First detection of the carbon chain molecules ¹³CCC and C¹³CC towards SgrB2(M)^★

¹ Laboratory for Astrophysics, Institute of Physics, University of Kassel, Kassel 34132, Germany
e-mail: t.giesen@uni-kassel.de
² Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
³ I. Physikalisches Institut, University of Cologne, Cologne, Germany

Received: 20 August 2019
Accepted: 18 November 2019

Abstract

Context. Carbon molecules and their ¹³C-isotopologues can be used to determine the ¹²C/¹³C abundance ratios in stellar and interstellar objects. C₃ 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 ¹³C-isotopologues of C₃ allow a selective search for the mono-substituted species ¹³CCC and C¹³CC based on accurate ro-vibrational frequencies.

Aims. We aim to provide the first detection of the ¹³C-isotopologues ¹³CCC and C¹³CC in space and to derive the ¹²C/¹³C 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 ¹³CCC and C¹³CC 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 and 1.9 THz, which were taken from the Herschel Science Data Archive.

Results. We report the first detection of the isotopologues ¹³CCC and C¹³CC. 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 × 10¹⁵ cm⁻². Assuming the excitation temperatures of C¹³CC and ¹³CCC to be the same as for CCC, we obtained column densities of the ¹³C-isotopologues of N(C¹³CC) = 2.1^+0.9_−0.6 × 10¹⁴ cm⁻² and N(¹³CCC) = 2.4^+1.2_−0.8 × 10¹⁴ cm⁻². The derived ¹²C/¹³C abundance ratio in the C₃ 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(¹³CCC)/N(C¹³CC) ratio to be 1.2 ± 0.1, which is shifted from the statistically expected value of two. We propose that the discrepant abundance ratio arises due to the lower zero-point energy of C¹³CC, which makes position-exchange reaction converting ¹³CCC to C¹³CC energetically favorable.