Alessia Gualandris

Ejection of hypervelocity stars from the Galactic Centre by intermediate‐mass black holes

We have performed N-body simulations of the formation of hypervelocity stars (HVS) in the centre of the Milky Way due to inspiralling intermediate-mass black holes (IMBHs). We considered IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc. We find that the IMBHs sink to the centre of the Galaxy due to dynamical friction, where they deplete the central cusp of stars. Some of these stars become HVS and are ejected with velocities sufficiently high to escape the Galaxy. Since the HVS carry with them information about their origin, in particular in the moment of ejection, the velocity distribution and the direction in which they escape the Galaxy, detecting a population of HVS will provide insight in the ejection processes and could therefore provide indirect evidence for the existence of IMBHs.

Three-body encounters in the Galactic Centre : the origin of the hypervelocity star SDSS J090745.0+024507

In the late 1980s Hills predicted that runaway stars could be accelerated to velocities greater than 1000kms-1 by dynamical encounters with the supermassive black hole (SMBH) in the Galactic Centre. The recently discovered hypervelocity star SDSS J090745.0+024507 (hereafter the HVS) is escaping the Galaxy at high speed and could be the first object in this class. With the measured radial velocity and the estimated distance to the HVS, we trace back its trajectory in the Galactic potential. Assuming it was ejected from the Centre, we find that a ~2masyr-1 proper motion is necessary for the star to have come within a few parsecs of the SMBH. We perform three-body scattering experiments to constrain the progenitor encounter that accelerated the HVS. As proposed recently by Yu & Tremaine, we consider the tidal disruption of binary systems by the SMBH and the encounter between a star and a binary black hole, as well as an alternative scenario involving intermediate-mass black holes. We find that the tidal disruption of a stellar binary ejects stars with a larger velocity compared to the encounter between a single star and a binary black hole, but has a somewhat smaller ejection rate due to the greater availability of single stars.

N-body simulations of stars escaping from the Orion nebula

We study the dynamical interaction in which the two single runaway stars, AE Aurigae and mu Columbae, and the binary iota Orionis acquired their unusually high space velocity. The two single runaways move in almost opposite directions with a velocity greater than 100 km s-1 away from the Trapezium cluster. The star iota Orionis is an eccentric (e~= 0.8) binary moving with a velocity of about 10 km s-1 at almost right angles with respect to the two single stars. The kinematic properties of the system suggest that a strong dynamical encounter occurred in the Trapezium cluster about 2.5 Myr ago. Curiously enough, the two binary components have similar spectral type but very different masses, indicating that their ages must be quite different. This observation leads to the hypothesis that an exchange interaction occurred in which an older star was swapped into the original iota Orionis binary. We test this hypothesis by a combination of numerical and theoretical techniques, using N-body simulations to constrain the dynamical encounter, binary evolution calculations to constrain the high orbital eccentricity of iota Orionis and stellar evolution calculations to constrain the age discrepancy of the two binary components. We find that an encounter between two low eccentricity (0.4 <~e<~ 0.6) binaries with comparable binding energy, leading to an exchange and the ionization of the wider binary, provides a reasonable solution to this problem.

On the origin of high-velocity runaway stars

We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary 50-100 M-circle dot star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of greater than or similar to 200-400 km s(-1) (typical of pulsars), while 3-4 M-circle dot stars can attain velocities of greater than or similar to 300-400 km s(-1) (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Wayss escape velocity.

