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981.
We present a thermal mid-infrared lightcurve of Asteroid 4 Vesta and use this to infer variations in thermophysical properties over the surface. Vesta was observed over three nights during the May 2007 opposition with the Infrared Telescope Facility on Mauna Kea. Mid-infrared observations are compared to a model based on the Standard Thermal Model which is draped over a Vesta shape model derived from Hubble Space Telescope observations.A visible lightcurve with similar aspect was used to estimate the albedo as Vesta rotates. Shape and albedo can explain some of the features observed in the mid-infrared lightcurve. However, variations in the thermophysical properties, such as the “beaming parameter,” over Vesta’s surface are required to completely explain the observations.In order to match the mid-infrared magnitudes observed of Vesta, a beaming parameter of ∼0.862 is required which is higher than other Main Belt Asteroids such as Ceres and Pallas (0.756), indicating a smoother and/or rockier surface on Vesta. Variations in the beaming parameter with longitude are invoked to reproduce the observed thermal variations. Surface materials with relatively high beaming values, indicating a smoother and/or rockier surface, in the eastern hemisphere of Vesta coincide with locations where impact excavations may have produced surfaces that are younger and brighter relative to the western hemisphere. 相似文献
982.
We study several high kinetic energy density jets observed during a traversal of the dayside magnetosheath by the Cluster spacecraft on March 17, 2001, at various distances from the magnetopause, generally characterised by anomalously high values of the local magnetosonic Mach number. We concentrate on two jets observed just outside the magnetopause, the first almost parallel to the GSM x axis and the second directed northward-tailward along the nominal magnetopause surface. We present evidence that none of them can be ascribed to magnetic reconnection at the magnetopause and show that the magnetopause is severely deformed by the jets, so that its local normal forms an angle of 97° with the quiet time magnetopause normal. On these grounds, we suggest that the indentation of the magnetopause is caused by an anti-sunward jet ramming into the magnetopause slightly equatorward of the northern cusp and that the northward-tailward jet is the result of its reflection at the deformed magnetopause. Finally, we briefly discuss our results by comparing them with past studies of events which in some way recall the one analysed herein. 相似文献
983.
V. V. Akimkin Y. N. Pavlyuchenkov A. I. Vasyunin D. S. Wiebe M. S. Kirsanova T. Henning 《Astrophysics and Space Science》2011,335(1):33-38
We study details of the UV radiation transfer in a protoplanetary disk, paying attention to the influence of dust growth and
sedimentation on the disk density and temperature. Also, we show how the dust evolution affects photoreaction rates of key
molecules, like CN and CS. 相似文献
984.
In the present article a model of well behaved charged superdense star with surface density 2×1014 gm/cm3 is constructed by considering a static spherically symmetric metric with t=const hypersurfaces as hyperboloid. So far well behaved model described by such metric could not be obtained. Maximum mass
of the star is found to be 0.343457M
⊙ and the corresponding radius is 9.57459 km. The red shift at the centre and on the surface are given as 0.068887 and 0.031726
respectively. 相似文献
985.
We propose a kinematic approach to searching for the stars that could be formed with the Sun in a common “parent” open cluster.
The approach consists in preselecting suitable candidates by the closeness of their space velocities to the solar velocity
and analyzing the parameters of their encounters with the solar orbit in the past in a time interval comparable to the lifetime
of stars. We consider stars from the Hipparcos catalog with available radial velocities. The Galactic orbits of stars have
been constructed in the Allen-Santillan potential by taking into account the perturbations from the spiral density wave. We
show that two stars, HIP 87382 and HIP 47399, are of considerable interest in our problem. Their orbits oscillate near the
solar orbit with an amplitude of ≈250 pc; there are short-term close encounters to distances <10 pc. Both stars have an evolutionary
status and metallicity similar to the solar ones. 相似文献
986.
D. Minniti P.W. Lucas J.P. Emerson R.K. Saito M. Hempel P. Pietrukowicz A.V. Ahumada M.V. Alonso J. Alonso-Garcia J.I. Arias R.M. Bandyopadhyay R.H. Barbá B. Barbuy L.R. Bedin E. Bica J. Borissova L. Bronfman G. Carraro M. Catelan J.J. Clariá G. Pietrzynski 《New Astronomy》2010,15(5):433-443
We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 h of observations with the 4-m VISTA telescope during 5 years (2010–2014), covering ~109 point sources across an area of 520 deg2, including 33 known globular clusters and ~350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9–2.5 μm) and a catalogue of more than 106 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars. 相似文献
987.
