Browsing by Subject "scorpius ob association"
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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 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 The Physical Structure Of Protoplanetary Disks: The Serpens Cluster Compared With Other Regions(2013-01) Oliveira, Isa; Merin, Bruno; Pontoppidan, Klaus M.; van Dishoeck, Ewine F.; Oliveira, IsaSpectral energy distributions are presented for 94 young stars surrounded by disks in the Serpens Molecular Cloud, based on photometry and Spitzer/IRS spectra. Most of the stars have spectroscopically determined spectral types. Taking a distance to the cloud of 415 pc rather than 259 pc, the distribution of ages is shifted to lower values, in the 1-3Myr range, with a tail up to 10 Myr. The mass distribution spans 0.2-1.2 M-circle dot, with median mass of 0.7 M-circle dot. The distribution of fractional disk luminosities in Serpens resembles that of the young Taurus Molecular Cloud, with most disks consistent with optically thick, passively irradiated disks in a variety of disk geometries (L-disk/L-star similar to 0.1). In contrast, the distributions for the older Upper Scorpius and. Chamaeleontis clusters are dominated by optically thin lower luminosity disks (L-disk/L-star similar to 0.02). This evolution in fractional disk luminosities is concurrent with that of disk fractions: with time disks become fainter and the disk fractions decrease. The actively accreting and non-accreting stars (based on H alpha data) in Serpens show very similar distributions in fractional disk luminosities, differing only in the brighter tail dominated by strongly accreting stars. In contrast with a sample of Herbig Ae/Be stars, the T Tauri stars in Serpens do not have a clear separation in fractional disk luminosities for different disk geometries: both flared and flat disks present wider, overlapping distributions. This result is consistent with previous suggestions of a faster evolution for disks around Herbig Ae/Be stars. Furthermore, the results for the mineralogy of the dust in the disk surface (grain sizes, temperatures and crystallinity fractions, as derived from Spitzer/IRS spectra) do not show any correlation to either stellar and disk characteristics or mean cluster age in the 1-10 Myr range probed here. A possible explanation for the lack of correlation is that the processes affecting the dust within disks have short timescales, happening repeatedly, making it difficult to distinguish long-lasting evolutionary effects.