Extremely faint high proper motion objects from SDSS stripe 82^*

Optical classification spectroscopy of about 40 new objects

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [rdscholz;jstorm;hzinnecker]@aip.de
² Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA e-mail: gk@astro.princeton.edu

Received: 29 September 2008
Accepted: 2 December 2008

Abstract

Aims. By pushing the magnitude limit of high proper motion surveys beyond the limit of photographic Schmidt plates, we aim to discover nearby and very fast low-luminosity objects of different classes: cool white dwarfs (CWDs), cool subdwarfs (sd), and very low-mass stars and brown dwarfs at the very faint end of the main sequence (MS).

Methods. The deep multi-epoch Sloan Digital Sky Survey data in a 275 square degree area along the celestial equator (SDSS stripe 82) allow us to search for extremely faint () objects with proper motions greater than 0.14 arcsec/yr. A reduced proper motion diagram clearly reveals three sequences (MS, sd, CWD) where our faintest candidates are representative of the still poorly known bottom of each sequence. We classify 38 newly detected objects with low-resolution optical spectroscopy using FORS1 @ ESO VLT. Together with our targets we observe six known L dwarfs in stripe 82, four (ultra)cool sd and one CWD as comparison objects. Distances and tangential velocities are estimated using known spectral type/absolute magnitude relations.

Results. All 22 previously known L dwarfs (and a few of the T dwarfs) in stripe 82 have been detected in our high proper motion survey. However, 11 of the known L dwarfs have smaller proper motions ( arcsec/yr). Although stripe 82 was already one of the best investigated sky regions with respect to L and T dwarfs, we are able to classify 13 new L dwarfs. Two previously known L dwarfs have been reclassified by us. We have also found eight new M 7.5–M 9.5 dwarfs. The four new CWDs discovered by us are about 1–2 mag fainter than those previously detected in SDSS data. All new L-type, late-M and CWD objects show thick disk and halo kinematics. Since our high-velocity late-M and L dwarfs do not show indications of low metallicity in their spectra, we conclude that there may be a population of ultracool halo objects with normal metallicities. There are 13 objects, mostly with uncertain proper motions, which we initially classified as mid-M dwarfs. Among them we have found 9 with an alternative subdwarf classification (sdM7 or earlier types), whereas we have not found any new spectra resembling the known ultracool (>sdM7) subdwarfs. Some M subdwarf candidates have been classified based on spectral indices with large uncertainties.

Conclusions. We failed to detect new nearby ( pc) L dwarfs, probably because the SDSS stripe 82 area was already well-investigated before. With our survey we have demonstrated a higher efficiency in finding Galactic halo CWDs than previous searches. The space density of halo CWDs is according to our results about 1.5– pc^-3.