Galactic<sup>12</sup>C/<sup>13</sup>C Ratios from Millimeter‐Wave Observations of Interstellar CN

Savage, C. M.; Apponi, A. J.; Ziurys, L. M.; Wyckoff, S.

doi:10.1086/342468

Cited by 114 publications

(130 citation statements)

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“…It would be interesting to compare the 12 C/ 13 C ratio in methanol and formaldehyde towards more of our sources. Savage et al (2002) present observations of the 12 C/ 13 C ratio in CN towards a number of galactic molecular clouds, including DR21 (OH) and W51 M (less than 10 from our targeted position in W51). They report a ratio of 35 ± 12 in W51 M, which is higher than for methanol, 18 ± 3, but within the errorbars for what is measured by Langer & Penzias (1990) for C 18 O, 46 ± 2.…”

Section: Discussionmentioning

confidence: 83%

Observational tests of interstellar methanol formation

Wirström

Geppert

Hjalmarson

et al. 2011

A&A

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Context. It has been established that the classical gas-phase production of interstellar methanol (CH 3 OH) cannot explain observed abundances. Instead it is now generally thought that the main formation path has to be by successive hydrogenation of solid CO on interstellar grain surfaces. Aims. While theoretical models and laboratory experiments show that methanol is efficiently formed from CO on cold grains, our aim is to test this scenario by astronomical observations of gas associated with young stellar objects (YSOs). Methods. We have observed the rotational transition quartets J = 2 K -1 K of 12 CH 3 OH and 13 CH 3 OH at 96.7 and 94.4 GHz, respectively, towards a sample of massive YSOs in different stages of evolution. In addition, the J = 1−0 transitions of 12 C 18 O and 13 C 18 O were observed towards some of these sources. We use the 12 C/ 13 C ratio to discriminate between gas-phase and grain surface origin: If methanol is formed from CO on grains, the ratios should be similar in CH 3 OH and CO. If not, the ratio should be higher in CH 3 OH due to 13 C fractionation in cold CO gas. We also estimate the abundance ratios between the nuclear spin types of methanol (E and A). If methanol is formed on grains, this ratio is likely to have been thermalized at the low physical temperature of the grain, and therefore show a relative over-abundance of A-methanol. Results. We show that the 12 C/ 13 C isotopic ratio is very similar in gas-phase CH 3 OH and C 18 O, on the spatial scale of about 40 , towards four YSOs. For two of our sources we find an overabundance of A-methanol as compared to E-methanol, corresponding to nuclear spin temperatures of 10 and 16 K. For the remaining five sources, the methanol E/A ratio is less than unity. Conclusions. While the 12 C/ 13 C ratio test is consistent with methanol formation from hydrogenation of CO on grain surfaces, the result of the E/A ratio test is inconclusive.

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Section: Discussionmentioning

confidence: 83%

Observational tests of interstellar methanol formation

Wirström

Geppert

Hjalmarson

et al. 2011

A&A

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“…In the case 13 CN, a case b βS coupling scheme is more applicable. Here, the electron and 13 C nuclear spins first combine together to generate quantum number F 1 , the rotational angular momentum adds in to create F 2 , and then the nitrogen spin couples in to produce F (see Savage et al 2002;Milam et al 2005 for a more detailed discussion).…”

Section: Resultsmentioning

confidence: 99%

“…If τ is determined to be thin, then the intensities of the strongest hf component of the 12 CN and 13 CN rotational transitions can be directly compared to derive an isotope ratio. This technique is described in detail by Savage et al (2002) and Milam et al (2005).…”

Section: Cnmentioning

confidence: 99%

“…The millimeter lines of 12 CO are well known to have large opacities, and 12 C/ 13 C ratios derived from carbon monoxide are usually model dependent (e.g., Schöier & Olofsson 2000). The CN radical, on the other hand, has a distinct hyperfine (hf) structure, which can be used to directly evaluate opacities (see Savage et al 2002;Milam et al 2005). Hence, values derived from 12 CN/ 13 CN serve as an additional constraint for this ratio.…”

