From Interstellar Polycyclic Aromatic Hydrocarbons and Ice to Astrobiology

Abstract

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible.Today the composition of dust in the diffuse ISM is reasonably well constrained to micron-sized cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In dense molecular clouds, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose major components are is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier interstellar chemistry standards, is widespread throughout the Universe. This paper presents a detailed summary of these disparate interstellar components and ends by considering both of them as important feedstock to the chemical inventory of the primordial Earth. Particular attention is paid to their possible role in the chemistry that led to the origin of life. An extensive reference list is given to allow the student entry into the full depth of the literature.The first part of this paper focuses on interstellar PAHs. The laboratory and theoretical underpinning which supports the PAH model is reviewed in some detail. This is followed by a few specific examples which demonstrate how these data can be used to analyze the interstellar spectra and probe local conditions in different emission zones. These examples include tracing the evolution of carbon as it passes from its birthsite in circumstellar shells through the ISM, determining specifics about the cosmic PAH population in many different environments including PAH size and structure, and probing local conditions in the different emission zones.The second part of this paper summarizes the laboratory and observational background leading to our current understanding of interstellar/precometary ices. Although the most abundant interstellar ice components are the very simple molecules such as H 2 O, CH 3 OH, CO, CO 2 , and NH 3 , more complex species including accreted PAHs and those formed by UV and cosmic ray processing within the ice must also be present. Here we give a detailed summary of the photochemical evolution on those ices 2 found in the densest regions of molecular clouds, the regions where stars and planetary systems are formed. Ultraviolet photolysis of these ices produces a host of new compounds, some of which show intriguing prebiotic behavior.The last part of this paper draws all this information together and considers the possible roles these compounds might have played in early Earth...

Detection of cometary amines in samples returned by Stardust

“…Indeed, complex organic compounds can be formed in laboratory-simulated interstellar/ cometary ices by UV or proton irradiation of simple ice mixtures that have been observed in the interstellar medium and in comets (Allamandola et al 1988;Moore and Hudson 1998;Bernstein et al 1995;Allamandola and Hudgins 2003). Analyses of the ice residues after irradiation using a variety of analytical techniques demonstrate that these materials contain a much more complex suite of volatile species than originally present in the ice as well as a refractory organic component.…”

Photoinduced products from cold coronene clusters

“…Other solid phase alternative laboratory analogues have been proposed for the carriers of the AIBs, such as hydrogenated amorphous carbon (HAC) (Duley & Williams 1981), or coal grains . Although some of these models are able to spectrally account for the observations from some particular regions , many strong arguments favor the PAH model (Allamandola & Hudgins 2003), mainly the scaling of IR bands with ISRF (Boulanger et al 1998;Uchida et al 2000) and the local variability in the spectra (Werner et al 2004). It is worth noting that the average size of the astro-PAHs estimated from ISO observational data is on the order of a few hundred Carbon atoms (Boulanger et al 1998), i.e.…”

Formation of benzene in the interstellar medium

“…astrochemistry | polycyclic aromatic hyrdrocarbons | reaction dynamics | bimolecular reaction P olycyclic aromatic hydrocarbons (PAHs) and related species such as (de)hydrogenated, ionized, and substituted PAHs are presumed to be omnipresent in the interstellar medium (ISM) (1,2). PAH-like species are suggested to account for up to 30% of the galactic interstellar carbon (2), have been implicated in the astrobiological evolution of the ISM (3), and provide nucleation sites for the formation of carbonaceous dust particles (4).…”