Variations in the HCN/HNC abundance ratio in the Orion molecular cloud

Goldsmith, Paul F.; Irvine, William M.; Hjalmarson, Å.; Elldér, J.

doi:10.1086/164692

Cited by 66 publications

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“…The HCN over HNC ratio has been used to determine time scales for the chemical evolution (Goldsmith et al 1986;Irvine et al 1987;Schilke et al 1992). It is beyond the scope of this work to perform a similar analysis here for the W 3 sources, but it is worthwhile to note that the values for the three W 3 sources lie somewhere between those of Orion IRc2 and the Ridge.…”

Section: Chemical Characteristicsmentioning

confidence: 98%

“…HNC is of interest for comparison with HCN since the HNC/HCN abundance ratio has been found to differ significantly between cold dark clouds, where it is close to unity, and warm clouds, where HCN can be more abundant by up to two orders of magnitude (Irvine et al 1987;Goldsmith et al 1986;Schilke et al 1992). In W 3, HCN is also observed to be more prominent than HNC.…”

Section: Hncmentioning

confidence: 99%

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Physical and chemical variations within the W3 star-forming region

Helmich

Jansen

Graauw³

et al. 1995

Lecture Notes in Physics

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Abstract. Results are presented of the 345 GHz spectral survey toward three sources in the W 3 Giant Molecular Cloud: W 3 IRS4, W 3 IRS5 and W 3(H 2 O). Nearly 90% of the atmospheric window between 334 and 365 GHz has been scanned using the James Clerk Maxwell Telescope (JCMT 1 ) down to a noise level of ∼ 80 mK per resolution element. These observations are complemented by a large amount of data in the 230 GHz atmospheric window. From this data set physical conditions and beam-averaged column densities are derived for more than 14 chemically different species (over 24 different isotopes). The physical parameters derived in Paper I (Helmich et al. 1994) are confirmed by the analysis of the excitation of other species, although there is evidence that the silicon-and sulfurbearing molecules exist in a somewhat denser and warmer environment. The densities are high, ≥ 10 6 cm −3 , in the three sources and the kinetic temperatures for the bulk of the gas range from 55 K for IRS4 to 220 K for W 3(H 2 O). The chemical differences between the three sources are very striking: silicon-and sulfur-bearing molecules such as SiO and SO 2 are prominent toward IRS5, whereas organic molecules like CH 3 OH, CH 3 OCH 3 and CH 3 OCHO are at least an order of magnitude more abundant toward W 3(H 2 O). Vibrationally excited molecules are also detected toward this source. Only simple molecules are found toward IRS4. The data provide constraints on the amount of deuterium fractionation and the ionization fraction in the observed regions as well. These chemical character- istics are discussed in the context of an evolutionary sequence, in which IRS5 is the youngest, W 3(H 2 O) somewhat older and IRS4, although still enigmatic, the oldest.

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Section: Chemical Characteristicsmentioning

confidence: 98%

Section: Hncmentioning

confidence: 99%

Physical and chemical variations within the W3 star-forming region

Helmich

Jansen

Graauw³

et al. 1995

Lecture Notes in Physics

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“…Churchwell et al 1984;Harju 1989), while in warmer and denser clouds associated with massive star formation the ratio can decrease to values of ∼0.01 (e.g. Goldsmith et al 1981Goldsmith et al , 1986Schilke et al 1992). It is thus highly surprising to find N(HNC)/N(HCN) ∼ 1 in the warm environment of a nuclear starburst, like that in NGC 4945.…”

Section: Ngc 4945 Orion and Tmc-1mentioning

confidence: 99%

Dense gas in nearby galaxies

Wang

Henkel²,

Chin

et al. 2004

A&A

129

184

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Abstract.A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945 has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are also presented. An analysis of CO and 1.3 mm continuum fluxes indicates that the conversion factor between H 2 column density and CO J = 1−0 integrated intensity is smaller than in the galactic disk by factors of 5−10. A large number of molecular species indicate the presence of a prominent high density interstellar gas component characterized by n H 2 ∼ 10 5 cm −3 . Some spectra show Gaussian profiles. Others exhibit two main velocity components, one at ∼450 km s −1 , the other at ∼710 km s −1 . While the gas in the former component has a higher linewidth, the latter component arises from gas that is more highly excited as is indicated by HCN, HCO + and CN spectra. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an "overabundance" of HNC in the nuclear environment of NGC 4945. While the HNC/HCN J = 1−0 line intensity ratio is ∼0.5, the HNC/HCN abundance ratio is ∼1. From a comparison of K a = 0 and 1 HNCO line intensities, an upper limit to the background radiation of 30 K is derived. While HCN is subthermally excited (T ex ∼ 8 K), CN is even less excited (T ex ∼ 3−4 K), indicating that it arises from a less dense gas component and that its N = 2−1 line can be optically thin even though its N = 1−0 emission is moderately optically thick. Overall, fractional abundances of NGC 4945 suggest that the starburst has reached a stage of evolution that is intermediate between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. Within the limits of uncertainty, carbon and oxygen isotope ratios appear to be the same in the nuclear regions of NGC 4945 and NGC 253. S ratios (6.4 ± 0.3, 195 ± 45, 105 ± 25 and 13.5 ± 2.5 in NGC 4945, respectively) appear to be characteristic properties of a starburst environment in which massive stars have had sufficient time to affect the isotopic composition of the surrounding interstellar medium.

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“…However, the abundance ratio between HNC and HCN is strongly dependent on the temperature. A good example is the Orion molecular cloud, where the HNC/HCN ratio was found to strongly decrease (by more than an order of magnitude) when going from the colder quiescent parts of the cloud to the warm plateau and hot-core regions where the ratio is much smaller than unity (Goldsmith et al 1986;cf. Schilke et al 1992).…”

Section: Hcn Hnc and Hn 13 C [Hydrogen (Iso-)cyanide]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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Variations in the HCN/HNC abundance ratio in the Orion molecular cloud

Cited by 66 publications

References 0 publications

Physical and chemical variations within the W3 star-forming region

Physical and chemical variations within the W3 star-forming region

Dense gas in nearby galaxies

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

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