Browsing by Author "Silverstone, M. D."

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    The disk and environment of a young Vega analog: HD 169142
    (IOP Publishing, 2007-08-20) Grady, C. A.; Schneider, G.; Hamaguchi, K.; Sitko, M. L.; Carpenter, W. J.; Hines, D.; Collins, K. A.; Williger, G. M.; Woodgate, B. E.; Henning, Th.; Menard, F.; Wilner, D.; Petre, R.; Palunas, P.; Quirrenbach, A.; Nuth, J. A., III; Silverstone, M. D.; Kim, J. S.; Eureka Scientific; National Aeronautics & Space Administration (NASA); NASA Goddard Space Flight Center; University of Arizona; Universities Space Research Association (USRA); University System of Ohio; University of Cincinnati; University of Louisville; Johns Hopkins University; Max Planck Society; Centre National de la Recherche Scientifique (CNRS); UDICE-French Research Universities; Communaute Universite Grenoble Alpes; Universite Grenoble Alpes (UGA); Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; University of Texas System; University of Texas Austin; Ruprecht Karls University Heidelberg; University of Alabama Tuscaloosa
    We trace the disk of HD169142 (A8 Ve) from 0.57" to 1.4" (approximate to 80-200 AU projected distance) in 1.1 mu m scattered light with HST NICMOS coronagraphy. The azimuthally symmetric disk has a peak azimuthally medianed surface brightness (SB) of approximate to 5 mJy arcsec(-2) at 0.57" from the star, and drops alpha r(-3). This radial SB profile is consistent with the presence of spatially resolved PAH emission and a Meeus group I IR SED only if the inner disk is either substantially flatter than the outer disk or partially devoid of material. Analysis of new HST ACS FUV imagery in tandem with archival IUE data indicates. M-acc <= 10(-9) M circle dot yr(-1). We estimate the age of HD 169142 to be 6(-3)(+6) Myr by identifying 2MASS 18242929-2946559, located 9.3" to the southwest, as a 130 mas separation weak-line T Tauri binary that is comoving with HD 169142 at the 4 sigma confidence level. We find no evidence for any additional stellar companion in either the ACS or Chandra ACIS-S data at r <= 1". HD 169142 has previously been interpreted as a slowly rotating, chemically peculiar star. However, by combining the disk inclination and v sin i from the literature, we find that the star has upsilon(equatorial) approximate to 240 km s(-1), making it a rapid rotator, similar to Altair or Vega. The UV data for HD 169142 are consistent with gravity darkening, while the X-ray luminosity and spectrum resembles early F stars at the age of the beta Pictoris moving group, rather than mid-A stars. In this context, spectral features previously interpreted as evidence for chemical peculiarity are more likely to reflect the presence of a strong photospheric latitudinal temperature gradient. With such a gradient, HD 169142 should closely resemble Vega at the epoch of central disk clearing.
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    Evolution of mid-infrared excess around sun-like stars: constraints on models of terrestrial planet formation
    (IOP Publishing, 2008-01-09) Meyer, M. R.; Carpenter, J. M.; Mamajek, E. E.; Hillenbrand, L. A.; Hollenbach, D.; Moro-Martin, A.; Kim, J. S.; Silverstone, M. D.; Najita, J.; Hines, D. C.; Pascucci, I.; Stauffer, J. R.; Bouwman, J.; Backman, D. E.; University of Arizona; California Institute of Technology; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; National Aeronautics & Space Administration (NASA); NASA Ames Research Center; Princeton University; National Optical Astronomy Observatory; Max Planck Society; University of Alabama Tuscaloosa
    We report observations from the Spitzer Space Telescope regarding the frequency of 24 mm excess emission toward Sun-like stars. Our unbiased sample is composed of 309 stars with masses 0.7-2.2 M-circle dot and ages from, < 3 Myr to > 3 Gyr that lack excess emission at wavelengths <= 8 mu m. We identify 30 stars that exhibit clear evidence of excess emission from the observed 24 mu m/8 mu m flux ratio. The implied 24 mu m excesses of these candidate debris disk systems range from 13% (the minimum detectable) to more than 100% compared to the expected photospheric emission. The frequency of systems with evidence for dust debris emitting at 24 mu m ranges from 8.5%-19% at ages < 300 Myr to < 4% for older stars. The results suggest that many, perhaps most, Sun-like stars might form terrestrial planets.
