Browsing by Subject "brown dwarfs"
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Item Accretion Onto Planetary Mass Companions of Low-Mass Young Stars(2014-03) Zhou, Yifan; Herczeg, Gregory J.; Kraus, Adam L.; Metchev, Stanimir; Cruz, Kelle L.; Kraus, Adam L.Measurements of accretion rates onto planetary mass objects may distinguish between different planet Formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC 06214-00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10(-9)-10(-11) M-circle dot yr(-1) for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the Formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of Ha luminosity over accretion luminosity for objects with low accretion rates suggest that searches for Ha emission may be an efficient way to find accreting planets.Item An Alma Constraint on the GSC 6214-210 B Circum-Substellar Accretion Disk Mass(2015-06) Bowler, Brendan P.; Andrews, Sean M.; Kraus, Adam L.; Ireland, Michael J.; Herczeg, Gregory; Ricci, Luca; Carpenter, John; Brown, Michael E.; Kraus, Adam L.We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of GSC 6214-210 A and B, a solar-mass member of the 5-10 Myr Upper Scorpius association with a 15 +/- 2 M-Jup companion orbiting at approximate to 330 AU (2 ''.2). Previous photometry and spectroscopy spanning 0.3-5 mu m revealed optical and thermal excess as well as strong H alpha and Pa beta emission originating from a circum-substellar accretion disk around GSC 6214-210 B, making it the lowest-mass companion with unambiguous evidence of a subdisk. Despite ALMA's unprecedented sensitivity and angular resolution, neither component was detected in our 880 mu m (341 GHz) continuum observations down to a 3 sigma limit of 0.22 mJy/beam. The corresponding constraints on the dust mass and total mass are <0.15M(circle plus) and <0.05 M-Jup, respectively, or <0.003% and <0.3% of the mass of GSC 6214-210 B itself assuming a 100:1 gas-to-dust ratio and characteristic dust temperature of 10-20 K. If the host star possesses a putative circum-stellar disk then at most it is a meager 0.0015% of the primary mass, implying that giant planet Formation has certainly ceased in this system. Considering these limits and its current accretion rate, GSC 6214210 B appears to be at the end stages of assembly and is not expected to gain any appreciable mass over the next few megayears.Item Characterizing The Cool KOIs. VII. Refined Physical Properties Of The Transiting Brown Dwarf LHS 6343 C(2015-02) Montet, Benjamin T.; Johnson, John A.; Muirhead, Phillip S.; Villar, Ashley; Vassallo, Corinne; Baranec, Christoph; Law, Nicholas M.; Riddle, Reed; Marcy, Geoffrey W.; Howard, Andrew W.; Isaacson, Howard; Vassallo, CorinneWe present an updated analysis of LHS 6343, a triple system in the Kepler field which consists of a brown dwarf transiting one member of a widely separated M+M binary system. By analyzing the full Kepler data set and 34 Keck/HIgh Resolution Echelle Spectrometer radial velocity observations, we measure both the observed transit depth and Doppler semiamplitude to 0.5% precision. With Robo-AO and Palomar/PHARO adaptive optics imaging as well as TripleSpec spectroscopy, we measure a model-dependent mass for LHS 6343 C of 62.1 +/- 1.2 M-Jup and a radius of 0.783 +/- 0.011 R-Jup. We detect the secondary eclipse in the Kepler data at 3.5 sigma, measuring e cos omega = 0.0228 +/- 0.0008. We also derive a method to measure the mass and radius of a star and transiting companion directly, without any direct reliance on stellar models. The mass and radius of both objects depend only on the orbital period, stellar density, reduced semimajor axis, Doppler semiamplitude, eccentricity, and inclination, as well as the knowledge that the primary star falls on the main sequence. With this method, we calculate a mass and radius for LHS 6343 C to a precision of 3% and 2%, respectively.Item Discovery Of A Low-Luminosity, Tight Substellar Binary At The T/Y Transition(2015-04) Dupuy, Trent J.; Liu, Michael C.; Leggett, S. K.; Dupuy, Trent J.We