James Liebert

The Two Micron All Sky Survey (2MASS)

Between 1997 June and 2001 February the Two Micron All Sky Survey (2MASS) collected 25.4 Tbytes of raw imagingdatacovering99.998%ofthecelestialsphereinthenear-infraredJ(1.25 � m),H(1.65 � m),andKs(2.16 � m) bandpasses. Observations were conducted from two dedicated 1.3 m diameter telescopes located at Mount Hopkins, Arizona,andCerroTololo,Chile.The7.8sofintegrationtimeaccumulatedforeachpointontheskyandstrictquality control yielded a 10 � point-source detection level of better than 15.8, 15.1, and 14.3 mag at the J, H, and Ks bands, respectively, for virtually the entire sky. Bright source extractions have 1 � photometric uncertainty of <0.03 mag and astrometric accuracy of order 100 mas. Calibration offsets between any two points in the sky are <0.02 mag. The 2MASS All-Sky Data Release includes 4.1 million compressed FITS images covering the entire sky, 471 million source extractions in a Point Source Catalog, and 1.6 million objects identified as extended in an Extended Source Catalog.

L Dwarfs and the Substellar Mass Function

Analysis of initial observations sky surveys has shown that the resulting photometric catalogs, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp 1999) indicates that these L dwarfs have effective temperatures between ≈2000 ± 100 K and 1500 ± 100 K, while the available trigonometric parallax data place their luminosities at between 10-3.5 and 10. Those properties, together with the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are available for most sources, but we can incorporate VLM/brown dwarf models in simulations of the solar neighborhood population and constrain Ψ(M) by comparing the predicted L dwarf surface densities and temperature distributions against observations from the Deep Near-Infrared Survey (DENIS) and 2 Micron All-Sky Survey (2MASS) surveys. The data, although sparse, can be represented by a power-law mass function, Ψ(M) ∝ M-α, with 1 M/M☉ > 0.01 brown dwarfs is 0.10 systems pc-3. In that case, brown dwarfs are twice as common as main-sequence stars but contribute no more than ~15% of the total mass of the disk.

Four Nearby L Dwarfs

We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000 A in the L0 dwarf 2MASS J0746+2000 and the L4 dwarf 2MASS J0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I λ4227 and Na I λλ5890/5896 are extremely strong, with the latter having an equivalent width of 240 A in the L4 dwarf. By spectral type L5, the D lines extend over ~1000 A and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband color. The K I resonance doublet at 7665/7699 A increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.

A 2MASS Ultracool M Dwarf Observed in a Spectacular Flare

2MASSW J0149090+295613 (hereafter 2M0149) is unique among the very late M/L dwarf counterparts to Two Micron All Sky Survey (2MASS) point sources in having shown, in the first set of spectra taken on 1997 December 7, a diverse emission-line spectrum at red wavelengths, which features an Hα equivalent width of 300 A. On four repeat observations, however, the spectrum was that of a more ordinary dMe object, albeit with a late spectral type of M9.5 V. Our interpretation is that the object underwent an extreme magnetic flare event. The continuum flux at the short-wavelength end of the spectrum was also several times stronger during the apparent flare than in quiescence. Comparison is made with midtype M dwarfs of higher mass that have undergone strong flares, and with another known M9.5 emission-line object PC 0025+0047. The 2MASS object adds to the evidence that magnetic dynamo eruptions may release more energy relative to Lbol as the stellar mass decreases; if the total flare luminosity scales with Hα in the same way as a well-observed event of the middle M dwarf AD Leo, 2M0149 may have had a flare luminosity that approached or exceeded its quiescent Lbol during the brief impulse phase. In contrast, its quiescent state shows an unremarkable level of chromospheric activity, which leads us to question whether it is unique at all; rather, it may be that many of the ultracool M and L dwarfs found by the Deep Near-Infrared Survey (DENIS) and 2MASS undergo similar flares.