A cosmic mystery: ESO telescope captures the disappearance of a massive
star

Date:
June 30, 2020
Source:
ESO
Summary:
Astronomers have discovered the absence of an unstable massive
star in a dwarf galaxy. Scientists think 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.



FULL STORY ========================================================================== [Very Large Telescope | Credit: (c) CPO / stock.adobe.com] Very Large
Telescope complex (stock image).

Credit: (c) CPO / stock.adobe.com [Very Large Telescope | Credit: (c)
CPO / stock.adobe.com] Very Large Telescope complex (stock image).

Credit: (c) CPO / stock.adobe.com Close Using the European Southern Observatory's Very Large Telescope (VLT), astronomers have discovered
the absence of an unstable massive star in a dwarf galaxy. Scientists
think 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 true," says team
leader and PhD student Andrew Allan of Trinity College Dublin, Ireland,
"this would be the first direct detection of such a monster star ending
its life in this manner."

========================================================================== 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 Ireland, Chile and the US wanted to find out
more about how very massive stars end their lives, and the object in
the Kinman Dwarf seemed like the perfect target. But when they pointed
ESO's VLT to the distant galaxy in 2019, they could no longer find the
telltale signatures of the star. "Instead, we were surprised to find out
that the star had disappeared!" says Allan, who led a study of the star published today in Monthly Notices of the Royal Astronomical Society.

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.

"It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion," says Allan.

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.

"We may have detected one of the most massive stars of the local
Universe going gently into the night," says team-member Jose Groh,
also of Trinity College Dublin. "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." Ireland became an ESO
member state in September 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 ESO Science Archive Facility enabled us
to find and use data of the same object obtained in 2002 and 2009,"
says 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 old data indicated that the
star in the Kinman Dwarf 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.

Based on their observations and models, the astronomers have suggested
two explanations for the star's disappearance and lack of a supernova,
related to this possible outburst. The outburst may have resulted in
the luminous blue variable being transformed into a less luminous star,
which could also be partly hidden 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 points to most of them ending their lives in
a supernova.

Future studies are needed to confirm what fate befell this star. 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, helping to solve cosmic mysteries such as this one.


========================================================================== Story Source: Materials provided by ESO. Note: Content may be edited
for style and length.


========================================================================== Related Multimedia:
*
Artist's_impression_of_the_disappearing_star;_views_of_the_sky_where_the
Kinman_Dwarf_galaxy_is_located ========================================================================== Journal Reference:
1. Jennifer E Andrews, Eoin J Farrell, Ioana Boian, Nathan Smith,
Andrea
Mehner, Jose H Groh, Andrew P Allan. The possible disappearance
of a massive star in the low-metallicity galaxy PHL 293B. Monthly
Notices of the Royal Astronomical Society, 2020; 496 (2): 1902 DOI:
10.1093/mnras/ staa1629 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200630072049.htm

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