Revealing the Chamaeleon: Young, low-mass stars surrounding eta and epsilon Cha

Abstract

The deep southern sky surrounding the Chamaeleon dark clouds is abundant with pre-main sequence stars of various ages. Because of their youth (5-10 Myr) and proximity (d~100 pc), members of the open cluster Eta Chamaeleontis and the nearby Epsilon Chamaeleontis Association are ideal laboratories to study the formation and evolution of extrasolar planetary systems. To better understand their role as potential planet hosts, this thesis explores the formation, dynamical evolution, accretion and disk properties of both groups' low-mass members.

The notable lack of low-mass stars in the young open cluster Eta Cha has long been puzzling. Two possible explanations have been suggested; a top-heavy initial mass function or dynamical evolution, which preferentially ejected the low-mass members. Previous efforts to find these stars several degrees from the cluster core have been unsuccessful. By undertaking a wider (95 sq deg) photometric and proper motion survey with extensive follow-up spectroscopy, we have identified eight low-mass stars that were ejected from Eta Cha over the past 5-10 Myr. Comparison with recent simulations shows our results are consistent with a dynamical origin for the current configuration of the cluster, without the need to invoke an initial mass function deficient in low-mass objects.

Two of the dispersed members exhibited strong, variable H-alpha emission during our observations, including a star which had an event suggestive of accretion from a circumstellar disk. New infrared photometry confirms the presence of the disk. This star demonstrates that infrequent, episodic accretion can continue at low levels long after most disks around `old' pre-main sequence stars have dissipated.

Another two confirmed non-members are slightly older than the cluster, but are only 42 arcseconds apart and share similar kinematics and distances. We show that they almost certainly form a wide (4000-6000 AU) ~10 Myr-old binary at 100-150 pc. The system is one of the widest pre-main sequence binaries known. Its isolation and dynamical fragility put strong constraints on any birthplace and mode of formation, which we propose was in a turbulent gas filament in the vicinity of the Scorpius-Centaurus OB Association.

In addition to Eta Cha, we have also examined membership of the unbound Epsilon Chamaeleontis Association, which lies some 10 degrees to the east and has a similar age, distance and kinematics. The two groups were almost certainly born in the outer regions of Sco-Cen only a few million years apart. Many members of Epsilon Cha have been proposed in the decade since its discovery. After considering the kinematics of candidates from the literature, we have confirmed 11 further stars as likely members. Many new members possess infrared spectral energy distributions attributable to circumstellar disks, including four stars with strong H-alpha and forbidden emission which are actively accreting material.

Our work on Eta and Epsilon Chamaeleontis has identified many interesting targets for follow-up studies of disk evolution, accretion, binarity, and other investigations that require samples of nearby pre-main sequence stars. Several avenues for future work are discussed, including the impact of photometry and astrometry from the forthcoming SkyMapper Southern Sky Survey.