White dwarfs
(Napiwotzki)
Most stars (i.e. all stars with a mass below about 8 solar masses) will end their lives as white dwarfs. These are small cooling bodies consisting mainly of carbon and oxygen with thin layers of hydrogen and helium on top, supported by degenerate electron gas. After having lost their envelopes in the final AGB and post-AGB phases no further nuclear energy sources are available and they will cool for billions of years.
White dwarfs have masses ranging from 50% to 100% the mass of our Sun, but matter in white dwarfs is so compressed that they are only the size of our Earth. The surface gravity of white dwarfs is so high that heavy elements diffuse downwards and only the lightest element (usually hydrogen or helium remains) at the surface.
Structure of a white dwarf: The figure (above) shows a cut through the interior of a typical white dwarf. The fusion processes in the progenitor star converted most of the matter into carbon and oxygen. On top of this core are thin layers of helium and hydrogen.
Progenitor systems of supernovae type Ia
Supernovae (SNe) mark the violent termination of a star's life in an explosion. They are classified according to their light curve as type I or II, with the type I SNe producing very similar light curves, while the SNe type II are more diverse. According to their spectral appearance the type I class can be further subdivided into Ia, Ib, and Ic.
SN 1994d in NGC 4526. This was a type Ia supernova. Since type Ia supernovae were identified as excellent distance indicators for cosmology and have provided indications of cosmic acceleration, it is extremely important to have a better understanding of their explosions and the systems that lead up to them. STScI, NASA
SNe type II and Ib,c are produced at the end of the life of short-lived massive stars (masses above 8 solar masses). Indeed, the occurrence of SN explosions and the formation of a neutron star remnant at the end of the nuclear lifetime of a massive stars are now relatively well understood processes. However, the question of SN Ia progenitors is not yet settled. SN Ia are observed in all types of galaxies, including elliptical galaxies containing only old stellar populations. This makes the thermonuclear explosion of a white dwarf the best candidate to explain the origin SN Ia explosions.
Most white dwarfs will never explode. To do so, they have to be forced into a density and temperature regime, where carbon and oxygen burn explosively and disrupt the star. Above the Chandrasekhar mass (1.4 solar masses) the electron degeneracy can no longer support white dwarfs. At this point the white dwarf either has to collapse to a neutron star or explode as a supernova. Since no physical process is known, which leads to such conditions in a single white dwarf, a companion star has to help. This general picture of binary white dwarfs as the progenitor stars for type Ia supernovae is the most commonly held view today.
The growth of a white dwarf to Chandrasekhar mass is a long-standing problem of observational astrophysics. Several channels have been identified as possibly yielding such a critical mass (see figure below). They can broadly be grouped into two classes. The single degenerate (SD) channel in which the white dwarf is accompanied by a regular star, either a main sequence star, a (super)giant, or a helium star, as mass donor and the double degenerate (DD) channel where the companion is another white dwarf. Close DDs radiate gravitational waves, which results in a shrinking orbit due to the loss of energy and angular momentum. If the initial separation is close enough (orbital periods below 10 h), a DD system could merge within a Hubble time, and if the combined mass exceeds the Chandrasekhar limit the DD would qualify as a potential SN Ia progenitor.
Possible evolutionary channels for the formation of a SN Ia progenitor via the double degenerate (DD) and the single degenerate (SD) scenarios. In both scenarios the evolution starts with a binary of two main sequence (MS) stars. The more massive star becomes a red giant and its envelope is ejected in a common envelope event. In the DD scenario the second MS stars evolves to a red giant with a second common envelope event and the formation of a close binary of two white dwarfs (WD). If the DD system is close enough and massive enough, gravitational wave radiation will cause it to merge and explode as a SN Ia. In the two variants of the SD scenario the secondary fills its Roche lobe while i) close to the main sequence or as red giant (RG) or ii) as a He-star after another common envelope phase. Mass is transferred onto the WD star and increases the WD mass until the Chandrasekhar limit is reached. Note that the SD scenarios predict the survival of the companion star.
The double-degenerate scenario for the progenitors was proposed many years ago. So far, no SN Ia progenitor has been identified, which is not really surprising considering the rareness of SNe Ia and the small volume that can be surveyed for white dwarfs. The orbital velocity of white dwarfs in potential SN Ia progenitor systems must be large (150 km/s) making radial velocity (RV) surveys of white dwarfs the most promising detection method. Several systematic RV searches for DDs were undertaken starting in the mid 1980's, but no SN Ia progenitors were discovered. This is not surprising, as theoretical simulations suggest that only a few percent of all DDs are potential SN Ia progenitors. It is obvious that larger samples are needed for statistically significant tests.
The surveys mentioned above were performed with 3...4 m class telescopes. A significant extension of the sample size without the use of larger telescopes would be difficult due to the limited number of bright white dwarfs. This situation changed after the ESO Very Large Telescope (VLT) became available. In order to perform a definitive test of the DD scenario we have embarked on a large spectroscopic survey of more than 1000 white dwarfs using the UVES spectrograph at the UT2 telescope (Kueyen) of VLT to search for RV variable white dwarfs (ESO SN Ia Progenitor surveY - SPY). SPY will overcome the main limitation of all efforts so far to detect DDs that are plausible SN Ia precursors: the samples of surveyed objects were too small. The aim of SPY was to observe more than 1000 white dwarfs brighter than V=16.5mag to perform a statistically significant test of the DD scenario. Stars observed during the survey observations are shown in the map (figure below). A large fraction of all known relatively bright white dwarfs was observed.
