The Behavior of Selected Diffuse Interstellar Bands with Molecular Fraction in Diffuse Atomic and Molecular Clouds

Abstract

Abstract:We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form (f H2 ), with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five "normal" DIBs (ll5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to E B-V , show a "Lambda-shaped" behavior: they increase at low f H2 , peak at f H2 ~ 0.3, and then decrease. The similarly normalized column densities of Ca, Ca + , Ti + , and CH + also decline for f H2 > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with f H2 , and the trends exhibited by the three C 2 DIBs (ll4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with f H2 are accompanied by cosmic scatter, the dispersion at any given f H2 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 besides ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest W l (5780)/W l (5797) ratios, characterizing the so-called "sigma-zeta effect", occur only at f H2 < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing f H2 . In order of increasing environmental density, we find the l6283.8 and l5780.5 DIBs, the l6196.0 DIB, the l6613.6 DIB, the l5797.1 DIB, and the C 2 DIBs.

Near-infrared diffuse interstellar bands towards Her 36

“…In 9 Sgr, it is a factor of at least two stronger than the diffuse ISM value. This is well within the known variability between lines of sight for visual DIBs, which can approach a factor of 10 (Fan et al 2017), but very different from the behaviour of the visual DIBs referred to above, all of which are weaker in 9 Sgr than in Her 36 .…”

Are the carriers of diffuse interstellar bands and extended red emission the same?

“…5A in Fan et al 2017), and it suggests that EW5797/EW5780 is primarily affected by the relative abundance of atomic and molecular hydro-gen, but only indirectly affected by the UV radiation which can dissociate the molecular hydrogen. With the correlation between f H 2 and EW5797/EW5780 (Fan et al 2017), the inferred f H 2 for star #46 is in the range 0.1-0.2, which is consistent with a low f H 2 under a high radiation field described in Vos et al (2011) (See Fig.20). So, atomic hydrogen is predominant in the line of sight towards star #46, where intense photo-processing dissociates molecular hydrogen into atomic form.…”

C60+ diffuse interstellar band correlations and environmental variations

“…The environmental data, in particular the colour excess E B−V , the column densities of neutral hydrogen and molecular hydrogen, N(H) and N(H 2 ), and the molecular fraction of hydrogen, f H 2 , were taken from Fan et al (2017), who studied the behaviour of eight relatively strong DIBs in different interstellar environments. In the Fan et al (2017) dataset, most of the spectra were taken with the ARC echelle spectrograph (ARCES) on the 3.5 m telescope at the Apache Point Observatory (APO). Fan et al (2017), however, did not include the 9577 and 9633 Å DIBs.…”

“…In the Fan et al (2017) dataset, most of the spectra were taken with the ARC echelle spectrograph (ARCES) on the 3.5 m telescope at the Apache Point Observatory (APO). Fan et al (2017), however, did not include the 9577 and 9633 Å DIBs. Therefore, most of the EWs for the 9633 and 9577 DIBs were taken from Galazutdinov et al (2021), who collected spectra of these bands using several high resolving power echelle spectrographs.…”

“…Krelowski et al 1992;Galazutdinov et al 2004;Friedman et al 2011), and the strengths of other DIBs (e.g. Snow 1973;Cami et al 1997;Moutou et al 1999;McCall et al 2010;Xiang et al 2012;Fan et al 2017;Bondar 2020;Smith et al 2021), often with the aim to group DIBs in families that could have similar (or even the same) carriers (Krelowski & Walker 1987;Westerlund & Krelowski 1989;Cami et al 1997;Wszołek & Godłowski 2003).…”