Ejection and Capture Dynamics in Restricted Three-Body Encounters Shiho Kobayashi1, Yanir Hainick2,3, Re’em Sari3,4, and Elena M. Rossi5 2 ABSTRACT 1 0 2 We study the tidal disruption of binaries by a massive point mass (e.g. the n black hole at the Galactic center), and we discuss how the ejection and capture a J preference between unequal-mass binary members depends on which orbit they 3 approach the massive object. We show that the restricted three-body approx- 2 imation provides a simple and clear description of the dynamics. The orbit of ] E a binary with mass m around a massive object M should be almost parabolic H with an eccentricity 1 e < (m/M)1/3 1 for a member to be captured, while h. the other is ejected.| In−de|ed∼, the energy≪change of the members obtained for a p - parabolic orbit can be used to describe non-parabolic cases. If a binary has an o r encounter velocity much larger than (M/m)1/3 times the binary rotationvelocity, t s it would be abruptly disrupted, and the energy change at the encounter can be a [ evaluated in a simple disruption model. We evaluate the probability distribu- 1 tions for the ejection and capture of circular binary members and for the final v energies. In principle, for any hyperbolic (elliptic) orbit, the heavier member 4 9 has more chance to be ejected (captured), because it carries a larger fraction of 7 the orbital energy. However, if the orbital energy is close to zero, the difference 4 1. between the two members becomes small, and there is practically no ejection 0 and capture preference. The preference becomes significant when the orbital en- 2 1 ergy is comparable to the typical energy change at the encounter. We discuss its : v implications to hypervelocity stars and irregular satellites around giant planets. i X Subject headings: binaries: general, Galaxy: kinematics and dynamics, planets r a and satellites: formation, Galaxy: Center, Galaxy: halo, Planets and satellites: individual (Triton) 1Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK 2Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel 3Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel 4Theoretical Astrophysics 350-17,California Institute of Technology, CA 91125, USA 5 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands – 2 – 1. Introduction The disruption of a star by a massive black hole (BH) is one of the most spectacular examples of the tidal phenomena (Komossa & Bade 1999; Donley et al. 2002; Grupe et al. 1995). A star that wanders too close to a massive BH is torn apart by gravitational forces. Almost half the debris would escape on hyperbolic orbits, while the other half would traverse elliptic orbits and returns to periapsis before producing a conspicuous flare (e.g. Rees 1988). The disruption process has been numerically investigated in detail (Evans & Kochanek 1989; Laguna et al. 1993; Ayal et al. 2000; Kobayashi et al. 2004; Guillochon et al. 2009), and the new generation of all-sky surveys are expected to detect many tidal flares (Strubbe & Quataert 2009, Lodat & Rossi 2011). Recently a possible discovery of the onset of the rapid BH accretion has been reported (Burrows et al. 2011; Zauderer et al. 2011; Levan et al. 2011; Bloom et al. 2011). Once astar gets deeply inside the tidal radiusof aBH, the tidal forcedominates over the self-gravity and thermal pressure of the star. A very simplified description of the disruption process could be the encounter between a star cluster (or a cluster of point masses) and a massive BH. The simplest case consists of a binary and a massive BH in which after the tidal disruption, one star would escape to the infinity, while the other could be captured by the BH. This is actually one of the leading models for the formation of hypervelocity stars (Hills 1988; Yu& Tremaine 2003). The captured stars may explain the S-stars in the Galactic center (Gould & Quillen 2003; Ginsburg & Loeb 2006; Ghez et al. 2005; Genzel et al. 2010). Hypervelocity stars are stars with a high velocity exceeding the escape velocity of the Galaxy. After the discovery of such stars in a survey of blue stars within the Galactic halo (Brown et al. 2005; Hirsch et al. 2005; Edelmann et al. 2005), many authors have predicted the properties of thehypervelocity stars (Gualandris et al. 