675 research outputs found
Alignment Timescale of the Microquasar GRO J1655-40
The microquasar GRO J1655-40 has a black hole with spin angular momentum
apparently misaligned to the orbital plane of its companion star. We
analytically model the system with a steady state disc warped by Lense-Thirring
precession and find the timescale for the alignment of the black hole with the
binary orbit. We make detailed stellar evolution models so as to estimate the
accretion rate and the lifetime of the system in this state. The secondary can
be evolving at the end of the main sequence or across the Hertzsprung gap. The
mass-transfer rate is typically fifty times higher in the latter case but we
find that, in both cases, the lifetime of the mass transfer state is at most a
few times the alignment timescale. The fact that the black hole has not yet
aligned with the orbital plane is therefore consistent with either model. We
conclude that the system may or may not have been counter-aligned after its
supernova kick but that it is most likely to be close to alignment rather than
counteralignment now.Comment: Accepted for publication in MNRA
The nature of the progenitor of the Type II-P supernova 1999em
We present high quality ground-based VRI images of the site of the Type II-P
SN1999em (in NGC1637) taken before explosion, which were extracted from the
CFHT archive. We determine a precise position of the SN on these images to an
accuracy of 0.17''. The host galaxy is close enough (7.5 +/- 0.5 Mpc) that the
bright supergiants are resolved as individual objects, however we show that
there is no detection of an object at the SN position before explosion that
could be interpreted as the progenitor star. By determining the sensitivity
limits of the VRI data, we derive bolometric luminosity limits for the
progenitor. Comparing these to standard stellar evolutionary tracks which trace
evolution up to the point of core carbon ignition, we initially derive an upper
mass limit of approximately 12M_sol. However we present evolutionary
calculations that follow 7-12M_sol stars throughout their C-burning lifetime
and show that we can restrict the mass of the progenitor even further. Our
calculations indicate that progenitors initially of 8-10M_sol, undergoing
expected mass loss, can also be excluded because a second dredge up sends them
to somewhat higher luminosities than a star of initially 12M_sol. These results
limit the progenitor's initial main-sequence mass to a very narrow range of 12
+/- 1 M_sol. We discuss the similarities between the Type II-P SNe 1999em and
1999gi and their progenitor mass limits, and suggest that SN Type II-P
originate only in intermediate mass stars of 8-12M_sol, which are in the red
supergiant region and that higher mass stars produce the other Type II
sub-types. (Abridged).Comment: Replaced with accepted version to appear in ApJ, 30 pages, inc. 6
figure
The Evolutionary Status of SS433
We consider possible evolutionary models for SS 433. We assume that
common-envelope evolution is avoided if radiation pressure is able to expel
most of a super-Eddington accretion flow from a region smaller than the
accretor's Roche lobe. This condition is satisfied, at least initially, for
largely radiative donors with masses in the range 4-12 solar masses. For donors
more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1
are indicated, thus tending to favor black-hole accretors. For lower mass
donors, evolutionary considerations do not distinguish between a neutron star
or black hole accretor. In all cases the mass transfer (and mass loss) rates
are much larger than the likely mass-loss rate in the precessing jets. Almost
all of the transferred mass is expelled at radii considerably larger than the
jet acceleration region, producing the "stationary" H-alpha line, the infrared
luminosity, and accounting for the low X-ray luminosity.Comment: 13 pages, Astrophysical Journal Letters, accepte
Gap solitons in Bragg gratings with a harmonic superlattice
Solitons are studied in a model of a fiber Bragg grating (BG) whose local
reflectivity is subjected to periodic modulation. The superlattice opens an
infinite number of new bandgaps in the model's spectrum. Averaging and
numerical continuation methods show that each gap gives rise to gap solitons
(GSs), including asymmetric and double-humped ones, which are not present
without the superlattice.Computation of stability eigenvalues and direct
