Dehydrogenated polycyclic aromatic hydrocarbons and UV bump

Abstract

Context. Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of polycyclic aromatic hydrocarbons (PAHs) in several ionisation states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C 24 H n , n ≤ 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. Aims. To assess the observational consequences of the latter hypothesis we have undertaken a systematic theoretical study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. Methods. We performed calculations using state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time-dependent DFT to evaluate the electronic excited-state properties. Results. We computed the absorption cross-section of the species C 24 H n (n = 12, 10, 8, 6, 4, 2, 0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fully dehydrogenated counterparts. Conclusions. π-electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the π → π * collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.

Diffuse Interstellar Bands and the Ultraviolet Extinction Curves: The Missing Link Revisited

“…Similar to Xiang et al (2011), we find that the 2175Å bump does not correlate with DIBs, except the λ6699Å DIB shows a tendency of correlating with the 2175Å bump. This suggests that it is unlikely for the extinction bump and DIBs to share a common carrier (see Figures 8d-14d for the selected DIBs), posing a challenge to the hypothesis of PAHs being responsible for both the 2175Å bump (Joblin et al 1992, Li & Draine 2001a, Cecchi-Pestellini et al 2008, Malloci et al 2008, Steglich et al 2010 and DIBs (Crawford et al 1985, Léger & d'Hendecourt 1985, van der Zwet & Allamandola 1985, Salama et al 1999.…”

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

“…For the other species we use the UV absorption cross section of the molecule with the closest number of H atoms within the same ionization state. We expect this to be a good approximation, based on the analysis of Malloci et al (2008). Even though the overall shape varies somewhat with dehydrogenation, when calculating the average photon energy absorbed in a given stellar radiation field, the absorbed average energy varies by less than 1 eV for the different hydrogenation states, being slightly higher for dehydrogenated states.…”

“…For the normally hydrogenated species, we have taken estimates from experiments when available . If no experimental values were available, we then adopted results from quantum theory (Malloci et al 2007(Malloci et al , 2008, or estimates based upon conducting disk behaviour (Bakes & Tielens 1994). See more details in Appendix A.…”

“…For dehydrogenated PAHs, we have followed Malloci et al (2008) who calculated the IPs for the even dehydrogenated states of coronene, i.e., C 24 H 2 n where n = 0−6. Based on their estimated change of IP with hydrogenation level of the PAH, ∆IP, we assume that for each PAH in charge state Z, and with N H number of H atoms, the following relation applies:…”

“…For the even dehydrogenated states of coronene, we use the DFT results from Malloci et al (2008). For the other species we use the UV absorption cross section of the molecule with the closest number of H atoms within the same ionization state.…”