arXiv subjects

James Liebert

Publications and source records attributed to James Liebert.

At least 37 records · Page 2Linked to original sources

arXiv2006

Model Atmosphere Analysis of the Weakly Magnetic DZ White Dwarf G165-7

A reanalysis of the strongly metal-blanketed DZ white dwarf G165-7 is presented. An improved grid of model atmospheres and synthetic spectra is used to analyze BVRI, JHK, and ugriz photometric observations as well as a high quality Sloan Digital Sky Survey spectrum covering the energy distribution from 3600 to 9000 A. The detection of splitting in several lines of Ca, Na, and Fe, suggesting a magnetic field of Bs~650 kG, is confirmed by spectropolarimetric observations that reveal as much as +/- 7.5% circular polarization in many of the absorption lines, most notably Na, Mg, and Fe. Our combined photometric and spectroscopic fit yields Teff = 6440 K, log g=7.99, log H/He=-3.0 and log Ca/He=-8.1. The other heavy elements have solar ratios with respect to calcium, with the exception of Na and Cr that had to be reduced by a factor of two and three, respectively. A crude polarization model based upon the observed local spectral flux gradient yields a longitudinal field of 165 kG, consistent with the mean surface field inferred from the Zeeman splitting. The inclusion of this weak magnetic field in our synthetic spectrum calculations, even in an approximate fashion, is shown to improve our fit significantly.

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arXiv2006

Hot DB White Dwarfs from the Sloan Digital Sky Survey

We present ugriz photometry and optical spectroscopy for 28 DB and DO white dwarfs with temperatures between 28,000K and 45,000K. About 10 of these are particularly well-observed; the remainder are candidates. These are the hottest DB stars yet found, and they populate the "DB gap" between the hotter DO stars and the familiar DB stars cooler than 30,000K. Nevertheless, after carefully matching the survey volumes, we find that the ratio of DA stars to DB/DO stars is a factor of 2.5 larger at 30,000 K than at 20,000 K, suggesting that the "DB gap" is indeed deficient and that some kind of atmospheric transformation takes place in roughly 10% of DA stars as they cool from 30,000 K to 20,000 K.

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arXiv2006

A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4

We present a catalog of 9316 spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey Data Release 4. We have selected the stars through photometric cuts and spectroscopic modeling, backed up by a set of visual inspections. Roughly 6000 of the stars are new discoveries, roughly doubling the number of spectroscopically confirmed white dwarfs. We analyze the stars by performing temperature and surface gravity fits to grids of pure hydrogen and helium atmospheres. Among the rare outliers are a set of presumed helium-core DA white dwarfs with estimated masses below 0.3 Msun, including two candidates that may be the lowest masses yet found. We also present a list of 928 hot subdwarfs.

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arXiv2006

Spitzer/IRAC Photometry of M, L, and T Dwarfs

We present the results of a program to acquire photometry for eighty-six late-M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_5.8 versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one parameter family in Teff. Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of five (~70 M_Jup to ~15 M_Jup), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 micron fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 microns. This suggests that equilibrium CH_4/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of Ks, IRAC 3.6 micron, and 4.5 micron bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_Ks versus Ks-[4.5] relation, which shows a smooth progression over spectral type and splits the M, L, and T types cleanly.

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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

A Hot DQ White Dwarf in the Open Star Cluster M35

We report the discovery of a hot DQ white dwarf, NGC 2168:LAWDS 28, that is a likely member of the 150-Myr old cluster NGC 2168 (Messier 35). The spectrum of the white dwarf is dominated by CII features. The effective temperature is difficult to estimate but likely > 20,000 K based on the temperatures of hot DQs with similar spectra. NGC2168:LAWDS 28 provides further evidence that hot DQs may be the ``missing'' high-mass helium-atmosphere white dwarfs. Based on published studies, we find that the DBA WD LP 475-242 is likely a member of the Hyades open cluster, as often assumed. These two white dwarfs are the entire sample of known He-atmosphere white dwarfs in open clusters with turnoff masses >2 solar masses. Based on the number of known cluster DA white dwarfs and a redetermination of the H-atmosphere:He-atmosphere ratio, commonly known as the DA:DB ratio, we re-examine the hypothesis that the H- to He-atmosphere ratio in open clusters is the same as the ratio in the field. Under this hypothesis, we calculate that five He-atmosphere WDs are expected to have been discovered, with a probability of finding fewer than three He-atmosphere white dwarfs of 0.08, or at the ~ 2-sigma level.

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arXiv2005

The White Dwarf Luminosity Function from SDSS Imaging Data

A sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from SDSS plus USNO-B combined data. Numerous SDSS and followup spectra (Kilic et al. 2005) are used to quantify completeness and contamination of the sample; kinematic models are used to understand and correct for velocity-dependent selection biases. A luminosity function is constructed covering the range 7 < M_bol < 16, and its sensitivity to various assumptions and selection limits is discussed. The white dwarf luminosity function based on 6000 stars is remarkably smooth, and rises nearly monotonically to M_bol = 15.3. It then drops abruptly, although the small number of low-luminosity stars in the sample and their unknown atmospheric composition prevent quantitative conclusions about this decline. Stars are identified that may have high tangential velocities, and a preliminary luminosity function is constructed for them.

