ZZ Ceti star

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The ZZ-Ceti stars are a class of pulsation-variable white dwarfs that are named after the prototype ZZ Ceti in the constellation Whale ( Latin: Cetus).

definition

The term ZZ-Ceti stars is used both as a synonym for all pulsation-variable white dwarfs and for a subgroup of pulsation-variable white dwarfs. The amplitude of the changes in brightness of all ZZ-Ceti stars is low with less than 0.3  mag , with periods of a few minutes. The vibrations in the atmospheres of all ZZ-Ceti stars are the result of non-radial g-waves .

classification

Pulsating white dwarfs are divided into several classes:

  • The DAV stars ( GCVS: ZZA ) are also known as classic ZZ-Ceti stars, since they were discovered as the first variable white dwarfs. Their position in the Hertzsprung-Russell diagram is the extension of the instability strip into the cooling path of white dwarfs. The kappa mechanism that controls the pulsations is located in the ionization zone of the hydrogen . The spectra show a characteristic hydrogen atmosphere with an effective temperature 11100-12500  K .
  • The DBV stars ( GCVS: ZZB ) or V777 Herculis stars show an almost pure helium atmosphere; the excitation mechanism of these stars is assumed to be in the helium ionization zone. The effective temperature of the DBV stars is between 19,000 and 25,000 K. The force of gravity is assumed to be the resetting mechanism , with period lengths of the pulsations between 100 and 1100 seconds.
  • The DQV stars show lines of neutral or simply ionized carbon or the Swan bands in their spectra . The unusual chemical composition of these white dwarfs could be due to late thermal pulses , or the precursor star was a super AGB star . The cause of the variability is unknown.
  • The GW Virginis (GCVS: ZZO ) or PG1159 stars have an even higher effective temperature between 75,000 and 200,000 K. These are Post-AGB stars that transform into white dwarfs. The kappa mechanism of the GW Vir stars is probably based on the cyclic ionization of carbon and oxygen . The PG1159 stars show a strong accumulation of helium and carbon in their atmospheres. This is interpreted as a consequence of a late thermal pulse. The term GW Virginis stars is also used for pulsating white dwarfs in cataclysmic binary star systems. These are close double stars made up of an accreting white dwarf and a mass-donating companion star. If the mass flow to the white dwarf is low, it is possible to observe and analyze the vibrations of the white dwarf.
  • White dwarfs with a mass of less than 0.35  solar masses (M ☉ ) and temperatures between 8500 and 10,000 K are known as ELMVs (extremely low mass variables) or EL Canum Venaticorum stars .
  • The white dwarf SDSS J184037.78 + 642312.3 has not yet been assigned to a class. It is the first pulsating white dwarf of very low mass (below 0.25 M ☉ ) with a helium nucleus and a thick hydrogen atmosphere. The pulsation periods are around 4700 seconds and the temperature around 9100 K. These white dwarfs with less than 0.25 M ☉ were found predominantly in binary star systems as companions of millisecond pulsars .

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists not quite a hundred stars with the abbreviations ZZ , ZZA , ZZB or ZZO , which means that not quite 0.2% of all stars in this catalog belong to the class of ZZ-Ceti stars or a sub-category.

Asteroseismology

Because of the short periods of a few seconds to minutes, the ZZ-Ceti stars are the preferred target of asteroseismology . The structure of the star is deduced from the analysis of the vibrations . Asteroseismology can determine the following quantities in white dwarfs:

Asteroseismology is a way of independently verifying simulations of star evolution . In the case of ZZ-Ceti stars, it is observed that only a few modes are excited. In contrast to this, according to theoretical calculations, a large number of vibration modes should pulsate. The analysis is made more difficult by the variability of the amplitude of the individual oscillation modes.

Examples

literature

Web links

Individual evidence

  1. ↑ Alejandro H. Corsico et al: Asteroseismology of the Kepler V777 Her variable white dwarf with fully evolutionary models . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1112.5882v1 .
  2. ↑ Kurtis A. Williams et al .: PHOTOMETRIC VARIABILITY IN A WARM, STRONGLY MAGNETIC DQ WHITE DWARF, SDSS J103655.39 + 652252.2 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.3165v1 .
  3. ↑ R. Gallino, O. Straniero, E. Zinner, M. Jadhav, L. Piersanti, p Cristallo, S. Bisterzo: Nucleosynthesis origin of PG 1159 stars, Sakurai's object and of rare subclasses of Presolar grains . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1107.0562v1 .
  4. ↑ Helena Uthas et al .: Two new accreting, pulsating white dwarfs: SDSS J1457 + 51 and BW Sculptoris . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1111.3956v1 .
  5. ^ SO Kepler, Alejandra D. Romero: Pulsating White Dwarfs . In: Astrophysics. Solar and Stellar Astrophysics . 2017, arxiv : 1706.07020v1 .
  6. ^ AH Corsico, AD Romero, LG Althaus, JJ Hermes: The seismic properties of low-mass He-core white dwarf stars . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1209.5107 .
  7. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved October 11, 2019 .
  8. ↑ JN Fu et al .: Asteroseismology of the ZZ Ceti star HS 0507 + 0434B . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1110.6226 .