Roche limit and stellar disruption in the Simpson--Visser spacetime

TL;DR

This work analyzes how the Simpson--Visser black-bounce spacetime modifies tidal forces and the Roche limit for extended bodies near black holes. By deriving the geodesic deviation in both static and radially infalling frames, the authors obtain explicit tidal components $K_1$ and $K_2$ that depend on the regularization parameter $a$ and, for infalling observers, on the energy $E$. They formulate the relativistic Roche limit equation $\dfrac{M_* r^5}{R_*^3}-2Mr^2+3Ma^2=0$ and apply it to neutron stars, white dwarfs, and Sun-like stars across stellar-m-mass to supermassive black holes, highlighting regimes where disruption is observable or horizon-hidden. The results show that larger $a$ generally weakens tides and can remove real Roche radii, with observable disruption possible around Sgr A* for some star types but often suppressed for M87*, offering a potential astrophysical probe of black-bounce spacetimes.

Abstract

Due to the tidal forces that a black hole can produce, certain types of compact objects may undergo disruption as they approach the black hole. This disruption point is known as the Roche limit (or Roche radius). In this work, we studied the tidal forces arising from the presence of the Simpson--Visser black bounce. We analyzed the tidal forces both for a static observer and for a radially infalling observer and showed that differences arise depending on the choice of observer. We used the tidal forces together with the stellar binding forces to determine the Roche radius for neutron stars, white dwarfs, and Sun-like stars, and to investigate how the Simpson--Visser regularization affects the tidal disruption of these astrophysical objects. We also examined whether, for astrophysical black holes such as M87* and Sgr~A*, these stellar disruption processes occur inside or outside the event horizon, and thus whether they are observable.

Figures (5)

• Figure 1: Comparison of the radius at which the angular component of the tidal force vanishes for both the static observer and the radially infalling observer as the energy is varied. In this case, we fix the throat radius to $a=0.5M$.
• Figure 2: Comparison of the radius at which the angular component of the tidal force has its minimum value for both the static observer and the radially infalling observer as the energy is varied. In this case, we fix the throat radius to $a=0.5M$.
• Figure 3: In the left panel, we compare the Roche radius of a neutron star for different values of $a$ with the event horizon radius as the black hole mass is varied. In the right panel, we show how the Roche radius behaves as the parameter $a$ is varied for different values of the black hole mass.
• Figure 4: In the left panel, we compare the Roche radius of a white dwarf for different values of $a$ with the event horizon radius as the black hole mass is varied. In the right panel, we show how the Roche radius behaves as the parameter $a$ is varied for different values of the black hole mass.
• Figure 5: In the left panel, we compare the Roche radius of a Sun-like star for different values of $a$ with the event horizon radius as the black hole mass is varied. In the right panel, we show how the Roche radius behaves as the parameter $a$ is varied for different values of the black hole mass.