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

TL;DR

This survey identifies low-mass stars and brown dwarf candidates in the Hyades cluster, revealing a deficit of such objects likely due to dynamical evolution, and enhances understanding of the cluster's mass function.

Contribution

The paper presents a comprehensive optical/IR survey combining photometry and proper motions to identify VLM and BD candidates in the Hyades, improving the cluster's mass function data.

Findings

14 proper motion members identified

Hy 6 classified as a brown dwarf candidate

Mass function continues to be shallow below 0.15 M_sun

Abstract

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. We search for new VLM stars and BD candidates in the Hyades to improve the present-day cluster mass function down to substellar masses. An imaging survey of the Hyades with a completeness limit of 21m.5 in the $R$ band and 20m.5 in the $I$ band was carried out with the 2kx2k 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 JHKs photometry. 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 14m.7<I<20m.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. With our results, the mass function of the Hyades continues to be shallow below 0.15 $M_\odot$ 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.