Michael W. McElwain

Characterization of Dusty Debris Disks: the IRAS and Hipparcos Catalogs

Dusty debris disks around main-sequence stars are signposts for the existence of planetesimals and exoplanets. From cross-correlating Hipparcos stars with the IRAS catalogs, we identify 146 stars within 120 pc of Earth that show excess emission at 60 μm. This search took special precautions to avoid false positives. Our sample is reasonably well distributed from late B to early K-type stars, but it contains very few later type stars. Even though IRAS flew more than 20 years ago and many astronomers have cross-correlated its catalogs with stellar catalogs, we were still able to newly identify debris disks at as many as 33 main-sequence stars; of these, 32 are within 100 pc of Earth. The power of an all-sky survey satellite like IRAS is evident when comparing our 33 new debris disks with the total of only 22 dusty debris disk stars first detected with the more sensitive, but pointed, satellite ISO. Our investigation focuses on the mass, dimensions, and evolution of dusty debris disks.

Integral Field Spectroscopy of a Candidate Disk Galaxy at z ~ 1.5 Using Laser Guide Star Adaptive Optics

We present 0.1 resolution near-infrared integral field spectroscopy of H? in a z = 1.4781 star-forming galaxy, Q2343-BM133. These observations were obtained with the OH Suppressing Infra-Red Imaging Spectrograph (OSIRIS) using the W. M. Keck Observatory laser guide star adaptive optics (LGS AO) system. H? emission is resolved over a 0.8(6.8 kpc) ? 0.5(4.3 kpc) region with a 0.1 spatial resolution. We find a global flux of 4.2 ? 0.6 ? 10-16 ergs s-1 cm-2, and detect a spatially resolved velocity gradient of ~134 km s-1 across the galaxy and a global velocity dispersion of 73 ? 9 km s-1. An upper limit of [N ]/H? 0.12 is inferred, which implies that this galaxy is not dominated by an active galactic nucleus and has a metallicity at or below 1/2 solar metallicity. We derive a star formation rate (SFR) of 47 ? 6 M? yr-1 and a dereddened SFR of 66 ? 9 M? yr-1. Two-dimensional kinematics for Q2343-BM133 fit well with an inclined disk model, with which we estimate an enclosed mass of 4.3 ? 109 M? within 5.5 kpc. A possible merger scenario is also presented, and cannot be fully ruled out. We derive a virial mass of 1.1 ? 1010 M? for a disk geometry, using the observed velocity dispersion. We propose that Q2343-BM133 is currently at an early stage of disk formation at a look-back time of 9.3 Gyr.

The 2MASS Wide-Field T Dwarf Search. IV. Hunting Out T Dwarfs with Methane Imaging

We present first results from a major program of methane filter photometry for low-mass stars and brown dwarfs. The definition of a new methane filter photometric system is described. A recipe is provided for the differential calibration of methane imaging data using existing Two Micron All Sky Survey (2MASS) photometry. We show that these filters are effective in discriminating T dwarfs from other types of stars, and we demonstrate this with Anglo-Australian Telescope observations using the IRIS2 imager. Methane imaging data and proper motions are presented for 10 T dwarfs identified as part of the 2MASS Wide-Field T Dwarf Search, seven of them initially identified as T dwarfs using methane imaging. We also present near-infrared moderate-resolution spectra for five T dwarfs newly discovered by this technique. Spectral types obtained from these spectra are compared to those derived from both our methane filter observations and spectral types derived by other observers. Finally, we suggest a range of future programs to which these filters are clearly well suited: the winnowing of T dwarf and Y dwarf candidate objects coming from the next generation of near-infrared sky surveys, the robust detection of candidate planetary-mass brown dwarfs in clusters, the detection of T dwarf companions to known L and T dwarfs via deep methane imaging, and the search for rotationally modulated time-variable surface features on cool brown dwarfs.

