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

Context. Carbon molecules and their C-13-isotopologues can be used to determine the C-12/C-13 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 C-13-isotopologues of C-3 allow a selective search for the mono-substituted species (CCC)-C-13 and (CCC)-C-13 based on accurate ro-vibrational frequencies. Aims. We aim to provide the first detection of the C-13-isotopologues (CCC)-C-13 and (CCC)-C-13 in space and to derive the C-12/C-13 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 (CCC)-C-13 and (CCC)-C-13 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 (CCC)-C-13 and (CCC)-C-13. 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(-3.9)(+4.7) K, and a total column density of N(CCC) = 3.88(-0.35)(+0.39) x 10(15) cm(-2). Assuming the excitation temperatures of (CCC)-C-13 and (CCC)-C-13 to be the same as for CCC, we obtained column densities of the C-13-isotopologues of N((CCC)-C-13) = 2.1(-0.6)(+0.9) x 10(14) cm(-2) and N((CCC)-C-13) = 2.4(-0.8)(+1.2) x 10(14) cm(-2). The derived C-12/C-13 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((CCC)-C-13)/N((CCC)-C-13) 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 (CCC)-C-13, which makes position-exchange reaction converting (CCC)-C-13 to (CCC)-C-13 energetically favorable.