The nearest cool white dwarf (d pc), the coolest M-type subdwarf (sdM9.5), and other high proper motion discoveries

Free access article

Issue		A&A Volume 425, Number 2, October II 2004


Page(s)		519 - 527
Section		Galactic structure, stellar clusters and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20041059

A&A 425, 519-527 (2004)
DOI: 10.1051/0004-6361:20041059

The nearest cool white dwarf (d $\mathsf{\sim4}$ pc), the coolest M-type subdwarf (sdM9.5), and other high proper motion discoveries

R.-D. Scholz¹, I. Lehmann², I. Matute² and H. Zinnecker¹

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: [rdscholz;hzinnecker]@aip.de
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
e-mail: [ile;matute]@mpe.mpg.de

(Received 8 April 2004 / Accepted 16 June 2004)

Abstract
We report the discovery of seven high proper motion stars with proper motions between about 0.7 and 2.2 arcsec/yr, all at relatively low Galactic latitudes ( $13^{\circ}<\vert b\vert<35^{\circ}$ ) and located in the southern sky. They were detected in a high proper motion search using multi-epoch positions in the optical SuperCOSMOS Sky Surveys and in the near-infrared sky surveys 2MASS and DENIS. Classification spectroscopy carried out for six of the objects reveals them to represent three different classes of cool objects in the solar neighbourhood: M dwarfs, M subdwarfs and cool white dwarfs. The star with the largest proper motion, SSSPM J1138-7722, is classified as a very nearby ( $d\sim$ 8 pc) M 5.5 dwarf with Galactic thin disk kinematics. A second star with ~2 arcsec/yr proper motion, SSSPM J1358-3938, is still lacking spectroscopic confirmation but can be classified from photometry as a thick disk ~M 3.5 dwarf. Three objects turn out to be cool subdwarf members of the Galactic thick disk or halo, including the first sdM9.5 object, SSSPM J1013-1356, which represents the currently coolest known M subdwarf, another ultra-cool subdwarf, SSSPM J1930-4311, of spectral type sdM7.0 as well as an earlier type (sdM1.5) star. The latter, SSSPM J1530-8146, has an extremely large space velocity with clear halo kinematics (heliocentric $(U,V,W)=(-534\pm78,-239\pm74,+188\pm23)$ km s ^-1). Two objects show featureless spectra classifying them as cool white dwarfs with $T_{{\rm eff}}<4500$ K. One of them, SSSPM J1549-3544, is an extremely nearby ( $d\sim4$ pc) thin disk object, the other one, SSSPM J1148-7458, has thick disk kinematics. SSSPM J1549-3544 is likely to be the nearest cool white dwarf and may be even the nearest isolated white dwarf, i.e. closer than van Maanen 2.

Key words: astrometry -- surveys -- stars: kinematics -- stars: low-mass, brown dwarfs -- stars: subdwarfs -- stars: white dwarfs

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