Theory 03/Physics, Cosmos & Matter /Stellar astrophysics
The Mass Gap Is a Helium Fraction
The gap is drawn on total mass. The instability may care about what is in the core.
The pair-instability mass gap in black hole masses is set by the helium core mass fraction rather than by total mass, so stars with unusual envelopes can produce remnants inside the gap.
01The evidence
The observed black hole mass distribution shows a deficit between roughly sixty and one hundred twenty solar masses, consistent with pair instability.
02The tension
The gap is modelled as a function of total mass and metallicity. That predicts a clean edge, and a few candidates sit inside it.
03The mechanism
Pair instability depends on the temperature and density reached in the helium core, which is set by the core mass rather than by the total. A star that loses its envelope retains a core mass that can fall outside the unstable range.
That makes the gap porous to stripped stars, which predicts that gap occupants should be progenitors with strong mass loss or binary stripping.
04What this is not
Those models use total mass. This uses core fraction, which predicts gaps occupants among stripped stars.
05Predictions
- P1Gap occupants should be stripped.
- P2Mass loss should open the gap.
- P3Metallicity should shift the boundary through the core.
06Refutation
Gap occupants showing unstripped envelopes would favour a total-mass boundary.
07What it changes
It would change how remnant populations are inferred.