Because of their compactness, neutron stars have an enormous gravitational pull around a billion times stronger than the Earth. This squashes every feature on the surface to miniscule dimensions, and means that the stellar remnant is an almost perfect sphere. Whilst they are billions of times smaller than on Earth, these deformations from a perfect sphere are nevertheless known as mountains. Past studies have suggested that neutron stars can sustain deviations from a perfect sphere of up to a few parts in one million, implying the mountains could be as large as a few centimeters. Those calculations assumed the neutron star was strained in such a way that the crust was close to breaking at every point. However, the new model indicates that such conditions are not physically realistic.
An artist’s impression of a neutron star. Image credit: Sci-News.com.
“Over the past few years, we have enjoyed a wide variety of gravitational-wave detections of compact binary coalescences,” said Fabian Gittins, a Ph.D. student at the University of Southampton, and his colleagues.
“However, the wait continues for the first observation of a rotating neutron star via gravitational waves and, so far, only upper limits on the size of the involved deformations have been obtained.”
“For these reasons, the maximum quadrupole deformation (or mountain) that a neutron star can sustain is of great interest.”
In the new research, the scientists used computational modeling to build realistic neutron stars and subject them to a range of mathematical forces to identify how the mountains are created.
They also studied the role of the ultra-dense nuclear matter in supporting the mountains.
They found that the largest mountains produced were only a fraction of a millimeter tall, one hundred times smaller than previous estimates.
“These results show how neutron stars truly are remarkably spherical objects,” Gittins said.
“Additionally, they suggest that observing gravitational waves from rotating neutron stars may be even more challenging than previously thought.”
“Although they are single objects, due to their intense gravitation, spinning neutron stars with slight deformations should produce ripples in the fabric of spacetime known as gravitational waves.”
“Gravitational waves from rotations of single neutron stars have yet to be observed, although future advances in extremely sensitive detectors such as advanced LIGO and Virgo may hold the key to probing these unique objects.”
Gittins and co-authors presented their research today at the Royal Astronomical Society’s National Astronomy Meeting 2021 (NAM 2021).
Fabian Gittins et al. Modelling neutron-star mountains. NAM 2021