The Hercules-Lyra association revisited^⋆,⋆⋆

New age estimation and multiplicity study

¹ Astrophysical Institute and University Observatory, Friedrich-Schiller Universität, Schillergässchen 2-3, 07745 Jena, Germany
e-mail: eisen@astro.uni-jena.de
² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
³ Georg-August-Universität, Institute for Astrophysics, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Max-Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
⁵ Department of Physics and Astronomy K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Faculty of Natural Sciences, University of Split, Teslina 12. 21000 Split, Croatia

Received: 29 October 2011
Accepted: 22 May 2013

Abstract

Context. The Hercules-Lyra association, a purported nearby young moving group, contains a few tens of zero age main sequence stars of spectral types F to M. The existence and the properties of the Her-Lyr association are controversial and have been discussed in the literature.

Aims. The present work reassesses the properties and the member list of the Her-Lyr association based on kinematics and age indicators. Many objects form multiple systems or have low-mass companions and so we need to properly account for multiplicity.

Methods. We use our own new imaging observations and archival data to identify multiple systems. The colors and magnitudes of kinematic candidates are compared to isochrones. We derive further information on the age based on Li depletion, rotation, and coronal and chromospheric activity. A set of canonical members is identified to infer mean properties. Membership criteria are derived from the mean properties and used to discard non-members.

Results. The candidates selected from the literature belong to 35 stellar systems, 42.9% of which are multiple. Four multiple systems (V538 Aur, DX Leo, V382 Ser, and HH Leo) are confirmed in this work by common proper motion. An orbital solution is presented for the binary system which forms a hierarchical triple with HH Leo. Indeed, a group of candidates displays signatures of youth. Seven canonical members are identified The distribution of Li equivalent widths of canonical Her-Lyr members is spread widely and is similar to that of the Pleiades and the UMa group. Gyrochronology gives an age of 257 ± 46 Myr which is roughly in between the ages of the Pleiades and the Ursa Major group. The measures of chromospheric and coronal activity support the young age. Four membership criteria are presented based on kinematics, lithium equivalent width, chromospheric activity, and gyrochronological age. In total, eleven stars are identified as certain members including co-moving objects plus additional 23 possible members while 14 candidates are doubtful or can be rejected. A comparison to the mass function, however, indicates the presence of a large number of additional low-mass members, which remain unidentified.