Detection of Interstellar H$_2$CCCHC$_3$N. (arXiv:2105.03347v1 [astro-ph.GA])

Detection of Interstellar H$_2$CCCHC$_3$N. (arXiv:2105.03347v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Shingledecker_C/0/1/0/all/0/1">C. N. Shingledecker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_K/0/1/0/all/0/1">K. L. K. Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wandishin_J/0/1/0/all/0/1">J. T. Wandishin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Balucani_N/0/1/0/all/0/1">N. Balucani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Burkhardt_A/0/1/0/all/0/1">A. M. Burkhardt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Charnley_S/0/1/0/all/0/1">S. B. Charnley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Loomis_R/0/1/0/all/0/1">R. Loomis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schreffler_M/0/1/0/all/0/1">M. Schreffler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siebert_M/0/1/0/all/0/1">M. Siebert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McCarthy_M/0/1/0/all/0/1">M. C. McCarthy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McGuire_B/0/1/0/all/0/1">B. A. McGuire</a>

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.

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.

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