The nearest cool white dwarf (d ∼4 pc), the coolest M-type subdwarf (sdM9.5), and other high proper motion discoveries

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^°<|b|<35^°) 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∼ 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±78,-239±74,+188±23) km s^-1). Two objects show featureless spectra classifying them as cool white dwarfs with T_eff<4500 K. One of them, SSSPM J1549-3544, is an extremely nearby (d∼4 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.

Based on the systematic search in archival data from the SuperCOSMOS Sky Surveys, 2MASS and DENIS, and on spectroscopic observations with the ESO NTT.

Figure \ref{8charts} is only available in electronic form at http://www.edpsciences.org