arXiv subjects

Michael C. Liu

Publications and source records attributed to Michael C. Liu.

At least 199 records · Page 11Linked to original sources

arXiv2008

BD-22 5866: A Low-mass Quadruple-lined Spectroscopic AND Eclipsing Binary

We report our discovery of an extremely rare, low mass, quadruple-lined spectroscopic binary BD-22 5866 (=NLTT 53279, integrated spectral type = M0 V), found during an ongoing search for the youngest M dwarfs in the solar neighborhood. From the cross-correlation function, we are able to measure relative flux levels, estimate the spectral types of the components, and set upper limits on the orbital periods and separations. The resulting system is hierarchical composed of K7 + K7 binary and a M1 + M2 binary with semi-major axes of asini_{A}<=0.06 AU and asini_{B}<=0.30 AU. A subsequent search of the SuperWASP photometric database revealed that the K7 + K7 binary is eclipsing with a period of 2.21 days and at an inclination angle of 85 degrees. Within uncertainties of 5%, the masses and radii of both components appear to be equal (0.59 Msun, 0.61 Rsun). These two tightly orbiting stars (a = 0.035 AU) are in synchronous rotation causing the observed excess Ca II, Halpha, X-ray and UV emission. The fact that the system was unresolved with published adaptive optics imaging, limits the projected physical separation of the two binaries at the time of the observation to d_{AB} < 4.1 AU at the photometric distance of 51 pc. The maximum observed radial velocity difference between the A and B binaries limits the orbit to asini_{AB}<=6.1 AU. As this tight configuration is difficult to reproduce with current formation models of multiple systems, we speculate that an early dynamical process reduced the size of the system such as the interaction of the two binaries with a circumquadruple disk. Intensive photometric, spectroscopic and interferometric monitoring as well as a parallax measurement of this rare quadruple system is certainly warranted.

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arXiv2008

HN Peg B: A Test of Models of the L to T Dwarf Transition

Luhman and collaborators recently discovered an early-T dwarf companion to the G0 dwarf star HN Peg, using Spitzer Infrared Array Camera (IRAC) images. Companionship was established on the basis of the common proper motion inferred from 1998 Two Micron All Sky Survey images and the 2004 IRAC images. In this paper we present new near-infrared imaging data which confirms the common proper motion of the system. We also present new 3 - 4 um spectroscopy of HN Peg B, which provides tighter constraints on both the bolometric luminosity determination and the comparison to synthetic spectra. New adaptive optics imaging data are also presented, which shows the T dwarf to be unresolved, providing limits on the multiplicity of the object. We use the age, distance and luminosity of the solar-metallicity T dwarf to determine its effective temperature and gravity, and compare synthetic spectra with these values, and a range of grain properties and vertical mixing, to the observed 0.8 - 4.0 um spectra and mid-infrared photometry. We find that models with temperature and gravity appropriate for the older end of the age range of the system (0.5 Gyr) can do a reasonable job of fitting the data, but only if the photospheric condensate cloud deck is thin, and if there is significant vertical mixing in the atmosphere. Dwarfs such as HN Peg B, with well-determined metallicity, radius, gravity and temperature will allow development of dynamical atmosphere models, leading to the solution of the puzzle of the L to T dwarf transition.

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arXiv2008

Subtle Signatures of Multiplicity in Late-type Dwarf Spectra: The Unresolved M8.5 + T5 Binary 2MASS J03202839-0446358

Evidence is presented that 2MASS J03202839-0446358, a late-type dwarf with discrepant optical (M8:) and near-infrared (L1) spectral types, is an as-yet unresolved stellar/brown dwarf binary with late-type M dwarf and T dwarf components. This conclusion is based on low-resolution, near-infrared spectroscopy that reveals a subtle but distinctive absorption feature at 1.6 micron. The feature, which is also present in the combined light spectrum of the M8.5 + T6 binary SCR 1845-6357, arises from the combination of FeH absorption from an M8.5 primary and pseudo-continuum flux from a T5+/-1 secondary, as ascertained from binary spectral templates constructed from empirical data. The binary templates provide a far superior match to the overall near-infrared spectral energy distribution of 2MASS J0320-0446 than any single comparison spectra. Laser guide star adaptive optics (LGS AO) imaging observations, including the first application of LGS AO aperture mask interferometry, fail to resolve a faint companion, restricting the projected separation of the system to less than 8.3 AU at the time of observation. 2MASS J0320-0446 is the second very low mass binary to be identified from unresolved, low-resolution, near-infrared spectroscopy, a technique that complements traditional high resolution imaging and spectroscopic methods.

