RU Camelopardalis, or RU Cam, is a W Virginis variable (

type II Cepheid Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days. They are population II stars: old, typically metal-poor, low mass objects. Like all Cepheid variables, Type IIs exhibit a relationship between the st ...

) in the constellation of Camelopardalis. It is also a Carbon star, which is very unusual for a Cepheid variable.

History

RU Cam was reported as a new variable star in 1907. It was quickly recognised as one of the Cepheid class of variable stars. The first detailed study of the spectrum of RU Cam showed that it changed during the brightness variations. From partway down the descending branch of the light curve to just after minimum brightness, the spectrum is class R with hydrogen absorption lines. The spectrum then develops hydrogen emission lines. For several days either side of maximum brightness, the spectrum becomes a relatively normal class K. RU Cam remained a somewhat unusual W Virginis variable until 1964, when the relatively regular pulsation of about 1 magnitude almost entirely stopped. Since then the pulsations have varied from cycle to cycle, with amplitudes changing from several tenths of a magnitude to almost zero. The light curve has a more sinusoidal shape than when it was pulsating at full amplitude and the period changes erratically between 17.4 and 26.6 days.

Properties

RU Camelopardalis is both a Carbon star and a

variable star. This is unusual but not unique. At least five other relatively bright examples are known, two of which are of the BL Herculis sub-type. The atmosphere contains more carbon than oxygen but is not deficient in hydrogen. This can be explained as the result of triple-alpha helium burning being processed through a

CNO cycle The CNO cycle (for carbon–nitrogen–oxygen; sometimes called Bethe–Weizsäcker cycle after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, ...

and convected to the surface. This process occurs in some of the more massive asymptotic giant branch (AGB) stars at the third

dredge-up A dredge-up is any one of several stages in the evolution of some stars. By definition, during a ''dredge-up'', a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion. Consequently, t ...

. W Virginis stars are typically metal-poor and enriched by s-process elements, but this is not the case for RU Cam which has near-solar metallicity and no heavy metal enhancement. W Virginis variables are thought to be AGB stars executing a

blue loop In the field of stellar evolution, a blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of the evolutionary track on a Hertzsprung–Russe ...

due to a thermal pulse from the helium burning shell. These stars cross the

instability strip The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillat ...

and undergo very regular pulsations. RU Cam fits this model reasonably well despite its peculiarities. Its temperature of around 5,000 K and luminosity several hundred times the sun's place it on or near the instability strip, and its mass about is typical of AGB stars. The brightness variations of RU Cam are caused by pulsations which cause both the temperature and radius to vary. The temperature has been estimated to vary between 3,800 K and 5,650 K, with a change in the radius of about an average size of . Even prior to 1965, the colour variations suggested a smaller temperature range of 4,220 K - 5,240 K. The maximum temperature occurs at the same time as the minimum radius, and this is when the star is near its brightest.

Evolution

The evolution of a star executing a blue loop from the AGB is expected to be rapid. Period changes in RU Cam before 1965 suggest that it would cross the entire instability strip in 31,000 years. Any secular period changes since then have been masked by irregularities. It is predicted that the temperature of RU Cam is increasing and it is approaching, or leaving, the bluer edge of the instability strip, in which case the pulsations would stop completely. A blueward crossing is the first crossing of the instability strip and would be followed by a second crossing when the star cools back towards the AGB.

References

{{DEFAULTSORT:RU Camelopardalis Camelopardalis (constellation) Camelopardalis, RU 056167 W Virginis variables Carbon stars 035681 J07214412+6940147 BD+69 417