John E. Gizis

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.

Dwarfs Cooler than “M”: The Definition of Spectral Type “L” Using Discoveries from the 2-Micron All-Sky Survey (2MASS)*

Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 sq. deg. of actual 2MASS survey data, we have identified another twenty such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W.M. Keck Observatory.

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

We present JHKs 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 four 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 pc of the Sun. A table of all known L and T dwarfs believed to lie within 25 pc—53 in total — is also presented. Using the distance measurement of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350 K 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 1300 K Teff 2000 K. 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-Ks, J-Ks) ≈ (1.3, 0.8, 2.1).

The Spectra of T Dwarfs. I. Near-Infrared Data and Spectral Classification

We present near-infrared spectra for a sample of T dwarfs, including 11 new discoveries made using the 2 Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1-2.5 μm spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH_4 and H_2O bands and the shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing T_(eff) scale, based on the evolution of CH_4 and H_2O bands and the properties of L and T dwarfs with known distances. However, we speculate that this scale is not linear with spectral type for cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T_(eff) ~ 1300-1500 K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of CH_4 as a minor absorber, shaping the K-band spectra of the latest L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J - K_s = -0.89 ± 0.24) due to suppression of the 2.1 μm peak. The feature is likely caused by enhanced collision-induced H_2 absorption in a high-pressure or low-metallicity photosphere.

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.

A Search for L Dwarf Binary Systems

We present analysis of Hubble Space Telescope (HST) planetary camera images of 20 L dwarfs identified in the course of the Two Micron All Sky Survey. Four of the targets, 2MASSW J0746425+200032, 2MASSs J0850359+105716, 2MASSW J0920122+351742, and 2MASSW J1146345+223053, have faint red companions at separations between 007 and 029 (1.6?7.6 AU). Ground-based infrared imaging confirms the last as a common proper motion companion. The surface density of background sources with comparable colors is extremely low, and we identify all four as physical binaries. In three cases, the bolometric magnitudes of the components differ by less than 0.3 mag. Since the cooling rate for brown dwarfs is a strong function of mass, similarity in luminosities implies comparable masses. The faint component in the 2M0850 system, however, is over 1.3 mag fainter than the primary in the I band and ~0.8 mag fainter in Mbol. Indeed, 2M0850B is ~0.8 mag fainter in I than the lowest luminosity L dwarf currently known, while the absolute magnitude we deduce at J is almost identical with MJ for Gl 229B. We discuss the implications of these results for the temperature scale in the L/T transition region. 2M0850 is known to exhibit 6708 ? Li I absorption, indicating that the primary has a mass less than 0.06 M?. Theoretical models predict that the magnitude difference implies a mass ratio of ?0.75. The apparent binary fraction of the current sample, 20%, is comparable to the results of previous surveys of late-type M dwarfs in the field and in the Hyades. However, the mean separation of the L dwarf binaries in the current sample is smaller than the M dwarf value by a factor of 2, and only one system would be detected at the distance of the Hyades. We discuss the likely binary frequency among L dwarfs in light of these new data.

Discovery of a Brown Dwarf Companion to Gliese 570ABC: A 2MASS T Dwarf Significantly Cooler than Gliese 229B

We report the discovery of a widely separated (258&farcs;3+/-0&farcs;4) T dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was initially identified from the Two Micron All-Sky Survey. Its near-infrared spectrum shows the 1.6 and 2.2 µm CH4 absorption bands characteristic of T dwarfs, while its common proper motion with the Gl 570ABC system confirms companionship. Gl 570D (MJ=16.47+/-0.07) is nearly a full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and estimates of L=&parl0;2.8+/-0.3&parr0;x10-6 L middle dot in circle and Teff=750+/-50 K make it significantly cooler and less luminous than any other known brown dwarf companion. Using evolutionary models by Burrows et al. and an adopted age of 2-10 Gyr, we derive a mass estimate of 50+/-20 MJup for this object.

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.

Near-infrared spectral classification of late-m and l dwarfs

We present near-infrared (1–2.5 μm), low-resolution spectroscopy of 11 ultracool dwarfs, with spectral types ranging from M7 to L8. Combining our observations with data published by Leggett et al. in 2001, we have measured equivalent widths for the strongest atomic features and constructed narrowband indices to gauge the strength of the strongest molecular features. Those measurements show that the behavior at near-infrared wavelengths is well correlated with spectral type, where the latter is defined from observations between 6300 and 10000 A. In particular, four indices designed to measure absorption in the wings of the 1.4 and 1.85 μm steam bands exhibit a linear, monotonic correlation with spectral type on the Kirkpatrick et al. system (published in 1999), allowing consistent calibration from both optical and near-infrared spectroscopy.

Brown Dwarf Companions to G-Type Stars. I. Gliese 417B and Gliese 584C

We present astrometric and spectroscopic observations confirming that two nearby G dwarf systems (Gliese 417 = BD+36°2162 and Gliese 584AB = η CrB AB) have a widely separated, L dwarf, substellar companion. Using age estimates of the G dwarf primaries, we estimate masses for these L dwarfs from theoretical evolutionary tracks. For the L4.5 dwarf Gl 417B we estimate an age of 0.08–0.3 Gyr and a mass of 0.035 ± 0.015 M_⊙. For the L8 dwarf Gl 584C we estimate an age of 1.0–2.5 Gyr and a mass of 0.060 ± 0.015 M_⊙. This latter object also shows evidence of spectrum variability, which may be due to surface inhomogeneities rotating into and out of view. These new companions are also compared to six other L dwarf and T dwarf companions previously recognized. For the L dwarf companions, ages implied by the presence or absence of lithium are consistent with ages inferred from the primaries alone.