OSIRIS: a diffraction limited integral field spectrograph for Keck

We present an overview of the OSIRIS integral field spectrograph which was recently commissioned on the Keck II Telescope. OSIRIS works with the Keck Adaptive Optics system and utilizes an infrared transmissive lenslet array to sample a rectangular field of view at close to the Keck diffraction limit. By packing the spectra close together (2 pixel rows per spectrum) and using the Rockwell Hawaii-2 detector (wavelengths between 1 and 2.5 microns), we achieve a relatively large field of view (up to 6.4) while maintaining full broad-band spectral coverage at a resolution of 3800. Among the challenges of the instrument are: a fully cryogenic design (approximately 250 kg are brought down to 55K); four spatial scales from 0.02 to 0.10; extremely low wavefront error (approximately 25 nm of non-common path error); large all aluminum optics for the spectrograph; extremely repeatable spectral formats; and a sophisticated data reduction pipeline. OSIRIS also serves as a starting point for our design of IRIS which is a planned integral field spectrograph for the Thirty Meter Telescope.

RESOLVED SPECTROSCOPY OF M DWARF/L DWARF BINARIES. I. DENIS J220002.05-303832.9AB

We present the discovery of the common proper motion M9+L0 binary DENIS J220002.05-303832.9AB, identified serendipitously with the SpeX near-infrared imager/spectrograph. Spectral types are derived from resolved near-infrared spectroscopy of the well-separated (109 ± 006) components and comparison to equivalent data for M and L dwarf spectral standards. Physical association is deduced from the angular proximity of the sources, their common proper motion, and their similar spectrophotometric distances (35 ± 2 pc). The estimated distance of this pair implies a projected separation of 38 ± 3 AU, wider than typical separations for other M dwarf/L dwarf binaries but consistent with the maximum separation/total system mass trend previously identified by Burgasser et al. We discuss the DENIS 2200-3038AB system in context with other low-mass binaries and its role in the study of dust formation processes and activity trends across the transition between the M and L dwarf spectral classes.

First High-Contrast Science with an Integral Field Spectrograph: The Substellar Companion to GQ Lupi

We present commissioning data from the OSIRIS integral field spectrograph (IFS) on the Keck II 10 m telescope that demonstrate the utility of adaptive optics IFS spectroscopy in studying faint close-in substellar companions in the halos of bright stars. Our R ≈ 2000 J- and H-band spectra of the substellar companion to the 1-10 Myr old GQ Lup complement existing K-band spectra and photometry and improve on the original estimate of its spectral type. We find that GQ Lup B is somewhat hotter (M6-L0) than reported in the discovery paper by Neuhauser and collaborators (M9-L4), mainly due to the surface gravity sensitivity of the K-band spectral classification indices used by the discoverers. Spectroscopic features characteristic of low surface gravity objects, such as lack of alkali absorption and a triangular H-band continuum, are indeed prominent in our spectrum of GQ Lup B. The peculiar shape of the H-band continuum and the difference between the two spectral type estimates is well explained in the context of the diminishing strength of H2 collision-induced absorption with decreasing surface gravity, as recently proposed for young ultracool dwarfs by Kirkpatrick and collaborators. Using our updated spectroscopic classification of GQ Lup B and a reevaluation of the age and heliocentric distance of the primary, we perform a comparative analysis of the available substellar evolutionary models to estimate the mass of the companion. We find that the mass of GQ Lup B is 0.010-0.040 M☉. Hence, it is unlikely to be a wide-orbit counterpart to the known radial velocity extrasolar planets, whose masses are 0.015 M☉. Instead, GQ Lup A/B is probably a member of a growing family of very low mass ratio widely separated binaries discovered through high-contrast imaging.

