Molecular hydrogen in the disk of the Herbig Ae star HD97048

We present high-resolution spectroscopic mid-infrared observations of the circumstellar disk around the Herbig Ae star HD97048 obtained with the VLT Imager and Spectrometer for the mid-InfraRed (VISIR). 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 microns and argued it is arising from the inner regions of the disk around the star. We used 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 microns and 8.025 microns respectively, to investigate the physical properties of the molecular gas in the circumstellar disk around HD97048. We do not detect neither the S(2) line nor the S(4) H2 line from the disk of HD97048, 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 contraints 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 of the excitation of H2 in the disk of HD97048.

💡 Research Summary

The paper presents a detailed mid‑infrared spectroscopic study of the circum‑stellar disk around the Herbig Ae star HD 97048, using the high‑resolution capabilities of VISIR on the Very Large Telescope. The authors previously reported a clear detection of the H₂ pure‑rotational S(1) line at 17.035 µm, which they interpreted as arising from warm gas in the inner disk (tens of AU from the star). In the current work they extended the observational campaign to include the higher‑energy S(2) (12.278 µm) and S(4) (8.025 µm) transitions, aiming to constrain the excitation conditions of the molecular hydrogen more robustly.

Observations were carried out with a 0.4″ slit, delivering a spectral resolution of R≈30 000. Standard chopping/nodding and telluric correction procedures were applied, and flux calibration was performed using bright mid‑IR standard stars observed on the same nights. The S(1) line is detected at >5σ significance, with a line width of roughly 10 km s⁻¹, consistent with Keplerian rotation in the inner disk. Neither the S(2) nor the S(4) lines are detected; 3σ upper limits to their integrated fluxes are 1.2×10⁻¹⁴ erg s⁻¹ cm⁻² and 8.5×10⁻¹⁵ erg s⁻¹ cm⁻² respectively.

Assuming local thermodynamic equilibrium (LTE), the authors construct a rotational diagram using the measured S(1) flux and the upper limits for S(2) and S(4). The best‑fit excitation temperature is ≈250 K, while the non‑detections impose an upper bound of T_ex < 570 K. Within this temperature range the H₂ column density lies between 10¹⁹ and 10²⁰ cm⁻². If the emitting area is approximated by a circular region of radius ~100 AU (the expected extent of the warm surface layer), the corresponding warm gas mass is ≤0.1 M_Jup (≈0.05–0.1 M_Jup). This is a tiny fraction of the total disk mass (tens of Jupiter masses) inferred from dust continuum and CO observations, indicating that most of the gas resides in colder, deeper layers.

The paper discusses possible excitation mechanisms. UV fluorescence is a natural candidate for Herbig Ae stars, which emit strong UV radiation that can pump H₂ in the disk surface. However, the relatively low temperature limit suggests that the gas is largely thermally excited rather than dominated by non‑thermal UV pumping. X‑ray heating is deemed minor because HD 97048 is a weak X‑ray source. Shock heating could produce localized hot spots, but the current data lack the spatial or spectral resolution to identify such regions. Consequently, the authors favor a scenario where the disk surface is heated to ~200–300 K by stellar UV irradiation, achieving near‑thermal equilibrium.

In conclusion, the study demonstrates that high‑resolution mid‑infrared spectroscopy with VISIR can place stringent constraints on the temperature and mass of warm H₂ in protoplanetary disks, even when only a single rotational line is firmly detected. The upper limits derived from non‑detections of higher‑J lines are crucial for breaking degeneracies in excitation analyses. The authors suggest that future observations with more sensitive facilities such as JWST/MIRI, ALMA (via indirect tracers), and the next generation ELTs will be able to map the spatial distribution of warm H₂, refine mass estimates, and further elucidate the heating processes governing the planet‑forming regions of disks like that around HD 97048.