Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21341202m&link_type=abstract
American Astronomical Society, AAS Meeting #213, #412.02; Bulletin of the American Astronomical Society, Vol. 41, p.217
Astronomy and Astrophysics
Astronomy
Scientific paper
We obtained spectra from the Spitzer Space Telescope Infrared Spectrometer (IRS) of lines of sight that probe large columns of diffuse interstellar medium (ISM) dust (PID 3616, J. Chiar). An absorption feature at 6.2 μm, that we attribute to polycyclic aromatic hydrocarbons (PAHs) in the cold ISM, is detected in nine of our spectra. PAHs are normally observed in emission near an exciting source, rather than in the cold ISM dust, however, Schutte et al. (1998, A&A, 337, 261) found the 6.2 μm absorption feature in spectra of WC-type Wolf-Rayet stars that probed moderate columns of diffuse ISM dust. However, it was later shown by Chiar et al. (2001, ApJ, 550, 207) that the feature could be attributed to circumstellar dust around these objects. A low limit was set on lack of detection in the diffuse ISM. Our new Spitzer spectra provide the first indisputable detections of the 6.2 μm PAH absorption feature toward stars that are not associated with circumstellar dust. Based on our nine detections and twenty detection limits, a positive correlation is suggested between the optical depth of the 6.2 μm absorption feature and visual extinction. If verified (with higher signal-to-noise data), this relationship would imply that PAHs are widespread components of cold ISM dust, similar to the well-known aliphatic hydrocarbons that peak at 3.4 μm. Assuming an elemental carbon abundance of C/H=3.7 x 10-4, we estimate that 30 to 40% of the interstellar carbon can be tied up in PAH dust. Future high signal-to-noise observations with SOFIA and/or the James Webb Space Telescope will be crucial to verify the nature and distribution of PAHs in cold ISM dust. This material is based upon work supported by the National Science Foundation under Grant No. 0552751.
Chiar Jean E.
Malsberger Rosalie
Sloan Greg C.
Tielens Alexander G. G. M.
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