Astronomers have discovered a red giant that has an invisible companion. The second object cannot be seen directly, but its presence is indicated by the gravitational influence on the star itself. The system was named J0008+0128, and the data for analysis were taken from the LAMOST telescope (a Chinese spectroscopic sky survey) and from the APOGEE project (a survey of Milky Way stars). The work was published in the journal Science Advances.
According to the researchers' estimates, the mass of the invisible companion is between 1.8 and 2.8 solar masses, and the most probable value is about 2.2 solar masses. The upper limit falls within the so-called mass gap - the interval of roughly 2.5 to 5 solar masses, where neither neutron stars nor black holes have yet been found.
Neutron stars are superdense remnants of exploded massive stars. Usually their mass is close to 1.4 solar masses, and all of it fits into a sphere about the size of a city, about 20 kilometers across. Not a single neutron star heavier than 2.5 solar masses is known so far. Black holes, the remnants of even larger stars, usually weigh no less than five solar masses. Astronomers do not know exactly what happens in the interval between these values: the mass limit of a neutron star depends on the behavior of matter under the enormous pressures inside it, but the equations of state describing this behavior are still poorly studied.
The J0008+0128 system is interesting not only for its mass. Its components complete an orbit in about 3.3 years, meaning they are far apart. At the same time, the orbit is surprisingly close to circular. No light other than the red giant's radiation has been recorded, so the companion is unlikely to be an ordinary star. The most likely candidate is a neutron star.
However, the combination of high mass, a wide and nearly circular orbit is unusual for binary systems with neutron stars. Many heavy neutron stars are in close pairs and probably gained mass by pulling matter from a neighbor. Here the distance is too great, and the typical signs of mass transfer are not visible.
The researchers ran POSYDON computer simulations - modeling of the evolution of isolated binary systems. None of them reproduced the combination of masses and orbital properties of J0008+0128. The authors write that the rarity of such systems is unlikely to be explained only by the limited sample of simulations: rather, it is due to evolutionary features. According to them, the formation of such a system is not excluded, but may require a rare combination of parameters - for example, very efficient common-envelope ejection, a large mass at the birth of the neutron star, and a weak kick during the supernova explosion.
Scientists do not rule out other explanations either. The system could be triple: for example, two close small neutron remnants plus a red giant, or a neutron star, a white dwarf and a red giant. It is possible that it was previously a triple system in which two small bodies merged. A very small black hole cannot be completely ruled out either, although such light black holes have not yet been found.
In any case, the object provides new data for understanding the boundary between neutron stars and black holes. The authors note: a wide orbit with low eccentricity (that is, weak elongation) is difficult to explain by standard paths of binary system evolution, so improved theoretical models are needed.