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311.
Astrophysical compact stars provide a natural laboratory for testing theoretical models which are otherwise difficult to prove from an experimental setup. In our present work we analyse an exact solution to the Einstein-Maxwell system for a charged anisotropic compact body in the linear regime. The charged parameter may be set to zero which gives us the case of neutral solutions. We have tuned the model parameters for the uncharged case so as to match with recent updated mass-radius estimates for five different compact objects. Then we make a systematic study of the effect of charge for the different parameter set that fits the observed stars. The effect of charge is clearly illustrated in the increase of mass. We show that the physical quantities for the objects PSR J1614-2230, PSR J1903+327, Vela X-1, SMC X-1, Cen X-3 are well behaved. 相似文献
312.
The discovery that newborn very low mass stars and brown dwarfs have optical forbidden line spectra similar to low mass young stars was a strong indication that these objects can also launch outflows. Forbidden lines are the traditional tracers of outflow activity in young stars and observations at these wavelengths have contributed much to the understanding of outflows. However in the case of brown dwarfs, the forbidden emission line regions observed are not well resolved spatially. Thus, their origin in an outflow could not be confirmed. Here, the technique of spectro-astrometry as a means of spatially probing the forbidden emission line regions of very low mass stars and brown dwarfs is introduced. Indeed spectro-astrometric data presented here demonstrates, for the first time, that young brown dwarfs that are actively accreting can drive outflows. Also discussed is the important role adaptive optics will play when it comes to spatially resolving the forbidden emission line regions of sub-stellar objects and the potential for developing spectro-astrometry to a 2D form through integral field spectroscopy. 相似文献
313.
Mark A. Norris Ray M. Sharples Harald Kuntschner 《Monthly notices of the Royal Astronomical Society》2006,367(2):815-824
We present Gemini Multi-Object Spectrograph longslit spectroscopy of the isolated S0 galaxy NGC 3115. We have determined kinematical data and Lick/IDS absorption line-strength indices for the major axis out to around 9 kpc and for the minor axis out to around 5 kpc (around 2R e ). Using stellar population models which include the effects of variable [α/Fe] ratios, we derive metallicities, abundance ratios and ages for the stellar population of NGC 3115. We find that [α/Fe] remains fairly constant with increasing radius at around [α/Fe]= 0.17 for the major axis but increases rapidly for the minor axis to around [α/Fe]= 0.3 . We also find that to first order, this behaviour can be explained by a simple spheroid + disc model, where the spheroid has [α/Fe]= 0.3 and the disc shows close to solar abundance ratios. The disc also appears considerably younger than the spheroid, having an age of around 6 Gyr compared to 12 Gyr for the spheroid. We compare these results to those previously presented for the globular cluster system of NGC 3115. 相似文献
314.
In light of recent reductions in sulphur (S) and nitrogen (N) emissions mandated by Title IV of the Clean Air Act Amendments of 1990, temporal trends and trend coherence in precipitation (1984–2001 and 1992–2001) and surface water chemistry (1992–2001) were determined in two of the most acid‐sensitive regions of North America, i.e. the Catskill and Adirondack Mountains of New York. Precipitation chemistry data from six sites located near these regions showed decreasing sulphate (SO42?), nitrate (NO3?), and base cation (CB) concentrations and increasing pH during 1984–2001, but few significant trends during 1992–2001. Data from five Catskill streams and 12 Adirondack lakes showed decreasing trends in SO42? concentrations at all sites, and decreasing trends in NO3?, CB, and H+ concentrations and increasing trends in dissolved organic carbon at most sites. In contrast, acid‐neutralizing capacity (ANC) increased significantly at only about half the Adirondack lakes and in one of the Catskill streams. Flow correction prior to trend analysis did not change any trend directions and had little effect on SO42? trends, but it caused several significant non‐flow‐corrected trends in NO3? and ANC to become non‐significant, suggesting that trend results for flow‐sensitive constituents are affected by flow‐related climate variation. SO42? concentrations showed high temporal coherence in precipitation, surface waters, and in precipitation–surface water comparisons, reflecting a strong link between S emissions, precipitation SO42? concentrations, and the processes that affect S cycling within these regions. NO3? and H+ concentrations and ANC generally showed weak coherence, especially in surface waters and in precipitation–surface water comparisons, indicating that variation in local‐scale processes driven by factors such as climate are affecting trends in acid–base chemistry in these two regions. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
315.
