The fastest-spinning white dwarf ever discovered is a shrinking cosmic vampire feasting on a stellar companion. A feeding process is pushing the dead star toward an imminent supernova explosion.
The fastest-spinning 'vampire star' we know of is shrinking. Soon, it will explode : Read more
Interesting, this will be a good candidate for testing hypothesis on what compact bodies do once they reach the Chandrasekhar limit.
1.
The Chandrasekhar Limit and Type Ia Supernovae
The Chandrasekhar limit (~1.4 solar masses for a non-rotating, electron-degenerate white dwarf) represents the maximum mass a white dwarf can sustain against gravitational collapse due to electron degeneracy pressure. When a white dwarf in a binary system accretes mass from a companion star and approaches this limit, the central density and temperature increase, leading to carbon-oxygen fusion and a runaway thermonuclear explosion—a Type Ia supernova.
This scenario is well-supported by observational evidence, such as the uniformity in the light curves of Type Ia supernovae, making them standard candles for cosmological distance measurements.
2.
Collapse to a Neutron Star Without a Supernova
There is a theory that a white dwarf could collapse directly into a neutron star without producing a typical Type Ia supernova is a fascinating alternative. This may occur under specific circumstances -
- Absence of a Thermonuclear Explosion - If the conditions for ignition of carbon fusion are not met (e.g., due to rapid heat dissipation or incomplete accretion), the star could undergo direct gravitational collapse.
- Electron Capture - In certain white dwarfs with an oxygen-neon-magnesium composition, electron capture on nuclei (e.g., e−+Mg→Na+νee^- + Mg > Na + \nu_ee−+Mg→Na+νe) might trigger core collapse before carbon ignition. This is thought to result in an "electron-capture supernova" or a quieter gravitational collapse to a neutron star.
- Rotational Effects - A rapidly spinning white dwarf might delay collapse by supporting itself with rotational kinetic energy. If it loses angular momentum abruptly (e.g., through magnetic braking), it could collapse directly without a dramatic explosion.
These scenarios might produce less luminous transients, or even none at all, making them challenging to observe.
3.
Exceeding the Chandrasekhar Limit Through Rapid Rotation
Rapid rotation can indeed allow a white dwarf to exceed the traditional Chandrasekhar limit. Here's why:
- Centrifugal Support - A spinning white dwarf experiences an outward centrifugal force that counteracts gravity, effectively increasing the mass it can support before collapsing.
- Magnetic Fields and Angular Momentum Transport - Over time, magnetic interactions or gravitational wave emission can redistribute or dissipate angular momentum, leading to eventual collapse.
In some cases, the collapse might be delayed long enough for other astrophysical processes to intervene, such as mass loss through a nova-like event or the shedding of angular momentum. This delay could influence the nature of the subsequent explosion or collapse.
Key Observational Challenges
- Detection of Non-Luminous Collapses - Direct collapses to neutron stars may produce weak or no electromagnetic signals, making them difficult to identify observationally.
- Variations in Type Ia Light Curves - There is ongoing research into "peculiar" Type Ia supernovae, which might hint at variations in progenitor systems, including rotational effects or direct collapses.
- Gravitational Waves - Advanced detectors like LIGO and Virgo may eventually identify signals from angular momentum redistribution in rapidly rotating white dwarfs or from quiet collapses to neutron stars.
While the Type Ia supernova pathway remains the dominant and most observationally supported model, alternative outcomes like direct collapse or rotation-supported over-limit states are active areas of research that could expand our understanding of stellar evolution and compact objects. Observational advances in transient astrophysics, gravitational wave astronomy, and multi-messenger observations are key to distinguishing these scenarios.
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