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A&A
Volume 530, June 2011
Article Number L2
Number of page(s) 5
Section Letters
DOI https://doi.org/10.1051/0004-6361/201116678
Published online 28 April 2011

© ESO, 2011

1. Introduction

Planets are formed in discs around young stars. The building of planets is intimately linked to the grain-growth process and the disc’s chemical evolution (Bergin et al. 2007). The chemical and temperature profiles of discs determine the chemical composition of the gas that will eventually be incorporated into a planet’s atmosphere, once a solid  ~10 Earth mass core is formed (Armitage 2010). Millimetre observations of molecules in discs probe the outer cold part (Dutrey et al. 1997; van Zadelhoff et al. 2001; Thi et al. 2004; Semenov et al. 2005; Henning et al. 2010; Öberg et al. 2010, e.g.). Minor species located in the inner disc, such as C2H2 and HCN, were observed with the Spitzer Space Telescope (Carr & Najita 2008; Pascucci & Sterzik 2009; Lahuis et al. 2006; Pontoppidan et al. 2010). Gas-phase carbon is believed to be locked into C+, C, and CO. However, a significant fraction of the carbon can be locked into CH and CH+, which are observed in the optical in diffuse clouds.

The Herschel Space Telescope permits observations of hydrides and their ions (OH, OH+, CH, CH+, ...), and warm light molecular gas (e.g., H2O), which all emit in the far-infrared. Rotational lines of CH+ at 300−500 K in the far-IR have been observed by ISO towards the NGC 7027 PDR (Cernicharo et al. 1997). The isotopologue 13CH+J = 1−0 was detected in absorption in the diffuse medium by Falgarone et al. (2005). The lowest rotational transition was detected in emission in the Orion Bar (Naylor et al. 2010; Habart et al. 2010) and in absorption towards DR 21 (Falgarone et al. 2010) with Herschel. The neutral counterpart CH has also been detected in the diffuse medium by Herschel (Qin et al. 2010; Gerin et al. 2010). In this letter, we discuss the detection of emission lines with Herschel that we assign to the rotational transition of CH+ from the disc around the 10 Myrs old Herbig Be star HD 100546 (M∗ = 2.2   M⊙, L∗ = 27   L⊙, Teff = 10   500 K) located at 103 pc (van den Ancker et al. 1997). HD 100546 shows a very rich gas and a solid spectrum in the infrared and millimetre ranges (Brittain et al. 2009; Sturm et al. 2010; Panić et al. 2010; Malfait et al. 1998).

thumbnail Fig. 1

Continuum-subtracted Herschel-PACS spectra of HD 100546 around the CH+J = 6−5, 4−3, 3−2, 2−1 (DIGIT data), and J = 5−4 (GASPS and DIGIT data) lines. The 3σ statistical error levels do not include the 30% calibration uncertainty. A rotational diagram using those transitions is plotted in the lower-right panel. The errors in the diagram are the quadratic sum of all the statistical and calibration errors (). Blended lines are considered as upper limits (red filled dots).

