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A spectroscopic survey of the youngest field stars in the solar neighborhood II. The optically faint sample

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2018-05-04
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EDP Sciences
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Context. Star formation in the solar neighborhood is mainly traced by young stars in open clusters, associations, and in the field, which can be identified, for example, by their X-ray emission. The determination of stellar parameters for the optical counterparts of X-ray sources is crucial for a full characterization of these stars. Aims. This work extends the spectroscopic study of the RasTyc sample, obtained by the cross-correlation of the Tycho and ROSAT All-Sky Survey catalogs, to stars fainter than V = 9.5 mag and aims to identify sparse populations of young stars in the solar neighborhood. Methods. We acquired 625 high-resolution spectra for 443 presumably young stars with four different instruments in the northern hemisphere. The radial and rotational velocity (v sin i) of our targets were measured by means of the cross-correlation technique, which is also helpful to discover single-lined (SB1), double-lined spectroscopic binaries (SB2), and multiple systems. We used the code ROTFIT to perform an MK spectral classification and to determine the atmospheric parameters (T-eff, log g, [ Fe/H]) and v sin i of the single stars and SB1 systems. For these objects, we used the spectral subtraction of slowly rotating templates to measure the equivalent widths of the H-alpha and Li I 6708 angstrom lines, which enabled us to derive their chromospheric activity level and lithium abundance. We made use of Gaia DR1 parallaxes and proper motions to locate the targets in the Hertzsprung-Russell (HR) diagram and to compute the space velocity components of the youngest objects. Results. We find a remarkable percentage (at least 35%) of binaries and multiple systems. On the basis of the lithium abundance, the sample of single stars and SB1 systems appears to be mostly (similar to 60%) composed of stars younger than the members of the UMa cluster. The remaining sources are in the age range between the UMa and Hyades clusters (similar to 20%) or older (similar to 20%). In total, we identify 42 very young (PMS-like) stars, which lie above or very close to the Pleiades upper envelope of the lithium abundance. A significant percentage (similar to 12%) of evolved stars (giants and subgiants) is also present in our sample. Some of these stars (similar to 36%) are also lithium rich (A(Li) > 1.4).
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© ESO 2018. This paper is dedicated to the memory of our colleague and friend Rubens Freire Ferrero. We thank the anonymous referee for useful suggestions. We are grateful to the TNG staff and, particularly, to Aldo F. Fiorenzano and Antonio Magazzu for conducting the service observations with SARG. We also thank the night assistants of the OHP and OAC observatories for their support and help with the observations. Support from the Italian Ministero dell'Istruzione, Universita e Ricerca (MIUR) is also acknowledged. D.M. acknowledges financial support from the Universidad Complutense de Madrid (UCM) and the Spanish Ministry of Economy and Competitiveness (MINECO) from project AYA2016-79425-C3-1-P. This research made use of SIMBAD and VIZIER databases, operated at the CDS, Strasbourg, France. This publication uses ROSAT data. This publication makes use of data products from the Widefield Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
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