The paper proposes a division of the local open clusters into two different disklike subpopulations on the base of the metallicity enhancement history, spatial distribution in the Galaxy and morphology of their C-M diagrams. The thin disk population objects exhibit higher metallicities and are more closely concentrated to the center and the disk of the Galaxy than the thick disk clusters. The former contain also frequently blue stragglers while the later exhibit the presence of Clump giant members, typical for the thick disk globular clusters. Kinematical properties of the two considered open cluster subpopulations remain unknown due to the lack of observational data. 相似文献
AC Cnc is a nova-like, eclipsing binary of period 7h13m. I chose it for observation because its eclipses are rather symmetrical. A photometric solution gives inclination i = 74.5° ± 0.8°, mass of white dwarf M1 = 0.74 ± 0.07 M, mass of the late-type companion, M2 = 0.97 ± 0.8 M. Temperature of the accretion disk varies approximately as inverse half-power of the radial distance, the temperature at the edge of the disk is 7600 K. Rate of mass transfer from the late-type star to the white dwarf is 7(−9) M/yr. The distance of AC Cnc is 500 ± 100 pc. 相似文献
In a previous paper, we have studied dynamical friction during a parabolic passage of a companion galaxy past a disk galaxy. This paper continues with the study of satellites in circular orbits around the disk galaxy. Simulations of orbit decay in a self gravitating disk are compared with estimates based on two-body scattering theories; the theories are found to give a satisfactory explanation of the orbital changes. The disk friction is strongly dependent on the sense of rotation of the companion relative to the rotation of the disk galaxy as well as on the amount of mass in a spherical halo. The greatest amount of dynamical friction occurs in direct motion if no spherical halo is present. Then the infall time from the edge of the disk is about one half of the orbital period of the disk edge. A halo twice as massive as the disk increases the infall time four fold. The results of Quinn and Goodman, obtained with a non-self-gravitating method, agree well with our experiments with massive halos (Q0 1.5), but are not usable in a more general case. We give analytic expressions for calculating the disk friction in galaxies of different disk/halo mass ratios. 相似文献
We have analyzed the optical (U BV) and ultraviolet (λ1000–2700 Å) observations of the nuclear variability of the Seyfert galaxy NGC 4151 in the period 1987–2001 (the second cycle of activity). The fast (tens of days) and slow (~10 years) components of the nuclear variability, F and S, respectively, are shown to be completely different, but thermal in nature. We associate the S component with the formation and evolution of an accretion disk and the F component (flares) with instabilities in the accretion disk and their propagation over the disk in the form of a shock wave. The S component is present not only in the optical, but also in the ultraviolet range, with its amplitude being comparable over the entire range λ1000–5500 Å under study. The amplitude of the average flare (the F component) doubles as the wavelength decreases from 5500 to 1000 Å, while the rise time of the brightness to its maximum Δt (the variability time scale) decreases from
to 6d ± 2d. The brightness decline (flare decay) time decreases by a factor of 16. The extinction in the ultraviolet is shown to have been grossly underestimated: beginning from the first IUE data, only the extinction in our Galaxy,
leads to a large increase in the luminosity of the variable source in the nucleus of NGC 4151: L = (6–8) × 1046 erg s?1. The spectral energy distribution for the variable source (λ950–5500 Å) agrees well with two Planck distributions: Te = 65 000 (λmax = 450 Å) and 8000 K. The radiation with Te = 8000 K is the reprocessing of the bulk of the ultraviolet radiation by the accretion disk with a lag of 0.5–0.6 days in the V band. The lag in the U-B variability of the slow component revealed the existence of an extended broad line region (EBLR) at an effective distance of 1.5 lt-years, as confirmed by spectroscopic data obtained at the Crimean Astrophysical Observatory. This yields the following mass of the central object in NGC 4151: Mc = (1–3) × 109M⊙. The luminosity of the variable source then accounts for 50–60% of LEdd rather than 1–2%, as has been thought previously. In general, the pattern of ultraviolet and optical variability in NGC 4151 agrees excellently with the theory of disk accretion instability for a supermassive black hole suggested by N. Shakura and R. Sunyaev 30 years ago: the energy release is at a maximumin the ultraviolet (in the case under consideration, at λ450 Å), the luminosity is ~1047 erg s?1 for Mc ~ 109M⊙ (several tens of percent of LEdd), and the variability time scale ranges from several days to many years.
