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Published November 10, 2006 | Published
Journal Article Open

Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks around Sun-like Stars


We have carried out a sensitive search for gas emission lines at IR and millimeter wavelengths for a sample of 15 young Sun-like stars selected from our dust disk survey with Spitzer. We have used mid-IR lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of ^(12)CO to probe the cold (~20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti & Hollenbach that confirm and extend our simple analysis. These models show that the [S I] 25.23 μm line can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04M_J of gas within a few AU from the disk inner radius for disk radii from 1 to ~40 AU. These gas mass upper limits even in the eight systems younger than ~30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, which is mainly traced by our CO data, are <2 M_⊕. If these systems are analogs of the solar system, they either have already formed Uranus- and Neptune-like planets or will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.

Additional Information

© 2006 American Astronomical Society. Received 2006 April 10; accepted 2006 June 26. It is a pleasure to thank all members of the FEPS team for their contributions to the project and to this study. I. P. wishes to thank D.Watson for suggestions on the data reduction of the IRS high resolution spectra and J. Muzerolle for helpful discussions on the mass accretion rate in young circumstellar disks. We would also like to thank the anonymous referee for a very careful and helpful review. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through the FEPS Legacy award issued by JPL/Caltech. Facilities: Spitzer

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