First detection of the carbon chain molecules¹³CCC and C¹³CC towards SgrB2(M)

Abstract

Context. Carbon molecules and their 13 C-isotopologues can be used to determine the 12 C/ 13 C abundance ratios in stellar and interstellar objects. C 3 is a pure carbon chain molecule found in star forming regions and in stellar shells of carbon-rich late-type stars. Latest laboratory data of 13 C-isotopologues of C 3 allow a selective search for the mono-substituted species 13 CCC and C 13 CC based on accurate ro-vibrational frequencies.Aims. Our aim was to provide the first detection of the 13 C-isotopologues 13 CCC and C 13 CC in space and to derive the 12 C/ 13 C ratio of interstellar gas in the massive star-forming region SgrB2(M) near the Galactic Center. Methods. We used the heterodyne receivers GREAT and upGREAT on board SOFIA to search for the ro-vibrational transitions Q(2) and Q(4) of 13 CCC and C 13 CC at 1.9 THz along the line of sight towards SgrB2(M). In addition, to determine the local excitation temperature we analyzed data from nine ro-vibrational transitions of the main isotopologue CCC in the frequency range between 1.6 -1.9 THz which were taken from the Herschel Science Data Archive. Results. We report the first detection of the isotopologues 13 CCC and C 13 CC. For both species the ro-vibrational absorption lines Q(2) and Q(4) have been identified, primarily arising from the warm gas physically associated with the strong continuum source SgrB2(M). From the available CCC ro-vibrational transitions we derived a gas excitation temperature of T ex = 44.4 +4.7 −3.9 K and a total column density of N(CCC)=3.88 +0.39 −0.35 × 10 15 cm −2 . Assuming the excitation temperatures of C 13 CC and 13 CCC to be the same as for CCC, we obtained column densities of the 13 C-isotopologues of N(C 13 CC) = 2.1 +0.9 −0.6 × 10 14 cm −2 and N( 13 CCC)=2.4 +1.2 −0.8 × 10 14 cm −2 . The derived 12 C/ 13 C abundance ratio in the C 3 molecules is 20.5±4.2, which is in agreement with the elemental ratio of 20, typically observed in SgrB2(M). However, we find the N( 13 CCC) / N(C 13 CC) ratio to be 1.2±0.1, which is shifted from the statistically expected value of 2. We propose that the discrepant abundance ratio arises due to the lower zero-point energy of C 13 CC which makes positionexchange reaction converting 13 CCC to C 13 CC energetically favorable.

Deexcitation rate coefficients of C3 by collision with H2 at low temperatures

“…Small carbon chains, such as C 2 , C 3 , and C 5 , have been observed in several regions of the interstellar medium (ISM) (Souza & Lutz 1977;Giesen, T. F. et al 2020;Maier et al 2001;Hinkle et al 1988;Bernath et al 1989). They are expected to be the building blocks of more complex organic molecules that could explain the origin of life.…”

High-resolution laser spectroscopy of the Ã1ΠuZhang1, Gu2, Doney3 et al. 2021Journal of Molecular Spectroscopy2000View full textAdd to dashboardCite

“…A gas and grain warm-up model proposed by Mookerjea et al has explained the abundance of C 3 in the warm envelopes of hot cores in a consistent manner [16]. Very recently, the detection of 13 C 12 C 12 C and 12 C 13 C 12 C along the line of sight towards Sgr B2(M) allowed to determine the 12 C/ 13 C element ratio in the C 3 molecules to be 20.5 AE 4.2 [17]. This value is in agreement with the elemental ratio of 20, typically observed in Sgr B2(M).…”

“…Spectroscopic constants were obtained and the C-C bond length was derived experimentally for each isotopologue. Furthermore, the precise line positions led to the detection of 13 C 12 C 12 C and 12 C 13 C 12 C towards Sgr B2(M) [17]. Besides all these experimental studies, important contributions came from precise theoretical work.…”

“…Besides the main isotopologue C 3 ; 13 C-isomers of C 3 have also been identified in the ISM recently. Giesen et al [17] reported the first detection of 13 C 12 C 12 C and…”

Isotopic fractionation study towards massive star-forming regions across the Galaxy

“…Recently, Giesen et al (2020) have reported the first detection of the 13 C-isotopologues of C 3 , 13 CCC and C 13 CC, towards the massive starforming region SgrB2(M), near the Galactic Center. They derived an average 12 C/ 13 C abundance ratio of 20.5±4.2, in agreement with the local value of 20 as derived from the Galactocentric trend by Milam et al (2005) for CN, CO and H 2 CO, taking into account the three molecules:…”