A Massive Star Has Disappeared Without a Trace

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A Massive Star Has Disappeared Without a Trace

Artist’s impression of the disappearing star.

 

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

 

An object inside the Kinman dwarf galaxy has disappeared from view, according to new research published today in Monthly Notices of the Royal Astronomical Society. This

 

massive and exceptionally bright blue star was hypothesized 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 two possible explanations: Either the star has experienced a dramatic drop in

 

luminosity and is now partially hiding behind some dust, or it transformed into a black hole without sparking a supernova explosion. If it’s the latter, it would represent just the

 

second known failed supernova.

 

The Kinman dwarf galaxy is located 75 million light-years from Earth, so it’s not close by any means. Astronomers cannot discern individual stars owing to the tremendous

 

distances involved, but the hypothesized star in question is a luminous blue variable (LBV), which is detectable at extreme distances. LBVs are massive and unpredictable stars at

 

the end of their lives. The variable nature of this star, through its dramatic shifts in spectra and brightness, can be spotted from Earth. Incredibly, this suspected 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 tiny galaxy is too far for scientists to pick out individual stars, so what look like stars in this Hubble image are

 

either stars in foreground or gigantic star clusters within the galaxy itself. Image: NASA, ESA/Hubble, J. Andrews

 

 

Observations gathered from 2001 to 2011 pointed to a late-stage LBV in the Kinman dwarf galaxy. In 2019, a team of astronomers wanted to take another look to see how it was

 

doing, and they did so using the European Southern Observatory’s Very Large Telescope. To their surprise, there was nothing to see—a result that was both exciting and

 

discouraging 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 the ESPRESSO instrument of ESO’S Very

 

Large Telescope,” Allan told Gizmodo. “We initially hoped for a higher-resolution observation that resembled the past observations, which we would use for our models.”

 

Figuring there was something wonky with ESPRESSO, Allan and his colleagues decided to take another look with the telescope’s X-shooter instrument.

 

“We rechecked the ESPRESSO observation a number of times but were unable to detect the star’s signature,” said Allan. “As the conditions were not perfect on the day this

 

observation was made, we wanted to make sure the signature was truly absent. This time we used the X-Shooter instrument of the Very Large Telescope and were happy to find

 

that this also pointed towards the star disappearing.”

 

With nothing new to see, and with a mystery that suddenly needed to be solved, the team dove into the archives, looking at previous observations of the dwarf galaxy. As it turns

 

out, the suspected massive star experienced a strong outburst period that came to end around 2011. LBVs are known to throw the odd temper tantrum, resulting in a sudden

 

loss of mass and a sharp increase in brightness.

 

In the wake of this particular outburst period, it’s possible that “we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter

 

effective temperatures, and some dust obscuration,” wrote the authors in the study. So the star might still be active, it’s just now too dim for us to detect 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 is formed, and it is not spinning very fast. However a collapse to 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 a supernova-less transition into a black hole is the case, it would mark the first known example of this happening to a massive star in a low metallicity galaxy, a finding that

 

“could hold important clues as to how stars could collapse to a black hole without producing a bright supernova,” said Allan.

 

“It’s a very interesting finding that is reported in the paper, with a very careful and well done analysis,” Beatriz Villarroel, a postdoc at IAC Tenerife and the Nordic Institute for

 

Theoretical Physics, told Gizmodo. “LBVs are unstable stars, and the analysis presented by the authors certainly contributes to the understanding of these quizzical objects. In

 

this particular case, it’s likely that they’ve observed the end of a strong eruption with a surviving star,” said Villarroel, who wasn’t involved in the new study.

 

As a relevant aside, the new paper is not to be confused with a similar paper co-authored by Villarroel from last year. Instead of tracking the disappearance of LBVs, Villarroel and

 

her colleagues tracked a phenomenon known as red transients, in which dim red dots get brighter and then recede from view.

 

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

 

“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 wasn’t involved in the new study, told

 

Gizmodo. “If the disappearance of the star is indeed due to its collapse to 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 thorough

 

search for similar disappearances are critical.”

 

To support the failed supernova hypothesis, Villarroel said, “we need to look for objects that stay missing for decades.” And given the “very short time scales involved in the

 

observations in the current paper, it makes me think we’re going see more [activity] from that star again,” she told Gizmodo.

 

That’s an exciting possibility, requiring astronomers to train their telescopes toward the Kinman dwarf galaxy. This mystery is far from being solved, but if Villarroel is correct,

 

there’s still potential for this star, if it still exists, to shine brightly once again.

 

 

Source

[flash13]

Astronomers Might Have Seen a Star Just Disappear. Turning Straight to a Black Hole Without a Supernova

 

Large stars have violent deaths. As they run out of hydrogen to fuse, the star’s weight squeezes its core to make it increasingly hot and dense. The star fuses heavier elements in

 

a last-ditch effort to keep from collapsing. Carbon to Silicon to Iron, each step generating heat and pressure. But soon it’s not enough. The fusion even heavier elements don’t

 

give the star more energy, and the core quickly collapses. The protons and neutrons of nuclei collide so violently that the resulting shock wave rips the star about. The outer

 

layers of the star are thrown outward, becoming a brilliant supernova. For a brief time, the star shines brighter than its entire galaxy, and its core collapses into a neutron star or

 

black hole. It was thought that all large stars end with a supernova, but new research finds that might not be the case.

 

A multi-wavelength layout of the Crab Nebula. Credit: (Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL/Caltech; Radio: NSF/NRAO/VLA; Ultraviolet:

ESA/XMM-Newton).

 

The evidence comes from a galaxy 75 million light-years away known as the Kinman Dwarf Galaxy. Between 2001 and 2011, several groups of astronomers were studying the

 

spectra of this galaxy because it has a particularly low metallicity. Because the galaxy is small and far away, astronomers can’t see all the individual stars, but the galactic spectra

 

allow them to identify some of the bright stars by particular emission lines. One of these was a luminous blue variable star.

 

Luminous Blue Variables (LBVs) are very large stars in their elder days. They have active and quiet periods and can shine 2.5 million times brighter than the Sun. The bright

 

emission lines of LBVs are unusual for a blue giant star, so they are easy to identify. Since astronomers saw these lines in the spectra of the Kinman Dwarf Galaxy, they knew the

 

galaxy contained such a star.

 

 

But when new spectra measurements of the Kinman Dwarf Galaxy were taken in 2019, the spectral lines of the LBV were gone. They had simply vanished as if the star wasn’t

 

there. It seemed the star had disappeared. But giant blue stars don’t just disappear. They should explode as a supernova before fading away. This particular star didn’t become a

 

supernova, so something strange is going on.

 

Image of the Kinman Dwarf galaxy, also known as PHL 293B. Credit: NASA, ESA/Hubble, J. Andrews (U. Arizona)

 

Since astronomers haven’t observed the star directly, it’s possible that the star did become a supernova and we happened to miss it. But that is highly unlikely. The Kinman Dwarf

 

Galaxy is regularly observed, and a supernova would be difficult to miss. It’s more likely that the star did fade from view, and the authors of this latest research have a couple of

 

ideas why.

 

One idea is that the star has entered a particularly quiet stage at the same time it happens to be obscured by dust in the galaxy. But a second idea is more interesting. Perhaps

 

the star underwent an unusual core collapse, forming a black hole without undergoing a supernova. There has been some debate over whether this is possible, and this new

 

work could be evidence to support the idea.

 

But this is only one star and not even one that was directly imaged. It will take much more work to determine what really happened.

 

 

Source

[Karlston]

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