Segregation of dust grains in dark clouds     

H. C. Bhatt  J. N. Desai 《Astrophysics and Space Science》1982,84(1):163-172

Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the cloud's center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.  相似文献   

On the effect of permeable magmatic foam on heat transfer in channelized lava flow     

Rosemarie Mohais  Balswaroop Bhatt 《Journal of Geochemical Exploration》2009

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Stardust-NExT,Deep Impact,and the accelerating spin of 9P/Tempel 1     

Michael J.S. Belton  Karen J. Meech  Steven Chesley  Jana Pittichová  Brian Carcich  Michal Drahus  Alan Harris  Stephen Gillam  Joseph Veverka  Nicholas Mastrodemos  William Owen  Michael F. A’Hearn  S. Bagnulo  J. Bai  L. Barrera  Fabienne Bastien  James M. Bauer  J. Bedient  B.C. Bhatt  Hermann Boehnhardt  H. Zhao 《Icarus》2011,213(1):345-368

The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13 year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 ± 0.025°/dy (41.335 ± 0.005 h) prior to the 2000 perihelion passage, 210.448 ± 0.016°/dy (41.055 ± 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 ± 0.030°/dy (40.783 ± 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 ± 0.012°/dy (40.827 ± 0.002 h) in the interval 2006–2010 following the 2005 perihelion passage. The period decreased by 16.8 ± 0.3 min during the 2000 passage and by 13.7 ± 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 ± 0.003°/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations.The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4–15) pole and longitude system. The possibility of a 180° error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42.We find that a net torque in the range of 0.3–2.5 × 10⁷ kg m² s^?2 acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than ～1° per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H₂O out-gassing from the surface is largely responsible for the observed spin-up.  相似文献   

X-ray Flares Observed from Six Young Stars Located in the Region of Star Clusters NGC 869 and IC 2602     

Himali Bhatt  J. C. Pandey  K. P. Singh  Ram Sagar  Brijesh Kumar 《Journal of Astrophysics and Astronomy》2014,35(1):39-54

We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10–40 min) and a slow decay (20–90 min). The X-ray luminosities during the flares in the energy band 0.3–7.5 keV are in the range of 10^29.9 to 10^31.7 erg s^?1. The strongest flare was observed with the ratio ～13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ～100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 10¹⁰ cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.  相似文献   

Photometric distances to dark clouds: cometary globule CG 12     

G. Maheswar  P. Manoj  H. C. Bhatt 《Monthly notices of the Royal Astronomical Society》2004,355(4):1272-1282

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Non-emission-line young stars of intermediate mass     

P. Manoj  G. Maheswar  H. C. Bhatt 《Monthly notices of the Royal Astronomical Society》2002,334(2):419-425

We present optical spectra of four intermediate-mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062, are members of the young Orion nebula cluster (ONC). This association constrains their ages to be ≲1 Myr. The lack of appreciable near-infrared excess in them suggests the absence of hot dust close to the central star. However, they do possess significant amounts of cold and extended dust as revealed by the large excess emission observed at far-infrared wavelengths. The fractional infrared luminosities  ( L _ir/ L _★)  and the dust masses computed from IRAS fluxes are systematically lower than those found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may thus represent the youngest examples of the Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star, as is evident from the low   L _ir/ L _★  , the scarcity of circumstellar dust, the low polarization, the presence of H α emission and near-infrared excess, and the far-infrared spectral energy distribution consistent with free–free emission similar to other well-known classical Be stars.  相似文献   

Emission-line CCD imaging of three southern symbiotic stars     

Harish C. Bhatt  Ram Sagar 《Journal of Astrophysics and Astronomy》1991,12(2):111-117

