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A massive star has disappeared without a trace

Artist's impression of the missing star.

Artist’s impression of the missing star.
Image: ESO

An unusually bright star in a nearby galaxy has disappeared, in a mystery of cosmic proportions.

An object within the dwarf galaxy Kinman has disappeared from sight, according to the new Research published today in the monthly Notices of the Royal Astronomical Society. This huge and exceptionally bright blue star was speculated to exist based on astronomical observations made between 2001 and 2011, but as of 2019, it is no longer detectable.

The study authors, led by PhD student Andrew Allan of Trinity College Dublin, evoked two possible explanations: Eeither the star has undergone a dramatic drop in brightness and is now partially hiding behind some dust, or has turned into a black hole without triggering a supernova explosion. If it is the second, it would represent only the second known failed supernova.

The dwarf galaxy Kinman is located at 75 million lightyears from Earth, so it’s not close at all. Astronomers cannot discern individual stars due to the huge distances involved, but the hypothesized star in question is a bright blue variable (LBV), detectable at extreme distances. LBV they are huge and unpredictable stars at the end of their life. The variable nature of this star, through its dramatic changes in spectra and brightness, can be identified from Earth. Incredibly, this suspicious star is 2.5 times brighter than our Sun.

Or at least it was.

Image of the dwarf galaxy Kinman, also known as PHL 293B. This small galaxy is too far away for scientists to locate individual stars, so what looks like stars in this Hubble image are foreground stars or gigantic star clusters within the galaxy itself.

Image of the dwarf galaxy Kinman, also known as PHL 293B. This small galaxy is too far away for scientists to locate individual stars, so what looks like stars in this Hubble image are foreground stars or gigantic star clusters within the galaxy itself.
Image: NASA, ESA / Hubble, J. Andrews

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The observations collected from 2001 to 2011 indicated an end-LBV stage in the dwarf galaxy Kinman. In 2019, a team of astronomers wanted to take another look to see how it was, and they did it using the Southern European Observatory’s Very Large Telescope. To their surprise, there was nothing to see: a result that was both exciting and daunting at the same time.

“We were all pleasantly surprised to find that the star’s signature was not present in our first observation taken in August 2019 using ESO’s ESPRESSO tool of the Very Large Telescope,” Allan told Gizmodo. “We initially hoped for aresolution observation that resembled past observations, that we would use for our models. ”

Thinking there was something strange with ESPRESSO, Allan and his colleagues he decided to take another look at the telescope X-shooter tool.

“We double-checked ESPRESSO’s observation several times but were unable to detect the star’s signature,” said Allan. “Since the conditions were not perfect on the day this observation was made, we wanted to make sure that the signature was really absent. This time we used the Very Large Telescope’s X-Shooter tool and were happy to find that this also pointed to the disappearance of the star. “

With nothing new to see and a mystery that suddenly had to be solved, the team dived into the archives, looking at previous observations of the dwarf galaxy. Apparently, the suspected massive star experienced a period of major explosion that ended around 2011. LBVs are known to launch bizarre irascibility, resulting in sudden mass loss and a sharp increase in brightness.

In the wake of this particular explosion period, it is possible that “we are witnessing the end of an LBV eruption of a surviving star, with a slight drop in brightness, a shift to warmer actual temperatures and a little darkening of the dust” , wrote the authors in the study. So the star may still be active, it is just too weak to be detected by Earth.

Another possible explanation is that the star has collapsed into a huge black hole without an accompanying supernova explosion, what astronomers call a failed supernova.

“This would be consistent with some of the current computer simulations that predict that some stars will not produce a bright supernova when they die,” Allan told Gizmodo. “This happens when a huge black hole is formed and it doesn’t turn very fast. However a collapse in a black hole without producing a supernova has only been observed once in the past, in the galaxy of NGC 6946 where a smaller massive star seemed to disappear without a bright supernova explosion. “

If it is the case of a transition without supernova in a black hole, it would mark the first known example of what happens to a massive star in a low metallization galaxy, a finding that “could contain important clues as to how stars could collapse into a black hole without producing a bright supernova,” said Allan.

“It is a very interesting discovery reported in the document, with a very careful and well done analysis,” Beatriz Villarroel, postdoc at IAC Tenerife and the Nordic Institute of Theoretical Physics, told Gizmodo. “LGWs are unstable stars and the analysis presented by the authors certainly contributes to the understanding of these quiet objects. In this particular case, they are likely to have observed the end of a strong eruption with a surviving star, “said Villarroel, who was not involved in the new study.

Illustration for the article entitled A massive star has disappeared without trace

Other than that, the new document should not be confused with a similar one paper co-author of Villarroel last year. Instead of tracing the disappearance of LBV, Villarroel and his colleagues followed a phenomenon known as red transients, in which the dim red spots become brighter and then move away from sight.

Imre Bartos, a University of Florida physicist, said that we have a lot to learn about massive stars and how they die, given their rarity and short life span.

“The current consensus is that stars cannot end their lives as black holes heavier than 65 times the mass of the Sun,” said Bartos, who was not involved in the new study. “If the star’s demise is really due to its collapse into a heavier black hole, we will have to rethink our understanding of how stars live and die.”

To which he added: “At this point, there are still uncertainties about this result and it is important to study this observation further, so further observations and a thorough search for similar disappearances are crucial. “

To support the failed supernova hypothesis, Villarroel said: “We have to look for objects that are missing for decades.” And given the “very short time scales involved in the observations in this document, it makes me think we’ll see more [activity] again from that star, “he told Gizmodo.

This is an exciting possibility, which requires astronomers to train their telescopes towards the dwarf galaxy Kinman. This mystery is far from being solved, but if Villarroel is right, there is still potential for this star, if it still exists, to shine once again.

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