have discovered that the brown dwarf WISE J014656.66+423410.0 is a close binary (0 ''.0875 +/- 0 ''.0021, 0.93(-0.16)(+0.12) AU) from Keck laser guide star adaptive optics imaging. Our photometry for this system reveals that both components are less luminous than those in any known substellar binary. Combining a new integrated-light spectrum (T9p) and resolved YJH-band photometry from Keck allows us to perform spectral decomposition and assign component types of T9 and Y0. Many of the unusual features in the spectrum might be explained by high surface gravity: Y-band peak broadened to the blue; J-band peak broadened to the red; H-band peak shifted slightly to the red; and red Y - J colors. Interestingly, the very low component luminosities imply that the T9 primary is unexpectedly cold (T-eff = 345 +/- 45 K assuming an age of 10 Gyr), making it approximate to 100 K cooler than any other late-T dwarf and comparable to Y dwarfs. One intriguing explanation for this apparent discrepancy is that the J- and H-band spectral features that trigger the transition from T to Y spectral types are highly gravity dependent. This can be tested directly in the very near future by orbit monitoring. We constrain the orbital period to be less than or similar to 10 yr by combining evolutionary-model-based mass estimates for the components (approximate to 12-21M(Jup), 1 sigma at 10 Gyr) with a statistical constraint on the semimajor axis (less than or similar to 1.3 AU). Such a period is shorter than any other known T/Y transition binary, meaning that WISE J0146+4234AB will likely yield a dynamical mass within the next few years.Item Fragmentation And Evolution Of Molecular Clouds. III. The Effect Of Dust And Gas Energetics(2012-09) Martel, Hugo; Urban, Andrea; Evans, Neal J.; Martel, Hugo; Evans, Neal J.Dust and gas energetics are incorporated into a cluster-scale simulation of star formation in order to study the effect of heating and cooling on the star formation process. We build on our previous work by calculating separately the dust and gas temperatures. The dust temperature is set by radiative equilibrium between heating by embedded stars and radiation from dust. The gas temperature is determined using an energy-rate balance algorithm which includes molecular cooling, dust-gas collisional energy transfer, and cosmic-ray ionization. The fragmentation proceeds roughly similarly to simulations in which the gas temperature is set to the dust temperature, but there are differences. The structure of regions around sink particles has properties similar to those of Class 0 objects, but the infall speeds and mass accretion rates are, on average, higher than those seen for regions forming only low-mass stars. The gas and dust temperature have complex distributions not well modeled by approximations that ignore the detailed thermal physics. There is no simple relationship between density and kinetic temperature. In particular, high-density regions have a large range of temperatures, determined by their location relative to heating sources. The total luminosity underestimates the star formation rate at these early stages, before ionizing sources are included, by an order of magnitude. As predicted in our previous work, a larger number of intermediate-mass objects form when improved thermal physics is included, but the resulting initial mass function (IMF) still has too few low-mass stars. However, if we consider recent evidence on core-to-star efficiencies, the match to the IMF is improved.Item Herschel Photometry Of Disks Around Low-Mass Stars In The R Cra Cloud(2014-11) Harvey, Paul M.; Henning, Thomas; Liu, Yao; Wolf, Sebastian; Harvey, Paul M.We report photometric results from a subset of a Herschel-PACS program to observe cool dust in disks around low-mass stars as a complement to our earlier program to measure far-infrared emission from brown dwarfs. In this latest study we observed five low-mass objects in the nearby R Corona Australis region and detected at least three at 70 mu m. Using a Monte Carlo radiative transfer