755 research outputs found
It takes a village to raise a tide: nonlinear multiple-mode coupling and mode identification in KOI-54
We explore the tidal excitation of stellar modes in binary systems using
Kepler observations of the remarkable eccentric binary KOI-54 (HD 187091; KIC
8112039), which displays strong ellipsoidal variation as well as a variety of
linear and nonlinear pulsations. We report the amplitude and phase of over 120
harmonic and anharmonic pulsations in the system. We use pulsation phases to
determine that the two largest-amplitude pulsations, the 90th and 91st
harmonics, most likely correspond to axisymmetric m=0 modes in both stars, and
thus cannot be responsible for resonance locks as had been recently proposed.
We find evidence that the amplitude of at least one of these two pulsations is
decreasing with a characteristic timescale of ~100 yr. We also use the
pulsations' phases to confirm the onset of the traveling wave regime for
harmonic pulsations with frequencies <~50 Omega_orbit, in agreement with
theoretical expectations. We present evidence that many pulsations that are not
harmonics of the orbital frequency correspond to modes undergoing simultaneous
nonlinear coupling to multiple linearly driven parent modes. Since coupling
among multiple modes can lower the threshold for nonlinear interactions,
nonlinear phenomena may be easier to observe in highly eccentric systems, where
broader arrays of driving frequencies are available. This may help to explain
why the observed amplitudes of the linear pulsations are much smaller than the
theoretical threshold for decay via three-mode coupling.Comment: Accepted for publication in MNRAS. Only minor corrections. 16 Pages;
8 Figures; 3 Table
Dynamical resonance locking in tidally interacting binary systems
We examine the dynamics of resonance locking in detached, tidally interacting
binary systems. In a resonance lock, a given stellar or planetary mode is
trapped in a highly resonant state for an extended period of time, during which
the spin and orbital frequencies vary in concert to maintain the resonance.
This phenomenon is qualitatively similar to resonance capture in planetary
dynamics. We show that resonance locks can accelerate the course of tidal
evolution in eccentric systems and also efficiently couple spin and orbital
evolution in circular binaries. Previous analyses of resonance locking have not
treated the mode amplitude as a fully dynamical variable, but rather assumed
the adiabatic (i.e. Lorentzian) approximation valid only in the limit of
relatively strong mode damping. We relax this approximation, analytically
derive conditions under which the fixed point associated with resonance locking
is stable, and further check these analytic results using numerical
integrations of the coupled mode, spin, and orbital evolution equations. These
show that resonance locking can sometimes take the form of complex limit cycles
or even chaotic trajectories. We provide simple analytic formulae that define
the binary and mode parameter regimes in which resonance locks of some kind
occur (stable, limit cycle, or chaotic). We briefly discuss the astrophysical
implications of our results for white dwarf and neutron star binaries as well
as eccentric stellar binaries.Comment: 16 pages, 11 figure
Tidal resonance locks in inspiraling white dwarf binaries
We calculate the tidal response of helium and carbon/oxygen (C/O) white dwarf
(WD) binaries inspiraling due to gravitational wave emission. We show that
resonance locks, previously considered in binaries with an early-type star,
occur universally in WD binaries. In a resonance lock, the orbital and spin
frequencies evolve in lockstep, so that the tidal forcing frequency is
approximately constant and a particular normal mode remains resonant, producing
efficient tidal dissipation and nearly synchronous rotation. We show that
analogous locks between the spin and orbital frequencies can occur not only
with global standing modes, but even when damping is so efficient that the
resonant tidal response becomes a traveling wave. We derive simple analytic
formulas for the tidal quality factor Q and tidal heating rate during a g-mode
resonance lock, and verify our results numerically. We find that Q ~ 10^7 for
orbital periods ~ 1 - 2 hr in C/O WDs, and Q ~ 10^9 for P_orb ~ 3 - 10 hr in
helium WDs. Typically tidal heating occurs sufficiently close to the surface
