Cyanoacetylene in the outflow/hot molecular core G331.512-0.103

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 13C isotopologues), and 14 in the lowest vibrationally excited state v7 = 1. Using local thermodynamic equilibrium (LTE)-based population diagrams for the beam-diluted v = 0 transitions, we determined Texc = 85 ± 4 K and N(HC3N) = (6.9 ± 0.8) × 1014cm-2, while for the beam-diluted v7 = 1 transitions we obtained Texc = 89 ± 10 K and N(HC3N) = (2 ± 1) × 1015 cm-2. Non-LTE calculations using H2 collision rates indicate that the HC3N emission is in good agreement with LTE-based results. From the non-LTE method, we estimated Tkin ≃90 K, n(H2) ≃ 2 × 107cm-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 13C isotopologues suggest that the main formation pathway of HC3N is C2H2 + CN→HC3N + H.