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

We present the results of a deep-imaging search for wide companions to low-mass stars and brown dwarfs using NSFCam on 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 $0.03 M_{\odot}$.Comment: 27 pages, 8 figures, 3 tables: accepted to the Astronomical Journa

Keck Imaging of Binary L Dwarfs

We present Keck near-infrared imaging of three binary L dwarf systems, all of which are likely to be sub-stellar. Two are lithium dwarfs, and a third exhibits an L7 spectral type, making it the coolest binary known to date. All have component flux ratios near 1 and projected physical separations between 5 and 10 AU, assuming distances of 18 to 26 pc from recent measurements of trigonometric parallax. These surprisingly similar binaries represent the sole detections of companions in ten L dwarf systems which were analyzed in the preliminary phase of a much larger dual-epoch imaging survey. The detection rate prompts us to speculate that binary companions to L dwarfs are common, that similar-mass systems predominate, and that their distribution peaks at radial distances in accord both with M dwarf binaries and with the radial location of Jovian planets in our own solar system. To fully establish these conjectures against doubts raised by biases inherent in this small preliminary survey, however, will require quantitative analysis of a larger volume-limited sample which has been observed with high resolution and dynamic range.Comment: LaTex manuscript in 13 pages, 3 postscript figures, Accepted for publication in the Letters of the Astrophysical Journal; Postscript pre-print version available at: http://www.hep.upenn.edu/PORG/papers/koerner99a.p

A Single Circumbinary Disk in the HD 98800 Quadruple System

We present sub-arcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0.8'' (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 microns show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess upon which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has a best-fit black-body temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most-likely values of disk properties in the ranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 = 2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_in is the inner radius, DeltaR is the radial extent of the disk, lambda_0 is the effective grain size, gamma is the radial power-law exponent of the optical depth, tau, and sigma_total is the total cross-section of the grains. The range of implied disk masses is 0.001--0.1 times that of the moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.Comment: 11 page Latex manuscript with 3 postscript figures. Accepted for publication in Astrophysical Journal Letters. Postscript version of complete paper also available at http://www.hep.upenn.edu/PORG/web/papers/koerner00a.p

The Substellar Mass Function: A Bayesian Approach

We report our efforts to constrain the form of the low-mass star and brown dwarf mass function via Bayesian inference. Recent surveys of M, L, and T dwarfs in the local solar neighborhood are an essential component of our study. Uncertainties in the age distribution of local field stars make reliable inference complicated. We adopt a wide range of plausible assumptions about the rate of galactic star formation and show that their deviations from a uniform rate produce little effect on the resulting luminosity function for a given mass function. We use a Bayesian statistical formalism to evaluate the probability of commonly used mass functions in light of recent discoveries. We consider three functional forms of the mass function, include a two-segment power law, a single power law with a low-mass cutoff, and a log-normal distribution. Our results show that, at a 60% confidence level, the power-law index, $\alpha$, for the low-mass arm of a two-segment power law has a value between -0.5 and 0.5 for objects with masses between $0.04 M_{\odot}$ and $0.10 M_{\odot}$. The best-fit index is $\alpha = 0.3\pm0.6$ at the 60% confidence level for a single-segment mass function. Current data require this function extend to at least $0.05 M_{\odot}$ with no restrictions placed on a lower mass cutoff. Inferences of the parameter values for a log-normal mass function are virtually unaffected by recent estimates of the local space density of L and T dwarfs. We find no preference among these three forms using this method. We discuss current and future capabilities that may eventually discriminate between mass-function models and refine estimates of their associated parameter values.Comment: 40 pages, 15 figures, 3 tables, accepted for publication in The Astrophysical Journa

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. I. Chamaeleon II Observed with MIPS

We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70, and 160 micron observed with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) and a 1.2 square degree millimeter map from SIMBA on the Swedish-ESO Submillimetre Telescope (SEST). The c2d Spitzer Legacy Team's data reduction pipeline is described in detail. Over 1500 24 micron sources and 41 70 micron sources were detected by MIPS with fluxes greater than 10-sigma. More than 40 potential YSOs are identified with a MIPS and 2MASS color-color diagram and by their spectral indices, including two previously unknown sources with 24 micron excesses. Our new SIMBA millimeter map of Cha II shows that only a small fraction of the gas is in compact structures with high column densities. The extended emission seen by MIPS is compared with previous CO observations. Some selected interesting sources, including two detected at 1 mm, associated with Cha II are discussed in detail and their SEDs presented. The classification of these sources using MIPS data is found to be consistent with previous studies.Comment: 44 pages, 12 figures (1 color), to be published in Ap

