Structure and chemistry of the solar neighborhood

The quest for understanding cosmic ray propagation in our Galaxy has led to the development of powerful 3D models to explain the plethora of observations. Yet, many simplifying assumptions still need to be made, that are only slowly relaxed to introduce a higher degree of realism to the models. One crucial ingredient is the density and chemical composition of the medium through which high-energy photons and particles travel, and in which they interact with the matter before reaching ground- or satellite-based detectors. In order to improve over the state-of-the-art we aim to determine the spatial distribution, amount and chemical composition of the gas and dust in the solar neighborhood out to distances of ~1 kpc from the Sun in a self- consistent way. Young OB-type stars will be employed as background sources for a large number of sightlines which will facilitate the characterization of the diffuse ISM gas and dust component, bringing to application breakthrough improvements in the quantitative spectroscopy of OB stars, recently introduced by the applicant. Optical and UV spectra that include both, stellar and interstellar absorption lines, and photo- metric data will be self-consistently analyzed for the first time to determine: the stellar chemical composition and distances, neutral hydrogen interstellar gas column densities, gas metal abundances, reddening, extinction, dust composition and dust-to-gas ratios. The strength of the project is the simultaneous characterization of stars, gas and dust from common sets of observational data, avoiding in this way any assumptions that trouble indirect methods that are usually employed to estimate some of the essential parameters required in the course of such investigations. Moreover, the project will allow the contribution of the massive stars to the UV/optical interstellar radiation field in the solar neighborhood to be refined. In this way, the project offers a tight synergy between the fields of astrophysics and astroparticle physics to be achieved, a connection that is not realized so far within the Austrian research community. We will provide the field of non-thermal high-energy particle and radiation propagation with an independent and highly detailed and accurate assessment of line-of-sight column densities and dust-to-gas ratios in the solar neighborhood, and a refined interstellar radiation field. In addition, tight observational constraints on the chemical composition of stars, gas and dust will shed light on the rather large range of currently proposed nuclear enhancement factors, allowing the associated systematic uncertainties in the interpretation of gamma-ray observations to be reduced. All these aspects have a profound impact on defining the cosmic-ray- induced gamma-ray flux and therefore also on our understanding of the propagation of cosmic rays in the Galaxy.