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Dense, compact star could be made of strange matter

With 80% of the mass of the Sun and a radius of 10.4 kilometers, the object could be made up of free quarks

The compact object XMMU J173203.3−344518 (at the center, in yellow) could be a strange star, made of quarks

Victor Doroshenko / Gerd Pühlhofer / ESA / XMM-Newton

Stars with a mass greater than eight times that of the Sun burn for a few tens of millions of years, then end with a powerful, luminous explosion called a supernova. During this event, marking the end of its ability to generate energy through nuclear fusion, the dying star expels its outermost layers, after which its remaining matter can follow two paths. If the initial total mass of the star was greater than the mass of 25 Suns, the inert core continues to collapse and becomes a black hole. If the star was between 10 and 25 solar masses, the central core survives the explosion and forms an object composed of only one type of particle, creating a star composed entirely of neutrons.

New studies suggest that a low-mass neutron star may become an even more exotic object, predicted in theory but never unequivocally observed: a star composed entirely of loose quarks, a kind of elementary, indivisible particle that is one of the fundamental components of matter. An article published by astrophysicists from the University of São Paulo (USP) and the Federal University of ABC (UFABC), in the April issue of the journal Astronomy & Astrophysics, points out that the mass, radius, and surface temperature of the compact object XMMU J173203. 3−344518, the official name of the star, match the theoretical models that predict the existence of this class of celestial body. These models use general relativity to infer gravitation and postulate various scenarios for the supernova core as it cools.

“We’re not saying that it is a strange star, but that its parameters are compatible with this category of object,” comments astronomer Jorge Horvath, from USP, who coordinated the group that conducted the study. “Strange star” is the technical name given to an object left over from a supernova, and is formed by strange matter, which contains quarks, especially strange quarks, which are not confined within ordinary particles. All particles classified as hadrons, the most stable of which are protons and neutrons, are composed of at least two or more quarks held together by the strong nuclear force. There are six types, or “flavors,” of quarks, each with an electrical charge and a specific mass: up, down, strange, charm, bottom, and top. Like neutron stars, strange stars do not emit visible light. The main clues to their existence are strong emissions at X-ray frequencies, remnants of the supernova activity that created them.

In nature, quarks should not exist unattached, they are normally trapped in the depths of protons and neutrons. In hypothetical celestial objects even smaller and denser than neutron stars, quarks, especially strange quarks, could nevertheless exist as isolated entities. “In a strange star, the neutrons will have dissolved and generated a soup of quarks,” Horvath explains. The main argument in favor of the idea that the compact object XMMU J173203.3−344518 could be made up of strange matter is the revised evaluation of its mass, which, in a study published at the end of 2022 in the journal Nature Astronomy, was reduced from 1.4 to 0.77 solar masses.

Although this calculation includes an estimated margin of error of around 20%, such a low mass is considered incompatible with the formation of a neutron star. According to the current theory, supported by observational data, a neutron star could not have less than 1.1 solar masses. “An object with almost 0.8 solar masses could be a strange star or it might still be a neutron star. But in this second case it would be an exceptionally light neutron star, which would also be a very interesting find,” says Russian astronomer Victor Doroshenko, from the University of Tübingen, Germany, in an interview with Pesquisa FAPESP. Doroshenko was the lead author of the study that recalculated the mass of the star last year. In that study, the radius of the celestial object was also revised, down to only 10.4 kilometers. This corrected value is quite close to the lower limit for neutron stars, whose radius varies from 10 to 20 kilometers.

The update to the intriguing star’s mass and radius derives from revising its distance from Earth. With new data from the Global Astrometric Interferometer for Astrophysics (GAIA) — the European space observatory working to create a more accurate three-dimensional map of the Milky Way by measuring the brightness and position of one billion stars — Doroshenko and his colleagues from Tübingen concluded that the star is 8,150 light-years from Earth, about 20% closer than previous calculations indicated. Rectifying a star’s distance leads to the revision of other parameters, such as its mass and radius. The closest star to our planet — not including the Sun — is Proxima Centauri, at 4.2 light-years away, which is about 1,900 times closer than the strange star candidate.

According to nuclear physicist Manuel Malheiro from the Instituto Tecnológico de Aeronáutica (ITA), one of the difficulties in defending the possible existence of quark stars is explaining the mechanism through which their matter cools. When it explodes, a supernova reaches temperatures in the range of billions of degrees kelvin (K). In neutron stars, the surface temperature is about 1 million K. Models predict that a strange star should be much cooler than this. But according to current estimates, XMMU J173203.3−344518 has temperatures of over 1 million K. “We don’t know how to explain why this star, which would be from 2,000 to 6,000 years old, didn’t cool off as quickly as the theory predicts,” comments Malheiro. The researchers are unanimous in saying that doubts about the existence of strange stars and quark stars — which would be even denser and more stable than neutron stars — will only be resolved when there are yet more precise data on their masses and radiuses.

Neutron stars: Structure, evolution, and gravitational waves (nº 20/08518-2) Grant Mechanism Regular Research Grant; Principal Investigator Jorge Horvath (USP); Investment R$103,206.59.

Scientific articles
HORVATH, J. E. et al. A light strange star in the remnant HESS J1731-347: Minimal consistency checks. Astronomy & Astrophysics. vol. 672. apr. 2023.
DOROSHENKO, V. et al. A strangely light neutron star within a supernova remnant. Nature Astronomy. vol. 6. dec. 2022.