A comparison of optical and radio wavelength observations of CH in the diffuse interstellar medium

Abstract

Listen

Translate article

view Abstract Citations (36) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A comparison of optical and radio wavelength observations of CH in the diffuse interstellar medium. Lang, K. R. ; Wilson, R. F. Abstract We report radio wavelength measurements and upper limits for CH emission from diffuse interstellar clouds in the direction of nearby bright stars whose optical wavelength spectra exhibit the absorption lines of CH. The F = 1-1 A-doublet transition at 3335.481 MHz was detected in the direction of 10 stars, and the F = 1-0 and F = 0-1 transitions at 3263.794 and 3349.193 MHz were detected in the direction of four stars. No departures from equilibrium line intensity ratios were observed. The velocities of the CH lines observed at optical and radio wavelengths are compatible with the assumption that in most cases the radio and optical transitions originate in the same diffuse interstellar cloud. The ratio of the column densities of CH inferred from the 10 measurements in the two spectral regions is NRAD/NopT = 1.68 + 0.34, where the error bar denotes one standard deviation. These measurements ensure the compatibility of the reduction procedures in the two regions, and indicate that the oscillator strengths and excitation temperatures used in the reductions cannot be in error by more than a factor of 2. The CH molecules must be well distributed within the diffuse interstellar clouds, and if clumping exists it is not substantial. The central velocities and the velocity dispersions of the CH and CH + profiles in the direction of Oph agree, suggesting that the two molecular species coexist throughout the diffuse interstellar cloud, and that neither molecule originates in separate compact or circumstellar regions. The column density, NoH, of the CH in the diffuse interstellar medium is strongly correlated with the extinction of starlight m = 3.0 , and the total hydrogen column density, (NH+ ), with Nc = 6.3 x l0' = (NH + 2NH2) for 0.6 mag and for extinctions smaller than 2 mag. The relative abundance NcH/NH is an order of magnitude higher than that predicted by the grain-formation theory of CH. When the linear relation found for extinctions less than 2 mag is extrapolated to the higher extinctions characteristic of dark clouds, column densities are predicted which are an order of magnitude higher than those observed. This difference may be due to uncertainties in the extinction estimates of dark clouds. Estimates of the relative column densities of CH and carbon monoxide, CO, obtained from radio wavelength measurements of the diffuse interstellar clouds agree with those obtained from observations at optical and ultraviolet wavelengths with NcH/Noo 0.3. The relative abundance of CH and OH in the diffuse interstellar medium is comparable to that in dark clouds with Nc /No 0.3, but the dark clouds appear to have a relative overabundance of CO or a relative underabundance of CH and OH with Nc /Nco 10- in dark clouds. Subject headings: interstellar: abundances - interstellar: molecules - radio sources: lines Publication: The Astrophysical Journal Pub Date: August 1978 DOI: 10.1086/156356 Bibcode: 1978ApJ...224..125L Keywords: Absorption Spectra; Hydrocarbons; Interstellar Matter; Molecular Absorption; Radio Sources (Astronomy); Abundance; Diatomic Molecules; Gaseous Diffusion; Interstellar Extinction; Line Spectra; Molecular Diffusion; Stellar Luminosity; Astrophysics; Interstellar Clouds:Molecules; Interstellar Clouds: Radio Observations full text sources ADS | data products SIMBAD (19)