What are RS CVn Stars?!
In 1976 the astronomer Douglas Holl defined one more class of spectroscopic binary stars, among the variety of the different binary systems. These are the RS CVn type stars. Because of the fact that the light curve of the RS CVn star itself looks like the change of the brightness of the Algol type systems, initially it was considered to be a part of this type of stars. The observation that followed showed that there are typical details in the light curve that make it not symmetrical. The brightness level before and after the main minimum is different. As a result of intensive observation during the following four years the presence of depression was confirmed with depth from 0,1m to 0,2m.
Artist's impression of a RS CVn star
RS CVn are class of detached binary typically composed of a chromospherically active G or K stars. The system generally rotate fast with typical orbital period from a few days to 20 days. The RS CVn binaries display a high level of activity with strong chromospheric line emissions - mainly H and K of CaII - which proves the chromospherical activity of solar type. X-ray and radio emissions are typical for these systems. For some of them a quasisinusoidal wave on the light curves in sections out of the eclipses, which changes their amplitude and position slowly in time is also very typical. The appearance of this wave (called distortion wave) is explained with the presence of groups of spots on the surface of the star. The rotational period of the groups is usually close to the orbital period (the period of eclipses), but it differs a bit. This causes the slow change (migration) of the phases of the minimum and the maximum of the distortion wave on the middle light curve. The variability of the wave's amplitude (reaching 0.m2 in V) is explained with the existence of long-periodical cycle of the star activity (similar to the Sun's 11-year cycle). The number and the total area of the spots on the star's surface are changed during this period.
A binary star system consists of two stars, both orbiting around their barycenter. The term "binary star" was apparently first coined by Sir William Herschel in 1802.
Binary star classifications: At present, binary stars are classified into four types according to their observable properties:
spectroscopic binaries - a binary star system in which the two components are so close together, that they cannot be resolved simply by looking at them, even through a powerful telescope. Their binary nature can, however, be established because of the Doppler shift of their spectral lines.. As the stars revolve around their common center of gravity, they alternately approach and recede in the line of sight and its spectral lines show blueshifts and redshifts as a result of this motion (e.g. we see a regular oscillation or doubling of the spectral lines).
Any star can belong to several of these classes, e.g., several spectroscopic binaries are also eclipsing binaries. There are also so called close binary systems - a binary star system in which the separation of the components is comparable to the diameter of the stars.
Another three-category classification is based on the distance of these stars:
detached binaries - neither star fills its Roche lobe, so that there is no significant mass transfer between the components
semidetached binaries - In a semidetached binary, one of the stars fills its Roche lobe, which results in this star losing material in a stream that either falls directly onto its companion, or, as is more usual, that enters an accretion disk
contact binaries - In a contact binary both components fill their Roche lobes or, more often, overflow them so that there is a common convective envelope
The red line in the following figure represents the equipotential surface of the Roche lobe
Algol star (eclipsing binary)
The most famous eclipsing binary star is visible to the naked eye and has a rich mythology associated with it...
Algol is a star in the constellation Perseus. Algol's variation in apparent - magnitude, from 2.06 to 3.28, is due to the fact that it is an eclipsing binary star, with one component revolving about the other with a period of 2 days, 20 hr, 49 min. Because the plane of revolution is almost parallel to the line of sight, the star dims noticeably when the dimmer component passes in front of, or eclipses, the brighter component, and dims again very slightly when the brighter component eclipses the dimmer one (see eclipse); the primary minimum, when the brighter component is eclipsed, lasts about 10 hr. Algol is of spectral class B8 V and is about 105 light-years from the earth. The name Algol comes from the Arabic Ras al Ghul, which means "demon's head," and the star is sometimes called the Demon Star.
The following animation illustrates the motion in the Algol system that leads to the eclipses
The orange line between the two stars is a schematic indication that there appears to be matter streaming between the components of the binary.
Capella - the brightest and the best learned RS CVn type of stars
Capella, brightest star in the constellation Auriga; Capella is a yellow giant star of spectral class G8 III and is also a spectroscopic binary star with a component of spectral class F. Its apparent magnitude of 0.06 makes it the 6th-brightest star in the sky. Capella is about 45 light-years from the earth. Its name is from the Latin for "little she-goat."
Capella is a source of X-rays, probably from surface magnetic activity similar to that seen on the surface of the Sun, but which star is responsible is uncertain.
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