Section: Introductionmentioning

confidence: 99%

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CIRCUMSTELLAR¹²C/¹³C ISOTOPE RATIOS FROM MILLIMETER OBSERVATIONS OF CN AND CO: MIXING IN CARBON- AND OXYGEN-RICH STARS

Milam

Woolf

Ziurys

2008

ApJ

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A survey of the 12 C/ 13 C ratio toward circumstellar envelopes has been conducted at millimeter wavelengths using the facilities of the Arizona Radio Observatory (ARO). The ratios were obtained for a sample of local C-and O-rich asymptotic giant branch and supergiant stars from observations of the 12 C and 13 C isotopologues of CO and CN, respectively. The J = 1 → 0 transitions of both molecules were observed at λ = 3 mm using the ARO 12 m telescope, while the J = 2 → 1 lines of the two species were measured using the ARO Sub-Millimeter Telescope (SMT) at λ = 1 mm. The 12 C/ 13 C ratios were determined from the CO data by modeling both transitions simultaneously with a circumstellar radiative transfer code, which can account for the high opacities present in the emission from this species. In the case of CN, the hyperfine structure was used to evaluate opacity effects. Ratios obtained independently from CO and CN are in good agreement. For the C-rich envelopes, the ratios fall in the range 12 C/ 13 C ∼ 25-90, while the O-rich shells have values of 10-35. Ratios of 12 C/ 13 C ∼ 3-14 are found for the supergiant stars, with the exception of VY CMa, where the values lie in the range 25-46. All ratios obtained in this study are 89, the solar value, suggesting that substantial carbon-13 enrichment may be currently occurring in the local interstellar medium. A qualitative model was constructed based on first and third dredge-up convective mixing that can reproduce the observed ratios. Substantial mixing of H-burning products must occur to explain the ratios in the O-rich objects, while a wide range of 12 C/ 13 C values can be generated by only a few percent mixing of He-burning ashes in the C-rich case. The 12 C/ 13 C ratios obtained in this study should help improve stellar yield models and contribute to the understanding of Galactic chemical evolution.

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“…The possible effects of selective photodissociation of CO have subsequently been emphasized (Glassgold et al 1985;Le Bourlot et al 1993;Visser et al 2009), which tend to increase the CO/ 13 CO ratio. The use of the CN radical as a tracer of 12 C/ 13 C isotopic ratio has been raised by Savage et al (2002), Milam et al (2005), who studied the corresponding gradient as a Appendices are available in electronic form at http://www.aanda.org function of the galactic distance. The actual value of the 12 C/ 13 C isotopic ratio in the local interstellar medium (ISM) is assumed to be 68 (Milam et al 2005).…”

Section: Introductionmentioning

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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Galactic¹²C/¹³C Ratios from Millimeter‐Wave Observations of Interstellar CN

Cited by 114 publications

References 31 publications

Observational tests of interstellar methanol formation

Observational tests of interstellar methanol formation

CIRCUMSTELLAR¹²C/¹³C ISOTOPE RATIOS FROM MILLIMETER OBSERVATIONS OF CN AND CO: MIXING IN CARBON- AND OXYGEN-RICH STARS

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

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Galactic12C/13C Ratios from Millimeter‐Wave Observations of Interstellar CN

Cited by 114 publications

References 31 publications

Observational tests of interstellar methanol formation

Observational tests of interstellar methanol formation

CIRCUMSTELLAR12C/13C ISOTOPE RATIOS FROM MILLIMETER OBSERVATIONS OF CN AND CO: MIXING IN CARBON- AND OXYGEN-RICH STARS

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

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Galactic¹²C/¹³C Ratios from Millimeter‐Wave Observations of Interstellar CN

CIRCUMSTELLAR¹²C/¹³C ISOTOPE RATIOS FROM MILLIMETER OBSERVATIONS OF CN AND CO: MIXING IN CARBON- AND OXYGEN-RICH STARS