1,776 research outputs found
Tidal Excitation of Modes in Binary Systems with Applications to Binary Pulsars
We consider the tidal excitation of modes in a binary system of arbitrary
eccentricity. For a circular orbit, the modes generally undergo forced
oscillation with a period equal to the orbital period (). For an eccentric
orbit, the amplitude of each tidally excited mode can be written approximately
as the sum of an oscillatory term that varies sinusoidally with the mode
frequency and a `static' term that follows the time dependence of the tidal
forcing function. The oscillatory term falls off exponentially with increasing
\b (defined as the ratio of the periastron passage time to the mode period),
whereas the `static' term is independent of \b. For small \b modes (\b
\approx 1), the two terms are comparable, and the magnitude of the mode
amplitude is nearly constant over the orbit. For large \b modes (\b \gta a
few), the oscillatory term is very small compared to the `static' term, in
which case the mode amplitude, like the tidal force, varies as the distance
cubed. For main sequence stars, , , and low order -modes generally
have large \b and hence small amplitudes of oscillation. High overtone
-modes, however, have small overlap with the tidal forcing function. Thus,
we expect an intermediate overtone -mode with \b \sim 1 to have the
largest oscillation amplitude. The dependence on mode damping and the stellar
rotation rate is considered, as well as the effects of orbital evolution. We
apply our work to the two binary pulsar system: PSR J0045-7319 and PSR
B1259-63.Comment: 28 pages of uuencoded compressed postscript. 9 postscript figures
available by anonymous ftp from ftp://brmha.mit.edu/ To be published in ApJ
GW170817 Most Likely Made a Black Hole
There are two outstanding issues regarding the neutron-star merger event
GW170817: the nature of the compact remnant and the interstellar shock. The
mass of the remnant of GW170817, 2.7 , implies the remnant could
be either a massive, rotating, neutron star, or a black hole. We report Chandra
Director's Discretionary Time observations made in 2017 December and 2018
January, and we reanalyze earlier observations from 2017 August and 2017
September, in order to address these unresolved issues. We estimate the X-ray
flux from a neutron star remnant and compare that to the measured X-ray flux.
If we assume that the spin-down luminosity of any putative neutron star is
converted to pulsar wind nebula X-ray emission in the 0.5-8 keV band with an
efficiency of , for a dipole magnetic field with G
< < G, a rising X-ray signal would result and would be brighter
than that observed by day 107, we therefore conclude that the remnant of
GW170817 is most likely a black hole. Independent of any assumptions of X-ray
efficiency, however, if the remnant is a rapidly-rotating, magnetized, neutron
star, the total energy in the external shock should rise by a factor
(to erg) after a few years, therefore, Chandra
observations over the next year or two that do not show substantial brightening
will rule out such a remnant. The same observations can distinguish between two
different models for the relativistic outflow, either an angular or radially
varying structure.Comment: 10 pages, 4 figures, accepted to ApJ
Differential rotation enhanced dissipation of tides in the PSR J0045-7319 Binary
Recent observations of PSR J0045-7319, a radio pulsar in a close eccentric
orbit with a massive B-star companion, indicate that the system's orbital
period is decreasing on a timescale of years, which is
much shorter than the timescale of 10^9 years given by the standard
theory of tidal dissipation in radiative stars. Observations also provide
strong evidence that the B-star is rotating rapidly, perhaps at nearly its
break up speed. We show that the dissipation of the dynamical tide in a star
rotating in the same direction as the orbital motion of its companion (prograde
rotation) with a speed greater than the orbital angular speed of the star at
periastron results in an increase in the orbital period of the binary system
with time. Thus, since the observed time derivative of the orbital period is
large and negative, the B-star in the PSR J0045-7319 binary must have
retrograde rotation if tidal effects are to account for the orbital decay. We
also show that the time scale for the synchronization of the B-star's spin with
the orbital angular speed of the star at periastron is comparable to the
orbital evolution time. From the work of Goldreich and Nicholson (1989) we
therefore expect that the B-star should be rotating differentially, with the
outer layers rotating more slowly than the interior. We show that the
dissipation of the dynamical tide in such a differentially rotating B-star is
enhanced by almost three orders of magnitude leading to an orbital evolution
time for the PSR J0045-7319 Binary that is consistent with the observations.Comment: 8 pages, tex. Submitted to Ap
The structure of the central disk of NGC 1068: a clumpy disk model
NGC 1068 is one of the best studied Seyfert II galaxies, for which the
blackhole mass has been determined from the Doppler velocities of water maser.
We show that the standard -disk model of NGC 1068 gives disk mass
between the radii of 0.65 pc and 1.1 pc (the region from which water maser
emission is detected) to be about 7x10 M (for ), more
than four times the blackhole mass, and a Toomre Q-parameter for the disk is
0.001. This disk is therefore highly self-gravitating and is subject to
large-amplitude density fluctuations. We conclude that the standard
-viscosity description for the structure of the accretion disk is
invalid for NGC 1068.
