A relatively small, dense object hidden in a cloud of its own exploded remains a few thousand light-years away challenges our understanding of stellar physics.
By all accounts, it appears to be a neutron star, although it is unusual at that. At only 77% of the mass of the Sun, it is the lowest mass ever measured for an object of this type.
Previously, the lightest neutron star ever measured was 1.17 times the mass of the Sun.
This more recent discovery is not only smaller, it is significantly smaller than the minimum neutron star mass predicted by theory. This suggests either that there is some gap in our understanding of these ultra-dense objects… or that what we are looking at is not a neutron star at all, but a peculiar, never-before-seen object known as the “strange” star name.
Neutron stars are among the densest objects in the entire Universe. It’s what’s left after a massive star about 8 to 30 times the mass of the Sun has reached the end of its life. When the star runs out of material to fuse into its core, it goes supernova, ejecting its outer layers of material into space.
No longer supported by the external pressure of fusion, the nucleus collapses in on itself to form an object so dense that the atomic nuclei smash together and the electrons are forced to become intimate with the protons long enough to they turn into neutrons.
Most of these compact objects are around 1.4 times the mass of the Sun, although theory suggests they could range from something as massive as around 2.3 solar masses to just 1.1 solar masses. All of this packed inside a sphere just packed into a sphere about 20 kilometers (12 miles) in diameter, which makes each teaspoonful of neutron star material weigh between 10 million and several billion tons.
Stars with higher and lower masses than neutron stars can also transform into dense objects. Heavier stars turn into black holes. Lighter stars turn into white dwarfs – less dense than neutron stars, with an upper mass limit of 1.4 solar masses, though still quite compact. It’s the eventual fate of our own Sun.
The neutron star that is the subject of this study lies at the center of a supernova remnant called HESS J1731-347, which was previously calculated to be more than 10,000 light-years away. One of the difficulties in studying neutron stars, however, lies in poorly constrained distance measurements. Without an accurate distance, it’s difficult to get accurate measurements of a star’s other features.
Recently, a second optically bright star was discovered hiding in HESS J1731-347. From there, using data from the Gaia map survey, a team of astronomers led by Victor Doroshenko from Eberhard Karls University in Tübingen in Germany was able to recalculate the distance to HESS J1731-347, and found it to be much closer than expected, about 8,150 light-years away.
This means that previous estimates of the neutron star’s other characteristics needed to be refined, including its mass. Combined with observations of X-ray light emitted by the neutron star (inconsistent with X-ray radiation from a white dwarf), Doroshenko and his colleagues were able to refine its radius to 10.4 kilometers and its mass to a level solar absolutely low of 0.77. masses.
This means that it may not actually be a neutron star as we know it, but a hypothetical object that has not yet been positively identified in nature.
“Our mass estimate makes the central compact object of HESS J1731-347 the lightest neutron star known to date, and potentially a more exotic object, i.e., a ‘strange star’ candidate. “, write the researchers in their article.
According to the theory, a strange star is very similar to a neutron star, but contains a greater proportion of fundamental particles called strange quarks. Quarks are fundamental subatomic particles that combine to form composite particles such as protons and neutrons. Quarks come in six different types, or flavors, called high, low, charm, strange, high, and low. Protons and neutrons are made up of up and down quarks.
The theory suggests that in the extremely compressed environment inside a neutron star, subatomic particles break down into their constituent quarks. According to this model, strange stars are made of matter composed of equal proportions of up, down and strange quarks.
Strange stars should form under large enough masses to really put the strain on, but since the rulebook for neutron stars disappears when enough quarks are involved, there’s essentially no lower limit either. This means that we cannot exclude the possibility that this neutron star is in fact a strange star.
That would be extremely cool; Physicists have been looking for quark matter and strange quark matter for decades. However, while a weird star is certainly possible, the greater likelihood is that what we’re looking at is a neutron star – and that too is extremely cool.
“The obtained constraints on mass and radius are still fully consistent with a standard interpretation of neutron stars and can be used to improve the astrophysical constraints on the cold dense matter equation of state under this assumption,” write the authors. researchers.
“Such a faint neutron star, whatever its supposed internal composition, seems to be a very intriguing object from an astrophysical point of view.”
It is difficult to determine how such a light neutron star could have formed with our current models. So, regardless, the dense object at the heart of HESS J1731-347 will have something to teach us about the mysterious afterlife of massive stars.
The team’s research has been published in natural astronomy.
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