Brown Dwarf-like Secondary Stars in Cataclysmic
Variables
Steve Howell
Planetary Science Institute, Tucson, Arizona.
David Ciardi
University of Florida
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predictions (Howell et al. 1997, MNRAS, 287, 921) that old, post-period
minimum cataclysmic variables contain brown dwarf-like secondary
stars. Assuming these donor stars are similar to field brown dwarfs,
we estimate a distance to both binary systems of only 30-40 pc;
LL And and EF Eri are virtually neighbors to our solar system.
To get a good feel for what these binaries are like, imagine placing
Jupiter where the moon is and having it orbit the Earth (which is
similar in size to a white dwarf) every 80 minutes! While the donor
stars do not appear to be identical analogues to field brown dwarfs,
their presence in a binary system is a boon for astronomers as further
observations will allow masses and radii for the stars to be well
determined, something that is essentially impossible to do for a
single star. More complete details of this work will appear in a
letter submitted to the Astrophysical Journal.
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Infrared (spectroscopic) observations made with CGS4 on the UKIRT
reveal the presence of brown dwarf-like mass donor stars in the
cataclysmic variables LL And and EF Eri. Cataclysmic Variables (CVs)
consist of a white dwarf primary and a less massive, cooler secondary
star. Theoretical calculations have shown that as a cataclysmic
variable becomes very old, the mass losing star will be whittled
down to a cold, Jupiter-sized body similar to a brown dwarf. An
artist's impression (courtesy of Dana Berry) of these end results
is shown on the front cover of this Newsletter!
Using observations obtained at the UKIRT late in 2000, with UKIRTstaff
scientists Chris Davis and Paul Hirst, we found direct
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evidence that such stars
indeed exist. During times when the mass transfer between the donor
star and its more massive white dwarf companion stops, infrared observations
of the cool companion star become possible. The presence of methane
absorption at 2.2 microns in the secondary star of LL And indicates
that it has an effective temperature near 1300K (see Figure 1), similar
to a "T" type methane browndwarf, while the secondary star
in EF Eridani is shown to be consistent with a slightly hotter L5
star - having a temperature of ~1650K. Both stars have theoretical
mass estimates near 0.03 solar masses, or about 40 times the mass
of Jupiter. These UKIRT spectroscopic observations of LL And and EF
Eri provide the first direct proof confirming theoretical |
