A survey for low-mass stellar and substellar members of the Hyades open cluster

Abstract

Context.Unlike young open clusters (with ages < 250 Myr), the Hyades cluster (age ∼ 600 Myr) has a clear deficit of very low-mass stars (VLM) and brown dwarfs (BD). Since this open cluster has a low stellar density and covers several tens of square degrees on the sky, extended surveys are required to improve the statistics of the VLM/BD objects in the cluster. Aims. We search for new VLM stars and BD candidates in the Hyades cluster to improve the present-day cluster mass function down to substellar masses.Methods. An imaging survey of the Hyades with a completeness limit of 21. m 5 in the R band and 20. m 5 in the I band was carried out with the 2k × 2k CCD Schmidt camera at the 2m Alfred Jensch Telescope in Tautenburg. We performed a photometric selection of the cluster member candidates by combining results of our survey with 2MASS JHK s photometry. Results. We present a photometric and proper motion survey covering 23.4 deg 2 in the Hyades cluster core region. Using optical/IR colour-magnitude diagrams, we identify 66 photometric cluster member candidates in the magnitude range 14. m 7 < I < 20. m 5. The proper motion measurements are based on several all-sky surveys with an epoch difference of 60-70 years for the bright objects. The proper motions allowed us to discriminate the cluster members from field objects and resulted in 14 proper motion members of the Hyades. We rediscover Hy 6 as a proper motion member and classify it as a substellar object candidate (BD) based on the comparison of the observed colour-magnitude diagram with theoretical model isochrones.Conclusions. With our results, the mass function of the Hyades continues to be shallow below ∼ 0.15M indicating that the Hyades have probably lost their lowest mass members by means of dynamical evolution. We conclude that the Hyades core represents the 'VLM/BD desert' and that most of the substeller objects may have already left the volume of the cluster.

A 3D view of the Hyades stellar and sub-stellar population

“…The surveys targeting the Hyades can be divided up into two main groups: the large-scale studies looking for a complete census of the cluster members over very large areas of the sky (Gizis et al 1999;Goldman et al 2013;Hanson 1975;Hogan et al 2008;Reid 1992;Röser et al 2011) or deeper surveys on small(er) patches in the cluster center to identify new members (Bouvier et al 2008;Dobbie et al 2002;Hanson 1975;Leggett et al 1994;Melnikov & Eislöffel 2018;Reid & Gizis 1997;Reid 1993;Stauffer et al 1995Stauffer et al , 1994. Subsequent spectroscopic follow-up has been conducted for many sources to confirm their membership via spectral typing, radial velocity and/or lithium content (Bryja et al 1994;Leggett & Hawkins 1989;Mermilliod et al 2009;Reid & Hawley 1999;Reid & Mahoney 2000;Soderblom et al 1995;Stauffer et al 1995Stauffer et al , 1994Tabernero et al 2012;White et al 2007).…”