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Register a new userDiscovery of Interstellar trans-cyanovinylacetylene (HCCCH=CHCN) and vinylcyanoacetylene (H$_2$C=CHC$_3$N) in GOTHAM Observations of TMC-1
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Added by
Kin Long Kelvin Lee
Publication date
2021
fields
Physics
and research's language is
English
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We report the discovery of two unsaturated organic species, trans-(E)-cyanovinylacetylene and vinylcyanoacetylene, using the second data release of the GOTHAM deep survey towards TMC-1 with the 100 m Green Bank Telescope. For both detections, we performed velocity stacking and matched filter analyses using Markov chain Monte Carlo simulations, and for trans-(E)-cyanovinylacetylene, three rotational lines were observed at low signal-to-noise (${sim}$3$sigma$). From this analysis, we derive column densities of $2times10^{11}$ and $3times10^{11}$ cm$^{-2}$ for vinylcyanoacetylene and trans-(E)-cyanovinylacetylene, respectively, and an upper limit of $<2times10^{11}$ cm$^{-2}$ for trans-(Z)-cyanovinylacetylene. Comparisons with G3//B3LYP semi-empirical thermochemical calculations indicate abundances of the [H$_3$C$_5$N}] isomers are not consistent with their thermodynamic stability, and instead their abundances are mainly driven by dynamics. We provide discussion into how these species may be formed in TMC-1, with reference to related species like vinyl cyanide (CH$_2$=CHCN). As part of this discussion, we performed the same analysis for ethyl cyanide (CH$_3$CH$_2$CN), the hydrogenation product of CH$_2$=CHCN. This analysis provides evidence -- at 4.17$sigma$ significance -- an upper limit to the column density of $<4times10^{11}$ cm$^{-2}$; an order of magnitude lower than previous upper limits towards this source.
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We present an overview of the GOTHAM (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 ($c$-C$_6$H$_5$CN) 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 (HCCCH$_2$CN) 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.
Polycyclic Aromatic Hydrocarbons (PAHs) have long been invoked in the study of interstellar and protostellar sources, but the unambiguous identification of any individual PAH has proven elusive until very recently. As a result, the formation mechanisms for this important class of molecules remain poorly constrained. Here we report the first interstellar detection of a pure hydrocarbon PAH, indene (C$_9$H$_8$), as part of the GBT Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey. This detection provides a new avenue for chemical inquiry, complementing the existing detections of CN-functionalized aromatic molecules. From fitting the GOTHAM observations, indene is found to be the most abundant organic ring detected in TMC-1 to date. And from astrochemical modeling with NAUTILUS, the observed abundance is greater than the models prediction by several orders of magnitude suggesting that current formation pathways in astrochemical models are incomplete. The detection of indene in relatively high abundance implies related species such as cyanoindene, cyclopentadiene, toluene, and styrene may be detectable in dark clouds.
We report the first detection in space of the cumulene carbon chain $l$-H$_2$C$_5$. A total of eleven rotational transitions, with $J_{up}$ = 7-10 and $K_a$ = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8$pm$0.5)$times$10$^{10}$ cm$^{-2}$. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that $l$-H$_2$C$_5$ is $sim$4.0 times less abundant than the larger cumulene carbene $l$-H$_2$C$_6$, while it is $sim$300 and $sim$500 times less abundant than the shorter chains $l$-H$_2$C$_3$ and $l$-H$_2$C$_4$. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C$_n$H$_2$ isomers with $n$,$>$,3.
The chemical pathways linking the small organic molecules commonly observed in molecular clouds to the large, complex, polycyclic species long-suspected to be carriers of the ubiquitous unidentified infrared emission bands remain unclear. To investigate whether the formation of mono- and poly-cyclic molecules observed in cold cores could form via the bottom-up reaction of ubiquitous carbon-chain species with, e.g. atomic hydrogen, a search is made for possible intermediates in data taken as part of the GOTHAM (GBT Observations of TMC-1 Hunting for Aromatic Molecules) project. Markov-Chain Monte Carlo (MCMC) Source Models were run to obtain column densities and excitation temperatures. Astrochemical models were run to examine possible formation routes, including a novel grain-surface pathway involving the hydrogenation of C$_6$N and HC$_6$N, as well as purely gas-phase reactions between C$_3$N and both propyne (CH$_3$CCH) and allene (CH$_2$CCH$_2$), as well as via the reaction CN + H$_2$CCCHCCH. We report the first detection of cyanoacetyleneallene (H$_2$CCCHC$_3$N) in space toward the TMC-1 cold cloud using the Robert C. Byrd 100 m Green Bank Telescope (GBT). Cyanoacetyleneallene may represent an intermediate between less-saturated carbon-chains, such as the cyanopolyynes, that are characteristic of cold cores and the more recently-discovered cyclic species like cyanocyclopentadiene. Results from our models show that the gas-phase allene-based formation route in particular produces abundances of H$_2$CCCHC$_3$N that match the column density of $2times10^{11}$ cm$^{-2}$ obtained from the MCMC Source Model, and that the grain-surface route yields large abundances on ices that could potentially be important as precursors for cyclic molecules.
We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH$_2$DC$_3$N. A total of fifteen rotational transitions, with $J$ = 8-12 and $K_a$ = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this deuterated isotopologue. We derive a column density of $(8.0pm 0.4) times 10^{10}$ cm$^{-2}$. The abundance ratio between CH$_3$C$_3$N and CH$_2$DC$_3$N is $sim$22. We also theoretically computed the principal spectroscopic constants of $^{13}$C isotopologues of CH$_3$C$_3$N and CH$_3$C$_4$H and those of the deuterated isotopologues of CH$_3$C$_4$H for which we could expect a similar degree of deuteration enhancement. However, we have not detected either CH$_2$DC$_4$H nor CH$_3$C$_4$D nor any $^{13}$C isotopologue. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas phase chemical model where the low temperature conditions favor deuteron transfer through reactions with H$_2$D$^+$.
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