LONG-TERM EVOLUTION OF MASSIVE BLACK HOLE BINARIES. IV. MERGERS OF GALAXIES WITH COLLISIONALLY RELAXED NUCLEI

We simulate mergers between galaxies containing collisionally relaxed nuclei around massive black holes (MBHs). Our galaxies contain four mass groups, representative of old stellar populations; a primary goal is to understand the distribution of stellar-mass black holes (BHs) after the merger. Mergers are followed using direct-summation N-body simulations, assuming a mass ratio of 1:3 and two different orbits. Evolution of the binary MBH is followed until its separation has shrunk by a factor of 20 below the hard-binary separation. During the galaxy merger, large cores are carved out in the stellar distribution, with radii several times the influence radius of the massive binary. Much of the pre-existing mass segregation is erased during this phase. We follow the evolution of the merged galaxies for approximately three central relaxation times after coalescence of the massive binary; both standard and top-heavy mass functions are considered. The cores that were formed in the stellar distribution persist, and the distribution of the stellar-mass BHs evolves against this essentially fixed background. Even after one central relaxation time, these models look very different from the relaxed, multi-mass models that are often assumed to describe the distribution of stars and stellar remnants near a massive BH. While the stellar BHs do form a cusp on roughly a relaxation timescale, the BH density can be much smaller than in those models. We discuss the implications of our results for the extreme-mass-ratio inspiral problem and for the existence of Bahcall-Wolf cusps.

Has the black hole in XTE J1118+480 experienced an asymmetric natal kick?

We explore the origin of the high Galactic latitude black hole X-ray binary XTE J1118+480 and in particular its birth location and the magnitude of the kick received by the black hole upon formation in the supernova explosion. Our analysis is constrained by the evolutionary state of the companion star, the observed limits on the orbital inclination, the Galactic position, and the peculiar velocity of the binary system. We constrain the age of the companion to the black hole using stellar evolution calculations between 2 and 5 Gyr, making an origin in a globular cluster unlikely. We therefore argue that the system was born in the Galactic disk, in which case the supernova must have propelled it in its current high-latitude orbit. Given the current estimates on its position in the sky, proper motion, and radial velocity, we back-trace the orbit of XTE J1118+480 in the Galactic potential to infer the peculiar velocity of the system at different disk crossings over the last 5 Gyr. Taking into account the uncertainties on the velocity components, we infer that the peculiar velocity required to change from a Galactic disk orbit to the currently observed orbit is 183+/-31 km s-1. The maximum velocity that the binary can acquire by symmetric supernova mass loss is about 100 km s-1, which is 2.7 sigma away from the mean of the peculiar velocity distribution. We therefore argue that an additional asymmetric kick velocity is required. By considering the orientation of the system relative to the plane of the sky, we derive a 95% probability for a nonnull component of the kick perpendicular to the orbital plane of the binary. The distribution of perpendicular velocities is skewed to lower velocities with an average of 93+55-60 km s-1. These estimates are independent of the age of the system but depend quite sensitively on the kinematic parameters of the system. A better constraint on the asymmetric kick velocity requires an order-of-magnitude improvement in the measurement of the current space velocity of the system.

PERTURBATIONS OF INTERMEDIATE-MASS BLACK HOLES ON STELLAR ORBITS IN THE GALACTIC CENTER

We study the short- and long-term effects of an intermediate mass black hole (IMBH) on the orbits of stars bound to the supermassive black hole (SMBH) at the center of the Milky Way. A regularized N-body code including post-Newtonian terms is used to carry out direct integrations of 19 stars in the S-star cluster for 10 Myr. The mass of the IMBH is assigned one of four values from 400 M ☉ to 4000 M ☉, and its initial semimajor axis with respect to the SMBH is varied from 0.3 to 30 mpc, bracketing the radii at which inspiral of the IMBH is expected to stall. We consider two values for the eccentricity of the IMBH/SMBH binary, e = (0, 0.7), and 12 values for the orientation of the binarys plane. Changes at the level of ~1% in the orbital elements of the S-stars could occur in just a few years if the IMBH is sufficiently massive. On timescales of 1 Myr or longer, the IMBH efficiently randomizes the eccentricities and orbital inclinations of the S-stars. Kozai oscillations are observed when the IMBH lies well outside the orbits of the stars. Perturbations from the IMBH can eject stars from the cluster, producing hypervelocity stars, and can also scatter stars into the SMBH; stars with high initial eccentricities are most likely to be affected in both cases. The distribution of S-star orbital elements is significantly altered from its currently observed form by IMBHs with masses greater than ~103 M ☉ if the IMBH/SMBH semimajor axis lies in the range 3-10 mpc. We use these results to further constrain the allowed parameters of an IMBH/SMBH binary at the Galactic center.

