Earthquakes below and starquakes above? The two coincide with each other

Author: Wang Weiyang (School of Astronomy and Space Science, University of Chinese Academy of Sciences)

As we all know, the solid earth itself is rigid. When the gravitational load reaches a certain critical value, it will break and cause earthquake disasters. Similarly, when the internal stress accumulation of some solid planets with a mass equivalent to that of the sun exceeds its tolerance limit, starquakes will naturally occur. Interestingly, current research believes that the causes of many extreme astrophysical events (including pulsar period jumps, fast radio bursts, gamma-ray bursts, neutron star explosions or flares, etc.) are likely to be closely related to such starquake events. Recently, the journal Science China: Physics, Mechanics and Astronomy published an article reviewing the progress of related research [1].

Everyday matter condenses its constituent units (i.e. atoms/molecules) through electromagnetic interactions and exists in liquid or solid forms; when the interactions between units are negligible, it appears in a gaseous state. It is generally believed that the nucleus of an atom is a liquid that is composed of nucleons (a general term for protons and neutrons) but is condensed by strong forces. However, if the formation and evolution of massive stars are considered, a dense celestial body with "atomic nuclei connected together" will be formed under the dominant effect of self-gravity. This type of celestial body has a partial or entire solid structure and often triggers stellar earthquake processes. Undoubtedly, mature solid geophysics theories and methods will find their place in the study of this type of stellar earthquakes and become "stones from other mountains".

The state of dense matter under strong gravitational fields is one of the focuses of current physics and astronomy, involving the low-energy behavior of fundamental strong interactions. When a massive main sequence star exhausts its nuclear energy, a celestial body composed of this type of dense matter will be born in its core region. If the mass of the dense celestial body is not large enough and its self-gravity is not strong enough to collapse into a black hole, all the atomic nuclei in the core of the star may be squeezed together and appear as what Landau called a "giant nucleus", that is, the observed pulsar.

Limited to the knowledge of low-energy strong interactions, although the nature of this type of extremely dense matter has not yet been determined, there is clear observational evidence that a wealth of extreme astrophysical events are closely related to giant nuclei. Similar to the Earth, pulsars often have a partial or even entire solid structure; when the internal stress accumulation reaches a certain critical value, starquakes will naturally occur. The difference is that pulsars have higher gravity, density and magnetic field than the Earth, so starquake activity will induce much more intense celestial activities. Here we call for the promotion of the cross-integration of seismology and astrophysics, and look forward to the ultimate revelation of the nature of giant nuclei with the help of higher-precision multi-messenger astronomical observations, thereby deepening the understanding of low-energy strong forces.

Figure 1. Triangle of light flavor quarks (u, d, and s) [1]. The grayscale in the triangle represents the charge-to-mass ratio R of the baryonic component. The nucleus is near point A, where R ≃ 1/2; the baryonic components inside neutron stars (n-point) and strange stars (s-point) are nearly electrically neutral. The basic unit located at point s, similar to the nucleus at point A, is called a strangeon.

In early 1931, Landau had a conversation with Bohr and Rosenfeld, and mentioned an interesting topic [2]: gravitational collapse causes atomic nuclei to squeeze together and eventually form a "giant nucleus", where electrons will be tightly bound to protons to avoid obtaining too much kinetic energy. This giant nucleus later became popular with the term "neutron star" and was considered to be the essence of "pulsar" (discovered in 1967). However, for giant nuclei, within the scope of the first generation of quarks, electrical neutrality and quark flavor symmetry cannot be achieved at the same time. With the development and success of the standard model of particle physics since the 1960s, strangeness has gradually received attention. The introduction of this degree of freedom can make giant nuclei have the best of both worlds: maintaining quark flavor symmetry and electrical neutrality at the same time, but the number of valence quark flavors needs to be extended from "2" to "3". We call the unit inside the giant nucleus, which is similar to the nucleon in the atomic nucleus but has a non-zero strangeness number, a "strangeon" (Figure 1). Admittedly, it is still unclear whether pulsars are traditional neutron stars or strangen stars.

The development of modern seismology originated in the early 20th century. A very famous earthquake example is the 1906 San Francisco earthquake (Figure 2). After this earthquake, people found that the earthquake produced huge cracks that stretched for hundreds of kilometers, and believed that the earthquake was caused by dislocations on the fault. After the San Francisco earthquake, the academic community [3] proposed the elastic rebound theory. This theory holds that: tectonic stress continuously loads the fault, and when the loading stress reaches a large enough level, the fault plane breaks due to instability. The vibration generated by the rupture propagates in the surrounding medium (seismic waves) and causes disasters. After the earthquake rupture, the media on both sides of the fault continue to be loaded by tectonic stress. After an earthquake cycle, the rupture of the next earthquake is formed. The earthquake cycle repeats itself, and after thousands of earthquake cycles, the fault forms the complex shape we see today.

(a) Surface rupture caused by the 1906 San Francisco earthquake. (b) Surface rupture caused by the 2016 Kaikoura earthquake in New Zealand. (c) Schematic diagram of the elastic rebound model [1].

Similar to earthquakes, starquakes also lead to the release of energy and a sudden change in the moment of inertia, which will manifest as a jump in the rotation period of the pulsar (glitch), which is believed to be related to the energy release of the shell [4], and solid strange stars are more conducive to explaining the amplitude and interval of period jumps than neutron stars with only solid shells. On December 11, 2021, a similar precursor radiation was detected for the short gamma-ray burst GRB 211211A, and quasi-periodic oscillations (QPO) were also observed in the precursor radiation [5].

By using this stellar earthquake model and combining it with the characteristic frequencies of some oscillation modes of solid-state strangelet stars, we can use the strangelet star-black hole merger process to understand the total energy release of the precursor radiation and the frequency of quasi-periodic oscillations [6]. Similar oscillation modes are also the main research content of global seismology. In addition, in many statistical studies of fast radio bursts, similar statistical relationships between the frequency and number of earthquakes have been found (such as the Gutenberg-Richter law and the Omori law [7]), indicating that stellar earthquakes of compact stars are very likely to trigger FRBs, accompanied by phenomena such as periodic jumps and quasi-periodic oscillations of compact stars.

The asteroseismic process of compact celestial bodies shows various details similar to those of earthquake activities, which makes people suspect that there is a close connection between the two. It is true that the discussion in this article is highly speculative, but in order to better compare the two, we need to collect more information from the universe. The new generation of astronomical equipment may bring us surprises and promote asteroseismological research. We expect and believe that with the accumulation of more and more observational phenomena, the asteroseismic model based on earthquake theory will be further improved and developed in understanding extreme astronomical events.

【References】

Lu Ruipeng, Gao Yong, Hu Yan, et al. From earthquakes to starquakes. Science in China: Physics, Mechanics and Astronomy, 2024, 54(8): 289501.

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Zhou EP, Gao Y, Zhou YR, et al. The precursor of GRB211211A: a tide-induced giant quake? RAA, 2024, 24, 025019.

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