2009ApJ...695.1302M

We present high-resolution spectroscopic mid-infrared observations of the circumstellar (CS) disk around the Herbig Ae star HD 97048 obtained with the VLT Imager and Spectrometer for the Mid-InfraRed. We conducted observations of mid-infrared pure rotational lines of molecular hydrogen (H₂) as a tracer of warm gas in the disk surface layers. In a previous paper, we reported the detection of the S(1) pure rotational line of H₂ at 17.035 µm, and argued that it is arising from the inner regions of the disk around the star. We used the VISIR on the VLT for a more comprehensive study based on complementary observations of the other mid-infrared molecular transitions, namely S(2) and S(4) at 12.278 µm and 8.025 µm respectively, to investigate the physical properties of the molecular gas in the CS disk around HD 97048. We do not detect either the S(2) line or the S(4) H₂ line from the disk of HD 97048, but we derive upper limits on the integrated line fluxes which allows us to estimate an upper limit on the gas excitation temperature, T _ex< 570 K. This limit on the temperature is consistent with the assumptions previously used in the analysis of the S(1) line, and allows us to set stronger constraints on the mass of warm gas in the inner regions of the disk. Indeed, we estimate the mass of warm gas to be lower than 0.1 M_Jup. We also discuss the probable physical mechanisms which could be responsible for the excitation of H₂ in the disk of HD 97048.