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SkyMapper Protected Science Project

Ultra-Cool White Dwarfs in the Southern Sky

Christian Wolf (RSAA), Mike Bessell (RSAA)

Ultra-cool white dwarfs (UCWDs) are rare and physically unexplained. Their SEDs are highly "non-Planckian", with very steep spectral slopes on either side of a maximum between g and r-band. The favoured physical explanation for these objects and their SEDs is that they are almost pure He white dwarfs with low-mass (0.2-0.3 M_sol) and T<3500 K, where collisionally-induced absorption (CIA) suppresses the far-red and NIR flux to produce extremely blue optical-NIR colours from spectral slopes that are much steeper than any Planck law can produce. They are thus potentially cooler than even the oldest CO white dwarfs, and might be the leftover He cores of former red giants that lost their envelopes by binary interaction (Justham, Wolf et al. 2009). Ten such UCWDs are known across the sky, of which six were found in SDSS (Gates et al. 2004). Only two are known in the Southern Sky, the relatively bright LHS 1402 and a faint specimen found by Wolf (2005). With SkyMapper we want to find all UCWDs in the Southern Sky, which should be possible using the SkyMapper filters even in the Galactic plane, as the survey sensitivity will limit us to a distance of ~100 parsec and thus dust extinction won't be a major factor. At 100 pc distance significant dust extinction would make them too faint to be found anyway. We expect to find 20-30 objects and will use the two known ones for verification of our search method.