152 research outputs found
The properties of dynamically ejected runaway and hyper-runaway stars
Runaway stars are stars observed to have large peculiar velocities. Two
mechanisms are thought to contribute to the ejection of runaway stars, both
involve binarity (or higher multiplicity). In the binary supernova scenario a
runaway star receives its velocity when its binary massive companion explodes
as a supernova (SN). In the alternative dynamical ejection scenario, runaway
stars are formed through gravitational interactions between stars and binaries
in dense, compact clusters or cluster cores. Here we study the ejection
scenario. We make use of extensive N-body simulations of massive clusters, as
well as analytic arguments, in order to to characterize the expected ejection
velocity distribution of runaways stars. We find the ejection velocity
distribution of the fastest runaways (>~80 km s^-1) depends on the binary
distribution in the cluster, consistent with our analytic toy model, whereas
the distribution of lower velocity runaways appears independent of the binaries
properties. For a realistic log constant distribution of binary separations, we
find the velocity distribution to follow a simple power law; Gamma(v) goes like
v^(-8/3) for the high velocity runaways and v^(-3/2) for the low velocity ones.
We calculate the total expected ejection rates of runaway stars from our
simulated massive clusters and explore their mass function and their binarity.
The mass function of runaway stars is biased towards high masses, and depends
strongly on their velocity. The binarity of runaways is a decreasing function
of their ejection velocity, with no binaries expected to be ejected with v>150
km s^-1. We also find that hyper-runaways with velocities of hundreds of km
s^-1 can be dynamically ejected from stellar clusters, but only at very low
rates, which cannot account for a significant fraction of the observed
population of hyper-velocity stars in the Galactic halo.Comment: Now matching published ApJ versio
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. 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 pc
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 over-predict 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 perturber 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.Comment: Comments are welcome
Hyper Velocity Stars and the Restricted Parabolic 3-body Problem
Motivated by detections of hypervelocity stars that may originate from the
Galactic Center, we revist the problem of a binary disruption by a passage near
a much more massive point mass. The six order of magnitude mass ratio between
the Galactic Center black hole and the binary stars allows us to formulate the
problem in the restricted parabolic three-body approximation. In this
framework, results can be simply rescaled in terms of binary masses, its
initial separation and binary-to-black hole mass ratio. Consequently, an
advantage over the full three-body calculation is that a much smaller set of
simulations is needed to explore the relevant parameter space. Contrary to
previous claims, we show that, upon binary disruption, the lighter star does
not remain preferentially bound to the black hole. In fact, it is ejected
exactly in 50% of the cases. Nonetheless, lighter objects have higher ejection
velocities, since the energy distribution is independent of mass. Focusing on
the planar case, we provide the probability distributions for disruption of
circular binaries and for the ejection energy. We show that even binaries that
penetrate deeply into the tidal sphere of the black hole are not doomed to
disruption, but survive in 20% of the cases. Nor do these deep encounters
produce the highest ejection energies, which are instead obtained for binaries
arriving to 0.1-0.5 of the tidal radius in a prograde orbit. Interestingly,
such deep-reaching binaries separate widely after penetrating the tidal radius,
but always approach each other again on their way out from the black
hole.[shortened]Comment: 10 pages, 10 Figures, Apj submitte
Tidal breakup of binary stars at the Galactic Center. II. Hydrodynamic simulations
In Paper I, we followed the evolution of binary stars as they orbited near
the supermassive black hole (SMBH) at the Galactic center, noting the cases in
which the two stars would come close enough together to collide. In this paper
we replace the point-mass stars by fluid realizations, and use a
smoothed-particle hydrodynamics (SPH) code to follow the close interactions. We
model the binary components as main-sequence stars with initial masses of 1, 3
and 6 Solar masses, and with chemical composition profiles taken from stellar
evolution codes. Outcomes of the close interactions include mergers, collisions
that leave both stars intact, and ejection of one star at high velocity
accompanied by capture of the other star into a tight orbit around the SMBH.
