(CN) — Astronomers believe a massive star within the Kinman Dwarf galaxy has disappeared, and there are two possible explanations: either the giant, unstable star has been obscured by cosmic dust, or it is the first known star to collapse into a black hole without going supernova.
This discovery was part of a study conducted by a team of collaborators in Ireland, Chile and the United States in which they observed this mysterious star from 2001 to 2011. Having observed that this star was nearing the end of its evolution, they took the opportunity to see exactly how these celestial bodies finish their lifespans. However, when the time came to use the European Southern Observatory’s Very Large Telescope (VLT) to watch the event in 2019, they could not detect the star’s signature anywhere.
“Instead, we were surprised to find out that the star had disappeared!” said doctoral student Andrew Allan of Trinity College Dublin, Ireland, leader of the study of the star published Tuesday in Monthly Notices of the Royal Astronomical Society.
The Kinman Dwarf galaxy is difficult for astronomers to study in detail as it sits about 75 light-years away within the Aquarius constellation, so to study individual stars they have to instead detect their signatures. During the 10-year study, the team found consistent evidence that this galaxy hosted what is known as a luminous blue variable (LBV) star, a notoriously unstable star that shines 2.5 million times brighter than the sun.
LBVs are incredibly unpredictable and are known to exhibit sudden and dramatic changes in their hue and brightness. Despite their variability, astronomers can still detect and track them based on the distinct signatures they leave. In the case of this star, those traces had vanished — leaving the team wondering what could have happened to it.
“It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion,” Allan said. “If true, this would be the first direct detection of such a monster star ending its life in this manner.”
During the study in 2019, the team used the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) instrument, beginning in August, as well as the VLT’s four 8-meter telescopes, and found no sign of the star. They tried again using an alternate instrument called the X-shooter a few months later and still were unable to find a trace.
“We may have detected one of the most massive stars of the local universe going gently into the night,” team member Jose Groh of Trinity College Dublin said. “Our discovery would not have been made without using the powerful ESO 8-meter telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO.”
The team then went over some older data collected with the X-shooter, as well as another instrument called the UVES in Chile’s Atacama Desert, and other telescopes for some more insight.
“The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009,” said Andrea Mehner, a staff astronomer at ESO 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 newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view.”
The team deduced from the older data that the star might have been going through a strong outburst period that would have ended around 2011, not unlikely for this type of star. LBVs are known for their massive outbursts, occurring throughout their lifespans, that can significantly increase their rate of mass loss and brightness.
The results of the study suggest two possible explanations for the absence of the star from the Kinman Dwarf galaxy without evidence of a supernova. The first is that the LBV could have become a less luminous star as a result of the supernova, allowing it to be shrouded from sight by dust. The second possibility is the star could have collapsed into a black hole, which would be a strange and rare occurrence as astronomers have long believed that massive stars always end in a supernova.
The authors said that further studies will be needed to determine the exact cause of this star’s disappearance, but ESO’s Extremely Large Telescope (ELT), able to observe stars in distant galaxies, will be online in 2025 and will be a major contribution to understanding these mysteries.