2005; Bromley et al. 2006; Sesana et al. 2007; Peretz et al. 2007; Kenyon et al. 2008; Tutukov & Fedrova 2009; Antonini et al. 2010; Zhang et al. 2011). These investigations so far have used three-body simulations or analytic methods that relied on results from three-body simulations. The six orders of magnitude mass ratio between the Galactic center BH and the binary stars allows us to formulate the problem in the restricted three-body approximation. In a previous paper (Sari et al. 2010, hereafter SKR), we have shown that the approximation is efficient and useful to understand how binary stars behave at the tidal breakup when the binary’s center of mass approaches the BH in a parabolic orbit. In this paper, we generalize the approximation for orbits with arbitrary eccentricity. This enables us to give a complete picture of the ejection and capture process. We also provide the ejection and capture probability distributions that can be simply rescaled in terms of binary masses, their – 3 – initial separation and the binary-to-black hole mass ratio when applied to a specific system. Our method is computationally more efficient than full three-body simulations, and it is easier to grasp the nature of the tidal interaction. In 2, we outline the restricted three-body approximation. In 3, we evaluate how much § § energy each member gains or loses at the tidal encounter and we discuss how the energy change evaluated for a parabolic orbit can be used to study non-parabolic orbit cases, and in 4, we give qualitative discussion on the ejection and capture preferences. In 5, we study § § high velocity encounters. In 6, the numerical results are discussed. In 7, we use our results § § to describe the capture process of Triton around Neptune. Finally, in 8, we summarize the § results. 2. The Restricted Three-Body Problem The equation of motion for each of the binary members is given by GM Gm 2 ¨r = r + (r r ), (1) 1 − r3 1 r r 3 2 − 1 1 1 2 | − | GM Gm 1 ¨r = r (r r ), (2) 2 − r3 2 − r r 3 2 − 1 2 1 2 | − | where r and r are the respective distance from the massive point mass with M. We will 1 2 call the point mass the black hole (BH), though the binary is assumed to travel well outside the event horizon and our results can be applied to any systems which include a Newtonian massive point mass. The equation for the distance between the two r r r is 2 1 ≡ − GM GM Gm ¨r = r + r r. (3) − r3 2 r3 1 − r3 2 1 where m = m +m M. We assume that the two masses are much closer to each other, 1 2 ≪ and to the trajectory of the center of mass of the binary r , than each of them to the BH. m Both energy and orbit obtained under the approximation are fairly accurate except a part of the orbit just around the periapsis passage (see SKR for the details). Linearizing the first two terms of equation (3) around the center of mass orbit r , we m find that the zero orders cancel out. Then, rescaling the distance between the bodies by (m/M)1/3r and the time by r3/GM where r is the distance of the closest approach p p p between the center of mass of thqe binary and the BH, we can re-write eq. (3) in terms of the dimensionless variables: η (M/m)1/3(r/r ) and t: p ≡ r 3 η η¨ = p [ η +3(ηˆr )ˆr ] , (4) r − m m − η 3 (cid:18) m(cid:19) | | – 4 – where ˆr is a unit vector pointing the center of mass of the binary. We define the orbit of the m center of mass to be a conic orbit r /r = (1+e)/(1+ecosf) where e is the eccentricity, and m p thetrueanomalyf istheanglefromthepointofclosestapproach. Sinceˆr = (cosf,sinf,0), m and we set η = (x,y,z), explicit equations in terms of dimensionless Cartesian coordinates reads (1+ecosf)3 x x¨ = [ x+3(xcosf +ysinf)cosf] , (5) (1+e)3 − − (x2 +y2 +z2)3/2 (1+ecosf)3 y y¨ = [ y +3(xcosf +ysinf)sinf] , (6) (1+e)3 − − (x2 +y2+z2)3/2 (1+ecosf)3 z z¨ = z . (7) − (1+e)3 − (x2 +y2 +z2)3/2 where the eccentricity 2r E p e = 1+ (8) GMm is related to the energy of the center of mass which is given by m GMm E = r˙ 2 . (9) m 2 | | − r m Using the dimensionless time, the conservation of the angular momentum can be expressed as f˙ = (1+e)−3/2(1+ecosf)2. (10) Analytically one has relations between r and t through a parameter which are given by m (e.g. Landau & Lifshitz 1976) E < 0 r /r = (1 e)−1(1 ecosξ), t = (1 e)−3/2(ξ esinξ), (11) m p − − − − E = 0 r /r = 