The O'Connell effect is an asymmetry in the photometric

light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

of certain close

eclipsing binary star A binary star or binary star system is a Star system, system of two stars that are gravity, 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 ...

s. It was named after the astronomer

Daniel Joseph Kelly O'Connell Daniel Joseph Kelly O'Connell was a seismologist, astronomer and Jesuit priest who is particularly known for his work in observational astronomy. He worked primarily observing binary star systems, and was involved in photographing the Green Flas ...

, SJ of Riverview College in

New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

who in 1951 studied this phenomenon and distinguished it from the so-called periastron effect described by earlier authors, as it does not necessarily appear near the

periastron An apsis (; ) is the farthest or nearest point in the orbit of a planetary-mass object, planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two maximum a ...

, when tidal effects and an increase in mutual radiation may cause an increase in

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

The effect

The out-of-eclipse brightness maxima of some binary stars are unequally high. This is contrary to expectations that the observed

of an eclipsing binary should be the same when its components switch positions every half period. The maximum following the primary minimum is nearly always brighter than the preceding one. This is called the positive O'Connell effect, the reverse case is referred to as the negative O'Connell effect. The difference increases with the

ellipticity Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening is f ...

of the stars, and the differences in their sizes and densities. Also,

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differences have been observed between subsequent maxima.

Attempts at explanation

In some systems where the phenomenon has been observed, such as in CG Cygni, RT Lacertae, XY Ursae Majoris, or YY Eridani, the luminosity difference between subsequent maxima has been found to be variable, in others relatively stable. Furthermore, it has been observed in a variety of configurations, such as over-contact, semi-detached, and near contact systems alike. These factors make an explanation difficult and suggest that various mechanisms may be responsible for the effect to manifest. Several reasons have thus been proposed: an asymmetric distribution of

starspot 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 ...

s, impacts of one-way gas streams between the components of the binary system, or the flow of circumstellar matter, asymmetrically deflected due to

Coriolis force In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motio ...

Examples

The O'Connell effect has been observed, among others, in the binary systems W Crucis, RT Lacertae, CX Canis Majoris, TU Crucis, AQ Monocerotis, DQ Velorum, and CG Cygni.

References

{{reflist - Unsolved problems in astronomy Periodic phenomena Optical phenomena