Low-resolution optical spectra of ultracool dwarfs with OSIRIS/GTC

Abstract

We present the results of low-resolution optical spectroscopy with OSIRIS/GTC (Optical System for Imaging and Low Resolution Integrated Spectroscopy / Gran Telescopio Canarias) for a sample of ultracool dwarfs. For a subsample of seven objects, based on 2MASS NIR photometric colours, a 'photometric' spectral type is determined and compared to the results of the optical spectroscopy. For the stars, showing H$\alpha$ line in emission, equivalent widths were measured, and the ratio of H$\alpha$ to bolometric luminosity were calculated. We find that two dwarfs show the presence of magnetic activity over long periods, LP 326-21 -- quasi-constant-like, and 2MASS J17071830+6439331 -- variable.

7 pages, 4 figures, 3 tables. arXiv admin note: text overlap with arXiv:astro-ph/0612604 by other authors

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