Royal Astronomical Society Press Notices
Date: 15 April 1997

Evidence of a Nearby Supernova -- 4 Million Years Ago

A team of astronomers led by Dr Martin Barstow of the University of Leicester has used a group of white dwarf stars to probe the structure of interstellar space in the vicinity of the Sun. Measurements made with the Extreme Ultraviolet Explorer (EUVE) Satellite have revealed that the local gas appears to be highly ionized in all directions. This can only have happened as the result of a nearby supernova explosion and the relative amounts of ionized gas indicate that it occurred about 4 million years ago.

The formation of the Solar System about 4500 million years ago, would have been profoundly affected by the conditions existing in the cloud of dust and gas from which it originated. Although this original nebula will have been largely dissipated by intense radiation from the young Sun and has probably been 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 distances of about 300 light years), is that the Sun is embedded in and near the edge of a wispy diffuse cloud, known as the Local Cloud (or Local Fluff). This cloud, which is only 20-30 light years across, is itself in a larger much less dense region called the Local Bubble. Interstellar gas near the Sun can only be studied directly by observing its effect on the extreme ultraviolet (EUV) radiation from other stars. The ROSAT EUV all- sky survey was able 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 the Local 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. The ionized material can only be detected in extreme ultraviolet spectra, recorded using NASA's Extreme Ultraviolet Explorer (EUVE).

A critical question is whether the interstellar gas is in equilibrium, with atoms being ionized at the same rate as the ions recombine with the electrons, or not. 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.

A team of astronomers led by Dr Martin Barstow and including Paul Dobbie (University of Leicester), Jay Holberg (University of Arizona), Ivan Hubeny (Goddard Space Flight Centre) and Thierry Lanz (University of Utrecht), have used the EUVE spectrometers to carry out detailed observations of 13 nearby white dwarfs, using the shadowing effect of the interstellar medium on the white dwarf spectra to measure the density and level of ionization.

Remarkably, while the gas density varies in different directions, the fraction of material ionized is highly uniform. This can be 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 hydrogen (27%) and helium (35%) indicate that the explosion occurred around 4 million years ago. This might be the same supernova which is 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.

Dr Barstow is presenting these results at an International Astronomical Union Colloquium on "The Local Bubble and Beyond", being held in Garching, Germany, from April 21st to 25th inclusive. A paper on the topic was also published in the 21 March issue of the Monthly Notices of the Royal Astronomical Society.

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