Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks around Intermediate-Aged Solar-type Stars

We report infrared spectroscopic observations of disks around nearby, intermediate-aged (3 Myr-300 Myr) solar-type stars. We have used the high spectral resolution mode (R ∼ 700) of the Infrared Spectrometer (IRS) aboard the Spitzer Space Telescope to search for gaseous emission lines from the disk. The observations reported here provide upper limits to the fluxes of the H₂ S(0) 28μ m, H₂ S(1) 17μ m, [FeII] 26μ m, [SiII] 35μ m, and [SI] 25μ m infrared emission lines.

The H₂ line upper limits directly constrain the mass of warm molecular gas in the disk: typically M(H_2)< 5, 0.1 and 0.01 M_J at T= 50, 100, and 200 K, respectively. We also compare the line flux upper limits to predictions from detailed thermal/chemical models of various gas distributions in the disk. These limits are considerably below the value for a minimum mass solar nebula, and indicate that less than 1 M_J of gas (at any temperature) exists in the 1-40 AU planet-forming region. Therefore, there is insufficient gas for giant planet formation to occur in these sources at this time. Either Jupiters have already formed or will never form. However, the models also indicate that if the inner disk radius is << 1 AU, then a considerable mass of cold gas could be present in the planet formation region of the disk without violating the line flux upper limits. This seems less likely because the gas would likely extend to the stellar surface, and viscous accretion onto the star would lead to observational indicators that are not observed.