ROSAT EUV observations of DA white dwarfs and late-type stars and the structure of the local interstellar medium

Abstract

The local interstellar medium (⁠|$\text{LISM}, D\lt150\enspace \text {pc}$|⁠) is observed to be generally deficient in cool dense material (⁠|$n\sim0.1,T\sim100\enspace \text K$|⁠) compared to surrounding areas of the Galactic plane. The location of the actual boundaries of this ‘local bubble’ is, however, uncertain. Extreme ultraviolet (EUV) observations are particularly sensitive to cool material and the recent ROSAT all-sky EUV survey provides a sample with which to investigate the distribution of this phase of the LISM. Neutral hydrogen columns (NH) are derived for a number of white dwarfs and late-type stars in the ROSAT EUV bright source catalogue by statistical fitting of appropriate absorbed model spectra. The NH data so derived are consistent with the bubble model of the LISM, and show that the bubble has a mean radius of 70 pc. The data also show that, in most directions, columns do not rise to greater than 1020 cm−2 within 100 pc, at odds with some other models. The existence of a large increase in NH towards the Galactic Centre is confirmed, but at a slightly greater distance than was previously believed. Significant inhomogeneities are apparent in the distribution of cool material inside the local bubble and there is evidence for some low column lines of sight.