Very Large Telescope captures the disappearance of a massive star

Using the Very Large Telescope (VLT) from the Southern European Observatory, astronomers have discovered the absence of a massive unstable star in a dwarf galaxy. Scientists think that this could indicate that the star became less bright and partially obscured by dust. An alternative explanation is that the star collapsed into a black hole without producing a supernova. “If it’s true,” says the team leader and Ph.D. Andrew Allan, a student at Trinity College in Dublin, Ireland, “this would be the first direct detection of a monster star that ends his life in this way.”

Between 2001 and 2011, several teams of astronomers studied the mysterious massive star, located in the dwarf galaxy Kinman, and their observations indicated that it was at a late stage in its evolution. Allan and his collaborators in Ireland, Chile, and the US wanted to know more about how very massive stars end their lives, and the object on the Dwarf Kinman seemed like the perfect target. But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the star’s tell-tale signatures. “Instead, we were surprised to discover that the star had disappeared.” says Allan, who led a study on the star published today in Monthly notices from the Royal Astronomical Society.

Located about 75 million light-years away in the constellation Aquarius, the dwarf Kinman galaxy is too far away for astronomers to see their individual stars, but they can detect the signatures of some of them. From 2001 to 2011, light from the galaxy consistently showed evidence that it harbored a ‘luminous blue variable’ star approximately 2.5 million times brighter than the Sun. Stars of this type are unstable and show occasional dramatic changes in their spectra and brightness. Even with those changes, the luminous blue variables leave specific footprints that scientists can identify, but they were missing from the data the team collected in 2019, leaving them wondering what happened to the star. “It would be very unusual for such a massive star to disappear without producing a brilliant supernova explosion,” says Allan.

The group first rotated the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-meter telescopes simultaneously. But they could not find the signs that previously pointed to the presence of the luminous star. A few months later, the group tested the X-shooter instrument, also on ESO’s VLT, and again found no trace of the star.

“We may have detected that one of the most massive stars in the local Universe is gently going into the night,” says team member Jose Groh, also from Trinity College Dublin. “Our discovery would not have been made without the use of ESO’s powerful 8-meter telescopes, their unique instrumentation, and quick access to those capabilities following Ireland’s recent agreement to join ESO.” Ireland became an ESO member state in September 2018.

The team then turned to older data collected with X-shooter and the UVES instrument at ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere. “ESO’s Scientific Archive Center allowed us to find and use data from the same object obtained in 2002 and 2009, “says Andrea Mehner, astronomer for ESO staff in Chile who participated in the study. “The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO’s new ESPRESSO high-resolution spectrograph was especially revealing, both from an astronomical and instrumentation point of view.”

Ancient data indicated that the star at Kinman Dwarf may have been experiencing a strong explosion period that probably ended sometime after 2011. Variable luminous blue stars like this are prone to experiencing giant explosions over the course of their lifetime, causing the stars’ mass loss rate to peak and their luminosity to increase dramatically.

Based on their observations and models, astronomers have suggested two explanations for the star’s disappearance and the lack of a supernova, in relation to this possible outburst. The outburst may have resulted in the luminous blue variable transforming into a less luminous star, which could also be partially obscured by dust. Alternatively, the team says the star may have collapsed into a black hole, without producing a supernova explosion. This would be a rare event: Our current understanding of how massive stars die suggests that most of them end their lives in a supernova.

Future studies are needed to confirm what fate happened to this star. Planned to start operating in 2025, ESO’s Extremely Large Telescope (ELT) will be able to resolve stars in distant galaxies like the Dwarf Kinman, helping to solve cosmic mysteries like this one.