code we have investigated the disk masses and geometry based on detailed spectral energy distribution (SED) modeling, and we compare these new results to those from our earlier larger sample of brown dwarfs. In particular, our SED analysis for these five objects shows again that disk geometries of brown dwarfs or low-mass stars are generally similar to their higher mass counterparts like T Tauri disks, but the range of disk mass extends to well below the value found in T Tauri stars.Item A Herschel Search for Cold Dust in Brown Dwarf Disks: First Results(2012-01) Harvey, Paul M.; Henning, Thomas; Menard, Francois; Wolf, Sebastian; Liu, Yao; Cieza, Lucas A.; Evans, Neal J., II; Pascucci, Ilaria; Merin, Bruno; Pinte, Christophe; Harvey, Paul M.; Evans, Neal J., IIWe report initial results from a Herschel program to search for far-infrared emission from cold dust around a statistically significant sample of young brown dwarfs. The first three objects in our survey are all detected at 70 mu m, and we report the first detection of a brown dwarf at 160 mu m. The flux densities are consistent with the presence of substantial amounts of cold dust in the outer disks around these objects. We modeled the spectral energy distributions (SEDs) with two different radiative transfer codes. We find that a broad range of model parameters provide a reasonable fit to the SEDs, but that the addition of our 70 mu m, and especially the 160 mu m, detection enables strong lower limits to be placed on the disk masses since most of the mass is in the outer disk. We find likely disk masses in the range of a few x 10(-6) to 10(-4) M(circle dot). Our models provide a good fit to the SEDs and do not require dust settling.Item A Herschel Survey Of Cold Dust In Disks Around Brown Dwarfs And Low-Mass Stars(2012-08) Harvey, Paul M.; Henning, Thomas; Liu, Yao; Menard, Francois; Pinte, Christopher; Wolf, Sebastian; Cieza, Lucas A.; Evans, Neal J.; Pascucci, Ilaria; Harvey, Paul M.We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs (BDs). We surveyed 50 fields containing 51 known or suspected BDs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70 mu m and 14 at 160 mu m with signal-to-noise ratio (S/N) greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]-[70] mu m colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 10(-6) M-circle dot up to 10(-3) M-circle dot with a median disk mass of the order of 3 x 10(-5) M-circle dot, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young BDs and low-mass stars are located span a range in estimated age from similar to 1-3 Myr to similar to 10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.Item Herschel/PACS View Of Disks Around Low-Mass Stars And Brown Dwarfs In The TW Hydrae Association(2015-01) Liu, Yao; Herczeg, Gregory J.; Gong, Munan A.; Allers, Katelyn N.; Brown, Joanna M.; Kraus, Adam L.; Liu, Michael C.; Shkolnik, Evgenya L.; van Dishoeck, Ewine F.; Kraus, Adam L.We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 mu m and 100 mu m and three targets at 160 mu m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M-disk proportional to M-star) observed mainly in low-mass stars may extend down to the brown dwarf regime.Item Hubble Space Telescope Spectroscopy Of Brown Dwarfs Discovered With The Wide-Field Infrared Survey Explorer(2015-05) Schneider, Adam C.; Cushing, Michael C.; Kirkpatrick, J. Davy; Gelino, Christopher R.; Mace, Gregory N.; Wright, Edward L.; Eisenhardt, Peter R.; Skrutskie, M. F.; Griffith, Roger L.; Marsh, Kenneth A.; Mace, Gregory N.We present a sample of brown dwarfs identified with the Wide-field Infrared Survey Explorer (WISE) for which we have obtained Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-infrared grism spectroscopy. The sample (22 in total) was observed with the G141 grism covering 1.10-1.70 mu m, while 15 were also observed with the G102 grism, which covers 0.90-1.10 mu m. The additional wavelength coverage provided by the G102 grism allows us to (1) search for spectroscopic features predicted to emerge at low effective