First Detection of A–X (0,0) Bands of Interstellar C₂ and CN

Abstract

We report the first detection of C 2 A 1 Π u -X 1 Σ + g (0,0) and CN A 2 Π u -X 2 Σ + (0,0) absorption bands in the interstellar medium. The detection was made using the near-infrared (0.91-1.35 µm) high-resolution (R = 20, 000 and 68,000) spectra of Cygnus OB2 No. 12 collected with the WINERED spectrograph mounted on the 1.3 m Araki telescope. The A-X (1,0) bands of C 2 and CN were detected simultaneously. These near-infrared bands have larger oscillator strengths, compared with the A-X (2,0) bands of C 2 and CN in the optical. In the spectrum of the C 2 (0,0) band with R = 68, 000, three velocity components in the line of sight could be resolved and the lines were detected up to high rotational levels (J ∼ 20). By analyzing the rotational distribution of C 2 , we could estimate the kinetic temperature and gas density of the clouds with high accuracy. Furthermore, we marginally detected weak lines of 12 C 13 C for the first time in the interstellar medium. Assuming that the rotational distribution and the oscillator strengths of the relevant transitions of 12 C 2 and 12 C 13 C are the same, the carbon isotope ratio was estimated to be 12 C/ 13 C = 50-100, which is consistent with the ratio in the local interstellar medium. We also calculated the oscillator strength ratio of the C 2 (0,0) and (1,0) bands from the observed band strengths. Unfortunately, our result could not discern theoretical and experimental results because of the uncertainties. High-resolution data to resolve the velocity components will be necessary for both bands in order to put stronger constraints on the oscillator strength ratios.

Theoretical studies of carbon isotopic fractionation in reactions of C with C2: dynamics, kinetics, and isotopologue equilibria

“…Despite some previous claims (Bakker & Lambert 1998), the present theoretical results strongly support the suggestion made by other authors (Roueff et al 2015;Colzi et al 2020) that the C + C 2 reactions (particularly (2) and ( 4)) may act as important routes in the overall C-fractionation chemistry, notably in lowtemperature C-rich environments. Besides providing key input data for astrochemical models of cold dense clouds (Furuya et al 2011;Roueff et al 2015;Colzi et al 2020;Loison et al 2020), the calculated rate constants over such a broad T range may also fulfill the needs of models of photo-dissociation regions (Röllig & Ossenkopf 2013), translucent clouds (Hamano et al 2019), protoplanetary disks (Woods & Willacy 2009), and circumstellar envelopes of evolved C-stars (Bakker & Lambert 1998). Apart from its astrophysical implications, this work is expected to provide safe grounds on which to base future methodological developments toward the calculation of theoretical rate constants of astrochemically relevant isotope-exchange reactions without resorting to (and avoid the burden of) quantum dynamics, while still recovering all intrinsic details of the interacting potentials between the colliding particles.…”

“…by means of a theoretical approach (see below). The motivation here is primarily grounded in the prevalence of C 2 , the smallest pure carbon cluster, throughout the ISM; it has been detected (via its Phillips (A 1 Π u -X 1 Σ + g ) and Swan (d 3 Π g -a 3 Π u ) bands) in a myriad of astronomical sources (Babb et al 2019), including diffuse (Souza & Lutz 1977;Snow & McCall 2006), translucent (Hamano et al 2019), and dense molecular clouds (Hobbs et al 1983) and is known to be the primary reservoir of gas-phase carbon in oxygen-poor regions (Souza & Lutz 1977). Besides being key for probing the physical conditions of interstellar clouds (Snow & McCall 2006), C 2 , together with C (+) , is thought to be the fundamental building block in the formation chemistry of larger hydrogen-deficient C-bearing species (Ehrenfreund & Charnley 2000;Kaiser 2002;Gu et al 2006), and therefore plays an active role in their 13 C enrichment.…”

“…Ever since the postulation of reaction (1) as the main C fractionation route in strongly shielded regions (Watson et al 1976), a notable contrast has emerged between the above general predictions (by chemistry models) of the strong 13 C depletion in C-containing molecules (Langer et al 1984) and the general absence of this observable effect in surveys conducted, for example, in abundant species such as CS (Liszt & Ziurys 2012), CN (Milam et al 2005), C 2 (Hamano et al 2019), CCS (Sakai et al 2007), HNC (Liszt & Ziurys 2012), C 3 (Giesen, T. F. et al 2020), and HC 3 N (Takano et al 1998) that are not formed directly from CO and whose 12 C/ 13 C ratios thus inferred are in agreement with (or even lower than) the gas elemental values. Such a conflict therefore opened up new avenues for the possibility of an overall 13 C enrichment in species other than CO, and led to the proposition of alternative isotope-exchange reactions (e.g., 13 C (+) +CN (Langer 1992;Roueff, E. et al 2015), 13 CO+HCO + (Smith & Adams 1980; Mladenović, M. & Roueff, E. 2017), 13 C+C 2 (Roueff, E. et al 2015), and 13 C+C 3 (Giesen, T. F. et al 2020;Colzi, L. et al 2020;Loison et al 2020)) and novel formation pathways (Takano et al 1998;Sakai et al 2007;Furuya et al 2011) deemed to contribute to the 13 C fractionation chemistry.…”

Full Spectroscopic Model and Trihybrid Experimental-Perturbative-Variational Line List for CN

“…These in turn rely on high-quality experimental data. In the case of CN, we need to consider at least the three lowest lying electronic states, the X 2 Σ + ground state, the A 2 Π state at 9243 cm −1 , and the B 2 Σ + state at 25752 cm −1 , as astrophysical observations of interactions between all of these electronic bands have been observed (Hamano et al 2019). All available experimental data was recently compiled for 12 C 14 N (Syme & McKemmish 2020b), which then used the Marvel (Furtenbacher et al 2007) procedure to extract empirical energy levels.…”

“…the X 2 Σ + , A 2 Π and the B 2 Σ + states. Uses of the Mollist line list have included; constraining the chemical evolution of the local disc with C and N abundances (Botelho et al 2020), modelling the impact of chemical hazes in exoplanetary atmospheres (Lavvas & Arfaux 2021), probing interstellar clouds (Welty et al 2020), chemical abundance in stars that may harbour rocky planets from TESS data (Tautvaišienė et al 2020), as well as observing the first A 2 Π-X 2 Σ + (0,0) band in the interstellar medium Hamano et al (2019).…”