The 2Mass Wide-Field T Dwarf Search. I. Discovery of a Bright T Dwarf within 10 Parsecs of the Sun

We present the discovery of a bright (J = 13.94 ± 0.03) T dwarf, 2MASS 1503+2525, identified in a new, wide-field search for T dwarfs using the recently completed Two Micron All Sky Survey (2MASS). The 1–2.5 μm spectrum of this object exhibits the strong H2O and CH4 bands characteristic of mid- and late-type T dwarfs, and we derive a spectral type of T5.5 using both the Burgasser et al. and the Geballe et al. classification schemes. Based on its spectral type and the absolute magnitudes of known T dwarfs, we estimate the distance of this object as 8 ± 3 pc if it is single, likely within 10 pc of the Sun. Our new 2MASS search, which covers 74% of the sky and greatly expands on earlier color constraints, should identify 15–25 new T dwarfs with J ≤ 16. Combined with the 20 known members of this class that already fall within our search criteria, our new sample will provide improved statistics for such key quantities as the binary fraction and the field substellar mass function. Furthermore, multiple detections from overlapping 2MASS scans provide multiple-epoch astrometry and photometry, and we present proper motions for five T dwarfs in our sample.

A New Brown Dwarf Desert? A Scarcity of Wide Ultracool Binaries

We present the results of a deep-imaging search for wide companions to low-mass stars and brown dwarfs using NSFCam on the Infrared Telescope Facility. We searched a sample of 132 M7-L8 dwarfs to magnitude limits of J ~ 20.5 and K ~ 18.5, corresponding to secondary-to-primary mass ratios of ~0.5. No companions were found with separations between 2 and 31 (~40 to ~1000 AU). This null result implies a wide companion frequency below 2.3% at the 95% confidence level within the sensitivity limits of the survey. Preliminary modeling efforts indicate that we could have detected 85% of companions more massive than 0.05 M⊙ and 50% above 0.03 M⊙.

THE 2MASS WIDE-FIELD T DWARF SEARCH. II. DISCOVERY OF THREE T DWARFS IN THE SOUTHERN HEMISPHERE

We present the discovery of three new southern hemisphere T dwarfs identified in the Two Micron All Sky Survey. These objects, 2MASS 0348-6022, 2MASS 0516-0445, and 2MASS 2228-4310, have classifications T7, T5.5, and T6.5, respectively. Using linear absolute magnitude/spectral type relations derived from T dwarfs with measured parallaxes, we estimate spectrophotometric distances for these discoveries; the closest, 2MASS 0348-6022, is likely within 10 pc of the Sun. Proper motions and estimated tangential velocities are consistent with membership in the Galactic disk population. We also list southern hemisphere T dwarf candidates that were either not found in subsequent near-infrared imaging observations and are most likely uncataloged minor planets, or have near-infrared spectra consistent with background stars.

Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J17072343−0558249AB

We present Infrared Telescope Facility SpeX observations of the M/L binary system 2MASS J17072343-0558249. SpeX imaging resolves the system into a 101 ± 017 visual binary in which both components have red near-infrared colors. Resolved low-resolution (R ~ 150) 0.8-2.5 μm spectroscopy reveals strong H2O, CO, and FeH bands and alkali lines in the spectra of both components, characteristic of late-type M and L dwarfs. A comparison to a sample of late-type field dwarf spectra indicates spectral types M9 and L3. Despite the small proper motion of the system (0100 ± 0009 yr-1), imaging observations over 2.5 yr provide strong evidence that the two components share common proper motion. Physical association is also likely due to the small spatial volume occupied by the two components (based on spectrophotometric distance estimates of 15 ± 1 pc) as compared to the relatively low spatial density of low-mass field stars. The projected separation of the system is 15 ± 3 AU, similar to other late-type M and L binaries. Assuming a system age of 0.5-5 Gyr, we estimate the masses of the binary components to be 0.072-0.083 and 0.064-0.077 M⊙, with an orbital period of roughly 150-300 yr. While this is nominally too long a baseline for astrometric mass measurements, the proximity and relatively wide angular separation of the 2MASS J1707-0558AB pair make it an ideal system for studying the M dwarf/L dwarf transition at a fixed age and metallicity.