An article published in the journal “Astronomy & Astrophysics” reports the creation of a detailed image of the red supergiant star WOH G64 and its surroundings, the first ever obtained of a star outside the Milky Way. A team of researchers led by astrophysicist Keiichi Ohnaka of the Universidad Andrés Bello, Chile, used the GRAVITY instrument on ESO’s VLTI in Chile to obtain details of this star in the Large Magellanic Cloud. This study may provide important insights into a star that is going through the death throes that will end with its explosion as a supernova. A dust cocoon and a possible torus surrounding WOH G64 show signs of that death throes.
WOH G64, also known as IRAS 04553-6825, is a red supergiant in the Large Magellanic Cloud, one of the Milky Way’s dwarf satellite galaxies. About 170,000 light years from Earth, it had already attracted the interest of astronomers because it’s in the last phase of its life but was less hot than expected. It’s about 1,500 times the size of the Sun because in that phase the star expanded enormously, the reason why over the years, it was nicknamed the Monster or the Behemoth.
Jacco van Loon, director of the Keele Observatory at Keele University, UK, started studying the star WOH G64 in the 1990s, when he studied it together with some colleagues discovering from it the first maser emissions from silicon monoxide coming from an extragalactic source.
In its red supergiant phase, a star ejects layers of its atmosphere, and Keiichi Ohnaka and his colleagues had already reported in an article published in April 2008 also in “Astronomy & Astrophysics” the discovery of a torus surrounding WOH G64. In that case, the researchers used the MIDI (MID-infrared Interferometric instrument) mounted on the Very Large Telescope (VLT).
The bottom image (ESO) shows the Large Magellanic Cloud as seen by NASA’s Spitzer Space Telescope, an inset with the position of the star WOH G64, and an artist’s impression of the star and the torus of material surrounding it.
Since 2016, the GRAVITY instrument has offered significant advances over previous interferometric instruments and allowed the four telescopes that make up the Very Large Telescope Interferometer (VLTI) to be used to their full potential. Its use enabled the new, much more detailed images of WOH G64 and the environment surrounding the star.
A surprise came when comparing observations made over several years because they indicate that the star WOH G64 has become fainter over the last decade. The processes that cause red giants and supergiants to eject the outer layers of their atmospheres can take millennia but those that we are observing from Earth are happening at great speed. WOH G64 may have already exploded but the light from the supernova will take much longer to reach Earth.
The cocoon surrounding the star WOH G64 is elongated, and this is another surprise since astronomers didn’t expect it. The ejection of materials could have distorted the cocoon or WOH G64 has a companion that hasn’t been detected yet.
According to the researchers, the ejection of materials by the star WOH G64 could be the cause of the decrease in its brightness. The new images suggest the presence of a ring around WOH G64 fainter than the central one which could be the inner ring of a dust torus. However, further observations are needed to obtain confirmation.
Even for the VLTI, it’s becoming increasingly difficult to obtain images of the star WOH G64 due to its decreasing brightness. However, an upgrade of its instruments is planned and GRAVITY+ will help to continue the observations as the improvements are gradually implemented. All this while waiting for WOH G64 to explode, an event that could happen at any time in the future.
