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Study of grain alignment efficiency and a distance estimate for small globule CB4

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Research in Astronomy and Astrophysics 2015年第7期15卷 953-962页

作者： Ajoy Barman himadri Sekhar das Department of Physics Assam University

We study the polarization efficiency（defined as the ratio of polarization to extinction） of stars in the background of the small, nearly spherical and isolated Bok globule CB4 to understand the grain alignment process. A decrease in polarization efficiency with an increase in visual extinction is noticed. This suggests that the observed polarization in lines of sight which intercept a Bok globule tends to show dominance of dust grains in the outer layers of the globule. This finding is consistent with the results obtained for other clouds in the past. We determined the distance to the cloud CB4 using near-infrared photometry（2MASS J H KScolors） of moderately obscured stars located at the periphery of the cloud. From the extinction-distance plot,the distance to this cloud is estimated to be（459 ± 85） pc.

关键词： polarization -- ISM: clouds -- dust extinction -- ISM: individual ob-jects: CB4

Correlation among extinction efficiency and other parameters in an aggregate dust model

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Research in Astronomy and Astrophysics 2017年第11期17卷 95-112页

作者： Tanuj Kumar Dhar himadri Sekhar das Department of Physics Assam University Silchar 788011 India

We study the extinction properties of highly porous Ballistic Cluster-Cluster Aggregate dust aggregates in a wide range of complex refractive indices （1.4 ≤ n 〈 2.0, 0.001 ≤ k 〈 1.0） and wavelengths （0.11 μm 〈 A 〈 3.4 μm）. An attempt has been made for the first time to investigate the correlation among extinction efficiency （Qext）, composition of dust aggregates （n, k）, wavelength of radiation （A） and size parameter of the monomers （x）. If k is fixed at any value between 0.001 and 1.0, Qext increases with increase of n from 1.4 to 2.0. Qext and n are correlated via linear regression when the cluster size is small, whereas the correlation is quadratic at moderate and higher sizes of the cluster. This feature is observed at all wavelengths （ultraviolet to optical to infrared）. We also find that the variation of Qext with n is very small when A is high. When n is fixed at any value between 1.4 and 2.0, it is observed that Qext and k are correlated via a polynomial regression equation （of degree 1, 2, 3 or 4）, where the degree of the equation depends on the cluster size, n and A. The correlation is linear for small size and quadratic/cubic/quartic for moderate and higher sizes. We have also found that Qext and x are correlated via a polynomial regression （of degree 3, 4 or 5） for all values of n. The degree of regression is found to be n and k-dependent. The set of relations obtained from our work can be used to model interstellar extinction for dust aggregates in a wide range of wavelengths and complex refractive indices.

关键词： light scattering -- ISM: dust extinction

Effect of pouring temperature on cooling slope casting of semi-solid Al-Si-Mg alloy

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Acta Metallurgica Sinica(English Letters) 2012年第5期25卷 329-339页

作者： Prosenjit das Sudip K. Samanta himadri Chattopadhyay Pradip Dutta Foundry Group CSIR-Central Mechanical Engineering Research Institute Durgapur-713209 India Department of Mechanical Engineering Jadavpur University Kolkata-70032 India Department of Mechanical Engineering Indian Institute of Science Bangalore-560012 India

Present trend of semi-solid processing is directed towards rheocasting route which allows manufacturing of near-net-shape cast components directly from the prepared semi-solid slurry. Generation of globular equi-axed grains during solidification of rheocast components, compared to the columnar dendritic structure of conventional casting routes, facilitates the manufacturing of components with improved mechanical properties and structural integrity. In the present investigation, a cooling slope has been designed and indigenously fabricated to produce semi solid slurry of A1-Si-Mg （A356） alloy and successively cast in a metallic mould. The scope of the present work discusses about development of a numerical model to simulate the liquid metal flow through cooling slope using Eulerian two-phase flow approach and to investigate the effect of pouring temperature on cooling slope semi-solid slurry generation process. The two phases considered in the present model are liquid metal and air. Solid fraction evolution of the solidifying melt is tracked at different locations of the cooling slope, following Schiel＇s equation. The continuity equation, momentum equation and energy equation are solved considering thin wall boundary condition approach. During solidification of the liquid metal, a modified temperature recovery scheme has been employed taking care of the latent heat release and change of fraction of liquid. The results obtained from simulations are compared with experimental findings and good agreement has been found.