Symbiotic stars that are strong radio sources and have cool dust emitting in the infrared are expected to have extended emission nebulae around them. In order to search for such emission nebulae, we have carried out CCD imaging of three symbiotic stars (R Aqr, RR Tel and H1-36) with narrow-band filters centred at the emission lines of [O III] λ5007, Hα λ6563, [N II] λ6584, [S II] λ6717 + 6731. RR Tel and H1-36 images do not show any extended nebulosities around them. The CCD image of the R Aqr nebulosity in the high excitation [O m] line is different from its image in Hα and the low excitation lines of [N II] and [S II] indicating ionization-stratification in the nebula. In H1-36 the optical nebulosity (if it exists) is smaller than ∽2 arcsec while the radio image size is known to be large (∽5 arcsec). This behaviour is opposite to that seen in R Aqr in which the radio emission comes from the core region of a much larger optical nebulosity. Interstellar and/or circumstellar extinctions are suggested to be responsible for this difference  相似文献   

Extension of the CSM‐FEMA440 to plan‐asymmetric real building structures     

Carlos Bhatt  R. Bento 《地震工程与结构动力学》2011,40(11):1263-1282

The capacity spectrum method (CSM) has established itself as one of the most used Nonlinear Static Procedures for the seismic assessment of structures, since its introduction in 1975, when it was first presented by Freeman. More recently, this procedure was implemented in the ATC40 guidelines and lately improved in the FEMA‐440 report. The first step of work addressed by this paper relates to the comparison between the two features of the CSM. In the second part, an extension of the FEMA‐440CSM version is proposed for plan‐asymmetric real RC building structures. The case studies under analysis are the SPEAR building—an irregular 3D structure representing typical old three‐storey buildings in the Mediterranean region, from the early 1970s—and two real Turkish buildings with five and eight storeys. The CSM‐ATC40, the CSM‐FEMA440 and the proposed extended CSM‐FEMA440 method are applied and the results obtained duly compared with nonlinear dynamicit timehistory analyses. For the latter, semi‐artificial ground motions are used to define the seismic action. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

FAO 56 Model and Remote Sensing for the Estimation of Crop-Water Requirement in Main Branch Canal of the Bhadra Command area, Karnataka State   总被引：1，自引：0，他引：1  

S. N. A. Abdul Karim  S. A. Ahmed  V. Nischitha  Surabhi Bhatt  S. Kiran Raj  K. N. Chandrashekarappa 《Journal of the Indian Society of Remote Sensing》2013,41(4):883-894

Remote sensing and FAO 56 crop water model are used for estimating crop water requirement for paddy crop located in the main branch canal of Bhadra Command Area in Karnataka, India. The estimation of crop-water requirement depends on the meteorological factors, soil type and crop coefficients. The result obtained showed that water requirements of rabi crops higher than those of the kariff crops. The total irrigated area estimated from the IRS image is 29,353 ha. It is found that the total paddy crop acreage is 18,257 ha covering 62 % in the total irrigated area of the command area, Arecanut 20 %, coconut 15 % and sugarcane with other crops 3 %. The water requirement for paddy is 1180.4 mm for its entire growth period. The total water requirement for irrigation supply for crops in the entire command area is 5,790 at a demand of 0.10501 cusecs per ha.  相似文献   

Stellar contents and star formation in the young star cluster Be 59     

A. K. Pandey  Saurabh Sharma  K. Ogura  D. K. Ojha  W. P. Chen  B. C. Bhatt  S. K. Ghosh 《Monthly notices of the Royal Astronomical Society》2008,383(3):1241-1258

We present   UBV  I _c   CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ∼10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between   E ( B − V ) ≃ 1.4  to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as  3.7 ± 0.3  . The distance of the cluster is found to be  1.00 ± 0.05 kpc  . Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between <1 and ∼2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ∼2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, Γ, in the mass range  2.5 < M /M_⊙≤ 28  is found to be  −1.01 ± 0.11  which is shallower than the Salpeter value (−1.35), whereas in the mass range  1.5 < M /M_⊙≤ 2.5  the slope is almost flat. The slope of the K -band luminosity function is estimated as  0.27 ± 0.02  , which is smaller than the average value (∼0.4) reported for young embedded clusters. Approximately 32 per cent of Hα emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.  相似文献