Unidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon ...(PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group-functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules.
In this paper, we propose a general formalism that allows for the estimation of radiolysis decomposition pathways and rate coefficients suitable for use in astrochemical models, with a focus on solid ...phase chemistry. Such a theory can help increase the connection between laboratory astrophysics experiments and astrochemical models by providing a means for modelers to incorporate radiation chemistry into chemical networks. The general method proposed here is targeted particularly at the majority of species now included in chemical networks for which little radiochemical data exist; however, the method can also be used as a starting point for considering better studied species. We here apply our theory to the irradiation of H
2
O ice and compare the results with previous experimental data.
A general theoretical framework is presented that allows for the addition of cosmic ray-driven radiation chemistry to astrochemical models.
In this paper, we present preliminary results illustrating the effect of cosmic rays on solid-phase chemistry in models of both TMC-1 and several sources with physical conditions identical to TMC-1 ...except for hypothetically enhanced ionization rates. Using a recent theory for the addition of cosmic-ray-induced reactions to astrochemical models, we calculated the radiochemical yields, called G values, for the primary dust grain ice-mantle constituents. We show that the inclusion of this nonthermal chemistry can lead to the formation of complex organic molecules from simpler ice-mantle constituents, even under cold core conditions. In addition to enriching ice mantles, we find that these new radiation-chemical processes can lead to increased gas-phase abundances as well, particularly for HOCO, NO2, HC2O, methyl formate (HCOOCH3), and ethanol (CH3CH2OH). These model results imply that HOCO-and perhaps NO2-might be observable in TMC-1. Future detections of either of these two species in cold interstellar environments could provide strong support for the importance of cosmic-ray-driven radiation chemistry. The increased gas-phase abundance of methyl formate can be compared with abundances achieved through other formation mechanisms such as pure gas-phase chemistry and surface reactions.
ABSTRACT
Complex organic molecules (COMs) have been detected in a variety of interstellar sources. The abundances of these COMs in warming sources can be explained by syntheses linked to increasing ...temperatures and densities, allowing quasi-thermal chemical reactions to occur rapidly enough to produce observable amounts of COMs, both in the gas phase, and upon dust grain ice mantles. The COMs produced on grains then become gaseous as the temperature increases sufficiently to allow their thermal desorption. The recent observation of gaseous COMs in cold sources has not been fully explained by these gas-phase and dust grain production routes. Radiolysis chemistry is a possible non-thermal method of producing COMs in cold dark clouds. This new method greatly increases the modelled abundance of selected COMs upon the ice surface and within the ice mantle due to excitation and ionization events from cosmic ray bombardment. We examine the effect of radiolysis on three C2H4O2 isomers – methyl formate (HCOOCH3), glycolaldehyde (HCOCH2OH), and acetic acid (CH3COOH) – and a chemically similar molecule, dimethyl ether (CH3OCH3), in cold dark clouds. We then compare our modelled gaseous abundances with observed abundances in TMC-1, L1689B, and B1-b.
In this work, we reexamine sulfur chemistry occurring on and in the ice mantles of interstellar dust grains, and report the effects of two new modifications to standard astrochemical models: namely, ...(a) the incorporation of cosmic-ray-driven radiation chemistry and (b) the assumption of fast, nondiffusive reactions for key radicals in the bulk. Results from our models of dense molecular clouds show that these changes can have a profound influence on the abundances of sulfur-bearing species in ice mantles, including a reduction in the abundance of solid-phase H2S and HS, and a significant increase in the abundances of OCS, SO2, as well as pure allotropes of sulfur, especially S8. These pure-sulfur species-though nearly impossible to observe directly-have long been speculated to be potential sulfur reservoirs and our results represent possibly the most accurate estimates yet of their abundances in the dense interstellar medium. Moreover, the results of these updated models are found to be in good agreement with available observational data. Finally, we examine the implications of our findings with regard to the as-yet-unknown sulfur reservoir thought to exist in dense interstellar environments.
