UV-driven chemistry in simulations of the interstellar medium

Levrier, F.; Petit, Franck; Hennebelle, P.; Lesaffre, Pierre; Gérin, M.; Falgarone, É.

doi:10.1051/0004-6361/201218865

Cited by 54 publications

(60 citation statements)

References 50 publications

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“…Previous observations show that CH traces H 2 well in the diffuse ISM. Steady-state models by Levrier et al (2012) reproduce the observed trend of CH versus H 2 , without the need to invoke dissipation of turbulence. This might be understood from the fact that the observations sample CH in regions with a significant fraction of hydrogen in H 2 , > f H 0.1 2 ( ) up to ∼1, while CH + prefers regions of the ISM with low hydrogen fractions as H 2 , f (H 2 )<0.5.…”

Section: Ch λ-Doubled Linesmentioning

confidence: 75%

“…A linear relation between CH and H 2 column densities, N(CH)=3.5×10 −8 N(H 2 ), has been derived by Sheffer et al (2008) to A V of around 5 mag. In steady-state chemical models, on the other hand CH traces H 2 at hydrogen densities greater than about 100 cm −3 (Liszt & Lucas 2002;Levrier et al 2012). Thus in warm and dense environments such as Orion BN/KL, the relationship between H 2 and CH could be more complex, because the abundances of CH may be at least partially a result of radiative association between C + and H 2 , which are more abundant in this environment than in the ISM.…”

Section: Introductionmentioning

confidence: 99%

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HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

Morris

Gupta

Nagy

et al. 2016

ApJ

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The CH + ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigate the formation of this highly reactive molecule with the first velocity-resolved spectral mapping of the CH + J= 1−0, 2−1 rotational transitions, three sets of CH Λ-doubled triplet lines, 12 C + and 13 C + P P , and CH is indicated to arise in the diluted gas, outside the explosive, dense BN/KL outflow. Our models show that UV irradiation provides favorable conditions for steady-state production of CH + in this environment. Surprisingly, no spatial or kinematic correspondences of the observed species are found with H 2 S(1) emission tracing shocked gas in the outflow. We propose that C + is being consumed by rapid production of CO to explain the lack of both C + and CH + in the outflow. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of the conditions leading to CH + formation and excitation is incomplete without including the important-possibly dominant-role of UV irradiation.

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Section: Ch λ-Doubled Linesmentioning

confidence: 75%

Section: Introductionmentioning

confidence: 99%

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

Morris

Gupta

Nagy

et al. 2016

ApJ

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“…Optical and FUV measurements towards bright stars have shown that the relationship between CH and H 2 is linear and exhibits a significant scatter of 0.2 dex (Sheffer et al 2008), larger than the measurement uncertainties, and which probably reflects small variations in the physical conditions along the probed sight-lines. As shown by Levrier et al (2012), averaging over long pathlengths tends to reduce the scatter, but local variations of the CH abundance relative to H 2 cannot be excluded. The same can be true for HF as well.…”

Section: Gas Phase Carbon Abundance and C + In The Ism Phasesmentioning

confidence: 98%

[C II] absorption and emission in the diffuse interstellar medium across the Galactic plane

Gérin

Ruaud

Goicoechea

et al. 2014

A&A

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120

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Aims. Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium (ISM) and its 158 μm fine structure transition [C ii] is the most important cooling line of the diffuse ISM. We combine [C ii] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM.Methods. We present high-resolution [C ii] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by single pointings in the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 μm with Herschel/PACS. Results. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution PACS data and that high spectral resolution spectra are needed to interpret the [C ii] and [O i] emission and the [C ii]/FIR ratio. This phenomenon may explain part of the [C ii]/FIR deficit seen in external luminous infrared galaxies where the bright emission from the nuclear regions may be partially canceled by absorption from diffuse gas in the foreground. The C + and C excitation in the diffuse gas is consistent with a median pressure of ∼5900 K cm −3 for a mean kinetic temperature of ∼100 K. A few higher pressure regions are detected along the lines of sight, as emission features in both fine structure lines of atomic carbon. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight. The derived median value of the filling factor is 2.4%, in good agreement with the properties of the Galactic cold neutral medium. The mean excitation temperature is used to derive the average cooling due to C + in the Galactic plane : 9.5 × 10 −26 erg −1 H −1 . Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 ± 0.4 × 10 −4 .

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“…We note that strictly speaking, this cooling function does not include molecular cooling in spite of the high densities reached in the simulation. However, it leads to temperatures that are entirely reasonable even at high densities and therefore appears to be sufficiently accurate, in particular because we are not explicitly solving for the formation of molecules in addition to H 2 (see Levrier et al 2012, for a quantitative estimate).…”

Section: Thermal Processesmentioning

confidence: 99%

H₂distribution during the formation of multiphase molecular clouds

Valdivia

Hennebelle

Gérin

et al. 2016

A&A

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Context. H 2 is the simplest and the most abundant molecule in the interstellar medium (ISM), and its formation precedes the formation of other molecules. Aims. Understanding the dynamical influence of the environment and the interplay between the thermal processes related to the formation and destruction of H 2 and the structure of the cloud is mandatory to understand correctly the observations of H 2 . Methods. We performed high-resolution magnetohydrodynamical colliding-flow simulations with the adaptive mesh refinement code RAMSES in which the physics of H 2 has been included. We compared the simulation results with various observations of the H 2 molecule, including the column densities of excited rotational levels. Results. As a result of a combination of thermal pressure, ram pressure, and gravity, the clouds produced at the converging point of HI streams are highly inhomogeneous. H 2 molecules quickly form in relatively dense clumps and spread into the diffuse interclump gas. This in particular leads to the existence of significant abundances of H 2 in the diffuse and warm gas that lies in between clumps. Simulations and observations show similar trends, especially for the HI-to-H 2 transition (H 2 fraction vs. total hydrogen column density). Moreover, the abundances of excited rotational levels, calculated at equilibrium in the simulations, turn out to be very similar to the observed abundances inferred from FUSE results. This is a direct consequence of the presence of the H 2 enriched diffuse and warm gas. Conclusions. Our simulations, which self-consistently form molecular clouds out of the diffuse atomic gas, show that H 2 rapidly forms in the dense clumps and, due to the complex structure of molecular clouds, quickly spreads at lower densities. Consequently, a significant fraction of warm H 2 exists in the low-density gas. This warm H 2 leads to column densities of excited rotational levels close to the observed ones and probably reveals the complex intermix between the warm and cold gas in molecular clouds. This suggests that the two-phase structure of molecular clouds is an essential ingredient for fully understanding molecular hydrogen in these objects.

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UV-driven chemistry in simulations of the interstellar medium

Cited by 54 publications

References 50 publications

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

[C II] absorption and emission in the diffuse interstellar medium across the Galactic plane

H₂distribution during the formation of multiphase molecular clouds

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UV-driven chemistry in simulations of the interstellar medium

Cited by 54 publications

References 50 publications

HERSCHEL/HIFI SPECTRAL MAPPING OF C+, CH+, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH+ FORMATION

HERSCHEL/HIFI SPECTRAL MAPPING OF C+, CH+, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH+ FORMATION

[C II] absorption and emission in the diffuse interstellar medium across the Galactic plane

H2distribution during the formation of multiphase molecular clouds

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HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

H₂distribution during the formation of multiphase molecular clouds