关键词： Rheocasting Semi-solid slurry Cooling slope A356 alloy Numerical simulation

Aggregate dust model to study the polarization properties of comet C/1996 B2 Hyakutake

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Research in Astronomy and Astrophysics 2010年第4期10卷 355-362页

作者： himadri Sekhar das Abinash Suklabaidya Saonli Datta Majumder Asoke Kumar Sen Department of Physics Assam University

The observed linear polarization data of comet Hyakutake are studied at wavelengths λ=0.365 μm,λ=0.485 μm and 0.684μm through simulations using Ballistic Particle-Cluster Aggregate and Ballistic Cluster-Cluster Aggregate aggregates of 128 spherical *** first found that the size parameter of the monomer,x ～ 1.56-1.70,turned out to be the most suitable which provides the best fits to the observed dust scattering properties at three wavelengths：λ=0.365 μm,0.485μm and 0.684μ***,the effective radius of the aggregate （r） lies in the range 0.45 μm ≤ r ≤ 0.49 μm at λ=0.365 μm;0.60 μm ≤ r ≤ 0.66 μm at λ=0.485 μm and 0.88 μm ≤ r ≤ 0.94 μm at λ=0.684 μ*** using superposition T-MATRIX code and the power-law size distribution,n（r）～ r-3,the best-fitting values of complex refractive indices are calculated for the observed polarization data at the above three *** best-fitting complex refractive indices （n,k） are found to be （1.745,0.095） at λ=0.365 μm,（1.743,0.100） at λ=0.485 μm and （1.695,0.100） at λ=0.684 μ*** refractive indices derived from the present analysis correspond to a mixture of both silicates and organics,which are in good agreement with the in situ measurement of comets by different spacecraft.

关键词： comets:general - dust extinction - scattering - polarization

The Relative Orientation between Local Magnetic Field and Galactic Plane in Low Latitude Dark Clouds

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Research in Astronomy and Astrophysics 2022年第7期22卷 84-98页

作者： Gulafsha B.Choudhury himadri S.das B.J.Medhi J.C.Pandey S.Wolf T.K.Dhar A.M.Mazarbhuiya Department of Physics Assam UniversitySilchar 788011India Department of Physics Gauhati UniversityGuwahati 781014India Aryabhatta Research Institute of Observational Sciences(ARIES) Nainital 263002India University of Kiel Institute of Theoretical Physics and AstrophysicsLeibnizstrasse 15D-24118KielGermany

In this work,we study the magnetic field morphology of selected star-forming clouds spread over the galactic latitude(b)range−10°to 10°.The polarimetric observations of clouds CB24,CB27 and CB188 are conducted to study the magnetic field geometry of those clouds using the 104 cm Sampurnanand Telescope(ST)located at ARIES,Manora Peak,Nainital,*** observations are combined with those of 14 further low latitude clouds available in the *** of these clouds are located within a distance range 140–500 pc except for CB3(∼2500 pc),CB34(∼1500 pc),CB39(∼1500 pc)and CB60(∼1500 pc).Analyzing the polarimetric data of 17 clouds,we find that the alignment between the envelope magnetic field(θ^(env)_(B))and galactic plane(GP)(θGP)of the low-latitude clouds varies with their galactic longitudes(l).We observe a strong correlation between the longitude(l)and the offset(θ_(off)=|θ^(env)_(B)-θ_(GP))which shows that θ^(env)_(B) is parallel to the GP when the clouds are situated in the region 115°250°.To check the consistency of our results,the stellar polarization data available in the Heiles catalog are overlaid on the DSS image of the clouds having mean polarization vector of field *** results are almost consistent with the Heiles data.A systematic discussion is presented in the *** effect of turbulence in the cloud is also studied which may play an important role in causing the misalignment phenomenon observed between θ^(env)_(B) and θ_(GP).We have used Herschel(Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.)SPIRE 500μm and SCUBA 850μm dust continuum emission maps in our work to understand the density structure of the clouds.

关键词： polarization (ISM:)dust extinction ISM:magnetic fields galaxies:magnetic fields