How Widespread Are Aromatic Cycles In Cold Clouds?

By Keith Cowing
Status Report
September 8, 2023
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How Widespread Are Aromatic Cycles In Cold Clouds?
Two individual lines of C6H5CN (left and middle panels) and the stacked spectrum of C6H5CN (right panels) for L1495B (top row), Lupus-1A (middle row), and L483 (bottom row), using the 130,13-120,12 line at 32833.813 MHz as a reference. The number of stacked lines and the significance level of the line stack signal (in units of σ) are indicated in the right panels. The systemic velocity of each source is indicated by a vertical dotted line. — astro-ph.GA

We report the detection of large hydrocarbon cycles toward several cold dense clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q band (31-50 GHz) using the Yebes 40m radiotelescope.

Using the line stack technique, we find statistically significant evidence of benzonitrile (C6H5CN) in L1495B, Lupus-1A, and L483 at the 31.8 sigma, 15.0 sigma, and 17.2 sigma levels, respectively, while there is no hint of C6H5CN in the fourth source, L1527. The column densities derived are in the range (1.8-4.0)e12 cm-2, which is somewhat below the value derived toward the cold dense cloud TMC-1. When we analyse together all the benzonitrile abundances derived toward cold clouds in this study and in the literature, a clear trend emerges in which the higher the abundance of HC7N, the more abundant C6H5CN is.

This indicates that aromatic cycles are specially favored in those interstellar clouds where long carbon chains are abundant, which suggests that the chemical processes that are responsible for the formation of linear carbon chains are also behind the synthesis of aromatic rings. We also searched for cycles other than benzonitrile, and found evidence of indene (C9H8), cyclopentadiene (C5H6), and 1-cyano cyclopentadiene (1-C5H5CN) at the 9.3 sigma, 7.5 sigma, and 8.4 sigma, respectively, toward L1495B, which shows the strongest signal from C6H5CN. The relative abundances between the various cycles detected in L1495B are consistent, within a factor of three, to those found previously in TMC-1. It is therefore likely that not only C6H5CN but also other large aromatic cycles are abundant in clouds rich in carbon chains.

M. Agundez, N. Marcelino, B. Tercero, J. Cernicharo

Comments: Accepted for publication in A&A Letters
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2308.15951 [astro-ph.GA] (or arXiv:2308.15951v1 [astro-ph.GA] for this version)
Submission history
From: Marcelino Agundez
[v1] Wed, 30 Aug 2023 11:00:31 UTC (83 KB)
Astrobiology, Astrochemistry

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