Abstract
Although mid-to-late type M dwarfs are the most common stars in our stellar
neighborhood, our knowledge of these objects is still limited. Open questions
include the evolution of their angular momentum, internal structures, dust
settling in their atmospheres, age dispersion within populations. In addition,
at young ages, late-type Ms have masses below the hydrogen burning limit and
therefore are key objects in the debate on the brown dwarf mechanism of
formation. In this work we determine and study in detail the physical
parameters of two samples of young, late M-type sources belonging to either the
Chamaeleon I Dark Cloud or the TW Hydrae Association and compare them with the
results obtained in the literature for other young clusters and also for older,
field, dwarfs. We used multi-wavelength photometry to construct and analyze
SEDs to determine general properties of the photosphere and disk presence. We
also used low resolution optical and near-infrared spectroscopy to study
activity, accretion, gravity and effective temperature sensitive indicators. We
propose a VO-based spectral index that is both temperature and age sensitive.
We derived physical parameters using independent techniques confirming the
already common feature/problem of the age/luminosity spread. In particular, we
highlight two brown dwarfs showing very similar temperatures but clearly
different surface gravity (explained invoking extreme early accretion). We also
show how, despite large improvement in the dust treatment in theoretical
models, there is still room for further progress in the simultaneous
reproduction of the optical and near-infrared features of these cold young
objects.
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