Study of the phase transition in the 3d Ising spin glass from out of equilibrium numerical simulations

Using the decay of the out equilibrium spin-spin correlation function we compute the equilibrium Edward-Anderson order parameter in the three dimensional binary Ising spin glass in the spin glass phase. We have checked that the Edward-Anderson order parameter computed from out of equilibrium numerical simulations follows with good precision the critical law as determined in experiments and in numerical studies at equilibrium. We have also studied the dependence of the order parameter with the lattice size. Finally we present a large time study of the scaling of the off-equilibrium fluctuation-dissipation relations.Comment: 14 pages, 7 Postscript figure

HST/NICMOS Imaging of Disks and Envelopes Around Very Young Stars

We present HST/NICMOS observations with 0.1" (15 AU) resolution of six young stellar objects in the Taurus star-formation region. The targets of our survey are three Class I IRAS sources (IRAS 04016+2610, IRAS 04248+2612, and IRAS 04302+2247) and three low-luminosity stars (DG Tau B, Haro 6-5B, and CoKu Tau/1) associated with Herbig Haro jets. The broad-band images show that the near-infrared radiation from these sources is dominated by light scattered from dusty circumstellar material distributed in a region 10 - 15 times the size of our solar system. Although the detailed morphologies of the individual objects are unique, the observed young stellar objects share common features. All of the circumstellar reflection nebulae are crossed by dark lanes from 500 - 900 AU in extent and from less than 50 to 350 AU in apparent thickness. The absorption lanes extend perpendicular to known optical and millimeter outflows in these sources. We interpret the dark lanes as optically thick circumstellar disks seen in silhouette against bright reflection nebulosity. The bipolar reflection nebulae extending perpendicular to the dust lanes appear to be produced by scattering from the upper and lower surfaces of the disks and from dusty material within or on the walls of the outflow cavities. Out of five objects in which the central source is directly detected, two are found to be subarcsecond binaries. This mini-survey is the highest resolution near-infrared study to date of circumstellar environments around solar-type stars with age <= 1 Myr.Comment: 34 pages, 4 figures; also available at http://spider.ipac.caltech.edu/staff/brandner/topics/disks/disks.html ; accepted for publication in AJ (March 1999 issue

A circumstellar dust disk around T Tau N: Sub-arcsecond imaging at 3 mm

We present high-resolution imaging of the young binary T Tauri in 3 mm continuum emission. Compact dust emission with integrated flux density 50 +/- 6 mJy is resolved in an aperture synthesis map at 0.5" resolution and is centered at the position of the optically visible component, T Tau N. No emission above a 3 sigma level of 9 mJy is detected 0.7" south of T Tau N at the position of the infrared companion, T Tau S. We interpret the continuum detection as arising from a circumstellar disk around T Tau N and estimate its properties by fitting a flat-disk model to visibilities at wavelengths of 1 and 3 mm and to the flux density at 7 mm. Given the data, probability distributions are calculated for values of the free parameters, including the temperature, density, dust opacity, and the disk outer radius. The radial variation in temperature and density is not narrowly constrained by the data. The most likely value of the frequency dependence of the dust opacity, beta = 0.53^{+0.27}_{-0.17}, is consistent with that of disks around other T Tauri stars in which grain growth is believed to have taken place. The outer radius, R = 41^{+26}_{-14} AU, is smaller than the projected binary separation, and may indicate truncation of the disk. The total mass estimated for the disk, log(M/M_sun) = {-2.4}^{+0.7}_{-0.6}, is similar to masses observed around many young single sources and to the minimum nebular mass required to form a planetary system like our own. This observation strongly suggests that the presence of a binary companion does not rule out the formation of a sizeable planetary system.Comment: Accepted by the Astrophysical Journal. 13 pages Latex (uses AASTeX macros) including 3 postscript figures. Also at http://astro.berkeley.edu/~rla