Distribution of all known white dwarfs south of a declination of +25 degrees and brighter than V=16.5. Red squares and green triangles indicate white dwarfs observed by SPY. Black dots are the remaining objects without a SPY observation. The yellow band indicates the position of the Galactic disk.
About 10% of the observed white dwarfs are radial velocity variable, i.e. they are residing in a close binary system. Three examples are shown in the figure below. Further follow-up observations are necessary to determine the orbital parameters of the binary. Knowledge of the orbital period is crucial, because only systems with periods below 10 hours will merge within a Hubble time (12 billion years).
Three single-lined RV variable DDs from our VLT survey. The green line marks the rest wavelength of H-alpha.
Other close binaries discovered by SPY have a main sequence or brown dwarf companion.
White dwarf populations: kinematics
Since white dwarfs are very faint they could contribute significantly to the mass of our Milky Way and other galaxies without contributing much to the luminosity (so called dark matter). A very large mass contribution of white dwarfs could be expected, if many old, cool and faint white dwarfs would be present in the so called halo population. Most stars we see in the sky, including our Sun, belong to the so-called thin disk. These stars move around the centre of our Galaxy in orbits not far away from the Galactic plane. The halo population fills a much larger spherical volume and only a small fraction of halo stars can be found in the solar neighbourhood.
The stellar populations in our Milky Way galaxy. Young stars, including our Sun, are found in the thin disk. Thick disk stars belong to an older population (about 10 billion years). The inner part of our galaxy is the so-called galactic bulge. The stars of the galactic halo fill a large spherical volume. Bulge and halo are the oldest components of the Milky Way (about 12 billion years). The yellow dot indicates the position of the Sun.
We used the SPY sample to constrain the contribution of halo white dwarfs to the mass of the Milky Way. We expect to find mainly white dwarfs from the disc populations in the SPY sample. However, because this is a large sample, white dwarfs from the halo population should be present in as well. The radial velocity measurements from the SPY are combined with proper motion measurements allowing us to evaluate the full 3-dimensional space motion of the stars. Stars from different populations are found in different positions in velocity diagrams like in the figure below.
UV diagram for main sequence stars. The velocities U, V and W describe the galactocentric velocities of stars. U: velocity component pointing in the direction of the galactic centre, V: component in the direction of galactic rotation, W: velocity perpendicular to the galactic disc. For main sequence stars the population membership can be inferred from an analysis of their chemical abundances. This is not possible for white dwarfs, because their surface abundances are strongly altered due to their very high surface gravity. Thus we use the position in the UV diagram calibrated with main sequence stars to determine their population membership. The smaller ellipse include the position of thin disc stars, the larger one that of thick disc stars. Stars outside the thick disc ellipse belong to the galactic halo.
We have now determined the population membership of 400 white dwarfs from the SPY sample using the UV diagram described in the figure above. The result is that 7% belong to the thick disk, 2% to the halo populations, the others are members of the thin disk. When computing the mass contributions one has to take into account that the volume inhabited by thick disc and halo white dwarfs is much larger than that of the thin disc. After correcting for this the contribution of the thick disc and halo white dwarfs become comparable to the thin disk, larger then thought previously. However, even so the combined contribution of all white dwarfs to the mass of the Milky Way is only modest.
White dwarfs with main sequence companions
Most stars reside in binaries. Thus it is not surprising that some white dwarfs have main sequence companions. Since a bright main sequence star outshines a white dwarf, it is easiest to find binaries consisting of a white dwarf and a low mass main sequence star (spectral type M) or a brown dwarf. The white dwarf can be seen in the blue part of the spectrum, while the M dwarf or brown dwarf shows up as excess radiation in the red or infrared.
- Some of these systems are close binaries with periods of only a few hours to a few days. Since the orbits are much smaller than the size of a red giant, this points to intensive interaction between both stars, in most cases a common envelope, in the final phases before the primary became a white dwarf. Common envelope means that the main sequence star is engulfed by the very extended envelope of the red giant. Frictional drag on the orbiting main sequence star causes the loss of energy and shrinkage of the orbit. Thus the outcome is a much closer binary than the initial system. This phase of binary evolution is only poorly understood and empirical data, like the period distribution, is important to improve our understanding.
- Wide binaries are important as benchmark systems. It is very difficult to determine the age of low mass stars or brown dwarfs. A white dwarf companion can help. It is straightforward to calculate the cooling age (the time since the formation of the white dwarf) once the temperature and mass of the white dwarf has been determined. This can be supplemented by an estimate of the main sequence lifetime of the progenitor to derive the total age of the white dwarf and its companion.
The SPY project discovered some 40 new white dwarf+main sequence binaries. Twenty of those show radial velocity variations indicating a short orbital period. This sample is very useful to investigate the effects of close binary evolution. Many more white dwarf binaries will be discovered in ongoing optical and infrared photometric surveys.