Dust Production Rates in the Fomalhaut Debris Disk from SOFIA/FORCAST Mid-infrared Imaging

We present the first spatially resolved mid-infrared (37.1 $\mu$m) image of the Fomalhaut debris disk. We use PSF fitting and subtraction to distinctly measure the flux from the unresolved component and the debris disk. We measure an infrared excess in the point source of $0.9 \pm 0.2$ Jy, consistent with emission from warm dust in an inner disk structure (Su et al. 2016), and inconsistent with a stellar wind origin. We cannot confirm or rule out the presence of a pileup ring (Su et al. 2016) near the star. In the cold region, the 37 $\mu$m imaging is sensitive to emission from small, blowout grains, which is an excellent probe of the dust production rate from planetesimal collisions. Under the assumptions that the dust grains are icy aggregates and the debris disk is in steady state, this result is consistent with the dust production rates predicted by Kenyon & Bromley (2008) from theoretical models of icy planet formation. We find a dust luminosity of $(7.9 \pm 0.8) \times 10^{-4}$ L$_\odot$ and a dust mass of 8 -- 16 lunar masses, depending on grain porosity, with $\sim 1$ lunar mass in grains with radius 1 $\mu$m -- 1 mm. If the grains are icy and highly porous, meter-sized objects must be invoked to explain the far-IR, submm, and mm emission. If the grains are composed of astronomical silicates, there is a dearth of blowout grains (Pawellek et al. 2014) and the mass loss rate is well below the predicted dust production values.
Comment: 8 pages, 8 figures, 2 tables, accepted for publication in ApJ