arXiv subjects

J. Davy Kirkpatrick

Publications and source records attributed to J. Davy Kirkpatrick.

At least 163 records · Page 9Linked to original sources

arXiv2007

A Cross-Match of 2MASS and SDSS: Newly-Found L and T Dwarfs and an Estimate of the Space Densitfy of T Dwarfs

We report new L and T dwarfs found in a cross-match of the SDSS Data Release 1 and 2MASS. Our simultaneous search of the two databases effectively allows us to relax the criteria for object detection in either survey and to explore the combined databases to a greater completeness level. We find two new T dwarfs in addition to the 13 already known in the SDSS DR1 footprint. We also identify 22 new candidate and bona-fide L dwarfs, including a new young L2 dwarf and a peculiar L2 dwarf with unusually blue near-IR colors: potentially the result of mildly sub-solar metallicity. These discoveries underscore the utility of simultaneous database cross-correlation in searching for rare objects. Our cross-match completes the census of T dwarfs within the joint SDSS and 2MASS flux limits to the 97% level. Hence, we are able to accurately infer the space density of T dwarfs. We employ Monte Carlo tools to simulate the observed population of SDSS DR1 T dwarfs with 2MASS counterparts and find that the space density of T0-T8 dwarf systems is 0.0070 (-0.0030; +0.0032) per cubic parsec (95% confidence interval), i.e., about one per 140 cubic parsecs. Compared to predictions for the T dwarf space density that depend on various assumptions for the sub-stellar mass function, this result is most consistent with models that assume a flat sub-stellar mass function dN/dM ~ M^0. No >T8 dwarfs were discovered in the present cross-match, though less than one was expected in the limited area (2099 sq. degrees) of SDSS DR1.

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arXiv2007

Clouds, Gravity and Metallicity in Blue L dwarfs: The Case of 2MASS J11263991-5003550

Optical and near-infrared spectroscopy of the newly discovered peculiar L dwarf 2MASS J11263991-5003550 are presented. Folkes et al. identified this source as a high proper motion L9+/-1 dwarf based on its strong H2O absorption at 1.4 micron. We find that the optical spectrum of 2MASS J1126-5003 is in fact consistent with that of a normal L4.5 dwarf with notably enhanced FeH absorption at 9896 A. However, its near-infrared spectrum is unusually blue, with strong H2O and weak CO bands similar in character to several recently identified ``blue L dwarfs''. Using 2MASS J1126-5003 as a case study, and guided by trends in the condensate cloud models of Burrows et al. and Marley et al., we find that the observed spectral peculiarities of these sources can be adequately explained by the presence of thin and/or large-grained condensate clouds as compared to normal field L dwarfs. Atypical surface gravities or metallicities alone cannot reproduce the observed peculiarities, although they may be partly responsible for the unusual condensate properties. We also rule out unresolved multiplicity as a cause for the spectral peculiarities of 2MASS J1126-5003. Our analysis is supported by examination of Spitzer mid-infrared spectral data from Cushing et al. which show that bluer L dwarfs tend to have weaker 10 micron absorption, a feature tentatively associated with silicate oxide grains. With their unique spectral properties, blue L dwarfs like 2MASS J1126-5003 should prove useful in studying the formation and properties of condensates and condensate clouds in low temperature atmospheres.

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arXiv2007

Near-Infrared Variability in the 2MASS Calibration Fields: A Search for Planetary Transit Candidates

The 2MASS photometric calibration observations cover ~6 square degrees on the sky in 35 "calibration fields" each sampled in nominal photometric conditions between 562 and 3692 times during the four years of the 2MASS mission. We compile a catalog of variables from the calibration observations to search for M dwarfs transited by extra-solar planets. We present our methods for measuring periodic and non-periodic flux variability. From 7554 sources with apparent Ks magnitudes between 5.6 and 16.1, we identify 247 variables, including extragalactic variables and 23 periodic variables. We have discovered three M dwarf eclipsing systems, including two candidates for transiting extrasolar planets.

