Highlights of Recent Research

What’s found

We construct models of supermassive stars in hydrostatic and thermal equilibrium. Their interiors contain a hot, nuclear-burning core surrounded by an envelope whose structure depends strongly on metallicity. Enhanced opacity can inflate the envelope, allowing metal-rich models to reach surface temperatures near 7,000 K.

The maximum stable mass is of the order $10^6 M_\odot$ (depending on core rotation). As the stars exhaust hydrogen on a timescale of the order 1 Myr, they can become unstable under general relativity and collapse into massive black holes.

Why it matters

This interpretation could connect little red dots to the formation of supermassive black holes. Cool stellar photospheres naturally produce observed rest-frame optical emission, while the predicted maximum luminosity is comparable to the observed bright-end luminosity function cutoff.

The strongest support for this model comes from the abundance of little red dots. Combined with a supermassive star’s nuclear lifetime of ~1 Myr, their observed number density predicts the number density of present-day supermassive black holes to within a factor of order unity, assuming that each little red dot leaves a black hole retaining most of the stellar mass.

Testing this picture requires more detailed calculations of the emergent spectra, especially the Balmer features and effects of departures from local thermodynamic equilibrium.

What’s found

We model a super-Eddington disk wind together with irradiation of the outer disk. Fits to Hubble photometry allow accretors from a $1.4M_\odot$ neutron star to a $\sim100M_\odot$ black hole. The wind’s color temperature depends weakly on mass, making these models difficult to distinguish in the UV/optical.

Why it matters

The plateau alone does not establish an unusually massive black hole. Infrared free-free emission and recombination lines offer tests of accretor mass and wind composition. The inferred disk properties are consistent with disruption of or merger with a companion star.

Earlier in Tsuna & Lu (2025), we propose a micro-tidal disruption event following a stripped-envelope supernova: a natal kick sends the newborn neutron star or black hole into its companion, disrupting it. Accretion of the stellar debris drives a powerful wind that collides with the supernova ejecta, providing a possible engine for LFBOTs.

For a broader introduction, our review Ho & Lu (2026) summarizes LFBOT observations, proposed origins, and promising ways to distinguish the models.

What’s found

We model repeated black-hole perturbations together with tidal interactions between the two stars. The perturbations can drive the binary to highly eccentric orbits, bringing the stars within a few stellar radii of each other. Their oscillations then grow chaotically, draining energy from the orbit and rapidly shrinking the binary.

In the modeled regime where binaries approach the tidal breakup boundary gradually (so-called “empty loss-cone”), up to half of initially wide binaries become close binaries instead of being disrupted while still wide. Their separations can shrink to less than ~10 stellar radii.

Why it matters

This mechanism creates close binaries in galactic nuclei and provides a channel for producing the fastest hypervelocity stars when the tightened binaries eventually break apart. It also connects binary evolution near massive black holes to repeating partial tidal disruptions and quasi-periodic eruptions. The fraction that undergoes this process depends on the initial binary properties and orbital environment.

What’s found

We simulate a short gamma-ray burst jet propagating through merger ejecta and follow the surrounding cocoon to the radius where photons diffuse out. Its cooling produces soft X-ray flashes lasting a few to ten seconds for off-axis observers. In our canonical model, Einstein Probe could detect a GW170817-like event at viewing angles up to 45 degrees, and events with smaller viewing angles of 10 degrees can be detected up to a distance of 1 Gpc.

Why it matters

These flashes could trigger rapid follow-up even without a detectable gamma-ray burst. Subsequent kilonova and jet-afterglow observations would test the merger interpretation. This is a predicted counterpart whose detectability depends on the jet power, ejecta density profile, and viewing angle.

This work extends our studies of jet-driven cocoons in stellar explosions. In Zheng & Lu (2026), we show that cooling cocoons from long gamma-ray burst jets viewed off axis can produce fast X-ray transients without detectable gamma-ray counterparts. In Zheng et al. (2025), we model EP240414a as an off-axis jet–cocoon system, connecting its X-ray flash, optical emission, and radio afterglow to a stellar explosion.

What’s found

We simulate a supernova in a close binary and show that the newborn neutron star can capture gas from its shock-inflated companion. The captured gas forms a disk. During a brief “propeller” phase, the rotating magnetosphere expels disk material and loses angular momentum, rapidly slowing the neutron star.

Our models produce both ordinary pulsars and ultra-long-period neutron stars. The longest periods require strong initial magnetic fields, typically above $10^{14}$ G.

Why it matters

This channel links binary stellar evolution to ultra-long-period pulsars. It offers a formation mechanism for slowly rotating neutron stars, while the origin of their radio emission remains a separate question.