In this paper we develop a new model for the accretion disk. The disk is
considered to be composed of gravitationally bound clumps; accretion in this
clumped disk model arises because of gravitational interaction of clumps with
each other and the dynamical frictional drag exerted on clumps from the stars
in the central region of the galaxy. The clumped disk model provides a
self-consistent description of the observations of NGC 1068. The computed
temperature and density are within the allowed parameter range for water maser
emission, and the rotational velocity in the disk falls off as .Comment: To appear in Ap
Angular momentum transport by gravity waves and its effect on the rotation of the solar interior
We calculate the excitation of low frequency gravity waves by turbulent
convection in the sun and the effect of the angular momentum carried by these
waves on the rotation profile of the sun's radiative interior. We find that the
gravity waves generated by convection in the sun provide a very efficient means
of coupling the rotation in the radiative interior to that of the convection
zone. In a differentially rotating star, waves of different azimuthal number
have their frequencies in the local rest frame of the star Doppler shifted by
different amounts. This leads to a difference in their local dissipation rate
and hence a redistribution of angular momentum in the star. We find that the
time scale for establishing uniform rotation throughout much of the radiative
interior of the sun is years, which provides a possible explanation
for the helioseismic observations that the solar interior is rotating as a
solid body.Comment: 10 pages, tex, 3 figures. To appear in ApJ lette
Antibacterial responses of retinal Müller glia: production of antimicrobial peptides, oxidative burst and phagocytosis
BACKGROUND: We have previously shown that, in response to microbial infection, activated Müller glia secrete inflammatory cytokines/chemokines and exhibit antimicrobial properties. The aim of this study is to understand the mechanisms and the key components involved in this response. METHODS: Immortalized human retinal Müller glia (MIO-M1 cells) were challenged with Staphylococcus (S) aureus, the leading cause of severe intraocular infection followed by RT(2) profile PCR array analysis. The expression of human β-defensin 1 (HBD1), 2 (HBD2), 3 (HBD3), hepcidine and cathelicidin LL37 was checked by RT-PCR and quantified by Taqman® qPCR. The expression of AMPs was confirmed at protein level by dot-blot analysis. The production of ROS was measured by dicholoro-dihydro-fluorescein diacetate (DCFH-DA) staining by flow cytometry as well as fluorescence microscopy. The level of nitric oxide (NO) was measured by measuring a stable metabolite, nitrite using the Griess reagent. In vitro killing assay was performed by Live/Dead® BacLight™ staining as well as by dilution plating in suspension and adherent conditions following S. aureus infection. Phagocytosis was measured by CFU enumeration following infection. RESULTS: PCR array data showed that, in comparison to uninfected control cells, bacterial challenge significantly (> two-fold) induced the expression of 26 genes involved in cytokine/chemokine, antimicrobials, Toll-like receptor, apoptotic, and NF-κB signaling. RT-PCR analysis showed time-dependent increased expression of HBD1, HBD2, HBD3, LL-37, and hepcidin mRNA in bacteria-challenged Müller glia. The expression of these antimicrobial molecules was also increased at the protein level in the culture supernatant, as detected by dot-blot analysis. Additionally, the bacteria-stimulated Müller glia were found to produce reactive oxygen (ROS) and reactive nitrogen (RNS) species. In vitro, killing assays revealed that Müller glia exhibited bactericidal activity against S. aureus in both adherent and suspension cultures. Furthermore, our data demonstrated that Müller glia can phagocytize and kill the bacteria in a time-dependent manner. CONCLUSIONS: These data suggest that retinal Müller glia behave like classical innate immune cells by producing a variety of antimicrobial molecules in response to bacterial challenge, suggesting their pivotal role in retinal innate defense
Tidal spin-up of stars in dense stellar cusps around massive black holes
We show that main-sequence stars in dense stellar cusps around massive black
holes are likely to rotate at a significant fraction of the centrifugal breakup
velocity due to spin-up by hyperbolic tidal encounters. We use realistic
stellar structure models to calculate analytically the tidal spin-up in soft
encounters, and extrapolate these results to close and penetrating collisions
using smoothed particle hydrodynamics simulations. We find that the spin-up
falls off only slowly with distance from the black hole because the increased
tidal coupling in slower collisions at larger distances compensates for the
decrease in the stellar density. We apply our results to the stars near the
massive black hole in the Galactic Center. Over their lifetime, ~1 Msol main
sequence stars in the inner 0.3 pc of the Galactic Center are spun-up on
average to ~10%--30% of the centrifugal breakup limit. Such rotation is ~20--60
times higher than is usual for such stars and may affect their subsequent
evolution and their observed properties.Comment: 25 pages, 7 figures. Submitted to Ap
X-ray Lines From Gamma-ray Bursts
X-ray lines have been recently detected in the afterglows of a few gamma-ray
bursts. We derive constraints on the physical conditions in the line-emitting
gas, using as an example the multiple K lines detected by Reeves et
al. (2002) in GRB 011211. We argue that models previously discussed in the
literature require either a very extreme geometry or too much mass in the
line-emitting region. We propose a new model in which gamma-rays and radiation
from the early x-ray afterglow are back-scattered by an electron-positron pair
screen at a distance of about cm from the source and
irradiate the expanding outer layers of the supernova ejecta, thereby producing
x-ray lines. The model suffers from fewer problems compared to previous models.
It also has the advantage of requiring only a single explosion to produce both
the GRB and the supernova ejecta, in contrast to most other models for the
lines which require the supernova to go off days or weeks prior to the GRB. The
model, however, has difficulty explaining the ergs of energy emitted
in the x-ray lines, which requires somewhat extreme choices of model
parameters. The difficulties associated with the various models are not
particular to GRB 011211. They are likely to pose a problem for any GRB with
x-ray lines.Comment: 19 pages, 1 figure, submitted to Ap.
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