The Dust and Gas Content of a Disk around the Young Star HR 4796A

We have used the James Clerk Maxwell Telescope in Hawaii to search at submillimeter wavelengths for continuum emission from dust, and spectral line emission from carbon monoxide (CO) gas, in the neighborhood of HR 4796A, This young star has a dusty disk with a central cavity, where planets may have formed, We detect the dust component at a wavelength of 850 mu m, and the inferred mass of solid material is greater than or equal to 0,25 M+. An upper limit for the CO J = 3-2 rotational line implies less than 1-7 M+ (less than or equal to 0.003-0.002 Jupiter masses) of molecular H-2 gas in the system. Thus, it is no longer possible to form new Jupiter-like gaseous giant planets around HR 4796A, If planet formation explains the observed dust cavity and lack of gas, then it must have occurred before the current stellar age of similar to 10 Myr.A search was also made for CO J = 3-2 emission around four other stars with dust excesses revealed by infrared measurements with the Infrared Astronomical Satellite (IRAS), Two were detected, both of them young sources with optical emission lines indicative of ongoing accretion of disk material onto the star, The gas mass lower limits are approximately 30 and 200 Earth masses, at least an order of magnitude higher than that for HR 4796A, illustrating the diversity of disk properties at ages of up to 10 Myr. (C) 2000 Academic Press.