Pulsars in the Gould belt?
The Gould belt is a region of young stars in the local part of the Galaxy. It was discovered by the American astronomer Benjamin Gould who noted a ring of stars passing through Orion, Carina, Crux, Centaurus, Scorpius, Taurus, Perseus, Cassiopeia and Cygnus. Gould realised that the stars traced out a flattened disk (within which the Sun is located) inclined at 20 degrees to the plane of the Galaxy. Many of the visible stars in this region are young, hot stars known as types O and B in the Morgan-Keenan classification system. These kinds of stars are often found in groups called OB associations, a well-known example is the cluster of stars at the centre of the Orion nebula. Detailed studies of nearby OB associations show that they are often part of this disk which is expanding and has motion relative to the rest of the Galaxy. Recent observations of the region have shown the presence of large numbers of x-ray bright, low mass stars whose distribution matches that of the high-mass stars and OB associations.
The Lund map of the Milky Way with the Gould belt superimposed on the top (yellow). CREDIT: Lund Observatory / Megan
One theory says that the Gould belt was triggered by the passage of a density wave associated with a spiral arm, causing gravitational collapse in a pre-existing giant molecular cloud. In a star forming region, large massive stars form. They use up their fuel very quickly and end their lives as supernovae. The explosions send shock waves into the surrounding gas cloud causing further collapse and star formation in a ring around the initial group of stars.
One possible consequence of a supernova explosion is a pulsar, a rotating neutron star with a high magnetic field and jets of radio emission emanating from the poles. Due to the amount of star formation in the Gould belt, it is likely that there have been many supernova in this region resulting in a number of relatively young pulsars. The EGRET catalogue of γ-ray sources contains around forty sources which are in the same direction as the Gould belt but which have no known counterparts, either pulsar or blazar, the two known kinds of γ-ray emitter.
A group of researchers at Jodrell Bank have used the Arecibo radio telescope to search the sky at some of these positions to see if there are young pulsars associated with 19 of the EGRET detections. They found one new pulsar associated (in position) with a γ-ray source, but it doesn't seem energetic enough to be the actual source of the highly energetic γ-rays. In the paper, the researchers find the distance to this pulsar (measured by a technique known as dispersion measure) is 3.5 kiloparsecs, but to be bright enough to have caused the observed γ-ray emission the pulsar would have to be at a distance of only 70 parsecs! They conclude that it is unlikely that this pulsar is associated with the EGRET source.
Why is all this important anyway? An understanding of nearby star formation processes and history will help when looking at other, more distant, regions of star formation such as those in the centre of starburst galaxies. Nearby regions of star formation are easier to observe because our telescopes can see more detail and if we understand them we can make educated guesses as to what is happening in more distant regions. This can help trace the star formation history of the Universe when looking at very distant starburst galaxies seen as they were in the early Universe.