Vilnius University Researchers Uncover New Clues to the Mystery of Hot Subdwarf Stars

There are myriad ways of “being a star”, and scientists spend years and years trying to uncover and understand them. One of these unusual types is the so-called hot subdwarfs – small stars that burn mysteriously too hot for their size. Scientists from the Faculty of Physics and the Faculty of Mathematics and Informatics at Vilnius University (VU) are currently doing their best to explain this strange phenomenon, and recently made some important breakthroughs.  

Can a star be too strange? 

The legendary space observatory Gaia, launched in 2013, catalogued more than half a billion stars. Out of all of them, only 60,000 belong to a peculiar class called hot subdwarfs. That’s less than one in a thousand. To understand what makes hot subdwarfs so unusual, one has to picture the standard map astronomers use to classify stars, a diagram where temperature runs along one axis and brightness along the other. 

“Most stars, including our Sun, sit along a broad band called the main sequence: stable, hydrogen-burning objects in the quiet middle of their lives. Hot subdwarfs don’t belong there. They’re hot and below this main branch. That’s how the name comes,” says Dr Markus Ambrosch, VU researcher at Astrospectroscopy and Exoplanets Group.  

He and his colleagues are trying to explain why subdwarfs are the way that they are. According to Dr Carlos Viscasillas Vázquez, who initiated the research project in the first place, hot subdwarfs are among the most fascinating objects in stellar astrophysics because they seem to borrow properties from many different kinds of stars, while belonging fully to none of them. 

“They are hot and blue like massive OB type stars, recognised for their extreme temperatures and brightness, but are much smaller, less luminous and extremely compact. They are exposed stellar cores, in most cases formed when a companion strips away most of the star’s outer envelope, changing its evolutionary path and leading it toward the white dwarf stage through a non-standard route,” he explains.  

Dr Carlos Viscasillo Vázquez, Prof. Ana Ulla, Dr Markus Ambrosch and Vladas Šatas. Photo from personal archive.

This “companion star” is the leading explanation for why subdwarfs came to be. Theory holds that hot subdwarfs are products of binary systems in which the more massive star swells into a red giant while the other remains compact and gravitationally hungry. The compact companion then strips away the giant’s outer hydrogen envelope, leaving behind just the bare core. 

How machine learning helps us detect subdwarf stars 

Understanding why hot subdwarfs behave in a certain way is one thing, but the problem is that to do that, you have to identify them in the first place. This is quite a complicated problem. As mentioned before, hot subdwarfs are incredibly rare. The last batch of Gaia data contained 60,000 hot subdwarf candidates for examination. The research group examined one-third of them that have XP spectrum. 

The regular approach of fitting models star by star, adjusting temperatures, and checking the fit was simply not feasible at that scale. To tackle this challenge, the team built a convolutional neural network. To train the algorithm, 2,500 stars with known classifications were presented to the network one by one, repeatedly. Once trained, the AI processed the remaining 17,500 unknown stars in seconds. 

“The network had taught itself, without any physical equations. It didn’t know any physics when it started. It was just randomly initialised. But through machine learning, it found out that this part of the spectrum is important. And this is what we’re glad to see, because it confirms what we know from physics. It makes sense. We would also do it in the same way,” says Dr Ambrosch.  

He also emphasises that one of the more rewarding aspects of the project was an opportunity to work with colleagues from different disciplines. For example, the training of the AI would’ve been impossible without the input of Dr Aidas Medžiūnas from the Faculty of Mathematics and Informatics.  

The 1.65-m telescope at the Molėtai Astronomical Observatory of the Institute of Theoretical Physics and Astronomy, Faculty of Physics, Vilnius University. Photo by A. Zigmantas.

“We focused on optimising the use of the XP Gaia spectra database by applying smoothing and dimensionality reduction methods. We also applied functional machine learning methods to improve model interpretability, which is crucial for physical interpretation. It was an intriguing challenge,” says Dr Medžiūnas.  

A spirit of collaboration 

Ana Ulla, one of the key members of the research team, a professor at the University of Vigo and a member of the Gaia Data Processing and Analysis Consortium, and one of the field’s leading experts, having conducted research on hot subdwarf stars for more than 30 years, emphasises the importance of the multidisciplinary nature of the process.  

“This collaboration is especially promising because the team is heterogeneous and truly complementary, combining expertise in many topics. For me, it is a beautiful example of how scientific ideas travel, evolve and take root in new places through international collaboration. This collaboration in particular has become a genuine success story,” she says.  

What began as a new research direction has quickly become a success story. In an incredibly short span of time, the team, researching hot subdwarfs, wrote two papers, obtained two grants, built strong international collaborations, and, importantly, observed hot subdwarfs from Lithuania for the first time at the VU Molėtai Astronomical Observatory.  

“I find this research interesting due to the complex nature of hot subdwarfs. They exhibit a mix of properties that make them difficult to analyse. However, this challenge makes the work rewarding, as every new finding reveals further directions for our research,” says Vladas Šatas, another member of the team.  

VU scientists emphasise the fact that this project has provided significant opportunities for professional growth within the broader science community. 

“With the new project that began in September, the release of Gaia DR4 in December and an expanding international team, we now have a unique opportunity to study these rare stars on a large scale and understand how binary companionship can change the fate of stars,” says Dr Vázquez.