The collisions between high-energy ions and solids can result in significant physical and chemical changes to the material. These effects are potentially important for better understanding the ...chemistry of interstellar and planetary bodies, which are exposed to cosmic radiation and the solar wind, respectively; however, modeling such collisions on a detailed microscopic basis has thus far been largely unsuccessful. To that end, a new model, entitled CIRIS: the Chemistry of Ionizing Radiation in Solids, was created to calculate the physical and chemical effects of the irradiation of solid materials. With the new code, we simulate O
2
ice irradiated with 100 keV protons. Our models are able to reproduce the measured ozone abundances of a previous experimental study, as well as independently predict the approximate thickness of the ice used in that work.
We simulate irradiated O
2
ice, both reproducing measured ozone abundances and predicting the ice thickness of a previous experiment.
Many current astrochemical models explicitly consider the species that comprise the bulk of interstellar dust grain ice mantles separately from those in the top few monolayers. Bombardment of these ...ices by ionizing radiation-whether in the form of cosmic rays, stellar winds, or radionuclide emission-represents an astrochemically viable means of driving a rich chemistry even in the bulk of the ice mantle, now supported by a large body of work in laboratory astrophysics. In this study, using an existing rate-equation-based astrochemical code modified to include a method of considering radiation chemistry recently developed by us, we attempted to simulate two such studies in which (a) pure O2 ice at 5 K and (b) pure H2O ice at 16 K and 77 K, were bombarded by keV H+ ions. Our aims were twofold: (1) to test the capability of our newly developed method to replicate the results of ice-irradiation experiments, and (2) to determine how bulk chemistry in such a well-constrained system is best handled using the same gas-grain codes that are used to model the interstellar medium. We found that our modified astrochemical model was able to reproduce both the abundance of O3 in the 5 K pure O2 ice, as well as both the abundance of H2O2 in the 16 K water ice and the previously noted decrease of hydrogen peroxide at higher temperatures. However, these results require the assumption that radicals and other reactive species produced via radiolysis react quickly and non-diffusively with neighbors in the ice.
Using data from the Green Bank Telescope (GBT) Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey, we report the first astronomical detection of the C10H− anion. The astronomical ...observations also provided the necessary data to refine the spectroscopic parameters of C10H−. From the velocity stacked data and the matched filter response, C10H− is detected at >9σ confidence level at a column density of 4.04−2.23+10.67×1011 cm−2. A dedicated search for the C10H radical was also conducted toward TMC-1. In this case, the stacked molecular emission of C10H was detected at a ∼3.2σ confidence interval at a column density of 2.02−0.82+2.68×1011 cm−2. However, as the determined confidence level is currently <5σ, we consider the identification of C10H as tentative. The full GOTHAM data set was also used to better characterize the physical parameters including column density, excitation temperature, line width, and source size for the C4H, C6H, and C8H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C10H−/C10H column density ratio is ∼2.0−1.6+5.9—the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain.
We present an overview of the GBT Observations of TMC-1: Hunting Aromatic Molecules Large Program on the Green Bank Telescope. This and a related program were launched to explore the depth and ...breadth of aromatic chemistry in the interstellar medium at the earliest stages of star formation, following our earlier detection of benzonitrile (c-C6H5CN) in TMC-1. In this work, details of the observations, use of archival data, and data reduction strategies are provided. Using these observations, the interstellar detection of propargyl cyanide (HCCCH2CN) is described, as well as the accompanying laboratory spectroscopy. We discuss these results, and the survey project as a whole, in the context of investigating a previously unexplored reservoir of complex, gas-phase molecules in pre-stellar sources. A series of companion papers describe other new astronomical detections and analyses.
Complex organic molecules such as sugars and amides are ubiquitous in star- and planet-forming regions, but their formation mechanisms have remained largely elusive until now. Here we show in a ...combined experimental, computational, and astrochemical modeling study that interstellar aldehydes and enols like acetaldehyde (CH₃CHO) and vinyl alcohol (C₂H₃OH) act as key tracers of a cosmic-ray-driven nonequilibrium chemistry leading to complex organics even deep within low-temperature interstellar ices at 10 K. Our findings challenge conventional wisdom and define a hitherto poorly characterized reaction class forming complex organic molecules inside interstellar ices before their sublimation in star-forming regions such as SgrB2(N). These processes are of vital importance in initiating a chain of chemical reactions leading eventually to the molecular precursors of biorelevant molecules as planets form in their interstellar nurseries.
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