Rare Objects

The extensive sky coverage and large number of spectral targets in the SEGUE 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

Brown dwarfs

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 M_sol, 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:

HE 0107-5240, 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.