The very low mass multiple system LHS 1070. A testbed for model atmospheres for the lower end of the main sequence

Abstract

Context. LHS 1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS 1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres. Aims. This work aims at determining the fundamental stellar parameters of LHS 1070 and to test recent model atmospheres: BT-Dusty, BT-Settl, DRIFT, and MARCS models. Methods. Unlike in previous studies, we have performed a χ 2 -minimization comparing well calibrated optical and infrared (IR) spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters T eff , radius, and log g for each of the three components of LHS 1070. Results. With exception of the MARCS models which do not include dust formation, the models are able to reproduce the observations and describe the main features of the visible to IR spectra. This is consistent with the fact that dust formation prevails in the B and C component atmospheres. The parameters obtained with the DRIFT models confirm the values determined in earlier studies. But important differences between models are observed, where the MARCS model is too bright in the H and K bands, and the BT-Settl and BT-Dusty models systematically yield up to 100 K higher T eff in the case of the B and C components. This confirms a trend for models without, or with less efficient cloud formation, to predict higher T eff than models richer in dust (DRIFT). Even models including cloud physics however still produce slightly too bright J band flux, showing as too blue J − K colors. The onset of dust formation remains therefore a particularly challenging regime to understand.

The effective temperature scale of M dwarfs

“…These grains produce a "veiling" by dust scattering over the optical band of the latest type M dwarfs. The BT-Settl models use therefore a slightly revised version of the Rossow (1978) cloud model (see Allard et al 2012a,b;Rajpurohit et al 2012b), for details on the model construction.…”

Spectral energy distribution of M-subdwarfs: A study of their atmospheric properties

“…Over the last decades tremendous development in the model atmospheres of cool low-mass stars, in particular M-dwarfs, has been achieved (Brott & Hauschildt 2005;Helling et al 2008;Allard et al 2012Allard et al , 2013) that has boosted the number of studies deriving accurate physical parameters of these stars both in the optical and in the near-infrared (Bayo et al 2011(Bayo et al , 2012Rajpurohit et al 2012Rajpurohit et al , 2013Neves et al 2014). Bayo et al (2014) have shown the differences between estimating the parameters in the optical and in the near-infrared, with spectra and photometry.…”

“…The synthetic spectra were distributed with resolution of R 200 000 via the PHOENIX web simulator 2 and are fully described in Allard et al (2012); Rajpurohit et al (2012) and Allard et al (2013). Oxide bands, which are stronger in M-dwarf spectra, are weaker in the subdwarfs where the hydride bands dominate.…”

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

“…For more details of the BT-Sett model atmosphere see Allard et al (2012Allard et al ( , 2013 and Rajpurohit et al (2012). The synthetic colours and spectra were distributed with a resolution of around R = 100 000 via the PHOENIX web simulator 2 .…”