"Vanished" -Mystery of a Massive Star in a Dwarf Galaxy

Rodiano Bonacci
Luglio 1, 2020

When the big ones die, they go out with a bang, so to speak, so when astronomers chose to check in on what they believed was a large star in the Kinman dwarf galaxy almost a decade after it was last studied, they were shocked to discover that it was gone. One explanation could be that the star dimmed considerably after its outburst, and was then further obscured by a thick veil of cosmic dust.

When researchers in 2019 made a decision to check in on the tiny galaxy they realized something was missing.

This could explain why the star appeared so bright during those early observations - still, it does not explain what happened after the outburst that caused the star to vanish. (Sometimes, this looksdownright lovely.) Following the blast, the dense core of leftover stellar material may collapse into a black hole or aneutron star - two of space's most massive and mysterious objects. Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness. The European Southern Observatory's (ESO's) Very Large Telescope, however, couldn't find the star during two separate observing sessions in 2019 with different spectrograph devices.

Also known as the Kinman Dwarf, PHL 293B lies approximately 75 million light-years away in the constellation of Aquarius.

"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". 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. The new telescope is scheduled to see first light in 2025. The massive star that had been previously documented was just plain gone.

"We may have detected one of the most massive stars of the local Universe going gently into the night", said Dr. Jose Groh, also of Trinity College Dublin.

The older observations seem to indicate that the star was experiencing giant eruptions, in which material is lost from the star.

Luminous blue variable stars are enormous by the standards of our Sun, and shine incredibly brightly. "The larger mass of the star we study as well as it being from a low metallicity galaxy makes the finding unique and could hold important clues as to how stars could collapse to a black hole without producing a bright supernova", Allen said.

Based on their observations, the researchers think that there are only two plausible explanations for the star's sudden disappearance and lack of a supernova.

Normally, when a star much larger than our sun reaches the end of its life, it erupts in an enormous supernova explosion.

Undated photo of the inman Dwarf galaxy, taken by the Hubbls Space Telescope. ESO also plays a leading role in promoting and organising cooperation in astronomical research.

ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive ground-based astronomical observatory by far. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become "the world's biggest eye on the sky".

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