White dwarfs within 10 parsecs

By early 2005, astronomers had found at least 21 white dwarfs within 10 parsecs (32.6 light-years) of Sol, although these objects are small and dim compared to OBAFGK stars. In 2002, a team of astronomers claimed that their census of white dwarfs within 13 parsecs (pc) of Sol appeared to be nearly complete, although their analysis of 109 white dwarfs within 20 parsecs (pc) indicated that around 50 additional white dwarfs may yet lie undiscovered within that distance, of which 129 total were identified by 2009 (Holberg et al, 2002; and Sion et al, 2009). Unfortunately, none are bright enough to observe with the unaided Human eye in Earth's night sky.

NASA, HST, WFPC 2,
H. Richer (UBC)


Larger image.


Dim, small white dwarfs
are common at the center
of old globular clusters
such as M4 (more).

In theory, white dwarfs are the remnants of low to intermediate mass stars that were born with less than or equal to 10 +/2 Solar-masses (Isern et al, 2008). Although some white dwarfs may have had their gaseous outer envelopes removed by companion objects, most are the naked cores of stars that have evolved off the main sequence and thrown off much of their outer layers of hydrogen and/or helium (some passing through the relatively brief planetary nebula stage). They become white dwars after their cores have consumed enough of their fusionable elements that they have undergone gravitational collapse into planet-sized objects, but not violently enough (through supernovae) to end up as small as neutron stars or even smaller black holes. While white dwarfs have been found to have more than 0.3 of a Solar-mass, all white dwarfs must mass less than 1.4 Solar-masses (the Chandrasekhar limit) in theory, so that electron degeneracy pressure acts to prevent them from further gravitational collapse. White dwarfs have radii around less than two percent of Sol's radius (<0.02 Rsun), which is around that of terrestrial planets like Earth. No nuclear fusion or gravitational contraction occurs in white dwarfs, and so they shine only by residual heat as their surfaces have very high temperatures and they radiate primarily in ultraviolet light.

NASA Observatorium





See a discussion of
"stellar remnants"
as part of stellar
evolution and death.

Controlling for age, the more massive white dwarfs are usually less luminous because greater gravitational contraction has reduced their radiative surface area. At least two (Sirus B and Procyon B) of the nearby white dwarfs that are now much dimmer and smaller than Sol were once also brighter, larger, and more massive. While stars with more than about eight Solar-masses become neutron stars and black holes through supernovae, those with less mass become white dwarfs. Over the 14 billion or so years since the birth of the known universe, many massive stars of BAF and early G spectral types have already evolved into ancient, cool white dwarfs. While some of these cool, dim objects were born in the Milky Way's disk like Sol, the oldest ones may be a major constituent of the galaxy's halo. Indeed, some astronomers believe that around 10 percent of the local dark matter halo may be in the form of very old, cool, white dwarfs (Ibata et al, 2000).

Modified Sion-type (1983) Classification System for White Dwarfs

In general, the spectral appearance (and therefore classification) of white dwarfs are affected by their evolution and final state as isolated objects and by those objects occurring in binary or multiple star systems. The strong gravity of white dwarfs causes a rapid settling of heavier elements so that hydrogen rises to the surface (called type DA), which can mask other elements, as is found in as many as three-quarters of all white dwarfs known. Sometimes, however, the outer hydrogen envelope is stripped away by companion objects or lost by some other evolutionary process. Without the heat generated by core fusion, however, newly born white dwarfs slowly cool over time and may accrete trace elements from the interstellar medium, comets, and other sources (Professor Shri Kulkarni's class notes on "White Dwarfs," in pdf). Although their outer surface may spectroscopically exhibit a hydrogen-rich "DA" surface when hot, white dwarfs change spectral class to non-DA species as they cool off (Holberg et al, 2002). (For more discussion about the classification of white dwarfs, see: Liebert and Sion, 1994.)

Unknown artist, NAU





Larger image.





While most white dwarfs are
believed to have Carbon-Oxygen
cores, many outer coatings of
hydrogen, neutral or ionized
helium, or a mixture of both.

The different types of white dwarfs are defined by the elements that dominate their surfaces as shown in their spectra. If material from a nearby companion star (or some other source) is accreting onto the surface of a white dwarf, the accreting substances can be detected through its spectra. Observational research indicate that white dwarfs with less than 0.4 of a Solar-mass have a core made of Helium, while those with more than 1.05 Solar-masses have a core made of Oxygen and Neon. Those with between 0.4 and 1.05 Solar-masses, the vast majority, have a core made of a mixture of Carbon and Oxygen. Three varieties (DA, DB and DO) of white dwarfs, however, have cores coated with an outer layer of nearly pure Hydrogen or Helium. PG 1159 dwarfs which have recently cast off their planetary nebulae appear to have partially exposed cores, and white dwarfs with mixtures of elements on their surfaces receive compound classifications. For example, DAB stars contain both Hydrogen and neutral Helium, while DAO stars have Hydrogen and ionized Helium. Most are coated with a thin Helium layer with a mass ranging between 0.01 and 0.0001 of a Solar-mass which, in turn, is surrounded by an even thinner layer of Hydrogen with a mass between 0.0001 and 10^-15 of a Solar-mass and are known as type DA, but about a quarter of white dwarfs do not have a Hydrogen envelopes. Because of the different opacities involved, DA white dwarfs cool down much more slowly than non-DA white dwarfs (Isern et al, 2009). Within the 10-parsec Solar neighborhood, Van Maanen's Star is a DZ white dwarf, an old stellar remnant that is now relatively cool with metallic but no hydrogen or helium lines in its spectrum (Weidemann and Koester, 1989).

The observed number of white dwarfs in the Solar neighborhood comprises only about one percent of its dynamical mass density (Liebert et al, 1988), but most white dwarf companions in binary systems with bright, main sequence stars have not been detected probably due to their relative faintness and because of the glare from the primary star (James Liebert, 1980). A recent survey of 146 DZ white dwarfs and their "externatlly polluted atmospheres" suggests that at least 3.5 percent of white dwarfs may harbor the remnants of terrestrial planetary systems, while another study of 129 white dwarfs within 20 parsecs (65.2 light-years) of the Solar System suggests that at least one fifth (20 percent) have "photospheric metals" that may have come from accretion of circumstallar debris disks which may have eventually produced planets or asteroid-like bodies (Farihi et al, 2010; and Sion et al, 2009). All the known white dwarfs within 20 parsecs (65.2 light-years) of the Solar System appear to be members of the thin disk, as none exhibit kinematic properties of the halo or extended thick disk Sion et al, 2009).