ASTRONOMY: Dark Matter & White Dwarfs

From: Robert J. Bradbury (
Date: Fri Mar 23 2001 - 21:09:30 MST

Yesterday Science came out with
  Astronomers Glimpse Galaxy's Heavy Halo
  Direct Detection of Galactic Halo Dark Matter
  (The second URL may require a subscription)

The abstract is:
> The Milky Way Galaxy contains a large, spherical component which is
> believed to harbor a substantial amount of unseen matter. Recent
> observations indirectly suggest that as much as half of this "dark
> matter" may be in the form of old, very cool white dwarfs, the
> remnants of an ancient population of stars as old as the Galaxy
> itself. We have conducted a new survey to find faint, cool white
> dwarfs with large space velocities, indicative of their membership
> in the Galaxy's spherical halo component. The survey reveals a
> substantial, directly observed population of old white dwarfs,
> too faint to be seen in previous surveys. This newly discovered
> population accounts for at least 3% of the halo dark matter. It
> provides a natural explanation for the indirect observations,
> and represents a direct detection of Galactic halo dark matter.

Now, the theoretical physicists have "proven" from known observations
and a very complex theory, that I do not claim to comprehend, that the
White Dwarfs cannot account for "most" of the Dark Matter. However
the White Dwarf "fans" (presumably including the authors of the above
article) want to argue that they may constitute a "significant"
fraction. How you read "3%" is of course debatable. The article
"pushes" the idea that this is a lower limit. Other constraints
based on the MACHO and EROS experiments suggest the high limit
may be 35%. (What's an order of magnitude among friends...).

However a careful read of the paper provides two very interesting
  "Two of the 38 new cool white dwarfs have unusual spectra (Fig. 2B).
   One, LHS 1402, has a spectrum very similar to those of the peculiar
   stars LHS 3250 (5,7) and SDSS 1337+00 (6), but with a steeper
   slope towards longer wavelengths, suggesting a *cooler* temperature.
   The other object, WD2356-209, possesses a bizare spectrum,
   *incomparable* to any other known object. We reanalyzed the
   data on this object several times and found no evidence for
   residual instrumental effects."
  "We still do not understand the nature of the three strange objects
   LHS 3250, LHS 1402 and SDS 1337+00 (5-7), all of which lie at
   R59F - I_N colors near -0.5 (well outside of Fig. 4). The
   kinematics of these stars, with the exception of LHS 1402, suggest
   that they may be disk objects. While they are certainly(?) white
   dwarfs, we know neither their effective temperatures nor their
   atmospheric compositions. They may represent another, perhaps
   unusual, stage in the spectral evolution of cool white dwafs."

[Emphasis *...* and (?) mine.]
(Fig. 4 contains a plot of the 'real' white dwarfs when measured
by two different optical characteristics).

The paper provides the spectrum of WD2356-209 and LHS-1402 and
they are very very different from the other spectra they show
from what they claim are 'valid' cool white dwarfs.

Now personally, I cannot imagine any artificial structures that
would match the spectra of these two objects. But its
pretty clear from the comments that the astronomers are having
a hard time matching any 'natural' phenomena to go with them as well.
Since they are dealing with only ~10% of the sky, this would
argue there are a couple of dozen "objects" within our observing
range that don't fit known stellar models. { They were using the
4m V.M. Blanco Telescope and we have telescopes 2.5x as large. }
Expect the plot to get thicker as references 6&7 (which are
still in press) become available.

What is interesting about this is that it seems when the
astronomers see something they don't expect, they go back
and check the instruments... :-) I wonder if the SIs think
of the astronomers as rats running a maze trying to find the


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