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A binary star just 1,500 light years away is heading for a spectacular end.
HD265435 consists of a type of dead star called a white dwarf and its binary companion; they are in orbit so close to each other that the white dwarf sucks up matter from the other star. Eventually, according to the theory, the white dwarf will gain so much mass that it is no longer stable, exploding into a huge supernova.
It won’t be for a while, but the discovery of such a doomed binary is rare, according to a team of scientists led by astronomer Ingrid Pelisoli of the University of Warwick in the UK; discovery can help us better understand the processes leading to these incredible events.
This is important, because the type of supernova this unstable star will cause is what we call a standard candle – one of the key tools we use to measure cosmic distances.
Stars spend their lives (what we call the main sequence) fusing elements with heavier elements in their cores, but they don’t have an infinite supply. Eventually, they’ll run out of things they can fuse together and die, ejecting their outer material. Depending on the mass of the star, several things can happen at this point.
For most stars, the nucleus will collapse into an ultra-dense object, and the nature of this object will depend on the mass of the main sequence progenitor star. For stars over 30 times the mass of the Sun, it will be a black hole. For stars between 8 and 30 solar masses, it will be a neutron star. And for stars with less than 8 solar masses (including our Sun), it will be a white dwarf.
These stars always glow with residual heat and take a very, very long time to cool down to darkness. The only thing preventing them from collapsing entirely under their own gravity is the degenerating pressure of the electrons. At a certain pressure level, electrons are stripped of their atomic nuclei. Because identical electrons cannot occupy the same space, these electrons provide the external pressure that keeps the star intact.
It also has a limit. Over about 1.4 times the mass of the Sun, or the Chandrasekhar limit, the white dwarf becomes so unstable that it dies again, exploding in a Type Ia supernova. This can happen when the white dwarf orbits so close to a binary companion that it siphons material from the other star, causing it to tip over the Chandrasekhar limit.
But there is a curious discrepancy in the number of observed Type Ia supernova remnants and the number of candidate Type Ia progenitors – we just haven’t found as many progenitors as there should be, based on the number of observable remains.
This is why HD265435 is so exciting. At its distance of 1,500 light years, it is the closest known ancestor of Type Ia, which means we have the opportunity to study it in detail.
“We can estimate how many supernovae are going to be in our galaxy by looking at many galaxies, or by what we know about stellar evolution, and that number is consistent,” Pelisoli said.
“But if we’re looking for objects that can become supernovae, we don’t have enough. This finding was very helpful in estimating what hot dwarf and white dwarf binaries can do. It still doesn’t seem like much, none of the channels that we observed do not seem to be enough. “
The binary itself is fascinating. It consists of the white dwarf and a hot sub-dwarf, the latter being a red giant after ejecting its outer layers and is about to start fusing helium, running out of hydrogen. This hot sub-dwarf is small, barely 0.6 times the mass of the Sun, but extremely bright – so bright that it completely eclipses the white dwarf. We don’t see the white dwarf at all.
Pelisoli and his team identified the binary by changes in brightness in the hot sub-dwarf. These changes suggest that the hot dwarf is being dragged away in a teardrop shape by something massive very close to it.
By carefully analyzing the changes in brightness, the researchers were able to deduce what was going on. A white dwarf of roughly the same mass as the Sun orbits the hot sub-dwarf about every 100 minutes, close enough to siphon material from the sub-dwarf and distort its atmosphere.
Together, the masses of the two objects exceed the Chandrasekhar limit, which means that a Type Ia supernova is expected to occur … in about 70 million years. Before that happens, the white sub-dwarf will run out of material to merge and transform into a second white dwarf star.
This discovery could help us understand a huge problem with cosmology. Because Chandrasekhar’s mass is within a known range, Type Ia supernovae have determinable intrinsic luminosity. This means that we can use them to map distances in the local Universe – but we use several methods to do this, and different methods give us different results for the rate of expansion of the Universe.
“The more we understand how supernovae work, the better we can calibrate our standard candles. This is very important right now because there is a gap between what we get from this type of standard candle and what we get from it. other methods, ”Pelisoli said. mentionned.
“The more we understand how supernovae are formed, the better we can understand whether this discrepancy we are observing is due to new physics that we are not aware of and taking into account, or simply because we are underestimating them. uncertainties of these distances. “
The research was published in Nature astronomy.
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