Towards a complete stellar mass function of the Hyades^⋆

I. Pan-STARRS1 optical observations of the low-mass stellar content

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: goldman@mpia.de
² Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Möchhofstrasse 12–14, 69120 Heidelberg, Germany
³ Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
⁴ National Astronomical Observatories of China, Chinese Academy of Sciences (NAOC/CAS), 20A Datun Lu, Chaoyang District, 100012 Beijing, PR China
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Graduate Institute of Astronomy, National Central University, 32054 Jhongli, Taiwan
⁷ Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
⁸ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

Received: 17 April 2013
Accepted: 10 July 2013

Abstract

Aims. The Hyades cluster is an ideal target to study the dynamical evolution of a star cluster over the entire mass range due to its intermediate age and proximity to the Sun.

Methods. We wanted to extend the Hyades mass function towards lower masses down to 0.1 M_⊙ and to use the full three-dimensional spatial information to characterize the dynamical evolution of the cluster.

Results. We performed a kinematic and photometric selection using the PPMXL and Pan-STARRS1 sky surveys, to search for cluster members up to 30 pc from the cluster centre. We determined our detection efficiency and field star contamination rate to derive the cluster luminosity and mass functions down to masses of 0.1 M_⊙. The thorough astrometric and photometric constraints minimized the contamination. A minimum spanning tree algorithm was used to quantify the mass segregation.

Conclusions. We discovered 43 new Hyades member candidates with velocity perpendicular to the Hyades motion up to 2 km s^-1. They have mass estimates between 0.43 and 0.09 M_⊙, for a total mass of 10 M_⊙. This doubles the number of Hyades candidates with masses smaller than 0.15 M_⊙. We provide an additional list of 11 possible candidates with velocity perpendicular to the Hyades motion up to 4 km s^-1. The cluster is significantly mass segregated. The extension of the mass function towards lower masses provided an even clearer signature than estimated in the past. We also identified as likely Hyades member an L0 dwarf previously assumed to be a field dwarf. Finally we question the membership of a number of previously published candidates, including a L2.5-type dwarf.