We present the first resolved images of the debris disk around the nearby K dwarf HD 92945 , obtained with the Hubble Space Telescope ’ 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 \farcs 0–3 \farcs 0 ( 43–65 AU ) from the star and an extended outer disk whose surface brightness declines slowly with increasing radius approximately 3 \farcs 0–5 \farcs 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 \lesssim 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 \sim 0.001 ~ { } M _ { \earth } .
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 .