A Galactic "shroud" of white dwarfs?
Our Milky Way is a typical
spiral galaxy containing a rapidly rotating disk of stars; like all known
spirals, measurements of its
rotation shows it to be
much heavier than it appears. It seems to be embedded in very large
amounts of "
dark matter ".
Part of the solution to this
so-called "dark matter problem" would be that
there are a lot of very dim stars out there which we have yet to detect
with our telescopes. A very thick disk of stars , "shrouding" the
galaxy's disk, has been proposed as a solution to the problem. The
visible components of the Milky Way are shown in the upper-right figure
in an image taken with DIRBE instrument on the COBE satellite. The
shroud is shown schematically in the second figure, in which the shroud
appears in grey and the visible Milky Way in red.
If such a shroud
of stars actually envelopes the Galaxy, then the stars must be very
faint in order never to have been seen before; the best candidate for
these stars are the so-called white dwarfs", stars which have run out
of fuel and are slowly cooling away to near invisibility. Whether the
proposed shroud could be made of such stars has now been addressed by
the
DARKSTAR
research team,
Chris
Flynn and Janne Holopainen of
Tuorla Observatory
The team created
a model of
the distribution of the low-mass stars around the Sun, including the
colours, luminosities and space motions of the stars. They compared
their model to two very large surveys of the fastest moving and
faintest detectible stars on the sky.
The team concluded that,
although a few quite interesting stars have turned up in these two
surveys, practically all the observations are well understood in terms
of our present knowledge of the Milky Way. Furthermore, they were able
to put strong constraints on how bright the putative white dwarfs could
be before significant numbers of them would have been detected in the
surveys; the results indicate that the white dwarfs must be very faint
indeed to have avoided detection. But there is still hope for
white dwarfs; they need only be a bit dimmer than the limits of the
existing surveys to have been missed. New large surveys are being
planned and may yet find them, if they are there.
The research has been
submitted to the Monthly Notices of the Royal Astronomical Society.
See also :
Flynn,
Holopainen and Holmberg, MNRAS 339, 81 (2003)
