A search for interstellar naphthalene and anthracene cations

Abstract

The evidence for naphthalene and anthracene cations reported by Iglesias-Groth and co-authors in 2008 and 2010 respectively in the spectra of a rather cool object, (A3V) Cernis 52, was carefully examined with the aid of high-resolving-power (30 000 less than or similar to R less than or similar to 90 000) spectra. No possible bands of these molecules were found in high signal-to-noise ratio spectra of HD 147889, HD 204827, HD 207538 and HD 281259 - these objects represent both weaker and stronger interstellar molecular features and diffuse bands than Cernis 52. The idea that possible naphthalene and anthracene feature detection has a connection to the anomalous microwave emission reported in the direction of Cernis 52 has been examined using high-quality spectra of HD 278942 and HD 281159 observed in the same area as Cernis 52. Despite the high microwave flux density in the direction of HD 281159 (similar to four times higher at 60 mu m than in the direction to Cernis 52), no expected features were detected. Accordingly, we consider these PAH discoveries as premature. Using the spectrum of HD 281159 we estimate the column density of interstellar naphthalene and anthracene normalized to E(B - V) = 1.0 as less than 2.16 x 1012 and 1.4 x 1012 respectively.