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arXiv2007

Discovery of 11 New T Dwarfs in the Two Micron All-Sky Survey, Including a Possible L/T Transition Binary

We present the discovery of 11 new T dwarfs, found during the course of a photometric survey for mid-to-late T dwarfs in the 2MASS Point Source Catalog and from a proper motion selected sample of ultracool dwarfs in the 2MASS Working Database. Using the NASA Infrared Telescope Facility SpeX spectrograph, we obtained low-resolution (R~150) spectroscopy, allowing us to derive near-infrared spectral types of T2-T8. One of these new T dwarfs, 2MASS J13243559+6358284, was also discovered independently by Metchev et al., in prep. This object is spectroscopically peculiar and possibly a binary and/or very young (<300 Myr). We specifically attempted to model the spectrum of this source as a composite binary to reproduce its peculiar spectral characteristics. The latest-type object in our sample is a T8 dwarf, 2MASS J07290002-3954043, now one of the four latest-type T dwarfs known. All 11 T dwarfs are nearby given their spectrophotometric distance estimates, with 1 T dwarf within 10 pc and 8 additional T dwarfs within 25 pc, if single. These new additions increase the 25 pc census of T dwarfs by ~14%. Their proximity offers an excellent opportunity to probe for companions at closer separations than are possible for more distant T dwarfs.

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arXiv2007

Outstanding Issues in Our Understanding of L, T, and Y Dwarfs

Since the discovery of the first L dwarf 19 years ago and the discovery of the first T dwarf 7 years after that, we have amassed a large list of these objects, now numbering almost six hundred. Despite making headway in understanding the physical chemistry of their atmospheres, some important issues remain unexplained. Three of these are the subject of this paper: (1) What is the role of "second parameters" such as gravity and metallicity in shaping the emergent spectra of L and T dwarfs? Can we establish a robust classification scheme so that objects with unusual values of log(g) or [M/H], unusual dust content, or unresolved binarity are easily recognized? (2) Which physical processes drive the unusual behavior at the L/T transition? Which observations can be obtained to better confine the problem? (3) What will objects cooler than T8 look like? How will we know a Y dwarf when we first observe one?

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arXiv2007

A New Population of Young Brown Dwarfs

We report the discovery of a population of late-M and L field dwarfs with unusual optical and near-infrared spectral features that we attribute to low gravity -- likely uncommonly young, low-mass brown dwarfs. Many of these new-found young objects have southerly declinations and distance estimates within 60 parsecs. Intriguingly, these are the same properties of recently discovered, nearby, intermediate-age (5-50 Myr), loose associations such as Tucana/Horologium, the TW Hydrae association, and the Beta Pictoris moving group. We describe our efforts to confirm cluster membership and to further investigate this possible new young population of brown dwarfs.

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arXiv2007

Moderate Resolution Spitzer Infrared Spectrograph (IRS) Observations of M, L, and T Dwarfs

We present 10 - 19 um moderate resolution spectra of ten M dwarfs, one L dwarf, and two T dwarf systems obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The IRS allows us to examine molecular spectroscopic features/lines at moderate spectral resolution in a heretofore untapped wavelength regime. These R~600 spectra allow for a more detailed examination of clouds, non-equilibrium chemistry, as well as the molecular features of H2O, NH3, and other trace molecular species that are the hallmarks of these objects. A cloud-free model best fits our mid-infrared spectrum of the T1 dwarf epsilon Indi Ba, and we find that the NH3 feature in epsilon Indi Bb is best explained by a non-equilibrium abundance due to vertical transport in its atmosphere. We examined a set of objects (mostly M dwarfs) in multiple systems to look for evidence of emission features, which might indicate an atmospheric temperature inversion, as well as trace molecular species; however, we found no evidence of either.

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arXiv2006

The NIRSPEC Brown Dwarf Spectroscopic Survey II: High-Resolution J-Band Spectra of M, L and T Dwarfs

We present a sequence of high resolution (R~20,000 or 15 km/s) infrared spectra of stars and brown dwarfs spanning spectral types M2.5 to T6. Observations of 16 objects were obtained using eight echelle orders to cover part of the J-band from 1.165-1.323 micron with NIRSPEC on the Keck II telescope. By comparing opacity plots and line lists, over 200 weak features in the J-band are identified with either FeH or H2O transitions. Absorption by FeH attains maximum strength in the mid-L dwarfs, while H2O absorption becomes systematically stronger towards later spectral types. Narrow resolved features broaden markedly after the M to L transition. Our high resolution spectra also reveal that the disappearance of neutral Al lines at the boundary between M and L dwarfs is remarkably abrupt, presumably because of the formation of grains. Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed by condensation. The neutral potassium (K I) doublets that dominate the J-band have pressure broadened wings that continue to broaden from ~50 km/s (FWHM) at mid-M to ~500 km/s at mid-T. In contrast however, the measured pseudo-equivalent widths of these same lines reach a maximum in the mid-L dwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines without extensive pressure-broadened wings, indicative of a lower gravity atmosphere. Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH and H2O features, confirming its status as a rapid rotator. In contrast to other late T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absence of potassium even at high resolution, which may be a metallicity effect or a result of a cooler, higher-gravity atmosphere.

