Lightest neutron star ever found could contain compressed quarks

A neutron star that is lighter than seems possible could be so small because it is made up of compressed versions of particles called strange quarks

By Alex Wilkins

The supernova remnant HESS J1731-347 (yellow and white) hosting a strangely small neutron star

Victor Doroshenko

The lightest neutron star ever found, at around 0.77 times the mass of the sun, no longer fits with our understanding of how these stars form. It might even contain exotic matter like strange quarks.

As far as we know, a neutron star forms after a star has spent its fuel and collapses under its own gravity, blasting most of its matter out in a shockwave called a supernova and leaving behind an ultra-dense core. Our best understanding of this process suggests that the minimum mass left behind can be about 1.17 times that of our own sun.

Astronomers have found some neutron stars that seem to be less heavy, however, called central compact objects, but it was thought that having a carbon atmosphere was making them appear smaller than they really were.

Now, Victor Doroshenko at the University of Tübingen in Germany and his colleagues have reanalysed one of these objects – found within supernova remnant HESS J1731-347 – using data from the European Space Agency’s Gaia star-mapping mission to better estimate its distance from us, which influences how we interpret the spectrum of light coming from a distant object. The team has found that its size can be best explained without a carbon atmosphere. This suggests the object really is around 0.77 times the mass of the sun and 20 kilometres wide.

The researchers were able to measure the neutron star’s distance from Earth because it had a shell of dust illuminated by a nearby regular star, which Gaia had mapped. Using this more accurate distance and previous X-ray data, the group used models to calculate various estimates for the mass and radius of the neutron star.

“If we look at the masses of neutron stars when they are measured precisely, they are all around 1.4 solar masses,” says Doroshenko. Understanding why this object is so much less massive will require a new theory of how they form, he says.

An alternative reason for its low mass could be that the neutron star is made up of elementary particles known as strange quarks that are in a highly compressed state, or a mixture of neutrons and quarks – but more data needs to be gathered to understand what it is made of, says Doroshenko.

“This presents a puzzle of: ‘How did this 0.7 solar mass neutron star begin its life and where did it come from?’ It’s much smaller than what we’d normally expect,” says Fabian Gittins at the University of Southampton, UK.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-022-01800-1

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