New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence
Abstract
We have combined 2MASS and POSS II data in a search for nearby ultracool (later than M6.5) dwarfs with Ks<12. Spectroscopic follow-up observations identify 53 M7-M9.5 dwarfs and seven L dwarfs. The observed space density is 0.0045+/-0.0008 M8-M9.5 dwarfs per cubic parsec, without accounting for biases, consistent with a mass function that is smooth across the stellar/substellar limit. We show the observed frequency of Hα emission peaks at ~100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute terms, however, as measured by the ratio of Hα 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 find that flaring is common among the coolest M dwarfs and estimate the frequency of flares at 7% or higher. We show that the kinematics of relatively active (EW>6 Å) 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 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 use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and 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.
- Publication:
-
The Astronomical Journal
- Pub Date:
- August 2000
- DOI:
- 10.1086/301456
- arXiv:
- arXiv:astro-ph/0004361
- Bibcode:
- 2000AJ....120.1085G
- Keywords:
-
- Galaxy: Solar Neighborhood;
- Stars: Activity;
- Stars: Kinematics;
- Stars: Low-Mass;
- Brown Dwarfs;
- Stars: Luminosity Function;
- Mass Function;
- Astrophysics
- E-Print:
- 39 pages including tables and figures, to appear in August 2000 A.J