Neutron stars are so named as a result of within the easiest of fashions they’re made from neutrons. They type when the core of a big star collapses, and the burden of gravity causes atoms to break down. Electrons are squeezed along with protons in order that the core turns into a dense sea of neutrons. However we now know that neutron stars aren’t simply gravitationally sure neutrons. For one factor, neutrons are comprised of quarks, which have their very own interactions each inside and between neutrons. These interactions are extraordinarily advanced, so the main points of a neutron star’s inside are one thing we don’t absolutely perceive.
The majority properties of neutron matter are finest described by the Tolman-Oppenheimer-Volkoff (TOV) equation of state. Primarily based on this, the higher mass restrict for a neutron star ought to be round 2.2 to 2.6 photo voltaic plenty, which appears to agree with commentary. The TOV equation additionally assumes that the neutrons throughout the neutron star stay neutrons. In atomic nuclei, you possibly can’t have a sea of free quarks due to the character of the sturdy nuclear pressure, so this looks as if an inexpensive assumption. However some physicists and astronomers have argued that throughout the dense coronary heart of a neutron star, quarks would possibly break away to create a quark star. Some have even prompt that quarks inside a neutron star would possibly work together so strongly that unusual quarks seem, making them unusual quark stars.
One strategy to discover these concepts is to take a look at pulsars. Since pulsars are rotating neutron stars the place their magnetic pole sweeps in our course, we will measure the speed of rotation by timing the radio pulses from a pulsar. So if a pulsar flashes each three seconds, we all know that’s how lengthy it takes for the neutron star to rotate as soon as. Pulsars are how we first discovered that neutron stars are, properly, neutron stars, as a result of the speed of an object’s rotation tells you the minimal density the article should have.
You possibly can consider it like a playground merry-go-round. If you happen to let a couple of kids climb on, then spin the merry-go-round actually quick, you possibly can watch the children fly off one after the other as they lose their grip. This is without doubt one of the causes playground merry-go-rounds are so uncommon nowadays. Since stars are held collectively by gravity, there may be an higher restrict on how briskly a star can rotate. Any sooner and gravity would lose its grip and the star would fly aside. So after we measure the rotation of a pulsar, we all know it should be beneath that higher restrict, generally known as the Kepler frequency. For the reason that floor gravity of a star depends upon its density, the rotation frequency tells us the minimal density of the star. When astronomers first found pulsars rotating a number of instances a second, they knew the density of the pulsar was better than a white dwarf, so it needed to be a neutron star.
There are some pulsars which have very excessive rotation frequencies. The quickest noticed pulsars, generally known as millisecond pulsars, can have frequencies above 700 Hz. It’s fairly astonishing when you consider it. An object with practically twice the mass of the Solar, however just a few kilometers throughout and making a whole bunch of rotations a second. Millisecond pulsars rotate so rapidly that they aren’t even spherical. They bulge out round their equators to change into oblate spheroids. This implies the density of their polar areas should be a lot greater than close to the equator. This raises the query of whether or not neutrons within the polar areas would possibly bear a section transition into quark matter.
To discover this concept, a crew checked out varied fashions of neutron stars. They modeled the equation of state for conventional neutron stars and in contrast them to so-called hybrid stars, the place the inside is a mixture of neutrons and quark matter. From this, they calculated the Kepler frequency because it pertains to the general mass of the star. They discovered that whereas all of the at the moment noticed millisecond pulsars may be described by the standard mannequin, the hybrid mannequin is a greater match for the quickest pulsars. Additionally they calculated that hybrid stars would push the higher restrict nearer to 1,000 rotations a second. So if we discover pulsars within the 800 Hz or greater vary, we all know they probably comprise quark matter of their cores.
One other strategy to take a look at the hybrid neutron star mannequin could be to search out extra millisecond pulsars with a variety of plenty. This might permit us to take a look at how the rotation frequency varies with mass on the higher restrict to see if Kepler frequencies agree extra strongly with a hybrid or conventional mannequin.
Reference: Gärtlein, Christoph, et al. “Quickest spinning millisecond pulsars: indicators for quark matter in neutron stars?” arXiv preprint arXiv:2412.07758 (2024).
