On the pulsation hypothesis for massive red supergiants

Red supergiant pulsar hypothesis using Q values derived from stellar luminosity, period, and spectru

Pulsational Instabilities in Hydrogen-poor Massive Blue Stars. 1

Structure and pulsational properties of hydrogen poor massive blue stars with helium core

Evolution of stars with suppressed core convection

Stellar evolution on the upper main sequence was computed for models of stars with cores assumed to be in radiative equilibrium, up to the point of central helium ignition. The role of the Schonberg-Chandrasekhar limit for an isothermal core is found to be critical for the evolutionary tracks. Observational data are used to rule out the hypothesis of evolution with radiative cores (in upper main-sequence stars) and, by implication, of magnetic fields that are sufficiently strong to have suppressed the core convention

Solar neutrinos and the influence of radiative opacities on solar models

Use of new radiative opacities based on the hot Thomas-Fermi model of the atom yields a predicted solar neutrino flux which is still considerably larger than the flux observed in Davis's Cl-37 experiment

Flood basalt eruptions, comet showers, and mass extinction events

A chronology of initiation dates of the major continental flood basalt episodes has been established from compilation of published K-Ar and Ar-Ar ages of basaltic flows and related basic intrusions. The dating is therefore independent of the biostratigraphic and paleomagnetic time scales, and the estimated errors of the inititation dates are approximately + or - 4 pct. There are 11 distinct episodes of continental flood basalts known during the past 250 Myr. The data show that flood basalt episodes are generally relatively brief geologic events, with intermittent eruptions during peak output periods lasting ony 2 to 3 Myr or less. Statistical analyses suggest that these episodes may have occurred quasi-periodically with a mean cycle time of 32 + or - 1 Myr. The initiation dates of the flood basalts are close to the estimated dates of marine mass extinctions and impact-crater clusters. Although a purely internal forcing might be argued for the flood basalt volcanism, quasi-periodic comet impacts may be the trigger for both the flood basalts and the extinctions. Impact cratering models suggest that large-body impactors lead to deep initial cratering, and therefore may cause mantle disturbances and initiate mantle plume activity. The flood basalt episodes commonly mark the initiation or jump of a mantle hotspot, and are often followed by continental rifting and separation. Evidence from dynamical studies of impacts, occurrences of craters and hotspots, and the geochemistry of boundary layers is synthesized to provide a possible model of impact-generated volcanism. Flood basalt eruptions may themselves have severe effects on climate, and possibly on life. Impacts might, as a result, have led to mass extinctions through direct atmospheric disturbances, and/or indirectly through prolonged flood basalt volcanism

CRITERIA FOR THE DISCOVERY OF BETA CEPHEI STARS ACCORDING TO THE MU -MECHANISM THEORY

Observational discriminants are proposed to test the μ-mechanism theory of β Cephei stars as advanced by Stothers and Simon (1969). These include: helium and nitrogen overabundances, far ultraviolet flux excesses (high surface temperature), narrow (occasionally broad) spectral line widths, and binary character. A list of potential β Cep candidates, comprising 24 spectroscopic binaries with spectral type O9-B4 III -V and a large orbital ratio of masses, has been compiled for the benefit of observers interested in searching for new β Cep stars. The data thus far accumulated are not in contradiction with, and occasionally positively support, the μ-mechanism theory

A Criterion for Nuclear-Energized Pulsational Instability

This Note will present briefly some general properties of nuclear-energized pulsations of stars as treated in the linear quasi-adiabatic theory. In particular, the results will be derived from a number of detailed calculations of stellar models. It is well known that an upper limit for the fundamental eigenfrequency of radial pulsation Ѡ0 is given by Ѡmax2 = 1ʃ0(3Γ1 ‒ 4)q/x dq / 1ʃ0 x2dq, Ѡ2= (2∏/Period)2 R3/GM (e.g., Ledoux and Walraven 1958). According to this expression, Ѡmax2 increases both with decreasing radiation pressure (i.e., increasing Γ1) and with increasing central condensation as measured by the ratio of integral

CRITERIA FOR THE DISCOVERY OF BETA CEPHEI STARS ACCORDING TO THE MU -MECHANISM THEORY

Observational discriminants are proposed to test the μ-mechanism theory of β Cephei stars as advanced by Stothers and Simon (1969). These include: helium and nitrogen overabundances, far ultraviolet flux excesses (high surface temperature), narrow (occasionally broad) spectral line widths, and binary character. A list of potential β Cep candidates, comprising 24 spectroscopic binaries with spectral type O9-B4 III -V and a large orbital ratio of masses, has been compiled for the benefit of observers interested in searching for new β Cep stars. The data thus far accumulated are not in contradiction with, and occasionally positively support, the μ-mechanism theory

Pulsational instability of yellow hypergiants

Instability of population I (X=0.7, Y=0.02) massive stars against radial oscillations during the post-main sequence gravitational contraction of the helium core is investigated. Initial stellar masses are in the range from 65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar oscillations we assumed that energy transfer in the envelope of the pulsating star is due to radiative heat conduction and convection. The convective heat transfer was treated in the framework of the theory of time-dependent turbulent convection. During evolutionary expansion of outer layers after hydrogen exhaustion in the stellar core the star is shown to be unstable against radial oscillations while its effective temperature is Teff > 6700K for Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability is due to the \kappa-mechanism in helium ionization zones and at lower effective temperature oscillations decay because of significantly increasing convection. The upper limit of the period of radial pulsations on this stage of evolution does not exceed 200 day. Radial oscillations of the hypergiant resume during evolutionary contraction of outer layers when the effective temperature is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot. Initially radial oscillations are due to instability of the first overtone and transition to fundamental mode pulsations takes place at higher effective temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for Mzams=90M_\odot). The upper limit of the period of radial oscillations of evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow hypergiants are stable against radial stellar pulsations during the major part of their evolutionary stage.Comment: 20 pages, 7 gigures. Accepted for publication in Astronomy Letter

Forehead Skin Blood Flow in Normal Neonates during Active and Quiet Sleep, Measured with a Diode Laser Doppler Instrument

Changes in forehead skin blood flow during active and quiet sleep were determined in 16 healthy neonates using a recently developed semi-conductor laser Doppler flow meter without light conducting fibres. Measurements were carried out at a postnatal age varying from 5 hours to 7 days. The two sleep states could be distinguished in 17 recordings. The mean skin blood flow values during active sleep were significantly higher (p<0.01) than those during quiet sleep, the mean increase being 28.1%. The variability of the flow signal, expressed as the coefficient of variation, changed significantly from 23.1% during active sleep to 18.2% during quiet sleep