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

TL;DR

This study uses low-resolution optical spectroscopy to analyze ultracool dwarfs, comparing photometric and spectroscopic types, and investigates magnetic activity through Hα emission over time.

Contribution

It provides new optical spectral data for ultracool dwarfs and compares photometric and spectroscopic classifications, revealing magnetic activity in some objects.

Findings

Two dwarfs show long-term magnetic activity.

Hα emission indicates magnetic activity variability.

Spectroscopic and photometric types are compared.

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.