temperatures (e.g., ammonia bands) and (2) construct a smooth spectral sequence across the T/Y boundary. We find no evidence of absorption due to ammonia in the G102 spectra. Six of these brown dwarfs are new discoveries, three of which are found to have spectral types of T8 or T9. The remaining three, WISE J082507.35+280548.5 (Y0.5), WISE J120604.38+840110.6 (Y0), and WISE J235402.77+024015.0 (Y1), are the 19th, 20th, and 21st spectroscopically confirmed Y dwarfs to date. We also present HST grism spectroscopy and reevaluate the spectral types of five brown dwarfs for which spectral types have been determined previously using other instruments.Item A Long-Period Massive Planet Around HD 106515A(2012-10) Desidera, S.; Gratton, R.; Carolo, E.; Fiorenzano, A. F. M.; Endl, M.; Mesa, D.; Cecconi, M.; Claudi, R.; Cosentino, R.; Scuderi, S.; Sozzetti, A.; Zurlo, A.; Endl, M.We have performed radial velocity (RV) monitoring of the components of the binary system HD 106515 over almost 11 years using the high-resolution spectrograph SARG at Telescopio Nazionale Galileo (TNG). The primary shows long-period radial velocity variations that indicate the presence of a low-mass companion whose projected mass is in the planetary regime (msin i = 9.33 M-J). The 9.8 year orbit is quite eccentric (e = 0.57), as is typical for massive giant planets. Our results confirm the previously made preliminary announcement of the planet by Mayor et al. (2011, A&A, submitted [arXiv:1109.2497]). The secondary instead does not show significant RV variations. The two components do not differ significantly in chemical composition, as was also found for other pairs of which one component hosts giant planets. Adaptive optics images obtained with TNG/AdOpt do not reveal additional stellar companions. From the analysis of the relative astrometry of the components of the wide pair we compute an upper limit on the mass of the newly detected companion of about 0.25 M-circle dot. State-of-the-art or near-future instrumentation can provide true mass determination, thanks to the availability of the wide companion HD106515B as reference. Therefore, HD 106515Ab will allow a deeper insight into the transition region between planets and brown dwarfs.Item The Mass Of Hd 38529c From Hubble Space Telescope Astrometry And High-Precision Radial Velocities(2010-05) Benedict, G. Fritz; McArthur, Barbara E.; Bean, Jacob L.; Barnes, Rory; Harrison, Thomas E.; Hatzes, Artie; Martioli, Eder; Nelan, Edmund P.; Benedict, G. Fritz; McArthur, Barbara E.Hubble Space Telescope Fine Guidance Sensor astrometric observations of the G4 IV star HD 38529 are combined with the results of the analysis of extensive ground-based radial velocity (RV) data to determine the mass of the outermost of two previously known companions. Our new RVs obtained with the Hobby-Eberly Telescope and velocities from the Carnegie-California group now span over 11 yr. With these data we obtain improved RV orbital elements for both the inner companion, HD 38529b, and the outer companion, HD 38529c. We identify a rotational period of HD 38529 (P(rot) = 31.65 +/- 0(d).17) with Fine Guidance Sensor photometry. The inferred star spot fraction is consistent with the remaining scatter in velocities being caused by spot-related stellar activity. We then model the combined astrometric and RV measurements to obtain the parallax, proper motion, perturbation period, perturbation inclination, and perturbation size due to HD 38529c. For HD 38529c we find P = 2136.1 +/- 0.3 d, perturbation semimajor axis alpha = 1.05 +/- 0.06 mas, and inclination i = 48 degrees.3 +/- 3 degrees.7. Assuming a primary mass M(*) = 1.48 M(circle dot), we obtain a companion mass M(c) = 17.6(1.2)(+1.5) M(Jup), 3 sigma above a 13 M(Jup) deuterium burning, brown dwarf lower limit. Dynamical simulations incorporating this accurate mass for HD 38529c indicate that a near-Saturn mass planet could exist between the two known companions. We find weak evidence of an additional low amplitude signal that can be modeled as a planetary-mass (similar to 0.17 M(Jup)) companion at P similar to 194 days. Including this component in our modeling lowers the error of the mass determined for HD 38529c. Additional observations (RVs and/or Gaia astrometry) are required to validate an interpretation of HD 38529d as a planetary-mass companion. If confirmed, the resulting HD 38529 planetary system may be an example of a >Packed Planetary System.