Once at the end of their lives, in a sort of baroud of honor, some stars explode in Supernovæ. Supernovæ play an important role in astronomy, especially thermonuclear power, called “IA type”. Their intrinsic brightness is known and particularly stable, which makes very good stallions to measure the cosmic distances (we can deduce their distance by comparing their apparent brightness and their intrinsic brightness). They are thus called “standard candles”. It is notably thanks to the IA supernovæ that physicists were able to measure the expansion of the universe. But the way these extreme events take place keeps a part of mystery. In particular, do these explosions all take place according to the same diagram? Thanks to new images of Very large telescope (VLT) From the Austral European Observatory (ESO), Priyam Das, from the University of New South Wales, and its colleagues, have established that these explosions sometimes take place in two stages, and for variable masses of the star. These results were published in the journal Nature astronomy July 2.
The remains of the supernova SNR 0509-67.5 scrutinized by the instruments of VLT are located at almost 160,000 light years, in the Grand Cloud of Magellan. These are the remains of the thermonuclear explosion of a so-called “white dwarf” star within a binary system, that is to say accompanied by another star. A white dwarf is the final stage of a low -mass star where nuclear fusion reactions of hydrogen and helium have died; It consists mainly of carbon and oxygen. In the binary system, the white dwarf tears from the material from its traveling companion. As the material accumulates, the temperature and the pressure climb … and when the white dwarf reaches the critical mass of 1.44 solar mass (called “Chandrasekhar mass”), the fusion reactions (this time carbon and oxygen) restart and get carried away in a few seconds, leading to the explosion. It is then a “cosmic fireworks”: the star disintegrates violently, producing heavy elements which are projected in space. Until recently, astrophysicists thought it was the only mechanism causing a thermonuclear supernova, and thus supported the scenario of the unique explosion.
But the data accumulated for decades show that IA supernovae are not all exactly identical, whether by their brightness or by their light spectrum.
Anterior simulations have shown that white dwarfs could, in theory, explode below the critical mass and, in this case, in two stages. At first, the white dwarf will accre a layer of helium torn from its companion. Under its own gravitational pressure, helium will merge, triggering a first detonation. The shock wave then compresses the elements of the heart of the white dwarf, and trigger their fusion, thus causing an even more powerful explosion (this is the same principle that is brought into play in hydrogen bombs, where the explosion of a fission bomb serves as a detonator for a thermonuclear explosion).
The distribution of calcium in the remains of the supernova reveals a structure in two layers. This structure is a sign of an explosion in two stages.
© that/p. Das et al.
The authors of the study identified in the images and the light spectrum of the SNR 0509-67.5 supernova SNC suggesting that these vestiges come from a two-step explosion. Indeed, the two detonations triggering different pressures with different fuels, the elements produced are not entirely the same. The simulations show that two distinct layers of calcium then form in the ejected debris, between which there are lighter elements such as silicon or sulfur. Using the Muse Specrograph (Multi-United Specrographic Explorer) of the VLTastronomers have highlighted in the spectrum of the supernova SN 0509-67.5 the wavelengths characteristic of these elements, thus confirming this structure in layers, and therefore the double explosion.
These measures suggest that the heart of the white dwarf represented a mass of approximately 1,028 solar mass, and the helium layer, of only 0.027 solar mass. The explosion would therefore have occurred when the star had a total mass of 1.06 solar mass, well below the critical mass for a usual explosion. This confirms that IA supernovae can have more diverse profiles than we thought.
By observing other recent supernovæ remains, astronomers hope to understand their origin better and characterize the variability of “standard candles” more precisely.
