Resonant Excitation of White Dwarf Oscillations in Compact Object Binaries: 1. The No Back Reaction Approximation

We consider the evolution of white dwarfs with compact object companions (specifically black holes with masses up to 10^6 solar masses, neutron stars, and other white dwarfs). We suppose that the orbits are initially quite elliptical and then shrink and circularise under the action of gravitational radiation. During this evolution, the white dwarfs will pass through resonances when harmonics of the orbital frequency match the stellar oscillation eigenfrequencies. As a star passes through these resonances, the associated modes will be excited and can be driven to amplitudes that are so large that there is a back reaction on the orbit which, in turn, limits the growth of the modes. A formalism is presented for describing this dynamical interaction for a non-rotating star in the linear approximation when the orbit can be treated as non-relativistic. A semi-analytical expression is found for computing the resonant energy transfer as a function of stellar and orbital parameters for the regime where back reaction may be neglected. This is used to calculate the results of passage through a sequence of resonances for several hypothetical systems. It is found that the amplitude of the l=m=2 f-mode can be driven into the non-linear regime for appropriate initial conditions. We also discuss where the no back reaction approximation is expected to fail, and the qualitative effects of back reaction.Comment: 14 pages, 4 figures. Figure 3 corrected. Accepted for publication in MNRA

A multi-dimensional, adiabatic, hydrodynamics code for studying tidal excitation

We have developed a parallel, simple, and fast hydrodynamics code for multi-dimensional, self-gravitating, adiabatic flows. Our primary motivation is the study of the non-linear evolution of white dwarf oscillations excited via tidal resonances, typically over hundreds of stellar dynamical times. Consequently, we require long term stability, low diffusivity, and high algorithmic efficiency. An explicit, Eulerian, finite-difference scheme on a regular Cartesian grid fulfills these requirements. It provides uniform resolution throughout the flow, as well as simplifying the computation of the self-gravitational potential, which is done via spectral methods. In this paper, we describe the numerical scheme and present the results of some diagnostic problems. We also demonstrate the stability of a cold white dwarf in three dimensions over hundreds of dynamical times. Finally, we compare the results of the numerical scheme to the linear theory of adiabatic oscillations, finding numerical quality factors on the order of 6000, and excellent agreement with the oscillation frequency obtained by the linear analysis.Comment: 11 pages, 10 figure

Resonant Excitation of White Dwarf Oscillations in Compact Object Binaries

White dwarfs are ubiquitous in the known Universe. They are frequently found in binary systems with ordinary stars, giants, or compact objects as companions. Depending upon their histories, such systems may have significantly eccentric orbits. Because of gravitational radiation, a white dwarf-compact object binary will shrink and circularize with time. If the system is initially close enough, then the inspiral will occur on a time-scale shorter than a Hubble time. As an eccentric system inspirals, it will pass through resonances when harmonics of the orbital period match one of the white dwarf's normal mode eigenfrequencies. At these tidal resonances, energy can be transferred from the orbit to the white dwarf normal modes, and the system will pass through a sequence of such resonances for each mode. If the amplitude of a mode is driven high enough, the modes may damp due to non-linear processes and heat the white dwarf. If the temperature of the white dwarf can be raised in this way to a critical value, then the star may undergo a thermonuclear detonation that results in a Type Ia supernova. In order to determine whether such a scenario is possible, and what other observable consequences of tidal resonances may be, it is necessary to understand the resonant energy transfer and the non-linear evolution of modes on a white dwarf in some detail. A variational approach to the excitation of dynamical tides is presented. This is then used to study the energy transfer in the resonant excitation of tides. The energy transfer problem is complicated by the fact that a mode perturbs the orbit as it is resonantly excited, effectively creating a non-linear feedback loop. We call this effect 'back reaction.' In the present work, the problem is considered both in the approximation when back reaction is neglected, and when it is included. It is found that back reaction changes the resonant energy transfer both qualitatively and quantitatively. In particular, unlike the no back reaction case, the energy transfer with back reaction is shown to be always positive to lowest order in the rate of dissipation by gravitational radiation, and any initial energy in the mode before resonance is shown to increase the energy transfer. Numerical simulations of resonant mode excitation and non-linear evolution of white dwarf oscillations are also considered. An adiabatic, parallel hydrodynamic code is described for this. Results from several test problems and preliminary simulations of resonant tidal excitation are presented. The formalism developed for resonant tidal excitation is applied to studying the feasibility of a tidally triggered supernova via resonant excitation of quadrupolar f-modes. It is found that a 1.4 solar mass companion to the white dwarf is not viable, which rules out double degenerates and white dwarf-neutron star binaries as potential progenitors. However, it is found that with a companion mass of ten to hundred thousand solar masses, there exist regions in the parameter space where the white dwarf can be detonated before tidal disruption. It is calculated that the ejecta from such a detonation would remain trapped in orbit around the companion for the majority of cases, and would presumably be accreted eventually. A preliminary calculation of the importance of tidal effects for gravitational wave observations of capture sources with central masses of about a million solar masses is also presented. The resonant excitation of f-modes is found to be unimportant because of the long orbital periods at the last stable orbits. It is, however, found that the excitation of g-modes could introduce significant errors in the parameter estimation for such systems, though it would probably not affect detection capability. The exact magnitude of the errors depends upon the density of resonances during the period of observation, and therefore depends upon details of the white dwarf model.</p

