Zand Z Andromedae variable stars

- Symbiotic variables of the Z Andromedae type. They are close binaries consisting of a hot star, a star of late type, and an extended envelope excited by the hot star's radiation. The combined brightness displays irregular variations with amplitudes up to 4"' in V. A very inhomogeneous group of objects. GCVS

Z And stars, also known as symbiotic stars, are interacting binary systems. The defining characteristic of this diverse group is that, in addition to erratic photometric variability, their spectra simultaneously show spectral signatures such as the molecular absorption features of a cool giant star. Studies over limited wavelength regions have often resulted in symbiotic stars being incorrectly classified as something else, most often as peculiar planetary nebulae. The molecular absorption features are frequently only present in infrared spectra.

The giant component of the binary system is usually of spectral type M or C. There are also a few so-called

7 A subclass of the U Geminorum type variable (dwarf novae) characterized by two types of outbursts called "normal" and "supermaxima" and by superposed "superhumps" during a super-maximum outburst.

"In Greek mythology, Andromeda was the daughter of King Cepheus and Queen Cassiopeia.

Observation

Mixed stars ^ Large amplitude* j^^ Mixed periods <5S> Visual, CCD/PEP

¡^ provided by the ySNET U«d wilh penwssio"

yellow symbiotics which have G-type spectra. The other star in the binary system may be a low-mass main sequence star or compact object, such as a subdwarf star, white dwarf or neutron star. The interaction that results in the symbiotic phenomenon, including erratic variability and high-excitation emission lines, begins when mass is transferred from the giant star to its partner. Most of the well-studied systems contain either a main sequence star which accretes by direct tidal overflow from the giant, or a white dwarf which accretes from the giant's stellar wind. Many Z And stars show evidence of an accretion disk. The transfer of mass will often produce a hot spot within the accretion disk. In many cases this hot spot provides the temperature necessary for ionizing part of the circumstellar environment and producing the emission lines. Symbiotic stars are closely related to the even rarer VV Cep systems within which a late-type supergiant interacts with an O or a B star.

Chapter 6

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