A research team in Italy has discovered a remarkable "odd couple", not mismatched room-mates but a peculiar pair of orbiting stars, writes Dick Ahlstrom.
Two powerful pieces of astronomical technology, the Hubble Space Telescope and the Parkes radio telescope, have identified a rarity, a companion-star pairing that includes a "millisecond pulsar". The pulsar spins at an astounding 274 times per second and, unusually, is coupled with a red giant star.
Astronomers have discovered more than 90 millisecond pulsars, but all are paired with white dwarf stars rather than red giants. More important, the scientists from the Bologna Astronomical Observatory who used the Hubble and the Parkes telescopes to identify the pair believe it confirms the way millisecond pulsars are spun up to such tremendous speeds.
Pulsars are most unusual entities whatever their partner. They are usually formed due to the collapse of an ageing high-mass star as it dies in a supernova explosion. Its central parts compact into a neutron star, a body so dense that a piece the size of a sugar cube would weight 200 billion kilograms.
The neutron star packs down into a ball perhaps 10-20 kilometres across, but the rotational energy of its original form is retained. Their small size means the rate of rotation is very fast, and the two poles of a neutron star emit radiation streams in two narrow beams, like the light from a lighthouse. If these point towards Earth, the energy signal tells us we are looking at a pulsar.
The first millisecond pulsar was identified in 1982, using the 300- metre Arecibo radio telescope, in Puerto Rico. All those discovered so far are matched with a white dwarf, but the new find, PSR J1740-5340, in the globular cluster NGC 6397, has a red-giant companion.
Red giants are old stars at the end of their days that puff out beyond their normal size for a time before burning down to become white dwarfs. The bloated red star in the new pulsar pairing has a radius about 100 times greater than that of a white dwarf and at least five times greater than that of a normal star of similar mass. The couple orbit one another in 1.35 days.
The "recycling scenario" describes the generally favoured theory of how a standard pulsar is spun into a millisecond pulsar. It holds that a slowly rotating neutron star begins to steal matter from its ageing companion star when this gets old enough to become a red giant. In its new, super-sized form, it gets a bit too close to the neutron star, enabling the neutron's gravitational pull to begin feeding on its partner.
The matter strikes the neutron star's surface, transferring energy to make it rotate faster. Over tens of millions of years, it can be accelerated to hundreds of revolutions per second, in the process turning the red giant into a white dwarf.
The Italian scientists believe they have caught a millisecond pulsar before it has finished lunching on its companion. "A system consisting of a millisecond pulsar and a star that is not a white dwarf has never been seen before," says Dr Francesco Ferraro, the lead astronomer. "Our favoured theory is that we are seeing the system before the bloated red star has been emptied of gas and turned into a white dwarf."
The scientists point to another clue that backs up this theory. Their observations indicate the abnormal presence of large amounts of gas in the system. Once the pulsar has been spun up, it can no longer absorb gas from the companion, even though gas continues to be released by the red giant. The gas will eventually be swept away by the spinning pulsar and should leave behind the pulsar's more typical pairing with a white dwarf.