Some pulsars and neutron stars could be formed of an exotic material unlike that found in normal stars. A new study suggests a way to find these strange quark objects.
Two astronomers in China have come up with a method to detect stars made of strange quark matter (SQM). What’s more, they say the technique could also reveal the existence of SQM exoplanets orbiting around them.
The intriguing idea has been suggested by Professors Yong-Feng Huang and Yong-Bo Yu from the University of Nanjing and Nanjing’s Key Laboratory of Modern Astronomy and Astrophysics.
Normal matter in the Universe is made of atoms that have nuclei comprised of protons and neutrons. Protons and neutrons themselves are further made of three particles each called quarks. Protons are made of up, up and down quarks and neutrons are made of up, down and down quarks.
According to the Standard Model of particle physics, along with up and down quarks there are four other types (called flavours): charm, strange, top and bottom. These are unstable and very quickly decay into up and down quarks, creating the protons and neutrons that we know. But two Soviet scientists hypothesised in the 1960s that strange quarks could remain stable in ‘quark stars’—SQM stars.
Shortly after Dmitri Ivanenko and D. Kurdgelaidze’s hypothesis, a neutron star was discovered in 1967 by Jocelyn Bell-Burnell and Anthony Hewish in Cambridge. Neutron stars are the dense cores of normal stars – at least eight times the mass of our Sun – which have consumed all of their fuel and exploded. Such an event literally crushes the core’s atoms into a giant nucleon.
About the size of a city and composed entirely of neutrons, they can have about twice the mass of our Sun. They also often rotate rapidly—hundreds of times a second. If their precessing, focused radio beams point towards us such that we can detect them then they’re known as pulsars.
The idea that neutron stars themselves may be SQMs isn’t new one, but in their paper published on 26th February, Yong-Feng Huang and Yong-Bo Yu say that observed pulsars deserve closer inspection.
“Currently about 2560 pulsars have been detected,” Huang told Skymania. Pulsars are still found by detecting the time of arrival (TOA) of their radio pulses. Huang adds: “TOA observations can reveal features such as spin period, spin-down evolution, glitches, etc. People are hoping to find new pulsars with very short periods, especially less than 1 millisecond.”
The fastest-known pulsar is PSR J1748-2446ad, which rotates at 716 times per second. At 16 kilometres in diameter, it would probably fly apart if it rotated any faster.
So finding a faster spinning object’s important for providing evidence for the SQM hypothesis. SQM stars would be denser than neutron stars due to their higher-mass strange quarks. Being smaller and more compact as a result, they’d rotate faster and be able to keep themselves together.
But the real test could be finding a SQM planet alongside. If a planet gets too close to a star, gravitational tidal forces often tear it apart. This also applies to pulsars. Anything made of normal matter within a pulsar’s ‘tidal disruption region’ wouldn’t survive.
Huang says: “If observers could find a pulsar-exoplanet system with the planet being very close–in, i.e. at a distance of less than 400,000 km from its host, then we can definitely say the planet must be a SQM object.” He adds: “In this case, there can be no other explanation for it.”
Although two confirmed pulsar exoplanets are known, Huang appeals to radio astronomers for more close-in searches, focusing on tidal disruption regions. Upcoming large-scale projects such as the Square Kilometre Array (SKA) and China’s Five Hundred Metre Aperture Telescope (FAST) could provide the required breakthrough.
Huang says: “SKA will be a powerful tool for studying pulsars, including revealing exoplanets around pulsars and in China, FAST has just been constructed. This telescope will also spend a significant portion of time observing pulsars. We are looking forward to new, breakthrough discoveries.”
(Kulvinder Singh Chadha is a UK-based freelance science writer and a former assistant editor of Astronomy Now magazine. He has an astrophysics degree from the University of Hertfordshire.)