The large- and small-scale CaII K structure of the Milky Way from observations of Galactic and Magellanic sightlines

TL;DR

This study investigates the small- and large-scale structure of the Milky Way's interstellar medium using CaII K line observations from multiple stellar sightlines, revealing variations in gas density and a new parallax relationship.

Contribution

It provides new measurements of CaII K line strength variations across different scales and introduces a functional form relating CaII column density to stellar parallax.

Findings

CaII K EW varies by factors of 1.8-3.0 over 0.05-9 pc scales.

Range of CaII K and NaI D EW consistent with spherical cloudlet ISM model.

Derived parallax-CaII column density relation differs from previous predictions.

Abstract

Aims: By utilising spectra of early-type stellar probes of known distances in the same region of the sky, the large and small-scale (pc) structure of the Galactic ISM can be investigated. This paper determines the variation in line strength of CaII at 3933.661 A, as a function of probe separation for a sample of stars, including many sightlines in the Magellanic Clouds. Methods: FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic & 4 Magellanic open clusters in CaII are used to obtain the velocity, EW, column density and line width of IS Galactic Ca for a total of 657 stars, of which 443 are Magellanic sightlines. In each cluster there are 43-110 stars observed. Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanic early-type stars are presented and combined with data from the literature to study the Ca column density/parallax relationship. Results: For the four Magellanic clusters studied with FLAMES, the strength of the Galactic IS CaII K EW over 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 min and max reduced EW for MW gas is found to lie in the range 35-125 mA & 30-160 mA for CaII K and NaI D, respectively. The range is consistent with a simple model of the ISM published by van Loon et al. (2009) consisting of spherical cloudlets of filling factor 0.3, although other geometries are not ruled out. Finally, the derived functional form for parallax and CaII column density is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derived parallax is 25 per cent lower than predicted by Megier et al. (2009) at a distance of 100 pc and 15% lower at a distance of 200 pc, reflecting inhomogeneity in the CaII distribution in the different sightlines studied.