that the energy should be observable as surface emission. Moreover, near an
orbital period of ~ 10 min, the tidal heating rate reaches ~ 10^{-2} L_\sun,
rivaling the luminosities of our fiducial WD models. Recent observations of the
13-minute double-WD binary J0651 are roughly consistent with our theoretical
predictions. Tides naturally tend to generate differential rotation; however,
we show that the fossil magnetic field strength of a typical WD can maintain
solid-body rotation down to at least P_orb ~ 10 min even in the presence of a
tidal torque concentrated near the WD surface.Comment: 24 pages, 8 figure
AN INSTABILITY DUE TO THE NONLINEAR COUPLING OF p-MODES TO g-MODES: IMPLICATIONS FOR COALESCING NEUTRON STAR BINARIES
A weakly nonlinear fluid wave propagating within a star can be unstable to three-wave interactions. The resonant parametric instability is a well-known form of three-wave interaction in which a primary wave of frequency ω [subscript a] excites a pair of secondary waves of frequency ω [subscript b] + ω [subscript c] [~ over bar] ω [subscript a]. Here we consider a nonresonant form of three-wave interaction in which a low-frequency primary wave excites a high-frequency p-mode and a low-frequency g-mode such that ω [subscript b] + ω [subscript c] >> ω [subscript a]. We show that a p-mode can couple so strongly to a g-mode of similar radial wavelength that this type of nonresonant interaction is unstable even if the primary wave amplitude is small. As an application, we analyze the stability of the tide in coalescing neutron star binaries to p-g mode coupling. We find that the equilibrium tide and dynamical tide are both p-g unstable at gravitational wave frequencies f [subscript gw] [> over ~] 20 Hz and drive short wavelength p-g mode pairs to significant energies on very short timescales (much less than the orbital decay time due to gravitational radiation). Resonant parametric coupling to the tide is, by contrast, either stable or drives modes at a much smaller rate. We do not solve for the saturation of the p-g instability and therefore we cannot say precisely how it influences the evolution of neutron star binaries. However, we show that if even a single daughter mode saturates near its wave breaking amplitude, the p-g instability of the equilibrium tide will (1) induce significant orbital phase errors (Δφ [> over ~] 1 radian) that accumulate primarily at low frequencies (f [subscript gw] [< over ~] 50 Hz) and (2) heat the neutron star core to a temperature of T ~ 10[superscript 10] K. Since there are at least ~100 unstable p-g daughter pairs, Δφ and T are potentially much larger than these values. Tides might therefore significantly influence the gravitational wave signal and electromagnetic emission from coalescing neutron star binaries at much larger orbital separations than previously thought.National Science Foundation (U.S.) (AST-0908873)United States. National Aeronautics and Space Administration (NNX09AF98G
The radial velocity signature of tides raised in stars hosting exoplanets
Close-in, massive exoplanets raise significant tides in their stellar hosts. We compute the radial velocity (RV) signal due to this fluid motion in the equilibrium tide approximation. The predicted RVs in the observed sample of exoplanets exceed 1 m s[superscript −1] for 17 systems, with the largest predicted signal being ∼30 m s[superscript −1] for WASP-18 b. Tidally induced RVs are thus detectable with present methods. Both the tidal fluid flow and the epicyclic motion of a slightly eccentric orbit produce an RV signal at twice the orbital frequency. If care is not taken, the tidally induced RV may, in some cases, be confused with a finite orbital eccentricity. Indeed, WASP-18 b is reported to have an eccentric orbit with small e= 0.009 and pericentre longitude w = -π/2. Whereas such a close alignment of the orbit and line of sight to the observer requires fine-tuning, this phase in the RV signal is naturally explained by the tidal velocity signature of an e= 0 orbit. Additionally, the equilibrium tide estimate for the amplitude is in rough agreement with the data. Thus, the reported eccentricity for WASP-18 b is instead likely a signature of the tidally induced RV in the stellar host. Measurement of both the orbital and tidal velocities for non-transiting planets may allow planet mass and inclination to be separately determined solely from RV data. We suggest that high-precision fitting of RV data should include the tidal velocity signal in those cases where it may affect the determination of orbital parameters.National Science Foundation (U.S.) (AST-0908873)United States. National Aeronautics and Space Administration (NNX09AF98G
