Interstellar C2, CH, and CN in translucent molecular clouds

Dishoeck, E. F. van; Black, J. H.

doi:10.1086/167391

Cited by 137 publications

(64 citation statements)

References 15 publications

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“…H 2 excitation can be used to get a handle on the I UV /n H ratio (Black & van Dishoeck 1987). Also, if present, C 2 rotational lines can be used to independently derive n H (Van Dishoeck & Black 1989). Possibly, more stringent results could be obtained from detailed modelling of specific Magellanic Cloud diffuse lines of sight.…”

Section: Discussion: Implications For the Pah-dib Carriermentioning

confidence: 99%

The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier

Cox

Spaans

2006

A&A

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Context. The unidentified diffuse interstellar bands (DIB) are observed throughout the Galaxy, the Local Group and beyond. Their carriers are possibly related to complex carbonaceous gas-phase molecules, such as (cationic) polycyclic aromatic hydrocarbons and fullerenes. Aims. In order to reveal the identity of the DIB carrier we investigate the effects of metallicity, radiation field and extinction curve on the PAH charge state distribution, and thus the theoretical emergent PAH spectrum, in diffuse interstellar clouds. This behaviour can then be linked to that of the DIB carrier, thus giving insight into its identity. Methods. We use radiative transfer and chemical models to compute the physical and chemical conditions in diffuse clouds with Galactic and Magellanic Cloud types of interstellar dust and gas. Subsequently, the PAH charge state distributions throughout these clouds are determined. Results. We find that the fraction of PAH cations is much higher in the Magellanic Cloud environments than in the Milky Way, caused predominantly by the respective lower metallicities, and mitigated by the steeper UV extinction curve. The fraction of anions is much lower in a low metallicity environment. The predicted DIB strength of cationic PAH carriers is similar to that of the Milk Way for the LMC and 40% for the SMC due to the overall metallicity. Stronger DIBs could be expected in the Magellanic Clouds if they emanate from clouds that are exposed to an average interstellar radiation field that is significantly stronger than in the Milky Way, although photo-destruction processes could possibly reduce this effect, especially for the smaller PAHs. Our results show that the presence and absence of DIB carriers in the Magellanic Cloud lines of sight can be tied to the PAH charge balance which is driven by metallicity, UV radiation and dust extinction effects.

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Section: Discussion: Implications For the Pah-dib Carriermentioning

confidence: 99%

The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier

Cox

Spaans

2006

A&A

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“…1) clearly indicate cold (dense) components in the diffuse medium (e.g. Van Dishoeck & Black 1989;Gredel 1999;Cecchi-Pestellini & Dalgarno 2002;Scappini et al 2002). The relative weakness of the 5780 Å DIB and the normal strength of the 5797 Å DIB (Table 4 and Fig.…”

Section: Ionisation Balancementioning

confidence: 91%

“…See e.g. Van Dishoeck & Black (1989) and references therein for transition levels and wavelengths. Table 2.…”

Section: Observed Dib Spectrum and Its Synthetic Counterpartmentioning

confidence: 99%

Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary 4U 1907+09

Cox¹,

Kaper²,

Foing

et al. 2005

A&A

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Abstract. High-resolution VLT/UVES spectra of the strongly reddened O supergiant companion to the X-ray pulsar 4U 1907+09 provide a unique opportunity to study the nature of the diffuse interstellar bands (DIBs) at unprecedented strength. We detect about 180 known DIBs, of which about 25 were listed as tentative and are now confirmed. A dozen new DIB candidates longwards of 6900 Å are identified. We show that the observed 5797 Å DIB strength is in line with the Galactic correlation with reddening, whereas the 5780 Å DIB strength is relatively weak. This indicates the contribution of denser regions, where the UV penetration is reduced. The presence of dense cloud cores is supported by the detection of C 2 rotational transitions. Members of one DIB family (5797, 6379 Å and 6196, 6613 Å) behave coherently, although one can make a distinction between the two correlated pairs. The broadened profiles of narrow DIBs are shown to be consistent with the premise that each of the main clouds in the line of sight discerned in the interstellar K  profile is contributing proportionally to the DIB profile. We complement and extend the relation of DIB strength with reddening E (B−V) , as well as with neutral hydrogen column density N(H ), respectively, using strongly reddened sightlines towards another four distant HMXBs. The 5780 Å DIB, and tentatively also the 5797 and 6613 Å DIBs, are better correlated to the gas tracer H  than to the dust tracer E (B−V) . The resulting relationship can be applied to any line of sight to obtain an estimate of the H  column density. In the search for the nature of the DIB carrier, this strongly reddened line of sight is a complementary addition to single cloud line of sight studies.

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“…Toward more reddened lines of sight, the correlation seems to break down. Two studies presented by van Dishoeck & Black (1989) and Gredel et al (1991) …”

Section: Introductionmentioning

confidence: 94%

Interstellar CN toward CH$\mathsf{^+}$-forming regions

Gredel¹,

Forêts²,

Federman³

2002

A&A

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Abstract. Measurements on interstellar CN absorption are presented for stars in three southern OB associations, NGC 2439, Vela OB1, and Cen OB1. CN is detected in 21 out of 31 stars observed. The doublet ratio for the R(1) and P (1) lines of the (0, 0) band of the B 2 Σ + -X 2 Σ + violet system and a comparison of violet system data with measurements of the (1, 0) and (2, 0) bands of the A 2 Π-X 2 Σ + red system are used to derive Doppler parameters and total column densities. Inferred CN column densities vary by more than an order of magnitude for lines of sight with similar CH column densities. Observations of the (0, 0) band of the CH B 2 Σ − -X 2 Π system are used to revise previously published CH column densities toward the lines of sight studied in CN. Together with earlier results on CH, CH + , and C2, the CN data presented here provide a homogeneous set of column densities and radial velocities of diatomic molecules in three individual translucent clouds. We use these data to study CN production via chemical models. Gas densities are inferred from models based on production via CH and C2 in cool gas. Most sightlines in our sample test densities typical for diffuse molecular gas (a few hundred cm −3 ) when the ultraviolet flux permeating the gas is between 1 and 5 times the average interstellar flux. A few lines of sight indicate that CN is produced under dark cloud conditions because relatively large densities are obtained or because this simple chemical scheme is unable to reproduce the observed CN columns. Low densities are indicated for directions with upper limits on CN. We add an ad hoc component of a number of low-velocity (<10 km s −1 ) criss-crossing MHD shocks to explain observed column densities of interstellar CH + . These shocks also produce about 10 to 30% of the total CH column along the line of sight.

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Interstellar C2, CH, and CN in translucent molecular clouds

Cited by 137 publications

References 15 publications

The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier

The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier

Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary 4U 1907+09

Interstellar CN toward CH$\mathsf{^+}$-forming regions

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