We present the first detection of the l-C3H+ hydrocarbon in the interstellar medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two positions, namely the photo-dissociation region (PDR) and the nearby shielded core, revealed
a consistent set of eight unidentified lines toward the PDR position. Six of them are detected with a signal-to-noise ratio from 6 to 19, while the two last ones are tentatively detected. Mostly noise appears at the same frequency toward the dense core, located less than 40 away. We simultaneously fit 1) the rotational and centrifugal distortion constants of a linear rotor, and 2) the Gaussian line shapes located at the eight predicted frequencies. The observed lines can be accurately fitted with a linear rotor model, implying a 1Sigma ground electronic state. The deduced rotational constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to constrain the chemistry of small hydrocarbons. A rotational diagram is then used to infer the excitation temperature and the column density. We finally compare the abundance to the results of the Meudon PDR photochemical model.
We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the
consequences for the understanding of the interstellar medium.
To understand the environment and extended structure of the host galactic gas whose molecular absorption line chemistry, we previously observed along the microscopic line of sight to the blazar/radiocontinuum source NRAO150 (aka B0355+508), we used t
he IRAM 30m Telescope and Plateau de Bure Interferometer to make two series of images of the host gas: i) 22.5 arcsec resolution single-dish maps of 12CO J=1-0 and 2-1 emission over a 220 arcsec by 220 arcsec field; ii) a hybrid (interferometer+singledish) aperture synthesis mosaic of 12CO J=1-0 emission at 5.8 arcsec resolution over a 90 arcsec-diameter region. CO components that are observed in absorption at a moderate optical depth (0.5) and are undetected in emission at 1 arcmin resolution toward NRAO 150 remain undetected at 6 arcsec resolution. This implies that they are not a previously-hidden large-scale molecular component revealed in absorption, but they do highlight the robustness of the chemistry into regions where the density and column density are too low to produce much rotational excitation, even in CO. Bright CO lines around NRAO150 most probably reflect the variation of a chemical process, i.e. the C+-CO conversion. However, the ultimate cause of the variations of this chemical process in such a limited field of view remains uncertain.
After a discussion about the need for observational benchmark for chemical models, we explain 1) why the Horsehead western edge is well suited to serve as reference for models and 2) the steps we are taking toward this goal. We summarize abundances obtained to date and we show recent results.
