Detection of vibronic bands of C3_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).Comment: 15 pages, 13 figures, submitted to MNRA

Diffuse band shifts: a possible explanation

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The relation between column densities of interstellar OH and CH molecules

We present a new, close relation between column densities of OH and CH molecules based on 16 translucent sightlines (six of them new) and confirm the theoretical oscillator strengths of the OH A--X transitions at 3078 and 3082 \AA (0.00105, 0.000648) and CH B--X transitions at 3886 and 3890 \AA, (0.00320, 0.00210), respectively. We also report no difference between observed and previously modelled abundances of the OH molecule.Comment: 4 pages, 0 figures, accepted for publication in MNRA

Dark Dust III: The high-quality single-cloud reddening curve sample. Scrutinizing extinction curves in the Milky Way

The nature of dust in the diffuse interstellar medium can be best investigated by means of reddening curves where only a single interstellar cloud lies between the observer and the background source. Published reddening curves often suffer from various systematic uncertainties. We merge a sample of 895 reddening curves of stars for which both FORS2 polarisation spectra and UVES high-resolution spectra are available. The resulting 111 sightlines toward OB-type stars have 175 reddening curves. For these stars, we derive their spectral type from the UVES high-resolution spectroscopy. To obtain high-quality reddening curves we exclude stars with composite spectra in the IUE/FUSE data due to multiple stellar systems. Likewise, we omit stars that have uncertain spectral type designations or stars with photometric variability. We neglect stars that show inconsistent parallaxes when comparing DR2 and DR3 from GAIA. Finally, we identify stars that show differences in the space and ground-based derived reddening curves between $0.28\,\mu$m and the $U$-band or in $R_V$. In total, we find 53 stars with one or more reddening curves passing the rejection criteria. This provides the highest quality Milky Way reddening curve sample available today. Averaging the curves from our high-quality sample, we find $R_V = 3.1 \pm 0.4$, confirming previous estimates. A future paper in this series will use the current sample of precise reddening curves and combine them with polarisation data to study the properties of Dark Dust.Comment: Recommended to acceptance by A&