Thomas R. Geballe

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity

We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs, and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices from Geballe et al. that measure the strengths of the near infrared water and methane bands. We identify nine new L8?9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954-0935066 as T9, the latest and coolest dwarf found to date. We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L?T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the Geballe et al. spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions, and thicknesses of the clouds. We have identified a small group of late-L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H2O, and K I absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late-T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H2 opacity. The correlation of H-K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages greater than108 yr and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15?75MJupiter. One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ~ 4.2?4.5 and mass ~ 10?15MJupiter.

Surface ices and the atmospheric composition of Pluto

Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.

Ices on the Surface of Triton

The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Tritons surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.

Detection of absorption by H2 in molecular clouds: A direct measurement of the H2:CO ratio

Vibrational absorption by H2 and CO has been searched for toward infrared sources embedded in molecular clouds. H2 was detected toward NGC 2024 IRS 2 and possibly toward NGC 2264 (GL 989). CO was detected toward both sources. The results are consistent with the H2 ortho:para ratio being equilibrated at the cloud temperature. Toward NGC 2024, H2:CO = (3700(sub -2600)(sup +3100)) (2 sigma limits), and toward NGC 2264, H2:CO less than 6000. Approximately one-third of all carbon is in gas-phase CO.

H3+ in Diffuse Interstellar Clouds: A Tracer for the Cosmic-Ray Ionization Rate

Author Institution: Department of Astronomy and Department of Chemistry, University of Illinois at; Urbana-Champaign, Urbana, IL 61801; Gemini Observatory, Hilo, HI 96720; Department of Astronomy \& Astrophysics and Department of Chemistry, University of; Chicago, Chicago, IL 60637

HOT AND DIFFUSE CLOUDS NEAR THE GALACTIC CENTER PROBED BY METASTABLE H~3^+

Using an absorption line from the metastable (J ; K) ¼ (3; 3) level of H þ together with other lines of H þ and CO observed along several sight lines, we have discovered a vast amount of high-temperature (T � 250 K) and lowdensity (n � 100 cm � 3 ) gas with a large velocity dispersion in the central molecular zone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately three-fourths of the H þ along the line of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the remaining H þ located in intervening spiral arms. About half of the H þ in the CMZ has velocities near �� 100 km s � 1 , indicating that it is associated with the 180 pc radius expanding molecular ring, which approximately forms the outer boundary of the CMZ. The other half,withvelocitiesof �� 50and � 0kms � 1 ,isprobablyclosertothecenter.COisnotveryabundantintheseclouds. Hot and diffuse gas in which the (3, 3) level is populated was not detected toward several dense clouds and diffuse clouds intheGalacticdiskwhere largecolumndensities ofcolder H þ have beenreportedpreviously.Thus,thenewly discovered environment appears to be unique to the CMZ. The large observed H þ column densities in the CMZ suggestanionizationratemuchhigher thaninthediffuseinterstellarmediumintheGalacticdisk.Ourfindingthatthe H þ in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor (f � 0:1) for n � 10 4 cm � 3 clouds in the CMZ is an overestimate by at least an order of magnitude. Subject headings: astrochemistry — Galaxy: center — ISM: clouds — ISM: molecules — molecular processes — radiation mechanisms: nonthermal

The Pistol Star

Results of an spectroscopic investigation of the Pistol star are presented. The near-infrared spectra and photometry data are fit with stellar wind models to find that the star is extraordinarily luminous, L = 106.7±0.5 L⊙, making it one of the most luminous stars known. Coupled with the relatively cool temperature, Teff = 10\(^{4.17_{ - 0.06}^{ + 0.19} } \) K, the star is clearly in violation of the Humphreys-Davidson limit. The derived line of sight velocity of the star assures its membership in the Quintuplet cluster. This, along with the inferred extinction, places the star at the Galactic Center.

The missing link: Early methane ("T") dwarfs in the sloan digital Sky Survey

We report the discovery of three cool brown dwarfs that fall in the effective temperature gap between the latest L dwarfs currently known, with no methane absorption bands in the 1-2.5 µm range, and the previously known methane (T) dwarfs, whose spectra are dominated by methane and water. The newly discovered objects were detected as very red objects in the Sloan Digital Sky Survey imaging data and have JHK colors between the red L dwarfs and the blue Gl 229B-like T dwarfs. They show both CO and CH(4) absorption in their near-infrared spectra in addition to H(2)O, with weaker CH(4) absorption features in the H and K bands than those in all other methane dwarfs reported to date. Due to the presence of CH(4) in these bands, we propose that these objects are early T dwarfs. The three form part of the brown dwarf spectral sequence and fill in the large gap in the overall spectral sequence from the hottest main-sequence stars to the coolest methane dwarfs currently known.

Molecular Abundances in the Atmosphere of the T Dwarf Gl 229B

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