Visual absorption lines in dark clouds - Highly reddened stars near the center of the Ophiuchus molecular cloud

Details Visual absorption lines in dark clouds - Highly reddened stars near the center of the Ophiuchus molecular cloud Visual absorption lines in dark clouds - Highly reddened stars near the center of the Ophiuchus molecular cloud

Mar 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...302..492c&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 302, March 1, 1986, p. 492-503.

Astronomy and Astrophysics

Astrophysics

45

Absorption Spectra, Interstellar Chemistry, Interstellar Extinction, Molecular Clouds, Ophiuchi Clouds, Stellar Spectra, Abundance, Cn Emission, Electron Density (Concentration), Molecular Spectra, Nebulae, Radial Velocity, Visible Spectrum

Scientific paper

New high-resolution spectroscopic observations of four stars deeply embedded in the ρ Ophiuchi dark cloud complex are presented. The stars are located near the center of the molecular cloud observed by Encrenaz, Falgarone, and Lucas (1975) and Myers et al. (1978) and include HD 147889, the most heavily obscurred star observed in the Ophiuchus cloud. The data include molecular lines of CN, CH, and CH+, as well as atomic lines of Fe I, Ca I, and Ca II. For HD 147889 and HD 147701 it is necessary to fit the CN lines with a smaller turbulent velocity than the CH lines if we require that the population of the two CN levels be controlled by the microwave (Texc = 2.8 K) background. However, if the CN lines are fitted to the CH curve of growth, we find Texc = 3.2 K, indicating an additional excitation source. The CH data for all four lines of sight are found to follow the relation N(CH) ∝ N(H2) shown by Federman (1982) and Danks, Federman, and Lambert (1984). For the lines of sight to HD 147701 and HD 147343, the relation N(CN) ∝ N(H2)3 as shown by Federman, Danks, and Lambert (1984) is extended to reddenings as high as E(B - V) = 0.7 [log N(H2) ≍ 21]. However, for the two most reddened stars, HD 147889 and HD 147648 [log N(H2) > 21], N(CN) is found to be "underabundant" by at least a factor of 10. This also appears to be true for the highly reddened Taurus dark cloud star HD 29647 (Crutcher 1985), indicating that the chemistry of CN formation for high reddenings (densities) is somewhat different. From the atomic data, ionization equilibrium considerations indicate that use of the attenuated diffuse radiation results in ne = 0.4 cm-3 from Ca II/Ca I, which is a factor of 5-10 larger than ne from Fe II/Fe I. Although this may in part be due to different line-of-sight distributions, we find that, owing to the anomalously low extinction of Ophiuchus dust at λ = 1600 Å (the location of the ionization edge of Fe), the diffuse radiation field underestimates the amount of ionizing flux penetrating the cloud by perhaps a factor of 3 or more. Finally, an examination of the depletion of Ca and Fe for these four lines of sight as well as for ρ Oph, &chi Oph, and σ Sco seems to indicate a definite trend of increasing depletion with density.

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