You Won’t Believe How Small These Neutron Stars Might Be!

When we think of neutron stars, the dense remnants of massive stars, we imagine objects so compact that a sugar cube of their material would weigh billions of tons. Typically, these stars have a mass range of 1.4 to 2.0 solar masses, confined within a radius of just about 10 kilometers. However, a new study challenges these well-established limits, suggesting the possible existence of neutron stars with masses below that of a white dwarf—a concept that defies long-standing astrophysical models. This groundbreaking research opens up new avenues for understanding stellar evolution and the very physics of matter under extreme conditions.

What Are Neutron Stars and Their Known Limits?

Neutron stars are the remnants of massive stars that have undergone a supernova explosion. These objects are composed almost entirely of neutrons, held together by gravity and supported against collapse by neutron degeneracy pressure. Traditionally, the mass of a neutron star falls between 1.4 solar masses—the Chandrasekhar Limit for white dwarfs—and around 2.17 solar masses, beyond which they collapse into black holes.

The Tolman–Oppenheimer–Volkoff (TOV) equation of state governs the behavior of neutron star matter, determining the mass limits based on the density and pressure of neutron matter. According to these models, neutron stars below 1.4 solar masses should not exist; such a star would collapse into a white dwarf instead. However, the recent study reveals that unique conditions during a supernova explosion might compress the stellar core rapidly enough to form a stable neutron star with a mass as low as 0.4 solar masses.

The Hypothesis: Could Low-Mass Neutron Stars Exist?

The existence of low-mass neutron stars hinges on how neutron matter behaves under extreme pressure and density. During a supernova explosion, the collapsing core could experience forces sufficient to create neutron matter without accumulating the mass typically required for such a transformation. If this process is rapid and intense, it could stabilize a neutron core with less than 1.4 solar masses.

The study explored these possibilities by tweaking the parameters of the TOV equation to reflect extreme conditions. Researchers simulated scenarios where neutron stars with masses as low as 0.4 solar masses could form and maintain stability. If confirmed, these findings would not only challenge existing theories but also refine the parameters of neutron star equations of state, contributing to a deeper understanding of stellar physics.

The Role of Gravitational Wave Observatories

Detecting low-mass neutron stars presents a significant observational challenge. Gravitational wave observatories like LIGO and Virgo are crucial in this effort. These instruments detect ripples in spacetime caused by cataclysmic events, such as neutron star mergers or collisions between neutron stars and black holes. These mergers produce distinct gravitational wave signatures, or “chirps,” that reveal the masses and properties of the colliding objects.

In their search for low-mass neutron stars, researchers simulated how tidal forces would deform these stars during a merger. These deformations would alter the gravitational chirp, providing a unique signature for low-mass neutron stars. While the study did not detect such signatures in the third observation run, it placed constraints on the frequency of these mergers, limiting them to fewer than 2,000 observable events per year for stars below 0.7 solar masses.

Why This Discovery Matters

The potential discovery of low-mass neutron stars could revolutionize our understanding of stellar evolution and the extreme physics governing compact objects. Such stars would redefine the lower mass limit for neutron stars and challenge the Chandrasekhar Limit, which has been a cornerstone of astrophysics for nearly a century. Moreover, these findings would have profound implications for the study of matter under extreme conditions, providing insights into the behavior of neutron-rich matter.

Understanding low-mass neutron stars could also enhance our knowledge of gravitational wave signals. These stars could serve as new laboratories for testing the limits of general relativity and the behavior of spacetime under intense gravitational forces.

The Future of Gravitational Wave Astronomy

As gravitational wave observatories become more sensitive, the chances of detecting low-mass neutron stars increase significantly. Upcoming advancements in technology will allow astronomers to detect fainter signals and distinguish them from background noise more effectively. Future missions, such as the proposed Einstein Telescope and LISA (Laser Interferometer Space Antenna), will expand the range of detectable gravitational wave sources, potentially uncovering elusive low-mass neutron stars.

These advancements will also refine our understanding of tidal deformations and gravitational wave signatures, enabling more precise modeling of neutron star mergers. This progress could confirm the existence of low-mass neutron stars or establish their nonexistence, thereby constraining the parameters of the TOV equation.

Implications for Astrophysics

The implications of low-mass neutron stars extend beyond stellar evolution. They could provide critical clues about the formation and evolution of compact objects in binary systems, the dynamics of supernova explosions, and the distribution of neutron stars in the galaxy. Additionally, these stars could offer insights into the formation of elements in the aftermath of supernovae, contributing to our understanding of nucleosynthesis.

Furthermore, the study of low-mass neutron stars could intersect with other fields of research, such as dark matter and high-energy astrophysics. For example, their unique properties might shed light on the behavior of matter at densities beyond those found in atomic nuclei, offering a glimpse into the fundamental forces shaping the universe.

Conclusion: Expanding the Horizons of Stellar Physics

The possibility of low-mass neutron stars represents an exciting frontier in astrophysics. While current observations have yet to confirm their existence, the groundwork laid by recent studies and advancements in gravitational wave astronomy paves the way for future discoveries. These stars could redefine our understanding of neutron star physics, challenge long-standing astrophysical models, and open new avenues for exploring the universe’s most extreme environments.

Reference:

A Search for Low-Mass Neutron Stars in the Third Observing Run of Advanced LIGO and Virgo