233 research outputs found
XMM-Newton X-ray Observations of the Wolf-Rayet Binary System WR 147
We present results of a 20 ksec X-ray observation of the Wolf-Rayet (WR)
binary system WR 147 obtained with XMM-Newton. Previous studies have shown that
this system consists of a nitrogen-type WN8 star plus an OB companion whose
winds are interacting to produce a colliding wind shock. X-ray spectra from the
pn and MOS detectors confirm the high extinction reported from IR studies and
reveal hot plasma including the first detection of the Fe K-alpha line complex
at 6.67 keV. Spectral fits with a constant-temperature plane-parallel shock
model give a shock temperature kT(shock) = 2.7 keV [T(shock) ~ 31 MK], close to
but slightly hotter than the maximum temperature predicted for a colliding wind
shock. Optically thin plasma models suggest even higher temperatures, which are
not yet ruled out. The X-ray spectra are harder than can be accounted for using
2D numerical colliding wind shock models based on nominal mass-loss parameters.
Possible explanations include: (i) underestimates of the terminal wind speeds
or wind abundances, (ii) overly simplistic colliding wind models, or (iii) the
presence of other X-ray emission mechanisms besides colliding wind shocks.
Further improvement of the numerical models to include potentially important
physics such as non-equilibrium ionization will be needed to rigorously test
the colliding wind interpretation.Comment: 8 pages, 7 figure
X-Rays From Massive OB Stars: Thermal Emission From Radiative Shocks
Chandra gratings spectra of a sample of 15 massive OB stars were analyzed
under the basic assumption that the X-ray emission is produced in an ensemble
of shocks formed in the winds driven by these objects. Shocks develop either as
a result of radiation-driven instabilities or due to confinement of the wind by
relatively strong magnetic field, and since they are radiative, a simple model
of their X-ray emission was developed that allows a direct comparison with
observations. According to our model, the shock structures (clumps, complete or
fractional shells) eventually become `cold' clouds in the X-ray sky of the
star. As a result, it is expected that for large covering factors of the hot
clumps, there is a high probability for X-ray absorption by the `cold' clouds,
resulting in blue-shifted spectral lines. Our analysis has revealed that such a
correlation indeed exists for the considered sample of OB stars. As to the
temperature characteristics of the X-ray emission plasma, the studied OB stars
fall in two groups: (i) one with plasma temperature limited to 0.1-0.4 keV;
(ii) the other wtih X-rays produced in plasmas at considerably higher
temperatures. We argue that the two groups correspond to different mechanisms
for the origin of X-rays: in radiative-driven instability shocks and in
magnetically-confined wind shocks, respectively.Comment: 11 pages, 4 figures, 2 tables; accepted for publication in MNRA
Harvard Glaucoma Fairness: A Retinal Nerve Disease Dataset for Fairness Learning and Fair Identity Normalization
Fairness (also known as equity interchangeably) in machine learning is
important for societal well-being, but limited public datasets hinder its
progress. Currently, no dedicated public medical datasets with imaging data for
fairness learning are available, though minority groups suffer from more health
issues. To address this gap, we introduce Harvard Glaucoma Fairness
(Harvard-GF), a retinal nerve disease dataset with both 2D and 3D imaging data
and balanced racial groups for glaucoma detection. Glaucoma is the leading
cause of irreversible blindness globally with Blacks having doubled glaucoma
prevalence than other races. We also propose a fair identity normalization
(FIN) approach to equalize the feature importance between different identity
groups. Our FIN approach is compared with various the-state-of-the-art fairness
learning methods with superior performance in the racial, gender, and ethnicity
fairness tasks with 2D and 3D imaging data, which demonstrate the utilities of
our dataset Harvard-GF for fairness learning. To facilitate fairness
comparisons between different models, we propose an equity-scaled performance
measure, which can be flexibly used to compare all kinds of performance metrics
in the context of fairness. The dataset and code are publicly accessible via
\url{https://ophai.hms.harvard.edu/datasets/harvard-glaucoma-fairness-3300-samples/}
Evolution of floral symmetry
