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241.
We present the results of a year-long monitoring campaign on J1819+3845.We interpret the results of this WSRT campaign to infer critical source parameters such as source lifetime and structure on tens of microarcseconds. The long lifetime of the source at such high brightness temperatures requires continuous energy injection or exotic emission processes. We have previously interpreted the extreme scintillation ofJ1819+3845 as due to a relatively nearby (∼ 20 pc) scattering screen. We show this screen has a velocity w.r.t. the LSR of ∼ 25 kms-1, as measured by the changing scintillation properties throughout the year: the `velocity parallax'. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
242.
We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M) black hole and is in a transitional state of duration shortward of 107 yr.  相似文献   
243.
244.
I discuss the effect of non-radial motions on the small-scale peculiar pairwise velocity dispersions (PVD) of galaxies in a cold dark matter (CDM) model and calculate the PVD for the SCDM model by means of the refined cosmic virial theorem (CVT), taking account of non-radial motions by means of the Del Popolo & Gambera model. I compare the results of the present model with the data from Davis & Peebles, the IRAS value at 1  h −1 Mpc of Fisher et al. and Marzke et al. I show that while the SCDM model disagrees with the observed values, as pointed out by several authors, taking account of non-radial motions produces smaller values for the PVD. At r ≤1  h −1 Mpc the result is in agreement with Bartlett & Blanchard. In the light of this last paper, the result may be also read as a strong dependence of the CVT prediction on the model chosen to describe the mass distribution around galaxies, suggesting that the CVT cannot be taken as a direct evidence for a low-density Universe. Similarly to what is shown by Del Popolo & Gambera and Del Popolo et al., the agreement of our model to the observational data is because of a scale-dependent bias induced by the presence of non-radial motions. As the assumptions on which CVT is based have been questioned by several authors, I also calculated the PVD using the redshift distortion in the redshift-space correlation function, ξ z( r p, π), and compared it with the PVD measured from the Las Campanas Redshift Survey by Jing et al. The result confirms that non-radial motions influence the PVD making them agree better with the observed data.  相似文献   
245.
CCD photometry in Johnson UBV and Strömgren uvby systems and medium-resolution spectroscopy of the galactic open cluster NGC 7128 are presented. Spectral types of the brightest 12 stars in the cluster field were determined based on equivalent widths of the H α and the He  i 6678-Å line. The spectroscopic observations also revealed two obvious and one probable Be-type stars showing H α emission. The analysis of the photometric diagrams gave a colour excess of E ( B − V )=1.03±0.06 mag, a distance modulus DM =13.0±0.2 mag and an age above 10 Myr. Time-resolved photometric observations obtained on one night resulted in the detection of short time-scale light variations of seven new and three already known variable stars in the cluster field.  相似文献   
246.
We use cosmological smooth particle hydrodynamical (SPH) simulations to study the effects of mergers in the star formation history of galactic objects in hierarchical clustering scenarios. We find that during some merger events, gaseous discs can experience two starbursts: the first one during the orbital decay phase, owing to gas inflows driven as the satellite approaches, and the second one when the two baryonic clumps collide. A trend for these first induced starbursts to be more efficient at transforming the gas into stars is also found. We detect that systems that do not experience early gas inflows have well-formed stellar bulges and more concentrated potential wells, which seem to be responsible for preventing further gas inward transport triggered by tidal forces. The potential wells concentrate owing to the accumulation of baryons in the central regions and of dark matter as the result of the pulling in by baryons. The coupled evolution of the dark matter and baryons would lead to an evolutionary sequence during which systems with shallower total potential wells suffer early gas inflows during the orbital decay phase that help to feed their central mass concentration, pulling in dark matter and contributing to build up more stable systems. Within this scenario, starbursts triggered by early gas inflows are more likely to occur at early stages of evolution of the systems and to be an important contributor to the formation of stellar bulges. Our results constitute the first proof that bulges can form as the product of collapse, collisions and secular evolution in a cosmological framework, and they are consistent with a rejuvenation of the stellar population in bulges at intermediate z with, at least, 50 per cent of the stars (in SCDM) being formed at high z .  相似文献   
247.
248.
We present a new mapping algorithm, the Accretion Stream Mapping (ASM), which uses the full phase-coverage of a light curve to derive spatially resolved intensity distributions along the accretion stream in magnetic cataclysmic variables of AM Herculis type (polars). The surface of the accretion stream is approximated as a 12-sided (duodecadon-shaped) tube. After successfully testing this method on artificial data we applied it to emission-line light curves of H β , H γ and He  ii λ 4686 of the bright eclipsing polar HU Aqr. We find hydrogen and helium line emission bright in the threading region of the stream where the stream couples on to magnetic field lines. It is particularly interesting that the stream is bright on the irradiated side facing the white dwarf, which highlights the interplay of collisional and radiative excitation/ionization.  相似文献   
249.
We develop a general formalism for analysing parameter information from non-Gaussian cosmic fields. The method can be adapted to include the non-linear effects in galaxy redshift surveys, weak lensing surveys and cosmic velocity field surveys as part of parameter estimation. It can also be used as a test of non-Gaussianity of the cosmic microwave background. Generalizing maximum-likelihood analysis to second order, we calculate the non-linear Fisher information matrix and likelihood surfaces in parameter space. To this order we find that the information content is always increased by including non-linearity. Our methods are applied to a realistic model of a galaxy redshift survey, including non-linear evolution, galaxy bias, shot-noise and redshift-space distortions to second order. We find that including non-linearities allows all of the degeneracies between parameters to be lifted. Marginalized parameter uncertainties of a few per cent will then be obtainable using forthcoming galaxy redshift surveys.  相似文献   
250.
Abstract— The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite‐rich lithic and mineral clasts embedded into a fine‐grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca‐rich plagioclase (An96.5–99.5), pyroxene (FS21.9–46.2Wo3.0–41.4), and olivine (Fa29.3–47.8); accessory phases include Fe‐Ni metal, ilmenite, and Ti‐Cr‐rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact‐melt rocks, microporphyritic impact‐melt breccias, dark fine‐grained impact‐melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the “glassy” material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MacAlpine Hills 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.  相似文献   
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