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

Abstract

Context.Carbon molecules and their13C-isotopologues can be used to determine the12C/13C abundance ratios in stellar and interstellar objects. C3is a pure carbon chain molecule found in star-forming regions and in stellar shells of carbon-rich late-type stars. Latest laboratory data of13C-isotopologues of C3allow a selective search for the mono-substituted species13CCC and C13CC based on accurate ro-vibrational frequencies.Aims.We aim to provide the first detection of the13C-isotopologues13CCC and C13CC in space and to derive the12C/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 transitionsQ(2) andQ(4) of13CCC 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 theHerschelScience Data Archive.Results.We report the first detection of the isotopologues13CCC and C13CC. For both species, the ro-vibrational absorption linesQ(2) andQ(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 ofTex= 44.4+4.7−3.9K, and a total column density ofN(CCC) = 3.88+0.39−0.35× 1015cm−2. Assuming the excitation temperatures of C13CC and13CCC to be the same as for CCC, we obtained column densities of the13C-isotopologues ofN(C13CC) = 2.1+0.9−0.6× 1014cm−2andN(13CCC) = 2.4+1.2−0.8× 1014cm−2. The derived12C/13C abundance ratio in the C3molecules is 20.5 ± 4.2, which is in agreement with the elemental ratio of 20, typically observed in SgrB2(M). However, we find theN(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 converting13CCC to C13CC energetically favorable.