Explore the vast range of titles available.

Aromatic molecules such as polycyclic aromatic hydrocarbons (PAHs) are known to exist in the interstellar medium owing to their characteristic infrared emission features. However, the infrared emission only indicates the general class of molecule, and identifying which specific molecular species are present is difficult. McGuire et al. used radio astronomy to detect rotational transitions of benzonitrile emitted from a well-known nearby cloud of interstellar gas (see the Perspective by Joblin and Cernicharo). This molecule may be a precursor to more complex PAHs. The identification of benzonitrile sheds light on the composition of aromatic material within the interstellar medium—material that will eventually be incorporated into new stars and planets. Science , this issue p. 202 ; see also p. 156 Radio astronomy is used to identify the aromatic molecule benzonitrile in the interstellar medium. Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are thought to be widespread throughout the universe, because these classes of molecules are probably responsible for the unidentified infrared bands, a set of emission features seen in numerous Galactic and extragalactic sources. Despite their expected ubiquity, astronomical identification of specific aromatic molecules has proven elusive. We present the discovery of benzonitrile ( c -C 6 H 5 CN), one of the simplest nitrogen-bearing aromatic molecules, in the interstellar medium. We observed hyperfine-resolved transitions of benzonitrile in emission from the molecular cloud TMC-1. Simple aromatic molecules such as benzonitrile may be precursors for polycyclic aromatic hydrocarbon formation, providing a chemical link to the carriers of the unidentified infrared bands.

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.

Much like six-membered rings, five-membered rings are ubiquitous in organic chemistry, frequently serving as the building blocks for larger molecules, including many of biochemical importance. From a combination of laboratory rotational spectroscopy and a sensitive spectral line survey in the radio band toward the starless cloud core TMC-1, we report the astronomical detection of 1-cyano-1,3-cyclopentadiene (1-cyano-CPD, c-C 5 H 5 CN), a highly polar, cyano derivative of cyclopentadiene. The derived abundance of 1-cyano-CPD is far greater than predicted from astrochemical models that well reproduce the abundance of many carbon chains. This finding implies that either an important production mechanism or a large reservoir of aromatic material may need to be considered. The apparent absence of its closely related isomer, 2-cyano-1,3-cyclopentadiene, may arise from that isomer’s lower stability or may be indicative of a more selective pathway for formation of the 1-cyano isomer, perhaps one starting from acyclic precursors. The absence of N-heterocycles such as pyrrole and pyridine is discussed in light of the astronomical finding of 1-cyano-CPD. A five-membered carbon ring molecule, cyanocyclopentadiene, has been detected in a molecular cloud at a higher abundance than expected. This result from the GOTHAM survey indicates a rich aromatic chemistry in molecular clouds that is not fully understood theoretically.

Benzonitrile (c-C 6 H 5 CN, where ‘c’ indicates a cyclic structure), a polar proxy for benzene (c-C 6 H 6 ), has the potential to serve as a highly convenient radio probe for aromatic chemistry, provided that this ring can be found in other astronomical sources beyond the molecule-rich prestellar cloud TMC-1. Here we present radio astronomical evidence of benzonitrile in four other prestellar, and possibly protostellar, sources: Serpens 1A, Serpens 1B, Serpens 2 and MC27/L1521F. These detections establish that benzonitrile is not unique to TMC-1; rather, aromatic chemistry appears to be widespread throughout the earliest stages of star formation, probably persisting at least until the initial formation of a protostar. The abundance of benzonitrile far exceeds predictions from models that well reproduce the abundances of carbon chains such as HC 7 N, a cyanpolyyne with the same heavy atoms, indicating that the chemistry responsible for planar carbon structures (as opposed to linear ones) in primordial sources is favourable but not well understood. The abundance of benzonitrile relative to carbon chain molecules displays sizable variations between sources within the Taurus and Serpens clouds, implying the importance of physical conditions and initial elemental reservoirs of the clouds themselves. Benzonitrile, a proxy for the aromatic ring molecule benzene, has now been detected at multiple locations in the Taurus and Serpens molecular clouds, suggesting a widespread aromatic chemistry in the interstellar medium. Chemical models underestimate the abundance of aromatic molecules, highlighting the need for further study.

As the inventory of interstellar molecules continues to grow, the gulf between small species, whose individual rotational lines can be observed with radio telescopes, and large ones, such as polycyclic aromatic hydrocarbons best studied in bulk via infrared and optical observations, is slowly being bridged. Understanding the connection between these two molecular reservoirs is critical to understanding the interstellar carbon cycle, but will require pushing the boundaries of how far we can probe molecular complexity while still retaining observational specificity. Towards this end, we present a method for detecting and characterizing new molecular species in single-dish observations towards sources with sparse line spectra. We have applied this method to data from the ongoing GOTHAM (GBT Observations of TMC-1: Hunting Aromatic Molecules) Green Bank Telescope large programme, discovering six new interstellar species. Here we highlight the detection of HC 11 N, the largest cyanopolyyne in the interstellar medium. The authors present a technique to detect (weak) molecular emission lines towards sources with sparse line spectra. This method supports the current GOTHAM survey of TMC-1, and is applied to the detection of the cyanopolyyne species HC 11 N.

