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 55 publications

(61 citation statements)

References 50 publications

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“…Indeed, the mean column density through the cube is about 10 21 cm −2 (corresponding to A V 0.6), which is comparable to the values in the simulation of Pelkonen et al (2009), but over a much larger volume (18 pc box compared to less than 1 pc); the bulk of the gas is therefore more fragmented and radiation penetrates more easily (Levrier et al 2012). That is why we take a uniform parameter p 0 .…”

Section: Simulated Planck Observationsmentioning

confidence: 70%

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Ade

Aghanim

Alina

et al. 2015

A&A

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126

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Polarized emission observed by Planck HFI at 353 GHz towards a sample of nearby fields is presented, focusing on the statistics of polarization fractions p and angles ψ. The polarization fractions and column densities in these nearby fields are representative of the range of values obtained over the whole sky. We find that: (i) the largest polarization fractions are reached in the most diffuse fields; (ii) the maximum polarization fraction p max decreases with column density N H in the more opaque fields with N H > 10 21 cm −2 ; and (iii) the polarization fraction along a given line of sight is correlated with the local spatial coherence of the polarization angle. These observations are compared to polarized emission maps computed in simulations of anisotropic magnetohydrodynamical turbulence in which we assume a uniform intrinsic polarization fraction of the dust grains. We find that an estimate of this parameter may be recovered from the maximum polarization fraction p max in diffuse regions where the magnetic field is ordered on large scales and perpendicular to the line of sight. This emphasizes the impact of anisotropies of the magnetic field on the emerging polarization signal. The decrease of the maximum polarization fraction with column density in nearby molecular clouds is well reproduced in the simulations, indicating that it is essentially due to the turbulent structure of the magnetic field: an accumulation of variously polarized structures along the line of sight leads to such an anti-correlation. In the simulations, polarization fractions are also found to anti-correlate with the angle dispersion function S. However, the dispersion of the polarization angle for a given polarization fraction is found to be larger in the simulations than in the observations, suggesting a shortcoming in the physical content of these numerical models. In summary, we find that the turbulent structure of the magnetic field is able to reproduce the main statistical properties of the dust polarization as observed in a variety of nearby clouds, dense cores excluded, and that the large-scale field orientation with respect to the line of sight plays a major role in the quantitative analysis of these statistical properties.

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Section: Simulated Planck Observationsmentioning

confidence: 70%

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Ade

Aghanim

Alina

et al. 2015

A&A

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126

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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%

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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“…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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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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UV-driven chemistry in simulations of the interstellar medium

Cited by 55 publications

References 50 publications

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

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

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UV-driven chemistry in simulations of the interstellar medium

Cited by 55 publications

References 50 publications

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

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

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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