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    The formation and evolution of planetary systems : Grain growth and chemical processing of dust in T Tauri systems
    (IOP Publishing, 2008-08-10) Bouwman, J.; Henning, Th.; Hillenbrand, L. A.; Meyer, M. R.; Pascucci, I.; Carpenter, J.; Hines, D.; Kim, J. S.; Silverstone, M. D.; Hollenbach, D.; Wolf, S.; Max Planck Society; California Institute of Technology; University of Arizona; National Aeronautics & Space Administration (NASA); NASA Ames Research Center; University of Alabama Tuscaloosa
    This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and growth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to sizes at least 10 times larger than observed in the interstellar medium and show evidence for a non-negligible (similar to 5% in mass fractions) contribution from crystalline species. These results are similar to those of other studies of protoplanetary disks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape of the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling of dust grains toward the disk midplane. Furthermore, we find a change in the relative abundance of the different crystalline species: more enstatite than forsterite is observed in the inner warm dust population at similar to 1 AU, while forsterite dominates in the colder outer regions at similar to 5-15 AU. This change in the relative abundances argues for a localized crystallization process rather than a radial mixing scenario in which crystalline silicates are being transported outwards from a single formation region in the hot inner parts of the disk. Finally, we report the detection of emission from polycyclic aromatic hydrocarbon ( PAH) molecules in five out of seven sources. We find a tentative PAH band at 8.2 mu m that was previously undetected in the spectra of disks around low-mass pre-main-sequence stars.
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    Formation and evolution of planetary systems: Upper limits to the gas mass in disks around Sun-like stars
    (IOP Publishing, 2006-11-10) Pascucci, I.; Gorti, U.; Hollenbach, D.; Najita, J.; Meyer, M. R.; Carpenter, J. M.; Hillenbrand, L. A.; Herczeg, G. J.; Padgett, D. L.; Mamajek, E. E.; Silverstone, M. D.; Schlingman, W. M.; Kim, J. S.; Stobie, E. B.; Bouwman, J.; Wolf, S.; Rodmann, J.; Hines, D. C.; Lunine, J.; Malhotra, R.; University of Arizona; University of California System; University of California Berkeley; National Aeronautics & Space Administration (NASA); NASA Ames Research Center; National Optical Astronomy Observatory; California Institute of Technology; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; Max Planck Society; University of Alabama Tuscaloosa
    We have carried out a sensitive search for gas emission lines at IR and millimeter wavelengths for a sample of 15 young Sun-like stars selected from our dust disk survey with Spitzer. We have used mid-IR lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of (CO)-C-12 to probe the cold (similar to 20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti & Hollenbach that confirm and extend our simple analysis. These models show that the [S-I]25.23 mu m line can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04M(J) of gas within a few AU from the disk inner radius for disk radii from 1 to similar to 40 AU. These gas mass upper limits even in the eight systems younger than similar to 30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, which is mainly traced by our CO data, are < 2 M-circle plus. If these systems are analogs of the solar system, they either have already formed Uranus- and Neptune-like planets or will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.
