Name/Affiliation: Simon J. Murphy (ARI/University of Heidelberg)
The low-mass membership of the Octans association
Thanks largely to the Hipparcos and ROSAT satellites, we now know that the immediate solar neighbourhood is bestrewn with a dozen or so sparse moving groups of young stars, including the well-known TW Hydrae, Beta Pictoris and AB Doradus associations. These groups are the evanescent products of nearby low-density star formation over the past 100 Myr. The under-studied Octans association is one of several such aggregates in the deep southern sky, well-separated from other groups in phase space. Unfortunately, none of Octans' members were observed by Hipparcos, leaving the distance, age and very existence of the group poorly constrained. To expand its membership and better determine these important parameters, we have begun a programme to identify new K and M-type Octans members in multi-wavelength all-sky surveys (e.g. SPM4, GALEX, 2MASS, WISE). By requiring candidates have proper motions and Galactic positions in agreement with existing members, as well as GALEX near-UV colours and elevated CMD positions consistent with youth, we have identified 61 possible low-mass members at kinematic distances of 100-200 pc. Our techniques are also applicable to other nearby young moving groups lacking parallaxes. Spectroscopic observations confirm several new late-K and early-M type stars with congruent radial velocities and photospheric lithium levels indicative of youth. Using this new sample of low-mass members we present an updated definition for Octans and discuss its age, structure and circumstellar disk properties, as well as the group's relationship to other associations in the solar neighbourhood.