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arXiv2006

Discovery of a High Proper Motion L Dwarf Binary: 2MASS J15200224-4422419AB

We report the discovery of the wide L1.5+L4.5 binary 2MASS J15200224-4422419AB, identified during spectroscopic followup of high proper motion sources selected from the Two Micron All Sky Survey. This source was independently identified by Kendall et al. in the SuperCOSMOS Sky Survey. Resolved JHK photometry and low resolution near-infrared spectroscopy demonstrate that this system is composed of two well-separated (1"174+/-0"016) L dwarfs. Component classifications are derived using both spectral ratios and comparison to near-infrared spectra of previously classified field L dwarfs. Physical association for the pair is deduced from the large (mu = 0"73+/-0"03 /yr) common proper motion of the components and their similar spectrophotometric distances (19+/-2 pc). The projected separation of the binary, 22+/-2 AU, is consistent with maximum separation/total system mass trends for very low mass binaries. The 2MASS J1520-4422 system exhibits both large tangential (66+/-7 km/s) and radial velocities (-70+/-18 km/s), and its motion in the local standard of rest suggests that it is an old member of the Galactic disk population. This system joins a growing list of well-separated (>0"5), very low mass binaries, and is an excellent target for resolved optical spectroscopy to constrain its age as well as trace activity/rotation trends near the hydrogen-burning limit.

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arXiv2006

Optical Spectroscopy of 2MASS Color-Selected Ultracool Subdwarfs

We present Gemini GMOS and Magellan LDSS-3 optical spectroscopy for seven ultracool subdwarf candidates color-selected from the Two Micron All Sky Survey. Five are identified as late-type subdwarfs, including the previously reported sdM9.5 SSSPM 1013-1356 and L subdwarf 2MASS 1626+3925, and a new sdM8.5 2MASS 0142+0523. 2MASS 1640+1231 exhibits spectral features intermediate between a late-type M dwarf and subdwarf, similar to the previously identified high proper motion star SSSPM 1444-2019, and we classify both sources as mild subdwarfs, d/sdM9. 2MASS 1227-0447 is a new ultracool extreme subdwarf, spectral type esdM7.5. Spectral model fits yield metallicities that are consistent with these metallicity classifications. Effective temperatures track with numerical subtype within a metallicity class, although they are not equivalent across metallicity classes. As a first attempt to delineate subtypes in the L subdwarf regime we classify 2MASS 1626+3925 and the previously identified 2MASS 0532+8246 as sdL4 and sdL7, respectively, to reflect their similarity to equivalently classified, solar metallicity L-type field dwarfs over the 7300-9000 A region. We also detail preliminary criteria for distinguishing L subdwarf optical spectra as a roadmap for defining this new spectral class. The strong TiO bands and Ca I and Ti I lines in the spectrum of 2MASS 1626+3925 provide further evidence that condensate formation may be inhibited in metal-deficient L subdwarfs. We conclude with a compendium of currently known, optically classified ultracool subdwarfs.

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arXiv2006

Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field

We present a 20-pc, volume-limited sample of M7-L8 dwarfs created through spectroscopic follow-up of sources selected from the Two Micron All Sky Survey (2MASS) Second Incremental Release Point Source Catalog. In this paper, we present optical spectroscopy of 198 candidate nearby ultracool dwarfs, including 12 late-M and L dwarfs likely to be within 20 pc of the Sun and 94 more distant late-type dwarfs. We have also identified five ultracool dwarfs with spectral signatures of low-gravity. Combining these data with previous results, we define a sample of 99 ultracool dwarfs in 91 systems within 20 pc. These are used to estimate the J- and K-band luminosity functions for dwarfs with optical spectral types between M7 and L8 (10.5<M_J<15, 9.5<M_K<13). We find a space density of 4.9 x 10^-3 pc^-3 for late-M dwarfs (M7-M9.5) and a lower limit of 3.8 x 10^-3 pc^-3 for L dwarfs.

