The ionization of the local interstellar medium

While most of our work has had the goal of understanding the structure and evolution of white dwarfs, EUV observations of these stars can also tell us much about the intervening interstellar material. Although the original nebula from which the Solar System formed, about 4500 million years ago, will have been largely dissipated by the intense radiation from the young Sun and also left behind by the motion of the Solar System through interstellar space, more recent encounters with interstellar material may have affected us directly and at the very least influenced our astronomical observations.

Our present picture of the local interstellar medium (the gas lying between the stars out to about 100pc) is that the Sun is embedded in and near the edge of a wispy diffuse "local" cloud (or local fluff). This cloud, which is only about 10pc across, is itself within a much less dense region called the "local bubble". The distribution of white dwarfs and late type stars in the ROSAT WFC survey has been used to map out the dimensions of the local bubble but could not tell us anything about the state of the gas inside. The current state of the gas in both the local cloud and bubble is expected to bear the imprint of recent nearby events, such as supernova explosions and radiation from hot young stars. As a result the interstellar gas should be ionized, with the electrons stripped from the constituent (mainly hydrogen and helium) atoms. A critical question is whether the interstellar gas is in equilibrium, with atoms being ionized at the same rate as they recombine with the electrons, or not.

A Hammer-Aitoff plot in galactic coordinates, showing the distribution of white dwarf positions. The sizes of the symbols correspond to the HI column density along the line of sight. Squares = observations where a 228 Angstrom edge is not detected; triangles = 228 Angstrom edge detections analysed with H+He models; circles = 228 Angstrom edge detections analysed with heavy element models. The contours included indicate the distance (in pc) to the edge of the local cavity, as determined by Warwick et al. (MNRAS, 262 , 289, 1993).

Ionized material can be detected by the "shadowing" effect it has on the EUV spectra of other objects. Since, white dwarfs have strong EUV continua, they are ideal targets with which to search for interstellar absorption from HeI and HeII.

We have used spectra obtained with the Extreme Ultraviolet Explorer to carry out these observations. From direct detection of the HeI and HeII features, the total amount of He and its fractional ionization can be simply calculated. Assuming a cosmic abundance ratio for the total amount of H and He (10:1), the H ionization fraction can also be inferred. Remarkably, while the gas density varies in different directions the fraction of ionized material is highly uniform ( Barstow et al., MNRAS, 286 , pp 58-76, 1997 ). At the observed level of ionization, the radiation from nearby stars is not enough to maintain an equilibrium but the shortfall could be made up of photons emitted by decaying dark matter. However, these results are best explained by a non-equilibrium scenario in which the local cloud was ionized by the shock wave from a nearby supernova explosion, since when the ions and electrons have been slowly recombining. The observed fractions of ionized H (35%) and He (27%) indicate that the explosion occurred around 4 million years ago. This might be the same supernova believed to have swept out the cavity we now identify with the local bubble. These results confirm that a source of decaying dark matter is not needed to explain the appearance of the local interstellar medium.

The observed fractional ionization of He for each star in the sample as a function of the total column density of hydrogen along the line of sight. The shaded shapes indicate those stars where HeII is detected directly, squares corresponding to objects analysed with H+He models (GD659, REJ1032, REJ2156, REJ0723) and circles objects analysed with heavy element models (PG1123 and GD246). The open diamonds represent those stars where HeII was not detected and only broad limits on the ionization fraction obtained.