1,772 research outputs found
Current concepts regarding the HTLV-1 receptor complex
The identity of the Human T lymphotropic Virus type 1 (HTLV-1) receptor remained an unsolved puzzle for two decades, until the recent demonstration that three molecules, Glucose Transporter 1, Neuropilin-1 and Heparan Sulfate Proteoglycans are involved in HTLV-1 binding and entry. Despite these advances, several questions remain unanswered, including the precise role of each of these molecules during virus entry. In light of the most recent data, we propose a model of the HTLV-1 receptor complex and discuss its potential impact on HTLV-1 infection
Towards relativistic orbit fitting of Galactic center stars and pulsars
The S stars orbiting the Galactic center black hole reach speeds of up to a
few percent the speed of light during pericenter passage. This makes, for
example, S2 at pericenter much more relativistic than known binary pulsars, and
opens up new possibilities for testing general relativity. This paper develops
a technique for fitting nearly-Keplerian orbits with perturbations from
Schwarzschild curvature, frame dragging, and spin-induced torque, to redshift
measurements distributed along the orbit but concentrated around pericenter.
Both orbital and light-path effects are taken into account. It turns out that
absolute calibration of rest-frame frequency is not required. Hence, if pulsars
on orbits similar to the S stars are discovered, the technique described here
can be applied without change, allowing the much greater accuracies of pulsar
timing to be taken advantage of. For example, pulse timing of 3 microsec over
one hour amounts to an effective redshift precision of 30 cm/s, enough to
measure frame dragging and the quadrupole moment from an S2-like orbit,
provided problems like the Newtonian "foreground" due to other masses can be
overcome. On the other hand, if stars with orbital periods of order a month are
discovered, the same could be accomplished with stellar spectroscopy from the
E-ELT at the level of 1 km/s.Comment: 22 pages, 9 figures, published in the Ap
Orbits Around Black Holes in Triaxial Nuclei
We discuss the properties of orbits within the influence sphere of a
supermassive black hole (BH), in the case that the surrounding star cluster is
nonaxisymmetric. There are four major orbit families; one of these, the pyramid
orbits, have the interesting property that they can approach arbitrarily
closely to the BH. We derive the orbit-averaged equations of motion and show
that in the limit of weak triaxiality, the pyramid orbits are integrable: the
motion consists of a two-dimensional libration of the major axis of the orbit
about the short axis of the triaxial figure, with eccentricity varying as a
function of the two orientation angles, and reaching unity at the corners.
Because pyramid orbits occupy the lowest angular momentum regions of phase
space, they compete with collisional loss cone repopulation and with resonant
relaxation in supplying matter to BHs. General relativistic advance of the
periapse dominates the precession for sufficiently eccentric orbits, and we
show that relativity imposes an upper limit to the eccentricity: roughly the
value at which the relativistic precession time is equal to the time for
torques to change the angular momentum. We argue that this upper limit to the
eccentricity should apply also to evolution driven by resonant relaxation, with
potentially important consequences for the rate of extreme-mass-ratio inspirals
in low-luminosity galaxies. In giant galaxies, we show that capture of stars on
pyramid orbits can dominate the feeding of BHs, at least until such a time as
the pyramid orbits are depleted; however this time can be of order a Hubble
time.Comment: 20 pages, 15 figure
The infrared imaging spectrograph (IRIS) for TMT: the science case
The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument being
designed for the Thirty Meter Telescope (TMT). IRIS is a combination of an
imager that will cover a 16.4" field of view at the diffraction limit of TMT (4
mas sampling), and an integral field unit spectrograph that will sample objects
at 4-50 mas scales. IRIS will open up new areas of observational parameter
space, allowing major progress in diverse fields of astronomy. We present the
science case and resulting requirements for the performance of IRIS.
