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Register a new userInfluence of the nano-grain depletion in photon-dominated regions: Application to the gas physics and chemistry in the Horsehead
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The large disparity in physical conditions from the diffuse interstellar medium (ISM) to denser clouds such as photon-dominated regions (PDRs) triggers an evolution of the dust properties (i.e. composition, size, and shape). The gas physics and chemistry are tightly connected to these dust properties and are therefore affected by dust evolution and especially the nano-grain depletion in the outer irradiated part of PDRs. We highlight the influence of nano-grain depletion on the gas physics and chemistry in the Horsehead nebula, a prototypical PDR. We used a model for atomic and molecular gas in PDRs, the Meudon PDR code, using diffuse ISM-like dust and Horsehead-like dust to study the influence of nano-grain depletion on the gas physics and chemistry, focusing on the impact on photoelectric heating and H2 formation and, therefore, on the H2 gas lines. We find that nano-grain depletion in the Horsehead strongly affects gas heating through the photoelectric effect and thus the gas temperature and the H2 formation, hence the H -> H2 position. Consequently, the first four pure rotational lines of H2 (e.g. 0-0 S(0), S(1), S(2), and S(3)) vary by a factor of 2 to 14. The 0-0 S(3) line that is often underestimated in models is underestimated even more when taking nano-grain depletion into account due to the decrease in gas heating through the photoelectric effect. This strongly suggests that our understanding of the excitation of H2 and/or of heating processes in the Horsehead, and more generally in PDRs, is still incomplete. Nano-grain depletion in the outer part of the Horsehead has a strong influence on several gas tracers that will be prominent in JWST observations of irradiated clouds. We therefore need to take this depletion into account in order to improve our understanding of the Horsehead, and more generally PDRs, and to contribute to the optimal scientific return of the mission.
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We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction between C$^+(^2P)$ and H$_2( u_{rm H_2}=1, 2)$, while the collisional (de)excitation and destruction of CH$^+$ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000~K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH$^+$ with H atoms at kinetic temperatures below 50~K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H$_2$ has a substantial effect on the excitation of CH$^+$ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH$^+$ toward the Orion Bar and the planetary nebula NGC~7027. Our results further suggest that the population of $ u_{rm H_2}=2$ might be significant in the photon-dominated region of NGC~7027.
Aims: We aim at deriving the excitation conditions of the interstellar gas as well as the local FUV intensities in the molecular cloud surrounding NGC 3603 to get a coherent picture of how the gas is energized by the central stars. Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1 and CO 4-3, 7-6 lines in a 2 x 2 region around the young OB cluster NGC 3603 YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100 micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2 observations show the presence of two molecular clumps located south-east and south-west of the cluster and confirm the overall structure already found by previous CS and C18O observations. We find a slight position offset of the peak intensity of CO and [CI], and the atomic carbon appears to be further extended compared to the molecular material. We used the HIRES far-infrared dust data to derive a map of the FUV field heating the dust. We constrain the FUV field to values of chi = 3 - 6 times 10^3 in units of the Draine field across the clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the [CII] 158 {mu}m cooling line observed by ISO. Applying LTE and escape probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K), column densities (N(MM1) = 0.9 times 10^22 cm^-2, N(MM2) = 2.5 times 10^22 cm^-2) and densities (n(MM1) = 3 times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3) for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is strongly interacting with the ambient molecular cloud, governing its structure and physical conditions. A stability analysis shows the existence of gravitationally collapsing gas clumps which should lead to star formation. Embedded IR sources have already been observed in the outskirts of the molecular cloud and seem to support our conclusions.
