A Spitzer Search For Planetary-Mass Brown Dwarfs With Circumstellar Disks: Candidate Selection

We report on initial results from a Spitzer program to search for very low-mass brown dwarfs in Ophiuchus. This program is an extension of an earlier study by Allers et al. which had resulted in an extraordinary success rate, 18 confirmed out of 19 candidates. Their program combined near-infrared and Spitzer photom- etry to identify objects with very cool photospheres together with circumstellar disk emission to indicate youth. Our new program has obtained deep IRAC pho- tometry of a 0.5 deg2 field that was part of the original Allers et al. study. We report 18 new candidates whose luminosities extend down to 10-4 L\cdot which sug- gests masses down to ~ 2 MJ if confirmed. We describe our selection techniques, likely contamination issues, and follow-on photometry and spectroscopy that are in progress

Testing Young Brown Dwarf Atmospheric Properties with L-Band Spectroscopy

One of the greatest challenges in the study of directly-imaged imaged exoplanets has been the determination of their atmospheric properties from observed spectroscopy and photometry. In particular, the extremely red near-IR colors and lack of methane for exoplanets have been difficult to model. There are several explanations for these properties of directly-imaged exoplanets, such as thermo-chemical instability, disequilibrium chemistry, and dust clouds. The difficulty in observing exoplanet atmospheres, however, have limited the wavelength coverage and detail with which these theories have been tested. With the intent to test these theories, we use young brown dwarf spectra as proxies for these exoplanet spectra. The objects we have chosen are of spectral types, masses, and ages that overlap with the red directly-imaged exoplanets. Specifically we are looking at the L-band where the 3.3 micron methane feature provides a sensitive probe for disequilibrium chemistry and the spectral slope probes for dust-grain size. We detect methane at spectral type L4 and later, which matches the onset of methane in field brown dwarfs. This work showcases the diagnostic power of L-band spectroscopy and the potential for observations with future facilities (such as JWST) to aid in our understanding of planetary atmospheres

A Stellar Census of the Tucana-Horologium Moving Group

We report the selection and spectroscopic confirmation of 129 new late-type (K3-M6) members of the Tuc-Hor moving group, a nearby (~40 pc), young (~40 Myr) population of comoving stars. We also report observations for 13/17 known Tuc-Hor members in this spectral type range, and that 62 additional candidates are likely to be unassociated field stars; the confirmation frequency for new candidates is therefore 129/191 = 67%. We have used RVs, Halpha emission, and Li6708 absorption to distinguish contaminants and bona fide members. Our expanded census of Tuc-Hor increases the known population by a factor of ~3 in total and by a factor of ~8 for members with SpT>K3, but even so, the K-M dwarf population of Tuc-Hor is still markedly incomplete. The spatial distribution of members appears to trace a 2D sheet, with a broad distribution in X and Y, but a very narrow distribution (+/-5 pc) in Z. The corresponding velocity distribution is very small, with a scatter of +/-1.1 km/s about the mean UVW velocity. We also show that the isochronal age (20--30 Myr) and the lithium depletion age (40 Myr) disagree, following a trend seen in other PMS populations. The Halpha emission follows a trend of increasing EW with later SpT, as seen for young clusters. We find that members have been depleted of lithium for spectral types of K7.0-M4.5. Finally, our purely kinematic and color-magnitude selection procedure allows us to test the efficiency and completeness for activity-based selection of young stars. We find that 60% of K-M dwarfs in Tuc-Hor do not have ROSAT counterparts and would be omitted in Xray selected samples. GALEX UV-selected samples using a previously suggested criterion for youth achieve completeness of 77% and purity of 78%. We suggest new selection criteria that yield >95% completeness for ~40 Myr populations.(Abridged)Comment: Accepted to AJ; 28 pages, 12 figures, 5 tables in emulateapj forma

R=100,000 Spectroscopy of Photodissociation Regions: H2 Rotational Lines in the Orion Bar

Ground state rotational lines of H2 are good temperature probes of moderately hot (200-1000 K) gas. The low A-values of these lines result in low critical densities while ensuring that the lines are optically thin. ISO observations of H2 rotational lines in PDRs reveal large quantities of warm gas that are difficult to explain via current models, but the spatial resolution of ISO does not resolve the temperature structure of the warm gas. We present and discuss high spatial resolution observations of H2 rotational line emission from the Orion Bar.Comment: 4 pages, 1 figure, Proceedings of the ESO Workshop on High Resolution Infrared Spectroscop

The First Spectrum of the Coldest Brown Dwarf

The recently discovered brown dwarf WISE 0855 presents our first opportunity to directly study an object outside the Solar System that is nearly as cold as our own gas giant planets. However the traditional methodology for characterizing brown dwarfs---near infrared spectroscopy---is not currently feasible as WISE 0855 is too cold and faint. To characterize this frozen extrasolar world we obtained a 4.5-5.2 $\mu$m spectrum, the same bandpass long used to study Jupiter's deep thermal emission. Our spectrum reveals the presence of atmospheric water vapor and clouds, with an absorption profile that is strikingly similar to Jupiter. The spectrum is high enough quality to allow the investigation of dynamical and chemical processes that have long been studied in Jupiter's atmosphere, but now on an extrasolar world.Comment: submitted to ApJ