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 submi llimeter wavelengths for continuum emission from dust, and spectral line em ission 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 i nferred mass of solid material is greater than or equal to 0,25 M+. An uppe r limit for the CO J = 3-2 rotational line implies less than 1-7 M+ (less t han or equal to 0.003-0.002 Jupiter masses) of molecular H-2 gas in the sys tem. Thus, it is no longer possible to form new Jupiter-like gaseous giant planets around HR 4796A, If planet formation explains the observed dust cav ity 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 Astronom ical Satellite (IRAS), Two were detected, both of them young sources with o ptical emission lines indicative of ongoing accretion of disk material onto the star, The gas mass lower limits are approximately 30 and 200 Earth mas ses, at least an order of magnitude higher than that for HR 4796A, illustra ting the diversity of disk properties at ages of up to 10 Myr. (C) 2000 Aca demic Press.