67 Additional L Dwarfs Discovered by the Two Micron All Sky Survey

Abstract

We present JHK s photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the latest L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Analysis of these new discoveries shows that Hα emission has yet to be convincingly detected in any L dwarf later than type L4.5, indicating a decline or absence of chromospheric activity in the latest L dwarfs. Further analysis shows that 16 (and possibly 4 more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until type ∼L6.5 V then declines for later types. This disappearance may be the first sign of depletion of atomic lithium as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements, we develop spectrophotometric relations to estimate distances to the other L dwarfs. Of the 67 new discoveries, 21 have photometric distances placing them within 25 parsecs of the Sun. A table of all known L and T dwarfs believed to lie within 25 parsecs -53 in total -is also presented. Using the distance measurement -2of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition objects have yet been uncovered. This evidence, combined with model fits to low-resolution spectra of late-M and early-L dwarfs, indicates that L-class objects span the range 1300K ∼ < T ef f ∼ < 2000K. The near-infrared color-color diagram shows that L dwarfs fall along a natural, redder extension of the well known M dwarf track. These near-infrared colors get progressively redder for later spectral types, with the L dwarf sequence abruptly ending near (J − H, H − K s , J − K s ) ≈ (1.3, 0.8, 2.1).

A deep, wide-field search for substellar members in NGC 2264

“…Of course, some fraction could be foreground field stars; the dash-dot line in Fig. 7 represents such a locus of observed dwarf colours given by Leggett (1992) and Kirkpatrick et al (2000). We do not yet have data to distinguish the cluster and field objects by, for example, gravity-sensitive near-infrared spectroscopy (Lucas et al 2001).…”

“…2 and 3. We have used observed loci for late-type dwarfs from compilations given by Leggett (1992), Leggett et al (1998), Bessell (1991) and Kirkpatrick et al (2000) to compare the location of our sample in the I − J, J − K s and J − H, H − K s colour-colour diagrams. In order to verify our methods we first checked our sample for obvious K s -band excesses, which would strongly affect estimates of the extinction which used the K s -magnitude.…”

Activity and Kinematics of M and L 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]. Observationally, a sample of nearby bright M and L dwarfs offers considerable advantages.…”

“…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. Together, near-infrared photometry, spectroscopy and astrometry provide a rich set of diagnostics.…”

“…1). Kirkpatrick et al's [13,14] data include both lithium and Hα measurements. Consider the L1-L4.5 dwarfs, where lithium is detectable even at low resolution.…”