>Item The Mass Of The Candidate Exoplanet Companion To HD 136118 From Hubble Space Telescope Astrometry And High-Precision Radial Velocities(2010-01) Martioli, Elder; McArthur, Barbara E.; Benedict, G. Fritz; Bean, Jacob L.; Harrison, Thomas E.; Armstrong, Amber; McArthur, Barbara E.; Benedict, G. Fritz; Armstrong, AmberWe use Hubble Space Telescope fine guidance sensor astrometry and high-cadence radial velocities for HD 136118 from the Hobby-Eberly Telescope with archival data from Lick to determine the complete set of orbital parameters for HD 136118 b. We find an orbital inclination for the candidate exoplanet of i(b) = 163 degrees.1 +/- 3 degrees.0. This establishes the actual mass of the object, M(b) = 42(-18)(+11) M(J), in contrast to the minimum mass determined from the radial velocity data only, M(b) sin i similar to 12 M(J). Therefore, the low-mass companion to HD 136118 is now identified as a likely brown dwarf residing in the "brown dwarf desert."Item The Mass-Luminosity Relation In The L/T Transition: Individual Dynamical Masses For The New J-Band Flux Reversal Binary SDSS J105213.51+442255.7Ab(2015-05) Dupuy, Trent J.; Liu, Michael C.; Leggett, S. K.; Ireland, Michael J.; Chiu, Kuenley; Golimowski, David A.; Dupuy, Trent J.We have discovered that SDSS J105213.51+442255.7 (T0.5 +/- 1.0) is a binary in Keck laser guide star adaptive optics imaging, displaying a large J- to K-band flux reversal (Delta J = -0.45 +/- 0.09 mag, Delta K= 0.52 +/- 0.05 mag). We determine a total dynamical mass from Keck orbital monitoring (88 +/- 5 M-Jup) and a mass ratio by measuring the photocenter orbit from CFHT/WIRCam absolute astrometry (M-B/M-A= 0.78 +/- 0.07). Combining these provides the first individual dynamical masses for any field L or T dwarfs, 49 +/- 3 M-Jup for the L6.5 +/- 1.5 primary and 39 +/- 3 M-Jup for the T1.5 +/- 1.0 secondary. Such a low mass ratio for a nearly equal luminosity binary implies a shallow mass-luminosity relation over the L/T transition (Delta log L-bol/Delta log M = 0.6(-0.8)(+0.6)). This provides the first observational support that cloud dispersal plays a significant role in the luminosity evolution of substellar objects. Fully cloudy models fail our coevality test for this binary, giving ages for the two components that disagree by 0.2 dex (2.0 sigma). In contrast, our observed masses and luminosities can be reproduced at a single age by "hybrid" evolutionary tracks where a smooth change from a cloudy to cloudless photosphere around 1300 K causes slowing of luminosity evolution. Remarkably, such models also match our observed JHK flux ratios and colors well. Overall, it seems that the distinguishing features SDSS J1052+ 4422AB, like a J-band flux reversal and high-amplitude variability, are normal for a field L/T binary caught during the process of cloud dispersal, given that the age (1.11(-0.20)(+0.17) Gyr) and surface gravity (log g = 5.0-5.2) of SDSS J1052+ 4422AB are typical for field ultracool dwarfs.Item The Mass-Radius Relation Of Young Stars. I. Usco 5, An M4.5 Eclipsing Binary In Upper Scorpius Observed By K2(2015-07) Kraus, Adam L.; Cody, Ann Marie; Covey, Kevin R.; Rizzuto, Aaron C.; Mann, Andrew W.; Ireland, Michael J.; Kraus, Adam L.; Rizzuto, Aaron C.; Mann, Andrew W.We present the discovery that UScoCTIO 5, a known spectroscopic binary in the Upper Scorpius star-forming region (P = 34 days, M-tot sin(i) = 0.64M(circle dot)), is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2. We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M-A = 0.329 +/- 0.002 M-circle dot, R-A = 0.834 +/- 0.006 R-circle dot, M-B = 0.317 +/- 0.002 M-circle dot, and R-B = 0.810 +/- 