Activity and Kinematics of Cool and Ultracool Dwarfs
Schmidt, Sarah Jane
MetadataShow full item record
The ages of cool and ultracool dwarfs are particularly important. For cool M dwarfs, accurate ages combined with their ubiquity in the stellar disk could lead to a new level of precision in age dating the Galaxy. A better understanding of the chromospheres of M dwarfs could provide important clues about the relationship between activity and age in these low mass stars. Ultracool (late-M and L) dwarfs have the distinction of including both warm, young brown dwarfs and stars with mean ages more representative of the stellar disk. Kinematics are a source of mean ages and could provide or confirm discriminating features between stars and brown dwarfs. This thesis is composed of several different projects, each investigating the activity or kinematics of cool or ultracool dwarfs. First, a sample of nearly 500 L dwarfs selected from SDSS DR7 photometry and spectroscopy is examined; we discovered 200 new L dwarfs and found evidence of a bias towards red J-K<sub>S</sub> colors in the entire population of previously known L dwarfs. Using the three-dimensional kinematics of 300 SDSS DR7 L dwarfs, we find that their kinematics are consistent with those of the stellar disk and include a previously undetected thick disk component. We also confirmed a relationship between age and J-K<sub>S</sub> color (due to our large sample of UVW motions and unbiased J-K<sub>S</sub> colors), with blue L dwarfs having hotter kinematics (consistent with older ages) and redder L dwarfs having colder, younger kinematics. The DR7 L dwarf sample showed no distinct kinematic difference between young brown dwarfs and disk-age stars, perhaps due to a bias towards early spectral types. In order to probe the kinematic distribution of L dwarfs in a volume-limited sample, we began a survey of radial velocities of nearby (d<20pc) L dwarfs using the TripleSpec instrument on the ARC 3.5-m telescope at APO. While several reduction packages were tested on the TripleSpec data, none were found to provide reductions of sufficient quality for the measurement of radial velocities. Another avenue for kinematic investigation was identified: the assembly of a large sample of late-M and L dwarfs using a combination of SDSS DR7 and BOSS photometry and spectroscopy, supplemented by photometry from 2MASS and WISE. In agreement with previous work, we find that colors based on a combination of SDSS and 2MASS photometry are well correlated with spectral type for early-L dwarfs, while infrared colors (from 2MASS and WISE alone) are well correlated with spectral type for the latest L dwarfs. Using the kinematics of this sample, we confirm that there is no relationship between velocity dispersions and spectral type, indicating that young brown dwarfs are not a significant component of early-L dwarfs in the field. The velocity dispersions of active and inactive dwarfs are also suggestive of an age activity relation for late-M and L dwarfs. The BOSS Ultracool Dwarf sample has also proven ideal for investigating the activity properties of M and L dwarfs using the presence and strength of H&alpha emission. We show that the fraction of objects which show H&alpha emission increases from early-M dwarfs through the L1 spectral type. Additionally, we use the NLTE radiative transfer code RH to investigate the ranges of temperature structure and filling factors of M and L dwarf chromospheres by generating the observed levels of H&alpha emission. To produce the observed levels of emission, the typically strongly emitting early-M dwarfs need hot chromospheres covering over 1% of their surfaces, while L dwarfs, which emit H&alpha at much weaker levels, need much less extended and/or cooler chromospheres. H&alpha can be difficult to detect in L dwarfs due to their faint optical luminosities, but they are relatively bright in the infrared. To begin a search for chromospheric indicators in the infrared, we began a monitoring program designed to detect infrared emission lines from M dwarf flares. We present the first reported detection of infrared emission lines during an M dwarf flare. Based on 50 hours of monitoring in the infrared, we estimate that M dwarfs spend ~3% of the time showing infrared emission lines. Using RH, we show that a very hot chromosphere (T~30,000K) is required to produce the line flux rations observed in optical and infrared spectra of our strongest flare.
- Astronomy