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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

The Age and Progenitor Mass of Sirius B

The Sirius AB binary system has masses that are well determined from many decades of astrometric measurements. Because of the well-measured radius and luminosity of Sirius A, we employed the TYCHO stellar evolution code to determine the age of the Sirius A,B binary system accurately, at 225-250 Myr. Note that this fit requires the assumption of solar abundance, and the use of the new Asplund et al. primordial solar metallicity. No fit to Sirius A's position is possible using the old Grevesse & Sauval scale. Because the Sirius B white dwarf parameters have also been determined accurately from space observations, the cooling age could be determined from recent calculations by Fontaine et al. or Wood to be 124 +/- 10 Myr. The difference of the two ages yields the nuclear lifetime and mass of the original primary star, 5.056 +0.374/-0.276 solar masses. This result yields in principle the most accurate data point at relatively high masses for the initial-final mass relation. However, the analysis relies on the assumption that the primordial abundance of the Sirius stars was solar, based on membership in the Sirius supercluster. A recent study suggests that its membership in the group is by no means certain.

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arXiv2005

Magnetic White Dwarfs from the SDSS II. The Second and Third Data Releases

Fifty-two magnetic white dwarfs have been identified in spectroscopic observations from the Sloan Digital Sky Survey (SDSS) obtained between mid-2002 and the end of 2004, including Data Releases 2 and 3. Though not as numerous nor as diverse as the discoveries from the first Data Release, the collection exhibits polar field strengths ranging from 1.5MG to ~1000MG, and includes two new unusual atomic DQA examples, a molecular DQ, and five stars that show hydrogen in fields above 500MG. The highest-field example, SDSSJ2346+3853, may be the most strongly magnetic white dwarf yet discovered. Analysis of the photometric data indicates that the magnetic sample spans the same temperature range as for nonmagnetic white dwarfs from the SDSS, and support is found for previous claims that magnetic white dwarfs tend to have larger masses than their nonmagnetic counterparts. A glaring exception to this trend is the apparently low-gravity object SDSSJ0933+1022, which may have a history involving a close binary companion.

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arXiv2005

Cool White Dwarfs in the Sloan Digital Sky Survey

A reduced proper motion diagram utilizing Sloan Digital Sky Survey (SDSS) photometry and astrometry and USNO-B plate astrometry is used to separate cool white dwarf candidates from metal-weak, high-velocity main sequence Population II stars (subdwarfs) in the SDSS Data Release 2 imaging area. Follow-up spectroscopy using the Hobby-Eberly Telescope, the MMT, and the McDonald 2.7m Telescope is used to demonstrate that the white dwarf and subdwarf loci separate cleanly in the reduced proper motion diagram, and that the contamination by subdwarfs is small near the cool white dwarf locus. This enables large statistically complete samples of white dwarfs, particularly the poorly understood cool white dwarfs, to be created from the SDSS imaging survey, with important implications for white dwarf luminosity function studies. SDSS photometry for our sample of cool white dwarfs is compared to current white dwarf models.

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arXiv2004

Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster

We present spectroscopic observations of the remaining four candidate white dwarfs in Praesepe. All four candidates are quasars with redshifts between 0.8 and 2.8. One quasar, LB 6072, is observed to have a strong metal-line absorption system blueward of the quasar redshift. The lack of additional white dwarfs in Praesepe leaves the total known white-dwarf population of the cluster at five, well below the number expected from commonly-assumed initial mass functions, though several undiscovered cluster WDs may lie in the outer regions of the cluster. All known Praesepe member white dwarfs are concentrated within 0.6 deg of the cluster center, and the radial profile of cluster white dwarfs is quite similar to the profile of massive cluster stars. This profile is mildly inconsistent with that of ~1Mo cluster stars and suggests that the white dwarfs did not receive a velocity kick during the progenitor star's mass loss phases. If complete, the observed Praesepe white dwarf population is consistent with a steeper high-end initial-mass function than commonly assumed, though the calculated slopes are inconsistent with the present-day mass function of Praesepe. Searches for white dwarfs outside the core of Praesepe and further study of the white dwarf populations of additional open clusters is necessary to constrain further the underlying cause of the white dwarf deficit.

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arXiv2004

Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.

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arXiv2004

A Helium White Dwarf of Extremely Low Mass

We analyze the spectrum of an unusually-low mass white dwarf, SDSSJ123410.37-022802.9 (0335-264-52000), found in our recent, white dwarf catalog from the SDSS DR1 data release. Two independent, model atmosphere fits result in an accurate determination of atmospheric and stellar parameters. The more hands-on analysis yields $T_{\rm eff}$ =17,470$\pm$750 K and $\log g$ = 6.38$\pm$0.05. We argue that the object cannot be a main sequence A star, a horizontal branch or subdwarf B star. Instead, it is interpreted as a very low mass white dwarf with a core composed of helium, with mass ($\sim$0.18-0.19$M_\odot$), similar to that published previously for the unusual companion to the millisecond pulsar J1012+5307. The star probably remains in a binary, perhaps even with an undiscovered or dead pulsar companion. However, the companion might be a more-ordinary star, provided Roche lobe mass transfer began shortly after the now-visible component left the main sequence. A second SDSS low mass white dwarf candidate is also analyzed, but the spectrum for this fainter object is of poorer quality. The sample appears to include additional, similar candidates, worthy of more accurate observation. Correct identification of any additional white dwarfs of extremely low mass requires careful observation and interpretation.

