears to be "something between a brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
and a planet." According to the definition of planet proposed by the owners of the exoplanet.eu database three years later, CoRoT-3b, being less massive than 25 Jupiter masses, is classified as an exoplanet. In an August 2010 paper, CoRoT detected the ellipsoidal and the relativistic beaming effects in the CoRoT-3 lightcurve.
* CoRot-7b, with a radius of 1.7 REarth and a mass of 7.3 MEarth, was the first confirmed rocky planet, with a density and composition which are close to those of the Earth.
Its orbital period (i.e. its local year) is very short since it lasts just 20.5 h; because the planet is very close to its star (an almost solar type star), its orbit is a mere 6 stellar radii. As the planet must be in synchronous rotation with its orbital motion because of the huge tidal forces it undergoes, it always presents the same hemisphere to the star: as a consequence, the two hemispheres, the enlightened and the dark, exhibit an extreme contrast in temperature (2200K vs 50K) and a huge ocean of lava must occupy a large part of the hot side. A continent of water and nitrogen dioxide ices is probably occupying the dark side. CoRoT-7b was also the first case of a system discovered by CoRoT, with two super-Earths, one in transit the other not; radial velocity measurements have indeed led to the discovery of CoRoT-7c, a planet of 8.4 MEarth and a period of 3.79 days. A third planet is even suspected.
* CoRoT-8b, a planet of the same class as Neptune, with a mass of 0.22 MJup;
* CoRoT-9b, the first planet that has earned the epithet of a temperate planet. With 80% of Jupiter mass, and an orbit similar to the Mercury (planet), Mercury one, this is the first transiting temperate planet found known to be similar to those within the Solar System. At the time of the discovery, it was the second longest period exoplanet found in transit, after HD 80606 b, HD80606 b.
* CoRoT-11b and CoRoT-2b, two inflated planets, with radius 1.4 and 1.5 RJup respectively: the theory does not yet provide a consistent model for such objects;
* CoRoT-15b, a bona fide brown dwarf in orbit;
* CoRoT-10b, CoRoT-16b, CoRoT-20b, CoRoT-23b, four hot Jupiters which are on an eccentric orbits, despite circularization is theoretically predicted for such small orbits: this is a clear constraint on Qp, the parameter that quantifies the energy dissipation by tidal forces;
* CoRoT-22b, notable for its small size, having less than half the mass of Saturn.
* CoRoT-24b and c, the second planetary system discovered by CoRoT, with two small planets of 0.10 and 0.17 MJup. The two planets are of Neptune size, and orbit the same star and represent the first multiple ''transiting'' system detected by CoRoT.
List of planets discovered
The following transiting planets have been announced by the mission.
''Light green rows indicate that the planet orbits one of the stars in a binary star system.''
Other discoveries
The following table illustrates brown dwarf detected by CoRoT as well as non-transiting planets detected in the follow-up program:
Global properties of the exoplanets discovered by CoRoT
All CoRoT planets were detected during long runs ''i.e.'' of at least 70 days. The ''detection team '' found on average between 200 and 300 cases of periodic events for each run, corresponding to 2–3% of the stars monitored. Of these, only 530 in total were selected as candidate planets (223 in the direction of the galactic anti-center and 307 towards the center). Only 30 of them were finally found to be true planets, ''i.e.'' about 6%, other cases being eclipsing binaries ( 46%) or unresolved cases (48%).
The detection capabilities of Corot are illustrated by the figure D showing the depth of the transits measured for all candidates, depending on the period and the brightness of the star: there is indeed a better ability to detect small planets (up to 1.5 R Earth ) for short periods (less than 5 days) and bright stars.
The CoRoT planets cover the wide range of properties and features found in the disparate family of exoplanets: for instance, the masses of CoRoT planets cover a range of almost four orders of magnitude, as shown on Figure.
Tracing the mass of the planet versus the mass of the star (Figure), one finds that the CoRoT data set, with its lower scatter than other experiments, indicates a clear trend that massive planets tend to orbit massive stars, which is consistent with the most commonly accepted models of planetary formation.
See also
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References
External links
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CoRoT on the CNES site
CoRoT N2 Public archive
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Satellites orbiting Earth
CNES
CoRoT,
European Space Agency satellites
Exoplanet search projects
Optical telescopes
Satellites of France
Space telescopes
Spacecraft launched by Soyuz-2 rockets
Spacecraft launched in 2006
Derelict space probes