How stars survive repeated encounters with black holes
Scientists unravel the survival mechanism of stars in repeated interactions with black holes
1The process of stars being destroyed by black holes
Supermassive black holes can have millions of times the mass of the sun and create incredibly strong gravitational forces. When a star gets too close, tidal forces can tear it apart. However, some stars survive partial destruction instead of being completely annihilated. This phenomenon is called a 'partial tidal disruption event' (pTDE).
2The secret of repeated encounters
Some stars encounter black holes repeatedly, generating new bursts of light each time. Astronomers call these events 'repeating partial tidal disruption events' (rpTDEs). Wide-field time-domain surveys enable observation of the same star interacting with the same black hole multiple times.
3The mystery of fading flares
About 10 repeating systems have been identified so far, with four showing progressively dimmer flares with each encounter. This contradicted theoretical models, as simulations predicted flares of similar brightness. Researchers at Syracuse University investigated stellar spin rates to solve this puzzle.
4The critical role of stellar spin
The study found that a star's rotation rate before its first encounter determines its behavior in subsequent interactions. Rapidly spinning stars can't be accelerated much in later encounters. This keeps the fallback time of stripped material relatively constant, causing flare brightness to gradually decrease.
5Differences in stellar structure
Low-mass stars have 'fluffier' internal structures, while high-mass stars have dense, onion-like cores. This difference affects how much material is stripped during encounters. Stars with dense cores can lose outer layers while keeping their cores intact.
6The Hills mechanism and binary systems
Researchers explain the existence of rapidly spinning stars through the 'Hills mechanism'. In this scenario, a close binary star system approaches a black hole. The black hole disrupts the system, ejecting one star while capturing the other. Close binaries can become tidally locked, causing rapid rotation.
7Importance for astronomy
These findings help us understand how matter behaves in extreme conditions near black holes. Repeating pTDEs offer unique opportunities to study black hole growth and evolution. They may also provide new insights about sources of gravitational waves.
8Roadmap for future research
The team plans to expand simulations to stars with different masses and spin rates. New telescopes like the Rubin Observatory could detect more rpTDEs to test these theories. These studies will deepen our understanding of how black holes affect stellar systems.