Pulsating white dwarf
Encyclopedia
A pulsating white dwarf is a white dwarf
star
whose luminosity
varies
due to non-radial gravity wave
pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ Ceti, stars, with hydrogen
-dominated atmospheres and the spectral type DA, pp. 891, 895; DBV, or V777 Her, stars, with helium
-dominated atmospheres and the spectral type DB, p. 3525; and GW Vir stars, with atmospheres dominated by helium, carbon
, and oxygen
, and the spectral type PG 1159
. (Some authors also include non-PG 1159 stars in the class of GW Vir stars.) GW Vir stars may be subdivided into DOV and PNNV stars;, §1.1, 1.2; they are not, strictly speaking, white dwarfs but pre-white dwarfs which have not yet reached the white dwarf region on the Hertzsprung-Russell diagram., § 1.1; A subtype of DQV stars, with carbon
-dominated atmospheres, has also been proposed.
These variables all exhibit small (1%–30%) variations in light output, arising from a superposition of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations gives asteroseismological
evidence about the interiors of white dwarfs.
Early calculations suggested that white dwarfs should vary with periods around 10 seconds, but searches in the 1960s failed to observe this., § 7.1.1; The first variable white dwarf found was HL Tau 76
; in 1965 and 1966, Arlo U. Landolt
observed it to vary with a period of approximately 12.5 minutes. The reason for this period being longer than predicted is that the variability of HL Tau 76, like that of the other pulsating variable white dwarfs known, arises from non-radial gravity wave
pulsations., § 7. In 1970, another white dwarf, Ross 548
, was found to have the same type of variability as HL Tau 76; in 1972, it was given the variable star
designation ZZ Ceti. The name ZZ Ceti also refers to this class of pulsating variable white dwarfs, which, as it consists of white dwarfs with hydrogen atmospheres, is also called DAV., pp. 891, 895. These stars have periods between 30 seconds and 25 minutes and are found in a rather narrow range of effective temperature
s between about 12,500 and 11,100 K
. The measurement of the rate of change of period with time for the gravity wave
pulsations in ZZ Ceti stars is a direct measurement of the cooling timescale for a DA white dwarf, which in turn can give an independent measurement of the age of the galactic disk.
was a variable DB, or DBV, white dwarf. This was the first prediction of a class of variable stars before their observation., p. 89. In 1985, this star was given the designation V777 Her, which is also another name for this class of variable stars.; , p. 3525 These stars have effective temperatures around 25,000K., p. 895.
., §1.1. This star (also the prototype for the class of PG 1159 star
s) was observed to vary in 1979, and was given the variable star designation GW Vir in 1985, giving its name to the class. These stars are not, strictly speaking, white dwarfs; rather, they are stars which are in a position on the Hertzsprung-Russell diagram between the asymptotic giant branch
and the white dwarf region. They may be called pre-white dwarfs., § 1.1; They are hot, with surface temperature
s between 75,000 K and 200,000 K, and have atmospheres dominated by helium
, carbon
, and oxygen
. They may have relatively low surface gravities (log g ≤ 6.5.), Table 1 It is believed that these stars will eventually cool and become DO white dwarfs., § 1.1.
The periods of the vibrational modes of GW Vir stars range from about 300 to about 5,000 second
s., Table 1 How pulsations are excited in GW Vir stars was first studied in the 1980s but remained puzzling for almost twenty years. From the beginning, the excitation mechanism was thought to be caused by the so-called κ-mechanism associated with ionized carbon
and oxygen
in the envelope below the photosphere, but it was thought this mechanism would not function if helium was present in the envelope. However, it now appears that instability can exist even in the presence of helium., §1.
suggest that SDSS J142625.71+575218.3 is such a white dwarf; if so, it would be the first member of a new, DQV, class, of pulsating white dwarfs. However, it is also possible that it is a white dwarf binary system
with a carbon
-oxygen
accretion disk.
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
whose luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
varies
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
due to non-radial gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ Ceti, stars, with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-dominated atmospheres and the spectral type DA, pp. 891, 895; DBV, or V777 Her, stars, with helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
-dominated atmospheres and the spectral type DB, p. 3525; and GW Vir stars, with atmospheres dominated by helium, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, and the spectral type PG 1159
PG 1159 star
A PG 1159 star, often also called a pre-degenerate, is a star with a hydrogen-deficient atmosphere which is in transition between being the central star of a planetary nebula and being a hot white dwarf...
. (Some authors also include non-PG 1159 stars in the class of GW Vir stars.) GW Vir stars may be subdivided into DOV and PNNV stars;, §1.1, 1.2; they are not, strictly speaking, white dwarfs but pre-white dwarfs which have not yet reached the white dwarf region on the Hertzsprung-Russell diagram., § 1.1; A subtype of DQV stars, with carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-dominated atmospheres, has also been proposed.
These variables all exhibit small (1%–30%) variations in light output, arising from a superposition of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations gives asteroseismological
Asteroseismology
Asteroseismology also known as stellar seismology is the science that studies the internal structure of pulsating stars by the interpretation of their frequency spectra. Different oscillation modes penetrate to different depths inside the star...
evidence about the interiors of white dwarfs.
DAV stars
| Types of pulsating white dwarf, §1.1, 1.2; | |
| DAV (GCVS: ZZA) | DA spectral type, having only hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... absorption lines in its spectrum |
| DBV (GCVS: ZZB) | DB spectral type, having only helium Helium Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table... absorption lines in its spectrum |
| GW Vir (GCVS: ZZO) | Atmosphere mostly C, He and O; may be divided into DOV and PNNV stars |
| DQV | DQ spectral type; hot, carbon Carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds... -dominated atmosphere |
Early calculations suggested that white dwarfs should vary with periods around 10 seconds, but searches in the 1960s failed to observe this., § 7.1.1; The first variable white dwarf found was HL Tau 76
HL Tau 76
HL Tau 76 is a variable white dwarf star of the DAV type. It was observed by G. Haro and W. J. Luyten in 1961, and was the first variable white dwarf discovered when, in 1968, Arlo U. Landolt found that it varied in brightness with a period of approximately 749.5 seconds, or 12.5 minutes...
