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 Spectroscopic follow-up observations identify 53 M7ÈM9.5 dwarfs and seven L K s \ 12. dwarfs. The observed space density is 0.0045^0.0008 M8ÈM9.5 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 Ha emission peaks at D100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute terms, however, as measured by the ratio of Ha 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 Ðnd that Ñaring is common among the coolest M dwarfs and estimate the frequency of Ñares at 7% or higher. We show that the kinematics of relatively active (EW [ 6 ultracool M dwarfs are consistent with an ordi-A ) nary 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 L2ÈL4. 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 Ðeld (brown) dwarfs are less active than many of the older, more massive Ðeld stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in Ðeld ultracool dwarfs can be an indication of comparative youth and substellar mass.

Activity and Kinematics of M and L Dwarfs

“…Like the Pleiades L brown dwarf, the σ Ori L-type brown dwarf has no detectable activity despite its youth. In the case of the ρOph brown dwarf, the mid-IR excess strongly suggests that the emission is due to a disk [16], and Gizis et al [10] suggest that the 'active' brown dwarfs are actually those with disks, while the relatively inactive brown dwarfs simply show the weak chromospheres expected by analogy with the field dwarfs.…”

“…Late-M and early-L dwarfs have been selected to K s < 12 on the basis of their colors without any kinematic bias [10]. This sample is supplemented by the large sample of fainter L dwarfs also selected on the basis of color [14].…”

“…This can be supplemented by astrometry, providing proper motions, trigonometric parallaxes, and hence tangential velocities. In the case of the late-M dwarfs, the sample is bright enough that the Palomar plates plus 2MASS allow measurements of the proper motions, which Gizis et al [10] combined with the photometric distance estimate to obtain v tan . The Kirkpatrick et al [13,14] L dwarfs are faint enough that this is not possible, but United States Naval Observatory has measured proper motions and trigonometric parallaxes for a representative sample using CCD astrometry.…”

“…The new result by Gizis et al [10] is to add the cooler (M7-L8) dwarfs, allowing dwarfs at and below the hydrogen-burning limit to be examined. This reveals (Fig.…”

“…Kinematics are also linked to age: Stars are born from molecular clouds with low random space velocities, but with time encounters other stars and clouds in the Galactic disk 'heat' the stellar velocity distribution. Most of the discussion in this paper is drawn from Gizis et al [10].…”