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

We report the first detection of the isotopologues 13CCC and C13CC. 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). 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). 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, and derived a gas excitation temperature of Tex = 44.4(+4.7/-3.9) K and a total column density of N(CCC)=3.88(+0.39/-0.35)x10^15 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)X10^14 cm^-2 and N(13CCC)=2.4(+1.2/-0.8)x10^14 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 2. 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.