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排序方式: 共有310条查询结果,搜索用时 46 毫秒
41.
A. K. Pandey K. Upadhyay K. Ogura Ram Sagar V. Mohan H. Mito H. C. Bhatt B. C. Bhatt 《Monthly notices of the Royal Astronomical Society》2005,358(4):1290-1308
UBVRI CCD photometry in a wide field around two young open clusters, NGC 663 and 654, has been carried out. Hα and polarimetric observations for the cluster NGC 654 have also been obtained. We use the photometric data to construct colour–colour and colour–magnitude diagrams, from which we can investigate the reddening, age, mass and evolutionary states of the stellar contents of the these clusters. The reddening across the cluster regions is found to be variable. There is evidence for anomalous reddening law in both clusters; however, more infrared and polarimetric data are needed to conclude about the reddening law. Both clusters are situated at about a distance of 2.4 kpc. Star formation in both clusters is found to be a continuous process. In the case of NGC 663, star formation seems to have taken place sequentially, in the sense that formation of low-mass stars precedes the formation of most massive stars. Whereas, in the case of NGC 654, formation of low-mass stars did not cease after the formation of most massive stars in the cluster. 相似文献
42.
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan Jayaram N. Chengalur 《Journal of Astrophysics and Astronomy》2001,22(1):35-50
Nearby interstellar clouds with high (|ν|≥10km s−1) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption)
in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio
sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random
velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds.
The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s−1, consistent with absorption arising from CNM clouds. 相似文献
43.
The system of Roche coordinates developed by Kopal to study the problems of stars in close binary systems has been used to study the problems of small oscillations of tidally distorted stars. 相似文献
44.
Kopal's method of representing the inner structure of a rotationally and tidally-distorted star by a rotationally-and tidally-distorted Roche model has been used in conjunction with an averaging concept introduced by Kippenhahn and Thomas of representing the equipotential surfaces of a rotationally and tidally distorted stellar model by the equipotential surfaces of an equivalent spherical model to determine the combined effects of rotation and tidal distortion on the periods of small adiabatic oscillations of a stellar model. 相似文献
45.
The current error of 0.0025 on the lunar homogeneity parameterI/MR
2 is dominated by the uncertainties in theC
20 andC
22 gravity harmonics. This error level is equivalent to a 4.20 gm cm–3 density uncertainty for a lunar interior model having a core 300 km in radius. Covariance analyses are performed using Doppler data from the relay satellite of the proposed Lunar Polar Orbiter mission to determine an optimum reduction strategy which obtains an order of magnitude improvement in the gravity estimates. Error studies show the long-arc reduction method obtains results which are an order of magnitude more accurate than the short-arc technique. The nominal 4000 km circular orbit of the relay satellite is very sensitive to the unmodeled effects of gravity harmonics of degree 5 through 9. Results from this orbital geometry indicate that it may not be possible to achieve the desired order of magnitude accuracy improvement. A modified orbit having the identical orbital conditions as the nominal one, but with a larger semi-major axis of 7000 km is studied. Results show the desired order of magnitude improvement can be achieved when a complete fourth degree and order model and some fifth and sixth degree terms are estimated while considering the unmodeled effects of the remaining harmonics through degree and order eight. Studies also show a 50% additional improvement inC
22 can be achieved if differential differenced Doppler is also processed with the direct Doppler. The improved uncertainty inI/MR
2 reduces the core density error from 4.20 gm cm–3 to 0.1 gm cm–3 for the case of a lunar density model having a 300 km core radius.Contribution #2885 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. 相似文献
46.
S. N. Charan E. V. S. S. K. Babu S. M. Naqvi J. G. Rana Prathap M. Ram Mohan D. Srinivasa Sarma 《Journal of the Geological Society of India》2009,73(3):371-378
The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced
at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features
characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These
restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue)
enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves
and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor
melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase
consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb,
K2O, MgO, FeO, TiO2 and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na2O, Al2O3, LREE, Th, U and enriched in K2O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphibole-plagioclase in these
amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50° C and pressure of 5 ± 1 Kbar. These data are
inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken
place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated
basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts. 相似文献
47.
