2014MNRAS.441.1134S

We report the detection of eight vibronic bands of C3, seven of which have been hitherto unobserved in astrophysical objects, in the translucent cloud towards HD 169454. Four of these bands are also found towards two additional objects: HD 73882 and HD 154368. Very high signal-to-noise ratio ( ∼ 1000 and higher) and high resolving power (R = 80000) UVES-VLT spectra (Paranal, Chile) allow for detecting novel spectral features of C3, even revealing weak perturbed features in the strongest bands. The work presented here provides the most complete spectroscopic survey of the so far largest carbon chain detected in translucent interstellar clouds. High-quality laboratory spectra of C3 are measured using cavity ring-down absorption spectroscopy in a supersonically expanding hydrocarbon plasma, to support the analysis of the identified bands towards HD 169454. A column density of N(C3) = (6.6±0.2)x10¹²/cm2 is inferred and the excitation of the molecule exhibits two temperature components; Texc = 22 ± 1 K for the low-J states and Texc = 187±25 K for the high-J tail. The rotational excitation of C3 is reasonably well explained by models involving a mechanism including inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. These models yield gas kinetic temperatures comparable to those found for Texc. The assignment of spectral features in the UV-blue range 3793-4054Å may be of relevance for future studies aiming at unravelling spectra to identify interstellar molecules associated with the diffuse interstellar bands (DIBs).