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Volume 16 N° 185 – MAIO/ 2021 POSSIBLE ROUTES FOR DOI: https://doi.org/10.29367/issn.1809-3957.16.2021.185.27 Revista SODEBRAS – Volume 16 N° 185 – MAIO/ 2021 POSSIBLE ROUTES FOR THE FORMATION OF PREBIOTIC MOLECULES IN THE HORSEHEAD NEBULA POSSÍVEIS ROTAS PARA A FORMAÇÃO DE MOLÉCULAS PREBIÓTICAS NA NEBULOSA CABEÇA DE CAVALO LUCIENE DA SILVA COELHO¹; AMÂNCIO CÉSAR DOS SANTOS FRIAÇA2; EDGAR MENDOZA2,3 1 – PLANETÁRIO JUAN BERNARDINO MARQUES BARRIO, INSTITUTO DE ESTUDOS SOCIOAMBIENTAIS, UNIVERSIDADE FEDERAL DE GOIÁS; 2 – INSTITUTO DE ASTRONOMIA, GEOFÍSICA E CIÊNCIAS ATMOSFÉRICAS, UNIVERSIDADE DE SÃO PAULO; 3 – OBSERVATÓRIO DO VALONGO, UNIVERSIDADE FEDERAL DO RIO DE JANEIRO [email protected]; [email protected]; [email protected] Abstract - This article presents the results of a theoretical study al. 2000). It is not surprising that the nitrogen atom concerning interstellar molecules which are useful for the participates in the composition of molecules that store bookkeeping of the organic content of the universe and for information. Since nitrogen, in contrast to carbon and providing a glimpse into prebiotic conditions on Earth and in other oxygen, has an odd number of valence electrons, and environments in the universe. Performing the PDR Meudon code, introduces asymmetries in the molecular structure of any we explored production channels for astrobiological relevant nitrogen-bearing cyclic molecules (N-heterocycles), e. g. pyrrole carbon compound. The existence of asymmetries is a and pyridine. The present simulations demonstrate how the necessary condition for information storage since a structure exploration of a few possible routes of production of N- formed by the repetition of identical sub-units would not heterocycles resulted in significant abundances for these species. allow the writing of instructional sequences (NELSON & One particularly efficient class of channels for the production of COX, 2012). N-heterocycles incorporates polycyclic aromatic hydrocarbons The cosmic abundance of the N atom is high enough to (PAHs) as catalysts. Thereby, an exploration of a variety of be found it in a huge variety of molecular forms, from the production paths should reveal more species to be target of simplest species like CN (ADAMS, 1941); formamide, astrophysical observations. NH2CHO (LÓPEZ-SEPULCRE et al. 2019) to most complex Keywords: Astrochemistry and Astrobiology: Cosmic Prebiotic structures, such as the polycyclic aromatic nitrogen Chemistry. Organic Molecules. ISM: PDRs. Simulations. heterocycles (PANHs), as some of which have been analyzed in Murchison meteorite samples (GLAVIN & BADA, 2004). Resumo – Este trabalho apresenta o resultado de um estudo PANHs are compounds derived from the polycyclic aromatic teórico sobre moléculas interestelares que são úteis para a hydrocarbons (PAHs) when N atoms are part of the aromatic contabilidade do conteúdo orgânico do universo e para fornecer rings. PAHs are widely spread in the ISM, from stellar um vislumbre das condições pré-bióticas na Terra e em outros atmospheres up to galaxies at high redshifts (SHIVAEI et al. ambientes do além do nosso planeta. Utilizando o código PDR 2017). They constitute an efficient form to accumulate carbon Meudon, nós exploramos rotas de formação de moléculas in solid phase across the Universe, and their abundance and heterocíclicas nitrogenadas, como o pirrol e a piridina, as quais possuem potencial astrobiológico. As simulações demonstraram ubiquity are partially due to the high stability against the como a exploração de poucas rotas possíveis de produção de dissociative effects of ionizing agents (ALLAMANDOLA et moléculas heterocíclicas nitrogenados resulta em abundâncias al. 1999; SALAMA et al. 1999; TIELENS, 2008, 2013). significantes destas espécies. Rotas particularmente efetivas para PAHs are so ubiquitous that most of the mid-infrared a produção de heterocíclicos nitrogenados envolvem os emission in galaxies with star formation is dominated by hidrocarbonetos policíclicos aromáticos como catalisadores. strong emission features generally attributed to PAH bands Assim, uma varredura por diversas rotas de produção deve revelar (SPOON et al. 2002; TIELENS et al. 2004; PEETERS et al. mais espécies a serem alvo de observações astrofísicas. 