simulation reveal the existence of completely stable families of fundamental
GSs filling the new gaps - also at negative frequencies, where the ordinary GSs
are unstable. Moving stable GSs with positive and negative effective mass are
found too.Comment: 7 pages, 3 figures, submitted to EP
Evolution in Binary and Triple Stars, with an application to SS Lac
We present equations governing the way in which both the orbit and the
intrinsic spins of stars in a close binary should evolve subject to a number of
perturbing forces, including the effect of a third body in a possibly inclined
wider orbit. We illustrate the solutions in some binary-star and triple-star
situations: tidal friction in a wide but eccentric orbit of a radio pulsar
about a B star, the Darwin and eccentricity instabilities in a more massive but
shorter-period massive X-ray binary, and the interaction of tidal friction with
Kozai cycles in a triple such as Algol (beta-Per), at an early stage in that
star's life when all 3 components were ZAMS stars. We also attempt to model in
some detail the interesting triple system SS Lac, which stopped eclipsing in
about 1950. We find that our model of SS Lac is quite constrained by the
relatively good observational data of this system, and leads to a specific
inclination (29 deg) of the outer orbit relative to the inner orbit at epoch
zero (1912). Although the intrinsic spins of the stars have little effect on
the orbit, the converse is not true: the spin axes can vary their orientation
relative to the close binary by up to 120 deg on a timescale of about a
century.Comment: 30 pages, 6 figure
Gravitational settling in pulsating subdwarf B stars and their progenitors
Diffusion of atoms can be important during quiescent phases of stellar
evolution. Particularly in the very thin inert envelopes of subdwarf B stars,
diffusive movements will considerably change the envelope structure and the
surface abundances on a short timescale. Also, the subdwarfs will inherit the
effects of diffusion in their direct progenitors, namely giants near the tip of
the red giant branch. This will influence the global evolution and the
pulsational properties of subdwarf B stars. We investigate the impact of
gravitational settling, thermal diffusion and concentration diffusion on the
evolution and pulsations of subdwarf B stars. Our diffusive stellar models are
compared with models evolved without diffusion. We constructed subdwarf B
models with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low
mass star ignited helium in an energetic flash, while the intermediate mass
star started helium fusion gently. For each progenitor type we computed series
with and without atomic diffusion. Atomic diffusion in red giants causes the
helium core mass at the onset of helium ignition to be larger. We find an
increase of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun
model. The effects on the red giant surface abundances are small after the
first dredge up. The evolutionary tracks of the diffusive subdwarf B models are
shifted to lower surface gravities and effective temperatures due to outward
diffusion of hydrogen. This affects both the frequencies of the excited modes
and the overall frequency spectrum. Especially the structure and pulsations of
the post-non-degenerate sdB star are drastically altered, proving that atomic
diffusion cannot be ignored in these stars.Comment: 10 pages, 6 figures, accepted for publication in A&
Optoacoustic solitons in Bragg gratings
Optical gap solitons, which exist due to a balance of nonlinearity and
dispersion due to a Bragg grating, can couple to acoustic waves through
electrostriction. This gives rise to a new species of ``gap-acoustic'' solitons
(GASs), for which we find exact analytic solutions. The GAS consists of an
optical pulse similar to the optical gap soliton, dressed by an accompanying
phonon pulse. Close to the speed of sound, the phonon component is large. In
subsonic (supersonic) solitons, the phonon pulse is a positive (negative)
density variation. Coupling to the acoustic field damps the solitons'
oscillatory instability, and gives rise to a distinct instability for
supersonic solitons, which may make the GAS decelerate and change direction,
ultimately making the soliton subsonic.Comment: 5 pages, 3 figure
Donor Stars in Black-Hole X-Ray Binaries
We study theoretically the formation of black-hole (BH) X-ray binaries.
