Observing a column-dependent<i>ζ</i>in dense interstellar sources: the case of the Horsehead nebula

Rimmer, Paul B.; Herbst, Eric; Morata, Ó.; Roueff, E.

doi:10.1051/0004-6361/201117048

Cited by 64 publications

(93 citation statements)

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“…Turner et al 2000). The dissociative recombination of the ions CH + (Rimmer et al 2012), C 2 H + 2 , and C 2 H + 3 also yield ethynyl molecules in dense molecular clouds (Mul & McGowan 1980). The C 2 H molecules can themselves be photodissociated back to form C 2 and C 2 H + (Fuente et al 1993).…”

Section: H (Ethynyl)mentioning

confidence: 99%

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

Miettinen

2014

A&A

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Context. Infrared dark clouds (IRDCs) provide a useful testbed in which to investigate the genuine initial conditions and early stages of massive-star formation. Aims. We attempt to characterise the chemical properties of a sample of 35 massive clumps of IRDCs through multi-molecular line observations. We also search for possible evolutionary trends among the derived chemical parameters.Methods. The clumps are studied using the MALT90 (Millimetre Astronomy Legacy Team 90 GHz) line survey data obtained with the Mopra 22 m telescope. The survey covers 16 different transitions near 90 GHz. The spectral-line data are used in concert with our previous LABOCA (Large APEX BOlometer CAmera) 870 μm dust emission data. Results. Eleven MALT90 transitions are detected towards the clumps at least at the 3σ level. Most of the detected species (SiO, C 2 H, HNCO, HCN, HCO + , HNC, HC 3 N, and N 2 H + ) show spatially extended emission towards many of the sources. Most of the fractional abundances of the molecules with respect to H 2 are found to be comparable to those determined in other recent similar studies of IRDC clumps. We found that the abundances of SiO, HNCO, and HCO + are higher in IR-bright clumps than in IR-dark sources, reflecting a possible evolutionary trend. A hint of this trend is also seen for HNC and HC 3 N. An opposite trend is seen for the C 2 H and N 2 H + abundances. Moreover, a positive correlation is found between the abundances of HCO + and HNC, and between those of HNC and HCN. The HCN and HNC abundances also appear to increase as a function of the N 2 H + abundance. The HNC/HCN and N 2 H + /HNC abundance ratios are derived to be near unity on average, while that of HC 3 N/HCN is ∼10%. The N 2 H + /HNC ratio appears to increase as the clump evolves, while the HNC/HCO + ratio shows the opposite behaviour. Conclusions. The detected SiO emission is probably caused by shocks driven by outflows in most cases, although shocks resulting from the cloud formation process could also play a role. Shock-origin for the HNCO, HC 3 N, and CH 3 CN emission is also plausible. The average HNC/HCN ratio is in good agreement with those seen in other IRDCs, but gas temperature measurements would be neeeded to study its temperature dependence. Our results support the finding that C 2 H can trace the cold gas, and not just the photodissociation regions. The HC 3 N/HCN ratio appears to be comparable to the values seen in other types of objects, such as T Tauri disks and comets.

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Section: H (Ethynyl)mentioning

confidence: 99%

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

Miettinen

2014

A&A

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“…Montmerle 2010) raised the questions about the origin of the CR flux that generates such a high ionisation rate and how to reconcile these values with those ones measured in denser clouds that are more than one order of magnitude lower. A number of studies approached this problem using different strategies, analysing the effects of Alfvén waves on CR streaming (Skilling & Strong 1976;Hartquist et al 1978;Padoan & Scalo 2005;Rimmer et al 2012), magnetic mirroring and focusing (Cesarsky & Völk 1978;Chandran 2000;Padovani & Galli 2011), or the possible existence of a low-energy flux of CR particles able to ionise diffuse but not dense clouds (Takayanagi 1973;Umebayashi & Nakano 1981;McCall et al 2003;Padovani et al 2009). …”