1+ξ2 , t = √2 ξ +ξ3/3 , (12) m p E > 0 r /r = (e 1)−1(ecoshξ 1), t = (e 1)−3/2(esinhξ ξ), (13) m p (cid:0) (cid:1) (cid:0) (cid:1) − − − − where the closest approach r = r happens at t = 0. m p 3. Energy change at the BH encounter We are interested in the fate of stars in a binary, following its encounter with a massive BH. In order to study the ejection and capture process, we evaluate the energies of the stars asfunctionsoftime. WhenthebinaryisatalargedistancefromtheBH,thebinarymembers rotate around their center of mass which gradually accelerates towards the BH. The specific – 5 – self-gravity energy ofthe binaryis about v2 Gm/a. Analytic arguments (SKR) suggest − 0 ≡ − that at the tidal breakup one member gets additional energy of the order of v v where v is m 0 m thevelocityofthecenterofmassatthetidalradiusr = (M/m)1/3a. Ifthebinaryapproaches t the BH with negligible orbital energy, the velocity is v = (GM/r )1/2 = v (M/m)1/3. m t 0 The additional energy is larger than the self-gravity energy by a factor of (M/m)1/3 1. ≫ Therefore, we will neglect the self-gravity term in the following energy estimates. This treatment is valid as long as the binary is injected into the orbit r at a radius much larger m than the tidal radius. The energy of one binary member m is given by i m GMm E = i r˙ 2 i. (14) i i 2 | | − r i Linearizing the kinetic and potential energy terms around the orbit of the center of mass r m and using the initial energy I (m /m)E, we obtain i i ≡ E = I +∆E , (15) i i i GMm ∆E m r˙ (r˙ r˙ )+ ir (r r ), (16) i ≡ i m i − m r3 m i − m m Since in our limit the total energy of the system is E, considering ∆E = ∆E , we get 2 1 − m m GM 1 2 ∆E = r˙ r˙ + r r . (17) 2 m m r3 m (cid:18) m (cid:19) Using our rescaled variables, the additional energy is given by 1/3 Gm m M ∆E = ∆E = 1 2 ∆E¯, (18) 2 1 − a m (cid:18) (cid:19) x˙ sinf +y˙(e+cosf) (1+ecosf)2 ∆E¯ D−1 − + (xcosf +ysinf) , (19) ≡ √1+e (1+e)2 (cid:20) (cid:21) where D = r /r is the penetration factor which is useful to characterize the tidal encounter. p t Once the binary dissolves, ∆E¯ becomes a constant because the body is eventually moving only under the conservative force of the BH. Hereafter, the energy change ∆E¯ means the constant value after the disruption, otherwise we specify it. The equation of motion (4) indicates that the negative of a solution r = r(t) is also a solution. The energies ∆E are i also linear in the coordinates. Therefore, another binary starting with a phase difference π will have the same additional energy in absolute value but opposite in sign. A uniform distribution in the binary phase implies that, when the binary is disrupted, each body has a 50% chance of gaining energy (and a 50% chance of losing energy). – 6 – As we have discussed, the typical energy change is larger than the self-gravity energy by a factor of (M/m)1/3, it is of order of (Gm m /a)(M/m)1/3. Then, the dimensionless 1 2 ∼ quantity ∆E¯ is an order-of-unity constant after the disruption. Its exact value depends on orbital parameters, but for qualitative discussion we just need to know that ∆E¯ is about unity. Later we will numerically show that ∆E¯ is an order-of-unity in the relevant parameter regime1, and numerical values will be used to estimate the ejection and capture probabilities. Rescaling energies by the typical value of the energy change, the energies of the binary members after the disruption are given by E¯ = I¯ ∆E¯, E¯ = I¯ +∆E¯, (20) 1 1 2 2 − where bar denotes energy scaled by (Gm m /a)(M/m)1/3. An interesting outcome of the 1 2 encounter between a binary system and a massive BH is the “three-body exchange reaction” (Heggie 1975; Hills 1975) where one member of the binary is expelled and its place is taken by the BH, i.e. one binary member is captured by the BH and the other is ejected to infinity. In order for a member m to escape fromthe BH, the initial binding energy should be smaller i than the energy gain: I¯ < ∆E¯ 1. The same condition is required when a member of i | | | | ∼ the binary in a hyperbolic orbit loses energy and it is bound around the BH. Therefore, when we discuss the ejection or capture process associated with a massive BH, the absolute value of the initial energy should be comparable or less than unit: I¯ < 1. i | | ∼ Since the