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Mid-IR emission lines of H2 are useful probes to determine the mass of warmgas present in the surface layers of disks. Numerous observations of HerbigAe/Be stars (HAeBes) have been performed, but only 2 detections of mid-IR H2toward HD97048 and AB Aur have been reported. We aim at tracing the warm gas inthe disks of 5 HAeBes with gas-rich environments and physical characteristicsclose to those of AB Aur and HD97048, to discuss whether the detections towardthese 2 objects are suggestive of peculiar conditions for the gas. We searchfor the H2 S(1) emission line at 17.035 \mu\m with VISIR, and complemented byCH molecule observations with UVES. We gather the H2 measurements from theliterature to put the new results in context and search for a correlation withsome disk properties. None of the 5 VISIR targets shows evidence for H2emission. From the 3sigma upper limits on the integrated line fluxes weconstrain the amount of optically thin warm gas to be less than 1.4 M_Jup inthe disk surface layers. There are now 20 HAeBes observed with VISIR and TEXESinstruments to search for warm H2, but only two detections (HD97048 and AB Aur)were made so far. We find that the two stars with detected warm H2 show at thesame time high 30/13 \mu\m flux ratios and large PAH line fluxes at 8.6 and11.3 \mu\m compared to the bulk of observed HAeBes and have emission CO linesdetected at 4.7 \mu\m. We detect the CH 4300.3A absorption line toward bothHD97048 and AB Aur with UVES. The CH to H2 abundance ratios that this wouldimply if it were to arise from the same component as well as the radialvelocity of the CH lines both suggest that CH arises from a surroundingenvelope, while the detected H2 would reside in the disk. The two detections ofthe S(1) line in the disks of HD97048 and AB Aur suggest either peculiarphysical conditions or a particular stage of evolution.

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