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Astron. Astrophys. 353, 691-706 (2000)

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The multiple system LHS 1070: a case study for the onset of dust formation in the atmospheres of very low mass stars *

Christoph Leinert 1, France Allard 2, Andrea Richichi 3 and Peter H. Hauschildt 4

1 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2 Centre de Recherche Astronomique de Lyon, Ecole Normale Supérieure, 46 Allée d'Italie, Lyon, 69364 France, Cedex 07
3 Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
4 University of Georgia, Department of Physics and Astronomy and Center for Simulational Physics, Athens, GA 30602-2451, USA

Received 21 July 1999 / Accepted 8 November 1999

Abstract

LHS 1070 (other common name: GJ 2005) is a nearby multiple system consisting of very low mass red dwarfs. We present the results of WFPC2 photometry and FOS spectroscopy for the three optically resolved components of this system acquired during HST cycle 5. These show (1) absolute brightnesses corresponding to theoretical masses of 0.080-0.083 [FORMULA] and 0.079-0.080 [FORMULA] for the faint pair, depending mainly on their age and metallicity; (2) a saturation of the optical TiO and VO absorption bands typical of the onset of photospheric dust formation, and (3) emission lines typical of moderate chromospheric activity in only the two most massive components. LiI lines are not seen. But also all other lines of the alkali elements are remakably weak or even absent in the two faint companions B and C. This appears to be an effect of dust formation. Comparison of the observations with model spectra, which account for dust formation and for the resulting opacities, yields good agreement for solar metallicity and effective temperatures and gravities (in [FORMULA] cm/s2) of 2950 K;5.3, 2400 K;5.5 and 2300 K;5.5 for the three components A,B and C, respectively. The existence of a fourth component, recently discovered in this system by HST Fine Guidance Sensor observations (Henry et al. 1999), has already been taken into account in the evaluation of the data for the main component. An effective temperature and gravity (in [FORMULA] cm/s2) for the fourth component of 2500 K;5.3 would best be compatible with our data. Then, based on our analysis the three components C, B and D of LHS 1070, in this order, are the faintest stars within 20 pc of the Sun for which dynamical determinations of mass appear possible within a decade. The system LHS 1070 thus has the potential to be the most important source of information for probing the low mass end of the main sequence.

Key words: stars: atmospheres – stars: binaries: general – stars: fundamental parameters – stars: low-mass, brown dwarfs – stars: luminosity function, mass function

* Based on observations with the Hubble Space Telescope, with additional observations obtained on the ESO-MPIA 2.2 m telescope on La Silla, Chile.

Send offprint requests to: Ch. Leinert (leinert@mpia-hd.mpg.de)

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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