EA 3 Persei stars

Observation

Key fa Mixed stars

^ Mixed amplitudes

A Mixed periods

- Binaries with spherical or slightly ellipsoidal components. It is possible to specify for their light curves the moments of the beginning and end of the eclipses. Between eclipses the light remains almost constant or varies insignificantly because of reflection effects, slight ellipsoidality of components, or physical variations. Secondary minima may be absent. An extremely wide range of periods is observed, from 01! 2 to > 104 days. Light amplitudes are also quite different and may reach several magnitudes. GCVS

Beta Persei stars are also known as Algol-type variables, named for the group's prototype star, are a subgroup of eclipsing binaries defined according to their distinctive light curve shape (morphology). Their brightness remains approximately constant between the eclipses, that is, variability due to the ellipticity effect and/or the reflection effect is relatively insignificant. As a result, the exact moments of the beginning and the end of the eclipses can be determined by carefully examining the light curve.

Eclipses can range from very shallow (01*01) if partial, to very deep (several magnitudes) if total. The two eclipses can be comparable in depth or can be unequal (Figure 7.3). In a few cases the secondary eclipse is too shallow to be measurable, for example when one star is very cool, or absent altogether when a high eccentric orbit is present.

Light curves of this shape are produced by an eclipsing binary in which both stars are nearly spherical or perhaps only very slightly ellipsoidal. One star may even be highly distorted to the point of filling its Roche lobe, provided it contributes relatively little to the system's total light. This is the case for at least half of the known EA variables.

Figure7.3.Ugh,CurVi of the EA-type variabL star, IQ Per. Data provided by the * I HIPPARCOS mission. Ij^ with permission.

Figure7.3.Ugh,CurVi of the EA-type variabL star, IQ Per. Data provided by the * I HIPPARCOS mission. Ij^ with permission.

Among the EAs you'll find binary systems of very different evolutionary status such as:

(a) systems containing two main sequence stars of any spectral type from O to M;

(b) systems in which one or both stars are evolved but have not yet overflowed their Roche lobe;

(c) systems with one star unevolved and the other overflowing its Roche lobe and causing mass transfer;

(d) systems with one star highly evolved, such as a hot subdwarf or a white dwarf, and the other less evolved;

(e) systems with stars not evolved at all.

Binary systems in the third evolutionary state -semidetached, one star evolved and one not, mass transfer in progress - are termed Algol-type binaries or Algol-like binaries. Such systems, if eclipsing, can have light curves of the EA or EB shape, or they may not eclipse at all. Ironically, ft Lyrae, the prototype of the EB light curve shape, is an Algol-type binary.

The first EA discovered, and the prototype of the group, was ft Persei. Its variability was known by the Chinese 2,000 years before John Goodricke in 1783 determined the strict period of it variability (2"?867) and first proposed eclipses as the mechanism. Algol has partial eclipses, is semidetached, undergoes mass transfer, has a chromospherically active secondary star which emits radio waves and X-rays, and belongs to a triple system.

For these systems, orbital periods range from extremely short such as a fraction of a day to very long, for example 27 years for e Aurigae. For an EA light curve shape, the stellar radius or radii must be a relatively small fraction of the star-to-star separation. Note that the brighter component in s Aurigae is a supergiant but its radius is still a small fraction of the large, 6000 solar radii, semimajor axis.

Orbital periods of EAs can be determined very accurately by timing the sharp eclipses. Period variations are found in many systems. Physical mechanisms responsible can be apsidal motion, orbit around a third body, mass loss and/or mass transfer, and solar-type magnetic cycles. The orbital period of Algol itself undergoes a 1.783 year cycle as it orbits around Algol C and it also has a 32-year magnetic cycle.

Observation

Mixed stars ^ Mixed amplitudes fl Mixed periods <g> Visual, CCD/PEP

0 0

Post a comment