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We report the detection of eight vibronic bands of C$_3$, seven of which havebeen hitherto unobserved in astrophysical objects, in the translucent cloudtowards HD~169454. Four of these bands are also found towards two additionalobjects: HD~73882 and HD~154368. Very high signal-to-noise ratio ($\sim$1000and higher) and high resolving power ($R=80,000$) UVES-VLT spectra (Paranal,Chile) allow for detecting novel spectral features of C$_3$, even revealingweak perturbed features in the strongest bands. The work presented hereprovides the most complete spectroscopic survey of the so far largest carbonchain detected in translucent interstellar clouds. High-quality laboratoryspectra of C$_3$ are measured using cavity ring-down absorption spectroscopy ina supersonically expanding hydrocarbon plasma, to support the analysis of theidentified bands towards HD~169454. A column density of N(C$_3$) = $(6.6 \pm0.2) \times 10^{12}$ cm$^{-2}$ is inferred and the excitation of the moleculeexhibits 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 rotationalexcitation of C$_3$ is reasonably well explained by models involving amechanism including inelastic collisions, formation and destruction of themolecule, and radiative pumping in the far-infrared. These models yield gaskinetic temperatures comparable to those found for $T_{exc}$. The assignment ofspectral features in the UV-blue range 3793-4054 \AA\ may be of relevance forfuture studies aiming at unravelling spectra to identify interstellar moleculesassociated with the diffuse interstellar bands (DIBs).

Detection of vibronic bands of C$_3$ in a translucent cloud towards HD 169454