1
A search for PAHs in the ISM: High-resolution UV observations confronted with laboratory spectra G. Rouillé, 1 R. Gredel, 2 Y. Carpentier, 1 M. Steglich, 1 F. Huisken, 1, 2 and Th. Henning 2 1 Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University, Institute for Solid State Physics, Jena, Germany 2 Max Planck Institute for Astronomy, Heidelberg, Germany Measurements in solutions… The absorption spectra of molecules in solutions cannot be compared with observational data of the ISM. Solutions, however, allow us to obtain quantitative absorption measurements over wide wavelength ranges using little sample. … in rare gas matrices … Similarly, the absorption spectra of molecules isolated in rare gas matrices cannot be used for comparison with spectra of the ISM. While quantitative measurements would be complex, the low temperature induces vibrationally resolved spectra. … and in supersonic jets (figure) Supersonic jets of rare gases seeded with the molecules of interest mimic the conditions of the ISM. Thus the spectra measured in such jets can be compared with observational data. The production of seeded jets with heated pulsed valves requires large amounts of sample. Obtaining quantitative data would be both complex and costly. A = L c c -1 τ -1 + ln R = dL j σ A absorbance R reflectance L c cavity length d density c speed of light L j jet thickness τ decay time σ molecular abs. cross section New spectra We have obtained high-resolution (R = 80 000) spectra toward the heavily reddened supergiants HD 169454, BD -14°5037, CPD -32°1734, CPD -33°1768, and HD 183143 using the UVES spectrograph at the VLT under programme 079.C-0597(A). Archival spectra Data from the UVES Paranal Observatory Project "A Library of High-Resolution Spectra of Stars across the Hertzsprung-Russell Diagram" [2] produced under ESO Director Discretionary Time (DDT) program 266.D-5655(A) were also exploited. The identification of the carriers of the diffuse interstellar bands (DIBs) is the longest standing problem in the study of the interstellar medium (ISM). 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 that can be used to identify individual species. The comparison of quantitative absorption spectra, obtained in the laboratory under conditions relevant to the ISM, with high signal-to-noise observational UV data has enabled us to establish new constraints on PAH abundances in translucent molecular clouds [1]. Introduction This project is supported by the Max Planck Institute for Astronomy and the Deutsche Forschungsgemeinschaft (DFG). We are grateful to Farid Salama for providing us with the jet-cooled spectrum of the S 2 ← S 0 transition of benzo[ghi]perylene. Acknowledgment and references [1] Gredel, R., Carpentier, Y., Rouillé, G., Steglich, M., et al. 2011, A&A, 530, A26. [2] Bagnulo, S., Jehin, E., Ledoux, C., et al. 2003, The Messenger, 114, 10. [3] Gummersbach, C. A. & Kauffer, A. 1993, The Hot Star Newsletter, 22, 16. [4] Salama, F., Galazutdinov, G. A., Krelowski, J., et al. 2011, ApJ, 728, 154. Quantitative laboratory absorption spectra of neutral small PAHs and observational UV absorption spectra of translucent molecular clouds have been measured. The comparison of the two sets of spectra has led to the determination of upper limits for the column densities and fractional abundances of PAHs. The latter are up to two orders of magnitude lower than estimated total PAH abundances in the ISM. This indicates that either neutral small PAHs are not abundant in translucent molecular clouds, possibly because small PAHs would be ionized or destroyed in such objects, or that a PAH population with a wide variety of molecules is present. The procedure for obtaining quantitative laboratory absorption spectra could be applied to other species that may be present in the ISM, e.g., substituted PAHs. Summary and outlook Observational data Laboratory absorption spectroscopy techniques A two-step procedure – Example: 2,3-benzofluorene Jet-cooled spectra are calibrated in terms of absorption cross section σ in two steps: (1) Quantitative absorption spectra measured in solution are used to calibrate the same spectra measured in Ne matrices by comparing the total area of the features in a chosen electronic transition. (2) Well separated bands observed in the newly calibrated matrix spectra are used to calibrate the jet-cooled spectra by comparing their areas in the respective measurements. Determination of absorption cross sections Absorption cross sections Laboratory wavelength position λ, full width at half maximum FWHM, both in Å, integrated molecular absorption cross section ∫σdλ expressed in units of 10 -16 cm 2 Å, and oscillator strength f for selected bands of a set of PAHs [1]: Species Band λ FWHM ∫σdλ f Phenanthrene S 1 (0) ← S 0 (0) 3409.21 0.20 0.16 0.00016 Anthracene S 1 (0) ← S 0 (0) 3610.74 0.34 19 0.016 Pyrene S 2 (0) ← S 0 (0) 3208.22 5.5 89 0.097 Benzofluorene S 1 (0) ← S 0 (0) 3344.16 0.18 8.4 0.0085 Benzoperylene S 2 (ν) ← S 0 (0) 3501.76 8.2 9.9 0.009 Comparison of laboratory spectra with observational data Laboratory absorption spectra (dotted blue curves), UVES spectra (solid black curves), and synthetic stellar spectra (solid red curves) obtained from the models of Gummersbach & Kauffer [3]: Upper limits for column densities and fractional abundances Upper limit for the column density N max toward five lines of sight expressed in units of 10 12 cm -2 and the order of magnitude of the corresponding maximum fractional abundance relative to hydrogen f max [1]: Species N max f max HD 169454 BD -14º5037 CPD -33º1768 CPD -32º1734 HD 183143 Phenanthrene 105 180 285 118 330 10 -8 Anthracene 0.8 1.7 2.4 1.4 2.8 10 -10 Pyrene 2.3 4.4 7.2 3.2 7.1 10 -10 Benzofluorene 2.7 3.7 6.8 2.8 7.9 10 -10 Benzoperylene 8.8 14.6 22 11 24 10 -9 Our results are in reasonable agreement with those of Salama et al. (2011) [4], which were inferred from theoretical oscillator strengths. Results Anthracene C 14 H 10 Pyrene C 16 H 10 Phenanthrene C 14 H 10 Benzoperylene C 22 H 12 Benzofluorene C 17 H 12 (1) (2) Heated pulsed valve (right). Cavity ring-down laser absorption spectroscopy (below). wavelength [Å] wavelength [Å] wavelength [Å] wavelength [Å] wavelength [Å]
