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The eccentricities of the barium stars 总被引:3,自引:0,他引:3
Amanda I. Karakas Christopher A. Tout John C. Lattanzio 《Monthly notices of the Royal Astronomical Society》2000,316(3):689-698
We investigate the eccentricities of barium (Ba ii ) stars formed via a stellar wind accretion model. We carry out a series of Monte Carlo simulations using a rapid binary evolution algorithm, which incorporates full tidal evolution, mass loss and accretion, and nucleosynthesis and dredge-up on the thermally pulsing asymptotic giant branch. We follow the enhancement of barium in the envelope of the accreting main-sequence companion and dilution into its convective envelope once the star ascends the giant branch.
The observed eccentricities of Ba ii stars are significantly smaller than those of an equivalent set of normal red giants but are nevertheless non-zero. We show that such a distribution of eccentricities is consistent with a wind accretion model for Ba ii star production with weak viscous tidal dissipation in the convective envelopes of giant stars. We successfully model the distribution of orbital periods and the number of observed Ba ii stars. The actual distribution of eccentricities is quite sensitive to the strength of the tides, so that we are able to confirm that this strength is close to, but less than, what is expected theoretically and found with alternative observational tests. Two systems – one very short-period but eccentric, and one long-period and highly eccentric – still lie outside the envelope of our models, and so require a more exotic formation mechanism. All our models, even those which were a good fit to the observed distributions, overproduced the number of high-period barium stars, a problem that could not be solved by some combination of the three parameters: tidal strength, tidal enhancement and wind accretion efficiency. 相似文献
The observed eccentricities of Ba ii stars are significantly smaller than those of an equivalent set of normal red giants but are nevertheless non-zero. We show that such a distribution of eccentricities is consistent with a wind accretion model for Ba ii star production with weak viscous tidal dissipation in the convective envelopes of giant stars. We successfully model the distribution of orbital periods and the number of observed Ba ii stars. The actual distribution of eccentricities is quite sensitive to the strength of the tides, so that we are able to confirm that this strength is close to, but less than, what is expected theoretically and found with alternative observational tests. Two systems – one very short-period but eccentric, and one long-period and highly eccentric – still lie outside the envelope of our models, and so require a more exotic formation mechanism. All our models, even those which were a good fit to the observed distributions, overproduced the number of high-period barium stars, a problem that could not be solved by some combination of the three parameters: tidal strength, tidal enhancement and wind accretion efficiency. 相似文献
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G. Fischer G. Karakas M. Blaas V. Ratmeyer N. Nowald R. Schlitzer P. Helmke R. Davenport B. Donner S. Neuer G. Wefer 《International Journal of Earth Sciences》2009,98(2):281-298
The ocean off NW Africa is the second most important coastal upwelling system with a total annual primary production of 0.33 Gt
of carbon per year (Carr in Deep Sea Res II 49:59–80, 2002). Deep ocean organic carbon fluxes measured by sediment traps are also fairly high despite low biogenic opal fluxes. Due
to a low supply of dissolved silicate from subsurface waters, the ocean off NW Africa is characterized by predominantly carbonate-secreting
primary producers, i.e. coccolithophorids. These algae which are key primary producers since millions of years are found in
organic- and chlorophyll-rich zooplankton fecal pellets, which sink rapidly through the water column within a few days. Particle
flux studies in the Mauretanian upwelling area (Cape Blanc) confirm the hypothesis of Armstrong et al. (Deep Sea Res II 49:219–236,
2002) who proposed that ballast availability, e.g. of carbonate particles, is essential to predict deep ocean organic carbon fluxes.
The role of dust as ballast mineral for organic carbon, however, must be also taken into consideration in the coastal settings
off NW Africa. There, high settling rates of larger particles approach 400 m day−1, which may be due to a particular composition of mineral ballast. An assessment of particle settling rates from opal-production
systems in the Southern Ocean of the Atlantic Sector, in contrast, provides lower values, consistent with the assumptions
of Francois et al. (Global Biogeochem Cycles 16(4):1087, 2002). Satellite chlorophyll distributions, particle distributions and fluxes in the water column off NW Africa as well as modelling
studies suggest a significant lateral flux component and export of particles from coastal shelf waters into the open ocean.
These transport processes have implications for paleo-reconstructions from sediment cores retrieved at continental margin
settings. 相似文献
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Karakas Gizem Nefeslioglu Hakan A. Kocaman Sultan Buyukdemircioglu Mehmet Yurur Tekin Gokceoglu Candan 《Landslides》2021,18(6):2193-2209
Landslides - On January 24, 2020, an earthquake with the Mw of 6.8 occurred on the East Anatolian Fault Zone in Elazig Province, Turkey, and triggered many landslides. Even though Turkey is in a... 相似文献
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A. Marcolini B. K. Gibson A. I. Karakas P. Sánchez-Blázquez 《Monthly notices of the Royal Astronomical Society》2009,395(2):719-735
We propose a new chemical evolution model aimed at explaining the chemical properties of globular clusters (GCs) stars. Our model depends upon the existence of (i) a peculiar pre-enrichment phase in the GC's parent galaxy associated with very low-metallicity Type II supernovae (SNe II) and (ii) localized inhomogeneous enrichment from a single Type Ia supernova (SN Ia) and intermediate-mass (4–7 M⊙ ) asymptotic giant branch field stars. GC formation is then assumed to take place within this chemically peculiar region. Thus, in our model the first low-mass GC stars to form are those with peculiar abundances (i.e. O-depleted and Na-enhanced), while 'normal' stars (i.e. O-rich and Na-depleted) are formed in a second stage when self-pollution from SNe II occurs and the peculiar pollution from the previous phase is dispersed. In this study, we focus on three different GCs: NGC 6752, 6205 (M 13) and 2808. We demonstrate that, within this framework, a model can be constructed which is consistent with (i) the elemental abundance anticorrelations, (ii) isotopic abundance patterns and (iii) the extreme [O/Fe] values observed in NGC 2808 and M 13, without violating the global constraints of approximately unimodal [Fe/H] and C+N+O. 相似文献
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