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arXiv2004

A Catalog of Spectroscopically Identified White Dwarf Stars in the First Data Release of the Sloan Digital Sky Survey

We present the full spectroscopic white dwarf and hot subdwarf sample from the SDSS first data release, DR1. We find 2551 white dwarf stars of various types, 240 hot subdwarf stars, and an additional 144 objects we have identified as uncertain white dwarf stars. Of the white dwarf stars, 1888 are non-magnetic DA types and 171, non-magnetic DBs. The remaining (492) objects consist of all different types of white dwarf stars: DO, DQ, DC, DH, DZ, hybrid stars like DAB, etc., and those with non-degenerate companions. We fit the DA and DB spectra with a grid of models to determine the Teff and log(g) for each object. For all objects, we provide coordinates, proper motions, SDSS photometric magnitudes, and enough information to retrieve the spectrum/image from the SDSS public database. This catalog nearly doubles the known sample of spectroscopically-identified white dwarf stars. In the DR1 imaged area of the sky, we increase the known sample of white dwarf stars by a factor of 8.5. We also comment on several particularly interesting objects in this sample.

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arXiv2004

2MASS J05185995-2828372: Discovery of an Unresolved L/T Binary

We present the peculiar near-infrared spectrum of the newly discovered brown dwarf 2MASS J05185995-2828372, identified in the Two Micron All-Sky Survey. Features characteristic of both L and T dwarfs are present, namely strong carbon monoxide absorption in K-band, strong methane absorption in J- and H-bands, and red near-infrared colors. We consider several scenarios that could produce these features and conclude that the object is most likely to be an unresolved L/T binary system. We discuss how the estimated photometric properties of this object are consistent with the observed J-band brightening of brown dwarfs between late-L and early-T dwarfs, making detailed study of this system an important probe of the L/T transition.

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arXiv2003

Meeting the Cool New Neighbors V: A 2MASS-Selected Sample of Ultracool Dwarfs

We present initial results of our effort to create a statistically robust, volume-limited sample of ultracool dwarfs from the 2MASS Second Incremental Data Release. We are engaged in a multifaceted search for nearby late-type dwarfs and this is the first installment of our search using purely photometric selection. The goal of this work is a determination of the low-mass star and brown dwarf luminosity function in the infrared. Here, we outline the construction of the sample, dubbed 2MU2, and present our first results, including the discovery of 186 M7--L6 dwarfs--47 of these are likely to be within 20 pc of the Sun. These results represent 66% of the ultracool candidates in our sample yet constitute an 127% increase in the number of ultracool dwarfs known within the volume searched (covering 40% of the sky out to 20 pc). In addition, we have identified 10 M4--M6.5 objects that are likely to be within 20 pc (or within 1$σ$). Finally, based on these initial data, we present a preliminary luminosity function and discuss several interesting features of the partial sample presented here. Once our sample is complete, we will use our measured luminosity function to constrain the mass function of low-mass stars and brown dwarfs.

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arXiv2003

The Spectra of T Dwarfs. II. Red Optical Data

We present 6300--10100 Å spectra for a sample of 13 T dwarfs observed using LRIS mounted on the Keck I 10m Telescope. A variety of features are identified and analyzed, including pressure-broadened K I and Na I doublets; narrow Cs I and Rb I lines; weak CaH, CrH, and FeH bands; strong H$_2$O absorption; and a possible weak CH$_4$ band. H$α$ emission is detected in three of the T dwarfs, strong in the previously reported active T dwarf 2MASS 1237+6526 and weak in SDSS 1254$-$0122 and 2MASS 1047+2124. None of the T dwarfs exhibit Li I absorption. Guided by the evolution of optical spectral features with near-infrared spectral type, we derive a parallel optical classification scheme, focusing on spectral types T5 to T8, anchored to select spectral standards. We find general agreement between optical and near-infrared types for nearly all of the T dwarfs so far observed, including two earlier-type T dwarfs, within our classification uncertainties ($\sim$1 subtype). These results suggest that competing gravity and temperature effects compensate for each other over the 0.6--2.5 $\micron$ spectral region. We identify one possible means of disentangling these effects by comparing the strength of the K I red wing to the 9250 Å H$_2$O band. One of our objects, 2MASS 0937+2931, exhibits a peculiar spectrum, with a substantial red slope and relatively strong FeH absorption, both consequences of a metal-deficient atmosphere. Based on its near-infrared properties and substantial space motion, this object may be a thick disk or halo brown dwarf.

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