Abstract

Using APEX-1 and APEX-2 observations, we have detected and studied the rotational lines of the HC3N molecule (cyanoacetylene) in the powerful outflow/hot molecular core G331.512-0.103. We identified 31 rotational lines at J levels between 24 and 39; 17 of them in the ground vibrational state v = 0 (9 lines corresponding to the main C isotopologue and 8 lines corresponding to the C-13 isotopologues), and 14 in the lowest vibrationally excited state v(7) = 1. Using local thermodynamic equilibrium (LTE)-based population diagrams for the beam-diluted v = 0 transitions, we determined T-exc = 85 +/- 4 K and N(HC3N) = (6.9 +/- 0.8) x 1014 cm(-2), while for the beam-diluted v7 = 1 transitions we obtained T-exc = 89 +/- 10 K and N(HC3N) = (2 +/- 1) x 10(15) cm(-2). Non-LTE calculations using H-2 collision rates indicate that the HC3N emission is in good agreement with LTE-based results. From the non-LTE method, we estimated T-kin similar or equal to 90 K, n(H-2) similar or equal to 2 x 10(7) cm(-3) for a central core of 6 arcsec in size. A vibrational temperature in the range from 130 to 145 K was also determined, values which are very likely lower limits. Our results suggest that rotational transitions are thermalized, while infrared radiative pumping processes are probably more efficient than collisions in exciting the molecule to the vibrationally excited state v7 = 1. Abundance ratios derived under LTE conditions for the C-13 isotopologues suggest that the main formation pathway of HC3N is C2H2 + CN -> HC3N + H.