A search for mid-infrared molecular hydrogen emission from protoplanetary disks^*

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: carmona@mpia.de
² European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei München, Germany
³ Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁵ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3030 Heverlee, Belgium

Received: 11 May 2007
Accepted: 15 October 2007

Abstract

We report on a sensitive search for mid-infrared molecular hydrogen emission from protoplanetary disks. We observed the Herbig Ae/Be stars UX Ori, HD 34282, HD 100453, HD 101412, HD 104237 and HD 142666, and the T Tauri star HD 319139, and searched for H emission at 12.278 micron and H emission at 17.035 micron with VISIR, ESO-VLT's high-resolution mid-infrared spectrograph. None of the sources present evidence for molecular hydrogen emission at the wavelengths observed. Stringent 3σ upper limits to the integrated line fluxes and the mass of optically thin warm gas ( 150, 300 and 1000 K) in the disks are derived. The disks contain less than a few tenths of Jupiter mass of optically thin H₂ gas at 150 K, and less than a few Earth masses of optically thin H₂ gas at 300 K and higher temperatures. We compare our results to a Chiang & Goldreich (1997, ApJ, 490, 368, CG97) two-layer disk model of masses 0.02  and 0.11 . The upper limits to the disk's optically thin warm gas mass are smaller than the amount of warm gas in the interior layer of the disk, but they are much larger than the amount of molecular gas expected to be in the surface layer. If the two-layer approximation to the structure of the disk is correct, our non-detections are consistent with the low flux levels expected from the small amount of H₂ gas in the surface layer. We present a calculation of the expected thermal H₂ emission from optically thick disks, assuming a CG97 disk structure, a gas-to-dust ratio of 100 and T_gas = T_dust. We show that the expected H₂ thermal emission fluxes from typical disks around Herbig Ae/Be stars are of the order of 10^-16 to 10^-17 erg s^-1 cm^-2 for a distance of 140 pc. This is much lower than the detection limits of our observations (5  10^-15 erg s^-1 cm^-2). H₂ emission levels are very sensitive to departures from the thermal coupling between the molecular gas and dust in the surface layer. Additional sources of heating of gas in the disk's surface layer could have a major impact on the expected H₂ disk emission. Our results suggest that in the observed sources the molecular gas and dust in the surface layer have not significantly departed from thermal coupling (T_gas/T_dust < 2) and that the gas-to-dust ratio in the surface layer is very likely lower than 1000.