PAH chemistry and IR emission from circumstellar disks

Visser, R.; Geers, V. C.; Dullemond, C. P.; Augereau, J.-C.; Pontoppidan, K. M.; Dishoeck, E. F. van

doi:10.1051/0004-6361:20066829

Cited by 100 publications

(132 citation statements)

References 80 publications

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“…Furthermore, it can explain the lower PAH flux contribution we found, because this emission is known to come from radii larger than the thermal silicate emission (see e.g. Habart et al 2004;van Boekel et al 2004;Lagage et al 2006;Visser et al 2007;Verhoeff 2009). …”

Section: Emerging Picturementioning

confidence: 88%

A mid-IR study of the circumstellar environment of Herbig Be stars

Verhoeff

Waters

Ancker

et al. 2012

A&A

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Context. The study of the formation of massive stars is complicated because of the short times scales, large distances, and obscuring natal clouds. There are observational and theoretical indications that the circumstellar environment of Herbig Be (HBe) stars is substantially different from that of their lower mass counterparts, the T Tauri and Herbig Ae stars. Aims. We map the spatial distribution and mineralogy of the warm circumstellar dust of a sample of HBe stars. We compare our results to a sample of less massive Herbig Ae stars. Methods. We used literature photometry to obtain optical extinctions and stellar parameters of the targets. We obtained N-band imaging and long-slit spectroscopic data with the VISIR instrument at the VLT and we analyzed these data. We performed photometry of the images and extracted spatial information. We corrected the spectra for extinction and performed mineralogical fits. We fitted Gaussian profiles to characterize the spatial extent of the spectra along the VISIR slit. Results. We find that the mid-infrared (IR) emission of the HBe stars is typically characterized by a circumstellar disk that efficiently reprocesses a substantial portion of the stellar flux. The mid-IR flux levels, the spatial compactness, and the dust composition are quite similar to those of the Herbig Ae stars. We find upper limits to the full-width-at-half-maximum (FWHM) size of the mid-IR emission of ∼500 AU. The main differences with the lower mass stars are the lower overall IR excess with a greater variety in shapes, the weaker PAH reprocessing power, and the lack of a silica-forsterite relation. The discrepancies between VISIR and IRAS photometry, the far-IR contributions and the large PAH sizes of HBe stars are attributed to natal clouds. Conclusions. Our results suggest that the Herbig Be disks are flatter than those around lower mass stars and they are likely truncated from the outside by photoevaporation.

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Section: Emerging Picturementioning

confidence: 88%

A mid-IR study of the circumstellar environment of Herbig Be stars

Verhoeff

Waters

Ancker

et al. 2012

A&A

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“…4.3. We assume PAHs to be photodestroyed in regions with integrated far ultraviolet (FUV) fluxes >10 6 ISRF, following the results of a more complete PAH chemistry by Visser et al (2007). This affects however only the upper layer of the disk and we find that the lines discussed in this work are unaffected by this assumption.…”

Section: Parameters For the Hd 100546 Modelmentioning

confidence: 97%

“…This emission however only traces regions where PAHs are excited by UV photons. An abundance and excitation study by Visser et al (2007) shows that in disks around Herbig Ae stars, most of the emission from intermediate-size PAHs emerge from the uppermost layer, above the τ V = 1 surface. Thus, PAHs do not trace down to regions where the C + /C transition is affected by the PAH charge exchange.…”

