Astrophysicists investigate a cosmic mystery – the disappearance of a massive star

A team of astronomers led by Professor Jose Groh’s group from Trinity is sleuthing for answers to a cosmic mystery having discovered the disappearance of an unstable massive star in a distant galaxy.

The absence, discovered with the European Southern Observatory’s Very Large Telescope (VLT), could indicate that the star either became less bright and partially obscured by dust, or that it collapsed into a black hole without producing a supernova.

Project leader, Andrew Allan, is a PhD student in Astrophysics in Trinity’s School of Physics, and is supported by a Provost’s PhD project award. He said:

“If true, this would be the first direct detection of such a monster star ending its life in this manner. It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion.

Between 2001 and 2011, various teams of astronomers studied the mysterious massive star, located in the Kinman dwarf galaxy, and their observations indicated it was in a late stage of its evolution. Allan and his collaborators in the European Southern Observatory and the US wanted to find out more about how very massive stars end their lives, and the object in the Kinman dwarf galaxy seemed like the perfect target.

But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the tell-tale signatures of the star.

Instead, we were surprised to find out that the star had disappeared!” says Allan, who led the study published today in Monthly Notices of the Royal Astronomical Society.

The Kinman Dwarf galaxy, also known as PHL 293B, taken with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 in 2011, before the disappearance of the massive star. Located some 75 million light-years away, the galaxy is too far away for astronomers to clearly resolve its individual stars, but in observations done between 2001 and 2011, they detected the signatures of the massive star. These signatures were not present in more recent data. Credit: NASA, ESA/Hubble, J. Andrews (U. Arizona)

Located some 75 million light-years away in the constellation of Aquarius, the Kinman dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them. From 2001 to 2011, the light from the galaxy consistently showed evidence that it hosted a ‘luminous blue variable’ star some 2.5 million times brighter than the Sun.

Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness. Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star.

The group first turned the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-metre telescopes simultaneously. But they were unable to find the signs that previously pointed to the presence of the luminous star. A few months later, the group tried the X-shooter instrument, also on ESO’s VLT, and again found no traces of the star.

Professor Jose Groh, Assistant Professor in Astrophysics at Trinity, said:

We may have detected one of the most massive stars of the local Universe going gently into the night. Our discovery would not have been made without using the powerful ESO 8-metre telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO as a member state in 2018.”

The team then turned to older data collected using X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere. The old data indicated that the star in the Kinman dwarf galaxy could have been undergoing a strong outburst period that likely ended sometime after 2011. Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically.

Professor Jose Groh added:

“We will likely need to wait a few years before confirming what fate befell this particular star. We will observe the galaxy again with the Hubble Space Telescope next year, which will provide new clues. Also, planned to begin operations in 2025, ESO’s Extremely Large Telescope – ELT – will be capable of resolving stars in distant galaxies such as the Kinman dwarf galaxy, which will enable us to solve cosmic mysteries such as this one.”