Stars and brown dwarfs in the sigma Orionis cluster IV. IDS/INT and OSIRIS/GTC spectroscopy and Gaia DR2 astrometry

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Caballero, J. A.
Burgos, A. de
Alonso Floriano, F.J.
Cabrera Lavers, A.
García Álvarez, D.
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Context. Only a few open clusters are as important for the study of stellar and substellar objects, and their formation and evolution, as the young σ Orionis cluster. However, a complete spectroscopic characterisation of its whole stellar population is still missing. Aims. We filled most of that gap with a large spectroscopic and astrometric survey of targets towards σ Orionis. Eventually, it will be one of the open clusters with the lowest proportion of interlopers and the largest proportion of confirmed cluster members with known uncontrovertible youth features. Methods. We acquired 317 low-resolution optical spectra with the Intermediate Dispersion Spectrograph (IDS) at the 2.5 m Isaac Newton Telescope (INT) and the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) at the 10.4 m Gran Telescopio Canarias (GTC). We measured equivalent widths of Li i, Hα, and other key lines from these spectra, and determined spectral types. We complemented this information with Gaia DR2 astrometric data and other features of youth (mid-infrared excess, X-ray emission) compiled with Virtual Observatory tools and from the literature. Results. Of the 168 observed targets, we determined for the first time spectral types of 39 stars and equivalent widths of Li i and Hα of 34 and 12 stars, respectively. We identified 11 close (ρ </≈ 3 arcsec) binaries resolved by Gaia, of which three are new, 14 strong accretors, of which four are new and another four have Hα emission shifted by over 120 km s^(−1) , two juvenile star candidates in the sparse population of the Ori OB1b association, and one spectroscopic binary candidate. Remarkably, we found 51 non-clustermembers, 35 of which were previously considered as σ Orionis members and taken into account in high-impact works on, for example, disc frequency and initial mass function.
© ESO 2019. We thank the anonymous referee for the careful review, J. Sanz-Forcada for helpful comments on X-rays, and R. Campillo for starting working on the data during his MSc thesis. This article is based on observations made in the Observatorios de Canarias del IAC with the Gran Telescopio Canarias and Isaac Newton Telescope of the Isaac Newton Group of Telescopes, both installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma, Spain, under programs INT15-07A, GTC55-12A, and GTC30-12B. This research made use of the SIMBAD, operated at Centre de Données astronomiques de Strasbourg, France, and NASA’s Astrophysics Data System. Financial support was provided by the Universidad Complutense de Madrid, the Comunidad Autónoma de Madrid, the Spanish Ministerio de Ciencia, Innovación y Universidades and the European FEDER/ERF funds under grants AYA2016-79425-C3-1/2-P and the Centre of Excellence “Severo Ochoa” and “María de Maeztu” awards to the Instituto de Astrofísica de Canarias (SEV-2015-0548) and Centro de Astrobiología (MDM2017-0737).
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