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Interest in the extreme lower main sequence has blossomed in recent years, largely because of its implications for star formation theory and dark matter candidates. Advances in spectroscopy and photometry, especially at infrared wavelengths, promise a wealth of new data on low-mass stars. There must be parallel improvements in models of cool stellar atmospheres if the full potential of these data is to be realised.
We will present state-of-the-art molecular and atomic line-blanketed convective models for the photospheres of cool low-mass stars, valid for a wide range of parameters encompassing the coolest known M dwarfs, M subdwarfs and brown dwarf candidates: \smallskip \centerline{$4000\,K \ge {\rm T}_{\rm eff} \ge 1800\,K$; $5.5 \ge \log\,g \ge 4.0$; and $-4.0 \le {\rm [M/H]} \le +0.5$.} \smallskip \noindent The atmospheres are characterised by a thick convective zone reaching optical depths as low as $\tau \sim 10^{-3}$ and by a nearly adiabatic temperature gradient. This latter feature leads to local temperatures in the photosphere which far exceed ${\rm T}_{\rm eff}$, unlike conventional grey atmosphere models. The model spectra yield close agreement with observations of M dwarfs across a wide spectral range from the blue to the near-IR, with one notable exception: the fit to the water bands. To illustrate the power of our new spectral synthesis code, we apply it to the observed energy distribution of a sample dM8 star, VB 10 (Gliese 752~B). We will also briefly discuss current efforts to incorporate water bands in cool stellar spectra.
Our synthetic spectra of M dwarfs with solar and near-solar abundances (${\rm [m/H]} \simeq 0.0\pm0.5$) show that the overall flux distribution depends surprisingly little on surface gravity, but the gravity sensitivity increases with decreasing metallicity. Therefore, broadband photometric surveys may have difficulty identifying brown dwarfs, but should prove effective in searches for low-mass subdwarfs. A more promising technique for solar-metallicity brown dwarfs is the use of lithium line strengths to discriminate between young brown dwarfs and older M dwarfs in which Li has been destroyed through mixing. Tests based on a full equation-of-state analysis demonstrate that undepleted Li abundances should be unambiguously recognisable in the spectrum of brown dwarfs.
