Radial velocities of pulsating subdwarf B stars: KPD 2109+4401 and PB 8783     

C. Simon Jeffery  Don Pollacco 《Monthly notices of the Royal Astronomical Society》2000,318(4):974-982

High-speed spectroscopy of two pulsating subdwarf B stars, KPD 2109+4401 and PB 8783, is presented. Radial motions are detected with the same frequencies as reported from photometric observations and with amplitudes of ∼2 km s⁻¹ in two or more independent modes. These represent the first direct observations of surface motion arising from multimode non-radial oscillations in subdwarf B stars. In the case of the sdB+F binary PB 8783, the velocities of both components are resolved; high-frequency oscillations are found only in the sdB star and not the F star. There also appears to be evidence for mutual motion of the binary components. If confirmed, it implies that the F-type companion is ≳1.2 times more massive than the sdB star, while the amplitude of the F-star acceleration over 4 h would constrain the orbital period to lie between 0.5 and 3.2 d.  相似文献   

The evolution of a rapidly accreting helium white dwarf to become a low-luminosity helium star     

Hideyuki Saio  C. Simon Jeffery 《Monthly notices of the Royal Astronomical Society》2000,313(4):671-677

We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the helium‐burning shell is approximately 0.25 M_⊙, the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models.
These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario.  相似文献   

Simultaneous modeling of thermopower and electrical conduction in olivine     

Steven Constable  Jeffery J. Roberts 《Physics and Chemistry of Minerals》1997,24(5):319-325

 Measurements of conductivity and thermopower as a function of oxygen fugacity (ƒ_{O 2}) are used to derive a model for conduction in olivine. Thermopower at 1000–1200 °C is between 50 and 400 μV/K and has a positive ƒ_{O 2} dependence, and electrical conductivity exhibits approximately a ¹/₁₁ power dependence on ƒ_{O 2}. However, small polarons, considered to be the conducting defect in olivine at these temperatures, would produce a larger thermopower than observed, with a negative ƒ_{O 2} dependence, as well as ¹/₆ power dependence of conductivity on ƒ_{O 2}. At least one other conducting defect species must be invoked to explain the observed magnitude and ƒ_{O 2} dependence of thermopower. An electron/polaron model cannot be made to fit the conductivity and thermopower data well, but a polaron/magnesium vacancy model fits the data if a constant polaron or magnesium vacancy term is included. Concentrations from our fits are consistent with predictions from theoretical models, and our analysis predicts a transition from polaron dominance in conduction to magnesium vacancy dominance at around 1300 °C, as has been previously inferred from other data. Received April 2, 1996 / Revised, accepted September 6, 1996  相似文献   

One-dimensional modelling of convective CO₂ exchange in the Tropical Atlantic   总被引：1，自引：0，他引：1  

C.D. Jeffery  D.K. Woolf  I.S. Robinson  C.J. Donlon 《Ocean Modelling》2007,19(3-4):161-182

Diurnal changes in seawater temperature affect the amount of air–sea gas exchange taking place through changes in solubility and buoyancy-driven nocturnal convection, which enhances the gas transfer velocity. We use a combination of in situ and satellite derived radiometric measurements and a modified version of the General Ocean Turbulence Model (GOTM), which includes the National Oceanic and Atmospheric Administration Coupled-Ocean Atmospheric Response Experiment (NOAA-COARE) air–sea gas transfer parameterization, to investigate heat and carbon dioxide exchange over the diurnal cycle in the Tropical Atlantic. A new term based on a water-side convective velocity scale (w_*w) is included, to improve parameterization of convectively driven gas transfer. Meteorological data from the PIRATA mooring located at 10°S10°W in the Tropical Atlantic are used, in conjunction with cloud cover estimates from Meteosat-7, to calculate fluxes of longwave, latent and sensible heat along with a heat budget and temperature profiles during February 2002. Twin model experiments, representing idealistic and realistic conditions, reveal that over daily time scales the additional contribution to gas exchange from convective overturning is important. Increases in transfer velocity of up to 20% are observed during times of strong insolation and low wind speeds (<6 m s⁻¹); the greatest enhancement from w_*w to the CO₂ flux occurs when diurnal warming is large. Hence, air–sea fluxes of CO₂ calculated using simple parameterizations underestimate the contribution from convective processes. The results support the need for parameterizations of gas transfer that are based on more than wind speed alone and include information about the heat budget.  相似文献   

Inferring marine distribution of Canadian and Irish Atlantic salmon (Salmo salar L.) in the North Atlantic from tissue concentrations of bio-accumulated caesium 137     

Spares  Aaron D.; Reader  Jeffery M.; Stokesbury  Michael J. W.; McDermott  Tom; Zikovsky  Lubomir; Avery  Trevor S.; Dadswell  Michael J. 《ICES Journal of Marine Science》2007,64(2):394-404

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Historic records of phosphorus levels in the reef-building coral Montastrea annularis from Tobago, West Indies     

Kishan Kumarsingh  Richard Laydoo  Jeffery K. Chen  Avril M. Siung-Chang   《Marine pollution bulletin》1998,36(12):1012-1018

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Investigations of the variability of dust particle sizes in the martian atmosphere using the NASA Ames General Circulation Model     

Melinda A. Kahre  Jeffery L. Hollingsworth  James R. Murphy 《Icarus》2008,195(2):576-597

We present a Mars General Circulation Model (GCM) numerical investigation of the physical processes (i.e., wind stress and dust devil dust lifting and atmospheric transport) responsible for temporal and spatial variability of suspended dust particle sizes. Measurements of spatial and temporal variations in airborne dust particles sizes in the martian atmosphere have been derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectral and emission phase function data [Wolff, M.J., Clancy, R.T., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002057. 1-1; Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002058. 2-1]. The range of dust particle sizes simulated by the NASA Ames GCM is qualitatively consistent with TES-derived observations of effective dust particle size variability. Model results suggest that the wind stress dust lifting scheme (which produces regionally confined dust lifting) is the process responsible for the majority of the dust particle size variability in the martian atmosphere. Additionally, model results suggest that atmospheric transport processes play an important role in the evolution of atmospheric dust particles sizes during substantial dust storms on Mars. Finally, we show that including the radiative effects of a spatially variable particle size distribution significantly influences thermal and dynamical fields during the dissipation phase of the simulated global dust storm.  相似文献   

A computational platform for considering the effects of aerodynamic and seismic load combination for utility scale horizontal axis wind turbines     

Mohammad-Amin Asareh  Ian Prowell  Jeffery Volz  William Schonberg 《地震工程与工程振动(英文版)》2016,15(1):91-102

The wide deployment of wind turbines in locations with high seismic hazard has led engineers to take into account a more comprehensive seismic design of such structures. Turbine specific guidelines usually use simplified methods and consider many assumptions to combine seismic demand with the other operational loads effecting the design of these structures. As the turbines increase in size and capacity, the interaction between seismic loads and aerodynamic loads becomes even more important. In response to the need for a computational tool that can perform coupled simulations of wind and seismic loads, a seismic module is developed for the FAST code and described in this research. This platform allows engineers working in this industry to directly consider interaction between seismic and other environmental loads for turbines. This paper details the practical application and theory of this platform and provides examples for the use of different capabilities. The platform is then used to show the suitable earthquake and operational load combination with the implicit consideration of aerodynamic damping by estimating appropriate load factors.  相似文献