Discovery of Very-Low-Mass Binary Stars and Circumstellar Disks in the Infrared

Abstract

We present results from three infrared observational studies investigating different aspects of stellar evolution. The first survey, conducted in the near-infrared with adaptive optics, measures for the first time the stellar multiplicity properties of field M6-M7.5 dwarf stars. We report that their binary fraction, separation distribution, and mass ratio distribution are very similar to those of later spectral type stars and brown dwarfs while distinct from more massive stellar binaries. These differences, when coupled with age, shed light on possibly different formation mechanisms and kinematic evolution between binary systems of different primary masses. I incorporate these results with those from all known very-lowmass binary systems (M(tot) ≤ 0.2 M⊙) and present their statistical properties. We also present the discovery of a very tight (66 mas) brown dwarf companion to a mid-L dwarf demonstrating the capabilities of laser guide star adaptive optics. In the second study we present mid-infrared Spitzer observations of members of the ∼ 50 Myr open stellar cluster IC 2391 where, using photometric techniques, we report that about a third of the solar-like stars (spectral types FGK) likely possess debris disks. With respect to several other stellar groups of known age, we show for the first time the evolution of the debris disk fraction of solar-like stars. We conclude that, along with more massive late-B and A stars, the formation of planetesimals around solar-like stars appears to be a universal process of star formation. Lastly, we present preliminary near- and mid-infrared Spitzer observations of stars in the direction of the ∼ 6 Myr open cluster IC 2395. Using photometric techniques, we identify upper main sequence cluster members and lower-mass candidate members with evidence of circumstellar disks at different stages of disk evolution - primordial, transition, and debris. We present for the first time the evolution of the median IRAC flux ratios emitted from the inner ∼ 0.2 AU regions of classic T Tauri stars. These results are possibly consistent with the processes of grain growth and dust settling as a mechanism for planetesimal formation.