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851.
V. Castellani S. Degl'Innocenti P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72
We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy. 相似文献
852.
M. A. Iatì C. Cecchi-Pestellini D. A. Williams F. Borghese P. Denti R. Saija S. Aiello 《Monthly notices of the Royal Astronomical Society》2001,322(4):749-756
Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities. 相似文献
853.
854.
We study the temporal intensity variations of Mgib bright features and investigate the corresponding Hα velocity pattern.
The network bright features are well visible in the continuum, in images averaged over the duration of the observations (130 min).
We detected `flashing' bright features, which appear and disappear within two to five minutes, while the rest of the bright
features undergo small variations of either their shape and/or their intensity. A power spectrum analysis reveals a 10-min
oscillation for approximately half of the stable bright features. The 5-min oscillations are detected mainly at the network
boundaries, where stable bright features are located, while 3-min oscillations coincide with few bright features, but are
also quite intense inside the network cells. The majority of bright features are associated with Hα downflows. The downflow
is very intense at the location of `flashing' bright features. 相似文献
855.
Reba M. Bandyopadhyay K.S. Wood P. Hertz M.N. Lovellette P.S. Ray M.T. Wolff B. Giebels E.D. Bloom 《Astrophysics and Space Science》2001,276(1):23-24
We present first results from a monitoring campaign of GRS 1915+105 undertaken with the USA X-ray timing experiment on the ARGOS satellite. A variety of behaviour has been observed, ranging from low, steady X-ray emission to rapid quasi-periodic flaring on timescales of approximately 10–120 seconds. 相似文献
856.
M. R.Burleigh F. J.Clarke S. T.Hodgkin 《Monthly notices of the Royal Astronomical Society》2002,331(4):L41-L45
We suggest that Jovian planets will survive the late stages of stellar evolution, and that white dwarfs will retain planetary systems in wide orbits (≳5 au). Utilizing evolutionary models for Jovian planets, we show that infrared imaging with 8-m class telescopes of suitable nearby white dwarfs should allow us to resolve and detect companions ≳3 M JUP . Detection of massive planetary companions to nearby white dwarfs would prove that such objects can survive the final stages of stellar evolution, place constraints on the frequency of main-sequence stars with planetary systems dynamically similar to our own and allow direct spectroscopic investigation of their composition and structure. 相似文献
857.
M. Stupar M. D. Filipović Q. A. Parker G. L. White T. G. Pannuti P. A. Jones 《Astrophysics and Space Science》2007,307(4):423-435
The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs)
at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample
of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide
reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within
carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from
the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam
together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature
values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria
are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution
from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin
are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs
which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution.
We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height. 相似文献
858.
S. S. Kaisin A. V. Kasparova A. Yu. Knyazev I. D. Karachentsev 《Astronomy Letters》2007,33(5):283-291
We present our Hα observations of 11 isolated southern galaxies: SDIG, PGC 51659, E 222-010, E 272-025, E 137-018, IC 4662, Sag DIG, IC 5052, IC 5152, UGCA 438, and E 149-003, with distances from 1 to 7 Mpc. We have determined the total Hα fluxes from these galaxies. The star formation rates in these galaxies range from 10?1 (IC 4662) to 10?4 M ⊙ yr?1 (SDIG) and the gas depletion time at the observed star formation rates lies within the range from 1/6 to 24 Hubble times H 0 ?1 . 相似文献
859.
Accretion disks orbiting black holes power high-energy systems such as X-ray binaries and Active Galactic Nuclei. Observations
are providing increasingly detailed quantitative information about such systems. This data has been interpreted using standard
toy-models that rely on simplifying assumptions such as regular flow geometry and a parameterized stress. Global numerical
simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions and, in principle,
lead to a genuinely predictive theory. In recent years we have developed a fully three-dimensional general relativistic magnetohydrodynamic
simulation code that evolves time-dependent inflows into Kerr black holes. Although the resulting global simulations of black
hole accretion are still somewhat simplified, they have brought to light a number of interesting results. These include the
formation of electro-magnetically dominated jets powered by the black hole’s rotation, and the presence of strong stresses
in the plunging region of the accretion flow. The observational consequences of these features are gradually being examined.
Increasing computer power and increasingly sophisticated algorithms promise a bright future for the computational approach
to black hole accretion. 相似文献
860.