Detection of CH⁺emission from the disc around HD 100546

Abstract

Despite its importance in the thermal balance of the gas and in the determination of primeval planetary atmospheres, the chemistry in protoplanetary discs remains poorly constrained with only a handful of detected species. We observed the emission from the disc around the Herbig Be star HD 100546 with the PACS instrument in the spectroscopic mode on board the Herschel Space Telescope as part of the GAS in Protoplanetary Systems (GASPS) programme and used archival data from the DIGIT programme to search for the rotational emission of CH + . We detected in both datasets an emission line centred at 72.16 μm that most likely corresponds to the J = 5−4 rotational emission of CH + . The J = 3−2 and 6−5 transitions are also detected albeit with lower confidence. Other CH + rotational lines in the PACS observations are blended with water lines. A rotational diagram analysis shows that the CH + gas is warm at 323 +2320 −151 K with a mass of ∼3 × 10 −14 −5 × 10 −12 M . We modelled the CH + chemistry with the chemo-physical code ProDiMo using a disc density structure and grain parameters that match continuum observations and near-and mid-infrared interferometric data. The model suggests that CH + is most abundant at the location of the disc rim at 10−13 AU from the star where the gas is warm, which is consistent with previous observations of hot CO gas emission.

DIGIT survey of far-infrared lines from protoplanetary disks

“…Compared to Thi et al (2011), the slab model analysis presented here indicates a lower excitation temperature, T ex ∼100 K versus 323 +2320 −151 K (Thi et al 2011). The CH + emitting region is also different: Thi et al (2011) find that most of the emission comes from a narrow rim at the cavity edge between 10−13 AU from the star through full thermo-chemical modeling, whereas the slab model suggests a much larger emitting area.…”

“…The CH + emitting region is also different: Thi et al (2011) find that most of the emission comes from a narrow rim at the cavity edge between 10−13 AU from the star through full thermo-chemical modeling, whereas the slab model suggests a much larger emitting area. Part of the discrepancy is due to a different flux calibration of the PACS spectra; the PACS spectrum presented here matches the PACS photometric points which have an accuracy of 5% in absolute flux.…”

“…For HD 100546 six rotational lines are detected (see also Thi et al 2011) while in the case of HD 97048 only the J = 6−5 and J = 5−4 transitions are seen. The line fluxes for HD 100546 differ from those reported by Thi et al (2011) by 10−50% due to updated flux calibration (see Sect. 2.2).…”

DIGIT survey of far-infrared lines from protoplanetary discs

“…Starting from CH + , a chain of reactions through CO + and HCO + then leads to CO. The species CH + is detected in both HD 97048 and HD 100546 (Thi et al 2011;Paper I). The CO lines discussed in this section form at large heights in the disc atmosphere and are not affected by any chemistry lower in the disc.…”

Astrochemistry of dust, ice and gas: introduction and overview

“…The most prominent example is that of CH + formation through reactions of C + with H 2 (v,J), where the H 2 ro-vibrational levels are pumped by UV radiation. These state-specific reactions were included in PDR models developed in the 1980s and 1990s but have recently been revived in the context of dense PDRs and the surface layers of protoplanetary disks [176][177][178] .…”