A sediment disk reactor was tested in once flow-through mode to retrieve kinetic parameters for the Monod rate law that describes sulfate reduction. The experimental method was compared with a previously described procedure by the authors where a sediment plug-flow reactor was operated in a recirculation mode. In recirculation mode, accumulation of metabolic byproducts in certain cases may result in negative feedback, thus preventing accurate determination of kinetic information. The method described in this article provides an alternative to the recirculation sediment plug-flow-through reactor technique for retrieving kinetic parameters of microbially mediated reactions in aquatic sediments.For sulfate reduction in a saltmarsh site, a maximum estimate of the half-saturation concentration, Ks, of 204±26 μM and a maximum reaction rate, Rm, of 2846±129 nmol cm(wetsediment)3 d−1 was determined. The Ks value obtained was consistent with the one estimated previously (Ks=240±20 μM) from a different site within the same saltmarsh mud flat using a recirculating reactor. From the Rm value and reduction rates determined using 35SO42− incubation experiments, we infer that sulfate reduction is limited in the field. Substrate availability is not the main contributor for the limitation, however. Competition from other microbes, such as iron reducers affects the activity of sulfate reducers in the suboxic to anoxic zones, whereas aerobes compete in the oxic zone. High sulfide concentration in the pore water may also have acted as a toxin to the sulfate reducers in the field. 相似文献
Summary We study the effects of discontinuity network parameters on the formation of removable wedges in rock slopes. Discontinuities
are simulated using the Poisson disk model, and removable wedges are identified using block theory. The formation of removable
wedges of different sizes is assumed to follow a Poisson process. Poisson regression and Monte Carlo simulation are then used
to identify statistically relevant parameters of the model, and to study the effects that variations in their values have
on formation of removable blocks. The sensitivity of the results as a function of the mean orientations of the discontinuity
sets forming the blocks is also studied by means of a parametric study. The volumetric intensity of discontinuities in the
rock mass is found to have a significant impact on the computed estimates of removable block formation. As predicted by theory,
our results indicate that, everything else being equal, the expected rate of formation of removable wedges is proportional
to the square of the intensity measure. Estimates are also sensitive to changes in discontinuity size, especially in cases
in which discontinuities are smaller than one to two times the height of the slope. The interaction between the mean size
of discontinuities and the coefficient of variation of discontinuity sizes is found to be significant as well. Finally, results
of our sensitivity analysis suggest that the orientation of discontinuity sets significantly affects the rate of formation
of removable blocks in rock slopes.
Author’s address: Dr. Rafael Jimenez-Rodriguez, ETS Ing. de Caminos, Canales y Puertos. Universidad Politecnica de Madrid,
Spain 相似文献
Our capacity to detect and interrogate patterns in nature depends on the use of standard methods for measuring biological units. Consensus methods to quantify the size of individual animals and characteristics of biological communities are critical for comparisons across time and space. Nowhere is this more important than when dealing with organisms such as sea anemones that display high plasticity in body shape. Despite the need for accurate measures of anemone size for ecological comparisons, there is little consensus on the accuracy and precision of size inferences for these animals. We assessed several morphological parameters to determine which in‐field measure accurately and reliably reflects the reference size of an anemone measured in the laboratory: (i) column height, (ii) column diameter, (iii) limbus diameter, (iv) pedal disc diameter, (v) pedal disc area or (vi) pedal disc perimeter. The results revealed large variability in the accuracy and precision amongst measures, which have implications for their suitability as a standard method for in situ measurements. In general, measures of diameter were preferable to those of height, area and perimeter; and those associated with attachment (i.e. the limbus and pedal disc diameter) performed the best. Overall, considering concurrence with measures obtained from two differing but useful reference states, pedal disc diameter was the most accurate parameter (mean percentage difference = 0.6) with which to estimate the size of sea anemones in the field, and we thus recommend its use as an effective, non‐destructive means of gaining insights into their behavioural and evolutionary ecology. 相似文献