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991.
V. R. MARTHI J. N. CHENGALUR Y. GUPTA G. C. DEWANGAN D. BHATTACHARYA 《Journal of Astrophysics and Astronomy》2011,32(4):451-455
We describe the serendipitous discovery of a very steep-spectrum radio point source in low-frequency Giant Metrewave Radio
Telescope (GMRT) images of the supernova remnant (SNR) G76.9+1.0. The steep spectrum, as well as the location of the point
source near the centre of this SNR confirm that this indeed is the pulsar J2022+3842. Archival Chandra X-ray data shows a point source coincident with the radio point source. However, no pulsed radio emission was detected despite
deep searches at 610 MHz and 1160 MHz – which can be understood to be due to temporal broadening of the pulses. Weak pulsed
emission has indeed been seen at 2 GHz with the Green Bank Telescope (GBT), establishing the fact that scattering is responsible
for its non-detection at low radio frequencies. We underline the usefulness of low-frequency radio imaging as a good technique
to prospect for pulsar candidates. 相似文献
992.
S. A. Borysenko Yu. V. Sizonenko I. V. Luk’yanyk A. V. Ivanova A. D. Voitsekhovskaya T. P. Sergeeva A. V. Golovin 《Kinematics and Physics of Celestial Bodies》2011,27(2):92-97
Photometric measurements of photographic images of comet C/1987 P1 Bradfield have been carried out with a flat-bed scanner
equipped with a slide module. Lengthwise and transverse photometric profiles of the cometary plasma tail have been obtained.
Magnetic field induction and some other physical characteristics of the cometary plasma tail observed in November 1987 have
been estimated with the use of the diffusion model for a cometary tail by Shul’man and Nazarchuk (1968). It has been shown
that the scanned images of comets can be used for estimating the physical characteristics of cometary tails. 相似文献
993.
Yu. A. Fadeyev 《Astronomy Letters》2011,37(6):403-413
Instability of population I (X = 0.7, Z = 0.02) massive stars against radial oscillations during the post-main-sequence gravitational contraction of the helium core
is investigated. Initial stellar masses are in the range 65M
⊙ ≤ M
ZAMS ≤ 90M
⊙. In hydrodynamic computations of self-exciting stellar oscillations we assumed that energy transfer in the envelope of the
pulsating star is due to radiative heat conduction and convection. The convective heat transfer was treated in the framework
of the theory of time-dependent turbulent convection. During evolutionary expansion of outer layers after hydrogen exhaustion
in the stellar core the star is shown to be unstable against radial oscillations while its effective temperature is T
eff > 6700 K for M
ZAMS = 65M
⊙ and T
eff > 7200 K for M
ZAMS = 90M
⊙. Pulsational instability is due to the κ-mechanism in helium ionization zones and at lower effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of evolution does not exceed ≈200 day. Radial
oscillations of the hypergiant resume during evolutionary contraction of outer layers when the effective temperature is T
eff > 7300 K for M
ZAMS = 65M
⊙ and T
eff > 7600 K for M
ZAMS = 90M
⊙. Initially radial oscillations are due to instability of the first overtone and transition to fundamental mode pulsations
takes place at higher effective temperatures (T
eff > 7700 K for M
ZAMS = 65M
⊙ and T
eff > 8200 K for M
ZAMS = 90M
⊙). The upper limit of the period of radial oscillations of evolving blueward yellow hypergiants does not exceed ≈130 day.
Thus, yellow hypergiants are stable against radial stellar pulsations during the major part of their evolutionary stage. 相似文献
994.
Yu. I. Fedorov 《Kinematics and Physics of Celestial Bodies》2011,27(1):1-16
Acceleration of cosmic rays interacting with the anisotropic magnetohydrodynamic turbulent medium is studied. Particle acceleration
is caused by a large-scale electric field arising in a turbulent medium due to the α-effect. A comparison is made of equilibrium
spectra of cosmic rays, characteristic of the specific acceleration mechanism, with the energy distribution of particles corresponding
to the statistical Fermi acceleration. 相似文献
995.
