A Rare "Ultraviolet" Supernova Discovered In The Constellation Of Draco
Astronomers have discovered an unusual supernova that is believed to have formed after a collision between a white dwarf and an ordinary star in the constellation Draco. Its study will help to understand how such "dead stars" explode, scientists write in the Astrophysical Journal.
"Supernovae of the first type occur quite often by themselves. Supernova SN2019yvq is unique because it was preceded by a powerful flash of ultraviolet radiation. This is the second such spike in the entire history of observations," said one of the authors of the article, an astrophysicist from Northwestern University (USA) Adam Miller.
White dwarfs are what astronomers call the cores of "burned-out" small stars that don't have their own energy sources. They occur in the last stages of the life of luminaries that are no more than ten times heavier than the Sun. Such stars do not explode as a supernova, but shed their outer shells, forming a nebula that is "illuminated" from the inside by the core of the former star.
Such "dead stars" are of interest to astrophysicists because they are considered the progenitors of relatively rare supernovae of the first type – large-scale space thermonuclear explosions. Thanks to such supernovae, distances in space can be estimated very accurately. Besides, scientists do not yet fully understand the nature, structure, and composition of the matter that makes up white dwarfs, which also spurs researchers.
Even more interesting are rare supernovae, which are preceded by a powerful flash of ultraviolet radiation. Presumably, they are formed after the collision of white dwarfs and ordinary stars, but scientists do not yet know how and why this happens. In the entire history of observations, only one flare of this kind is known – the supernova iPTF14atg. Astronomers spotted it in the galaxy IC 831 six years ago.
The mysteries of white dwarfs
Analyzing the data collected by the automated ZTF telescope, Miller and his colleagues recorded the second example of such flares. This supernova was even more inexplicable than the iptf14atg flash.
In December of last year, ZTF detected a relatively weak type I supernova flare in the constellation Draco. It originated in the lenticular galaxy NGC 4441, which is located at a relatively short distance from the milky Way – 140 million light-years.
Initially, astronomers believed that it was an ordinary supernova of the first type. However, a study of photos of the galaxy NGC 4441, which were taken at the same time by the Swift space telescope, showed that a powerful flash of ultraviolet radiation occurred in the same area of the Universe shortly before it – the same as in the case of iPTF14atg. The supernova was named SN2019yvq.
Further observations showed that this supernova did not fade away in the same way, as it usually does with type one supernovae. Also, scientists discovered several unique properties that distinguished it from iPTF14atg.
In particular, SN2019yvq was significantly dimmer in optical telescope images than iPTF14atg and all other Type I supernova flares. At the same time, SN2019yvq and faded faster than they did. The changes in its spectrum were not similar to the evolution of any other flare that occurred due to white dwarf explosions; this indicates an unusual isotopic and chemical composition of the traces of this supernova.
All these oddities, as the researchers note, can not yet be explained by any theory that describes the explosion of single white dwarfs. Such outbreaks probably occur in different scenarios. Miller and his colleagues hope to figure out these scenarios in the coming year when the cloud of hot plasma and dust that surrounds SN2019yvq becomes transparent to telescopes.