2015A&A...582A..59S

The large and small-scale (pc) structure of the Galactic interstellar medium can be investigated by utilising spectra of early-type stellar probes of known distances in the same region of the sky. This paper determines the variation in line strength of CaII at 3933.661Å as a function of probe separation for a large sample of stars, including a number of sightlines in the Magellanic Clouds. FLAMES-GIRAFFE data taken with the Very Large Telescope towards early-type stars in 3 Galactic and 4 Magellanic open clusters in CaII are used to obtain the velocity, equivalent width, column density, and line width of interstellar Galactic calcium for a total of 657 stars, of which 443 are Magellanic Cloud sightlines. In each cluster there are between 43 and 111 stars observed. Additionally, FEROS and UVES CaII K and Nai D spectra of 21 Galactic and 154 Magellanic early-type stars are presented and combined with data from the literature to study the calcium column density - parallax relationship. For the four Magellanic clusters studied with FLAMES, the strength of the Galactic interstellar CaII K equivalent width on transverse scales from ∼0.05-9pc is found to vary by factors of ∼1.8-3.0, corresponding to column density variations of ∼0.3-0.5dex in the optically-thin approximation. Using FLAMES, FEROS, and UVES archive spectra, the minimum and maximum reduced equivalent widths for Milky Way gas are found to lie in the range ∼35-125mÅ and ∼30-160mÅ for CaII K and NaI D, respectively. The range is consistent with a previously published simple model of the interstellar medium consisting of spherical cloudlets of filling factor ∼0.3, although other geometries are not ruled out. Finally, the derived functional form for parallax (π) and CaII column density (N_CaII) is found to be π(mas)=1/(2.39x10^–13xN_CaII(cm^–2)+0.11). Our derived parallax is ∼25 per cent lower than predicted by Megier et al. (2009A&A...507..833M) at a distance of ∼100pc and ∼15 percent lower at a distance of ∼200pc, reflecting inhomogeneity in the CaII distribution in the different sightlines studied.