Physicists in Canada and the UK have had a rare glimpse at the atmosphere of a neutron star just 330 years after it was formed in a violent explosion. Instead of resembling more mature neutron stars, which are surrounded by hydrogen, this baby star is blanketed in carbon gas – a discovery that could provide important new insights into the evolution of neutron stars.

Craig Heinke of the University of Alberta and Wynn Ho of Southampton University came to this conclusion by reinterpreting observations of the neutron star Cassiopeia A, which were made over the past 10 years by the Chandra X-ray Observatory.

Located about 11,000 light-years away, the star is believed to have formed in the remains of a supernova that was observed about 330 years ago, making it the youngest known neutron star. Such stars are created from the collapsed cores of massive stars that have exploded in a supernova. They retain much of their former mass but shrink to around 20 km in diameter, giving them densities comparable to atomic nuclei.

'It's not expected'

"The gravitational field is so strong that the star tends to stratify," Heinke explained. The lightest elements should rise to the surface, as seen in mature neutron stars, which have an outer atmosphere of hydrogen. "This is the first time we've seen a carbon atmosphere on top of a neutron star," Heinke said. "It's not expected."

Earlier studies of X-rays emitted by Cassiopeia A suggested that the star's radius is much smaller than expected – assuming it had a hydrogen atmosphere. Heinke and Ho tried to resolve this problem by modelling a helium atmosphere, but that failed to produce a convincing match with the X-ray spectrum. The scientists then tried carbon without much hope of success, but discovered that neutron stars with diameters ranging from 8–17 km and a carbon atmosphere would produce the observed radiation.

"It modelled it perfectly and produced a radius in the right range," Heinke recalled. "It worked out much better than we expected." The scenario also suggests that this neutron star has a 1.6 × 106 K effective surface temperature, with the gaseous carbon atmosphere lying just 10 cm thick around it.

Mellowing with age

The researchers believe that the absence of hydrogen in Cassiopeia A is related to the extremely high temperatures associated with a supernova. While the internal temperature of a neutron star is in the million-Kelvin range, that is low compared with the billions of Kelvin immediately after a supernova. "At this extremely hot time in its life, nuclear fusion on the surface fuses all of the hydrogen and helium into carbon," Heinke told Physics World.

However as time progresses, the immense gravitational field of Cassiopeia A should draw in lighter elements that are present in the surrounding supernova remnant. Heinke predicts that around 1000 to 2000 years after the supernova, the star will cool to below fusion temperatures and then allow these elements to precipitate on the surface – eventually producing the familiar hydrogen atmosphere.

Well formed models

Although Heinke's theory has not been disputed by other astronomers, it will be difficult to further substantiate because of a lack of other similar-aged neutron stars to compare against. Not surprisingly, Heinke says that a search for such stars is high on his agenda.

George Pavlov at Penn State University says that there is little to dispute in Ho and Heinke's findings. However, Pavlov, whose data and modelling of the Cassiopeia A the pair reinterpreted, does not consider the idea proved yet either. He asks: "Why carbon? You are more likely to burn off the hydrogen and leave helium. But it is this that makes it interesting to me. It is an unusual finding."

This research appears in the latest edition of Nature.