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.
