A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti

Abstract

The A star 49 Ceti, at a distance of 61 pc, is unusual in retaininga substantial quantity of molecular gas while exhibiting dust propertiessimilar to those of a debris disk. We present resolved observationsof the disk around 49 Ceti from the Submillimeter Array in the J=2-1rotational transition of CO with a resolution of 1.0''×1.2''. Theobserved emission reveals an extended rotating structure viewed approximatelyedge-on and clear of detectable CO emission out to a distance of~90 AU from the star. No 1.3 mm continuum emission is detected ata 3 σ sensitivity of 2.1 mJy beam-1. Models of disk structure andchemistry indicate that the inner disk is devoid of molecular gas,while the outer gas disk between 40 and 200 AU from the star is dominatedby photochemistry from stellar and interstellar radiation. We determineparameters for a model that reproduces the basic features of thespatially resolved CO J=2-1 emission, the spectral energy distribution,and the unresolved CO J=3-2 spectrum. We investigate variations indisk chemistry and observable properties for a range of structuralparameters. The star 49 Ceti appears to be a rare example of a systemin a late stage of transition between a gas-rich protoplanetary diskand a tenuous, virtually gas-free debris disk.