; in 1965 and 1966, Arlo U. Landolt
Arlo U. Landolt
Arlo U. Landolt is an American astronomer. Landolt has worked principally in photometry and has published a number of widely used lists of standard stars. In 1995, he received the George van Biesbroeck Prize from the American Astronomical Society...
observed it to vary with a period of approximately 12.5 minutes. The reason for this period being longer than predicted is that the variability of HL Tau 76, like that of the other pulsating variable white dwarfs known, arises from non-radial gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
pulsations., § 7. In 1970, another white dwarf, Ross 548
Ross 548
Ross 548 is a variable white dwarf star of the DAV type. It was found to be variable in 1970; in 1972, it was given the variable star designation ZZ Ceti, which also refers to the entire class of DAV variable white dwarfs., pp. 891, 895....
, was found to have the same type of variability as HL Tau 76; in 1972, it was given the variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
designation ZZ Ceti. The name ZZ Ceti also refers to this class of pulsating variable white dwarfs, which, as it consists of white dwarfs with hydrogen atmospheres, is also called DAV., pp. 891, 895. These stars have periods between 30 seconds and 25 minutes and are found in a rather narrow range of effective temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...
s between about 12,500 and 11,100 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. The measurement of the rate of change of period with time for the gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
pulsations in ZZ Ceti stars is a direct measurement of the cooling timescale for a DA white dwarf, which in turn can give an independent measurement of the age of the galactic disk.
DBV stars
In 1982, calculations by D. E. Winget and his coworkers suggested that helium-atmosphere DB white dwarfs with surface temperatures around 19,000 K should also pulsate., p. L67. Winget then searched for such stars and found that GD 358GD 358
GD 358 is a variable white dwarf star of the DBV type. Like other pulsating white dwarfs, its variability arises from non-radial gravity wave pulsations within itself. GD 358 was discovered during the 1958–1970 Lowell Observatory survey for high proper motion stars in the Northern Hemisphere...
was a variable DB, or DBV, white dwarf. This was the first prediction of a class of variable stars before their observation., p. 89. In 1985, this star was given the designation V777 Her, which is also another name for this class of variable stars.; , p. 3525 These stars have effective temperatures around 25,000K., p. 895.
GW Vir stars
The third known class of pulsating variable white dwarfs is the GW Vir stars, sometimes subdivided into DOV and PNNV stars. Their prototype is PG 1159-035PG 1159-035
PG 1159-035 is the prototypical PG 1159 star after which the class of PG 1159 stars was named. It was discovered in the Palomar-Green survey of ultraviolet-excess stellar objects and, like the other PG 1159 stars, is in transition between being the central star of a planetary nebula and being a...
., §1.1. This star (also the prototype for the class of PG 1159 star
PG 1159 star
A PG 1159 star, often also called a pre-degenerate, is a star with a hydrogen-deficient atmosphere which is in transition between being the central star of a planetary nebula and being a hot white dwarf...
s) was observed to vary in 1979, and was given the variable star designation GW Vir in 1985, giving its name to the class. These stars are not, strictly speaking, white dwarfs; rather, they are stars which are in a position on the Hertzsprung-Russell diagram between the asymptotic giant branch
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
and the white dwarf region. They may be called pre-white dwarfs., § 1.1; They are hot, with surface temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...
s between 75,000 K and 200,000 K, and have atmospheres dominated by helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. They may have relatively low surface gravities (log g ≤ 6.5.), Table 1 It is believed that these stars will eventually cool and become DO white dwarfs., § 1.1.
The periods of the vibrational modes of GW Vir stars range from about 300 to about 5,000 second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s., Table 1 How pulsations are excited in GW Vir stars was first studied in the 1980s but remained puzzling for almost twenty years. From the beginning, the excitation mechanism was thought to be caused by the so-called κ-mechanism associated with ionized carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the envelope below the photosphere, but it was thought this mechanism would not function if helium was present in the envelope. However, it now appears that instability can exist even in the presence of helium., §1.
DQV stars
A new class of white dwarfs, with spectral type DQ and hot, carbon-dominated atmospheres, has recently been discovered by Patrick Dufour, James Liebert and their coworkers. Theoretically, such white dwarfs should pulsate at temperatures where their atmospheres are partially ionized. Observations made at McDonald ObservatoryMcDonald Observatory
The McDonald Observatory is an astronomical observatory located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Fowlkes and Mount Locke in the Davis Mountains of West Texas...
suggest that SDSS J142625.71+575218.3 is such a white dwarf; if so, it would be the first member of a new, DQV, class, of pulsating white dwarfs. However, it is also possible that it is a white dwarf binary system
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
with a carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
accretion disk.
External links and further reading
- Variable White Dwarf Data Tables, Paul A. Bradley, 22 March 2005 version. Accessed online June 7, 2007.
- A Progress Report on the Empirical Determination of the ZZ Ceti Instability Strip, A. Gianninas, P. Bergeron, and G. Fontaine, arXiv:astro-ph/0612043.
- Asteroseismology of white dwarf stars, D. E. Winget, Journal of Physics: Condensed Matter 10, #49 (December 14, 1998), pp. 11247–11261. DOI 10.1088/0953-8984/10/49/014.