Spectral analysis of M dwarfs

We compare 1.16-1.22 μm spectra of cool low-mass dwarfs with synthetic spectra calculated with a stellar atmosphere code. The synthetic spectra give a representation of the overall spectral features which has not been possible in previous comparisons with cool low-mass dwarf spectra, although the interpretation of the observed spectra is severely hampered by the quality of atomic and molecular input data. The observed spectral region is rich in absorption features of which the strongest are K, Fe, Mg and FeH. Each observed spectrum is compared with a grid of synthetic spectra extending well outside the expected parameter space for M dwarfs. The effective temperatures, metallicities and gravities derived are compared with expectations based on previous work. For the cooler objects the parameters show broad agreement; however, for the hotter objects the poor fit of Fe lines leads to best fits to models with low gravity and metallicity. All features show a strong sensitivity to temperature, especially the K and FeH. Metallicity and gravity effects are relatively smaller. For the hotter stars, Mg and Fe absorption lines are the most useful discriminators between gravity and metallicity. For the cooler objects the complicated dependence of modelled equivalent widths on metallicity and gravity makes it awkward to separate effects due to a single model parameter. The comparisons suggest a similar spread in metallicities to that anticipated, although for our sample neither kinematic motion nor membership of a particular photometric class is, on its own, a reliable indicator of metallicity. Comparison of the observed strong K lines in GD 165B with synthetic spectra indicate that it is relatively metal rich. This result together with an improved measurements of its distance means that within the best available constraints it is likely to be a brown dwarf.