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arXiv2007

Four Faint T Dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Southern Stripe

We present the optical and near-infrared photometry and spectroscopy of four faint T dwarfs newly discovered from the UKIDSS first data release. The sample, drawn from an imaged area of ~136 square degrees to a depth of Y=19.9 (5-sigma, Vega), is located in the SDSS Southern Equatorial Stripe, a region of significant future deep imaging potential. We detail the selection and followup of these objects, three of which are spectroscopically confirmed brown dwarfs ranging from type T2.5 to T7.5, and one is photometrically identified as early T. Their magnitudes range from Y=19.01 to 19.88 with derived distances from 34 to 98 pc, making these among the coldest and faintest brown dwarfs known. The sample brings the total number of T dwarfs found or confirmed by UKIDSS data in this region to nine, and we discuss the projected numbers of dwarfs in the future survey data. We estimate that ~240 early- and late-T dwarfs are discoverable in the UKIDSS LAS data, falling significantly short of published model projections and suggesting that IMFs and/or birthrates may be at the low end of possible models. Thus, deeper optical data has good potential to exploit the UKIDSS survey depth more fully, but may still find the potential Y dwarf sample to be extremely rare.

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arXiv2007

Physical and Spectral Characteristics of the T8 and Later-Type Dwarfs

We use new and published near-IR spectra, with synthetic spectra, to derive physical properties of three of the latest-type T dwarfs. A new R~1700 spectrum of the T7.5 dwarf HD 3651B, with existing data, allows a detailed comparison to the well-studied and very similar dwarf, Gl 570D. We find that HD 3651B has both higher gravity and metallicity than Gl 570D, with Teff=820-830K, log g= 5.4-5.5, [m/H]= +0.2 and Kzz=10^4cm^2/s. Its age is 8-12 Gyr and its implied mass is 60-70 M_Jup. We perform a similar analyis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data, comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that the two dwarfs have the same Teff as the reference dwarf, and similar gravities, but lower metallicities. The parameters are Teff=725-775K and [m/H]= -0.3; log g=5.3-5.45 for 2MASS J09393548-2448279 and log g=5.0-5.3 for 2MASS J11145133- 261823. The age and mass are ~10Gyr and 60M_Jup for 2MASS J09393548-2448279, and ~5 Gyr and 40M_Jup for 2MASS J11145133-261823. A serious limitation is the incompleteness of the line lists of CH4 and NH3 at lambda <1.7um. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas, suggest that NH3 features in the Y- and J-bands may be useful as indicators of the next cooler spectral type, and not features in the H- and K-bands as previously thought. However large uncertainties remain, as the abundance of NH3 is likely to be significantly below the chemical equilibrium value, and inclusion of laboratory NH3 opacities predicts band shapes that are discrepant with existing data. It is possible that the T spectral class will have to be extended to low temperatures around 400K, when water clouds condense in the atmosphere [abridged].

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arXiv2007

The Late-T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra

(Abridged) We present near-IR photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We find a spectral type of T7.5+/-0.5, a temperature of 780-840 K, and a mass of 40-72 Mjup; its bolometric luminosity and effective temperature are among the lowest measured for any brown dwarf. We estimate an age of 3-12 Gyr for the primary star HD 3651A and find that it is ~3x older than the K4V star Gl 570A (1-5 Gyr), the host star of the T7.5 dwarf Gl 570D. HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low-temperature objects. Given their similar temperatures (within ~30 K) and metallicities (within ~0.1 dex) but different ages, a comparison of HD 3651B and GL 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to GL 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to GL 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

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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

A Re-appraisal of the Habitability of Planets Around M Dwarf Stars

Stable, hydrogen-burning, M dwarf stars comprise about 75% of all stars in the Galaxy. They are extremely long-lived and because they are much smaller in mass than the Sun (between 0.5 and 0.08 MSun), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially-habitable, wet planets residing within their habitable zones, which are only ~ 1/5 to 1/50 of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 MEarth orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone doesn't necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity, or thermal and non-thermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life.