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    Multi-wavelength modeling of the spatially resolved debris disk of HD107146
    (EDP Sciences, 2001-09-15) Ertel, S.; Wolf, S.; Metchev, S.; Schneider, G.; Carpenter, J. M.; Meyer, M. R.; Hillenbrand, L. A.; Silverstone, M. D.; University of Kiel; State University of New York (SUNY) System; State University of New York (SUNY) Stony Brook; University of Arizona; California Institute of Technology; Swiss Federal Institutes of Technology Domain; ETH Zurich; University of Alabama Tuscaloosa
    Aims. We aim to constrain the location, composition, and dynamical state of planetesimal populations and dust around the young, sun-like (G2V) star HD107146. Methods. We consider coronagraphic observations obtained with the Advanced Camera for Surveys (HST/ACS) onboard the Hubble Space Telescope (HST) in broad V (lambda(c) approximate to 0.6 mu m) and broad I (lambda(c) approximate to 0.8 mu m) filters, a resolved 1.3 mm map obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA), Spitzer/IRS low resolution spectra in the range of 7.6 mu m to 37.0 mu m, and the spectral energy distribution (SED) of the object at wavelengths ranging from 3.5 mu m to 3.1 mm. We complement these data with new coronagraphic high resolution observations of the debris disk using the Near Infrared Camera and Multi-Object Spectrometer (HST/NICMOS) aboard the HST in the F110W filter (lambda(c) approximate to 1.1 mu m). The SED and images of the disk in scattered light as well as in thermal reemission are combined in our modeling using a parameterized model for the disk density distribution and optical properties of the dust. Results. A detailed analytical model of the debris disk around HD107146 is presented that allows us to reproduce the almost entire set of spatially resolved and unresolved multi-wavelength observations. Considering the variety of complementary observational data, we are able to break the degeneracies produced by modeling SED data alone. We find the disk to be an extended ring with a peak surface density at 131 AU. Furthermore, we find evidence for an additional, inner disk probably composed of small grains released at the inner edge of the outer disk and moving inwards due to Poynting-Robertson drag. A birth ring scenario (i.e., a more or less broad ring of planetesimals creating the dust disk trough collisions) is found to be the most likely explanation of the ringlike shape of the disk.
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    STRUCTURE AND COMPOSITION OF TWO TRANSITIONAL CIRCUMSTELLAR DISKS IN CORONA AUSTRALIS
    (IOP Publishing, 2010-08-13) Hughes, A. M.; Andrews, S. M.; Wilner, D. J.; Meyer, M. R.; Carpenter, J. M.; Qi, C.; Hales, A. S.; Casassus, S.; Hogerheijde, M. R.; Mamajek, E. E.; Wolf, S.; Henning, T.; Silverstone, M. D.; Harvard University; Smithsonian Astrophysical Observatory; Smithsonian Institution; Swiss Federal Institutes of Technology Domain; ETH Zurich; California Institute of Technology; National Radio Astronomy Observatory (NRAO); Universidad de Chile; Leiden University; Leiden University - Excl LUMC; University of Rochester; University of Kiel; Max Planck Society; Eureka Scientific; University of Alabama Tuscaloosa
    The late stages of evolution of the primordial circumstellar disks surrounding young stars are poorly understood, yet vital to constraining theories of planet formation. We consider basic structural models for the disks around two similar to 10 Myr old members of the nearby RCrA association: RX J1842.9-3532 and RX J1852.3-3700. We present new arc second-resolution maps of their 230 GHz continuum emission from the Submillimeter Array and unresolved CO(3-2) spectra from the Atacama Submillimeter Telescope Experiment. By combining these data with broadband fluxes from the literature and infrared fluxes and spectra from the catalog of the Formation and Evolution of Planetary Systems Legacy program on the Spitzer Space Telescope, we assemble a multiwavelength data set probing the gas and dust disks. Using the Monte Carlo radiative transfer code RADMC to model simultaneously the spectral energy distribution and millimeter continuum visibilities, we derive basic dust disk properties and identify an inner cavity of radius 16 AU in the disk around RX J1852.3-3700. We also identify an optically thin 5 AU cavity in the disk around RX J1842.9-3532, with a small amount of optically thick material close to the star. The molecular line observations suggest an intermediate disk inclination in RX J1842.9-3532, consistent with the continuum emission. In combination with the dust models, the molecular data allow us to derive a lower CO content than expected, suggesting that the process of gas clearing is likely underway in both systems, perhaps simultaneously with planet formation.