G29-38
Encyclopedia
Giclas 29-38 is a variable
white dwarf
star
of the DAV, or ZZ Ceti
,
whose variability is due to large-amplitude
, non-radial pulsations, known as gravity wave
s. It was first reported to be variable by Shulov and Kopatskaya in 1974. DAV stars are like normal white dwarfs but have luminosity variations with amplitudes as high as 30%, arising from a superposition of vibrational modes with periods from 100 to 1,000 seconds. Large-amplitude DAVs generally differ from lower-amplitude DAVs by having lower temperatures, longer primary periodicities, and many peaks in their vibrational spectra
with frequencies which are sums of other vibrational modes.
G29-38, like other complex, large-amplitude DAV variables, has proven difficult to understand. The power spectrum
or periodogram
of the light curve varies over times which range from weeks to years. Usually, one strong mode dominates, although many smaller-amplitude modes are often observed. The larger-amplitude modes, however, fluctuate in and out of observability; some low-power areas show more stability. Asteroseismology
uses the observed spectrum of pulsations from stars like G29-38 to infer the structure of their interiors.
progenitor, and therefore the excess was naturally explained by emission from a Jupiter
-like brown dwarf with a temperature of 1200 K
and a radius of 0.15 solar radius. However, later observations, including speckle interferometry, failed to detect a brown dwarf.
Infrared observations made in 2004 by NASA
's Spitzer Space Telescope
indicated the presence of a dust cloud around G29-38, which may have been created by tidal disruption of a comet passing close to the white dwarf. This may mean that G29-38 is still orbit
ed by a ring of surviving comets and, possibly, outer planets. This is the first observation supporting the idea that comet
s persist to the white dwarf stage of stellar evolution
.
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...
white dwarf
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...
of the DAV, or ZZ Ceti
Pulsating white dwarf
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,...
,
whose variability is due to large-amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
, non-radial pulsations, known as 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....
s. It was first reported to be variable by Shulov and Kopatskaya in 1974. DAV stars are like normal white dwarfs but have luminosity variations with amplitudes as high as 30%, arising from a superposition of vibrational modes with periods from 100 to 1,000 seconds. Large-amplitude DAVs generally differ from lower-amplitude DAVs by having lower temperatures, longer primary periodicities, and many peaks in their vibrational spectra
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
with frequencies which are sums of other vibrational modes.
G29-38, like other complex, large-amplitude DAV variables, has proven difficult to understand. The power spectrum
Spectral density
In statistical signal processing and physics, the spectral density, power spectral density , or energy spectral density , is a positive real function of a frequency variable associated with a stationary stochastic process, or a deterministic function of time, which has dimensions of power per hertz...
or periodogram
Periodogram
The periodogram is an estimate of the spectral density of a signal. The term was coined by Arthur Schuster in 1898 as in the following quote:...
of the light curve varies over times which range from weeks to years. Usually, one strong mode dominates, although many smaller-amplitude modes are often observed. The larger-amplitude modes, however, fluctuate in and out of observability; some low-power areas show more stability. Asteroseismology
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...
uses the observed spectrum of pulsations from stars like G29-38 to infer the structure of their interiors.
Debris disk
The circumstellar environment of G29-38 first attracted attention in the late 1980s during a near-infrared survey of 200 white dwarfs conducted by Ben Zuckerman and Eric Becklin to search for low mass companion stars and brown dwarfs. G29-38 was shown to radiate substantial emission between 2 and 5 micrometres, far in excess of that expected from extrapolation of the visual and near infrared spectrum of the star. Like other young, hot white dwarfs, G29-38 is thought to have formed relatively recently (600 million years ago) from its AGBAsymptotic 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....
progenitor, and therefore the excess was naturally explained by emission from a Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
-like brown dwarf with a temperature of 1200 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...
and a radius of 0.15 solar radius. However, later observations, including speckle interferometry, failed to detect a brown dwarf.
Infrared observations made in 2004 by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
indicated the presence of a dust cloud around G29-38, which may have been created by tidal disruption of a comet passing close to the white dwarf. This may mean that G29-38 is still orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
ed by a ring of surviving comets and, possibly, outer planets. This is the first observation supporting the idea that comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s persist to the white dwarf stage of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
.