Early-type stars observed in the ESO UVES Paranal Observatory Project - V. Time-variable interstellar absorption

TL;DR

This study investigates the small-scale structure of the diffuse interstellar medium by comparing multi-epoch spectroscopic data of O- and B-type stars, finding limited but notable temporal variations in certain absorption lines.

Contribution

It provides the first systematic search for time-variable interstellar absorption lines over scales from 0 to 360 au using high-resolution spectra from VLT-UVES and McDonald Observatory.

Findings

Detected variations in 5 out of 104 sightlines, with changes up to a factor of 3.6 in column density.

Most sightlines show no variation, with upper limits of 0.1-0.3 dex in column density changes.

Variations suggest the presence of fine-scale structure in the ISM, but such features are not very common.

Abstract

The structure and properties of the diffuse interstellar medium (ISM) on small scales, sub-au to 1 pc, are poorly understood. We compare interstellar absorption-lines, observed towards a selection of O- and B-type stars at two or more epochs, to search for variations over time caused by the transverse motion of each star combined with changes in the structure in the foreground ISM. Two sets of data were used: 83 VLT- UVES spectra with approximately 6 yr between epochs and 21 McDonald observatory 2.7m telescope echelle spectra with 6 - 20 yr between epochs, over a range of scales from 0 - 360 au. The interstellar absorption-lines observed at the two epochs were subtracted and searched for any residuals due to changes in the foreground ISM. Of the 104 sightlines investigated with typically five or more components in Na I D, possible temporal variation was identified in five UVES spectra (six components), in Ca II, Ca I and/or Na I absorption-lines. The variations detected range from 7\% to a factor of 3.6 in column density. No variation was found in any other interstellar species. Most sightlines show no variation, with 3{\sigma} upper limits to changes of the order 0.1 - 0.3 dex in Ca II and Na I. These variations observed imply that fine-scale structure is present in the ISM, but at the resolution available in this study, is not very common at visible wavelengths. A determination of the electron densities and lower limits to the total number density of a sample of the sightlines implies that there is no striking difference between these parameters in sightlines with, and sightlines without, varying components.