A Formation Crisis of Repeating Partial Tidal Disruption Events

TL;DR

This work analyzes the formation channels of repeating partial tidal disruption events (rpTDEs), comparing the traditional loss-cone channel against the Hills mechanism (binary disruption) around supermassive black holes. It derives an SMBH-mass–period bound $M_\bullet,max(T_{obt})$ and an overarching inequality $M_\bullet \\lesssim 4\times 10^6 M_\\odot (T_{obt}/10 \\ { m yr})^{4/9}$ that must hold for rpTDEs in the loss-cone channel, finding that most reported rpTDE candidates violate this limit and suggesting an alternative formation path. The Hills channel can generate observable rpTDEs if a substantial fraction of binaries are hard and if captured stars originate from near-contact binaries; this channel also predicts hypervelocity stars (HVSs) with velocities up to ~$3.6\times 10^3$ km s$^{-1}$. The paper discusses tensions with Milky Way HVS observations and advocates a complete HVS survey to test the Hills scenario, highlighting the need for improved data on binary properties in galactic nuclei.

Abstract

A number of candidate repeating partial tidal disruption events (rpTDEs) have been reported in recent years. If these events are confirmed, the high fraction of observed rpTDEs among all tidal disruption events (TDEs) is in tension with prediction of the loss cone channel. We further point out an inequality $M_\bullet \lesssim 4\times 10^6 M_\odot (T_{\rm obt}/10\ {\rm yr})^{4/9}$ that must be satisfied for rpTDEs of solar type stars in the loss cone channel, where $M_\bullet$ is the central supermassive black hole (SMBH) mass and $T_{\rm obt}$ is the orbital period of the star. However the majority of reported rpTDE candidates potentially violate this inequality, indicating an alternative formation channel. In the commonly invoked Hills mechanism, the captured stars produced by tidal disruption of near-contact binaries can evade this inequality and may be the dominant source of rpTDEs. If the same process operates in the Galactic Center, there should exist a population of hypervelocity stars (HVSs) ejected with velocities as high as $3.6\times 10^3 (M_\bullet/10^6 M_\odot)^{1/6}\ {\rm km\ s}^{-1}$, which however have not been detected. A complete search for HVSs in the Milky Way will be critical for testing this prediction.

Figures (9)

• Figure 1: Schematic plot showing the formation of full TDEs, non-repeating pTDEs, rpTDEs and stellar EMRIs in a nuclear stellar cluster around a SMBH with $M_\bullet=10^6 M_\odot$, where $r_{\rm p}$ and $a$ are the pericenter distance and semi-major axis of the stellar orbit, respectively. The line $t_J=T_{\rm obt}$ marks the boundary between the full and empty loss cone regimes. The region on the right-hand side of the black diamond ($\hbox{o}rigin=c]{45}{$$\blacksquare$$}$) dot, $a > a_{\rm full}$ (see Eq. \ref{['eq:full_empty']}) is where full TDEs and non-repeating pTDEs are expected to occur. The line $t_J=t_{\rm GW}$ marks the boundary between GW emission and 2-body scattering dominated regimes. The region between the black square ($\blacksquare$) and the black bullet ($\medbullet$) dots, $a_{\rm GW}(r_{\rm p}=2r_{\rm t}) < a < a(T_{\rm obt}=10\ {\rm yr})$ (see Eq. \ref{['eq:a_JGW']}), is where observable rpTDEs (with period less than 10 years) are expected to occur.
• Figure 2: Fraction of rpTDEs $f_{< T_{\rm obt}}(M_\bullet)$ among all star disruptions as a function of SMBH mass for $T_{\rm obt}\in\{1, 3, 10\}$ yr obtained from Eqs. (\ref{['eq:f_rpTDE']}-\ref{['eq:f_rpTDE2']}), where we have used $\beta_{\rm rpTDE}=0.5$.
• Figure 3: Average rpTDE fraction in the loss cone channel is insensitive to the stellar type or the exact loss cone boundary $\beta_{\rm rpTDE}$. The four “diagonal" lines are obtained from Eqs. (\ref{['eq:f_rpTDE2']}, \ref{['eq:f_avg']}). In addition, dynamical tides may drive stars from larger “initial” orbital period $T_{\rm obt, i}$ to shorter orbital period $T_{\rm obt}$, where the stars repeatedly get disrupted and are observed. As a result, dynamical tides increase the rate of observable rpTDEs by a factor of a few (see Section \ref{['subsec:dynamical tides']}). Here the heavy blue line is obtained from the light blue line by assuming $T_{\rm obt}=T_{\rm obt,i}/10$.
• Figure 4: Statistics of rpTDE candidates in the $T_{\rm obt}-M_\bullet$ space (see Table. \ref{['table']}). The shadowed region is forbidden by the loss cone channel, and the boundary is obtained from Eq. (\ref{['eq:M_max']}) assuming solar type stars and different penetration factors of rpTDEs $\beta_{\rm rpTDE}$ .
• Figure 5: A cartoon illustrating the outcomes of binary disruption, leading to rpTDEs and stellar EMRI. In the 1st stage, a stellar binary is randomly scattered by background stars and gets disrupted when the pericenter distance $r_{\rm p}$ approaches $r_{\rm t,b}$. Immediately after the binary disruption, the captured star has a pericenter distance larger than the stellar tidal radius $r_{\rm t}$. In the 2nd stage, the captured star is either (i) randomly scattered by background stars (when $t_J\lesssim t_{\rm GW}$) and finally ends as a rpTDE when it diffuses to the loss cone boundary of rpTDEs $r_{\rm p}\simeq 2r_{\rm t}$, or (ii) driven by gravitational radiation (when $t_{\rm GW}\lesssim t_{J}$) to become a stellar EMRI.