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J.E. Gizis, J.D. Kirkpatrick (IPAC/Caltech), I.N. Reid (U. Penn), D.G. Monet (USNO/Flagstaff), J. Liebert (U. Arizona)
We present results of a study of chromospheric activity in very low mass stars and brown dwarfs. We show the observed frequency of H\alpha emission peaks at ~100% for M7 dwarfs and then decreases for cooler M and L dwarfs. In absolute terms, however, as measured by the ratio of H\alpha to bolometric luminosity, none of the ultracool M dwarfs can be considered very active compared to earlier M dwarfs, and we show that the decrease that begins at spectral type M6 continues to the latest L dwarfs. We show that the kinematics of relatively active ultracool M dwarfs are consistent with an ordinary old disk stellar population, while the kinematics of inactive ultracool M dwarfs are more typical of a young (substellar?) 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen burning limit is near spectral types L2-L4. We compare the field M and L dwarfs to cluster brown dwarfs of known age. We use the available data on M and L dwarfs to show that chromospheric activity drops sharply with temperature, and that at a given (M8 or later) spectral type, the younger field (brown) dwarfs are less active than many of the older, more massive field stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in field ultracool dwarfs can be an indication of comparative youth and substellar mass.
This publication makes use of data products from 2MASS, which is a joint project of the University of Massachusetts and IPAC, funded by NASA and NSF.
The author(s) of this abstract have provided an email address for comments about the abstract: gizis@ipac.caltech.edu