2. Herschel observations and rotational diagram

We observed HD 100546 with PACS (Poglitsch et al. 2010) on board the Herschel Space Telescope (Pilbratt et al. 2010) in the 72−73 μm spectroscopic scan range (Δv = 163 km s-1), as part of the open-time key programme GASPS (GAS in Protoplanetary Systems, obsid 1342188437), which aims at a systematic study of gas and dust in discs around stars aged between 0.5 and 30 Myr. The observations were reduced using the standard pipeline in HIPE 4.2. We detected a line at 72.14 μm that we assign to the rotational J = 5−4 emission of CH+ (see Fig. 1). We also detected the same line in the science demonstration data spectrum taken by the DIGIT team (Sturm et al. 2010), together with the J = 6−5 (60.24 μm) and J = 3−2 (119.68 μm) lines but with a lower signal-to-noise ratio. It was not possible to strengthen our analysis with detection of either the lowest rotational transition J = 1−0, which lies in the SPIRE/HIFI range, or the J = 2−1 and J = 4−3 lines in the DIGIT dataset, which are blended with water emission lines at the PACS resolution. The ortho H2O 212-101 emission at 179.52 μm is blended with the CH+J = 2−1 line at 179.61 μm, and the para-H2O 322−211 at 89.98 μm is blended with the CH+J = 4−3 line at 90.02 μm. The flux calibration accuracy of the Herschel-PACS instrument is currently 30% in addition to the statistical noise. In addition, flux uncertainties are introduced by difficulties in determining the continuum level in the DIGIT spectra. A new reduction with HIPE 7.0.822, which includes division by the spectral response function and flatfielding, does not improve the noise level in the continuum. The line ratio  [O   I]  63 μm/CH+J = 5−4 is 80. We fitted a Gaussian profile to each line. The observed CH+,  [O   I] ,  [C   II]  (Sturm et al. 2010), and CO 3–2 (Panić et al. 2010) fluxes are all reported in Table 1.

We constructed a rotational diagram by assuming LTE population and optically thin emission (see lower right panel of Fig. 1). The level energies, line frequencies, and Einstein spontaneous emission coefficients were computed by Müller (2010) based on the spectroscopic analysis of Amano (2010). We analysed the diagram using censored data regression and bootstrapping methods. We derived an excitation temperature of 323 K, a disc-averaged CH+ column density of 4.3  ×  1012 cm-2, consistent with the upper limit found in observing CH+ at 4232.7 Å (Martin-Zaïdi et al. 2011), and a lower limit to the CH+ mass of 3.3 M⊙. The CH+ emitting gas is warm, which suggests that CH+ is located within 100 AU of the star. The CH+ abundance is  ≥ 3.9 × 10-9 relative to an upper limit of warm molecular gas from the VISIR H2S(1) 17 μm observation of M(Tgas = 300   K) ≤ 8.4 × 10-5 M⊙ (Martin-Zaïdi et al. 2010).

Table 1

Observed and modelled line fluxes.

3. Modelling the physics and chemistry of CH+

We first fitted the spectral energy distribution (SED) and infrared interferometric VLT-AMBER and MIDI data to constrain the dust properties and the disc structure with a hydrostatic disc using the MCFOST radiative transfer code (Pinte et al. 2006, 2009). The details of the fit are discussed in Benisty et al. (2010) and Tatulli et al. (2011). The disc is composed of three parts: the inner disc, the outer disc, and an upper layer on the top of the outer disc (see Table 2, Fig. A.1, and the sketch in Benisty et al. 2010). The inner and outer discs are separated by a gap (Bouwman et al. 2003; Grady et al. 2005). The fit to the SED in Tatulli et al. (2011) suggests a scale height value of 10 AU at 100 AU, which is consistent with the scattered-light images, but equally good fits can also be obtained with other values. Therefore we tried several disc structures by varying the scale height H100   AU. Using the disc structures derived from the fit to the SED, we modelled the gas chemistry and radiative transfer in the HD 100546 disc with the ProDiMo code (Woitke et al. 2009; Kamp et al. 2010). The ProDiMo code computes the chemistry and heating-cooling balance of the gas self-consistently. We assumed a non-viscous gas (α = 0) and a PAH mass (C150H30) that is consistent with the strong PAH emissions in the IR (Keller et al. 2008). Large PAHs ( ≥ 100 carbon atoms) exhibit a strong feature between 10.9 and 11.3 μm (Bauschlicher et al. 2009) as seen in the spectrum of HD 100546. The amount of PAH influences both the thermal state of the disc and the ionisation state of the upper layers. The non-thermal line width is constrained by the resolved CO 3−2 profile. We used a dereddened FUSE and IUE UV spectrum as input to the gas modelling (Martin-Zaïdi et al. 2008). We also ran a ProDiMo model where the disc vertical structure is calculated according to the gas pressure and found that the height of the rim at 13 AU is consistent with the fixed three-part structure.

