Abundances of PAHs in the ISM: confronting observations with experimental results

Abstract

Context. The identification of the carriers of the diffuse interstellar bands (DIBs) is the longest standing problem in the study of the interstellar medium. Here we present recent UV laboratory spectra of various polycyclic aromatic hydrocarbons (PAHs) and explore the potential of these molecules as carriers of the DIBs. Whereas, in the near IR range, the PAHs exhibit vibrational bands that are not molecule-specific, their electronic transitions occurring in the UV/vis provide characteristic fingerprints. The comparison of laboratory spectra calibrated in intensity with high signal-to-noise observational data in the UV enables us to establish new constraints on PAH abundances. Aims. From a detailed comparison of the gas-phase and Ne-matrix absorption spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, benzo[ghi]perylene, and hexabenzocoronene with new interstellar spectra, we aim to infer the abundance of these PAHs in the interstellar medium. Methods. We present spectra of PAHs measured at low temperature in the gas phase and in an Ne matrix, and present methods to derive absolute absorption cross sections for the matrix and gas phase spectra. We have obtained high signal to noise (S/N > 100) absorption spectra toward five lines of sight with reddenings of E B−V = 1−1.6 mag. The spectra cover the 3050−3850 Å wavelength region where the PAHs studied here show prominent absorption features. Results. From the observations, we infer upper limits in the fractional abundances of the PAHs studied here. Upper limits in the column densities of anthracene of 0.8−2.8 × 10 12 cm −2 and of pyrene and 2,3-benzofluorene ranging from 2−8 × 10 12 cm −2 are inferred. Upper limits in the column densities of benzo [ghi]perylene are 0.9−2.4 × 10 13 and 10 14 cm −2 for phenanthrene. The measurements indicate fractional abundances of anthracene, pyrene, and 2,3-benzofluorene of a few times 10 −10 . Upper limits in the fractional abundance of benzo [ghi]perylene of a few times 10 −9 and of phenanthrene of few times 10 −8 are inferred. Toward CPD −32 • 1734, we found near 3584 Å an absorption line of OH + , which was discovered in the interstellar medium only very recently. Conclusions. The fractional abundances of PAHs inferred here are up to two orders of magnitude lower than estimated total PAH abundances in the interstellar medium. This indicates that either neutral PAHs are not abundant in translucent molecular clouds or that a PAH population with a wide variety of molecules is present.

The smoothness of the interstellar extinction curve in the UV

“…However, the matrix-induced broadening usually affects only those bands that involve the first one or two electronic transitions in a closed-shell molecule, i.e., S 0 → S 1,2 . The bands of these transitions can be very narrow in the gas phase and would appear substantially broadened in a Ne matrix (see, e.g., Gredel et al 2011). Nevertheless, as inferred from the aromatic infrared emission bands, the bulk of interstellar PAHs are probably larger than the species we studied here ( > ∼ 40 C atoms; Tielens 2008).…”

“…Recently, astronomers tried to detect the spectral signatures of individual PAHs in the UV part (λ < 400 nm) of the interstellar extinction curve (Clayton et al 2003;Gredel et al 2011;Salama et al 2011). While Clayton et al 2003 used low-resolution observations in the 157−318 nm range, which were obtained with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope, the surveys reported by Gredel et al (2011) Salama et al (2011) covered high-resolution spectra for wavelengths longer than 305 nm recorded with the UVES spectrograph of the Very Large Telescope. However, besides the well-known UV bump at 217.5 nm, some atomic lines, and bands from small, mostly diatomic molecules, such as CH, CH + , CN, OH + , or NH, no narrow bands related to large gas-phase molecules were found.…”

“…However, besides the well-known UV bump at 217.5 nm, some atomic lines, and bands from small, mostly diatomic molecules, such as CH, CH + , CN, OH + , or NH, no narrow bands related to large gas-phase molecules were found. Based on the signal-to-noise ratio in these observations (S /N > 100), very low upper fractional abundances for specific PAHs in the diffuse interstellar medium (ISM) on the order of N(PAH)/N(H) ≈ 10 −10 -10 −8 (Gredel et al 2011) were derived. On the other hand, inferred from the aromatic infrared emission bands, PAHs are expected to occur in large quantities in the ISM (estimated total PAH abundances on the order of 10 −7 ; Habart et al 2004;Tielens 2008).…”

Polyynyl-Substituted PAH Molecules and DIB Carriers

“…They took a higher thermodynamic stability as an example of selection criterion, which would favor populations of fully-benzenoid PAHs such as hexa-peri-hexabenzocoronene. Note that an attempt to detect this species in several astronomical objects was unsuccessful (Gredel et al 2011). Fundamentally, however, the selection results from the balance between the formation and destruction processes of the molecules (Duley 2006).…”

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

“…The DIBs are a set of several hundred absorption features spanning the wavelength range of the near ultraviolet (UV) at λ 4000Å (e.g., see York et al 2006, Gredel et al 2011, Salama et al 2011, Bhatt & Cami 2015 to the near infrared (IR) at λ 1.8 µm (e.g., see Joblin et al 1990, Geballe et al 2011, Cox et al 2014, Zasowski et al 2015, Hamano et al 2015, with most of the bands falling in the visible wavelength range. The DIBs were originally discovered in 1919 by Herger (1922) and their interstellar nature was established based on their stationary nature as observed toward spectroscopic binaries.…”