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arXiv2006

A Sensitive Search for Variability in Late L Dwarfs: The Quest for Weather

We have conducted a photometric monitoring program of 3 field late-L brown dwarfs looking for evidence of non-axisymmetric structure or temporal variability in their photospheres. The observations were performed using Spitzer/IRAC 4.5 and 8 micron bandpasses and were designed to cover at least one rotational period of each object. One-sigma RMS (root mean squared) uncertainties of less than 3 mmag at 4.5 micron and around 9 mmag at 8 micron were achieved. Two out of the three objects studied exhibit some modulation in their light curves at 4.5 micron - but not 8 micron - with periods of 7.4 hr and 4.6 hr and peak-to-peak amplitudes of 10 mmag and 8 mmag. Although the lack of detectable 8 micron variation suggests an instrumental origin for the detected variations, the data may nevertheless still be consistent with intrinsic variability since the shorter wavelength IRAC bandpasses probe more deeply into late L dwarf atmospheres than the longer wavelengths. A cloud feature occupying a small percentage (1-2 %) of the visible hemisphere could account for the observed amplitude of variation. If, instead, the variability is indeed instrumental in origin, then our non-variable L dwarfs could be either completely covered with clouds or objects whose clouds are smaller and uniformly distributed. Such scenarios would lead to very small photometric variations. Followup IRAC photometry at 3.6 and 5.8 micron bandpasses should distinguish between the two cases. In any event, the present observations provide the most sensitive search to date for structure in the photospheres of late-L dwarfs at mid-IR wavelengths, and our photometry provides stringent upper limits to the extent to which the photospheres of these transition L dwarfs are structured.

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arXiv2006

Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition

We present the results of a Hubble Space Telescope NICMOS imaging survey of 22 T-type field brown dwarfs. Five are resolved as binary systems with angular separations of 0"05-0"35, and companionship is established on the basis of component F110W-F170M colors (indicative of CH4 absorption) and low probabilities of background contamination. Prior ground-based observations show 2MASS 1553+1532AB to be a common proper motion binary. The properties of these systems - low multiplicity fraction (11[+7][-3]% resolved, as corrected for sample selection baises), close projected separations (a = 1.8-5.0 AU) and near-unity mass ratios - are consistent with previous results for field brown dwarf binaries. Three of the binaries have components that span the poorly-understood transition between L dwarfs and T dwarfs. Spectral decomposition analysis of one of these, SDSS 1021-0304AB, reveals a peculiar flux reversal between its components, as its T5 secondary is ~30% brighter at 1.05 and 1.27 micron than its T1 primary. This system, 2MASS 0518-2828AB and SDSS 1534+1615AB all demonstrate that the J-band brightening observed between late-type L to mid-type T dwarfs is an intrinsic feature of this spectral transition, albeit less pronounced than previously surmised. We also find that the resolved binary fraction of L7 to T3.5 dwarfs is twice that of other L and T dwarfs, an anomaly that can be explained by a relatively rapid evolution of brown dwarfs through the L/T transition, perhaps driven by dynamic (nonequilibrium) depletion of photospheric condensates.

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arXiv2006

Discovery of the Coolest Extreme Subdwarf

We report the discovery of LEHPM 2-59 as the coolest extreme M subdwarf (esdM) found to date. Optical and near infrared spectroscopy demonstrate that this source is of later spectral type than the esdM7 APMPM 0559-2903, with the presence of strong alkali lines (including Rb I), VO absorption at 7400 A and H2O absorption at 1.4 microns. Current optical classification schemes yield a spectral type of esdM8, making LEHPM 2-59 one of only two ultracool esdMs known. The substantial space velocity of this object (V_galactic ~ -180 km/s) identifies it as a halo star. Spectral model fits to the optical and near infrared spectral data for this and four other late-type esdMs indicate that LEHPM 2-59 is the coolest esdM currently known, with Teff = 2800-3000 K and -1.5 <~ [M/H] <~ -2.0. Comparison of Teff determinations for M dwarfs and esdMs based on spectral model fits from this study and the literature demonstrate a divergence in Teff scales beyond spectral types M5/esdM5, as large as 600-800 K by types M8/esdM8. While this divergence is likely an artifact of the underlying classification scheme, it may lead to systematic errors in the derived properties of intermediate metallicity subdwarfs. We comment on the future of ultracool subdwarf classification, and suggest several ideas for addressing shortcomings in current (largely extrapolated) schemes.

