What Happens to a Star that Captures a Primordial Black Hole?

Why It Matters

If PBHs constitute a fraction of dark matter, their capture by stars offers a novel astrophysical probe, linking transient surveys and gravitational‑wave detectors to dark‑matter physics.

Key Takeaways

• Three-body interactions enable primordial black hole capture by stars
• Captured PBH migrate to stellar core, forming “Hawking star.”
• Disk formation triggers either explosive transient or quiet consumption
• Explosive branch produces X‑ray flash, UV burst, possible gamma‑ray burst
• Quiet branch may yield massive remnant and low‑mass gravitational‑wave source

Pulse Analysis

Primordial black holes have long been a speculative candidate for the Universe’s missing mass, but their elusive nature makes direct detection difficult. Formed seconds after the Big Bang from density fluctuations, PBHs could span a vast mass range—from asteroid‑size objects to those comparable to stars. If they exist in sufficient numbers, they would permeate galactic halos, occasionally intersecting stellar systems. Traditional searches focus on microlensing or cosmic‑microwave‑background signatures, yet stars themselves provide a complementary laboratory: a captured PBH would alter the host’s evolution in ways that can be observed across the electromagnetic spectrum and through gravitational waves.

In a new arXiv paper, MIT physicist Ore Gottlieb and collaborators present the first global framework that couples stellar evolution codes with three‑dimensional magnetohydrodynamic simulations of PBH capture. Their results overturn the assumption that direct dynamical friction inside a star can trap a PBH; instead, a three‑body encounter with a planet or binary companion efficiently places the black hole on a star‑crossing orbit that spirals inward. Once the PBH reaches the core, it accretes material and may form an accretion disk. Disk formation is the “point of no return,” dictating whether the star ends in a rapid, multi‑wavelength explosion or a quiet, steady consumption that leaves a massive, low‑mass remnant.

The bifurcation yields two distinct observational pathways. An explosive “Hawking‑star” transient would appear as a brief X‑ray flash, a day‑long UV/blue cooling pulse, and possibly a low‑luminosity gamma‑ray burst, followed by a synchrotron afterglow—signatures that differ from conventional supernovae and could be flagged by time‑domain surveys such as LSST or SVOM. The quieter branch may produce a high‑spin, sub‑solar black hole whose merger with another compact object would generate an anomalous gravitational‑wave signal, offering a direct probe of PBH‑driven stellar death. Detecting either channel would tighten constraints on PBH contributions to dark matter and open a new frontier in multimessenger astrophysics.