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Published July 2011 | Published
Journal Article Open

Hubble and Spitzer Space Telescope Observations of the Debris Disk around the nearby K Dwarf HD 92945


We present the first resolved images of the debris disk around the nearby K dwarf HD 92945, obtained with the Hubble Space Telescope's (HST 's) Advanced Camera for Surveys. Our F606W (Broad V) and F814W (Broad I) coronagraphic images reveal an inclined, axisymmetric disk consisting of an inner ring about 2".0-3".0 (43-65 AU) from the star and an extended outer disk whose surface brightness declines slowly with increasing radius approximately 3".0-5".1 (65-110 AU) from the star. A precipitous drop in the surface brightness beyond 110 AU suggests that the outer disk is truncated at that distance. The radial surface-density profile is peaked at both the inner ring and the outer edge of the disk. The dust in the outer disk scatters neutrally but isotropically, and it has a low V-band albedo of 0.1. This combination of axisymmetry, ringed and extended morphology, and isotropic neutral scattering is unique among the 16 debris disks currently resolved in scattered light. We also present new infrared photometry and spectra of HD 92945 obtained with the Spitzer Space Telescope's Multiband Imaging Photometer and InfraRed Spectrograph. These data reveal no infrared excess from the disk shortward of 30 μm and constrain the width of the 70 μm source to ≾180 AU. Assuming that the dust comprises compact grains of astronomical silicate with a surface-density profile described by our scattered-light model of the disk, we successfully model the 24-350 μm emission with a minimum grain size of a_(min) = 4.5 μm and a size distribution proportional to a^(–3.7) throughout the disk, but with maximum grain sizes of 900 μm in the inner ring and 50 μm in the outer disk. Together, our HST and Spitzer observations indicate a total dust mass of ~0.001M _⊕. However, our observations provide contradictory evidence of the dust's physical characteristics: its neutral V-I color and lack of 24 μm emission imply grains larger than a few microns, but its isotropic scattering and low albedo suggest a large population of submicron-sized grains. If grains smaller than a few microns are absent, then stellar radiation pressure may be the cause only if the dust is composed of highly absorptive materials like graphite. The dynamical causes of the sharply edged inner ring and outer disk are unclear, but recent models of dust creation and transport in the presence of migrating planets support the notion that the disk indicates an advanced state of planet formation around HD 92945.

Additional Information

© 2011 American Astronomical Society. Received 2011 March 18; accepted 2011 May 3; published 2011 June 14. Based in part on guaranteed observing time awarded by the National Aeronautics and Space Administration (NASA) to the Advanced Camera for Surveys Investigation Definition Team and the Multiband Imaging Photometer for Spitzer Instrument Team. We gratefully acknowledge Paul Smith from the University of Arizona for his assistance with MIPS SED data processing. We also thank Glenn Schneider and collaborators for sharing the results of their NICMOS observations of HD 92945 prior to publication. ACS was developed under NASA contract NAS 5-32865, and this research has been supported by NASA grant NAG5-7697 to the ACS Investigation Definition Team. Additional support for John Krist and Karl Stapelfeldt was provided by NASA through grants HST-GO-10539 and HST-GO-10854. This research was partially supported by NASA through JPL/Caltech contract 1255094 to the University of Arizona. It made use of Tiny Tim/Spitzer, developed by John Krist for the Spitzer Science Center, which is managed by Caltech under a contract with NASA. The Space Telescope Science Institute is operated by AURA Inc., under NASA contract NAS5-26555.

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