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arXiv2005

Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574

While following up L dwarf candidates selected photometrically from the Two Micron All Sky Survey, we uncovered an unusual object designated 2MASS J01415823-4633574. Its optical spectrum exhibits very strong bands of vanadium oxide but abnormally weak absorptions by titanium oxide, potassium, and sodium. Morphologically such spectroscopic characteristics fall intermediate between old, field early-L dwarfs (log(g)~5) and very late M giants (log(g)~0), leading us to favor low gravity as the explanation for the unique spectral signatures of this L dwarf. Such a low gravity can be explained only if this L dwarf is much lower in mass than a typical old field L dwarf of similar temperature and is still contracting to its final radius. These conditions imply a very young age. Further evidence of youth is found in the near-infrared spectrum, including a triangular-shaped H-band continuum reminiscent of young brown dwarf candidates discovered in the Orion Nebula Cluster. Using the above information along with comparisons to brown dwarf atmospheric and interior models, our current best estimate is that this L dwarf has an age of 1-50 Myr and a mass of 6-25 M_Jupiter. The location of 2MASS 0141-4633 on the sky coupled with a distance estimate of ~35 pc and the above age estimate suggests that this object may be a brown dwarf member of either the 30-Myr-old Tucana/Horologium Association or the ~12-Myr-old beta Pic Moving Group.

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arXiv2005

A Method for Determining the Physical Properties of the Coldest Known Brown Dwarfs

We present a method for measuring the physical parameters of the coldest T-type brown dwarfs using low resolution near infrared spectra. By comparing H_2O- and H_2-sensitive spectral ratios between empirical data and theoretical atmosphere models, and calibrating these ratios to measurements for the well-characterized 2-5 Gyr companion brown dwarf Gliese 570D, we derive estimates of the effective temperatures and surface gravities for 13 mid- and late-type field T dwarfs. We also deduce the first quantitative estimate of subsolar metallicity for the peculiar T dwarf 2MASS 0937+2931. Derived temperatures are consistent with prior estimates based on parallax and bolometric luminosity measurements, and examination of possible systematic effects indicate that the results are robust. Two recently discovered late-type T dwarfs, 2MASS 0939-2448 and 2MASS 1114-2618, both appear to be >50 K cooler than the latest-type T dwarf, 2MASS 0415-0935, and are potentially the coldest and least luminous brown dwarfs currently known. We find that, in general, higher surface gravity T dwarfs have lower effective temperatures and luminosities for a given spectral type, explaining previously observed scatter in the Teff/spectral type relation for these objects. Masses, radii and ages are estimated for the T dwarfs in our sample using the evolutionary models of Burrows et al.; we also determine masses and radii independently for eight T dwarfs with measured luminosities. These two determinations are largely consistent, lending support to the validity of evolutionary models at late ages. Our method is well suited to large samples of faint brown dwarfs, and can ultimately be used to directly measure the substellar mass function and formation history in the Galaxy.

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arXiv2005

SDSS J042348.57-041403.5AB: A Brown Dwarf Binary Straddling the L/T Transition

We present the discovery of SDSS J042348.57-041403.5 as a closely-separated (0"16) brown dwarf binary, resolved by the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer. Physical association is deduced from the angular proximity of the components and constraints on their common proper motion. SDSS0423-0414AB appears to be composed of two brown dwarfs with spectral types L6+/-1 and T2+/-1. Hence, this system straddles the transition between L dwarfs and T dwarfs, a unique evolutionary phase of brown dwarfs characterized by substantial shifts in spectral morphology over an apparently narrow effective temperature range. Binarity explains a number of unusual properties of SDSS 0423-0414, including its overluminosity and high effective temperature compared to other early-type T dwarfs, and possibly its conflicting spectral classifications (L7.5 in the optical, T0 in the near infrared). The relatively short estimated orbital period of this system (~15-20 yr) and the presence of Li I absorption in its combined light spectrum make it an ideal target for both resolved spectroscopy and dynamical mass measurements. SDSS 0423-0414AB joins a growing list of late-L/early-T dwarf binaries, the high percentage of which (~50%) may provide a natural explanation for observed peculiarities across the L/T transition.

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arXiv2005

A Unified Near Infrared Spectral Classification Scheme for T Dwarfs

A revised near infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser et al. and Geballe et al., and defined following the precepts of the MK Process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0 to T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion and accessibility from both northern and southern hemispheres. The classification of T dwarfs is formally made by the direct comparison of near infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H$_2$O and CH$_4$ bands in the 1-2.5 micron region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.

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