Tidal capture and repeating partial tidal disruption events of giant stars

Abstract

When an object is scattered near a supermassive black hole (SMBH), tidal oscillations excited within it reduce its orbital energy, leading to capture by the SMBH. This process, called tidal capture, can also occur when the object approaches even closer to the SMBH, resulting in a partial tidal disruption event (pTDE). Previous studies on pTDEs of main-sequence stars have shown that as the disruption intensifies, dynamical effects dominate over tidal oscillations, causing the remnant material to acquire a kick velocity instead of being captured by the SMBH. In this work, we performed hydrodynamic numerical simulations of pTDEs involving giant stars. We found that for weaker disruptions, the dynamical behavior of the remnant material resembles that of main-sequence stars. However, as the disruptions deepen, the remnant material transitions from gaining energy to losing energy, leading to capture by the SMBH. This behavior markedly differs from that of main-sequence stars, demonstrating that the presence of a compact core significantly influences the dynamical processes in pTDEs. Our simulations reveal that the energy change of the remnant material strongly correlates with asymmetric mass -- lossspecifically, the difference in mass outflow between the Lagrange points L1 and L2. This suggests that the energy change stems from asymmetric mass loss, consistent with conclusions from previous studies on main-sequence stars. However, quantitative analysis contradicts earlier models, indicating that the dynamical model of pTDEs requires further refinement. Finally, we discuss the characteristics of repeating pTDEs produced by this process and their potential observability, as well as the implications for the long-term orbital evolution of high eccentricity extreme mass ratio inspiral systems.

Figures (9)

• Figure 1: Specific orbital energy evolution with different $\beta$ for profile RG2. The specific orbital energy $\epsilon_c$ and time are normalized to $GM_{\star}/R_{\star}$ and $t_{\mathrm{dyn}}=\sqrt{R_{\star}^3/GM_{\star}}$, respectively. Curves of different colors represent simulations with different $\beta$.
• Figure 2: Final specific orbital energy $\epsilon_c$ with different $\beta$. Upper and lower panel correspond to $\epsilon_c$ normalized to $GM_{\star}/R_{\star}$ and $GM_{\odot}/R_{\odot}$, respectively. The zoom in panel show results of $\beta\lesssim 1$. In the zoomed panel in the upper panel, the red and blue dashed lines represent main-sequence star (MS) disruptions for $\gamma=5/3$ and $\gamma=4/3$ cases by 2024ApJ...977...80C, respectively (see Text).
• Figure 3: Evolution of the asymmetric mass loss and the specific orbital energy of the remnant material for RG2 with $\beta=3$. The red and blue lines represent the asymmetric mass loss and the specific orbital energy of the remnant core normalized to $GM_{\star}/R_{\star}$, respectively.
• Figure 4: Relationship between the averaged specific orbital energy and the asymmetric mass loss. The symbol colors correspond to different values of $\beta$, matching the scheme in Fig. \ref{['fig: epsilon_vs_time']}. The solid red line represents the best linear fit for cases where $\beta\le 1$, with the red shaded area indicating the 95% confidence interval. Note that the averaged specific orbital energy is normalized to $GM_{\star}/R_{\star}$
• Figure 5: Dependence of $\epsilon_cM_{\star,rem}/(\Delta m_1 -\Delta m_2)$ on $\beta$ in our simulations, where $\epsilon_c$ is normalized to $GM_{\star}/R_{\star}$. The blue solid line and shaded region represent the best-fit linear relationship between $\epsilon_c M_{\star}/(\Delta m_1-\Delta m_2)$ and $\beta$ in logarithmic coordinates and the corresponding 95% confidence interval, respectively. The red solid line and shaded region show the equivalent relationship for $\epsilon_c M_{rem}/(\Delta m_1-\Delta m_2)$.