The extensive sky coverage and large number of spectral targets in the
survey offered a unique opportunity to discover rare objects. In
particular SEGUE searched for:
- Brown dwarfs
- Cool white dwarfs
- Extremely low metallicity [Fe/H] < -5.0 stars
Because they are not in equilibrium, brown dwarfs cool after formation
and their spectral properties depend on mass, age and metallicity. SEGUE data
greatly increased the known number of these objects and extend our
knowledge of the IMF below about 0.08 solar masses.
Cool white dwarfs
White dwarf stars are the end-stage remnants of main sequence stars with masses
less than about 8 Msol, and the coolest white dwarfs represent
some of the
oldest objects in the galaxy. These fossil remains of an ancient
stellar population offer a window into the early stages of the
galaxy and its formation. They can be used to place lower limits
on the ages of various galactic components, extend our knowledge of stellar
evolution, and explore star formation processes
during the Galaxy's earliest epochs.
Very cool white dwarfs with hydrogen in the
atmosphere exhibit a unique spectral signature due to
collision induced absorption (CIA) by molecular hydrogen.
In pure H-atmosphere white dwarfs,
CIA produces a flux suppression in the infrared at temperatures
below about 5000 K.
For T < 3000 K CIA becomes strong enough to affect the optical
colors of the star and
results in a bluer spectrum, with a significant flux suppression redward
of about 6000 Å, relative to a blackbody SED.
For mixed H-He atmospheres, infrared flux suppression becomes apparent
at temperatures below about 6000 K and the optical colors are affected
at temperatures below 4000 K.
Prior to 2004, only two ultracool white dwarfs with strong CIA were known
to exist, and three with milder CIA suppression. Six new CWDs
(and two previously known CWDs) found in SDSS data have since been published
(Harris et al 1999
Gates et al 2004
highlighting the ability of SDSS photometric and spectroscopic data to
identify these rare objects.
SEGUE used a cut on reduced
proper motion to target CWDs.
Extremely low metallicity stars
Perhaps the rarest of all halo objects are the stars with metallicities
[Fe/H] < -5.0. As of 2006, only two such stars have been found:
a halo giant with [Fe/H] = -5.2
Christlieb et al. 2002)
HE 1327-2326, [Fe/H] = -5.4
( Frebel et al 2005)
Such stars provide
the most direct means of deciphering the metal content and chemistry of the
early Universe, and hence are of enormous interest. The SEGUE survey
increased the number of observed very metal-poor stars ([Fe/H] < -2.0)
by a factor of 10 or so over previous surveys
Beers et al. 2005) -- among which may
be additional examples of extremely metal-poor stars.