For the first time, we follow the evolution of the collision products for many
() orbits around the SMBH. Stars that are initially too small to
be tidally disrupted by the SMBH can be puffed up by close encounters or
collisions, with the result that tidal stripping occurs in subsequent periapse
passages. In these cases, mass loss occurs episodically, sometimes for hundreds
of orbits before the star is completely disrupted. Repeated tidal flares, of
either increasing or decreasing intensity, are a predicted consequence. In
collisions involving a low-mass and a high-mass star, the merger product
acquires a high core hydrogen abundance from the smaller star, effectively
resetting the nuclear evolution "clock" to a younger age. Elements like Li, Be
and B that can exist only in the outermost envelope of a star are severely
depleted due to envelope ejection during collisions and due to tidal forces
from the SMBH. In the absence of collisions, tidal spin-up of stars is only
important in a narrow range of periapse distances, with the tidal disruption radius.Comment: ApJ accepted, 22 pages, 19 figures. Version with high-resolution
figures, and additional animations, available at this url:
http://astrophysics.rit.edu/fantonini/tbbs2
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 Msun to 4000 Msun, and its initial semi-major axis with respect to the SMBH
is varied from 0.3-30 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 binary's
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 time scales
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 1000 Msun if the IMBH/SMBH semi-major 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.Comment: 11 pages, 13 figures, revised versio
The spatial and velocity distributions of hypervelocity stars
Hypervelocity stars (HVSs) found in the Galactic halo are probably the
dynamical products of interactions between (binary) stars and the massive black
hole(s) (MBH) in the Galactic center (GC). It has been shown that the detected
HVSs are spatially consistent with being located on two thin disks (Lu et al.),
one of which has the same orientation as the clockwise-rotating stellar disk in
the GC. Here we perform a large number of three-body experiments of the
interactions between the MBH and binary stars bound to it, and find that the
probability of ejecting HVSs is substantially enhanced by multiple encounters
between the MBH and binary stars at distances substantially larger than the
initial tidal breakup radii. Assuming that the HVS progenitors are originated
from the two disks, the inclination distribution of the HVSs relative to the
disk planes can be reproduced by either the mechanism of tidal breakup of
binary stars or the mechanism of ejecting HVSs by a hypothetical binary black
hole (BBH) in the GC. However, an isotropical origination of HVS progenitors is
inconsistent with the observed inclination distribution. Assuming that the HVSs
were ejected out by the tidal breakup mechanism, its velocity distribution can
be reproduced if their progenitors diffuse onto low angular momentum orbits
slowly and most of the progenitors were broken up at relatively large distances
due to multiple encounters. Assuming that the HVSs were ejected out by a BBH
within the allowed parameter space in the GC, our simulations produce
relatively flatter velocity spectra compared to the observed ones; however, the
BBH mechanism cannot be statistically ruled out, yet. Future surveys of HVSs
and better statistics of their spatial and velocity distributions should enable
to distinguish the ejection mechanisms of HVSs and shed new light on the
dynamical environment of the MBH.(abridged)Comment: 19 pages, 16 figure
Metal-poor hypervelocity star candidates from the Sloan Digital Sky Survey
Hypervelocity stars are believed to be ejected out from the Galactic center
through dynamical interactions of (binary) stars with the central massive black
hole(s). In this letter, we report 13 metal-poor F-type hypervelocity star
candidates selected from 370,000 stars of the data release 7 of the Sloan
Digital Sky Survey. With a detailed analysis of the kinematics of these stars,
we find that seven of them were likely ejected from the Galactic center (GC) or
the Galactic disk, four neither originated from the GC nor the Galactic disk,
and the other two were possibly ejected from either the Galactic disk or other
regions. Those candidates which unlikely originated from the GC or the Galactic
disk, may be explained by other mechanisms, like the tidal disruption of the
Milky Way's dwarf galaxies in the Galactic potential, or the gravitational
interactions with a massive black hole at the center of M31 or M32
Tidal break-up 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 binary's first passage. This process can lead to
stellar coalescence and eventually serve as an important source of young stars
at the galactic center.Comment: accepted for publication in the Astrophysical Journa
Hypervelocity Planets and Transits Around Hypervelocity Stars
The disruption of a binary star system by the massive black hole at the
Galactic Centre, SgrA*, can lead to the capture of one star around SgrA* and
the ejection of its companion as a hypervelocity star (HVS). We consider the
possibility that these stars may have planets and study the dynamics of these
planets. Using a direct -body integration code, we simulated a large number
of different binary orbits around SgrA*. For some orbital parameters, a planet
is ejected at a high speed. In other instances, a HVS is ejected with one or
more planets orbiting around it. In these cases, it may be possible to observe
the planet as it transits the face of the star. A planet may also collide with
its host star. In such cases the atmosphere of the star will be enriched with
metals. In other cases, a planet is tidally disrupted by SgrA*, leading to a
bright flare.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in MNRA
MMT Hypervelocity Star Survey. II. Five New Unbound Stars
We present the discovery of five new unbound hypervelocity stars (HVSs) in
the outer Milky Way halo. Using a conservative estimate of Galactic escape
velocity, our targeted spectroscopic survey has now identified 16 unbound HVSs
as well as a comparable number of HVSs ejected on bound trajectories. A
Galactic center origin for the HVSs is supported by their unbound velocities,
the observed number of unbound stars, their stellar nature, their ejection time
distribution, and their Galactic latitude and longitude distribution. Other
proposed origins for the unbound HVSs, such as runaway ejections from the disk
or dwarf galaxy tidal debris, cannot be reconciled with the observations. An
intriguing result is the spatial anisotropy of HVSs on the sky, which possibly
reflects an anisotropic potential in the central 10-100 pc region of the
Galaxy. Further progress requires measurement of the spatial distribution of
HVSs over the southern sky. Our survey also identifies seven B supergiants
associated with known star-forming galaxies; the absence of B supergiants
elsewhere in the survey implies there are no new star-forming galaxies in our
survey footprint to a depth of 1-2 Mpc.Comment: 10 pages, submitted to Ap
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