PAH charge state distribution and DIB carriers: Implications from the line of sight toward HD 147889

Abstract

Abstract.We have computed physical parameters such as density, degree of ionization and temperature, constrained by a large observational data set on atomic and molecular species, for the line of sight toward the single cloud HD 147889. Diffuse interstellar bands (DIBs) produced along this line of sight are well documented and can be used to test the PAH hypothesis. To this effect, the charge state fractions of different polycyclic aromatic hydrocarbons (PAHs) are calculated in HD 147889 as a function of depth for the derived density, electron abundance and temperature profile. As input for the construction of these charge state distributions, the microscopic properties of the PAHs, e.g., ionization potential and electron affinity, are determined for a series of symmetry groups. The combination of a physical model for the chemical and thermal balance of the gas toward HD 147889 with a detailed treatment of the PAH charge state distribution, and laboratory and theoretical data on specific PAHs, allow us to compute electronic spectra of gas phase PAH molecules and to draw conclusions about the required properties of PAHs as DIB carriers. We find the following. 1) The variation of the total charge state distribution of each specific class (series) of PAH in the translucent cloud toward HD 147889 (and also of course for any other diffuse/translucent cloud) depends strongly on the molecular symmetry and size (number of π electrons). This is due to the strong effects of these parameters on the ionization potential of a PAH. 2) Different wavelength regions in the DIB spectrum are populated preferentially by different PAH charge states depending on the underlying PAH size distribution. 3) The PAH size distribution for HD 147889 is constrained by the observed DIB spectrum to be Gaussian with a mean of 50 carbon atoms.

H2formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

“…All photoprocesses are mitigated by dust extinction at increasing column densities, and for H 2 photodissociation we also include H 2 self-shielding. Ionization processes are counteracted by electron recombination, for which we use rates from Ruiterkamp et al (2005) and the online database KIDA 1 (Wakelam et al 2012). The formation of H 2 on interstellar dust grains (silicates and amorphous carbon) as well as on PAHs is considered in our model.…”

“…with T the temperature, s (e) the electron sticking coefficient and n e the number density of free electrons (Ruiterkamp et al 2005). From Ruiterkamp et al (2005) we adopt an electron sticking coefficient of s (e) = 10 −3 for coronene.…”

Diffuse interstellar bands in Upper Scorpius: probing variations in the DIB spectrum due to changing environmental conditions

“…DIB carriers seem to reflect the evolutionary cycle of molecular carbon species (such as aromatic molecules) through formation, ionisation, recombination, and destruction (Cami et al 1997;Ruiterkamp et al 2005).…”

“…Many possible carriers have been proposed, ranging from grain impurities and exotic molecules to H 2 . In the past two decades the field has converged towards larger carbonaceous molecules, like the fullerenes and polycyclic aromatic hydrocarbons (PAHs), which have electronic transitions in the optical (see for example Salama et al 1999;Ruiterkamp et al 2005;Kokkin & Schmidt 2006;Zhou et al 2006;and Salama et al 2011). New diffuse bands have been detected in one line-of-sight which appear to match with naphthalene and anthracene cations (Iglesias-Groth et al 2008 and the weak 5450 Å DIB is found to match with an absorption band arising from a hydrocarbon plasma created in the laboratory (Linnartz et al 2010).…”

Linear and circular spectropolarimetry of diffuse interstellar bands

“…Cami et al 1997;. Their strength variation could reflect the local charge state balance of the carrier molecules (Ruiterkamp et al 2005;Cox & Spaans 2006). Therefore, specific groups of stable UV resistant molecules (such as PAHs, fullerenes and carbon chains) are commonly postulated carrier candidates (Herbig 1995).…”