White Dwarfs

[m2q]

The Galaxy’s large spherical halo (see GALACTIC METALPOOR

HALO and HALO, GALACTIC) may harbor as many as

several hundred billion WHITE DWARFS, a population as

large in number as the total number of stars in the

Galaxy’s disk (see DISK GALAXIES and GALACTIC THIN DISK).

Although this assertion is controversial, several astronomical

surveys provide strong support for it and the

implications affect fields of astronomical inquiry as

diverse as dark matter and star formation. The reason

that this population of white dwarfs may be related to

dark matter is tied to the fact that white dwarfs cool and

fade as they age. This means that a white dwarf as old as

the ancient halo is extremely faint. Current large-scale

imaging surveys can only see relatively nearby white

dwarfs, and only a tiny fraction of the halo’s mass exists

near the Sun, while the vast majority of the Galaxy’s halo

is thousands of light-years away.

The theory of gravity requires that the mass of the

halo must dominate that of the Galaxy’s disk. Since the

halo is largely invisible, it is almost entirely made of dark

matter (see DARK MATTER IN GALAXIES). Indeed, the known

population of normal stars in the halo constitutes a trivial

fraction of the total mass. However, those stars exhibit

properties distinct from disk stars, including rapid

motion relative to the Sun. As a result a halo star that

happens to be within the solar neighborhood can be distinguished

from a disk star by examining the rapidity

with which it passes through the solar neighborhood.

This is a statistical criterion, meaning that for any given

star one can assign a probability that it belongs to the

disk or halo. For a survey of many stars with such probabilities

one can estimate the actual number of stars in

the disk and halo populations.

The first hint that white dwarfs may be a significant

part of the halo emerged in 1996, when a group conducting

a MICROLENSING experiment claimed to detect indirectly

a large population of objects in the Galaxy’s halo

all with masses characteristic of white dwarfs. Instead of

actually seeing the white dwarfs themselves, they

observed the effect they have on images of stars in the

Milky Way’s companion galaxy the Large Magellanic

Cloud (see WIMPS AND MACHOS). The microlensing results,

now backed by several independent, similar experiments,

have found that at most 20% of the total mass of

the Galaxy’s halo may be in this supposed population of

white dwarfs.

In 2001 a search for moving stars in an archive of

photographic plates taken between 1950 and 1998

revealed about 20 nearby white dwarfs which are moving

so fast that they must be members of the Galaxy’s

halo. Statistical analysis of the data showed that at least

1% of the halo’s mass is due to a population of white

dwarfs. Alternative interpretations of this survey suggest

that a fraction of this population may be part of the

Galaxy’s thick disk. In all of the interpretations, however,

there still appears to be an unexpectedly large number

of white dwarfs in the halo, constituting at least 0.5% of

the halo’s mass.

With 0.5–20% of the halo’s mass accounted for by

white dwarfs, a fraction of the Galaxy’s dark matter has

finally been identified. Furthermore, the halo white

dwarf population is important to STAR FORMATION theory.

Ongoing star formation creates many low-mass stars for

every high-mass star. If the same proportions of highand

low-mass stars formed 13 billion years ago, when the

halo formed, only one to five halo white dwarfs should

have been found by the recent survey. This is known

because the low-mass halo stars have not had enough

time to evolve into white dwarfs, and we know the number

of low-mass stars in the halo. That number, assuming

a universal star formation process, determines the number

of high-mass stars that have evolved into white

dwarfs in the halo. The actual numbers observed imply

that high-mass stars formed more readily in the halo

than they do at present.