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

LHS 1070 (other common name: GJ 2005) is a nearby multiple system consistin g of very low mass red dwarfs. We present the results of WFPC2 photometry a nd FOS spec -troscopy for the three optically resolved components of this s ystem acquired during HST cycle 5, These show (1) absolute brightnesses cor responding to theoretical masses of 0.080-0.083 M. and 0.079-0.080 M. for t he faint pair, depending mainly on their age and metallicity; (2) a saturat ion of the optical TiO and VO absorption bands typical of the onset of phot ospheric dust formation, and (3) emission lines typical of moderate chromos pheric activity in only the two most massive components. Li I lines are not seen. But also all other lines of the alkali elements are remakably weak o r even absent in the two faint companions B and C. This appears to be an ef fect of dust formation. Comparison of the observations with model spectra, which account for dust formation and for the resulting opacities, yields go od agreement for solar metallicity and effective temperatures and gravities (in log: cm/s(2)) of 2950 K;5,3, 2400 K;5,5 and 2300 K;5.5 for the three c omponents A,B and C, respectively. The existence of a fourth component, rec ently discovered in this system by HST Fine Guidance Sensor observations (H enry 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 log cm/s(2)) for the fourth component of 2500 K;5.3 would best be compatibl e 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 Su n for which dynamical determinations of mass appear possible within a decad e. The system LHS 1070 thus has the potential to be the most important sour ce of information for probing the low mass end of the main sequence.