Ellipsoid variable star

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The ellipsoidal variable stars belong to the group of rotationally variable stars . Their change of light is the result of the deviation from the spherical shape. This deviation occurs when the two stars of a close binary star system deform ellipsoidally due to their mutual gravitational attraction . As a result, the area visible from the earth changes during an orbit and the brightness fluctuates with low amplitude .

Classic ellipsoidal variables

With the classic ellipsoidal variables, the stars of a binary star system revolve around the common center of mass on a circular path. The shape of the stars takes on an egg shape due to the gravitation of the other star. Furthermore, there is the phenomenon of gravity darkening , i.e. a change in the temperature of the star surface that faces away from the other star.

Together, these effects result in a light curve in which two minima and two maxima occur per revolution. The period of most ellipsoid variables is no more than 5 days for main sequence stars . The amplitude of the changes in brightness is less than 0.1  mag . Ellipsoidal light change often occurs in combination with coverage variability and reflection light change .

Ellipsoidal light change in red giants

Ellipsoidal change of light has also been demonstrated for red giants and AGB stars in binary star systems. The orbital period of these stars is up to 1000 days. Since these stars all show semi-regular changes in brightness , the separation between the ellipsoid and the pulsation- induced light change is difficult.

Heartbeat stars

Heartbeat stars are so named because the shape of their light curve is similar to the heartbeat on an electrocardiogram . The light change of these stars is caused by elliptical orbits around the common center of mass.

The elliptical orbits lead to changes in the shape of the stars during each orbit and thus trigger oscillations in the stars. The duration of the oscillations is an integral part of the travel time, it is a resonance phenomenon similar to the orbit resonance .

The vibrations together with the ellipsoidal light change lead to a great variety of light curves, some showing a minimum before the heartbeat, others after ; W- or M-shaped changes in brightness have also been observed, as well as an overlay with variability of coverage. The shape of the light curves can be modeled very well using the parameters inclination , eccentricity and apsidal angle .

The amplitudes are very small with a few hundred parts per million . The periods of rotation of the systems discovered so far are between 2 and 20 days.

Examples

Individual evidence

  1. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  2. ↑ CP Nicholls and PR Wood: Eccentric Ellipsoidal Red Giant Binaries in the LMC: Complete Orbital Solutions and Comments on Interaction at Periastron . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.1043v1 .
  3. ↑ CP Nicholls, PR Wood and M.-RL Cioni: Ellipsoidal Variability and the Difference between Sequence D and E Red Giants . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1002.3751v1 .
  4. ↑ William F. Welsh, Jerome A. Orosz, Conny Aerts, Timothy M. Brown, Erik Brugamyer, William D. Cochran, Ronald L. Gilliland, Joyce Ann Guzik, DW Kurtz, David W. Latham, Geoffrey W. Marcy, Samuel N. Quinn, Wolfgang Zima, Christopher Allen, Natalie M. Batalha, Steve Bryson, Lars A. Buchhave, Douglas A. Caldwell, Thomas N. Gautier III, Steve B. Howell, K. Kinemuchi, Khadeejah A. Ibrahim, Howard Isaacson , Jon M. Jenkins, Andrej Prsa, Martin Still, Rachel Street, Bill Wohler, David G. Koch, William J. Borucki: KOI-54: The Kepler Discovery of Tidally-Excited Pulsations and Brightenings in a Highly Eccentric Binary . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1102.1730 .
  5. ↑ Susan E. Thompson, Mark Everett, Fergal Mullally, Thomas Barclay, Steve B. Howell, Martin Still, Jason Rowe, Jessie L. Christiansen, Donald W. Kurtz, Kelly Hambleton, Joseph D. Twicken, Khadeejah A. Ibrahim, Bruce D. Clarke: A Class of Eccentric Binaries with Dynamic Tidal Distortions Discovered with Kepler . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.6115 .
  6. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .