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With the start of a new Great Observatories era, there is renewed concern that the demand for these forefront facilities, through proposal pressure, will exceed conventional peer-review management's capacity for ensuring an unbiased and efficient selection. There is need for new methods, strategies, and tools to facilitate those reviews. Here, we describe PACMan2, an updated tool for proposal review management that utilizes machine learning models and techniques to topically categorize proposals and reviewers, to match proposals to reviewers, and to facilitate proposal assignments, mitigating some conflicts of interest. We find that the classifier has cross-validation accuracy of $80.0\pm2.2\%$ on proposals for time on the Hubble Space Telescope and the James Webb Space Telescope., Comment: 10 pages, 7 figures

The binary brown dwarf WISE J104915.57$-$531906.1 (also Luhman 16AB), composed of a late L and early T dwarf, is a prototypical L/T transition flux reversal binary located at only 2 pc distance. Luhman 16B is a known variable whose light curves evolve rapidly. We present spatially resolved spectroscopic time-series of Luhman 16A and B covering 6.5 h using HST/WFC3 at 1.1 to 1.66 $\mu$m. The small, count-dependent variability of Luhman 16A at the beginning of the observations likely stems from instrumental systematics; Luhman 16A appears non-variable above $\approx$0.4%. Its spectrum is well fit by a single cloud layer with intermediate cloud thickness (f_sed=2, Teff=1200 K). Luhman 16B varies at all wavelengths with peak-to-valley amplitudes of 7-11%. The amplitude and light curve shape changes over only one rotation period. The lowest relative amplitude is found in the deep water absorption band at 1.4 $\mu$m, otherwise it mostly decreases gradually from the blue to the red edge of the spectrum. This is very similar to the other two known highly variable early T dwarfs. A two-component cloud model accounts for most of the variability, although small deviations are seen in the water absorption band. We fit the mean spectrum and relative amplitudes with a linear combination of two models of a warm, thinner cloud (Teff=1300 K, fsed=3) and a cooler, thicker cloud (Teff=1000-1100 K, f_sed=1), assuming out-of-equilibrium atmospheric chemistry. A cloud as for Luhman 16A but with holes cannot reproduce the variability of Luhman 16B, indicating more complex cloud evolution through the L/T transition. The projected separation of the binary has decreased by $\approx$0.3'' in 8 months., Comment: 14 pages, 12 figures. Accepted for publication in ApJ

The Proposal Auto-Categorizer and Manager (PACMan) tool was written to respond to concerns on subjective flaws and potential biases in some aspects of the proposal review process for time allocation for the {\it Hubble Space Telescope} (HST), and to partially alleviate some of the anticipated additional workload from the {\it James Webb Space Telescope} (JWST) proposal review. PACMan is essentially a mixed-method Naive Bayesian spam filtering routine, with multiple pools representing scientific categories, that utilizes the Robinson method for combining token (or word) probabilities. PACMan was trained to make similar programmatic decisions in science category sorting, panelist selection, and proposal-to-panelists assignments to those made by individuals and committees in the Science Policies Group (SPG) at Space Telescope Science Institute. Based on training from the previous cycle's proposals, PACMan made the same science category assignments for proposals in Cycle 24 as did the SPG, an average of 87\% of the time. Tests for similar science categorizations, based on training using proposals from additional cycles, show that this accuracy can be further improved, to the $>95\%$ level. This tool will be used to augment or replace key functions in the TAC review processes in future HST and JWST cycles., 9 pages, 7 figures

Proposal success rates are calculated for HST Cycles 11 through 21 as a function of the gender of the Principal Investigator (PI). In each cycle, proposals with male PIs have a higher success rate, with the disparity greatest for Cycles 12 and 18. The offsets are small enough that they might be ascribed to chance for any single cycle, but the consistent pattern suggests the presence of a systematic effect. Closer inspection of results from Cycles 19, 20, and 21 shows that the systematic difference does not appear to depend on the geographic origin of the proposal nor does it depend on the gender distribution on the review panels. Segregating proposals by the seniority of the PI, the success rates by gender for more recent graduates (Ph.D. since 2000) are more closely comparable. There is also a correlation between success by gender and the average seniority of the review panel for Cycles 19 and 20, but not Cycle 21. We discuss these results and some consequent changes to the proposal format and additions to the HST TAC orientation process.

We report the discovery of a tight substellar companion to the young solar analog PZ Tel, a member of the Beta Pictoris moving group observed with high contrast adaptive optics imaging as part of the Gemini NICI Planet-Finding Campaign. The companion was detected at a projected separation of 16.4 +/- 1.0 AU (0.33 +/- 0.01") in April 2009. Second-epoch observations in May 2010 demonstrate that the companion is physically associated and shows significant orbital motion. Monte Carlo modeling constrains the orbit of PZ Tel B to eccentricities > 0.6. The near-IR colors of PZ Tel B indicate a spectral type of M7+/-2 and thus this object will be a new benchmark companion for studies of ultracool, low-gravity photospheres. Adopting an age of 12 +8 -4 Myr for the system, we estimate a mass of 36 +/- 6 Mjup based on the Lyon/DUSTY evolutionary models. PZ Tel B is one of few young substellar companions directly imaged at orbital separations similar to those of giant planets in our own solar system. Additionally, the primary star PZ Tel A shows a 70 um emission excess, evidence for a significant quantity of circumstellar dust that has not been disrupted by the orbital motion of the companion., 15 pages, 4 figures, to appear in ApJ Letters

We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic Globular Clusters to construct global present-day mass functions for 17 globular clusters utilizing multi-mass King models to extrapolate from our observations to the global cluster behavior. The global present-day mass functions for these clusters are well matched by power laws from the turnoff, 0.8 M ☉, to 0.2-0.3 M ☉ on the lower main sequence. The slopes of those power-law fits, α, have been correlated with an extensive set of intrinsic and extrinsic cluster properties to investigate which parameters may influence the form of the present-day mass function. We do not confirm previous suggestions of correlations between α and either metallicity or Galactic location. However, we do find a strong statistical correlation with the related parameters central surface brightness, μ V , and inferred central density, ρ0. The correlation is such that clusters with denser cores (stronger binding energy) tend to have steeper mass functions (a higher proportion of low-mass stars), suggesting that dynamical evolution due to external interactions may have played a key role in determining α. Thus, the present-day mass function may owe more to nurture than to nature. Detailed modeling of external dynamical effects is therefore a requisite for determining the initial mass function for Galactic globular clusters.

We detect radio emission in an additional two epochs of the ultracool dwarf binary LP 349- 25, finding that the observed emission is broadband and steady on timescales between 10s and 10.7 hours, as well as on timescales of 0.6 and 1.6 years. This system is unusual for ultracool dwarfs with detectable radio emission, in exhibiting a lack of any large scale variability, particularly the bursting (periodic or aperiodic) behavior exhibited by the other objects with detectable levels of radio emission. We explore the constraints that the lack of variability on long and short timescales, and flat spectral index, imply about the radio-emitting structures and mechanism. The temporal constraints argue for a high latitude emitting region with a large inclination so that it is always in view, and survives for at least 0.6 year. Temporal constraints also limit the plasma conditions, implying that the electron density be n_e, 28 pages, two figures, accepted for publication in ApJ

Using data from the 2 Micron All Sky Survey All-Sky Point Source Catalogue, we have extended our census of nearby ultracool dwarfs to cover the full celestial sphere above Galactic latitude of 15°. Starting with an initial catalog of 2,139,484 sources, we have winnowed the sample to 467 candidate late-type M or L dwarfs within 20 pc of the Sun. Fifty-four of those sources already have spectroscopic observations confirming them as late-type dwarfs. We present optical spectroscopy of 376 of the remaining 413 sources, and identify 44 as ultracool dwarfs with spectroscopic distances less than 20 pc. Twenty-five of the 37 sources that lack optical data have near-infrared spectroscopy. Combining the present sample with our previous results and data from the literature, we catalog 94 L dwarf systems within 20 pc. We discuss the distribution of activity, as measured by Hα emission, in this volume-limited sample. We have coupled the present ultracool catalog with data for stars in the northern 8 pc sample and recent (incomplete) statistics for T dwarfs to provide a snapshot of the current 20 pc census as a function of spectral type.

We present a spectroscopic analysis of over 38,000 low-mass stars from the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Analysis of this unprecedentedly large sample confirms the previously detected decrease in the fraction of magnetically active stars (as traced by H-alpha emission) as a function of vertical distance from the Galactic Plane. The magnitude and slope of this effect varies as a function of spectral type. Using simple 1-D dynamical models, we demonstrate that the drop in activity fraction can be explained by thin disk dynamical heating and a rapid decrease in magnetic activity. The timescale for this rapid activity decrease changes according to the spectral type. By comparing our data to the simulations, we calibrate the age-activity relation at each M dwarf spectral type. We also present evidence for a possible decrease in the metallicity as a function of height above the Galactic Plane. In addition to our activity analysis, we provide line measurements, molecular band indices, colors, radial velocities, 3-D space motions and mean properties as a function of spectral type for the SDSS DR5 low-mass star sample.

We present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular Clusters. Our deep (F606W~26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised usingboth isochrone and main-sequence fitting to (m-M)_0=17.27 and E(B-V)=0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, 2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (0.1-0.8 Gyr old), more metal-rich ([Fe/H]\sim0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting., Comment: Accepted to ApJ Letter; 11 pages, 2 figures; figure 1 uploaded as jpg; paper in ApJ format with full-resolution figures available at: http://www.astro.ufl.edu/~ata/public_hstgc/paperIV/paperIV.ps

Using data from the 2MASS All-Sky Point Source Catalogue, we have extended our census of nearby ultracool dwarfs to cover the full celestial sphere above Galactic latitute 15 degrees. Starting with an initial catalogue of 2,139,484 sources, we have winnowed the sample to 467 candidate late-type M or L dwarfs within 20 parsecs of the Sun. Fifty-four of those sources already have spectroscopic observations confirming them as late-type dwarfs. We present optical spectroscopy of 376 of the remaining 413 sources, and identify 44 as ultracool dwarfs with spectroscopic distances less than 20 parsecs. Twenty-five of the 37 sources that lack optical data have near-infrared spectroscopy. Combining the present sample with our previous results and data from the literature, we catalogue 94 L dwarf systems within 20 parsecs. We discuss the distribution of activity, as measured by H-alpha emission, in this volume-limited sample. We have coupled the present ultracool catalogue with data for stars in the northern 8-parsec sample and recent (incomplete) statistics for T dwarfs to provide a snapshot of the current 20-parsec census as a function of spectral type.

We present the results of a deep-imaging search for wide companions to low-mass stars and brown dwarfs using NSFCam on IRTF. We searched a sample of 132 M7-L8 dwarfs to magnitude limits of $J \sim 20.5$ and $K \sim 18.5$, corresponding to secondary-primary mass ratios of $\sim 0.5$. No companions were found with separations between $2{\arcsec}$ to $31{\arcsec}$ ($\sim$40 AU to $\sim$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 $0.05 M_{\odot}$ and 50% above $0.03 M_{\odot}$., 27 pages, 8 figures, 3 tables: accepted to the Astronomical Journal

We present results of a high-resolution, near-infrared survey of 41 nearby, young ( = M6 very low-mass, VLM) populations. We find that the separation distribution of the binaries in this sample peaks at 13+14-9 AU, which is consistent with previous measurements of early-M binaries. Hence, early-M binaries seem to occur in--on average--tighter systems than G binaries. At the same time they are significantly wider than field VLM binary stars. The distribution of mass ratios q of primary and secondary stars was found to show an intermediate distribution between the strongly q-->1 peaked distribution of field VLM systems and the almost flat distribution of earlier-type stars. Consequently, we show evidence for relatively young, early-M binaries representing a transition between the well known earlier star distributions and the recently examined field VLM population characteristics. Despite the fact that this survey was dedicated to the search for faint brown dwarf and planetary mass companions, all planetary mass candidates were background objects. We exclude the existence of physical companions with masses greater than 10 Jupiter masses (M_Jup) at separations of >~40 AU and masses greater than 24 M_Jup for separations >~10 AU around 37 of the 41 observed objects.

We present new observations of the LP 261‐75/2MASSW J09510549+3558021 M4.5/L6 wide, common‐proper‐motion binary system. Optical spectroscopy of LP 261‐75 shows strong Hα emission, and the star may also be associated with the ROSAT FSC source 1RXS J095102.7+355824. The derived chromospheric and coronal activity levels are consistent with those of Pleiades stars of the same spectral type, and we infer an age of 100–200 Myr for the system. In that case, theoretical models suggest that the L6 dwarf 2MASSW J09510549+3558021 is a low‐mass brown dwarf, with \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr...

NICMOS images of the nearby late-type L dwarf 2MASS J22521073-1730134 (DENIS-P J225210.73-173013.4) show that it is a close double, separation 013. The companion is 1.0 mag fainter than the primary in the F110W passband and 1.55 mag fainter in the F170M images. The latter passband is centered on the 1.6 μm methane band. The small separation and the absence of an optical counterpart suggest that the two sources are associated; the relatively blue infrared color suggests that 2MASS J22521073-1730134B is a T-type binary companion of the late-L primary. This hypothesis is supported by infrared spectroscopy, which shows weak methane absorption in both the H and K passbands. We estimate a distance of 13.6 pc to the system and a projected linear separation of 1.75 AU. We consider the potential for measuring dynamical masses of the two components.

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., 6 pages, 2 figures, accepted for publication in Astrophysical Journal Letters

▪ Abstract A number of recent investigations suggest that cool white dwarfs are more numerous than predicted by conventional Galactic models and that those stars make a significant contribution to the mass budget of the Milky Way. In particular, there is speculation that cool white dwarfs are linked with the dark-matter halos. This review examines those recent results and matches them against current understanding of the properties of the stellar populations that make up the Milky Way, taking due account of the relative star formation histories of the disk and the thick disk. The new white dwarf observations do not require any additions to the conventional stellar populations of the Milky Way. There is no credible evidence of either an underlying population of ancient white dwarfs or a link between high-velocity degenerates and dark matter. Nonetheless, placed in the proper context, these high-velocity white dwarfs provide interesting insight on the likely history of the Milky Way. We show that the thick disk is likely to contribute ∼20% of Solar Neighborhood white dwarfs. If the thick disk is an old, single-burst population, as favored by most investigations, then those white dwarfs dominate at faint absolute magnitudes. As a result, analyses of the low-luminosity cutoff in the local white dwarf–luminosity function actually set limits on the age of the thick disk, rather than the thin disk. In absolute terms, these results imply that the thick disk is no older than 10 Gyrs, whereas major star formation in the thin disk may not have started until 7 to 8 Gyrs ago. Moreover, the enhanced [α/Fe] ratios in thick-disk and halo main-sequence stars suggest that those two populations have similar ages, implying a relatively young age for the field halo.

We present low-resolution spectroscopic observations of faint proper-motion stars from the LHS Catalogue, concentrating on stars with mr > 16.5 and yr–1. The present paper includes observations and spectral classifications for 294 M dwarfs, M subdwarfs (sdM), and extreme M subdwarfs (esdM). We also identify white dwarfs among the faintest LHS stars. We have cross-referenced this sample against the Two Micron All Sky Survey (2MASS) sources, and list data for the detected objects. We discuss stars of individual interest, as well as the characteristics of the overall sample. As expected, a significant number of the stars in this proper-motion-selected sample are halo subdwarfs, including an esdM dwarf, LHS 3481, that is likely to lie within 20 pc of the Sun. None of the subdwarfs show Hα emission.

We present the discovery of seven new T dwarfs identified in the Two Micron All Sky Survey. Low-resolution (R$\sim$150) 0.8--2.5 micron spectroscopy obtained with the IRTF SpeX instrument reveal the characteristic H$_2$O and CH$_4$ bands in the spectra of these brown dwarfs. Comparison to spectral standards observed with the same instrument enable us to derive classifications of T3 to T7 for the objects in this sample. Moderate-resolution (R$\sim$1200) near-infrared spectroscopy for a subset of these discoveries reveal K I line strengths consistent with previously observed trends with spectral type. Follow-up imaging observations provide proper motion measurements for these sources, ranging from $, Comment: 27 pages incl. 9 figures and 8 tables, accepted for publication AJ, May 2004; note that findercharts are provided separately as jpg files

The Galactic Anticenter Stellar Structure (GASS) has been identified with excess surface densities of field stars in several large area sky surveys, and with an unusual, string-like grouping of five globular clusters. At least two of these are diffuse, young ``transitional'' clusters between open and globular types. Here we call attention to the fact that four {\it younger} open or transitional clusters extend the previously identified, string-like cluster grouping, with at least one having a radial velocity consistent with the previously found GASS velocity-longitude trend. All nine clusters lie close to a plane tipped 17$\arcdeg$ to the Galactic plane. This planar orientation is used to forage for additional potential cluster members in the inner Galaxy, and a number are found along the same plane and string-like sequence, {\it including almost all fifteen known outer, old open clusters}. Tidal accretion of a dwarf satellite galaxy on a low inclination orbit -- perhaps the GASS system -- appears to be a plausible explanation for the origin of the outer, old open and transitional clusters of the Milky Way. We use these clusters to explore the age-metallicity relation of the putative accreted GASS progenitor. Finally, we provide the first radial velocity of a star in the cluster BH 176 and discuss its implications., 4 pages, 3 postscript figures, emulateapj.sty (accepted ApJL)

(abridged) We present the discovery of two T dwarf binaries, 2MASS 1225-2739AB and 2MASS 1534-2952AB, identified in a sample of ten T dwarfs imaged with the Hubble Space Telescope Wide Field Planetary Camera 2. The separations of the two binary systems are 0{\farcs}282$\pm$0{\farcs}005 and 0{\farcs}065$\pm$0{\farcs}007, implying projected separations of 3.17$\pm$0.14 and 1.0$\pm$0.3 AU, respectively. The observed binary fraction of our HST sample, 20$^{+17}_{-7}$%, is consistent with results obtained for late-M and L field dwarfs, and implies a bias-corrected binary fraction of 9$^{+15}_{-4}$% for $a \gtrsim 1$ AU and $q \gtrsim 0.4$, significantly lower than the binary fractions of F--G and early-type M dwarf stars. Neither of the T binaries have separations $a \gtrsim 10$ AU, consistent with results from other brown dwarf binary searches. We conclude that tidal disruption by passing stars or Giant Molecular Clouds, which limits the extent of wide stellar binaries, plays no role in eliminating wide brown dwarf binaries, implying either disruption very early in the formation process (ages $\lesssim 1-10$ Myr) or a formation mechanism which precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale as M$_{total}^2$, a possible clue to the physical mechanism which restricts wide substellar systems.

The comparison of optical and 2MASS near-infrared photometry for large samples of catalogued proper motion stars has the potential to discover previously unrecognized nearby objects of rare type. In this paper we present the discovery of two new carbon dwarfs, LSR 2105+2514 and LP 758-43, which were drawn from proper motion lists and which lie in a sparsely populated part of optical/near-IR color-color space. Their optical spectra, exhibiting absorptions by C_2 and/or CN, are discussed. LSR 2105+2514 is believed to lie within 200 pc and would have M_(K_s) < 6.7, making it lower in luminosity than any carbon dwarf with a measured trigonometric parallax. LP 758-43, which is not as red but still probably cooler than the best studied carbon dwarfs, is believed to lie within 360 pc. Using our optical/near-infrared selection technique on published lists of proper motion stars, we hope in the near future to expand further the current sample of carbon dwarfs, which numbers only 31 objects at this writing., 6 pages, 4 figs, Accepted by ApJ Letters

We present observations of 2MASSI J1835379+325954, a previously unrecognized late-type M dwarf within 6 pc of the Sun. Identified based analysis of the Two Micron All Sky Survey optical spectroscopy and photometry indicate a spectral type of M8.5. The star has a proper motion of 0759 yr-1 and is clearly visible on both POSS I and POSS II photographic plate material, but it may have escaped detection in previous surveys through its proximity to the Galactic plane. We discuss potential implications for the completeness of the local stellar census.

We have used new astrometric and spectroscopic observations to refine the volume-complete sample of M dwarfs defined in previous papers in this series. With the addition of Hipparcos astrometry, our revised VC2 sample includes 558 main-sequence stars in 448 systems. Analysis of that data set shows no evidence of any systematic kinematic bias. Combining those data with a Hipparcos-based sample of AFGK dwarfs within 25 pc of the Sun, we have derived the solar neighborhood luminosity function, ?(MV), for stars with absolute magnitudes between -1 and +17. Using empirical and semiempirical mass-MV relations, we transform ?(MV) to the present-day mass function, ?(M) (=dN/dM). Depending on the mass-luminosity calibration adopted, ?(M) can be represented by either a two-component or a three-component power law. In either case, the power-law index ? has a value of ~1.3 at low masses (0.1 M? 4) stars in the Hipparcos 25 pc sample are well represented by two-component Gaussian distributions, with ~10% of the stars in the higher velocity dispersion component. We suggest that the latter component is the thick disk, and we offer a possible explanation for the relatively low velocity dispersions shown by ultracool dwarfs.

(abridged) We present high-resolution echelle spectroscopy of a photometricaly-selected sample if thirty-nine dwarfs with spectral types between M6.5 and L0.5. Two stars, 2MASSI 0253202+271333 and 2MASSW 0952219-192431, are double-lined spectroscopic binaries. We have used our observations to search for Li I 6708 A absorption, characteristic of sub-stellar mass; estimate the level of chromospheric activity through measurement of H-alpha emission fluxes; measure rotational velocities via line broadening; and determine radial velocities and Galactic space motions. Two dwarfs have strong lithium absorption, the previously-known brown dwarf, LP 944-20, and 2MASSI J0335020+234235, which we identify as a probable 0.06 M_sun brown dwarf, age ~1 Gyr. We have investigated the prospect of using the observed frequency of lithium absorption amongst ultracool M dwarfs (M7 to M9.5) as a probe of the initial mass function. The available observations are difficult to reconcile with Salpeter-like power-law mass functions (alpha > 2) for masses below 0.1M_Sun. A comparison between the rotational velocities and -alpha fluxes shows no evidence for significant correlation. Velocity dispersions are significantly lower than those measured for nearby M dwarfs, but show remarkable similarity to results for earlier-type emission-line (dMe) dwarfs. The latter are generally assigned ages of less than ~3 Gyrs., 53 pages, 18 figures Accepted for AJ

While quality checking a new motion-aware co-addition of all 12.5 months of WISE data, the source WISE J070720.48+170533.0 was found to have moved 0.9 arc-sec in 6 months. Backtracking this motion allowed us to identify this source as 2MASS J07071961+1705464 and with several entries in the USNO B catalog. An astrometric fit to these archival data gives a proper motion of 1793 +/- 2 mas/yr and a parallax of 35 +/- 42 mas. Photometry from WISE, 2MASS and the POSS can be fit reasonably well by a blackbody with T = 3658 K and an angular radius of 4.36E-11 radians. No clear evidence of H_2 collision-induced absorption is seen in the near-IR. An optical spectrum shows broad deep CaH bands at 638 and 690 nm, broad deep Na D at 598.2 nm, and weak or absent TiO, indicating that this source is an ultra-subdwarf M star with a radial velocity about -21 +/- 18 km/sec relative to the Sun. Given its apparent magnitude, the distance is about 39 +/- 9 pc and the tangential velocity is probably about 330 km/sec, but a more precise parallax is needed to be certain., Accepted by the Astronomical Journal, 10 pages LaTex with 9 embedded figures. UCAC and CMC data added, fits updated

In this study we compare the photometric data of 34 Milky Way globular clusters, observed within the ACS Treasury Program (PI: Ata Sarajedini) with the corresponding ground-based data, provided by the Photometric Standard Field Catalogs of Stetson (2000, 2005). We focus on the transformation between the HST/ACS F606W to V-band and F814W to I-band only. The goal is to assess the validity of the filter transformation equations by Sirianni et al.(2005) with respect to their dependence on metallicity, Horizontal Branch morphology, mass and integrated (V-I) colour of the various globular clusters. Such a dependence is expected due to the fact that the transformation equations are based on the observations of only one globular cluster, i.e., NGC 2419. Surprisingly, the correlation between offset and metallicity is found to be weak, with a low level significance. The correlation between offset and Horizontal Branch structure, as well as total cluster mass is still weaker. Based on the available data we do not find the photometric offset to be linked to multiple stellar populations, e.g., as found in NGC 0288, NGC 1851, and NGC 5139. The results of this study show that there are small systematic offsets between the transformed ACS- and observed ground based photometry, and that these are only weakly correlated, if at all, with various cluster parameters and their underlying stellar populations. As a result, investigators wishing to transform globular cluster photometry from the Sirianni et al.(2005) ground-based V, I system onto the Stetson (2000) system simply need to add 0.040 (+/-0.012) to the V-band magnitudes and 0.047 (+/-0.011) to the I-band magnitudes. This in turn means that the transformed ACS (V-I) colours match the ground-based values from Stetson (2000) to within ~0.01 mag., 28 pages, 14 figures, accepted for publication in ApJS

We present analyses of high resolution and medium resolution spectra of early L dwarfs. We used our latest set of model atmospheres to reproduce and analyze the observed features. We can model the optical flux and the atomic line profiles with the best accuracy to date. The models used to reproduce the observations include dust condensation and dust opacities. Compared to previous studies using older models we find that our dust treatment is much improved. The derived parameters for the objects are well in the expected range for old very low mass objects. This is also supported by the absence of Li in most of the objects. For the objects showing Li we can be almost certain that those are brown dwarfs. However, a spectral analysis in general, and this one in particular can only very roughly determine mass and age., AASTeX5.0. 26 pages, including all figures, Accepted for ApJ

We present BVRI photometry of 180 bright, southern nearby-star candidates. The stars were selected from the New Luyten Two-Tenths proper motion catalogue based on optical/infrared colours, constructed by combining Luytens's m(r) estimates with near-infrared photometry from the 2-Micron All Sky Survey. Photometric parallaxes derived from (V-K), (V-I) and (I-J) colours, combined with the limited available astrometry, show that as many as 108 stars may lie within 20 parsecs of the Sun. Of these, 53 are new to nearby star catalogues, including three within 10 parsecs of the Sun., Comment: 24 pages, 6 figures, to be published in The Astronomical Journal. More information can be found at http://www.stsci.edu/~inr/nstars.html

We present the mass distribution, gravitational redshifts, radial velocities, and space motions of white dwarf stars in common proper motion binary systems. The mass distribution we derive for the 41 DA white dwarfs in this study has a mean of 0.68 ± 0.04 M⊙. This distribution has a slightly higher mean and larger dispersion than most previous white dwarf studies. We hypothesize that this is due to a higher fraction of cool (average Teff ~ 10,000 K), hence old, white dwarfs in our sample. Our results indicate that samples made up of predominantly cool, old white dwarf stars tend to have a bimodal distribution with a second mass peak at ~1.0 M⊙, which skews the mean toward a higher mass. Both the mean and individual white dwarf masses we report here are in better agreement with those determined from model atmosphere spectroscopic fits to line profiles than with most previous gravitational redshift studies of cool white dwarfs. Our results indicate that measurement biases and weak geocoronal emission lines in the observed spectra may have affected previous gravitational redshift measurements. These have been minimized in our study. We present measurements for some previously unobserved white dwarfs, as well as independent new measurements for some that have been reported in the literature. A list of complete space motions for 50 wide binary white dwarfs is presented, derived from radial velocity measurements of their nondegenerate companions. We find that the UVW space motions and dispersions of the common proper motion binaries that contain white dwarf components are consistent with those of old, metal-poor disk stars.

We have applied the astrometric techniques devised by Murray for analysis of 48-inch Schmidt photographic data to SuperCOSMOS scans of UK Schmidt plates centred on Kapteyn's Selected Area 94 (α=2h 53m, δ=0°). In this preliminary study, we combine astrometric data from four short-exposure V-band plates, taken in 1987 August (2 plates) and 1993 August (2 plates), with BVRI photometry from sky-limited plate material, to identify stars with proper motions exceeding 0.1 arcsec yr−1. This paper discusses the completeness of the resulting sample and presents spectroscopy of 30 stars with μ>0.2 arcsec yr−1. Based on the latter observations and the distribution in the [HV, (V−I)] reduced proper-motion diagram, we have classified stars in the complete sample as candidate white dwarfs, main-sequence dwarfs and halo subdwarfs, and derived estimates of the disc and halo luminosity functions.

We present JHKs photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the latest L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Analysis of these new discoveries shows that H-alpha emission has yet to be convincingly detected in any L dwarf later than type L4.5, indicating a decline or absence of chromospheric activity in the latest L dwarfs. Further analysis shows that 16 (and possibly 4 more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until roughly type L6.5 V then declines for later types. This disappearance may be the first sign of depletion of atomic lithium as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements, we develop spectrophotometric relations to estimate distances to the other L dwarfs. Of the 67 new discoveries, 21 have photometric distances placing them within 25 parsecs of the Sun. A table of all known L and T dwarfs believed to lie within 25 parsecs - 53 in total - is also presented. Using the distance measurement of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition objects have yet been uncovered. This evidence, combined with model fits to low-resolution spectra of late-M and early-L dwarfs, indicates that L dwarfs span the range 1300K < Teff < 2000K.

We present the results of a Hubble Space Telescope snapshot program to search for very low mass stellar companions to nearby M subdwarfs. None of our nine targetted metal‐poor primaries has companions more massive than the hydrogen burning limit, implying that the halo binary fraction is equal to or less than the Galactic disk binary fraction below 0.3 M⊙. In addition, the more distant tertiary VB 12, an sdM3.0 companion to an F subdwarf double, is also unresolved. We show that the relation between WFPC2 F555W and F850LP photometry and ground‐based V, I photometry is consistent with theoretical expectations. We also report that two recently observed Hyades M dwarfs appear single.

We report the detection of H{$\alpha$} emission in the T dwarf (methane brown dwarf) 2MASSW J1237392+652615 over three days using the Keck Low Resolution Imaging Spectrograph. The measured line flux, log(L$_{H{\alpha}}$/L$_{bol}$) = $-$4.3, is roughly consistent with early M dwarf activity levels and inconsistent with decreasing activity trends in late M and L dwarfs. Similar emission is not seen in two other T dwarfs. We speculate on several mechanisms that may be responsible for emission, including a strong magnetic field, continuous flaring, acoustic heat generation, and a close ($a$ $\sim$ 4 - 20 R$_{Jup}$) interacting binary, with the cooler component overflowing its Roche lobe. We suggest that the M9.5Ve PC 0025+0447 could be a warm analogue to 2MASS J1237+65, and may be powered by the latter mechanism., Comment: 14 pages, 3 figures, accepted by AJ

We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000\AA in the L0 dwarf 2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I 4227\AA and Na I 5890/5896\AA are extremely strong, with the latter having an equivalent width of 240\AA in the L4 dwarf. By spectral type L5, the D lines extend over $\sim1000$\AA and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband colour. The KI resonance doublet at 7665/7699\AA increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres., Comment: to appear in AJ, January 2000; 27 pages, including 3 tables and 7 figures embedded in the text

We report the discovery of four field methane ("T"-type) brown dwarfs using Two Micron All-Sky Survey (2MASS) data. One additional methane dwarf, previously discovered by the Sloan Digital Sky Survey, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 μm CH_4 absorption bands characteristic of objects with T_(eff) ≾ 1300 K as well as broadened H_2O bands at 1.4 and 1.9 μm. Comparing the spectra of these objects with that of Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, 2MASS J1225-27, 2MASS J1047+21, and 2MASS J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs deg^(-2), or ~90 warm T dwarfs over the whole sky detectable to J < 16. The resulting space density upper limit, 0.01 T dwarfs pc^(-3), is comparable to that of the first L dwarf sample from Kirkpatrick et al.

▪ Abstract The completion and publication of the Hipparcos astrometric catalogue has revitalized studies in many fundamental areas of Galactic structure and stellar evolution. This article reviews the impact of the new parallax results on our understanding of the location of the main-sequence as a function of abundance, of the luminosity calibration of primary distance indicators and of the Galactic distance scale. Many of these issues remain to be resolved.

Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 sq. deg. of actual 2MASS survey data, we have identified another twenty such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W.M. Keck Observatory.

2MASSW J0149090+295613 (hereafter 2M0149) is unique among the very late M/L dwarf counterparts to Two Micron All Sky Survey (2MASS) point sources in having shown, in the first set of spectra taken on 1997 December 7, a diverse emission-line spectrum at red wavelengths, which features an Hα equivalent width of 300 A. On four repeat observations, however, the spectrum was that of a more ordinary dMe object, albeit with a late spectral type of M9.5 V. Our interpretation is that the object underwent an extreme magnetic flare event. The continuum flux at the short-wavelength end of the spectrum was also several times stronger during the apparent flare than in quiescence. Comparison is made with midtype M dwarfs of higher mass that have undergone strong flares, and with another known M9.5 emission-line object PC 0025+0047. The 2MASS object adds to the evidence that magnetic dynamo eruptions may release more energy relative to Lbol as the stellar mass decreases; if the total flare luminosity scales with Hα in the same way as a well-observed event of the middle M dwarf AD Leo, 2M0149 may have had a flare luminosity that approached or exceeded its quiescent Lbol during the brief impulse phase. In contrast, its quiescent state shows an unremarkable level of chromospheric activity, which leads us to question whether it is unique at all; rather, it may be that many of the ultracool M and L dwarfs found by the Deep Near-Infrared Survey (DENIS) and 2MASS undergo similar flares.

We have obtained photometric and spectroscopic observations of low-mass stars in the young cluster NGC 2516 and the older cluster NGC 3680. Our observations extend the membership surveys approximately 6 mag fainter than previous studies in these clusters. Using fiducial main sequences representative of the cluster metallicities, we present candidate membership lists and use these to investigate the magnetic activity and luminosity functions in the clusters. NGC 2516 has many active M dwarfs and their properties are similar to those in other young clusters. The dMe stars tend to lie above/red of the fiducial main sequence in an MV versus V-I color-magnitude diagram. Using spectroscopic observations of the CaOH band at 6230 ?, we show that CaOH is shallower in dMe stars compared with dM stars at the same color. The absolute magnitudes of the dMe stars and dM stars can be brought into agreement when CaOH is used as the temperature indicator, which indicates that the magnetic activity is affecting the V-I color in these stars. The activity strength, measured by log (LH?/Lbol), increases toward lower mass stars, in agreement with X-ray and chromospheric results in other young clusters. A few anomalous stars with low activity strength were found, in contrast to other young clusters. Comparison with a recent X-ray survey of NGC 2516 revealed very few low-mass stars with X-ray emission, indicating that low-mass cluster stars probably cannot explain the many X-ray sources without optical counterparts in the survey. The luminosity function for NGC 2516 shows that the cluster is quite extended on the sky, and mass segregation and preferential evaporation of certain low-mass stars are probably occurring. The NGC 3680 observations are consistent with a truncation of the cluster at ~1 M? (MV 5?6) stars. The absence of low-mass members indicates extensive dynamical evolution or a deviation from a typical initial mass function for this cluster.

We have used both the Low-Resolution Imaging Spectrograph and the HIRES echelle spectrograph on the Keck telescopes to obtain spectra of twelve candidate members of the Hyades cluster identified by Leggett and Hawkins (1988, 1989). All of the objects are chromospherically-active, late-type M-dwarfs, with H$\alpha$ equivalent widths varying from 1 to 30\AA. Based on our measured radial velocities, the level of stellar activity and other spectroscopic features, only one of the twelve stars has properties consistent with cluster membership. We consider how this result affects estimates of the luminosity and mass function of the Hyades cluster. Five of the eleven field stars have weak K I 7665/7699\AA and CaH absorption as compared with M-dwarf standards of the same spectral type, suggesting a lower surface gravity. Two of these sources, LH0416+14 and LH0419+15, exhibit significant lithium 6708 \AA absorption. Based partly on parallax measurements by the US Naval Observatory (Harris et al, 1998), we identify all five as likely to be young, pre-main sequence objects in or near the Taurus-Auriga association at distances of between 150 and 250 parsecs. A comparison with theoretical models of pre-main sequence stars indicates masses of less than 0.05 M$_\odot$., Comment: to appear in AJ, January 1999; 34 pages, (Latex format), including 10 embedded postscript figures and two tables

We used the HIRES echelle spectrograph on the Keck telescope to search for metals in the 3700-5200 A spectra of white dwarfs with hydrogen-rich photospheres. The most popular models that account for the presence of metals are interstellar accretion and comet impact. The data are not easily reconciled with either model, but binarity might play a role. We comment on the implications for Oort-like comet clouds and certain types of planets around main-sequence stars.

The color-magnitude diagram of the lower main sequence, as measured from a volume-limited sample of nearby stars, shows an abrupt downward jump between Mv = 12 and 13. This jump indicates that the observed mass-radius relationship steepens between 0.3 and 0.2 solar masses, but theoretical models show no such effect. It is difficult to isolate the source of this disagreement: the observational mass-radius relationship relies upon transformations that may not be sufficiently accurate, while the theoretical relationship relies upon stellar models that may not be sufficiently complete, particularly in their treatment of the complex physics governing the interior equation-of-state. If the features in the observationally derived mass-radius relationship are real, their existence provides a natural explanation for the well-known gap in the orbital period distribution of cataclysmic variables. This explanation relies only upon the observed mass-radius relationship of low-mass stars, and does not require ad hoc changes in magnetic braking or in the structure of cataclysmic variable secondaries. If correct, it will allow broader application of cataclysmic variable observations to problems of basic stellar physics., Comment: 27 pages, 10 postscript figures, AASTeX (aaspp4), accepted by ApJ

Condensate clouds strongly impact the spectra of brown dwarfs and exoplanets. Recent discoveries of variable L/T transition dwarfs argued for patchy clouds in at least some ultracool atmospheres. This study aims to measure the frequency and level of spectral variability in brown dwarfs and to search for correlations with spectral type. We used HST/WFC3 to obtain spectroscopic time series for 22 brown dwarfs of spectral types ranging from L5 to T6 at 1.1-1.7 $��$m for $\approx$40 min per object. Using Bayesian analysis, we find 6 brown dwarfs with confident $(p>95\%)$ variability in the relative flux in at least one wavelength region at sub-percent precision, and 5 brown dwarfs with tentative $(p>68\%)$ variability. We derive a minimum variability fraction $f_{min}=27^{+11}_{-7}\%$ over all covered spectral types. The fraction of variables is equal within errors for mid L, late L and mid T spectral types; for early T dwarfs we do not find any confident variable but the sample is too small to derive meaningful limits. For some objects, the variability occurs primarily in the flux peak in the J or H band, others are variable throughout the spectrum or only in specific absorption regions. Four sources may have broad-band peak-to-peak amplitudes exceeding 1%. Our measurements are not sensitive to very long periods, inclinations near pole-on and rotationally symmetric heterogeneity. The detection statistics are consistent with most brown dwarf photospheres being patchy. While multiple-percent near-infrared variability may be rare and confined to the L/T transition, low-level heterogeneities are a frequent characteristic of brown dwarf atmospheres., Accepted for publication in ApJ, 16 pages, plus 25 pages appendix

We have completed a high-contrast direct imaging survey for giant planets around 57 debris disk stars as part of the Gemini NICI Planet-Finding Campaign. We achieved median H-band contrasts of 12.4 mag at 0.5" and 14.1 mag at 1" separation. Follow-up observations of the 66 candidates with projected separation < 500 AU show that all of them are background objects. To establish statistical constraints on the underlying giant planet population based on our imaging data, we have developed a new Bayesian formalism that incorporates (1) non-detections, (2) single-epoch candidates, (3) astrometric and (4) photometric information, and (5) the possibility of multiple planets per star to constrain the planet population. Our formalism allows us to include in our analysis the previously known Beta Pictoris and the HR 8799 planets. Our results show at 95% confidence that 5MJup planet beyond 80 AU, and 3MJup planet outside of 40 AU, based on hot-start evolutionary models. We model the population of directly-imaged planets as d^2N/dMda ~ m^alpha a^beta, where m is planet mass and a is orbital semi-major axis (with a maximum value of amax). We find that beta < -0.8 and/or alpha > 1.7. Likewise, we find that beta < -0.8 and/or amax < 200 AU. If we ignore the Beta Pic and HR 8799 planets (should they belong to a rare and distinct group), we find that < 20% of debris disk stars have a > 3MJup planet beyond 10 AU, and beta < -0.8 and/or alpha < -1.5. Our Bayesian constraints are not strong enough to reveal any dependence of the planet frequency on stellar host mass. Studies of transition disks have suggested that about 20% of stars are undergoing planet formation; our non-detections at large separations show that planets with orbital separation > 40 AU and planet masses > 3 MJup do not carve the central holes in these disks., Accepted to ApJ on June 24, 2013. 67 pages, 17 figures, 12 tables

We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around high-mass stars (~1.5-2.5 M_sun) conducted to date and includes the planet hosts beta Pic and Fomalhaut. We obtained follow-up astrometry of all candidate companions within 400 AU projected separation for stars in uncrowded fields and identified new low-mass companions to HD 1160 and HIP 79797. We have found that the previously known young brown dwarf companion to HIP 79797 is itself a tight (3 AU) binary, composed of brown dwarfs with masses 58 (+21, -20) M_Jup and 55 (+20, -19) M_Jup, making this system one of the rare substellar binaries in orbit around a star. Considering the contrast limits of our NICI data and the fact that we did not detect any planets, we use high-fidelity Monte Carlo simulations to show that fewer than 20% of 2 M_sun stars can have giant planets greater than 4 M_Jup between 59 and 460 AU at 95% confidence, and fewer than 10% of these stars can have a planet more massive than 10 M_Jup between 38 and 650 AU. Overall, we find that large-separation giant planets are not common around B and A stars: fewer than 10% of B and A stars can have an analog to the HR 8799 b (7 M_Jup, 68 AU) planet at 95% confidence. We also describe a new Bayesian technique for determining the ages of field B and A stars from photometry and theoretical isochrones. Our method produces more plausible ages for high-mass stars than previous age-dating techniques, which tend to underestimate stellar ages and their uncertainties., Comment: 84 pages, 26 figures, 10 tables. ApJ in press

We have conducted a kinematic study of 165 young M dwarfs with ages of, Accepted for publication in the Astrophysical Journal