The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC

Turner, B. E.; Pirogov, L. E.; Minh, Y. C.

doi:10.1086/304228

Cited by 91 publications

(77 citation statements)

References 27 publications

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“…An IRAS point source is located at southern edge of the cloud which is detected at 100µm only (Clemens (1991) showed that CB4 is cold (≈ 7K) and does not contain a dense core (n > 10 4 cm −3 ) (Clemens et al (1991), Kane et al (1994) and Turner et al (1997)). …”

Section: Cb4mentioning

confidence: 99%

Study of grain alignment efficiency and a distance estimate for small globule CB4

Barman

Das

2015

Res. Astron. Astrophys.

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We study the polarization efficiency (defined as the ratio of polarization to extinction) of stars in the background of the small, nearly spherical and isolated Bok globule CB4 to understand the grain alignment process. A decrease in polarization efficiency with an increase in visual extinction is noticed. This suggests that the observed polarization in lines of sight which intercept a Bok globule tends to show dominance of dust grains in the outer layers of the globule. This finding is consistent with the results obtained for other clouds in the past. We determined the distance to the cloud CB4 using near-infrared photometry (2MASS JHKS colors) of moderately obscured stars located at the periphery of the cloud. From the extinction-distance plot, the distance to this cloud is estimated to be (459 ± 85) pc.

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Section: Cb4mentioning

confidence: 99%

Study of grain alignment efficiency and a distance estimate for small globule CB4

Barman

Das

2015

Res. Astron. Astrophys.

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“…On the other hand, the HNC/HCN ratio is observed to decrease at increasingly high densities in interstellar clouds (Turner et al 1997). The relatively low HNC/HCN ratio observed in PNe could then be due to the combination of moderate temperatures and densities prevailing in the molecular envelopes.…”

Section: Chemistry Of N-bearing Moleculesmentioning

confidence: 99%

The chemistry of compact planetary nebulae

Josselin

Bachiller

2002

A&A

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Abstract. We report high-sensitivity millimetre observations of several molecular species ( 13 CO, HCN, HNC, CN, HCO + and N 2 H + ) in a sample of compact planetary nebulae. Some species such as HCO + and CN are particularly abundant compared to envelopes around AGB stars or even interstellar clouds. We have estimated the following average values for the column densities ratios: CN/HCN ∼ 2.6, HCO + /HCN ∼ 0.5, and HNC/HCN ∼ 0.4. Thus, the chemical composition of the molecular envelopes in these compact PNe appears somewhat intermediate between the composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such as the Ring, M4-9, or the Helix). From observations of the CO isotopomers, we have estimated that the 12 C/ 13 C ratio is in the range 10 < ∼ 12 C/ 13 C < ∼ 40. These values are below those expected from standard asymptotic giant branch models and suggest non-standard mixing processes. The observed molecular abundances are compared to very recent modelling work, and we conclude that the observations are well explained, in general terms, by time-dependent gas-phase chemical models in which the ionization rate is enhanced by several orders of magnitude with respect to the average interstellar value. Thus, our observations confirm that the chemistry in the neutral shells of PNe is essentially governed by the high energy radiation from the hot central stars. The complexity of the chemical processes is increased by numerous factors linked to the properties of the central star and the geometry and degree of clumpiness of the envelope. Several aspects of the PN chemistry that remains to be understood are discussed within the frame of the available chemical models.

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“…In fact, the decrease of this ratio in our models may be misleading: both HCN and HNC fractional abundances do indeed increase when nitrogen is hydrogenated and released from grains but while HCN is efficiently destroyed, HNC remains constant. The observational result of Turner et al (1997) therefore suggests that the UMIST95 rate file, which we have used, may not contain all the possible destruction routes for HNC. Indeed the new UMIST99 database (Le Teuff et al 2000) does add several new reactions for HNC.…”

Section: −9mentioning

confidence: 99%

“…A puzzling result is, however, the variation of HCN/HNC ratio among models: in models where hydrogenation and ejection of nitrogen occur, the HNC column density increases, while that of HCN stays roughly the same as in Models A and B, leading to a decrease in the HCN/HNC ratio (making it much less than unity) for Models C-F3; this result is not supported by observations. Indeed, Turner et al (1997) find the ratio HCN/HNC to be "significantly above unity in translucent clouds". In fact, the decrease of this ratio in our models may be misleading: both HCN and HNC fractional abundances do indeed increase when nitrogen is hydrogenated and released from grains but while HCN is efficiently destroyed, HNC remains constant.…”

Section: −9mentioning

confidence: 99%

Surface hydrogenation in diffuse and translucent interstellar clouds

O'Neill

Viti

Williams

2002

A&A

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Abstract. We investigate the effects of grain-surface hydrogenation reactions on gas phase chemical models of diffuse and translucent clouds in the interstellar medium. Models in which gas phase species freeze out on to dust without release do not match observed column densities well. Expanding on previous work we extend our models to include the release of C, N, O, S and CO following hydrogenation on the grain surface. The results show that such mechanisms do improve the ability of chemical models to reproduce observed abundances, not only through the release of otherwise frozen-out species but also because of the additional hydrogenating reactions. For example, the predicted column densities of NH in diffuse clouds and H2S and NH3 in translucent clouds match observations better in models with grain-surface hydrogenation.

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The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC

Cited by 91 publications

References 27 publications

Study of grain alignment efficiency and a distance estimate for small globule CB4

Study of grain alignment efficiency and a distance estimate for small globule CB4

The chemistry of compact planetary nebulae

Surface hydrogenation in diffuse and translucent interstellar clouds

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