Showing papers by "William T. Reach published in 2018"
University College London1, University of Cambridge2, Wesleyan University3, Arizona State University4, University of North Carolina at Chapel Hill5, University of Arizona6, Smithsonian Astrophysical Observatory7, University of Warwick8, Embry-Riddle Aeronautical University, Daytona Beach9, Space Telescope Science Institute10, Ames Research Center11
TL;DR: In this article, the infrared dust emission from the white dwarf GD 56 was found to rise and fall by 20 per cent peak-to-peak over 11.2yr, and is consistent with ongoing dust production and depletion, indicating that dust is produced and depleted within a fixed range of orbital radii.
Abstract: The infrared dust emission from the white dwarf GD 56 is found to rise and fall by 20 per cent peak-to-peak over 11.2 yr, and is consistent with ongoing dust production and depletion. It is hypothesized that the dust is produced via collisions associated with an evolving dust disc, temporarily increasing the emitting surface of warm debris, and is subsequently destroyed or assimilated within a few years. The variations are consistent with debris that does not change temperature, indicating that dust is produced and depleted within a fixed range of orbital radii. Gas produced in collisions may rapidly re-condense onto grains, or may accrete onto the white dwarf surface on viscous timescales that are considerably longer than Poynting–Robertson drag for micron-sized dust. This potential delay in mass accretion rate change is consistent with multi-epoch spectra of the unchanging Ca II and Mg II absorption features in GD 56 over 15 yr, although the sampling is sparse. Overall, these results indicate that collisions are likely to be the source of dust and gas, either inferred or observed, orbiting most or all polluted white dwarfs.
43 citations
TL;DR: In this paper, the authors study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds.
Abstract: We study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds. Maps of the clouds were constructed from WISE data and directly compared to the maps of dust optical depth from Planck. The mid-infrared emission per unit dust optical depth has a significant trend toward lower values at higher optical depths. The trend can be quantitatively explained by extinction of starlight within the clouds. The relative amounts of PAH and very small grains traced by WISE, compared to large grains tracked by Planck, are consistent with being constant. The temperature of the large grains significantly decreases for clouds with larger dust optical depth; this trend is partially due to dust property variations but is primarily due to extinction of starlight. We updated the prediction for molecular hydrogen column density, taking into account variations in dust properties, and find it can explain the observed dust optical depth per unit gas column density. Thus the low gas-to-dust ratios in the clouds are most likely due to `dark gas' that is molecular hydrogen.
13 citations
26 Dec 2018
TL;DR: The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint project between NASA and the German Aerospace Center DLR that provides access to observations of the infrared and sub-millimeter un...
Abstract: The Stratospheric Observatory for Infrared Astronomy (SOFIA), a joint project between NASA and the German Aerospace Center DLR, provides access to observations of the infrared and sub-millimeter un...
6 citations