Rembrandt D. Scholz

Optical spectroscopy of high proper motion stars : new M dwarfs within 10 pc and the closest pair of subdwarfs

ABSTRACT We present spectra of 59 nearby stars candidates, M dwarfs and white dwarfs, pre-viously identified using high proper motion catalogues and the DENIS database. Wereview the existing spectral classification schemes and spectroscopic parallax calibra-tions in the near-infrared J-band and derive spectral types and distances of the nearbycandidates. 42 stars have spectroscopic distances smaller than 25 pc, three of thembeing white dwarfs. Two targets lie within 10 pc, one M8 star at 10.0 pc (APMPMJ0103-3738), and one M4 star at 8.3 pc (LP 225-57). One star, LHS 73, is found to beamong the few subdwarfs lying within 20 pc. Furthermore, together with LHS 72, itprobably belongs to the closest pair of subdwarfs we know.Key words: Galaxy: solar neighbourhood – stars: late type – white dwarfs – subd-warfs 1 INTRODUCTIONRecent discoveries of cool objects, such as M stars orbrown dwarfs, closer than 5 parsecs show that even theimmediate solar neighbourhood sample is still incomplete(Delfosse et al. 2001; Scholz et al. 2003; Teegarden et al.2003; Hambly et al. 2004).Henry et al. (1997) estimated that about 130 systemsover 359 (36%) are missing within 10 pc. The missing frac-tion is even larger within 25 pc (63%) with a deficit of about3500 systems over the 5500 expected ones (Henry et al.2002). Statistical comparisons from the local sample in thenorthern hemisphere led to a less pessimistic result, the cur-rent 10 pc sample being ∼75% complete (Reid et al. 2003a).New surveys in the near infrared such as DENIS(Epchtein et al. 1997) and 2MASS (Cutri et al. 2003) pro-vide unprecedented data for a systematic search for low lu-minosity cool dwarfs. The use of these data together withhigh proper motion catalogues is a powerful tool for discover-ing our neighbours. Hundreds of stars closer than 25 parsecshave been discovered this way (e.g. Phan-Bao et al. 2001,2003; Reid & Cruz 2002; Reid et al. 2002; Reyl´e et al. 2002;Reid et al. 2003b, 2004; Hambly et al. 2004; Reyl´e & Robin2004; Lodieu et al. 2005; Scholz et al. 2005).As spectroscopy provides much more information thanphotometry alone, spectroscopic observations were also car-ried out to identify and classify nearby stars. Recent studies

New nearby stars selected in a high proper motion survey by DENIS photometry

We present new nearby stars extracted from a proper motion catalogue and having a DENIS counterpart. Their distances and spectral type are estimated using the DENIS colours. 107 stars are within 50 pc. 31 stars among them have previously measured distances. In addition, 40 stars may enter within the 50 pc limit depending on which population they belong to. 6 stars among them have already measured distances. 5 objects, LHS5045, L225-57, LP831-45, LHS1767, and WT792, are probably closer than 15 pc, with L225-57 at 9.5 pc. Most of these stars are M-type while 4 stars are white dwarfs. 88 M-dwarfs are disc stars, 14 belong to the thick disc and 1 to the spheroid.

High-resolution spectroscopic atlas of M subdwarfs. Effective temperature and metallicity

Context. M subdwarfs are metal poor and cool stars. They are important probes of the old galactic populations. However, they remain elusive due to their low luminosity. Observational and modeling efforts are required to fully understand their physics and to investigate the effects of metallicity in their cool atmospheres. nAims. We perform a detailed study of a sample of subdwarfs to determine their stellar parameters and constrain the stat-of-the art atmospheric models. nMethods. We present UVES/VLT high resolution spectra of three late-K subdwarfs and 18 M subdwarfs. Our atlas covers the optical region from 6400 AA up to the near infrared at 8900 AA. We show spectral details of cool atmospheres at very high resolution (R= 40 000) and compare with synthetic spectra computed from the recent BT-Settl atmosphere models. nResults. Our comparison shows that molecular features (TiO, VO, CaH), and atomic features (Fe I, Ti I, Na I, K I) are well fitted by current models. We produce an effective temperature versus spectral type relation all over the subdwarf spectral sequence. Thanks to the high resolution of our spectra, we perform a detailed comparison of line profiles of individual elements such as Fe I, Ca II, Ti I, and are able to determine accurate metallicities of these stars. These determinations contribute to calibrate the relation between metallicity and molecular band strength indices from low-resolution spectra. nConclusions. This work shows that the new generation of models are able to reproduce various spectral features of M subdwarfs. Working with these high resolution spectra allowed us to disentangle the atmospheric parameters (effective temperature, gravity, metallicity), which is not possible when using low resolution spectroscopy or photometry.