D. F. Webb H. Cremades A. C. Sterling C. H. Mandrini S. Dasso S. E. Gibson D. A. Haber R. W. Komm G. J. D. Petrie P. S. McIntosh B. T. Welsch S. P. Plunkett 《Solar physics》2011,274(1-2):57-86
The Whole Heliosphere Interval (WHI) was an international observing and modeling effort to characterize the 3-D interconnected ??heliophysical?? system during this solar minimum, centered on Carrington Rotation 2068, March 20??C?April 16, 2008. During the latter half of the WHI period, the Sun presented a sunspot-free, deep solar minimum type face. But during the first half of CR 2068 three solar active regions flanked by two opposite-polarity, low-latitude coronal holes were present. These departures from the quiet Sun led to both eruptive activity and solar wind structure. Most of the eruptive activity, i.e., flares, filament eruptions and coronal mass ejections (CMEs), occurred during this first, active half of the interval. We determined the source locations of the CMEs and the type of associated region, such as active region, or quiet sun or active region prominence. To analyze the evolution of the events in the context of the global solar magnetic field and its evolution during the three rotations centered on CR 2068, we plotted the CME source locations onto synoptic maps of the photospheric magnetic field, of the magnetic and chromospheric structure, of the white light corona, and of helioseismological subsurface flows. Most of the CME sources were associated with the three dominant active regions on CR 2068, particularly AR 10989. Most of the other sources on all three CRs appear to have been associated with either isolated filaments or filaments in the north polar crown filament channel. Although calculations of the flux balance and helicity of the surface magnetic features did not clearly identify a dominance of one region over the others, helioseismological subsurface flows beneath these active regions did reveal a pronounced difference among them. These preliminary results suggest that the ??twistedness?? (i.e., vorticity and helicity) of subsurface flows and its temporal variation might be related to the CME productivity of active regions, similar to the relationship between flares and subsurface flows. 相似文献
996.
Cassini VIMS detected carbon dioxide on the surface of Iapetus during its insertion orbit. We evaluated the CO2 distribution on Iapetus and determined that it is concentrated almost exclusively on Iapetus’ dark material. VIMS spectra show a 4.27-μm feature with an absorption depth of 24%, which, if it were in the form of free ice, requires a layer 31 nm thick. Extrapolating for all dark material on Iapetus, the total observable CO2 would be 2.3 × 108 kg.Previous studies note that free CO2 is unstable at 10 AU over geologic timescales. Carbon dioxide could, however, be stable if trapped or complexed, such as in inclusions or clathrates. While complexed CO2 has a lower thermal volatility, loss due to photodissociation by UV radiation and gravitational escape would occur at a rate of 2.6 × 107 kg year−1. Thus, Iapetus’ entire inventory of surface CO2 could be lost within a few decades.The high loss/destruction rate of CO2 requires an active source. We conducted experiments that generated CO2 by UV radiation of simulated icy regolith under Iapetus-like conditions. The simulated regolith was created by flash-freezing degassed water, crushing it into sub-millimeter sized particles, and then mixing it with isotopically labeled amorphous carbon (13C) dust. These samples were placed in a vacuum chamber and cooled to temperatures between 50 K and 160 K. The samples were irradiated with UV light, and the products were measured using a mass spectrometer, from which we measured 13CO2 production at a rate of 2.0 × 1012 mol s−1. Extrapolating to Iapetus and adjusting for the solar UV intensity and Iapetus’ surface area, we calculated that CO2 production for the entire surface would be 1.1 × 107 kg year−1, which is only a factor of two less than the loss rate. As such, UV photochemical generation of CO2 is a plausible source of the detected CO2. 相似文献
997.
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
998.
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
999.
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
1000.
Marit Øieroset David A. Brain David L. Mitchell Jasper S. Halekas Mario H. Acuña 《Icarus》2010,206(1):189-198
More than 490 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) in 1997 and 1998 provide unprecedented coverage of the solar wind in the vicinity of the orbits of the martian moons Phobos and Deimos. We have performed a comprehensive survey of magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. A total of 1246 solar wind disturbance events were identified and their distribution was examined relative to Phobos, the Phobos orbit, and the Deimos orbit. We find that the spatial distribution of solar wind perturbations does not increase near or downstream of Phobos, Phobos’ orbit, or Deimos’ orbit, which would have been expected if there is significant outgassing or dust escape from the martian moons. Of the 1246 magnetic field perturbation events found in the MGS data set, 11 events were found within 2000 km of the Phobos orbit, while three events were found within 2000 km of the Deimos orbit. These events were analyzed in detail and found to likely have other causes than outgassing/dust escape from the martian moons. Thus we conclude that the amount of gas/dust escaping the martian moons is not significant enough to induce detectable magnetic field perturbations in the solar wind. In essence we have not found any clear evidence in the MGS magnetic field data for outgassing or dust escape from the martian moons. 相似文献