An article published in the “Monthly Notices of the Royal Astronomical Society” reports the mysterious disappearance of a massive star of the luminous blue variable type in the Kinman dwarf galaxy. A team of researchers led by Andrew Allan of Trinity College in Dublin, Ireland, detected the disappearance of that star using ESO’s Very Large Telescope (VLT) in Chile. Additional observations and research of archival data to try to better understand the events led to believe that the most likely possibilities are a transformation that led to a sharp drop in brightness, maybe dimmed even more due to the dust that covered it, and its direct collapse into a black hole without a supernova explosion.
About 75 million light-years away from Earth, the Kinman dwarf galaxy, also known as PHL 293B, has a low metallicity, which means a low percentage of heavy elements. Among the stars hosted in this galaxy, there’s a luminous blue variable, a massive star in an advanced stage of its life which is consequently unstable. This type of star is also called S Doradus variable referring to the first of this type discovered.
At the distance of the Kinman dwarf galaxy, it’s impossible to see single stars directly, but spectrographic analyzes reveal traces like that of the star detected in observations between 2001 and 2011, which at the time was about 2.5 million times brighter than the Sun. The significant changes in brightness they’re subject to should be detected as well, but observations from 2019 have no longer found any trace of it.
To search for more data, Andrew Allan’s team used the ESLESS (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) instrument mounted on the VLT using all its four telescopes without finding traces of the luminous blue variable star. A few months later they tried to use another VLT instrument, X-shooter, with the same results.
The analysis of the collected data led the researchers to two possible explanations for the disappearance of the luminous blue variable star: a transformation that led to a sharp drop in brightness, maybe dimmed even more due to the dust that covered it, and the collapse headed into a black hole without the supernova explosion.
Old data collected by the VLT with the UVES (Ultraviolet and Visual Echelle Spectrograph) and X-shooter instruments indicate that the luminous blue variable star may have passed through powerful outburst periods that probably ended after 2011. These are typical events for stars of that type that lead to significant mass losses. This means that we’re now seeing it in a state where its brightness might have dropped significantly and it’s also possible that it’s covered at least in part by dust.
A very intriguing possibility is that the luminous blue variable star collapsed directly into a black hole instead of exploding into a supernova. It’s a case predicted in theory, and the star N6946-BH1 is already considered another candidate for this type of end.
The distance increases the difficulties in understanding what really happened. X-ray emissions from the area where the luminous blue variable star was detected would indicate that there’s now a black hole. Research will continue in the coming years and ESO is building the Extremely Large Telescope (ELT), which could be a perfect instrument for this type of research.