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Title:
New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence
Authors:
Gizis, John E.; Monet, David G.; Reid, I. Neill; Kirkpatrick, J. Davy; Liebert, James; Williams, Rik J.
Affiliation:
AA(Infrared Processing and Analysis Center, 100-22, California Institute of Technology, Pasadena, CA 91125; ), AB(US Naval Observatory, P.O. Box 1149, Flagstaff, AZ 86002), AC(Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104-6396), AD(Infrared Processing and Analysis Center, 100-22, California Institute of Technology, Pasadena, CA 91125), AE(Steward Observatory, University of Arizona, Tucson, AZ 85721), AF(Department of Astronomy, MSC 152, California Institute of Technology, Pasadena, CA 91126-0152)
Publication:
The Astronomical Journal, Volume 120, Issue 2, pp. 1085-1099. (AJ Homepage)
Publication Date:
08/2000
Origin:
UCP
AJ Keywords:
Galaxy: Solar Neighborhood, Stars: Activity, Stars: Kinematics, Stars: Low-Mass, Brown Dwarfs, Stars: Luminosity Function, Mass Function
DOI:
10.1086/301456
Bibliographic Code:
2000AJ....120.1085G

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.
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