0.006 R-circle dot. The radii are broadly consistent with pre-main-sequence ages predicted by stellar evolutionary models, but none agree to within the uncertainties. All models predict systematically incorrect masses at the 25%-50% level for the HR diagram position of these mid-M dwarfs, suggesting significant modifications to mass-dependent outcomes of star and planet formation. The form of the discrepancy for most model sets is not that they predict luminosities that are too low, but rather that they predict temperatures that are too high, suggesting that the models do not fully encompass the physics of energy transport (via convection and/or missing opacities) and/or a miscalibration of the SpT-T-eff scale. The simplest modification to the models (changing T-eff to match observations) would yield an older age for this system, in line with the recently proposed older age of Upper Scorpius (tau similar to 11 Myr).Item Nature Of Transition Circumstellar Disks. I. The Ophiuchus Molecular Cloud(2010-04) Cieza, Lucas A.; Schreiber, Matthias R.; Romero, Grisela A.; Mora, Marcelo D.; Merin, Bruno; Swift, Jonathan J.; Orellana, Mariana; Williams, Jonathan P.; Harvey, Paul M.; Evans, Neal J.; Harvey, Paul M.; Evans, Neal J.We have obtained millimeter-wavelength photometry, high-resolution optical spectroscopy, and adaptive optics near-infrared imaging for a sample of 26 Spitzer-selected transition circumstellar disks. All of our targets are located in the Ophiuchus molecular cloud (d similar to 125 pc) and have spectral energy distributions (SEDs) suggesting the presence of inner opacity holes. We use these ground-based data to estimate the disk mass, multiplicity, and accretion rate for each object in our sample in order to investigate the mechanisms potentially responsible for their inner holes. We find that transition disks are a heterogeneous group of objects, with disk masses ranging from <0.6 to 40 M(JUP) and accretion rates ranging from <10(-11) to 10(-7) M(circle dot) yr(-1), but most tend to have much lower masses and accretion rates than "full disks" (i.e., disks without opacity holes). Eight of our targets have stellar companions: six of them are binaries and the other two are triple systems. In four cases, the stellar companions are close enough to suspect they are responsible for the inferred inner holes. We find that nine of our 26 targets have low disk mass (<2.5 M(JUP)) and negligible accretion (<10(-11) M(circle dot) yr(-1)), and are thus consistent with photoevaporating (or photoevaporated) disks. Four of these nine non-accreting objects have fractional disk luminosities <10(-3) and could already be in a debris disk stage. Seventeen of our transition disks are accreting. Thirteen of these accreting objects are consistent with grain growth. The remaining four accreting objects have SEDs suggesting the presence of sharp inner holes, and thus are excellent candidates for harboring giant planets.Item New Evidence For A Substellar Luminosity Problem: Dynamical Mass For The Brown Dwarf Binary Gl 417BC(2014-08) Dupuy, Trent J.; Liu, Michael C.; Ireland, Michael J.; Dupuy, Trent J.We present new evidence for a problem with cooling rates predicted by substellar evolutionary models that implies that model-derived masses in the literature for brown dwarfs and directly imaged planets may be too high. Based on our dynamical mass for Gl 417BC (L4.5+L6) and a gyrochronology system age from its young, solar-type host star, commonly used models predict luminosities 0.2-0.4 dex lower than we observe. This corroborates a similar luminosity-age discrepancy identified in our previous work on the L4+L4 binary HD 130948BC, which coincidentally has nearly identical component masses (approximate to 50-55 M-Jup) and age (approximate to 800 Myr) as Gl 417BC. Such a luminosity offset would cause systematic errors of 15%-25% in model-derived masses at this age. After comparing different models, including cloudless models that should not be appropriate for mid-L dwarfs like Gl 417BC and HD 130948BC but actually match