Reactive open-shell species, such as radicals and biradicals, are key intermediates in the formation of (poly)cyclic hydrocarbon species in a variety of interstellar environments, ranging from cold molecular clouds to the outflows of carbon-rich stars. In this work, we identify the products of the o-benzyne + methyl radical reaction isomer-selectively by photoion mass-selected threshold photoelectron spectroscopy. We assign the benzyl (C7H7⋅) radical as the sole intermediate of the association reaction. Subsequent hydrogen-atom loss from benzyl yields the five-membered ring species fulvenallene (FA), 1-ethynylcyclopentadiene (1ECP) and 2-ethynylcyclopentadiene (2ECP), which have recently been detected in the cold molecular cloud TMC-1. We report a comprehensive C7H7 potential energy surface of the title reaction and show that the products form via direct barrierless addition followed by ring contraction and hydrogen elimination. A statistical model predicts 89% 1ECP, 8% FA and 3% 2ECP branching ratios at 0 K. Astrochemical simulations of TMC-1 incorporating this reaction result in the excellent reproduction of the abundance of a five-membered ring species, 1ECP, and provide strong evidence for the in situ ‘bottom-up’ formation of small cyclic species in cold cores. Last, we put the results in context of the recent detection of fulvenallene in TMC-1.Hydrocarbons containing five-membered rings have recently been detected in the cold Taurus Molecular Cloud. Here the authors show that the reaction involving ortho-benzyne and the methyl radical plays a critical role in the bottom-up formation of these complex hydrocarbons.

Most interstellar and planetary environments are suffused by a continuous flux of several types of ionizing radiation, including cosmic rays, stellar winds, x-rays, and gamma-rays from radionuclide decay. There is now a large body of experimental work showing that these kinds of radiation can trigger significant physicochemical changes in ices, including the dissociation of species (radiolysis), sputtering of surface species, and ice heating. Even so, modeling the chemical effects that result from interactions between ionizing radiation and interstellar dust grain ice mantles has proven challenging due to the complexity and variety of the underlying physical processes. To address this shortcoming, we have developed a method whereby such effects could easily be included in standard rate-equations-based astrochemical models. Here, we describe how such models, thus improved, can fruitfully be used to simulate experiments in order to better understand bulk chemistry at low temperatures.

Polycyclic aromatic hydrocarbons (PAHs) are among the most widespread compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent electric dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. Here we report the detection of 2-cyanopyrene and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in the Taurus molecular cloud (TMC-1). Our detections of the cyanopyrene isomers suggest that small PAHs like pyrene must be either formed in or transported to the cold interstellar medium, challenging assumptions about the origin and fate of PAHs in space. Wenzel et al. detect radio signatures of two forms of cyanopyrene, a small molecular sheet of carbon, which can be used as indicators of the abundance of pyrene. Their findings suggest that small polycyclic aromatic hydrocarbons must be formed in or transported to the cold interstellar medium.

This work aims to spectroscopically characterize and provide for the first time direct experimental frequencies of the ground vibrational state and two excited states of the simplest alkynyl thiocyanate (HCCSCN) for astrophysical use. Both microwave (8-16~GHz) and millimeter wave regions (50-120~GHz) of the spectrum have been measured and analyzed in terms of Watson's semirigid rotor Hamiltonian. A total of 314 transitions were assigned to the ground state of HCCSCN and a first set of spectroscopic constants have been accurately determined. Spectral features of the molecule were then searched for in Sgr B2(N), NGC 6334I, G+0.693-0.027 and TMC-1 molecular clouds. Upper limits to the column density are provided.

Cosmic rays are ubiquitous in interstellar environments, and their bombardment of dust-grain ice mantles is a possible driver for the formation of complex, even prebiotic molecules. Yet, critical data that are essential for accurate modeling of this phenomenon, such as the average radii of cosmic-ray tracks in amorphous solid water (ASW) remain unconstrained. It is shown that cosmic ray tracks in ASW can be approximated as a cylindrical volume with an average radius that is mostly independent of the initial particle energy. Interactions between energetic ions and both a low-density amorphous (LDA) and high-density amorphous (HDA) ice target are simulated using the Geant4-DNA Monte Carlo toolkit, which allows for tracking secondary electrons down to subexcitation energies in the material. We find the peak track core radii, \\(r_\\mathrm{cyl}\\), for LDA and HDA ices to be 9.9 nm and 8.4 nm, respectively - somewhat less than double the value of 5 nm often assumed in astrochemical models.