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It has been an enigma that in the Tieschitz, H3, and other unequilibrated chondrites the silicates show quench textures yet their metallic minerals, according to the Wood [6] model, appear to have cooled extremely slowly.In Tieschitz, spherical metallic chondrules up to 500 μm in diameter, with textures indicating an origin as liquid droplets, consist of polycrystalline intergrowths of α(kamacite), γ(taenite) and troilite. Interface Ni compositions of contiguous α (～5 wt.%) and γ (～50 wt.%) grains define equilibrium tie-line relationships in the Fe-Ni system indicating equilibration to ～350°C (620 K). Polycrystalline γ(taenite) is multi-zoned with respect to Ni and is interpreted as the relict of a primary solidification structure. A mechanism whereby Ni compositional heterogeneities were produced in γ(taenite) by the rapid, non-equilibrium cooling of FeNiS melts during chondrule formation is discussed.Comparisons with lunar metal globules indicate solidification rates for Tieschitz metallic chondrules in the range 1–106 K/s. It is suggested that before or during aggregation, sub-solidus cooling in the temperature range ～700–1400°C with cooling times of days to weeks allowed the preservation of a relict solidification structure in metallic chondrules. At a temperature of ～700°C accretion and shallow burial (1–10 m) on the surface of the Tieschitz parent body provided insulation with slower cooling required to nucleate and grow α(kamacite) from the heterogeneous γ(taenite) under equilibrium conditions by the process of solid state diffusion proposed by Wood [6]. The cooling rate (1 K/106 yr) through 500°C derived using the Wood model is shown to be an underestimate of the real cooling rate of Tieschitz metal through that temperature, since it does not take into account Ni heterogeneities produced at higher temperatures. A rough estimate of the post-accretional cooling rate is obtained from the average size of α(kamacite) grains(<100 μm) andTeqα ～ 350°C indicating a cooling rate of the order of<1K/103yr through 500°C. 相似文献
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E. A. Corbett A. Robinson D. J. Axon J. H. Hough 《Monthly notices of the Royal Astronomical Society》2000,311(3):485-492
We present a series of new spectroscopic observations of BL Lacertae. The observations were obtained over a period of 30 months and sample the violent continuum flare of 1997 June–August. The equivalent width of the recently discovered broad H α line varies approximately inversely with the optical continuum flux, implying that the broad-line-emitting gas does not respond significantly to large variations in the beamed synchrotron continuum which dominates the spectrum at optical wavelengths. The simplest and most plausible interpretation of this result is that the broad-line region in BL Lacertae is 'unaware' of the synchrotron beam and is photoionized by another source, a likely candidate being a hot accretion disc. Alternatively, similar behaviour would be observed if the broad-line region is indeed photoionized by the synchrotron beam but is predominantly matter-bounded so that it absorbs only a small fraction of the incident ionizing photon flux. 相似文献
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J. W. Atkinson J. L. Collett A. Marconi D. J. Axon A. Alonso-Herrero D. Batcheldor J. J. Binney A. Capetti C. M. Carollo L. Dressel H. Ford J. Gerssen M. A. Hughes D. Macchetto W. Maciejewski M. R. Merrifield C. Scarlata W. Sparks M. Stiavelli Z. Tsvetanov R. P. van der Marel 《Monthly notices of the Royal Astronomical Society》2005,359(2):504-520
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Preliminary results of radio continuum observations of selected Sérsic-Pastoriza galaxies are presented. Subjects reported are their radio properties at 6 and 20 cm, estimates of linear polarization and spectral indices and a discussion of possible relationships between nuclear morphology and radio luminosity. 相似文献
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S. Young D. J. Axon J. H. Hough A. C. Fabian & M. J. Ward 《Monthly notices of the Royal Astronomical Society》1998,294(3):478-484
We present spectropolarimetric observations of the radio galaxy 3C 234 at optical and, for the first time, near-IR wavelengths. In agreement with previous observations we detect broad Hα in total and polarized flux, consistent with the scenario in which the central active nucleus is hidden from view and observed via scattered radiation.
We model the flux and polarization properties at optical and near-IR wavelengths, which result in a point-source, cone-based scattering geometry in the optical, and an extended source scattering in the near-IR, with a dichroic view to the emission regions, which becomes important only at the longer wavelengths. From this model we calculate an intrinsic Hα luminosity of 4.9×1044 erg s−1 , and an extinction to the near-IR emission region of 60 mag for AV . The scattered radiation also undergoes extinction, this time by 1 mag for AV , assuming a source function of a typical stellar-subtracted Seyfert 1 galaxy. Our modelling does not require the presence of a second featureless continuum (FC2).
Additionally, we require that the scatterers are in bulk radial outflow at approximately 600 km s−1 to explain an observed increase in the intrinsic polarization of the scattered broad Hα line in the red wing, and blueshifted narrow-line components in polarized flux. 相似文献
We model the flux and polarization properties at optical and near-IR wavelengths, which result in a point-source, cone-based scattering geometry in the optical, and an extended source scattering in the near-IR, with a dichroic view to the emission regions, which becomes important only at the longer wavelengths. From this model we calculate an intrinsic Hα luminosity of 4.9×10
Additionally, we require that the scatterers are in bulk radial outflow at approximately 600 km s