their luminosities better, we speculate the observed overluminosity could be caused by opacity holes (i.e., patchy clouds) in these objects. Moreover, from hybrid substellar evolutionary models that account for cloud disappearance, we infer the corresponding phase of overluminosity may extend from a few hundred million years up to a few gigayears and cause masses to be overestimated by up to 25%, even well after clouds disappear from view entirely. Thus, the range of ages and spectral types affected by this potential systematic shift in luminosity evolution would encompass most known directly imaged gas-giants and field brown dwarfs.Item New H-2 Collision-Induced Absorption And Nh3 Opacity And The Spectra Of The Coolest Brown Dwarfs(2012-05) Saumon, Didier; Marley, Mark S.; Abel, Martin; Frommhold, Lothar; Freedman, Richard S.; Abel, Martin; Frommhold, LotharWe present new cloudy and cloudless model atmospheres for brown dwarfs using recent ab initio calculations of the line list of ammonia (NH3) and of the collision-induced absorption of molecular hydrogen (H-2). We compare the new synthetic spectra with models based on an earlier description of the H-2 and NH3 opacities. We find a significant improvement in fitting the nearly complete spectral energy distribution of the T7p dwarf Gliese 570D and in near-infrared color-magnitude diagrams of field brown dwarfs. We apply these new models to the identification of NH3 absorption in the H-band peak of very late T dwarfs and the new Y dwarfs and discuss the observed trend in the NH3-H spectral index. The new NH3 line list also allows a detailed study of the medium-resolution spectrum of the T9/T10 dwarf UGPS J072227.51-054031.2 where we identify several specific features caused by NH3.Item New X-Ray-Selected Pre-Main-Sequence Members Of The Serpens Molecular Cloud(2013-11) Oliveira, Isa; van der Laan, Margriet; Brown, Joanna M.; Oliveira, IsaThe study of young stars no longer surrounded by disks can greatly add to our understanding of how protoplanetary disks evolve and planets form. We have used VLT/FLAMES optical spectroscopy to confirm the youth and membership of 19 new young diskless stars in the Serpens Molecular Cloud, identified at X-ray wavelengths. Spectral types, effective temperatures, and stellar luminosities were determined using optical spectra and optical/near-infrared photometry. Stellar masses and ages were derived based on pre-main-sequence evolutionary tracks. The results yield remarkable similarities for age and mass distribution between the diskless and disk-bearing stellar populations in Serpens. We discuss the important implications these similarities may have on the standard picture of disk evolution.Item On The Binary Frequency Of The Lowest Mass Members Of The Pleiades With Hubble Space Telescope Wide Field Camera 3(2015-05) Garcia, E. V.; Dupuy, Trent J.; Allers, Katelyn N.; Liu, Michael C.; Deacon, Niall R.; Dupuy, Trent J.We present the results of a Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging survey of 11 of the lowest mass brown dwarfs in the Pleiades known (25-40M(Jup)). These objects represent the predecessors to T dwarfs in the field. Using a semi-empirical binary point-spread function (PSF)-fitting technique, we are able to probe to 0 ''.03 (0.75 pixel), better than 2x the WFC3/UVIS diffraction limit. We did not find any companions to our targets. From extensive testing of our PSF-fitting method on simulated binaries, we compute detection limits which rule out companions to our targets with mass ratios of greater than or similar to 0.7 and separations greater than or similar to 4 AU. Thus, our survey is the first to attain the high angular resolution needed to resolve brown dwarf binaries in the Pleiades at separations that are most common in the field population. We constrain the binary frequency over this range of separation and mass ratio of 25-40M(Jup) Pleiades brown dwarfs to be < 11% for 1 sigma (< 26% at 2s). This binary frequency is consistent with both younger and older brown dwarfs in this mass range.