Table 2

Model parameters.

The main CH+ formation reaction C+ + H2  →  CH+ + H (reaction 1) is endothermic by 0.398 eV (4537 K). The reaction is efficient only at a few 100 K found in shocked or turbulent regions or when H2 is vibrationally excited (Hierl et al. 1997; Agúndez et al. 2010). The rate for reaction 1 in Herbst & Knudson (1981) underestimates the experimental data by a factor of 2−3, and the UMIST 2006 rate (Woodall et al. 2007) is a factor 7–8 lower than the experimental values. Initial models with a low rate for reaction 1 underpredict the CH+ abundance by more than a factor 10. We adopted the value suggested by Hierl et al. (1997): k1(T) = (7.4 ± 0.8) × 10-10e−4537/Tcm3  s-1. We also take the enhanced rate for vibrationally-excited molecular hydrogen into account by assuming that the rate is equal to the Langevin collision value of 1.6    ×    10-9 cm3 s-1 (Agúndez et al. 2010). The alternative radiative association reaction C +  + H → CH +  + hν has an extremely low rate of  ~10-16−10-17 cm-3 s-1 (Barinovs & van Hemert 2006). The warm and UV-excited gas at the disc atmosphere around a Herbig AeBe star renders reaction 1 fast. CH+ reacts with H2 to form CH, which in turn reacts with H2 to form CH. As both exothermic reactions are fast, an enhanced abundance of CH+ is accompanied by a high abundance of CH and CH, which have not been detected yet. CH+ can also dissociatively recombine with an electron into C+ and H or exchange its charge with PAHs. CH+ is also destroyed upon absorption of UV photons. The absorption cross-section peaks at  ~950 Å. Therefore, the photodissociation around a B9.5V star without strong UV excess due to accretion is not as fast as with an interstellar UV field. The photodissociation and photoionisation rates are computed inside the disc using cross-sections taken from van Dishoeck et al. (2006). The steady-state chemistry includes 188 gas and solid species.

We ran several models with disc gas mass between 10-4 and 10-2 M⊙ and H100 AU between 10 and 20 AU (Fig. A.2). In Table 1 we show the three detected line fluxes for CH+ out of the 15 rotational transitions in the model, the OI and CII fine-structure lines, and the CO 3–2 transition for our preferred models, which have a disc mass of (0.5−1) × 10-3 M⊙ and H100   AU = 14 AU, for the outer disc. All the modelled lines agree within a factor 2 with the observations. In particular, the sum of the p-H2O 322−211 and CH+J = 4−3 fluxes is consistent with the observed value. The collision rates of CH+ with electrons were computed by Lim et al. (1999). In the absence of published collision rates with H2, we scaled the collision rates with He by 1.39 for J ≤ 6 transitions (Hammami et al. 2009; Turpin et al. 2010).

thumbnail Fig. 2

CH+ abundance structure in the HD 100546 disc with respect to the number of H-nuclei. The H2 abundance relative to total H-nuclei abundance contours in log-scale are overplotted in blue, while the Tgas = 300 and 3000 K contours are overplotted in white. The line-of-sight CH+ observations in the UV would cross the disc at z/r = 0.9 (42°).

4. Discussion

The models suggest that CH+ is mostly located on the rim (10−13 AU) of the outer disc and at the disc surface (Fig. 2). Although CH+ is abundant in the tenuous inner disc, its amount is insufficient to contribute to the 72.14 μm emission. CH+ is located where the gas is mostly heated by collisional de-excitations of nascent and UV-pumped H. Part of the CH+ is formed by the reaction with excited H, but the main fraction comes from the reaction of C+ with thermal H2 at Tgas >  400 K. CH+ reaches a maximum abundance of  ~ 10-7 just behind the rim and has a total mass of 10-13−10-12 M⊙. The disc average CH+ abundance is (0.7−7) × 10-9 relative to H-nuclei, consistent with the simple analysis in Sect. 2. The high CH+ abundance in the rim region and upper disc layers where hydrogen is in atomic form does not affect the C+ abundance much, which reaches 10-4. However below the disc atmosphere, H2 self-shielding is efficient, and C+ is converted rapidly into CH+. The formation reaction of CH+ (C++H2) competes with the recombination reaction of C+ to form neutral carbon.

