Current Research

Low-mass star formation in Scorpius-Centaurus

Sco-Cen OB Association
The southern Milky Way through the constellations of Centaurus, Crux and Carina. Credit: Akira Fujii

Lying at a distance of 100-150 pc from the Sun, the Scorpius-Centaurus OB Association is the closest site of recent massive star formation. Despite its proximity, little is currently known of its stellar population below 1 M⊙.

Without these low-mass members, the spatial structure, star-formation history and mass function of the most important star-forming event in the Solar neighbourhood are still ill-constrained. We are therefore undertaking a deep, wide-field survey for new M-type Sco-Cen members over the entire 2000 sq. deg association (5% of the sky).

Using proper motions from the updated PPMXL and UCAC4 catalogues, combined with photometry from the WISE satellite and the new SkyMapper Southern Sky Survey, we have selected candidates with kinematics and colour-magnitude positions consistent with membership in Sco-Cen. A new Bayesian selection method allows us to greatly reduce contamination from non-members over the wide fields of our survey. We are in the process of observing these candidates on the ANU 2.3-m and Anglo-Australian telescopes at Siding Spring to determine robust temperatures, ages, masses and radial velocities, as well as checking for signs of accretion from circumstellar disks.

We will use our large new sample of low-mass members to examine the star formation history, substructure and mass function of Sco-Cen, with a particular interest in its older and more dispersed Upper Centaurus Lupus and Lower Centaurus Crux subgroups, and their role in the birth of the nearby β Pictoris, TW Hydrae and ε Chamaeleontis groups. With accurate proper motions and radial velocities we also have a robust sample to investigate the kinematics of the association.

Stay tuned for early results!

The young groups η and ε Chamaeleontis

Chamaeleon region
Infrared Astronomical Satellite (IRAS) 25/60/100 micron colour composite of the Chamaeleon region, showing the young open cluster η Cha, the ε Cha Association and the background Chamaeleon dark clouds.

Two of the most intriguing young moving groups associated with Sco-Cen are the open cluster η Cha and the ε Cha Association. Both are 5-10 Myr old and only ~100 pc away, at opposite ends of the southern constellation of Chamaeleon.

We are conducting the first systematic investigation of the low-mass population of ε Cha, combining several proposed groups in the region into a coherent membership. On the basis of their kinematics, photometry and spectroscopic youth indicators, we have so far confirmed 11 stars as members, with estimated kinematic distances of 95–135 pc. The remaining "members" proposed in the literature are likely members of the Cha I and II background cloud populations, as well as the Lower Centaurus Crux subgroup of Sco-Cen. Contrary to previous studies, we propose η and ε Cha were not born together, but were separated by approximately 30 pc at the time η Cha formed.

As well as studying ε Cha, my Ph.D. thesis also examined:

  • The low-mass stellar halo around η Cha
    The central core of η Cha appears to be missing many of its low-mass members, ostensibly due to dynamical evolution which dispersed the stars to large radii. From a wide-field (95 sq. deg) photometric and proper motion survey around η Cha, combined with extensive follow-up spectroscopy, we have identified several new young, low-mass stars up to 5 degrees from η Cha. These stars have proper motions and radial velocities consistent with ejection or evaporation from the cluster over the past 5-10 Myr.
  • Episodic accretion in 'old' pre-main sequence clusters
    Several members of η and ε Cha show strong and variable H-alpha emission in our multi-epoch ANU 2.3-m/WiFeS spectra, including one star which underwent an isolated event indicative of accretion from a circumstellar disk. WISE infrared photometry confirms the presence of a disk around the star and demonstrates that episodic accretion can continue at low levels long after the majority of circumstellar material has dissipated.
  • A new wide pre-main sequence binary born in Sco-Cen
    Two of the outlying η Cha candidates are noticeably older than the cluster and only 42 arcsec apart. We have demonstrated that they almost certainly form an isolated, wide (4000–5000 AU) 10 Myr-old binary at a distance of 100–150 pc. The dynamical fragility of the system puts strong constraints on its birthplace, which we believe was in the outskirts of Sco-Cen.

Background image credit: Akira Fujii