Flowers can be classified into two basic types according to their symmetry: regular flowers have more than one plane of symmetry and irregular flowers have only a single plane of symmetry. The irregular condition is thought to have evolved many times independently from the regular one: most commonly through the appearance of asymmetry along the dorso-ventral axis of the flower. In most cases, the irregular condition is associated with a particular type of inflorescence architecture. To understand the molecular mechanism and evolutionary origin of irregular flowers, we have been investigating genes controlling asymmetry in Antirrhinum. Several mutations have been described in Antirrhinum, a species with irregular flowers, that reduce or eliminate asymmetry along the dorso-ventral axis. We describe the nature of these mutations and how they may be used to analyse the molecular mechanisms underlying floral evolution
Evaluation of Mycobacterium avium subsp. paratuberculosis isocitrate lyase (IcL) and ABC transporter (BacA) knockout mutants as vaccine candidates
There has been little success in controlling Johne’s disease, caused by Mycobacterium avium subsp. paratuberculosis, due to suboptimal diagnostics and the ineffectiveness of available vaccines. By knocking out BacA and IcL, genes required for MAP survival in dairy calves, two live-attenuated vaccine candidates were created. This study evaluated the host-specific attenuation of MAP IcL and BacA mutants in mouse and calf models, as well as the elicited immune responses. Deletion mutants were generated in MAP strain A1-157 through specialized transduction and found viable in vitro. First, the mutants’ attenuation and elicited cytokine secretion were assessed in a mouse model, 3 weeks after intraperitoneal inoculation with MAP strains. Later, vaccine strains were assessed in a natural host infection model where calves received 109CFU oral dose of MAP wild-type or mutant strains at 2 weeks old. Transcription levels of cytokines in PBMCs were evaluated at 12-, 14-, and 16-weeks post-inoculation (WPI) and MAP colonization in tissue was assessed at 4.5 months after inoculation. Whereas both vaccine candidates colonized mouse tissues similarly to wild-type strain, both failed to persist in calf tissues. In either mouse or calf models, gene deletion did not reduce immunogenicity. Instead, inoculation with ΔBacA induced a greater upregulation of proinflammatory cytokines than ΔIcL and wild-type in both models and a greater expansion of cytotoxic and memory T-cells than uninfected control in calves. ΔBacA and wild-type strains significantly increased secretion of IP-10, MIG, TNFα, and RANTES in mice serum compared to uninfected control. This agreed with upregulation of IL-12, IL-17, and TNFα in calves inoculated with ΔBacA at all time points. The ΔBacA also gave rise to greater populations of CD4+CD45RO+, and CD8+ cells than uninfected control calves at 16 WPI. Low survival rate of MAP in macrophages co-incubated with PBMCs isolated from the ΔBacA group indicated that these cell populations are capable of killing MAP. Overall, the immune response elicited by ΔBacA is stronger compared to ΔIcL and it is maintained over two different models and over time in calves. Further investigation is warranted to evaluate the BacA mutant's protection against MAP infection as a live attenuated vaccine candidate
An actuator surface model to simulate vertical axis turbines
An actuator surface model (ASM) to be employed to simulate the effect of a vertical axis turbine on the hydrodynamics in its vicinity, particularly its wake is introduced. The advantage of the newly developed ASM is that it can represent the complex flow inside the vertical axis turbine’s perimeter reasonably well, and hence, is able to predict, with a satisfying degree of accuracy, the turbine’s near-wake, with a low computational cost. The ASM appears to overcome the inadequacy of actuator line models to account for the flow blockage of the rotor blades when they are on the up-stream side of the revolution, because the ASM uses a surface instead of a line to represent the blade. The ASM was used on a series of test cases to prove its validity, demonstrating that first order flow statistics—in our study, profiles of the stream-wise velocity—in the turbine’s vicinity, can be produced with reasonable accuracy. The prediction of second order statistics, here in the form of the turbulent kinetic energy (TKE), exhibited dependence on the chosen grid; the finer the grid, the better the match between measured and computed TKE profiles
Anisotropic mass ejection in binary mergers
We investigate the mass loss from a rotationally distorted envelope following
the early, rapid in-spiral of a companion star inside a common envelope. For
initially wide, massive binaries (M_1+M_2=20M_{\odot}, P\sim 10 yr), the
primary has a convective envelope at the onset of mass transfer and is able to
store much of the available orbital angular momentum in its expanded envelope.
