Observations of Brown Dwarfs at Low Temperatures and Low Masses

Open Access
Esplin, Taran LeRoy
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
June 02, 2017
Committee Members:
  • Kevin Luhman, Dissertation Advisor
  • Kevin Luhman, Committee Chair
  • Lawrence William Ramsey, Committee Member
  • Eric D Feigelson, Committee Member
  • Sarah Elizabeth Shandera, Outside Member
  • Jason Thomas Wright, Committee Member
  • open clusters and associations: general
  • astrometry
  • brown dwarfs
  • infrared: stars
  • planets and satellites: atmospheres
  • solar neighborhood
  • stars: low-mass
  • planetary systems
  • protoplanetary disks
  • stars: formation
  • stars: luminosity function
  • stars: pre-main sequence
We have contributed to the characterization and discovery of brown dwarfs at low temperatures and low masses. Because the coolest brown dwarfs overlap with giant planets in temperature, they can be used as proxies for studying planetary atmospheres. In multiple temperature regimes for brown dwarfs, condensates are predicted to form clouds. Based on observations of both terrestrial planets and gas giants in our solar system, clouds of water ice are likely to be a prominent constituent in the atmospheres of some brown dwarfs. If these clouds are present, they should be patchy, which should produce a variable light curve. In the last few years, a few brown dwarfs have been discovered that should be cool enough to harbor water ice clouds (< 375 K). We have searched for evidence of such clouds in the coldest known brown dwarf (~250 K) in light curves obtained at 3.6 and 4.5 microns with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We did detect variability, but its low amplitude is difficult to explain using the theoretical models of water ice clouds unless there is only a very small deviation in cloud coverage between hemispheres. Alternatively, the similarity in mid-infrared variability amplitudes between this object and warmer T and Y dwarfs may suggest that they share a common origin for their variability (i.e., not water clouds). Because brown dwarfs are the product of the process of star formation, detecting the minimum mass at which they form is a fundamental test of star formation theories. The most thorough surveys to detect this minimum mass are complete down to ~10 MJup, but have found objects with masses as low as ~5 MJup. Thus, the minimum mass has not been detected. Nearby star-forming regions are the most promising sites where large numbers of young (and hence bright) brown dwarfs <10 MJup can be found. Proper motion measurements can be used to identify new brown dwarfs in these regions. Because IRAC images are available at multiple epochs spanning a decade for most nearby sites of star formation, proper motions can be measured with these data. However, the expected motions of these regions over this time frame range from 50–200 mas and the error in a typical IRAC astrometric measurement is ∼100 mas. To more fully realize IRAC’s astrometric capabilities, we have measured new distortion corrections for the 3.6 and 4.5 micron arrays. Using these corrections, we have identified a method for measuring astrometry of point sources in IRAC images that can achieve a total astrometric error of 20 mas, which is sufficiently accurate for identifying candidate brown dwarfs in nearby star-forming regions. We have performed a search for planetary-mass brown dwarfs in the Chamaeleon I and Taurus star-forming regions using proper motions measured with IRAC and other wide-field surveys and using optical and near-infrared photometry. Through near-infrared spectroscopy, we have confirmed 6 and 18 candidates as new late-type members of Chamaeleon I and Taurus, respectively. These objects include the faintest known members in both regions, which should have masses of 3–6 MJup according to evolutionary models. Because the new brown dwarfs that we have found extend below the completeness limits of our surveys, deeper observations are needed to measure the minimum mass of star-formation in these regions.