H. S. Jeon S. Cho Y. S. Kwak J. K. Chung J. U. Park D. K. Lee M. Kuzmicz-Cieslak 《Astrophysics and Space Science》2011,332(2):341-351
The atmospheric mass density of the upper atmosphere from the spherical Starlette satellite’s Precise Orbit Determination
is first derived with Satellite Laser Ranging measurements at 815 to 1115 km during strong solar and geomagnetic activities.
Starlette’s orbit is determined using the improved orbit determination techniques combining optimum parameters with a precise
empirical drag application to a gravity field. MSIS-86 and NRLMSISE-00 atmospheric density models are compared with the Starlette
drag-derived atmospheric density of the upper atmosphere. It is found that the variation in the Starlette’s drag coefficient
above 800 km corresponds well with the level of geomagnetic activity. This represents that the satellite orbit is mainly perturbed
by the Joule heating from geomagnetic activity at the upper atmosphere. This result concludes that MSIS empirical models strongly
underestimate the mass density of the upper atmosphere as compared to the Starlette drag-derived atmospheric density during
the geomagnetic storms. We suggest that the atmospheric density models should be analyzed with higher altitude acceleration
data for a better understanding of long-term solar and geomagnetic effects. 相似文献
988.
L. Zaninetti 《Astrophysics and Space Science》2011,333(1):99-113
A classical and a relativistic law of motion for a supernova remnant (SNR) are deduced assuming an inverse power law behavior
for the density of the interstellar medium and applying the thin layer approximation. A third equation of motion is found
in the framework of relativistic hydrodynamics with pressure, applying momentum conservation. These new formulas are calibrated
against a decade of observations of SN 1993J. The existing knowledge of the diffusive processes of ultrarelativistic electrons
is reviewed in order to explain the behavior of the ‘U’ shaped profile of intensity versus distance from the center of SN 1993J. 相似文献
989.
R. Chandra B. Schmieder C. H. Mandrini P. Démoulin E. Pariat T. Török W. Uddin 《Solar physics》2011,269(1):83-104
We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501
on 20 November 2003. Both flares displayed four homologous Hα ribbons and were both accompanied by coronal mass ejections
(CMEs). The central flare ribbons were located at the site of an emerging bipole in the centre of the active region. The negative
polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ≈ 110° within 32 hours.
We model the coronal magnetic field and compute its topology, using as boundary condition the magnetogram closest in time
to each flare. In particular, we calculate the location of quasi-separatrix layers (QSLs) in order to understand the connectivity between the flare ribbons. Though several polarities were present in AR 10501,
the global magnetic field topology corresponds to a quadrupolar magnetic field distribution without magnetic null points.
For both flares, the photospheric traces of QSLs are similar and match well the locations of the four Hα ribbons. This globally
unchanged topology and the continuous shearing by the rotating bipole are two key factors responsible for the flare homology.
However, our analyses also indicate that different magnetic connectivity domains of the quadrupolar configuration become unstable
during each flare, so that magnetic reconnection proceeds differently in both events. 相似文献
990.
Based on the extended Greenwich – NOAA/USAF catalogue of sunspot groups, it is demonstrated that the parameters describing
the latitudinal width of the sunspot generating zone (SGZ) are closely related to the current level of solar activity, and
the growth of the activity leads to the expansion of the SGZ. The ratio of the sunspot number to the width of the SGZ shows
saturation at a certain level of the sunspot number, and above this level the increase of the activity takes place mostly
due to the expansion of the SGZ. It is shown that the mean latitudes of sunspots can be reconstructed from the amplitudes
of solar activity. Using the obtained relations and the group sunspot numbers by Hoyt and Schatten (Solar Phys.
179, 189, 1998), the latitude distribution of sunspot groups (“the Maunder butterfly diagram”) for the eighteenth and the first half of
the nineteenth centuries is reconstructed and compared with historical sunspot observations. 相似文献