Consistency of the models with the observed relative numbers of systems with
low-mass (<2 M_sun) and intermediate-mass (~2 M_sun - M_{BH}) donors leads to
severe constraints on the evolutionary parameters of the progenitors. In
particular, we find that (i) BH progenitor masses cannot exceed about 2 M_{BH};
(ii) high values of the common-envelope efficiency parameter (alpha_{CE} > 1)
are required, implying that energy sources other than orbital contraction must
be invoked to eject the envelope; (iii) the mass-loss fraction in helium-star
winds is limited to be <50%. Outside of this limited parameter space for
progenitors we find that either BH X-ray binary formation cannot occur at all
or donors do not have the full range of observed masses. We discuss the
implications of these results for the structure of massive hydrogen-rich stars,
the evolution of helium-stars, and BH formation. We also consider the possible
importance of asymmetric kicks.Comment: 29 pages, 6 figures, accepted for publication in The Astrophysical
Journa
The Cepheid Distance to NGC 1637: A Direct Test of the EPM Distance to SN 1999em
Type II-plateau supernovae (SNe II-P) are the classic variety of
core-collapse events that result from isolated, massive stars with thick
hydrogen envelopes intact at the time of explosion. Their distances are now
routinely estimated through two techniques: the expanding photosphere method
(EPM), a primary distance-determining method, and the recently developed
standard-candle method (SCM), a promising secondary technique. Using Cycle 10
HST observations, we identify 41 Cepheid variable stars in NGC 1637, the host
galaxy of the most thoroughly studied SN II-P to date, SN 1999em. Remarkably,
the Cepheid distance that we derive to NGC 1637, D = 11.7 +/- 1.0 Mpc, is
nearly 50% larger than earlier EPM distance estimates to SN 1999em. This is the
first direct comparison between these two primary distance determining methods
for a galaxy hosting a well-observed, spectroscopically and photometrically
normal, SN II-P. Extensive consistency checks show strong evidence to support
the Cepheid distance scale, so we are led to believe that either SN 1999em is
in some heretofore unsuspected way an unusual SN II-P, or that the SN II-P
distance scale must be revised. Assuming the latter, this one calibration
yields H_0(EPM) = 57 +/- 15 km/s/Mpc and H_0(SCM) = 59 +/- 11 km/s/Mpc;
additional calibrating galaxies are clearly desirable in order to test the
robustness of both determinations of H_0. The HST observations of NGC 1637 also
captured the fading SN 1999em two years after explosion, providing the latest
photometry ever obtained for an SN II-P. Through comparison with photometry of
SN 1987A at similar epochs, we conclude that a slightly greater amount of
radioactive Ni-56, ~0.09 M_sun, was ejected by SN 1999em than was derived for
SN 1987A (0.075 M_sun).Comment: Accepted for publication in The Astrophysical Journal; Version with
full figures available at http://astron.berkeley.edu/~leonard/papers
Autoantibodies against C1q as a diagnostic measure of lupus nephritis:systematic review and meta-analysis
This is a freely-available open access publication. Please cite the published version which is available via the DOI link in this record.Objectives: To evaluate the diagnostic accuracy of C1q autoantibodies in identifying lupus nephritis (LN) in patients with systemic lupus erythematosus (SLE). Data Sources and methods: Citation indexes were searched and 370 articles published from 1977 to 2013 were evaluated. The 31 selected studies included in the meta-analysis were cross-sectional in design. Among the 31 studies, 28 compared anti-C1q antibodies in 2769 SLE patients including those with (n = 1442) and without a history of LN (n = 1327). Nine studies examined anti-C1q in 517 SLE patients with active (n = 249) and inactive LN (n = 268). Hierarchical summary receiver operating characteristic (HSROC) random effects models were fitted to pool estimates of accuracy across the studies. Results: Anti-C1q antibodies discriminated between patients with and without a history of LN, with a median specificity of 73.5%. The HSROC model estimated the corresponding sensitivity to be 70.4%. A hypothetical patient with a 55% prior probability of having a history of LN as opposed to no history (the median prevalence across 28 eligible studies) would have a post-test probability of 76.4% following a positive test result (positive predictive value) or 33.0% following a negative test result (negative predictive value). For differentiating active from inactive LN the median specificity of anti-C1q antibodies was 80%, with a corresponding estimated sensitivity value 75.7% based on the HSROC model. A hypothetical patient with a 56% prior probability of active as opposed to inactive LN (the median prevalence across the 9 eligible studies) would have a post-test probability of 82.8% following a positive test result or 27.9% following a negative test result. Conclusions: Although C1q antibodies are associated with lupus nephritis the post-test probabilities are not sufficiently convincing to provide reasonable certainty of the presence or absence of history of disease/active disease.Arthritis Research UKPeninsula Collaboration for Leadership in Applied
Health Research and Care (CLAHRC)National Health Service
South West, funded by the National Institute for Health Research, U
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