A case study of atmospheric boundary layer features during winter over a tropical inland station — Kharagpur (22.32°N, 87.32°E) 总被引:1,自引:0,他引:1
Denny P. Alappattu P. K. Kunhikrishnan Marina Aloysius M. Mohan 《Journal of Earth System Science》2009,118(4):281-293
The local weather and air quality over a region are greatly influenced by the atmospheric boundary layer (ABL) structure and
dynamics. ABL characteristics were measured using a tethered balloon-sonde system over Kharagpur (22.32°N, 87.32°E, 40m above
MSL), India, for the period 7 December 2004 to 30 December 2004, as a part of the Indian Space Research Organization-Geosphere
Biosphere Program (ISRO-GBP) Aerosol Land Campaign II. High-resolution data of pressure, temperature, humidity, wind speed
and wind direction were archived along with surface layer measurements using an automatic weather station. This paper presents
the features of ABL, like ABL depth and nocturnal boundary layer (NBL) depth. The sea surface winds from Quikscat over the
oceanic regions near the experiment site were analyzed along with the NCEP/NCAR reanalysis winds over Kharagpur to estimate
the convergence of wind, moisture and vorticity to understand the observed variations in wind speed and relative humidity,
and also the increased aerosol concentrations. The variation of ventilation coefficient (V C), a factor determining the air pollution potential over a region, is also discussed in detail. 相似文献
48.
Sanjay Kumar S. Priyadarshi S. Gopi Krishna A. K. Singh 《Astrophysics and Space Science》2012,339(1):165-178
The paper is based on the ionospheric variations in terms of vertical total electron content (VTEC) for the low solar activity
period from May 2007 to April 2009 based on the analysis of dual frequency signals from the Global Positioning System (GPS)
satellites recorded at ground stations Varanasi (Geographic latitude 25°16′ N, Longitude 82°59′ E), situated near the equatorial
ionization anomaly crest and other two International GNSS Service (IGS) stations Hyderabad (Geographic latitude 17°20′ N,
longitude 78°30′ E) and Bangalore (Geographic latitude 12°58′ N, longitude 77°33′ E) in India. We describe the diurnal and
seasonal variations of total electron content (TEC), and the effects of a space weather related event i.e. a geomagnetic storm
on TEC. The mean diurnal variation during different seasons is brought out. It is found that TEC at all the three stations
is maximum during equinoctial months (March, April, September and October), and minimum during the winter months (November,
December, January and February), while obtaining intermediate values during summer months (May, June, July and August). TEC
shows a semi-annual variation. TEC variation during geomagnetic quiet as well as disturbed days of each month and hence for
each season from May 2007 to April 2008 at Varanasi is examined and is found to be more during disturbed period compared to
that in the quiet period. Monthly, seasonal and annual variability of GPS-TEC has been compared with those derived from International
Reference Ionosphere (IRI)-2007 with three different options of topside electron density, NeQuick, IRI01-corr and IRI 2001.
A good agreement is found between the GPS-TEC and IRI model TEC at all the three stations. 相似文献
49.
We present NeV/MgV and SiVII/MgVII theoretical line intensity ratios as a function of electron densityN
e
and temperatureT
e
. These are shown in the form of ratio-ratio diagrams, which should in principle allow bothN
e
andT
e
to be deduced for the emitting region of the solar plasma. We apply these diagnostics in the solar atmosphere, and discuss
the available observations made from space. In most cases, however, we deduceN
e
andT
e
from the computed absolute line intensities in a spherically symmetric model atmosphere of the Sun. Possible future applications
of this investigation to spectral data from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory
(SOHO) are briefly discussed. 相似文献
50.