a massive star has disappeared

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Wednesday 1 July 2020 – 11:18

ESO, cosmic mystery: a massive star has disappeared

Maybe collapsed into a black hole without producing a supernova

Rome, July 1 (askanews) – Using the VLT (Very Large Telescope) of the European Southern Observatory (ESO), some astronomers have discovered the absence of an unstable massive star in a dwarf galaxy. Scientists think this may indicate that the star has become less bright and partially obscured by dust. An alternative explanation is that the star collapsed to form a black hole without producing a supernova. “If true,” says PhD student Andrew Allan of Trinity College in Dublin, Ireland, at the head of the team, “this would be the first direct detection of a giant star that ends its life.”

Between 2001 and 2011, – explains the European Southern Observatory – various groups of astronomers studied this mysterious massive star, located in the dwarf galaxy Kinman, and their observations indicated that it was in an advanced evolutionary phase. Allan and his collaborators in Ireland, Chile and the United States of America wanted to find out more about how the life of very massive stars ends and the object in the dwarf galaxy Kinman seemed the perfect target. But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find indications of the star’s presence. “Instead, we were surprised to find that the star had disappeared!” says Allan, who conducted a study on the star published on June 30 by the magazine “Monthly Notices of the Royal Astronomical Society”.

Located about 75 million light years away from us in the Aquarius constellation, the dwarf galaxy Kinman is too far away for astronomers to see its individual stars, but they can detect clues to the presence of some of them. From 2001 to 2011, the light received from the galaxy continued to show the presence of a “bright blue variable” star approximately 2.5 million times brighter than the Sun. Stars of this type are unstable and occasionally show dramatic changes in the spectrum and in brightness. Even with these changes, however, the bright blue variables leave specific traces that scientists can identify. These were absent from the data collected by the team in 2019, leaving the doubt of what had happened to the star. “It would be extremely unusual for such a massive star to disappear without producing a bright supernova explosion,” says Allan.

The group first oriented the ESPRESSO instrument towards the star in August 2019, using the four 8-meter telescopes of the VLT simultaneously, without being able to find the signs that previously indicated the presence of the bright star. A few months later, the team tried the X-shooter tool, also installed on ESO’s VLT, and once again found no traces of the star. “We may have noticed the moment when one of the heaviest stars in the local universe enters the night gently,” says team member Jose Groh, also from Trinity College Dublin. “Our discovery would not have been possible without ESO’s powerful 8-meter telescopes, their unique instrumentation and access to these capabilities following Ireland’s recent agreement to join ESO.” Ireland became a member of ESO in September 2018.

The team then turned to the older data collected using the X-shooter and the UVES tool installed on ESO’s VLT, located in the Chilean Atacama desert, and other telescopes elsewhere. “ESO’s scientific archive has allowed us to find and use data from the same object obtained in 2002 and 2009,” said Andrea Mehner, an astronomer from ESO in Chile, who participated in the study. “The comparison of the 2002 high resolution UVES spectra with our observations obtained in 2019 with the most recent ESO ESPRESSO high resolution spectrograph was particularly revealing, both from an astronomical and an instrumental point of view.” The old data indicated that the star in the dwarf galaxy Kinman could have been in a period of strong ejection of matter that probably ended some time after 2011. Bright blue variable stars like this are prone to experience giant explosions throughout their lives , causing the mass loss rate and brightness to rise dramatically.

Based on the observations and their models, astronomers have suggested two explanations for the disappearance of the star and the lack of a supernova, in relation to this possible expulsion period. The expulsion of matter could have led to the transformation of the bright blue variable into a less bright star, which could also be partially hidden by dust. Alternatively, the team says the star could have collapsed directly into a black hole, without producing a supernova explosion. This would be a rare event: our current understanding of how mass stars die indicates that most of them end their lives in a supernova. Future studies are needed to confirm the fate of this star. Designed to begin operations in 2025, ESO’s Extremely Large Telescope (ELT) will be able to solve single stars in distant galaxies such as the dwarf galaxy Kinman, helping solve cosmic mysteries of this type.

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