A team of astronomers led by IAC has reported evidence for the presence of naphthalene cation in a region of the interestellar medium with anomalous microwave emission. This identification adds support to the hypothesis that this type of molecules can be the carriers of both diffuse interestellar bands and anomalous microwave emission, and can play an important role in the formation of prebiotic molecules present in the protoplanetary material.

Polycyclic aromatic hydrocarbons (PAHs) have been postulated as the natural carriers for (1) the mid-infrared emission features widely observed in the interestellar medium; (2) the diffuse interestellar bands (DIBs), hundreds of absoprtion bands in the optical and near-infrared spectra of stars reddened by interestellar material; and (3) the so-called anomalous microwave emission, a diffuse emission detected at high Galactic latitude by several cosmic microwave experiments. In order to explore a possible connection between the carriers of the last two phenomena, the astronomers decided to investigate some DIBs toward the Perseus molecular cloud complex where recent observations demonstrated the existence of regions of anomalous microwave emission.

They searched for DIBs by performing high-resolution optical spectroscopy of Cernis 52 (BD +31 640), a reddened star located at the line of sight of maximum anomalous microwave emission, and at a distance of 240 pc, where most of the dust extinction is known to concentrate in the Perseus OB2 dark cloud complex. They identified numerous DIBs toward Cernis 52 and found two new bands of likely interestellar origin with wavelengths and strenghts consistent with those of the naphthalene cation (C₁₀H₈⁺). Naphthalene is the simplest PAH, and the tentative identification of this cation reinforces the hyphotesis that this class of molecules could be carriers of the anomalous microwave emission.

Spectra of star Cernis 52 showing three independent detections of the λ6707.4 naphthalene band from different telescopes. The red line in the second spectrum shows the lower resolution data obtained with ISIS at the William Herschel Telescope in service time (programme reference SW2007b10). The narrow feature at 6707.7Å is associated with interstellar lithium. The known DIBs at 6699.32, 6702.02, and 6709.43Å are marked with wavelength and can be also identified in the spectrum of the hotter reference star HD 23180 (displayed at the bottom). A synthetic spectrum (Synt A3 V) computed using a suitable model atmosphere for Cernis 52 is plotted for two values of Li abundance. The synthetic spectra were convolved assuming a rotational velocity of 80 km s-1, adequate for the star. Star HIP 86032 is plotted for comparison (extracted from ApJL, 685, 55). [ JPG ]

In molecular clouds naphthalene is expected to condense onto refractory dust grains. A large variety of ice mantles can form, but H₂O will very probably be a dominant species in the ices. Recent studies have shown how, under astrophysical conditions, UV radiation on naphthalene in H₂O ice produces naphthoquinones. These molecules, if further functionalized with a methyl group and a long isoprene chain, play essential roles in biochemical functions. Since quinones and aromatic alcohols are already found in meteoritic material, the detection of naphthalene in molecular clouds shows a possible path for these biogenic compounds from the chemistry of clouds, in which new stars form, to the prebiotic molecules present in the protoplanetary material.

More information:

• Iglesias-Groth, S., Manchado, A., García-Hernández, D. A., González Hernández, J. I., Lambert, D. L., 2008, "Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission", ApJ, 685, L55.
• "Descubren en el espacio interestelar moléculas clave para la formación de estructuras básicas de la vida", IAC Press Release, 19th September 2008.