Ultimately, the spectrograph will enable very well-resolved and sensitive
studies of the kinematics and internal chemical abundances of high-redshift
galaxies, shedding light on many scenarios for the evolution of galaxies at
early times. With unprecedented imaging and spectroscopy of exoplanets, IRIS
will allow detailed exploration of a range of planetary systems that are
inaccessible with current technology. By revealing details about resolved
stellar populations in nearby galaxies, it will directly probe the formation of
systems like our own Milky Way. Because it will be possible to directly
characterize the stellar initial mass function in many environments and in
galaxies outside of the the Milky Way, IRIS will enable a greater understanding
of whether stars form differently in diverse conditions. IRIS will reveal
detailed kinematics in the centers of low-mass galaxies, allowing a test of
black hole formation scenarios. Finally, it will revolutionize the
characterization of reionization and the first galaxies to form in the
universe.Comment: to appear in Proc. SPIE 773
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
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
The nuclear star cluster of the Milky Way
The nuclear star cluster of the Milky Way is a unique target in the Universe.
Contrary to extragalactic nuclear star clusters, using current technology it
can be resolved into tens of thousands of individual stars. This allows us to
study in detail its spatial and velocity structure as well as the different
stellar populations that make up the cluster. Moreover, the Milky Way is one of
the very few cases where we have firm evidence for the co-existence of a
nuclear star cluster with a central supermassive black hole, Sagittarius A*.
The number density of stars in the Galactic center nuclear star cluster can be
well described, at distances pc from Sagittarius A*, by a power-law
of the form with an index of .
In the central parsec the index of the power-law becomes much flatter and
decreases to . We present proper motions for more than 6000
stars within 1 pc in projection from the central black hole. The cluster
appears isotropic at projected distances pc from Sagittarius A*.
Outside of 0.5 pc and out to 1.0 pc the velocity dispersion appears to stay
constant. A robust result of our Jeans modeling of the data is the required
presence of of extended (stellar) mass in the
central parsec of the Galaxy.Comment: To appear in the proceedings of "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Journal of Physics:Conference
Series (IOP; http://www.iop.org/EJ/conf) This version has been slightly
modified (e.g. double-log plot in right hand panel of Figure 5
Explaining the Orbits of the Galactic Center S-Stars
The young stars near the supermassive black hole at the galactic center
follow orbits that are nearly random in orientation and that have an
approximately thermal distribution of eccentricities, N(e) ~ e. We show that
both of these properties are a natural consequence of a few million years'
interaction with an intermediate-mass black hole (IBH), if the latter's orbit
is mildly eccentric and if its mass exceeds approximately 1500 solar masses.
Producing the most tightly-bound S-stars requires an IBH orbit with periastron
distance less than about 10 mpc. Our results provide support for a model in
which the young stars are carried to the galactic center while bound to an IBH,
and are consistent with the hypothesis that an IBH may still be orbiting within
the nuclear star cluster.Comment: 4 pages, 3 figure
Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars
We use N-body simulations to study the evolution of the orbital
eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black
hole (MBH), starting from initial conditions motivated by two competing models
for their origin: formation in a disk followed by inward migration; and
exchange interactions involving a binary star. The first model predicts modest
eccentricities, lower than those observed in the S-star cluster, while the
second model predicts higher eccentricities than observed. The N-body
simulations include a dense cluster of 10 M_sun stellar black holes (SBHs),
expected to accumulate near the MBH by mass segregation. Perturbations from the
SBHs tend to randomize the stellar orbits, partially erasing the dynamical
signatures of their origin. The eccentricities of the initially highly
eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that
is consistent at the ~95 % level with the observed eccentricity distribution.
In contrast, the eccentricities of the initially more circular orbits fail to
evolve to the observed values in 20 Myr, arguing against the disk migration
scenario. We find that 20 % - 30 % of the S-stars are tidally disrupted by the
MBH over their lifetimes, and that the S-stars are not likely to be ejected as
hypervelocity stars outside the central 0.05 pc by close encounters with
stellar black holes.Comment: 6 pages, 2 figures. Minor corrections, Sumitted to Ap
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