Each star orbits the center of our Galaxy, and its speed and position depend on its history of interactions with dust and other stars in the Galactic plane. As stars age, they have more and more kinematic interactions, which overall increase the total stellar velocity and result in older stars spending more time further from the plane of the Galaxy. Because the changes in orbital motion are a stochastic process, kinematics is only a reliable age indicator when considering populations or groups of stars.
I mainly use kinematics to examine the usefulness of other age indicators. As part of my dissertation, I looked at the J-K colors of L dwarfs compared to kinematics, finding that older stars have bluer J-K colors than younger stars, presumably due to the effects of lower metallicity in older stars or thicker clouds in younger stars.
Left panel: The velocity dispersion in the three dimensions of motion as a function of color difference from the mean J-K color. The bluer dwarfs (smaller values of J-K) have a larger velocity dispersion (consistent with older ages), while redder dwarfs have a smaller velocity dispersion. From
Schmidt et al. (2010)
Right panel: The position of dwarfs in the BUD sample relative to the Galactic plane, separated by spectral type.
I am currently working with my BOSS Ultracool Dwarfs sample of 13000 late-M and L dwarfs with spectra from SDSS and Galactic kinematics to examine other properties related to age. Initial results suggest that several colors are related to age, and that magnetic activity and age are related even at these late spectral types where spin-down is less efficient.
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The Effect of Atmospheric Cooling on Vertical Velocity Dispersion and Density Distribution of Brown Dwarfs
Jr. Ryan, Paul A. Thorman, Sarah J. Schmidt, Seth H. Cohen, Nimish P. Hathi, Benne W. Holwerda, Jonathan I. Lunine, Nor Pirzkal, Rogier A. Windhorst, and Erick Young
ApJ, Sep 2017
We present a Monte Carlo simulation designed to predict the vertical velocity dispersion of brown dwarfs in the Milky Way. We show that since these stars are constantly cooling, the velocity dispersion has a noticeable trend with the spectral type. With realistic assumptions for the initial mass function, star formation history, and the cooling models, we show that the velocity dispersion is roughly consistent with what is observed for M dwarfs, decreases to cooler spectral types, and increases again for the coolest types in our study (̃T9). We predict a minimum in the velocity dispersions for L/T transition objects, however, the detailed properties of the minimum predominately depend on the star formation history. Since this trend is due to brown dwarf cooling, we expect that the velocity dispersion as a function of spectral type should deviate from the constancy around the hydrogen-burning limit. We convert from velocity dispersion to vertical scale height using standard disk models and present similar trends in disk thickness as a function of spectral type. We suggest that future, wide-field photometric and/or spectroscopic missions may collect sizable samples of distant (̃ 1 kpc) dwarfs that span the hydrogen-burning limit. As such, we speculate that such observations may provide a unique way of constraining the average spectral type of hydrogen burning. Support for program #13266 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under the NASA contract NAS 5-26555.
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The Brown Dwarf Kinematics Project (BDKP). IV. Radial Velocities of 85 Late-M and L Dwarfs with MagE
Adam J. Burgasser, Sarah E. Logsdon, Jonathan Gagné, John J. Bochanski, Jaqueline K. Faherty, Andrew A. West, Eric E. Mamajek, Sarah J. Schmidt, and Kelle L. Cruz
ApJs, Sep 2015
Radial velocity measurements are presented for 85 late M- and L-type very low-mass stars and brown dwarfs obtained with the Magellan Echellette spectrograph. Targets primarily have distances within 20 pc of the Sun, with more distant sources selected for their unusual spectral energy distributions. We achieved precisions of 2-3 km s-1, and combined these with astrometric and spectrophotometric data to calculate UVW velocities. Most are members of the thin disk of the Galaxy, and velocity dispersions indicate a mean age of 5.2±0.2 Gyr for sources within 20 pc. We find signficantly different kinematic ages between late-M dwarfs (4.0±0.2 Gyr) and L dwarfs (6.5±0.4 Gyr) in our sample that are contrary to predictions from prior simulations. This difference appears to be driven by a dispersed population of unusually blue L dwarfs which may be more prevalent in our local volume-limited sample than in deeper magnitude-limited surveys. The L dwarfs exhibit an asymmetric U velocity distribution with a net inward flow, similar to gradients recently detected in local stellar samples. Simulations incorporating brown dwarf evolution and Galactic orbital dynamics are unable to reproduce the velocity asymmetry, suggesting non-axisymmetric perturbations or two distinct L dwarf populations. We also find the L dwarfs to have a kinematic age-activity correlation similar to more massive stars. We identify several sources with low surface gravities, and two new substellar candidate members of nearby young moving groups: the astrometric binary DENIS J08230313-4912012AB, a low-probability member of the β Pictoris Moving Group; and 2MASS J15104786-2818174, a moderate-probability member of the 30-50 Myr Argus Association.
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Colors and Kinematics of L Dwarfs from the Sloan Digital Sky Survey
Sarah J. Schmidt, Andrew A. West, Suzanne L. Hawley, and J. Sebastian Pineda
AJ, May 2010
We present a sample of 484 L dwarfs, 210 of which are newly discovered from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic database. We combine this sample with known L dwarfs to investigate their izJHKS colors. Our spectroscopically selected sample has 0.1 mag bluer median J - KS color at a given spectral type (for L0-L4) than previously known L dwarfs, which reflects a bias toward redder L dwarfs in past selection criteria. We present photometric distance relations based on i - z and i - J colors and derive distances to our L dwarf sample. We combine the distances with SDSS/2MASS proper motions in order to examine the tangential velocities. For the majority of our spectroscopic sample, we measured radial velocities and present three-dimensional kinematics. We also provide Hα detections for the fraction of our sample with sufficient quality spectra. Comparison of the velocities of our L dwarf sample to a kinematic model shows evidence for both cold and hot dynamical populations, consistent with young and old disk components. The dispersions of these components are similar to those found for M dwarfs. We also show that J - KS color is correlated with velocity dispersion, confirming a relationship between J - KS color and age.
