C-12/C-13 isotope ratio across the Galaxy from observations of C-13/O-18 in molecular clouds

Langer, W. D.; Penzias, A. A.

doi:10.1086/168935

Cited by 318 publications

(316 citation statements)

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“…Their observations showed a slight enhancement of C 18 O/ 13 C 18 O = 75 and no enhancement of [ 13 Ci]/ [Ci] relative to the standard elemental abundance while the chemical models predicted the opposite. The systematic study of the C 18 O/ 13 C 18 O by Langer & Penzias (1990 showed a systematic gradient with Galactocentric radius, i.e. significantly higher 13 C abundances in the inner Milky Way, but also variations between 57 and 78 at the solar circle.…”

Section: Introductionmentioning

confidence: 92%

Carbon fractionation in photo-dissociation regions

Röllig

Ossenkopf

2013

A&A

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We upgraded the chemical network from the UMIST Database for Astrochemistry 2006 to include isotopes such as 13 C and 18 O. This includes all corresponding isotopologues, their chemical reactions and the properly scaled reaction rate coefficients. We study the fractionation behavior of astrochemically relevant species over a wide range of model parameters, relevant for modelling of photo-dissociation regions (PDRs). We separately analyze the fractionation of the local abundances, fractionation of the total column densities, and fractionation visible in the emission line ratios. We find that strong C + fractionation is possible in cool C + gas. Optical thickness as well as excitation effects produce intensity ratios between 40 and 400. The fractionation of CO in PDRs is significantly different from the diffuse interstellar medium. PDR model results never show a fractionation ratio of the CO column density larger than the elemental ratio. Isotope-selective photo-dissociation is always dominated by the isotope-selective chemistry in dense PDR gas. The fractionation of C, CH, CH + and HCO + is studied in detail, showing that the fractionation of C, CH and CH + is dominated by the fractionation of their parental species. The light hydrides chemically derive from C + , and, consequently, their fractionation state is coupled to that of C + . The fractionation of C is a mixed case depending on whether formation from CO or HCO + dominates. Ratios of the emission lines of [C ii], [C i], 13 CO, and H 13 CO + provide individual diagnostics to the fractionation status of C + , C, and CO.

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Section: Introductionmentioning

confidence: 92%

Carbon fractionation in photo-dissociation regions

Röllig

Ossenkopf

2013

A&A

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“…As long as there is still a relatively high carbon-atom concentration in the gas phase, there is enough free 13 C to allow strong enrichment of CN through the 13 C + CN reaction. When CO molecules become the reservoir of carbon, even if in that case the 13 C concentration is low, the 13 C + + 12 CO → 12 C + + 13 CO reaction still leads to a small 13 CO enrichment (Langer & Penzias 1990;Milam et al 2005). Although this small excess is not measurable in CO, significant amounts of 13 C are locked up in CO, and most of the other carbon containing species become depleted in 13 C, as found for CH and other carbon chains.…”

Section: C Chemistrymentioning

confidence: 99%

Isotopic fractionation of carbon, deuterium, and nitrogen: a full chemical study

Roueff

Loison

Hickson

2015

A&A

171

349

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Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The 14 N/ 15 N ratio is difficult to measure directly for molecules containing carbon. Aims. Using a time-dependent gas-phase chemical model, we check the underlying hypothesis that the 13 C/ 12 C ratio of nitriles and isonitriles is equal to the elemental value. Methods. We built a chemical network that contains D, 13 C, and 15 N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolutionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15 N, which agrees with observations. The 14 N/ 15 N ratio in N 2 H + is found to be close to the elemental value, in contrast to previous models that obtain a significant enrichment, because we found that the fractionation reaction between 15 N and N 2 H + has a barrier in the entrance channel. The high values of the N 2 H + / 15 NNH + and N 2 H + /N 15 NH + ratios derived in L1544 cannot be reproduced in our model. Finally, we find that nitriles and isonitriles are in fact significantly depleted in 13 C, thereby challenging previous interpretations of observed C 15 N, HC 15 N, and H 15 NC abundances from 13 C containing isotopologues.

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“…The solar ratio is C12/C 13 = 90. The ISM value (Crane et al 1990;Langer and Penzias 1990) rises from 24 near the galactic center through 30 at 5 kpc and 60 here to about 70 at 12 kpc (where O/H is definitely less than solar). This is just about the only evidence we have that interstellar gas has continued to evolve chemically since the solar system formed.…”

Section: Interstellar Matter and The Cosmic Ray Source Compositionmentioning

confidence: 73%

“…First, CI2/C 13 can be extracted from ratios in CH, CO, and other molecules. One has to worry about mass-dependent chemical fractionation and about saturated lines, the latter being alleviated by using, e.g., CO ~8 (Langer and Penzias 1990). The solar ratio is C12/C 13 = 90.…”

Section: Interstellar Matter and The Cosmic Ray Source Compositionmentioning

confidence: 99%

The origin and abundances of the chemical elements revisited

Trimble

1991

The Astron Astrophys Rev

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C-12/C-13 isotope ratio across the Galaxy from observations of C-13/O-18 in molecular clouds

Cited by 318 publications

References 0 publications

Carbon fractionation in photo-dissociation regions

Carbon fractionation in photo-dissociation regions

Isotopic fractionation of carbon, deuterium, and nitrogen: a full chemical study

The origin and abundances of the chemical elements revisited

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