Hot young stars put theory into a spin
Stellar find Astronomers have found a new type of hot blue star that regularly changes in brightness, providing new insights into how massive stars evolve.
The discovery, to appear in the journal Astronomy & Astrophysics and published on the pre-press website ArXiv.org, contradicts existing theoretical models for these stars, which predict no periodic variation in their brightness.
Researchers from Switzerland's University of Geneva Observatory, including lead author Dr Nami Mowlavi made the discovery while studying an open star cluster called NGC 3766, located 7000 light years away in the constellation Centaurus.
"We find 36 main sequence variable stars ... where no pulsation is predicted to occur based on standard stellar models," the authors write.
"These form a new group of variables with properties distinct from those of the adjacent [stars]."
Astronomers use the term 'main sequence' to describe stars shining by nuclear fusion of hydrogen in their core.
Blue stars have twice the mass and are at least twice as big as the Sun, and appear blue-white in colour.
Using a relatively small 1.2-metre telescope at the European Southern Observatory in Chile, Mowlavi and colleagues meticulously measured minute variations in brightness coming from over 3000 stars in the cluster.
Many stars vary in brightness over time because of changes in their internal dynamics or surrounding environment.
Mowlavi and colleagues detected 127 variable stars of various known types in the cluster.
However the discovery of 36 variable blue stars that shouldn't display any pulsations, came as a complete surprise.
The researchers were able to detect regular variations of approximately 0.1 per cent in stellar brightness in these 36 stars over periods ranging from two to twenty hours.
They found the variations remained consistent over the seven-year observing period.
Although the cause of variability in this new group remains unknown, the researchers point to a subset of four stars in the group, which they identify as rapid rotators.
These are stars that spin at over half of their critical velocity, a threshold where stars become unstable and throw off material.
"In those conditions, the fast spin will have an important impact on their internal properties, but we are not able yet to adequately model their light variations," says Mowlavi.
The idea of rapidly spinning stars pulsating isn't new according to astro-seismologist Professor Tim Bedding of the University of Sydney.
"There's some theoretical work which predicted these stars should exist. But if they rotate rapidly enough, that rotation can trigger them into pulsating," says Bedding.
"It's possible this discovery, if confirmed, vindicates these theoretical calculations."
Bedding also points to a blue star in the constellation Pleiades, the Seven Sisters, which he says has also shown signs of varying in brightness.
"There's a class of stars called Maia stars that were postulated in the 1950s, and that is this same sort of star," says Bedding.
"So it's possible this isn't a new discovery, but a confirmation of something that was suggested many years ago."