Astrophys. J., 850, 194-194 (2017/December-1)
The behavior of selected diffuse interstellar bands with molecular fraction in diffuse atomic and molecular clouds.
FAN H., WELTY D.E., YORK D.G., SONNENTRUCKER P., DAHLSTROM J.A., BASKES N., FRIEDMAN S.D., HOBBS L.M., JIANG Z., RACHFORD B., SNOW T.P., SHERMAN R. and ZHAO G.
Abstract (from CDS):
We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form (fH2), with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five "normal" DIBs (λλ5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to EB–V, show a "lambda-shaped" behavior: they increase at low fH2, peak at fH2 ∼ 0.3, and then decrease. The similarly normalized column densities of Ca, Ca+, Ti+, and CH+ also decline for fH2 > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with fH2, and the trends exhibited by the three C2 DIBs (λλ4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with fH2 are accompanied by cosmic scatter, the dispersion at any given fH2 being significantly larger than the individual errors of measurement. The lambda-shaped trends suggest the balance between creation and destruction of the DIB carriers differs dramatically between diffuse atomic and diffuse molecular clouds; additional processes aside from ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest Wλ(5780)/Wλ(5797) ratios, characterizing the so-called "sigma-zeta effect," occur only at fH2 < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing fH2. In order of increasing environmental density, we find the λ6283.8 and λ5780.5 DIBs, the λ6196.0 DIB, the λ6613.6 DIB, the λ5797.1 DIB, and the C2 DIBs.
© 2017. The American Astronomical Society. All rights reserved.
dust, extinction - ISM: clouds - ISM: lines and bands - ISM: molecules - ISM: molecules
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