energy, penetration factor (periapsis radius) and eccentricity are related by eq (8) or equivalently: m 1/3 m m e = 1+2DE¯ 1 2 , (21) M m m (cid:16) (cid:17) (cid:16) (cid:17)(cid:16) (cid:17) only two of them are the independent parameters to describe the binary orbit. Considering I¯ < 1togetherwiththemassratio(m/M)1/3 1andthetidaldisruptionconditionD < 1, i | | ∼ ≪ ∼ the eccentricity should be almost unity 1 e < D(m/M)1/3(m /m) for a member m to par i | − | ∼ be ejected or captured where m is the mass of the partner (m = m for i = 1 and m par par 2 1 for i = 2). If we use the semi-major axis r r D/(1 e), the condition can be rewritten a t ≡ − as r /r > (M/m)1/3(m/m ) where r is negative for hyperbolic orbits. a t par a | | ∼ Such orbits differ very little from parabolic orbits with the same periapsis distance, especially around thetidal radiusand inside it. Therefore, the energy change ∆E¯ is expected to be almost identical to that for the parabolic case. As long as we study the exchange 1For prograde orbits with D 0.1, the energy change is as large as ∆E¯ 30 in a very narrow range ∼ ∼ of the binary phase (see figure 7 in SKR) where the binary members once come close to each other before they break up. However,the phase-averagedvalue ∆E¯ is still an order-of-unityand it is a more relevant h| |i quantity for the discussion on the ejection and capture probabilities and the final energies – 7 – reaction, we can approximate ∆E¯ by the parabolic results ∆E¯ . However, e 1 does e=1 ∼ ¯ not necessarily mean E 1. In general, we need to take into account the offset of the | | ≪ final energy due to the non-zero initial energy, which would affect the ejection and capture probabilities. The final energies are approximately given by E¯ = I¯ ∆E¯ , E¯ = I¯ +∆E¯ . (22) 1 1 e=1 2 2 e=1 − 4. Which gets kicked out? We here consider a simple question: Which member is ejected or captured if an unequal- mass binary is tidally disrupted by a massive BH? If E¯ > 0 (hyperbolic orbits), one binary member could lose energy and get captured by the BH, the other flies away with a larger energy. Assuming a uniformdistribution in thebinary phase, each member hasa 50%chance of losing energy (and gaining energy). However, since the lighter one (the secondary) has a smaller initial energy, it is preferentially captured and the heavier one (the primary) has more chance to be ejected. For elliptical orbits, by considering a plausible semi-major axis r , we can obtain tighter a constraints on the eccentricity and energy, compared to the requirements from the exchange reaction. This is particularly relevant for studies of hypervelocity stars. If r is around the a radius of influence of the BH r GM/σ2 where σ is the local stellar velocity dispersion, h ∼ for the Galactic center, it is about r a few parsecs 105r for a several solar radii. a t ∼ ∼ ∼ Then, we get 1 e = D(r /r ) < 10−5 and E¯ (r /r )(M/m)1/3(m/m )(m/m ) 10−3. t a t a 1 2 − ∼ | | ∼ ∼ Our previous estimates based on parabolic orbits are appropriate to study the production of hypervelocity stars for which an equal ejection chance is expected (SKR). When the semi- major axis is as small as r (M/m)1/3(m/m )r , the initial energy I¯ would be of the a par t i ∼ | | order of unity as we have discussed and it affects the ejection preference. Since the secondary has less negative initial energy, it is preferentially ejected. Recently, Antonini et al. (2011) performed N-body simulations of unequal mass binaries with m = 6M , m = 1 or 3M and a = 0.1AU in elliptical orbits around a supermassive 1 ⊙ 2 ⊙ BH M = 4 106M . They find that the initial distance of the binary from the central BH ⊙ × plays a fundamental role in determining which member is ejected: for a large initial distance d = 0.1pcorequivalently r 3 103r , theejectionprobabilityisalmost independent onthe a t ∼ × stellar mass, while for d = 0.01pc or r 3 102r , the lighter star is preferentially ejected. a t ∼ × Considering that the ejection probability significantly decreases if r becomes smaller than a (M/m)1/3(m/m )r 80(m/m )r , these results are consistent with our analysis. par t par t ∼ ∼ These ejection preferences for hyperbolic and elliptic orbits are naturally understood if – 8 – we consider a large mass ratio for the binary members. The energy of the primary