Quark Star, A “Strange” Companion (Planetary Science)

Do you remember Gw 190814, the gravitational signal detected by Ligo and Virgo in August 2019? To produce it was the fusion of two compact objects: a black hole with 23 solar masses and an object of about 2.6 solar masses whose nature is still shrouded in mystery. According to a study published in Physical Review Letters, it may have been a strange quark star. We talk about it with Alessandro Drago of the University of Ferrara, Infn associate and co-author of the study

On August 14, two years ago, the gravitational wave interferometers Ligo and Virgo announced the detection of Gw 190814 , a gravitational wave signal produced by the merger of two compact objects in a binary system. The following year, in an article published in The Astrophysical Journal Letters , ( we  also talked about it here on  Media Inaf ), the same collaboration announces the details of the two objects whose fusion generated Gw 190814.

The first member of the pair, according to the study, was a black hole of 23 solar masses. The second, however, was a compact object of about 2.6 solar masses whose nature is still shrouded in mystery. The reason is that its mass falls into what astronomers call the mass gap: an interval between 2.5 and 5 solar masses in which a neutron star or a black hole has never been observed, the two eligible candidates to form the other component of the duet that generated the signal. The heaviest neutron star known to date does not exceed 2.5 solar masses, and the lightest known black hole is about 5 solar masses. Hence the hypothesis that the low-mass companion that merged with the 23-solar-mass black hole, producing a black hole about 25 times the mass of the Sun, was either the heaviest neutron star or the black hole. lighter than you know.

However, according to a study published last week in Physical Review Letters , this member of the couple in question may not be either object. Or rather, it could be a kind of “modified” neutron star, a celestial body called a quark star . Weird quarks , to be precise. To find out more, Media Inaf reached Alessandro Drago of the University of Ferrara, associated with the Infn and co-author of the study together with Ignazio Bombaci (University of Pisa and Infn of Pisa), Domenico Logoteta (University of Pisa and Infn of Pisa), Giuseppe Pagliara (University of Ferrara and Infn of Ferrara) and Isaac Vidaña (Infn of Catania).

Professor, let’s start from the beginning: where does the problem of the so-called ‘mass gap’ arise when we talk about neutron stars or black holes in a compact binary system?

«The maximum mass of a neutron star (or a quark star) is fixed by the interactions between its constituents, that is, by nuclear physics. There are insurmountable limits, linked for example to the principle of causality, which prevent having masses greater than about 3 solar masses, but realistic models for nuclear interaction suggest even more stringent limits. For this reason, until recently it was considered unlikely the existence of neutron stars with a mass much greater than 2 solar masses. Similarly, it is difficult to imagine astrophysical processes leading to the formation of very light black holes, since black holes of few solar masses are produced in unsuccessful supernova explosions. and in those cases the quantity of material falling on the central object is rather large and tends to form black holes of more than five solar masses. One way to form black holes of about 3 solar masses could be based on the merger of two “light” neutron stars. TheHowever, the rate of this type of event has not yet been studied ».

Alessandro Drago, professor at the University of Ferrara, at the Department of Physics and Earth Sciences, and associated with the Infn section of Ferrara © INAF

In your study you investigated the possibility that the least massive object in the binary system that produced the Gw 190814 signal is a star with strange quarks. What are these bodies? 

“Strange quark stars rely heavily on the validity of the so-called Witten hypothesis regarding the stability of strange matter . This hypothesis suggests that the most stable phase of matter is not iron, but a mixture of quark up, down and strange (strange quarks are not present in ordinary matter). Ordinary matter does not decay into this more stable state, as the process requires many simultaneous weak transitions and therefore has a time scale much larger than the age of the universe. But strange matter can form at the extreme densities reached in compact stars. A star of strange quarks is made up almost integrally of deconfined quarks, with the exception of a possible crust of ordinary matter similar to the so-called “outer crust” of ordinary neutron stars. A quark star can form when the density at the center of a neutron star exceeds a threshold value beyond which many hyperons (hadrons containing strange quarks) form at the center of the neutron star. At that point the deconfinement process can begin and can turn the entire neutron star into a quark star in seconds . There are three main ways of forming a quark star: by mass accretion from a companion, by merger of two lighter stars or by the explosion of a large-mass progenitor through a mechanism that facilitates the explosion by using the energy associated with the deconfinement of quarks ».

