The large- and small-scale Ca II K structure of the Milky Way from observations of Galactic and Magellanic sightlines^⋆,⋆⋆

¹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
e-mail: jsmoker@eso.org
² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN, UK
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 20 October 2014
Accepted: 18 June 2015

Abstract

Aims. 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 Ca ii at 3933.661 Å  as a function of probe separation for a large sample of stars, including a number of sightlines in the Magellanic Clouds.

Methods. FLAMES-GIRAFFE data taken with the Very Large Telescope towards early-type stars in 3 Galactic and 4 Magellanic open clusters in Ca ii 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 Ca ii K and Na i 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.

Results. For the four Magellanic clusters studied with FLAMES, the strength of the Galactic interstellar Ca ii K equivalent width on transverse scales from ~0.05−9 pc is found to vary by factors of ~1.8−3.0, corresponding to column density variations of ~0.3−0.5 dex 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 ~35−125 mÅ  and ~30−160 mÅ  for Ca ii K and Na i 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 Ca ii column density (N_CaII) is found to be π(mas) = 1 / (2.39 × 10^-13 × N_CaII (cm^-2) + 0.11). Our derived parallax is ~25 per cent lower than predicted by Megier et al. (2009, A&A, 507, 833) at a distance of ~100 pc and ~15 percent lower at a distance of ~200 pc, reflecting inhomogeneity in the Ca ii distribution in the different sightlines studied.