Section: Introductionmentioning

confidence: 99%

Cosmic-ray ionisation in collapsing clouds

Padovani

Hennebelle

Galli

2013

A&A

104

118

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Context. Cosmic rays play an important role in dense molecular cores, affecting their thermal and dynamical evolution and initiating the chemistry. Several studies have shown that the formation of protostellar discs in collapsing clouds is severely hampered by the braking torque exerted by the entrained magnetic field on the infalling gas, as long as the field remains frozen to the gas. Aims. In this paper we examine the possibility that the concentration and twisting of the field lines in the inner region of collapse can produce a significant reduction of the ionisation fraction. Methods. To check whether the cosmic-ray ionisation rate can fall below the critical value required to maintain good coupling, we first study the propagation of cosmic rays in a model of a static magnetised cloud varying the relative strength of the toroidal/poloidal components and the mass-to-flux ratio. We then follow the path of cosmic rays using realistic magnetic field configurations generated by numerical simulations of a rotating collapsing core with different initial conditions. Results. We find that an increment of the toroidal component of the magnetic field, or, in general, a more twisted configuration of the field lines, results in a decrease in the cosmic-ray flux. This is mainly due to the magnetic mirroring effect that is stronger where larger variations in the field direction are present. In particular, we find a decrease of the cosmic-ray ionisation rate below 10 −18 s −1 in the central 300-400 AU, where density is higher than about 10 9 cm −3 . This very low value of the ionisation rate is attained in the cases of intermediate and low magnetisation (mass-to-flux ratio λ = 5 and 17, respectively) and for toroidal fields larger than about 40% of the total field. Conclusions. Magnetic field effects can significantly reduce the ionisation fraction in collapsing clouds. We provide a handy fitting formula to compute approximately the attenuation of the cosmic-ray ionisation rate in a molecular cloud as a function of the density and the magnetic configuration.

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“…This last point has indeed been explored in Rimmer et al (2012), who studied the influence of the cosmic-ray ionization rate and showed that high value of ζ -either a fixed ζ ∼ 10…”

Section: Small Hydrocarbon Casementioning

confidence: 88%

“…On the other hand, in the PAH region the hydrocarbon abundances seem not to depend much on ζ, probably becauce in this region, the hydrocarbon chemistry is dominated by photon interactions than by cosmic-rays'. It should thus be worthwhile to further explore the impact of the cosmic-ray ionization rate in our model to see how it impacts the chemistry and if it helps to reproduce better the small hydrocarbons in the PAH region, even though Rimmer et al (2012) found with their model that it does not.…”

Section: Small Hydrocarbon Casementioning

confidence: 99%

“…The dust grain temperature profile has been found to stay relatively low, from ∼ 12 K in the Core to almost 30 K at the outer layers of the PDR (Goicoechea et al 2009a). This low dust temperature profile can be explained as a result of the combination of the moderately low UV-photon radiation field of 60 × that of the ISRF (Mathis et al 1983;Habart et al 2005;Rimmer ).…”

Section: Introductionmentioning

confidence: 99%

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A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Gal

Herbst

Dufour

et al. 2017

A&A

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A wide variety of molecules have recently been detected in the Horsehead nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain chemistries should both contribute to the formation of organic molecules, and (ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas phase of grain surface species. In order to tackle these specific problems and more generally in order to better constrain the chemical structure of these types of environments we present a study of the Horsehead nebula gas-grain chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with an appropriate physical structure computed with the Meudon PDR Code and compared our modeled outcomes with published observations and with previously modeled results when available. The use of a large set of chemical reactions coupled with the time-dependent code Nautilus allows us to reproduce most of the observations well, including those of the first detections in a PDR of the organic molecules HCOOH, CH 2 CO, CH 3 CHO and CH 3 CCH, which are mostly associated with hot cores. We also provide some abundance predictions for other molecules of interest. Understanding the chemistry behind the detection of these organic molecules is crucial to better constrain the environments these molecules can probe.

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Observing a column-dependentζin dense interstellar sources: the case of the Horsehead nebula

Cited by 64 publications

References 65 publications

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

Cosmic-ray ionisation in collapsing clouds

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

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