Azithromycin Synergizes with Cationic Antimicrobial Peptides to Exert Bactericidal and Therapeutic Activity Against Highly Multidrug-Resistant Gram-Negative Bacterial Pathogens
AbstractAntibiotic resistance poses an increasingly grave threat to the public health. Of pressing concern, rapid spread of carbapenem-resistance among multidrug-resistant (MDR) Gram-negative rods (GNR) is associated with few treatment options and high mortality rates. Current antibiotic susceptibility testing guiding patient management is performed in a standardized manner, identifying minimum inhibitory concentrations (MIC) in bacteriologic media, but ignoring host immune factors. Lacking activity in standard MIC testing, azithromycin (AZM), the most commonly prescribed antibiotic in the U.S., is never recommended for MDR GNR infection. Here we report a potent bactericidal action of AZM against MDR carbapenem-resistant isolates of Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii. This pharmaceutical activity is associated with enhanced AZM cell penetration in eukaryotic tissue culture media and striking multi-log-fold synergies with host cathelicidin antimicrobial peptide LL-37 or the last line antibiotic colistin. Finally, AZM monotherapy exerts clear therapeutic effects in murine models of MDR GNR infection. Our results suggest that AZM, currently ignored as a treatment option, could benefit patients with MDR GNR infections, especially in combination with colistin
Tidal asteroseismology: Kepler's KOI-54
We develop a general framework for interpreting and analyzing high-precision
lightcurves from eccentric stellar binaries. Although our methods are general,
we focus on the recently discovered Kepler system KOI-54, a face-on binary of
two A stars with and an orbital period of 42 days. KOI-54 exhibits
strong ellipsoidal variability during its periastron passage; its lightcurve
also contains ~20 pulsations at perfect harmonics of the orbital frequency, and
another ~10 nonharmonic pulsations. Analysis of such data is a new form of
asteroseismology in which oscillation amplitudes and phases rather than
frequencies contain information that can be mined to constrain stellar
properties. We qualitatively explain the physics of mode excitation and the
range of harmonics expected to be observed. To quantitatively model observed
pulsation spectra, we develop and apply a linear, tidally forced, nonadiabatic
stellar oscillation formalism including the Coriolis force. We produce temporal
power spectra for KOI-54 that are semi-quantitatively consistent with the
observations. Both stars in the KOI-54 system are expected to be rotating
pseudosynchronously, with resonant nonaxisymmetric modes providing a key
contribution to the total torque; such resonances provide a possible
explanation for the two largest-amplitude harmonic pulsations observed in
KOI-54, although we find quantitative problems with this interpretation. We
show in detail that the nonharmonic pulsations observed in KOI-54 can be
produced by nonlinear three-mode coupling. The methods developed in this paper
can be generalized in the future to determine the best-fit stellar parameters
given pulsation data. We also derive an analytic model of KOI-54's ellipsoidal
variability, including both tidal distortion and stellar irradiation, which can
be used to model other similar systems.Comment: 26 pages, 9 figures. Accepted to MNRA
Global burden of peripheral artery disease and its risk factors, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019
