A massive star has disappeared without a trace

Artist's impression of the missing star.

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

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

An object inside the dwarf galaxy Kinman has disappeared from view, according to new investigation published today in Monthly Notices of the Royal Astronomical Society. This massive and exceptionally bright blue star is believed to exist based on astronomical observations made between 2001 and 2011, but as of 2019, It is no longer detectable.

The authors of the study, led by PhD student Andrew Allan of Trinity College Dublin, have conjured up two possible explanations: EWhether the star has experienced a dramatic drop in luminosity and is now partially hiding behind some dust, or transformed into a black hole without causing a supernova explosion. If it is the latter, it would represent only the second known failed supernova.

Kinman’s dwarf galaxy is 75 million light.years from Earth, so it is not close in any way. Astronomers cannot discern individual stars due to the tremendous distances involved, but the hypothetical star in question is a luminous blue variable (LBV), which is detectable at extreme distances. LBV they are massive and unpredictable stars at the end of their lives. The variable nature of this star, through its dramatic changes in spectra and brightness, can be seen from Earth. Incredibly, this suspicious star is 2.5 times brighter than our Sun.

Or at least it was.

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

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

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Observations collected between 2001 and 2011 pointed to a delayLBV stage in the Kinman dwarf galaxy. In 2019, a team of astronomers wanted to take a look to see how it was, and they did so using the Very Large Telescope of the European Southern Observatory. To his surprise, there was nothing to see, a result that was both exciting and daunting.

“We were pleasantly surprised to discover that the star’s signature was not present in our first observation taken in August 2019 using the ESPRESSO instrument from ESO’s Very Large Telescope,” Allan told Gizmodo. “Initially we expected more-resolution observation that resembled past observations, that we would use for our models. ”

Imagining that there was something strange with ESPRESSO, Allan and his colleagues decided to take another look with the telescope X-shooter instrument.

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

With nothing new to see, and with a mystery that suddenly needed to be solved, the team dipped into the files, looking at previous observations. of the dwarf galaxy. As a result, the suspicious massive star experienced a strong explosion period that ended around 2011. LBVThey are known to cause a strange tantrum, resulting in a sudden loss of mass and a sharp increase in brightness.

Following this particular burst period, we may “be seeing the end of a surviving LBV eruption of a star, with a slight drop in luminosity, a change to higher effective temperatures, and some darkening of the dust,” they wrote. the authors. in the study. So the star could still be active, it is now too dim for us to detect it from Earth.

Another possible explanation is that the star collapsed into a massive 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 massive black hole forms and it doesn’t spin very fast. However, a collapse to a black hole without producing a supernova has only been observed once in the past, in the NGC 6946 galaxy where a smaller massive star seemed to disappear without a brilliant supernova explosion. “

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

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

As a relevant aside, the new document should not be confused with a similar document paper co-author of Villarroel from last year. Instead of tracking the disappearance of LBV, Villarroel and his colleagues tracked a phenomenon known as red transients, in which the red dots become brighter and then disappear from view.

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

“The current consensus is that stars cannot end their lives as black holes heavier than about 65 times the mass of the Sun,” Bartos, who was not involved in the new study, told Gizmodo. “If the star’s disappearance 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 additional observations and a comprehensive search for similar disappearances are critical. “

To support the failed supernova hypothesis, Villarroel said: “We need to search for objects that are missing for decades.” And given the “very short time scales involved in observations in the current document, it makes me think that we are going to see more [activity] of that star again, “he said to Gizmodo.

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