Are they stars that “shine”?

“Their electromagnetic properties have not yet been studied in detail, but they are certainly not very different from those of ordinary neutron stars: in particular they can emit X-rays and can host strong magnetic fields and therefore emit radio waves”.

Could they have planets orbiting around them, perhaps habitable?

“It is quite difficult for neutron stars and similarly for quark stars to have planets, as they are formed by supernova explosions which can very easily destroy them. As is well known, however, some planets have been observed around neutron stars and therefore could also exist around stars with strange quarks. I would rule out that they could be habitable due to the extreme conditions associated with the formation of compact stars ».

Why did you refer to quark stars?

“It is difficult to obtain a neutron star with a mass of 2.5-2.6 solar masses without violating some limits of nuclear physics or some previous astrophysical observations. This problem has been noted in several articles, for example in a study by F. Fattoyev et al. published in 2020 of Physical Review C . The main reason is that to support such large masses nuclear matter would have to be extremely rigid, but collision experiments between heavy ions and the limits set by the study of Gw 170817they are difficult to reconcile with such requests. Furthermore, such rigid neutron stars would exclude the possibility of having radii significantly lower than about 12 km for stars of mass around 1.4-1.5 solar masses ( DA Godzieba et al. , 2021 ), as instead suggested by some observational analyzes (for example ApJ  887 (2019) 1, 48 ). The simultaneous existence of neutron stars and strange quark stars circumvents these problems, since quark stars can easily reach large masses (matter consisting exclusively of deconfined quarks is not soft at all, see for example A. Kurkela et al. , 2010) and at the same time the stars made up of neutrons can have radii even much lower than 12 km, satisfying laboratory and observational limits ».

What results have you achieved in your study ?

«In the scheme we studied, in which neutron stars coexist with strange quark stars, we have identified some specific signatures of the existence of strange stars, for example in the merger processes of two compact stars. Our prediction is of significantly fewer merger processes leading to the so-called kilonova signal than if only neutron stars exist. In general, the phenomenology of merger processes described in our scenario is quite different from that in which only neutron stars exist and can be easily tested through multi-messenger observations ( A. Drago and G. Pagliara, 2018 and R. De Pietri et al. , 2019). It is worth noting that there is observational evidence that the mass distribution of “neutron” stars is bimodal, with a first peak at around 1.4 solar masses and a second peak at around 1.8 solar masses ( Tauris et al. 2017 ). This division corresponds very well to what we hypothesized in the scheme we studied in which neutron stars with masses lower than about 1.6 solar masses and rather small rays coexist with strange quark stars having larger masses and radii that can reach 13-14 km ” .

So what are, according to your hypothesis, the possible paths that led to the formation of a strange quark star in the fusion that produced Gw 190814?

“There are two possibilities: the strange quark star could have been formed by the merger of two lower-mass neutron stars or it could have been produced in a supernova explosion in which the deconfinement of quarks helped the parent star to explode, such as discussed previously ‘.

Has the existence of similar stars already been confirmed? If not, how could they be?

“The existence of strange quark stars is a hypothesis for now. An indirect confirmation of their existence could derive from the observation of compact stars with radii of the order of 11.5 km or less (which would hardly be reconcilable with the existence of large-mass stars, if only neutron stars exist, while they could exist in our scenario since neutron stars do not have to reach large masses). Quark stars can reach large masses with large radii. From this point of view, the very recent announcement by NASA that a star of mass about 2.1 times that of the Sun would have radii of the order of 12.4-13.7 km is very interesting . Those data are perfectly consistent with the hypothesis that the object in question is a quark star ».

An open and very interesting question is whether dark matter can be at least partly made up of strange matter . What do you think?

«This hypothesis was already advanced in Witten’s 1984 paper and continues to be valid ( S. Burdin et al. , 2015 ; DM Jacobs et al. , 2015 ). Recently, together with Marco Casolino of the Infn of Tor Vergata, we proposed to test the existence of lumps of strange matter through a network of lunar seismographs, which could reveal the impact of these lumps in a much more “silent” environment than Earth’s surface”.

Featured image Credits: Ligo / Caltech / Mit / R. Hurt (Ipac)


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