| 1. |
- Gehren, Thomas, et al.
(author)
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Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
- 2006
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In: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; :451, s. 1065-1079
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Journal article (pop. science, debate, etc.)abstract
- Aims. Parameters for 55 nearby metal-poor stars are determined using high-resolution spectroscopy. Together with similar data takenfrom a recent analysis, they are used to show trends of their Galactic evolution with stellar [Fe/H] or [Mg/H] abundances. Theseparation of abundance ratios between disk and halo stars is used as a basic criterion for population membership.Methods. After careful selection of a clean subsample free of suspected or known binaries and peculiar stars, abundances of Mg, Na and Al are based on NLTE kinetic equilibrium calculations applied to spectrum synthesis methods.Results. The relation between [Na/Mg] and [Fe/H] is a continuous enrichment through all three Galactic populations spanning a rangeof values between a metal-poor plateau at [Na/Mg] = −0.7 and solar values. [Al/Mg] displays a step-like difference between starsof the Galactic halo with [Al/Mg] ∼ −0.45 and the two disk populations with [Al/Mg] ∼ +0.10. [Al/Mg] ratios, together with the[Mg/Fe] ratios, asymmetric drift velocities V, and stellar evolutionary ages, make possible the individual discrimination between stars of the thick disk and the halo. At present, this evidence is limited by the small number of stars, and by the theoretical and empirical uncertainties of stellar age determinations, but it achieves a high significance.Conclusions. While the stellar sample is not complete with respect to space volume, the resulting abundances indicate the necessityto revise current models of chemical evolution to allow for an adequate production of Al in early stellar generations.
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| 2. |
- Zhao-Hai, He, et al.
(author)
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Characters of the magnetotail plasma injection surveyed from Cluster observation
- 2008
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In: Chinese Journal of Geophysics. - 0001-5733. ; 51:2, s. 307-315
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Journal article (peer-reviewed)abstract
- The properties of proton (0 eV < E < 40 keV) in the plasma sheet are examined by means of a superposed epoch analysis, using 115 magnetotail plasma injection events which are identified from Cluster magnetotail orbit time in between 2001 and 2004. All events distribute in magnetic local time from 20 p.m. to 04 a. in. Five classes of magnetotail injection events are found to be similar with the geosynchronous observation: (1) pure ion injections; (2) ion injections followed a few minutes later by an electron injection; (3) simultaneous ion and electron injections; (4) electron injections followed a few minutes later by an ion injection; (5) pure electron injections. Proton shows a significant increase in temperature and density at the onset, and injects earthward with an increasing velocity more than the pre-injection average one. Super-posed epoch analysis on the simultaneous observation data of dusk-dawn electric field from the EFW (Electric Field and Waves) instrument, we found two different electric field configurations: ( I) electric field increases suddenly at the onset and the value is positive; (2) electric field changes the direction at the onset, and turns into a negative value. The simulation results of velocity vector after injection in equatorial plain, calculated in static magnetic (T89c) and electric (Volland-Stern) field models, agree with the statistical results mostly, and that suggests the electric drift caused by dawn-dusk convection electric field is one of the mechanisms of the particles injected earthward in magnetotail(-18 R-E < R < -10R(E)).
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