Detection of vibronic bands of C$_3$ in a translucent cloud towards HD
  169454
  We report the detection of eight vibronic bands of C$_3$, 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 ($\sim$1000
and higher) and high resolving power ($R=80,000$) UVES-VLT spectra (Paranal,
Chile) allow for detecting novel spectral features of C$_3$, even revealing
weak perturbed features in the strongest bands. The work presented here
provides the most complete spectroscopic survey of the so far largest carbon
chain detected in translucent interstellar clouds. High-quality laboratory
spectra of C$_3$ 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(C$_3$) = $(6.6 \pm
0.2) \times 10^{12}$ cm$^{-2}$ is inferred and the excitation of the molecule
exhibits two temperature components; $T_{exc}= 22 \pm 1$ K for the low-$J$
states and $T_{exc}= 187 \pm 25$ K for the high-$J$ tail. The rotational
excitation of C$_3$ 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 $T_{exc}$. The assignment of
spectral features in the UV-blue range 3793-4054 \AA\ may be of relevance for
future studies aiming at unravelling spectra to identify interstellar molecules
associated with the diffuse interstellar bands (DIBs).
