The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction. I. Probing small molecular PAHs

Astrophysics, abstract
astro-ph/0304025

From: Geoffrey Clayton <gclayton@fenway.phys.lsu.edu>

Date: Tue, 1 Apr 2003 16:11:36 GMT   (81kb)

Authors:
Geoffrey C. Clayton,
Karl D. Gordon,
Adolf N. Witt,
L.J. Allamandola,
Peter G. Martin,
F. Salama,
T. P. Snow,
D.C.B. Whittet,
Michael J. Wolff,
Tracy L. Smith

Comments: 16 pages, 3 figure, ApJ in press

We have obtained new STIS/HST spectra to search for structure in the
ultraviolet interstellar extinction curve, with particular emphasis on a search
for absorption features produced by polycyclic aromatic hydrocarbons (PAHs).
The presence of these molecules in the interstellar medium has been postulated
to explain the infrared emission features seen in the 3-13 $mu$m spectra of
numerous sources. UV spectra are uniquely capable of identifying specific PAH
molecules. We obtained high S/N UV spectra of stars which are significantly
more reddened than those observed in previous studies. These data put limits on
the role of small (30-50 carbon atoms) PAHs in UV extinction and call for
further observations to probe the role of larger PAHs. PAHs are of importance
because of their ubiquity and high abundance inferred from the infrared data
and also because they may link the molecular and dust phases of the
interstellar medium. A presence or absence of ultraviolet absorption bands due
to PAHs could be a definitive test of this hypothesis. We should be able to
detect a 20 AA wide feature down to a 3$sigma$ limit of $sim$0.02 A$_V$. No
such absorption features are seen other than the well-known 2175 AA bump.

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