practically does not change at the tidal encounter. Whether it is ejected or captured after the tidal breakup simply depends on the initial energy E, while the secondary might have a chance ∼ to make a transition between a bound and unbound orbits around the BH (Bromley et al. 2006). In the large mass ratio limit, the exchange reaction condition (i.e. the transition condition for the secondary) is E < (Gm2/a)(M/m)1/3 or equivalently r > (M/m)2/3a. a | | ∼ | | ∼ 5. High Energy Regime If a binary has a large orbital energy E¯ 1, both members are ejected after the BH ≫ encounter as a binary system or two independent objects 2. Although the high energy regime is not important in the context of the three-body exchange reaction, we discuss the regime to clarify the parameter dependence of the numerical results in the next section. A high orbital energy E¯ (M/m)1/3(m/m )(m/m ) affects the velocity of their center of mass at the 1 2 ≫ encounter v (E/m + GM/r )1/2 = (M/m)1/3v (e 1)/2D+r /r . Then, the tidal m m 0 t m ∼ − disruption radius (i.e. where a binary is disrupted) can be defined in three different ways. p We here order them from a large to small radius. (a) Relative acceleration: the radius at which the BH tidal force becomes comparable to the mutual gravity of the binary. This is r . t (b) Relative velocity: the radius at which the tidal force induces the relative velocity between the binary members comparable to the binary escape velocity v . (c) Relative position: the 0 radiusatwhich thedifferenceinpositionincreases bymorethantheinitial binaryseparation. The duration that the center of mass is around r is of order of ∆t r /v . During m m m ∼ this period, the tidal acceleration of the relative motion of the binary members by the BH is of order of A GMa/r3 . The two radii (b) and (c) can be estimated from two conditions: ∼ m ∆v = A∆t v and ∆x = A∆t2 a, provided that the duration of the encounter is 0 ∼ ∼ comparable or shorter than the binary rotation time-scale: ∆t < a/v . If the energy is high 0 (e 1)/D = 2E¯(m/M)1/3(m /m)(m /m) 1, these conditions∼give r = r D1/4/(e 1)1/4 1 2 m t − ≫ − and r D/(e 1), respectively. Since they should be larger than the periapsis distance, only t − the cases that satisfy D < (e 1)−1/3 for the radius (b) or e < 2 for the radius (c) lead to ∼ − ∼ the disruption. The radius (c) is basically the place at which the orbit of the center of mass makes its turn (i.e. the periapsis). If the energy is low (e 1)/D 1, all the estimates give − ≪ the original tidal radius r . t When we discuss the energy change ∆E at the tidal encounter, there are two important 2 If E¯ is a large negative value, both members are captured after the disruption. Since the velocity at the tidal radius is reduced vm < (M/m)1/3v0, the energy change should be smaller ∆E¯ < 1. ∼ | |∼ – 9 – points which we should emphasize. First, the energy of each of the binary members in the BH frame changes only due to the mutual force between the binary members. Secondly most of the work done by one member on the other, which is ∆E, is done outside the tidal radius r . The mutual force is of order of Gm m /a2. During the binary rotation t 1 2 time-scale a/v , the force acts over a length (v /v )a in the BH frame. Therefore, the 0 m 0 ∼ work is W (m m /m)v v . Since the direction of the mutual force changes with the 1 2 m 0 ∼ binary rotation, ∆E(t) oscillates with the amplitude of W. When the binary is disrupted, ∆E becomes a constant value which is basically determined by the binary phase at the disruption. Then, we might expect that the final value of ∆E is a sinusoidal function of the binary phase for circular binaries. As we will see later, this is actually the case for the high energy encounters. Even with the largest estimate of the tidal radius (i.e. r ), the duration t of the encounter ∆t r /v is shorter than the binary rotation time-scale by a factor of t m ∼ D/(e 1) 1. The work during the encounter is negligible compared to the work W ∼ − ≪ which has been done outside r . On the other hand, in the low energy regime, the duration p t of the encounter is comparable to the binary rotation time-scale. Considering that at the encounter the orbits of the members in the comoving frame of their center of mass should be significantly deformed from the original orbits (e.g. circular orbits) before they finally break up, the work during the encounter could induce deviation of ∆E(φ) from a simple sinusoidal function. However, the typical value is still expected to be about ∆E (m m /m)v v . 