Section: Varying Pah Abundancementioning

confidence: 99%

The warm gas atmosphere of the HD 100546 disk seen byHerschel

Bruderer

Dishoeck

Doty

et al. 2012

A&A

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Context. With the Herschel Space Observatory, lines of simple molecules (C + , O, and high-J lines of CO, J up 14) have been observed in the atmosphere of protoplanetary disks. When combined with ground-based data on [C i], all principle forms of carbon can be studied. These data allow us to test model predictions for the main carbon-bearing species and verify the presence of a warm surface layer. The absence of neutral carbon [C i], which is predicted by models to be strong, can then be interpreted together with ionized carbon [C ii] and carbon monoxide. Aims. We study the gas temperature, excitation, and chemical abundance of the simple carbon-bearing species C, C + , and CO, as well as O by the method of chemical-physical modeling. Using the models, we explore the sensitivity of the lines to the entering parameters and constrain the region from which the line radiation emerges. Methods. Numerical models of the radiative transfer in the lines and dust are used together with a chemical network simulation and a calculation of the gas energetics to obtain the gas temperature. We present our new model, which is based on our previous models but includes several improvements that we report in detail, together with the results of benchmark tests. Results. A model of the disk around the Herbig Be star HD 100546 is able to reproduce the CO ladder together with the atomic finestructure lines of [O i] and either [C i] or [C ii]. We find that the high-J lines of CO can only be reproduced by a warm atmosphere with T gas T dust . The low-J lines of CO, observable from the ground, are dominated by the outer disk with a radius of several 100 AU, while the high-J CO observable with Herschel-PACS are dominated from regions within some tens of AU. The spectral profiles of high-J lines of CO are predicted to be broader than those of the low-J lines. We study the effect of several parameters including the size of the disk, the gas mass of the disk, the PAH abundance and distribution, and the amount of carbon in the gas phase. Conclusions. The main conclusions of our work are (i) only a warm atmosphere with T gas T dust can reproduce the CO ladder. (ii) The CO ladder together with [O i] and the upper limit to [C i] can be reproduced by models with a high gas/dust ratio and a low abundance of volatile carbon. These models however produce too small amounts of [C ii]. Models with a low gas/dust ratio and more volatile carbon also reproduce CO and [O i], are in closer agreement with observations of [C ii], but overproduce [C i]. Owing to the uncertain origin of the [C ii] emission, we prefer the high gas/dust ratio models, indicating a low abundance of volatile carbon.

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“…The models described in this paper use circumcoronene (C 54 H 18 ) as the representative PAH. Circumcoronene is a large and compact PAH (pericondensed, superaromatic) and the electron delocalization adds to its stability against photodissociation (Tielens et al 1987;Visser et al 2007;De Becker 2013).…”

Section: A6 Pah Chemistrymentioning

confidence: 99%

Consistent dust and gas models for protoplanetary disks

Kamp

Thi

Woitke

et al. 2017

A&A

109

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Aims. We define a small and large chemical network which can be used for the quantitative simultaneous analysis of molecular emission from the near-IR to the submm. We revise reactions of excited molecular hydrogen, which are not included in UMIST, to provide a homogeneous database for future applications. Methods. We use the thermo-chemical disk modeling code ProDiMo and a standard T Tauri disk model to evaluate the impact of various chemical networks, reaction rate databases and sets of adsorption energies on a large sample of chemical species and emerging line fluxes from the near-IR to the submm wavelength range. Results. We find large differences in the masses and radial distribution of ice reservoirs when considering freeze-out on bare or polar ice coated grains. Most strongly the ammonia ice mass and the location of the snow line (water) change. As a consequence molecules associated to the ice lines such as N 2 H + change their emitting region; none of the line fluxes in the sample considered here changes by more than 25% except CO isotopologues, CN and N 2 H + lines. The three-body reaction N+H 2 +M plays a key role in the formation of water in the outer disk. Besides that, differences between the UMIST 2006 and 2012 database change line fluxes in the sample considered here by less than a factor 2 (a subset of low excitation CO and fine structure lines stays even within 25%); exceptions are OH, CN, HCN, HCO + and N 2 H + lines. However, different networks such as OSU and KIDA 2011 lead to pronounced differences in the chemistry inside 100 au and thus affect emission lines from high excitation CO, OH and CN lines. H 2 is easily excited at the disk surface and state-to-state reactions enhance the abundance of CH + and to a lesser extent HCO + . For sub-mm lines of HCN, N 2 H + and HCO + , a more complex larger network is recommended. Conclusions. More work is required to consolidate data on key reactions leading to the formation of water, molecular ions such as HCO + and N 2 H + as well as the nitrogen chemistry. This affects many of the key lines used in the interpretation of disk observations. Differential analysis of various disk models using the same chemical input data will be more robust than the interpretation of absolute fluxes.

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PAH chemistry and IR emission from circumstellar disks

Cited by 100 publications

References 80 publications

A mid-IR study of the circumstellar environment of Herbig Be stars

A mid-IR study of the circumstellar environment of Herbig Be stars

The warm gas atmosphere of the HD 100546 disk seen byHerschel

Consistent dust and gas models for protoplanetary disks

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