Publication: Discovery of new members of the nearby young stellar association in Cepheus
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Context. Young field stars are hardly distinguishable from older ones because their space motion rapidly mixes them with the stellar population of the Galactic plane. Nevertheless, a careful target selection allows for young stars to be spotted throughout the sky. Aims. We aim to identify additional sources associated with the four young comoving stars that we discovered towards the CO Cepheus void and to provide a comprehensive view of the Cepheus association. Methods. Based on multivariate analysis methods, we have built an extended sample of 193 young star candidates, which are the optical and infrared counterparts of ROSAT All-Sky Survey and XMM-Newton X-ray sources. From optical spectroscopic observations, we measured their radial velocity with the cross-correlation technique. We derived their atmospheric parameters and projected rotational velocity with the code ROTFIT. We applied the subtraction of inactive templates to measure the lithium equivalent width, from which we infer their lithium abundance and age. Finally, we studied their kinematics using the second Gaia data release. Results. Our sample is mainly composed of young or active stars and multiple systems. We identify two distinct populations of young stars that are spatially and kinematically separated. Those with an age between 100 and 300 Myr are mostly projected towards the Galactic plane. In contrast, 23 of the 37 sources younger than 30 Myr are located in the CO Cepheus void, and 21 of them belong to the stellar kinematic group that we previously reported in this sky area. We report a total of 32 bona fide members and nine candidates for this nearby (distance = 157±10 pc) young (age = 10–20 Myr) stellar association. According to the spatial distribution of its members, the original cluster is already dispersed and partially mixed with the local population of the Galactic plane.
© ESO 2020. We thank the anonymous referee for useful suggestions. This work is supported by the Universidad Complutense de Madrid and Spanish Ministerio de Ciencia e Innovación y Universidades (MICINN) under grant AYA2016-79425-C3-1-P. A.K. and D.M. were also supported by AstroMadrid (CAM S2009/ESP-1496), and MICINN under grants AYA2008-00695 and AYA2008-06423-C03-03. Part of this study is also supported by the Italian Ministero dell’Istruzione, Università e Ricerca (MIUR), the French Centre National d’Études spatiales (CNES), and the Région Alsace. This research made use of the SIMBAD database, the VIZIER catalog access and the X-Match service, which are operated at the Centre de Données astronomiques de Strasbourg (CDS). This publication makes use of the data products from ROSAT and XMM-Newton X-ray observatories, the Two Micron All Sky Survey, and the European Space Agency (ESA) missions Gaia and Planck. The ROSAT All-Sky Survey catalogs were produced by the ROSAT Scientific Data Center at the MaxPlanck-Institut für Extraterrestrische Physik (MPE), Garching (Germany). The 2XMMi-DR3 catalog is the fifth publicly released XMM-Newton X-ray source catalog produced by the XMM-Newton Survey Science Centre (SSC) consortium on behalf of ESA. The 2XMMi-DR3 is one of two incremental versions of the 2XMM catalog. The Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, was funded by the National Aeronautics and Space Administration and the National Science Foundation. The data from the ESA mission Gaia (https://www.cosmos.esa.int/gaia) was 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. Planck (http://www.esa.int/Planck) is an ESA science mission with instruments and contributions directly funded by ESA Member States, NASA, and Canada. This publication used the POLLUX database (http://pollux.graal.univ-montp2.fr) operated at LUPM (Laboratoire Univers et Particules de Montpellier, Université Montpellier II – CNRS, France) with the support of the French Programme National de Physique Stellaire and Institut national des sciences de l’Univers.