Three-dimensional smoothed particle hydrodynamics calculations show that mass
loss is enhanced at mid-latitudes due to shock reflection from a torus-shaped
outer envelope. Mass ejection in the equatorial plane is completely suppressed
if the shock wave is too weak to penetrate the outer envelope in the equatorial
direction (typically when the energy deposited in the star is less than about
one-third of the binding energy of the envelope). We present a parameter study
to show how the geometry of the ejecta depends on the angular momentum and the
energy deposited in the envelope during a merging event. Applications to the
nearly axisymmetric, but very non-spherical nebulae around SN1987A and Sheridan
25 are discussed, as well as possible links to RY Scuti and the Small
Magellanic Cloud object R4.Comment: 10 pages, 11 figures, accepted for publication in MNRAS. Figs 1, 2
and 10 reduced in siz
Theoretical X-ray Line Profiles from Colliding Wind Binaries
We present theoretical X-ray line profiles from a range of model colliding
wind systems. In particular, we investigate the effects of varying the stellar
mass-loss rates, the wind speeds, and the viewing orientation. We find that a
wide range of theoretical line profile shapes is possible, varying with orbital
inclination and phase. At or near conjunction, the lines have approximately
Gaussian profiles, with small widths (HWHM ~ 0.1 v_\infty) and definite blue-
or redshifts (depending on whether the star with the weaker wind is in front or
behind). When the system is viewed at quadrature, the lines are generally much
broader (HWHM ~ v_\infty), flat-topped and unshifted. Local absorption can have
a major effect on the observed profiles - in systems with mass-loss rates of a
few times 10^{-6} Msol/yr the lower energy lines (E <~ 1 kev) are particularly
affected. This generally results in blueward-skewed profiles, especially when
the system is viewed through the dense wind of the primary. The orbital
variation of the line widths and shifts is reduced in a low inclination binary.
The extreme case is a binary with i = 0 degrees, for which we would expect no
line profile variation.Comment: 15 pages, 15 figures. To appear in MNRA
Weak wind effects in CNO driven winds of hot first stars
During the evolution of rotating first stars, which initially consisted of
only hydrogen and helium, CNO elements may emerge to their surface. These stars
may therefore have winds that are driven only by CNO elements. We study weak
wind effects (Gayley-Owocki heating and multicomponent effects) in stellar
winds of first generation stars driven purely by CNO elements. We apply our
NLTE multicomponent models and hydrodynamical simulations. The multicomponent
effects (frictional heating and decoupling) are important particularly for low
metallicity winds, but they influence mass loss rate only if they cause
decoupling for velocities lower than the escape velocity. The multicomponent
effects also modify the feedback from first stars. As a result of the
decoupling of radiatively accelerated metals from hydrogen and helium, the
first low-energy cosmic ray particles are generated. We study the interaction
of these particles with the interstellar medium concluding that these particles
easily penetrate the interstellar medium of a given minihalo. We discuss the
charging of the first stars by means of their winds. Gayley-Owocki heating,
frictional heating, and the decoupling of wind components occur in the winds of
evolved low-metallicity stars and the solar metallicity main-sequence stars.Comment: 10 pages, accepted for publication in Astronomy & Astrophysic
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