Background: Peripheral artery disease is a growing public health problem. We aimed to estimate the global disease burden of peripheral artery disease, its risk factors, and temporospatial trends to inform policy and public measures. Methods: Data on peripheral artery disease were modelled using the Global Burden of Disease, Injuries, and Risk Factors Study (GBD) 2019 database. Prevalence, disability-adjusted life years (DALYs), and mortality estimates of peripheral artery disease were extracted from GBD 2019. Total DALYs and age-standardised DALY rate of peripheral artery disease attributed to modifiable risk factors were also assessed. Findings: In 2019, the number of people aged 40 years and older with peripheral artery disease was 113 million (95% uncertainty interval [UI] 99·2–128·4), with a global prevalence of 1·52% (95% UI 1·33–1·72), of which 42·6% was in countries with low to middle Socio-demographic Index (SDI). The global prevalence of peripheral artery disease was higher in older people, (14·91% [12·41–17·87] in those aged 80–84 years), and was generally higher in females than in males. Globally, the total number of DALYs attributable to modifiable risk factors in 2019 accounted for 69·4% (64·2–74·3) of total peripheral artery disease DALYs. The prevalence of peripheral artery disease was highest in countries with high SDI and lowest in countries with low SDI, whereas DALY and mortality rates showed U-shaped curves, with the highest burden in the high and low SDI quintiles. Interpretation: The total number of people with peripheral artery disease has increased globally from 1990 to 2019. Despite the lower prevalence of peripheral artery disease in males and low-income countries, these groups showed similar DALY rates to females and higher-income countries, highlighting disproportionate burden in these groups. Modifiable risk factors were responsible for around 70% of the global peripheral artery disease burden. Public measures could mitigate the burden of peripheral artery disease by modifying risk factors. Funding: Bill & Melinda Gates Foundation
Global, regional, and national sex-specific burden and control of the HIV epidemic, 1990-2019, for 204 countries and territories: the Global Burden of Diseases Study 2019
Background: The sustainable development goals (SDGs) aim to end HIV/AIDS as a public health threat by 2030. Understanding the current state of the HIV epidemic and its change over time is essential to this effort. This study assesses the current sex-specific HIV burden in 204 countries and territories and measures progress in the control of the epidemic.
Methods: To estimate age-specific and sex-specific trends in 48 of 204 countries, we extended the Estimation and Projection Package Age-Sex Model to also implement the spectrum paediatric model. We used this model in cases where age and sex specific HIV-seroprevalence surveys and antenatal care-clinic sentinel surveillance data were available. For the remaining 156 of 204 locations, we developed a cohort-incidence bias adjustment to derive incidence as a function of cause-of-death data from vital registration systems. The incidence was input to a custom Spectrum model. To assess progress, we measured the percentage change in incident cases and deaths between 2010 and 2019 (threshold >75% decline), the ratio of incident cases to number of people living with HIV (incidence-to-prevalence ratio threshold <0·03), and the ratio of incident cases to deaths (incidence-to-mortality ratio threshold <1·0).
Findings: In 2019, there were 36·8 million (95% uncertainty interval [UI] 35·1–38·9) people living with HIV worldwide. There were 0·84 males (95% UI 0·78–0·91) per female living with HIV in 2019, 0·99 male infections (0·91–1·10) for every female infection, and 1·02 male deaths (0·95–1·10) per female death. Global progress in incident cases and deaths between 2010 and 2019 was driven by sub-Saharan Africa (with a 28·52% decrease in incident cases, 95% UI 19·58–35·43, and a 39·66% decrease in deaths, 36·49–42·36). Elsewhere, the incidence remained stable or increased, whereas deaths generally decreased. In 2019, the global incidence-to-prevalence ratio was 0·05 (95% UI 0·05–0·06) and the global incidence-to-mortality ratio was 1·94 (1·76–2·12). No regions met suggested thresholds for progress. Interpretation: Sub-Saharan Africa had both the highest HIV burden and the greatest progress between 1990 and 2019. The number of incident cases and deaths in males and females approached parity in 2019, although there remained more females with HIV than males with HIV. Globally, the HIV epidemic is far from the UNAIDS benchmarks on progress metrics.
Funding: The Bill & Melinda Gates Foundation, the National Institute of Mental Health of the US National Institutes of Health (NIH), and the National Institute on Aging of the NIH
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