Molecular line observations may serve as diagnostics of the degree to which the number density of cosmic ray protons, having energies of 10s to 100s of MeVs each, is enhanced in starburst galaxies and galaxies with active nuclei. Results, obtained with the UCL_PDR code, for the fractional abundances of molecules as functions of the cosmic-ray induced ionisation rate, $zeta$, are presented. The aim is not to model any particular external galaxies. Rather, it is to identify characteristics of the dependencies of molecular abundances on $zeta$, in part to enable the development of suitable observational programmes for cosmic ray dominated regions (CRDRs) which will then stimulate detailed modelling. For a number density of hydrogen nuclei of of $10^4$ cm$^{-3}$, and high visual extinction, the fractional abundances of some species increase as $zeta$ increases to $10^{-14}$ s$^{-1}$, but for much higher values of $zeta$ the fractional abundances of all molecular species are significantly below their peak values. We show in particular that OH, H$_{2}$O, H$_{3}^{+}$, H$_{3}$O$^{+}$ and OH$^{+}$ attain large fractional abundances ($geqslant 10^{-8}$) for $zeta$ as large as $10^{-12}$ s$^{-1}$. HCO$^{+}$ is a poor tracer of CRDRs when $zeta > 10^{-13}$ s$^{-1}$. Sulphur-bearing species may be useful tracers of CRDRs gas in which $zeta sim 10^{-16}$ s$^{-1}$. Ammonia has a large fractional abundance for $zeta leqslant 10^{-16}$ s$^{-1}$ and nitrogen appears in CN-bearing species at significant levels as $zeta$ increases, even up to $sim 10^{-14}$ s$^{-1}$. In this paper, we also discuss our model predictions, comparing them to recent detections in both galactic and extragalactic sources. We show that they agree well, to a first approximation, with the observational constraints.
Grain surface chemistry is key to the composition of protoplanetary disks around young stars. The temperature of grains depends on their size. We evaluate the impact of this temperature dependence on the disk chemistry. We model a moderately massive disk with 16 different grain sizes. We use POLARIS to calculate the dust grain temperatures and the local UV flux. We model the chemistry using the 3-phase astrochemical code NAUTILUS. Photoprocesses are handled using frequency-dependent cross-sections, and a new method to account for self and mutual shielding. The multi-grain model outputs are compared to those of single-grain size models (0.1 $mu$m), with two different assumptions for their equivalent temperature. We find that the Langmuir-Hinshelwood (LH) mechanism at equilibrium temperature is not efficient to form H$_2$ at 3-4 scale heights ($H$), and adopt a parametric fit to a stochastic method to model H$_2$ formation instead. We find the molecular layer composition (1-3 $H$) to depend on the amount of remaining H atoms. Differences in molecular surface densities between single and multi-grain models are mostly due to what occurs above 1.5 $H$. At 100 au, models with colder grains produce H$_2$O and CH$_4$ ices in the midplane, and warmer ones produce more CO$_2$ ices, both allowing efficient depletion of C and O as soon as CO sticks on grain surfaces. Complex organic molecules (COMs) production is enhanced by the presence of warmer grains in the multi-grain models. Using a single grain model mimicking grain growth and dust settling fails to reproduce the complexity of gas-grain chemistry. Chemical models with a single grain size are sensitive to the adopted grain temperature, and cannot account for all expected effects. A spatial spread of the snowlines is expected to result from the ranges in grain temperature. The amplitude of the effects will depend on the dust disk mass.
We present CI 3P1-3P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M83 and M51 obtained at the JCMT. This data is complemented with maps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of CII (158 micron), OI (63 micron), and NII (122 micron) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the NII line, we estimate that between 15% and 30% of the observed CII emission originate from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ~0.21% in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) of Kaufman et al. (1999) with the standard ratios OI(63)/CII_PDR and (OI(63)+CII_PDR) vs. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, CII_PDR/CI(1-0), CI(1-0)/CO(3-2), CO(3-2)/CO(1-0), CII/CO(3-2), and, OI(63)/CII_PDR. The best fits to the latter ratios yield densities of 10^4 cm^-3 and FUV fields of ~G_0=20-30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in perfect agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4-5 at the nuclei, indicating the presence of other mechanisms heating the dust.
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