The secular tidal disruption of stars by low-mass Super Massive Black Holes secondaries in galactic nuclei

Résumé : Stars passing too close to a super massive black hole (SMBH) can be produce tidal disruption events (TDEs). Since the resulting stellar debris can produce a strong electromagnetic flare, TDEs are believed to probe the presence of single SMBHs in galactic nuclei, which otherwise remain dark. A few TDE candidates have so far been observed. In this paper, we show how stars orbiting an IMBH secondary are perturbed by an SMBH primary by means of high-precision direct N-body simulations. We find that the evolution of the stellar orbits are severely affected by the primary SMBH due to secular effects. As a result, those regions of parameter space corresponding to large stellar orbital inclinations with respect to the SMBH-IMBH orbital plane are depleted. Stars orbiting with high inclinations end their lives as TDEs due to Kozai-Lidov oscillations, hence illuminating the secondary SMBH/IMBH. Above a critical SMBH mass of $\approx 1.15 \times 10^8$ M$_{\odot}$, no TDE event can occur for typical stars in an old stellar population since the Schwarzschild radius exceeds the tidal disruption radius. Consequently, any TDEs due to such massive SMBHs will remain dark. It follows that no TDEs should be observed in galaxies more massive than $\approx 4.15\times 10^{10}$ M$_{\odot}$, unless a lower-mass secondary SMBH or IMBH is also present. The secular mechanism for producing TDEs considered here therefore offers a useful probe of SMBH-SMBH/IMBH binarity in the most massive galaxies. We further show that the TDE rate can be as high as $\approx 10^{-2}-10^{-1}$ yr$^{-1}$, and that most TDEs occur on a timescale $\approx 0.5$ Myr. Finally, we show that stars may be ejected with velocities up to thousands of km s$^{-1}$, which could contribute to the observed population of Galactic hypervelocity stars.