Itai Linial, Eliot Quataert
8 min
What is the observational signature and physical evolution of a star undergoing Roche lobe overflow while on a nearly circular orbit around a supermassive black hole? While traditional tidal disruption events involve stars on highly eccentric orbits, the authors examine a separate evolutionary channel driven by angular momentum loss that brings stars onto circularized, tight orbits.
Using analytical modeling of stellar mass loss, the authors track the orbital evolution of a star as it begins to fill its Roche lobe. They calculate the resulting mass transfer rate and the subsequent super-Eddington accretion onto the central black hole. The paper models the radiative output of the resulting optically thick, radiation-dominated outflows, while also considering the late-time viscous evolution of the stellar debris remaining after the star is fully shredded.
The authors propose that these circular tidal disruption events are distinct from parabolic ones. Key signatures include a decade-long soft X-ray precursor caused by sub-Eddington accretion that builds as the star approaches the point of no return. As the mass transfer rate accelerates into a runaway phase, the system produces a bright, blue optical/UV flare lasting weeks or months, fueled by a powerful, optically thick wind. The authors note that these events might naturally explain some observed luminous fast blue optical transients and potential jetted tidal disruption events, as the tidal deformation of the star prior to disruption could generate the magnetic fields necessary to power jets. They also extend their model to white dwarfs orbiting intermediate-mass black holes, suggesting these could serve as multi-messenger sources in the LISA gravitational wave era.
This work expands our understanding of the variety of nuclear transients occurring in galactic centers. By distinguishing circular tidal disruption events from standard parabolic disruptions, it provides a new theoretical framework for interpreting observations of high-energy phenomena in galactic nuclei and offers potential new sources for future gravitational wave observatories.
We consider Roche lobe overflow (RLO) from a low-mass star on a nearly circular orbit, onto a supermassive black hole (SMBH). If mass transfer is unstable, its rate accelerates in a runaway process, resulting in highly super-Eddington mass accretion rates, accompanied by an optically-thick outflow emanating from the SMBH vicinity. This produces a week-month long, bright optical/Ultraviolet flare, accompanied by a year-decade long X-ray precursor and post-cursor emitted from the accretion flow onto the SMBH. Such ``Circular Tidal Disruption Events (TDEs)" represent a new class of nuclear transients, occurring at up to $1-10\%$ of the canonical parabolic tidal disruption event rate. Near breakup rotation and strong tidal deformation of the star prior to disruption could lead to strong magnetic fields, making circular-TDEs possible progenitors of jetted TDEs. Outflows prior to the final stellar disruption produce a circum-nuclear environment (CNM) with $\sim \rm 10^{-2} \, M_\odot$ at distances of $\sim 0.01-0.1 \, \rm pc$, likely leading to bright radio emission, and also similar to the CNM inferred for jetted TDEs. We discuss broader connections between circular TDEs and other recently identified classes of transients associated with galactic nuclei, such as repeating-TDEs and Quasi-Periodic X-ray Eruptions, as well as possible connections to luminous fast blue optical transients such as AT2018cow. We also discuss observational signatures of the analogous RLO of a white dwarf around an intermediate mass BH, which may be a multi-messenger source in the LISA era.
Alex: Give us one concrete system, so we can attach those stages to something.
Sam: Their illustrative calculation uses a Sun-like star orbiting a black hole with a million times the Sun’s mass. Initially, little gas reaches the hole. As the feeding increases, a bright disk forms and emits soft X-rays.
Alex: What determines when that disk becomes bright enough to matter observationally?
Sam: They organize the evolution around the Eddington limit, the luminosity where outward radiation balances inward gravity. They define a corresponding feeding rate using an assumed radiation efficiency. In their example, the radiatively efficient disk phase begins about nine years before final disruption.
Alex: That gives the long warning. What makes the final flare blue rather than simply producing an even brighter X-ray source?
Sam: As feeding becomes highly super-Eddington, meaning far above that reference rate, the model develops a dense outflow. It is opaque enough to trap radiation near the black hole. The effective emitting surface expands outward, and the escaping emission becomes cooler.
Alex: So a fading X-ray source could accompany increasing total luminosity. The energy is emerging in a different band, not necessarily disappearing.
Sam: That is the predicted transition. Around destruction, optical and ultraviolet emission strengthens while X-rays decline. Later, as the supply drops and the outflow subsides, the inner disk becomes the source of soft X-rays again.
Alex: The star’s loss rate is accelerating toward destruction. What prevents the predicted brightness from increasing without limit?
Sam: Gas cannot move through the disk instantaneously. Its transport timescale, called the viscous time, limits how quickly the black hole receives the stripped material. The authors’ illustrative value is about sixteen days, which sets the characteristic duration near peak.
Alex: Does that bottleneck still allow an unusually high feeding rate, compared with the stable-transfer picture?
Sam: Their runaway model reaches peak feeding rates above roughly a thousand times the Eddington rate. But luminosity rises much more slowly than feeding. In their wind calculation, much of the supplied power ends up as motion of the outflow, not escaping radiation.
Alex: We have several specific predictions now. What is the evidence behind them: observations, simulations, or an analytical construction?
Sam: Primarily an analytical construction, with calculated evolutionary tracks and lightcurves. They combine a prescription for stellar mass loss, disk evolution, and an idealized radiating wind. There is no observed sample demonstrating that this sequence occurs.
Alex: Then the decisive question is whether nature actually permits the runaway. Do the authors treat that as settled?
Sam: No. They discuss a proposal that interaction between the star and its own accretion flow could halt the runaway. Whether that equilibrium exists depends sensitively on uncertain disk structure. This paper explores the opposite limit, where those interactions do not stop it.
Alex: That is more fundamental than uncertainty in peak brightness. If transfer stabilizes, the destructive sequence they calculate need not happen.
Sam: And there are uncertainties even if transfer runs away. The viscous time is uncertain by about a factor of ten. Their wind is also assumed spherical and steady, while the simulations they cite suggest a more angular, hourglass-like flow.
Alex: You mentioned residual orbital eccentricity earlier. Why does that matter for a model described as nearly circular?
Sam: Even small departures from circularity can generate substantial tidal heating. That heating can inflate the star and start overflow farther out than otherwise expected. The authors allow for an earlier onset, but a heated star’s internal structure could also change whether transfer is stable.
Alex: With those caveats, what should observers look for that a normal disruption would not already explain?
Sam: The strongest proposed discriminator is the long soft X-ray precursor, not the blue flare alone. The optical event could resemble an ordinary disruption. The authors estimate circular events could occur at up to one to ten percent of the canonical disruption rate, but that rate is conditional.
Alex: Conditional on stars surviving long enough to reach overflow, as well as on the runaway itself?
Sam: Yes. Encounters with disks from other stellar disruptions might strip or destroy an inspiraling star first. The paper also extends the model to white dwarfs, compact stellar remnants, around smaller black holes. Those events evolve faster and could combine a light signal with detectable gravitational waves.
Alex: Who should read the full document, and where should they start to judge whether this channel matters for their work?
Sam: Researchers studying galactic-center transients, stellar mass transfer, or black hole accretion. Start with “Lightcurves” and the discussion comparing circular and parabolic disruptions. Then read “Inspiral and Mass Transfer” to examine the instability assumption, and the tidal-heating appendix for the stellar-structure caveat.
Alex: And what should everyone else carry away, without treating a model prediction as a confirmed class?
Sam: Earlier X-rays might be the warning of stellar destruction, not a reason to reject the flare.
Alex: That is a history worth checking.