Pulsating white dwarf

The ZZ Ceti stars are a class of pulsating variables white dwarfs, which are named after the prototype ZZ Ceti in the constellation Cetus (Latin Cetus ).

Definition

The term of the ZZ Ceti star is used both as a synonym for all pulsationsveränderliche white dwarfs as well as for a subset of the pulsating variables white dwarfs. The amplitude of the brightness variations of all ZZ Ceti star is low at less than 0.3 may at periods of a few minutes. The vibrations in the atmosphere of all ZZ Ceti stars are the result of non-radial G waves.

Classification

Pulsating white dwarfs are divided into three classes:

• The DAV stars are also classic ZZ Ceti star named because they were discovered as the first variable white dwarfs. Your position in the Hertzsprung -Russell diagram is the extension of the instability strip in the cooling track of white dwarfs and the pulsations controlling kappa mechanism lies in the ionization of hydrogen. The spectra show a characteristic atmosphere of hydrogen at an effective temperature of 12,500 to 11,100 degrees Kelvin.
• The DBV star or V777 - Herculis stars show an almost pure helium atmosphere and the excitation mechanism is suspected in the ionization of helium. The effective temperature of the DBV star is 19000-25000 Kelvin. When resetting mechanism, the gravitational force is assumed in period lengths of the pulsations 100-1100 seconds.
• The DQV -star show lines of neutral or ionized carbon or the Swan bands in their spectra. The unusual chemical composition of these white dwarfs could by late thermal pulses occur or the progenitor star was a super - AGB star. The cause of the variability is not known.
• The GW Virginis stars and PG1159 stars have an even higher effective temperature 75000-200000 Kelvin. These are post-AGB stars, which transform into white dwarf. The kappa mechanism of the GW Vir star probably based on the cyclical ionization of carbon and oxygen. The PG1159 stars show a strong enrichment of helium and carbon in their atmospheres. This is interpreted as a result of a thermal pulse late. The term of the GW Virginis stars is also used for pulsating white dwarfs in cataclysmic binary systems. This is to close binary systems with a white dwarf accreting and a mass -giving companion star. If the mass flow to the white dwarf low so it is possible to observe the oscillations of the white dwarf and analyze.
• Was still no class assigned to the white dwarf SDSS J184037.78 642312.3. This is the first pulsating white dwarf of very low mass with a helium nucleus and a thick atmosphere of hydrogen. The mass is less than 0.25 solar masses. The pulsation periods will be around 4700 seconds and the temperature at 9100 K. These white dwarfs with less than 0.25 solar masses have been found predominantly in binary systems as a companion to Millisekundenpulsaren.

Asteroseismology

Because of the short periods of a few seconds to minutes, the ZZ Ceti star preferred destination of asteroseismology. This is inferred from the analysis of the vibration to the structure of the star. The asteroseismology can determine in white dwarfs, the density distribution, the radius, mass, the absolute luminosity, rotation speed, the chemical composition and indirectly age. This is one way to verify the stellar evolution simulations independently. It is observed in ZZ Ceti stars that only a few modes are excited as opposed to theoretical calculations, which should pulsate a variety of modes of oscillation. Complicating the analysis is the variability of the amplitude of the individual modes of vibration.