Where is the warm H2? A search for H2 emission from disks around Herbig Ae/Be stars

Context. Mid-infrared (mid-IR) emission lines of molecular hydrogen (H₂) are useful probes to determine the mass of warm gas present in the surface layers of circumstellar disks. In the past years, numerous observations of Herbig Ae/Be stars (HAeBes) have been performed, but only two detections of H₂ mid-IR emission toward HD 97048 and AB Aur have been reported.
Aims: We aim at tracing the warm gas in the circumstellar environment of five additional HAeBes with gas-rich environments and/or physical characteristics close to those of AB Aur and/or HD 97048, to discuss whether the detections toward these two objects are suggestive of peculiar conditions for the observed gas.
Methods: We search for the H₂ S(1) emission line at 17.035 μm using high-resolution mid-IR spectra obtained with VLT/VISIR, and complemented by CH molecule observations with VLT/UVES. We gather the H₂ measurements from the literature to put the new results in context and search for a correlation with some disk properties.
Results: None of the five VISIR targets shows evidence for H₂ emission at 17.035 μm. From the 3σ upper limits on the integrated line fluxes we constrain the amount of optically thin warm (>150 K) gas to be less than ~ 1.4 M_Jup in the disk surface layers. There are now 20 HAeBes observed with VISIR and TEXES instruments to search for warm H₂, but only two detections (HD 97048 and AB Aur) were made so far. We find that the two stars with detected warm H₂ show at the same time high 30/13 μm flux ratios and large PAH line fluxes at 8.6 and 11.3 μm compared to the bulk of observed HAeBes and have emission CO lines detected at 4.7 μm. We detect the CH 4300.3 Åabsorption line toward both HD 97048 and AB Aur with UVES. The CH to H₂ abundance ratios that this would imply if it were to arise from the same component as well as the radial velocity of the CH lines both suggest that CH arises from a surrounding envelope, while the detected H₂ would reside in the disk.
Conclusions: The two detections of the S(1) line in the disks of HD 97048 and AB Aur suggest either peculiar physical conditions or a particular stage of evolution. New instruments such as Herschel / PACS should bring significant new data for the constraints of thermodynamics in young disks by observing the gas and the dust simultaneously.