TIDAL BREAKUP OF BINARY STARS AT THE GALACTIC CENTER AND ITS CONSEQUENCES

The tidal breakup of binary star systems by the supermassive black hole (SMBH) in the center of the galaxy has been suggested as the source of both the observed sample of hypervelocity stars (HVSs) in the halo of the Galaxy and the S-stars that remain in tight orbits around Sgr A*. Here, we use a post-Newtonian N-body code to study the dynamics of main-sequence binaries on highly elliptical bound orbits whose periapses lie close to the SMBH, determining the properties of ejected and bound stars as well as collision products. Unlike previous studies, we follow binaries that remain bound for several revolutions around the SMBH, finding that in the case of relatively large periapses and highly inclined binaries the Kozai resonance can lead to large periodic oscillations in the internal binary eccentricity and inclination. Collisions and mergers of the binary elements are found to increase significantly for multiple orbits around the SMBH, while HVSs are primarily produced during a binarys first passage. This process can lead to stellar coalescence and eventually serve as an important source of young stars at the galactic center.

Very massive runaway stars from three-body encounters

Very massive stars preferentially reside in the cores of their parent clusters and form binary or multiple systems. We study the role of tight very massive binaries in the origin of the field population of very massive stars. We performed numerical simulations of dynamical encounters between single (massive) stars and a very massive binary with parameters similar to those of the most massive known Galactic binaries, WR20a and NGC3603-A1. We found that these three-body encounters could be responsible for the origin of high peculiar velocities (> 70 kms) observed for some very massive (> 60 − 70M⊙) runaway stars in the Milky Way and the Large Magellanic Cloud (e.g., λCep, BD+43 3654, Sk−6722, BI 237, 30Dor 016), which can hardly be explained within the framework of the binary-supernova scenario. The production of high-velocity massive stars via three-body encounters is accompanied by the recoil of the binary in the opposite direction to the ejected star. We show that the relative position of the very massive binary R145 and the runaway early B-type star Sk−69 206 on the sky is consistent with the possibility that both objects were ejected from the central cluster, R136, of the star-forming region 30Doradus via the same dynamical event – a three-body encounter.

DYNAMICAL CONSTRAINTS ON THE ORIGIN OF THE YOUNG B-STARS IN THE GALACTIC CENTER

Regular star formation is thought to be inhibited close to the massive black hole (MBH) in the Galactic center. Nevertheless, tens of young main-sequence B-stars have been observed in an isotropic distribution close to it. These stars are observed to have an apparently continuous distribution from very close to the MBH (<0.01 pc) and up to at least ~0.5 pc, suggesting a common origin. Various models have been suggested for the formation of the B-stars closest to the MBH (<0.05 pc; the S-stars), typically involving the migration of these stars from their original birthplace to their currently observed position. Here, we explore the orbital phase space distribution of the B-stars throughout the central parsec expected from the various suggested models for the origin of the B-stars. We find that most of these models have difficulties in explaining, by themselves, both the population of the S-stars (<0.05 pc) and the population of the young B-stars further away (up to 0.5 pc). Most models grossly overpredict the number of B-stars up to 0.5 pc, given the observed number of S-stars. Such models include the intermediate-mass black hole assisted cluster inspiral scenario, Kozai-like perturbations by two disks, spiral density waves migration in a gaseous disk, and some of the eccentric disk instability models. We focus on one of the other models, the massive perturbers induced binary disruption, which is consistent with both the S-stars and the extended population of B-stars further away. For this model, we use analytical arguments and N-body simulations to provide further observational predictions. These could be compared with future observations to further support this model, constrain it, or refute it. These predictions include the radial distribution of the young B-stars, their eccentricity distribution, and its dependence on distance from the MBH (higher eccentricities at larger distances from the MBH), as well as less specific expectations regarding their mass function.