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arXiv2006

Characterizing Young Brown Dwarfs using Low Resolution Near-IR Spectra

We present near-infrared (1.0-2.4 micron) spectra confirming the youth and cool effective temperatures of 6 brown dwarfs and low mass stars with circumstellar disks toward the Chamaeleon II and Ophiuchus star forming regions. The spectrum of one of our objects indicates that it has a spectral type of ~L1, making it one of the latest spectral type young brown dwarfs identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and giant stars, we define a 1.49-1.56 micron H2O index capable of determining spectral type to within 1 sub-type, independent of gravity. We have also defined an index based on the 1.14 micron sodium feature that is sensitive to gravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14 micron Na index can be used to distinguish young cluster members (t <~ 5 Myr) from young field dwarfs, both of which may have the triangular H-band continuum shape which persists for at least tens of Myr. Using effective temperatures determined from the spectral types of our objects along with luminosities derived from near and mid-infrared photometry, we place our objects on the H-R diagram and overlay evolutionary models to estimate the masses and ages of our young sources. Three of our sources have inferred ages (t ~= 10-30 Myr) significantly older than the median stellar age of their parent clouds (1-3 Myr). For these three objects, we derive masses ~3 times greater than expected for 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources. The large discrepancies in the inferred masses and ages determined using two separate, yet reasonable methods, emphasize the need for caution when deriving or exploiting brown dwarf mass and age estimates.

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arXiv2006

Astronomical Science with Laser Guide Star Adaptive Optics: A Brief Review, a Current Snapshot, and a Bright Future

We briefly discuss the past, present, and future state of astronomical science with laser guide star adaptive optics (LGS AO). We present a tabulation of refereed science papers from LGS AO, amounting to a total of 23 publications as of May 2006. The first decade of LGS AO science (1995-2004) was marked by modest science productivity (~1 paper/year), as LGS systems were being implemented and commissioned. The last two years have seen explosive science growth (~1 paper/month), largely due to the new LGS system on the Keck II 10-meter telescope, and point to an exciting new era for high angular resolution science. To illustrate the achievable on-sky performance, we present an extensive collection of Keck LGS performance measurements from the first year of our brown dwarf near-IR imaging survey. We summarize the current strengths and weaknesses of LGS compared to Hubble Space Telescope, offer a list of desired improvements, and look forward to a bright future for LGS given its wide-scale implementation on large ground-based telescopes.

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arXiv2006

SDSS J1534+1615AB: A Novel T Dwarf Binary Found with Keck Laser Guide Star Adaptive Optics and the Potential Role of Binarity in the L/T Transition

We have resolved the newly discovered T dwarf SDSS J1534+1615 into a 0.11'' binary using the Keck sodium laser guide star adaptive optics system. With an integrated-light spectral type of T3.5, this binary provides a new benchmark for studying the distinctive J-band brightening previously noted among early and mid-T dwarfs, using two brown dwarfs with different spectral types but having a common metallicity and age and very similar surface gravities. We estimate spectral types of T1.5+/-0.5 and T5.5+/-0.5 for the two components based on their near-IR colors, consistent with modeling the integrated-light spectrum as the blend of two components. The observed near-IR flux ratios are unique compared to all previously known substellar binaries: the component that is fainter at H and K' is brighter at J. This inversion of the near-IR fluxes is a manifestation of the J-band brightening within this individual binary system. Therefore, SDSS 1534+1615 demonstrates that the brightening can be intrinsic to ultracool photospheres (e.g., arising from cloud disruption and/or rapid increase in cloud sedimentation) and does not necessarily result from physical variations among the observed ensemble of T dwarfs (e.g., a range in masses, ages and/or metallicities). We suggest that the apparently large amplitude of the J-band brightening may be due to a high incidence of unresolved binaries and that the true amplitude of the phenomenon could be more modest. This scenario would imply that truly single objects in these spectral subclasses are relatively rare, in agreement with the small effective temperature range inferred for the L/T transition.

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arXiv2005

Kelu-1 is a Binary L Dwarf: First Brown Dwarf Science from Laser Guide Star Adaptive Optics

(Abridged) We present near-IR imaging of the nearby L dwarf Kelu-1 obtained with the Keck sodium laser guide star adaptive optics (LGS AO) system as part of a high angular resolution survey for substellar binaries. Kelu-1 was one of the first free-floating L dwarfs identified, and the origin of its overluminosity compared to other similar objects has been a long-standing question. Our images clearly resolve Kelu-1 into a 0.29'' (5.4 AU) binary, and a previous non-detection by HST demonstrates that the system is a true physical pair. Binarity explains the properties of Kelu-1 that were previously noted to be anomalous compared to other early-L dwarfs. We estimate spectral types of L1.5-L3 and L3-L4.5 for the two components, giving model-derived masses of 0.05-0.07 Msun and 0.045-0.065 Msun for an estimated age of 0.3-0.8 Gyr. More distant companions are not detected to a limit of 5-9 Mjup. The presence of lithium absorption indicates that both components are substellar, but the weakness of this feature relative to other L dwarfs can be explained if only Kelu-1B is Li-bearing. Determining whether both or only one of the components possesses lithium could constrain the age of Kelu-1 (and other Li-bearing L binaries) with higher precision than is possible for most ultracool field objects. These results are the first LGS AO observations of brown dwarfs and demonstrate the potential of this new instrumental capability for substellar astronomy.

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arXiv2005

Near-IR Surface Brightness Fluctuations and Optical Colours of Magellanic Star Clusters

This work continues our efforts to calibrate model surface brightness luminosities for the study of unresolved stellar populations, through the comparison with data of Magellanic Cloud star clusters. We present here the relation between absolute K_s-band fluctuation magnitude and (V - I) integrated colour, using data from the 2MASS and DENIS surveys, and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. (2002). We find that intermediate-age to old clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the barM_{K_s} vs. (V - I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V - I_c)_0 < 1.25, ours holds in the interval -5 >= barM_{K_s} >= -9, 0.3 <= (V - I) <= 1.25. This implies, according to Bruzual & Charlot (2003) and Mouhcine & Lancon (2003) models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, there is a slight offset between the galaxies and the star clusters [the latter are ~ 0.7 mag fainter than the former at a given value of (V - I)], caused by the difference in metallicity of roughly a factor of two. The confrontation between models and galaxy data also suggests that galaxies with K_s fluctuation magnitudes that are brighter than predicted, given their (V - I) colour, might be explained in part by longer lifetimes of TP-AGB stars. (Abridged version.)

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arXiv2004

Substructure in the Circumstellar Disk around the Young Star AU Mic (GJ 803)

Keck adaptive optics imaging with a physical resolution of 0.4 AU resolves the inner (15-80 AU) disk of AU Mic (GJ 803), the nearest known scattered light disk to Earth. The inner disk is asymmetric and possesses a sharp change in structure at 35 AU. The disk also shows spatially localized enhancements and deficits at 25-40 AU separations. The overall morphology points to the influence of unseen larger bodies and resembles structures expected from recent planet formation. AU Mic is coeval with the archetypical debris disk system beta Pic, and the similarities between their two disks point to synchronous disk evolution. Multiple indications of substructure appear to be common in circumstellar disks at an age of ~12 Myr.

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arXiv2004

IR Surface Brightness Fluctuations of Magellanic Star Clusters

We present surface brightness fluctuations (SBFs) in the near--IR for 191 Magellanic star clusters available in the Second Incremental and All Sky Data releases of the Two Micron All Sky Survey (2MASS), and compare them with SBFs of Fornax Cluster galaxies and with predictions from stellar population models. Our goals are twofold. First, to provide an empirical calibration of near--IR SBFs, given that existing stellar population synthesis models are particularly discrepant in the near--IR. Second, whereas most previous SBF studies have focused on old, metal rich populations, this is the first application to a system with such a wide range of ages (~ 10^6 to more than 10^10 yr, i.e., 4 orders of magnitude), at the same time that the clusters have a very narrow range of metallicities (Z ~ 0.0006 -- 0.01, ie., 1 order of magnitude only). Since stellar population synthesis models predict a more complex sensitivity of SBFs to metallicity and age in the near--IR than in the optical, this analysis offers a unique way of disentangling the effects of age and metallicity. We find a satisfactory agreement between models and data. We also confirm that near--IR fluctuations and fluctuation colors are mostly driven by age in the Magellanic cluster populations, and that in this respect they constitute a sequence in which the Fornax Cluster galaxies fit adequately. Moreover, fluctuation colors display a tendency to redden with age that can be fit by a straight line. Finally, we use for the first time a Poissonian approach to establish the error bars of the fluctuation measurements, instead of the customary Monte Carlo simulations.

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arXiv2004

A Sub-Millimeter Search of Nearby Young Stars for Cold Dust: Discovery of Debris Disks around Two Low-Mass Stars

(Abridged) We present results from a JCMT/SCUBA 850 um search for cold dust around nearby young stars belonging to the beta Pic (t~12 Myr) and the Local Association (t~50 Myr) moving groups. Unlike most past sub-mm studies, our sample was chosen on the basis of stellar age. Our observations achieve about an order of magnitude greater sensitivity in dust mass compared to previous work in this age range. We detected two of the three M dwarfs in our sample at 850 um, GJ 182 and GJ 803. GJ 182 may also possess a 25 um excess, indicative of warm dust in the inner few AU of its disk. For GJ 803 (AU Mic), sub-mm mapping finds that the 850 um emission is unresolved. A non-detection of the CO 3-2 line indicates the system is gas-poor, and the SED suggests the presence of a large inner disk hole (~17 AU = 1.7 arcsec in radius). These are possible indications that planets at large separations can form around M dwarfs within \~10 Myr. In a companion paper (Kalas, Liu & Matthews 2004), we confirm the existence of a dust disk around GJ 803 using optical coronagraphic imaging. Given its youthfulness, proximity, and detectability, the GJ 803 disk will be a valuable system for studying disk, and perhaps planet, formation in great detail. Overall, sub-mm measurements of debris disks point to a drop in dust mass by a factor of about 10^3 within the first ~10 Myr, with the subsequent decline in the masses of sub-mm detected disks consistent with t^{-0.5} to t^{-1}.

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arXiv2004

Discovery of a large dust disk around the nearby star AU Microscopium

We present the discovery of a circumstellar dust disk surrounding AU Microscopium (AU Mic, GJ 803, HD 197481). This young M star at 10 parsec has the same age and origin as beta Pictoris, another nearby star surrounded by a dust disk. The AU Mic disk is detected between 50 AU and 210 AU radius, a region where dust lifetimes exceed the present stellar age. Thus, AU Mic is the nearest star where we directly observe the solid material required for planet formation. Since 85% of stars are M-type, the AU Mic disk provides new clues on how the majority of planetary systems might form and evolve.

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arXiv2003

Detection of cool dust around the G2V star HD 107146

We report the detection of dust emission at sub-millimeter wavelengths from HD 107146, a G2V star with an age estimated to lie between 80 and 200 Myr. The emission is resolved at 450um with a size 300 AU x 210 AU. A fit to the spectral energy distribution gives a dust temperature of 51 K and dust mass of 0.10 Earth masses. No excess emission above the photosphere was detected at 18um showing that there is very little warm dust and implying the presence of a large inner hole, at least 31 AU (~ 1'') in radius, around the star. The properties of this star-disk system are compared with similar observations of other systems. We also discuss prospects for future observations that may be able to determine whether the inner hole is maintained by the dynamical effect of an unseen orbiting companion.

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