A swirl spotted in the turbulent disc of dust encircling a young star called AB Aurigae was revealed last week in high resolution, and to much fanfare in the astronomy community. Such a distinctive disturbance, researchers suggested, could be the signature of a baby exoplanet in the process of forming.
Although this baby exoplanet was widely reported by many outlets as an unquestionable interpretation of the data, that ‘could’ is key. Now, a new paper neatly demonstrates why: a separate team of researchers has found that the swirling dust could be caused by a different object – a forming star, a binary companion to AB Aurigae.
The new interpretation has been described in a paper submitted to the Monthly Notices of the Royal Astronomical Society, and is awaiting peer review.
“AB Aurigae is a compendium of interesting features,” one of the researchers, astronomer Pedro Poblete from the Pontificia Universidad Católica de Chile, explained to ScienceAlert.
“We have a huge central cavity, spirals, dusty clumps, among others. The planet scenario has been proposed to explain some of those features.
“It is true that a planet can explain the dusty clumps, but not the huge cavity, and also it is true that it can explain spirals, but not the spirals that we observe in AB Aurigae,” Poblete added. “In contrast, a stellar binary scenario can easily explain all of those features.”
AB Aurigae is one of the closest stars of its kind. It’s very young, only around 4 million years old (the Sun is about 4.6 billion years old), and it’s still surrounded by a thick, complex protoplanetary disc – the material left over from the star’s formation.
Such stars are of great interest and importance, because we believe that they can reveal how exoplanets are formed from leftover stardust. And AB Aurigae in particular is very interesting. In 2017, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile revealed a gap in the dust cloud, containing what seemed to be curling, spiral features.
“AB Aurigae belongs to a class of discs labelled ‘transition discs’, which are ‘discs with holes in the middle’,” Monash University astronomer Daniel Price, co-author of the latest paper, told ScienceAlert.
“There has been a lot of speculation about what creates these central cavities (or ‘holes’) – whether binary stars, or planets, or dust clearing by some other mechanism.”
Exoplanets were the explanation proposed in last week’s paper, led by astronomer Anthony Boccaletti of the Observatoire de Paris in France. His team’s modelling suggested that the swirling spirals inside AB Aurigae’s cloud could have been produced by a forming exoplanet between four and 13 times the mass of Jupiter.
But not everyone agrees. “A planet alone would not be massive enough to create the strong spiral arms and cleared-out central cavity seen in AB Aurigae,” Price told ScienceAlert.
A binary companion, on the other hand, could fit the observed features. And it’s not a wild idea. Binary stars are extremely common – up to 85 percent of all stars could be in multiple-star systems, and there’s evidence to suggest that all stars are born in pairs, with some losing their companions later (like the Sun’s long-lost twin).
In one model of binary formation, the circumstellar disc around a young star fragments, and part of it gravitationally collapses in on itself to form a second star, pulling the surrounding material around itself in a second, smaller disc within the larger disc.
In the model proposed by Poblete and his team, the spiral observed in AB Aurigae’s dust cavity could have been produced by a small star, around half the mass of the Sun, to AB Aurigae’s two solar masses – if the binary companion is on a highly inclined orbit between 60 and 90 degrees, around AB Aurigae’s poles, and with a high orbital eccentricity.
That’s not as strange as it may sound. As Price notes, star formation is a messy process, which can result in orbital weirdness.
“We expect to find a ‘weird’ configuration in the early stages of binary star formation,” Poblete said.
“We know stars are formed in a giant gas cloud. This cloud is a factory of stars. When stars are born, they always interact with others, and in some cases, two or more stars can [become] bound in a multiple system.”
“All this happens when the stars are still absorbing gas from an accretion disc; for that reason, you can get [secondary] discs eccentric and inclined with respect to such discs,” Poblete added. “A binary system is also going to tend to have an unstable stable configuration; then, as time passes, the binary will become more and more stable.”
The observed spiral arms in AB Aurigae’s disc, the team notes in the paper, are similar to those observed in other forming binary systems such as [BHB2007] 11 and FS Tau A, the latter of which has an orbital inclination between 35 and 60 degrees.
But, as we can’t see the object – which isn’t actually that strange, given the brightness of AB Aurigae, and the amount of material swirling around – the presence of a binary companion is still hypothetical.
So, the exoplanet cannot be ruled out. As Boccaletti explained to ScienceAlert, although Poblete’s team’s model is more complex than the work of his own team, his team had access to more information.
“We have new data from SPHERE,” Boccaletti told ScienceAlert, referring to the adaptive optics system attached to the Very Large Telescope in Chile.
“ALMA and SPHERE are not observing the same light and are not sensitive to the same component. SPHERE is sensitive to the scattered light by the dust from the star. It is also very sensitive to stars themselves.”
“Although we did not make any analysis,” he added, “there is a good chance that if a stellar companion were responsible for the spiral it would have been an obvious signal in the SPHERE image, unless the obscuration by the gas and dust is huge.”
Neither team is making a firm claim of a discovery at this point. We simply don’t have enough data. There is no detection of either an exoplanet or a protostar – merely ripples in the gas that, depending on which model and dataset you apply, could be interpreted as one or the other.
“We cannot claim discovery based on a modelling paper,” Price said. “What we can say is that the claimed protoplanet is implausible based on our modelling, because we think there is something much bigger there (and not in the proposed location). It does not mean our claims are correct either, it is just a healthy debate.”
This is how science works at its best. Observations are made. Scientists interpret the data and propose explanations. This is the point at which we currently find ourselves. The next step is further observations, which can shed more light on the phenomenon and help reach a firmer conclusion.
Both teams are working towards this end. According to Price, a binary companion would slightly displace AB Aurigae in the sky. If that’s the case, that displacement should be detectable in the next release of data from the Gaia satellite, which is mapping the Milky Way.
Meanwhile, Boccaletti’s team is requesting more data from SPHERE to see if closer observations of the system can resolve AB Aurigae’s identity crisis.
“AB Aurigae is definitely a very interesting system to understand planetary formation and a lot of work is still to be done,” Boccaletti said.
The new research has been submitted to the Monthly Notices of the Royal Astronomical Society and is available on arXiv.