Constraining the size of the carrier of the λ5797.1 diffuse interstellar band

Abstract

The diffuse interstellar band (DIB) at 5797.1Å is simulated based on three premises: (1) The carrier of the DIB is polar as concluded by T. Oka et al. from the anomalous spectrum toward Herschel 36. (2) The sharp central feature observed by P. J. Sarre's group is the Qbranch of a parallel band of a prolate top. (3) The radiative temperature of the environment is Tr = 2.73 K.A 2 Π ← 2 Π transition of a linear radical is simulated. Results depend on 10 parameters, with the rotational constant B being the most critical. Comparisons of calculated spectra with observed data constrain B, which in turn constrains the number of heavy atoms to 5 ≤ n ≤ 7. Upper limits based on the Her 36 spectrum and lower limits based on stability against photodissociation are also discussed. The latter is based on the assumption that the DIB molecules are produced top-down from the breakdown of dust rather than bottom-up by chemical reactions.A difficulty with this limit is that J. P. Maier's laboratory has observed many molecules within this size range, some of which have been tested astronomically. Candidates are discussed in light of many prolate tops observed by the P. Thaddeus and M. C. McCarthy microwave group.

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

“…The low abundance of the fullerenes means, however, that they are likely not the only kind of DIB carrier, especially for the strong bands studied here (Omont 2016). The limits on carrier size for the l5797.1 and l5780.5 DIBs inferred by Oka et al (2013) and Huang & Oka (2015) also raise the question of whether PAHs and fullerenes can be the carriers for those DIBs, and more generally, what the dominant ionization state of the DIB carriers might be in any given part of the cloud. It is not certain that the radiation field is the only factor governing the behavior of the DIBs, as other parameters, such as density, may contribute as well.…”

Dust evolution, a global view I. Nanoparticles, nascence, nitrogen and natural selection … joining the dots

“…This approach is also not without problems because, for example:

— it is not clear that large planar, polycyclic aromatic species will retain their inherent structure in the harsh environments of the ISM [56–62];

— the DIB correlation with dust is stronger than with small radicals (e.g. C 2 , C 3 , CN, CH, …), implying that interstellar chemistry is more consistent with a top-down process than with bottom-up formation [63,64]; and

— the extrapolation of PAH absorption cross-sections to nanoparticle dimensions appears to be somewhat problematic [4,13]. …”

VISIBLE ABSORPTIONS OF POTENTIAL DIFFUSE ISM HYDROCARBONS: C9H9 AND C9H5 RADICALS

“…Analysis of the apparent rotational profiles of a few DIBs with resolved structure has led Huang & Oka (2015) to conclude that their most likely carriers are smaller molecules with five to eight heavy atoms. The initially evident systems, such as C 6 H and others, have already been studied, albeit with negative results (Motylewski et al 2000).…”