A new type of variable star has been discovered by astronomers in Switzerland. The team says that its observations reveal previously unknown properties of variable stars that defy current theories and raise more questions about the origins of the luminosity variation in stars. The team's results are based on a seven-year-long study of regular measurements of the brightness of more than 3000 stars in the open star cluster NGC 3766, using the European Southern Observatory's 1.2 m Euler telescope at the La Silla Observatory in Chile.

Variable stars are those with a brightness that appears to fluctuate or "vary" when they are observed from Earth. They are divided into two broad categories depending on the cause of the variation. If it is caused by a change in the physical properties of the star, then they are called "intrinsic variables", whereas "extrinsic variables" fluctuate thanks to external factors, such as an eclipsing orbiting companion. "Our group didn't know what would come out of the observations, but knew the potential of observing open clusters regularly on a long [period of time] to improve our understanding of known classes of variable stars...but none of us was expecting to find a new class," says Nami Mowlavi of Geneva Observatory, who is the current leader of the research team. Mowlavi says that the team's findings were so surprising that the researchers spent more than six months trying to understand and make sense of the results, but that ultimately "the quality of the data and of the analysis" convinced the researchers of the reality of the results.

Varying varieties

During the study, the team found 36 of the new variety of variable stars, which represent 20% of stars with similar magnitudes within the observed cluster. Mowlavi explains that these provide sufficient evidence of a new type because all the stars were observed in a single cluster. This means that they all have the same stellar properties, including their surface temperatures. Hence, what is so surprising about the results is that periodic light variations occur in stars with those specific temperatures.

"Were it only for their variability properties, these stars could have been considered as 'standard' variable stars, like some pulsating stars that are already known," says Mowlavi. But knowing that they are main-sequence stars – that burn hydrogen in their core, such as the Sun – with surface temperatures of between 9000 and 11000 K makes them very special. This is because main-sequence stars at these temperatures are not expected to pulsate, or to have any other physical characteristic that would lead to periodic variations of their luminosity, according to current theories.

Unexpected and unexplained?

Mowlavi, along with Shopie Saesen, who is also an astronomer at Geneva Observatory, and colleagues, has considered three possible scenarios to explain these unexpected variations. The first looks at the possibility of a binary companion. "If the star is part of a binary system, then the total light emitted by the star could be modulated by its orbital motion around its companion," says Mowlavi. "But about one-third of the 36 stars are multiperiodic. This means that more than one frequency is detected in their light signal, which cannot be explained by binarity," he explains.

The second scenario relates to stellar pulsation that is consistent with the multiperiodicity, as well as with some other properties exhibited by the new class. Unfortunately, stellar pulsation is not expected in these stars. The team's observations found that four of the 36 stars are characterized by very high rotational velocities – spinning at more than 50% of their critical velocity (the velocity above which the star would break up). "Fast rotation might alter the internal conditions of a star enough to sustain stellar pulsations. But we actually don't know. There is currently no stellar model that can predict whether pulsation can be sustained in very fast rotating stars," explains Mowlavi.

The third option takes into account the presence of "spots" on the surface of such rotating stars and that these spots would induce light variations as the star rotates. But hot stars are not expected to be active, and no theory can currently explain how spots could be produced on the surface of such stars. "So, the origin of these light variations is mysterious, and we do not exclude any possibility, even others not mentioned here. We plan further observations to better characterize these stars," says Mowlavi.

The researchers have also observed other clusters during the seven-year study, and are currently analysing those data. Mowlavi told physicsworld.com that "since the stellar populations are different from one cluster to another, we may or may not find representatives of this new type of variable stars in other clusters". He points out that whatever the result, it will provide the team with further clues to the origin of these light variations "by relating their presence – or their absence – with the clusters' properties".

The team hopes that its results will encourage specialists in stellar pulsation to provide predictions for very fast rotating stars. Mowlavi says that other collaborations at Geneva University with specialists in this domain believe that this is a "very difficult task".

The research is published in Astronomy and Astrophysics.