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

Abstract

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.

Abundances of PAHs in the ISM: confronting observations with experimental results

“…Toward both lines of sight, the radial velocities of the NH absorption lines agree with those of CH, CH + , and CN. Table 4) are consistent with the measurements of Gredel et al (1991) and Gredel et al (2002) toward those lines of sight.…”

“…The CH and CN absorption lines are resolved into two velocity components at similar heliocentric velocities (cf. Table 4 (Gredel et al 2002, and references therein). From observations of C 2 , gaskinetic temperatures of T kin = 85 K, densities of n > 1000 cm −3 , and column densities of N(C 2 ) ≈ 10 14 cm −2 were inferred toward both lines of sight.…”

First Detection of 13 CH + ( J = 1-0)

“…We thus conclude that the line arises in a gas phase In several respects, the existence of in the diffuse inter-ϩ CH stellar medium has remained for several decades among the most intractable puzzles in astrophysics (e.g., Bates & Spitzer 1951;Black & Dalgarno 1973;Black et al 1975;Falgarone et al 1995;Gredel et al 2002): needs H 2 to form but, once formed, it ϩ CH recombines rapidly with electrons or reacts with H 2 to form . However, the radiative association process…”

“…Last, the existence of ϩ CH 3 small amounts of warm gas intimately associated with the CNM is supported by an increasing body of observations: e.g., Gredel et al (2002) and Gredel (1997) for extensive detections in ϩ CH the visible; Neufeld et al (2002) and Plume et al (2004) for H 2 O; and Gry et al (2002), Lacour et al (2005), and Falgarone et al (2005b) for traces of rotationally excited H 2 in the CNM that cannot be explained by UV excitation.…”

The au-scale structure in diffuse molecular gas towardsζ Persei

“…Let us first discuss shock models, as described by , Gredel et al (2002), and more recently by Lesaffre et al (2013). One difficulty in connecting their predictions to observations is that the 3D geometry of these regions is not well defined.…”