1 2 0 m ∼ In both the low and high energy regime, the typical energy change is given in a dimen- sionless form by m 1/3 m m e 1 r ∆E¯ 1+E¯ 1 2 = 1+ − = 1 t , (23) ∼ M m m 2D − 2r r r r a (cid:16) (cid:17) (cid:16) (cid:17)(cid:16) (cid:17) where we have assumed r = r to estimate v . In the high energy regime, the disruption m t m might happen at a smaller radius, but v is determined by the orbital energy and it is m insensitive to the choice of r . When (e 1)/D 1, the energy change becomes much m − ≫ larger than unity. However, the energy gain is not significant compared to the original energy E¯, and one finds that the tidal encounter is not an efficient acceleration process anymore. In the high energy regime, the energy change eq (19) can be evaluated by assuming that a binary is abruptly disrupted at the tidal radius r , since the work during the tidal t encounter is negligible. For a circular coplanar binary: (x,y) = D−1(cosφ ,sinφ ) and t t (x˙,y˙) = D1/2( sinφ ,cosφ ), we obtain t t ± − 1 1+e/cosf ∆E¯ = 1 sinf sinφ + 1 t cosf cosφ (24) t t t t ± D(1+e)! ± D(1+e) ! p p – 10 – where φ is the binary phase at the tidal radius, f is the negative value solution of 1 + t t ecosf = (1+e)D and the signature indicates a prograde (+) or retrograde (-) orbit. This t is a sinusoidal function of the binary phase as we expected, and the square of its amplitude is 3 r /r 2 (1+e)D which is larger for prograde orbits and the difference between t a − ± prograde and retrograde orbits becomes smaller for deep penetrators D 1, because in this p ≪ limit the binary center of mass approaches the BH in an almost radial fashion. If the disruption is abrupt, the ejection and capture preference could be roughly il- lustrated in terms of velocity (e.g Morbidelli 2006; Agnor & Hamilton 2006). The binary members rotate around their center of mass, such that their own motion is half of time with and half of time against, the motion of the center of mass r˙ . The net velocity of the m members relative to the BH is accordingly increased or reduced. Since the secondary has a higher rotation velocity, it has more chance that the net velocity exceeds or drops below the escape velocity from the BH. Then, it is preferentially ejected to infinity or captured in a boundorbit. However, for thefull discussion of theprocess, we also need to take into account the variation in the escape velocity or the variation in the potential. The displacement of order a in the position of each member of the binary, at a distance of about r from the BH, t results in a change in gravitational energy of GMa/r2 v2(M/m)1/3, this is comparable t ∼ 0 to the variation in the kinetic energy. As we have done, it should be easier to discuss the overall effect in the energy domain. In our formula, the energy change (17) includes both the variation of kinetic energy and potential energy. For prograde orbits, the kinetic and potential terms cooperate and the net energy change is larger, the member on the “outside track” is expected to be ejected and its partner is captured (the “outside track” could be well defined especially when the orbital energy is large because the duration of the encounter is much shorter than the binary rotation time-scale). On the other hand, for retrograde orbits the variation in the gravitational energy would counteract that in the kinetic energy. 6. Numerical Results In this paper we focus on results for circular coplanar binaries, though our formulae can be used to study the evolution of a binary with arbitrary orbital parameters. The orbit of a binary is assumed to be initially circular in the comoving frame of the binary center of mass. The center of mass of the binary is in a prograde or retrograde orbit around the BH (see SKR for the details of the numerical setup). For M/m 1 the problem can be reduced to the motion of a single particle in a time- ≫ dependent potential (“the restricted three-body approximation”) described by the equations (5)-(7) and (10). The energy change, eq (19), depends only on the penetration factor D,