2004). PANHs are especially relevant for astrobiology and they Palavras-chave: Astroquímica e Astrobiologia: Química Prebiótica Cósmica. Moléculas Orgânicas. ISM: PDRs. are closely related to the purine (C5H4N4) and pyrimidine Simulações. (C4H4N2) nucleobases, which are part of the DNA and RNA molecular structures. These species are probably formed from I. INTRODUCTION the polymerization of small molecules such as acetylene (C2H2), nitrogen atoms being incorporated via the One of the minimal traits of any living system is the substitution of acetylene by cyanic acid (HCN) (PRASAD & presence of some mechanism for information storage, reading HUNTRESS, 1980; FRENKLACH & FEIGELSON, 1989; and self-replication. In terrestrial life, nucleic acids (DNA HUDGIN et al., 2005; CHEN et al., 2008). The and RNA) are part of the machinery responsible for the computational study of Ricca et al. (2001) has shown that informational function already mentioned (SECKBACH et minor units of PANHs could be synthesized via successive Volume 16 – n. 185 – Maio/2021 27 ISSN 1809-3957 reactions of C2H2 and HCN under the stimulus of UV photons grains forming icy mantles (ZHANG & JIN, 2015). Thus, or cosmic rays. Laboratory experiments have also observational and semi-empirical studies reveal important demonstrated that UV photoirradiation of icy samples, aspects on the presence and distribution of molecules in disks. containing water (H2O), ammonia (NH3), and benzene (C6H6) This work aims to conduct a study about the possibility or naphthalene (C10H8), leads to the formation of N and O- of the formation of prebiotic N-heterocycles and other COMS heterocycles, such as pyridine (C5H5N) and phthalide in the interstellar medium, specifically, in the Horsehead (C8H6O2) (MATERESE et al., 2015). In addition, PAHs also Nebula, considering the physical conditions of molecular have a potential to synthetize amino acids. Chen et al. (2008) clouds and photodissociation regions (PDRs). identified 13 different amino acids produced from the irradiation of ices containing C10H8, H2O and NH3. II. METHOD The spectroscopy of PANHs requires high sensitivity to We used the PDR Meudon code to calculate the UV- differentiate their spectral signatures from those corresponding to PAHs. Hudgins et al. (2005) reported a shift driven chemistry of interstellar clouds considering the in the peak position of PAHs at 6.2 μm, mainly due to typical physical and chemical conditions of environments such as PDRs. The PDR Meudon code allows simulating regions as signatures of PANHs, when a C atom is replaced by N. stationary plane-parallel slabs of gas and dust illuminated by Laboratory measures have elucidated the different vibrational radiation fields, which can be the Interstellar Standard modes and spectroscopy of PANHs, whose peak positions are Radiation Field. Heating (Photoelectric effect on grains, ranged between 6.7 – 10 micron (MATTIODA et al. 2005, 2017). The identification of a peak shift in that band, towards cosmic rays) and cooling (infrared and millimeter emission) sources as the Horsehead Nebula, might reveal the presence, processes contribute to the thermal balance. The output of the PDR Meudon includes gas properties like temperature, in the interstellar medium, of PANHs, which have already ionized fraction, chemical abundances and column densities been found in meteorites (STOKS & SCHWARTZ, 1981) (e.g., LE BOURLOT et al., 1993; LE PETIT et al., 2006; and suggested to be present in starburst-dominated galaxies (CANELO et al., 2018). GONZALEZ GARCIA et al., 2008). If the substituted N is located on the periphery of the The Horsehead Nebula was chosen as the benchmark for this study because of its relatively simple physics and PAH, the resulting PANH is quite reactive, which may lead geometry. It is a PDR, with some regions having a nearly to the production of other N-heterocycles, as pyrroles, plane-parallel geometry. We have assumed a fixed pyridines, pyrimidines or purines (e. g. MATERESE et al. temperature of 15 K and parameters representative of the 2015). Going back further than the RNA world, there could be a PANH world, or an aromatic world (EHRENFREUND Horsehead Nebula, i.e., G = 60 G0, where G0 is rather the flux et al. 2006). In this pre/protobiotic scenario, assembling of ISM as measured by Habing (1968), AV = 10 mag to the cloud center, and a total hydrogen density of 10⁴ cm⁻³. In PAH/PANH rich material could perform the transition from addition, the classic parameterized modeling by Cordiner et nonliving to living matter. For instance, PANHs may al. (2007) for this object, well known for its rich atomic, constitute a precursor to biological information carriers. molecular and dust emission observed from visible to sub- So far, the search for N-heterocycles has not yielded a conclusive identification in the gas phase of the interstellar mm wavelength (GERIN et al., 2009), make this nebula an medium (ISM), despite the multiple surveys performed in the ideal target to test any model for the formation of the molecular lines. Likewise, it is possible to extend the studies sub-mm domain of the electromagnetic spectrum (SIMON & on the region, based on the results obtained by Goicoechea et SIMON, 1973). al. (2009) and Gerin et al. (2009) considering this region as Kuan et al. (2003)
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