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[80.06] Faint Carbon Stars Discovered by the Sloan Digital Sky Survey

B. Margon, S. F. Anderson, E. W. Deutsch (U. Washington), H. C. Harris (USNO), SDSS Collaboration

Although faint high latitude carbon (FHLC) stars range in luminosity by up to factors of 10⁵, both the high and low luminosity versions are interesting. The apparently faintest of the luminous giants are located at >100~kpc, and aside from raising issues of stellar evolution, are also excellent probes of the dynamics of the distant halo, at a distance which encompasses all of the dark matter. The intrinsically faint dwarf carbon stars constitute a class of objects just recognized a decade ago, yet now understood to be the numerical majority of all stars with C₂ in their spectra. The faintest of the FHLC (R>16) have thus far been discovered either serendipitously, or in very small numbers by heterogeneous techniques. We have long hoped that the Sloan Digital Sky Survey (SDSS) might provide a large, homogeneously-selected sample of FHLCs via color-selection from the 5-color imaging data base, and followup fiber spectroscopy as an adjunct to other SDSS primary programs. However this is a challenging task, as these rare objects are expected to be only slightly separated from the 10⁸ normal stars in SDSS photometry, even in 5-color space. In particular, most FHLCs are not terribly red.

Here we report initial successful results on the search for FHLC stars in SDSS commissioning data. Two such objects have thus far been discovered in a very informal search of a small fraction of the data. The fainter object, at R ~ 19, is one of the faintest FHLC yet known, boding well for the prospect of a respectable number (>10²) of new objects in the entire Survey. The brighter object (R ~ 17) is most certainly a dwarf: it displays proper motion of 0.24''~yr^-1, one of the highest motions known for any dC star. Curiously, the faint object also shows evidence for a small but significant proper motion, and thus dwarf classification. It now appears likely that SDSS will fulfill long-standing hopes as a source of a large number of uniformly-selected and well-characterized FHLCs. In a metaphor for these enigmatic objects, however, it remains unclear if we will be chiefly studying stars at a distance of 100~pc or 100~kpc.