Discovery of the C₇N⁻ anion in TMC-1 and IRC +10216

Abstract

We report on the discovery of the C 7 N − anion towards the starless core TMC-1 and towards the carbon-rich evolved star IRC +10216. We used the data of the QUIJOTE 1 line survey towards TMC-1 and found six lines in perfect harmonic frequency relation from J=27-26 up to J=32-31. The frequency of the lines can be reproduced with a rotational constant and a distortion constant of B=582.68490±0.00024 MHz and D=4.01±0.13 Hz, respectively. The standard deviation of the fit is 4 kHz. Towards IRC +10216, we identify 17 lines from J=27-26 up to J=43-42; their frequencies are also in harmonic relation, providing B=582.6827±0.00085 MHz and D=3.31±0.31 Hz. The nearly exact coincidence of the rotational and distortion constants in both sources points unambiguously to a common molecular carrier. Taking into account the chemical peculiarities of both sources, the carrier could be a radical or an anion. The radical can be discarded, as the observed lines belong to a singlet species. Hence, the most plausible carrier is an anion. High-level ab initio calculations indicate that C 7 N − , for which we compute a rotational constant of B=582.0 MHz and a dipole moment of 7.5 D, is the carrier of the lines in both sources. We predict the neutral C 7 N to have a ground electronic state 2 Π and a dipole moment of ∼1 D. Because of this low value of µ and to its much larger rotational partition function, its lines are expected to be well below the sensitivity of our data for both sources.

Discovery of two metallic cyanoacetylides in IRC +10216: HMgCCCN and NaCCCN

“…Our current data of IRC +10216 have never before reached a sensitivity (σ) as low as 0.15 mK at some frequencies in the T * A scale for a spectral resolution of 229 kHz. This exceptional data quality has already been shown in recent publications presenting the detection of MgC 5 N and MgC 6 H (Pardo et al 2021), and of C 7 N − (Cernicharo et al 2023). The data show a large number of lines coming from isotopologues and vibrationally excited states of abundant species that are identified using the catalogues MADEX (Cernicharo 2012), CDMS (Müller et al 2005), and JPL (Pickett et al 1998).…”

“…New receivers, built within the Nanocosmos 1 project and installed at the Yebes 40m radiotelescope, were used for the observations of IRC +10216 (α J2000 = 9 h 47 m 57.36 s and δ J2000 = +13 • 16 ′ 44.4 ′′ ) as part of the Nanocosmos survey of evolved stars (Pardo et al 2022). Data from the QUIJOTE 2 line survey (Cernicharo et al 2021a(Cernicharo et al , 2022(Cernicharo et al , 2023 towards TMC-1 were also (a) Observed frequency assuming a V LS R = −26.5 km s −1 . (b) Observed minus calculated frequencies in MHz.…”

The spatial distribution of an aromatic molecule, C6H5CN, in the cold dark cloud TMC-1

“…The detection technique in QUIJOTE is the classical, and reliable, line-by-line detection without any spectral stacking. The present sensitivity of QUIJOTE is 0.12-0.25 mK (Cernicharo et al 2023).…”

Computed Rotational Collision Rate Coefficients for Recently Detected Anionic Cyanopolyynes

“…Interestingly, very recently, two additional (C,N)-bearing chains have been detected. They involve the two longest sequences of the linear-chain series observed so far: C 10 H − (Remijan et al 2023) and C 7 N − (Cernicharo et al 2023).…”