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

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.

A copy of the reduced spectra is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/633/A120