Charles S. Cockell Pascal Lee Gordon Osinski Gerda Horneck Paul Broady 《Meteoritics & planetary science》2002,37(10):1287-1298
Abstract— Asteroid and comet impacts on Earth are commonly viewed as agents of ecosystem destruction, be it on local or global scales. However, for some microbial communities, impacts may represent an opportunity for habitat formation as some substrates are rendered more suitable for colonization when processed by impacts. We describe how heavily shocked gneissic crystalline basement rocks exposed at the Haughton impact structure, Devon Island, Nunavut, Arctic Canada, are hosts to endolithic photosynthetic microorganisms in significantly greater abundance than lesser‐shocked or unshocked gneisses. Two factors contribute to this enhancement: (a) increased porosity due to impact fracturing and differential mineral vaporization, and (b) increased translucence due to the selective vaporization of opaque mineral phases. Using biological ultraviolet radiation dosimetry, and by measuring the concentrations of photoprotective compounds, we demonstrate that a covering of 0.8 mm of shocked gneiss can provide substantial protection from ultraviolet radiation, reducing the inactivation of Bacillus subtilis spores by 2 orders of magnitude. The colonisation of the shocked habitat represents a potential mechanism for pioneer microorganisms to invade an impact structure in the earliest stages of post‐impact primary succession. The communities are analogous to the endolithic communities associated with sedimentary rocks in Antarctica, but because they occur in shocked crystalline rocks, they illustrate a mechanism for the creation of microbial habitats on planetary surfaces that do not have exposed sedimentary units. This might have been the case on early Earth. The data have implications for the microhabitats in which biological signatures might be sought on Mars. 相似文献
316.
Gordon R. Osinski Pascal Lee John G. Spray John Parnell Darlene S. S. Lim Theodore E. Bunch Charles S. Cockell Brian Glass 《Meteoritics & planetary science》2005,40(12):1759-1776
Abstract— The Haughton impact structure has been the focus of systematic, multi‐disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ?1880 m. New 40Ar–39Ar dates place the impact event at ?39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ?23 km, with an estimated rim (final crater) diameter of ?16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater‐fill impact melt breccias contain a calcite‐anhydrite‐silicate glass groundmass, all of which have been shown to represent impact‐generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater‐fill impactites provided a heat source that drove a post‐impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra‐crater lacustrine sediments of the Haughton Formation during the Miocene. Present‐day intra‐crater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present‐day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain. 相似文献
317.
Conway (Celest. Mech. 39, 199–211, 1986) drew attention to the circumstance that when the Newton–Raphson algorithm is applied
to Kepler's equation for very high eccentricities there are certain apparently capricious and random values of the eccentricity
and mean anomaly for which convergence seems not to be easily reached when the starting guess for the eccentric anomaly is
taken to be equal to the mean anomaly. We examine this chaotic behavior and show that rapid convergence is always reached
if the first guess for the eccentric anomaly is π. We present graphs and an empirical formula for obtaining an even better
first guess. We also examine an unstable situation where iterations oscillate between two in correct results until the instability
results in sudden convergence to the unique correct solution.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
318.
F. Bacciotti T.P. Ray D. Coffey J. Eislöffel J. Woitas 《Astrophysics and Space Science》2004,292(1-4):651-658
Many magneto-hydrodynamic (MHD) models have been developed to describe the acceleration and collimation of stellar jets, in the framework of an infall/outflow process. Thanks to high angular resolution instrumentation, such as the one on-board the Hubble Space Telescope (HST), we are finally able to test observationally the proposed ideas. We present the results obtained by us from the first 0”.1 resolution spectra of the initial portion (within 100–200 AU from the source) of the outflows from visible T Tauri stars, taken with the Space Telescope Imaging Spectrograph (STIS). We obtain the jet morphology, kinematics and excitation in different velocity intervals, and we derive the jet mass and momentum fluxes. These results confirm the predictions of magneto-centrifugal models for the jet launch. Recently we have also found indications for rotation in the peripheral regions of several flows. The derived rotational motions appear to be in agreement with the expected extraction of angular momentum from the star/disk system caused by the jet, which in turn allows the star to accrete up to its final mass. Improvements to resolution are expected from observations with STIS in the ultraviolet, and with the forthcoming AMBER spectrometer to be mounted at the VLTI. 相似文献
319.
320.