Risk Factors for Perinatal Transmission of Hepatitis C Virus

OBJECTIVE: To estimate the rate of perinatal transmission of hepatitis C virus (HCV) infection, to identify risk factors for perinatal transmission of HCV infection, and to determine the viremic threshold for perinatal transmission. METHODS: This was a prospective, multicenter, observational study of pregnant individuals at less than 24 weeks of gestation screened for HCV infection from 2012 to 2018 in the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Individuals found to be HCV antibody-positive were followed throughout pregnancy. Children were followed for evidence of perinatal transmission at 2-6 months (HCV RNA testing) and at 18-24 months (HCV RNA and antibody testing) of life. The primary outcome was perinatal transmission, defined as positive test results at either follow-up time point. RESULTS: A total of 109,379 individuals were screened for HCV infection. Of the 1,224 participants who screened positive, 772 (63.1%) enrolled and 432 of those 772 (56.0%) had data available to assess primary outcome. The overall rate of perinatal transmission was 6.0% (26/432, 95% CI 4.0-8.7%). All children with HCV infection were born to individuals with demonstrable viremia. In viremic participants (n=314), the perinatal transmission rate was 8.0% (95% CI 5.2-11.5%). Risk factors for perinatal transmission included HCV RNA greater than 106 international units/mL (adjusted odds ratio [aOR] 8.22, 95% CI 3.16-21.4) and vaginal bleeding reported at any time before delivery (aOR 3.26, 95% CI 1.32-8.03). A viremic threshold for perinatal transmission could not be established. CONCLUSION: Perinatal transmission of HCV infection was limited to viremic individuals. High viral loads and antepartum bleeding were associated with perinatal transmission

Global variation in postoperative mortality and complications after cancer surgery: a multicentre, prospective cohort study in 82 countries

© 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 licenseBackground: 80% of individuals with cancer will require a surgical procedure, yet little comparative data exist on early outcomes in low-income and middle-income countries (LMICs). We compared postoperative outcomes in breast, colorectal, and gastric cancer surgery in hospitals worldwide, focusing on the effect of disease stage and complications on postoperative mortality. Methods: This was a multicentre, international prospective cohort study of consecutive adult patients undergoing surgery for primary breast, colorectal, or gastric cancer requiring a skin incision done under general or neuraxial anaesthesia. The primary outcome was death or major complication within 30 days of surgery. Multilevel logistic regression determined relationships within three-level nested models of patients within hospitals and countries. Hospital-level infrastructure effects were explored with three-way mediation analyses. This study was registered with ClinicalTrials.gov, NCT03471494. Findings: Between April 1, 2018, and Jan 31, 2019, we enrolled 15 958 patients from 428 hospitals in 82 countries (high income 9106 patients, 31 countries; upper-middle income 2721 patients, 23 countries; or lower-middle income 4131 patients, 28 countries). Patients in LMICs presented with more advanced disease compared with patients in high-income countries. 30-day mortality was higher for gastric cancer in low-income or lower-middle-income countries (adjusted odds ratio 3·72, 95% CI 1·70–8·16) and for colorectal cancer in low-income or lower-middle-income countries (4·59, 2·39–8·80) and upper-middle-income countries (2·06, 1·11–3·83). No difference in 30-day mortality was seen in breast cancer. The proportion of patients who died after a major complication was greatest in low-income or lower-middle-income countries (6·15, 3·26–11·59) and upper-middle-income countries (3·89, 2·08–7·29). Postoperative death after complications was partly explained by patient factors (60%) and partly by hospital or country (40%). The absence of consistently available postoperative care facilities was associated with seven to 10 more deaths per 100 major complications in LMICs. Cancer stage alone explained little of the early variation in mortality or postoperative complications. Interpretation: Higher levels of mortality after cancer surgery in LMICs was not fully explained by later presentation of disease. The capacity to rescue patients from surgical complications is a tangible opportunity for meaningful intervention. Early death after cancer surgery might be reduced by policies focusing on strengthening perioperative care systems to detect and intervene in common complications. Funding: National Institute for Health Research Global Health Research Unit