Warm OH (Sturm et al. 2010) and fluorescent ro-vibrational CO emission (Brittain et al. 2009; van der Plas et al. 2009) have also been detected in HD 100546 . OH is mostly formed by the reaction of atomic oxygen O with thermally hot H2 and vibrationally excited H. The presence of hot CO, CH+ and OH supports the idea that hot and excited gas chemical reactions occur on disc surfaces and at the inner rim.

5. Conclusions

We detected the J = 5−4 rotational transition of CH+ at 72.16 μm in two Herschel datasets. We also tentatively detected other lines from CH+ at 60.25 and 119.87 μm. Other CH+ lines are blended with water lines. We modelled the CH+ line fluxes using the most recent chemical and collisional rates. Searches for CH+ in other Herbig AeBe discs are warranted to test whether the presence of CH+ is unique to HD 100546 or if it is related to the presence of a high temperature rim of gas, or both.

Online material

Appendix A: Disc density and gas temperature structure

We provide for completeness the disc density and the dust and gas temperature structures for model 2 (Figs. A.1–A.3).

thumbnail Fig. A.1

The input disc density structure for model 2 constrained by the fit to the SED (see Tatulli et al. 2011, for details). The gap is filled with a gas at density of nH = 100 cm-3.

thumbnail Fig. A.2

The disc dust temperature profile computed by MCFOST for model 2.

thumbnail Fig. A.3

The disc gas temperature profile computed by ProDiMo for model 2.

Acknowledgments

The Grenoble group acknowledges PNPS, CNES, and ANR (contract ANR-07-BLAN-0221). C. Eiroa and G. Meeus are partly supported by the Spanish grant AYA 2008-01727. C. Pinte acknowledges the funding from the EC 7th Framework Programme (PIEF-GA-2008-220891, PERG06-GA-2009-256513). I. Pascucci, M. Grady, S. Brittain, C. Howard, and G. Sandell acknowledge NASA/JPL.

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All Tables

Table 1

Observed and modelled line fluxes.

In the text
Table 2

Model parameters.

In the text

All Figures

thumbnail Fig. 1

Continuum-subtracted Herschel-PACS spectra of HD 100546 around the CH+J = 6−5, 4−3, 3−2, 2−1 (DIGIT data), and J = 5−4 (GASPS and DIGIT data) lines. The 3σ statistical error levels do not include the 30% calibration uncertainty. A rotational diagram using those transitions is plotted in the lower-right panel. The errors in the diagram are the quadratic sum of all the statistical and calibration errors (). Blended lines are considered as upper limits (red filled dots).
In the text
thumbnail Fig. 2

CH+ abundance structure in the HD 100546 disc with respect to the number of H-nuclei. The H2 abundance relative to total H-nuclei abundance contours in log-scale are overplotted in blue, while the Tgas = 300 and 3000 K contours are overplotted in white. The line-of-sight CH+ observations in the UV would cross the disc at z/r = 0.9 (42°).
In the text
thumbnail Fig. A.1

The input disc density structure for model 2 constrained by the fit to the SED (see Tatulli et al. 2011, for details). The gap is filled with a gas at density of nH = 100 cm-3.
In the text
thumbnail Fig. A.2

The disc dust temperature profile computed by MCFOST for model 2.
In the text
thumbnail Fig. A.3

The disc gas temperature profile computed by ProDiMo for model 2.
In the text

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