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Analysis of Interstellar Dust Using Linear Polarization

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Analysis of Interstellar Dust Using Linear Polarization AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH © 2010, Science Huβ, http://www.scihub.org/AJSIR ISSN: 2153-649X doi:10.5251/ajsir.2010.1.2.180.189 Analysis of interstellar dust using linear polarization Ali Abdul Sattar Jaber AL – Edhari Department of Physics, College of Science, Al- Muthanna University, Iraq E-mail: aliabdsatar@yahoo.com Tel: +964-7811238920 ABSTRACT This paper, presents the study and analysis of the optical properties of interstellar dust grains. Attempt is made to select a suitable model which defines the structure and composition of these grains, by studying the linear polarization features .We shall present the application of the Serkowski law fit of linear polarization models. In this work we shall present and analysis the data for linear polarization of 6 regions HD142250 , HD34078 , HD46202, HD48099, HD147889 and HD154445 . The hollow needle shape cylindrical bacteria can be fit with reasonable agreement to the normalized linear polarization data in infrared to visible region of the spectrum. Serkowski law fit is still the excellent representation of linear polarization of the star lights of all the regions. Keywords: ISM, Galaxy, Interstellar, matter – Polarization, Microbial Model INTRODUCTION is fixed unless polarization is caused by different Observation of differing physical conditions of materials in different directions (Clayton and Mathis, interstellar dust provides valuable information about 1988). the size, shape and composition of the grains. The The wavelength dependence of interstellar linear polarization was first recognized by Serkowski, et al first information concerning the nature of the (1975). They found that interstellar linear polarization interstellar dust grains came from observations of the can be fitted well by an empirical relation ,Figure(1), characteristics on polarization of star lights (Clayton which is now universally accepted and considered and Martin, 1985). as excellent interpretation of normalized linear Polarization produced by non-spherical interstellar polarization of starlight . The polarization of starlight is generally attributed to interstellar dust. It is another aligned grains is also widely observed in the lines of important feature attributed to the scattering sight to (OB) stars over a similar spectral range properties of the dust. The linear and circular (Clayton and Martin, 1983). polarization of star light was accidentally discovered Linear polarizations are integrals over the same over fifty years ago. It is well established that particle size distributions for the aligned grains. The interstellar polarization is produced by non spherical integrands involve the product of the particle size interstellar dust grains aligned by Galactic magnetic distributions times the appropriate cross sections for field. polarization. The cross sections are determined by the material and geometry of the grain. So their ratio 1.0 0.8 0.6 0.4 0.2 0.0 0.00.51.01.52.02.53.03.54.04.5 Fig 1: Normalized Serkowski Linear Polarization Curve ,λmax=0.55µm , k=0.923 Am. J. Sci. Ind. Res., 2010, 1(2): 180-189 Linear Polarization: Sometimes the light received polarization was first recognizes by Serkowski et.al from celestial objects is polarized. This polarization (1975). They found that a good fit to this dependence can be detected by placing a polarized filter in front of can be made by using the following empirical relation The detector. The filter only passes radiation whose 2 electric field vector is parallel to the polarization P/Pmax=exp{-K Ln (λmax /λ)} …………..4 direction of the filter. Rotating the filter, different polarizations of the incoming light are passed. If the where Pmax and λma are previously defined and K is incoming light is unpolarized , the amount of light some constant. Although the relation is empirical, it is coming through will not depend on the angle through universally accepted and considered as excellent which the filter has been rotated. If the incoming interpretation of linear polarization and is known as light is completely linearly polarized, there will be one Serkowski law fit (Clayton and Martin, 1983). Clayton position of the filter for which the image can be seen et. al (1992) have found that the linear interstellar at full brightness and another position 90° away, for polarization can be fitted well by modified Serkowski which no image can be seen. If the incoming light is law fit from short far Ultraviolet to long infrared partially polarized, a maximum brightness can be wavelengths. For Galactic stars, it has been found seen at one position and a minimum brightness that K and λma are closely related, the most recent when the filter is rotated by 90°. Greater the amount determination from observations of 105 stars being of polarization, greater the contrast between the maximum and the minimum Kutner (1987). When K = 1.66 λma + 0.01 …………………….5 unpolarized starlight passes through interstellar dust clouds, it can emerge with a slight degree of linear Interstellar linear polarization is now generally polarization. This means that the polarization must be thought to be produced by non spherical dust grains caused by the dust itself Clayton et. al (1992) . If Imax aligned by the Galactic magnetic field ,Several and Imjn are the maximum and minimum intensities models have been put forward that attempt to fit most recorded as the polaroid is rotated , the polarization P or all of the known extinction and polarization is defined Wickramasinghe(1967) by features , but no one model is completely successful. Silicates are presented in all models because the P= [Imax- Imin] / [Imax - Imin] …1 polarization of the l0µm feature is attributed to aligned silicate grains Massa et. al. (1983) .In most o By this definition P=0 if Imax= Imin, i.e. corresponding models the 2175 A extinction bump is produced by to unpolarized light and P=1.0 if Imin =0.0, i.e. small graphite grains (Szomoru and corresponding to total linear polarization. For partial Guhatiiakutta,1999). Recent studies have shown that plane polarization, P lies between zero and one .It the wavelength dependence of the extinction and can be shown that the linear polarization is related to polarization Bradely et. al. (1999) is in the red and the differences in optical depths ( ∆τ ) for light with infrared may be the same from one line of sight to electric vector in two perpendicular directions by the another. The observed variations seem to be relation confined to the blue and Ultraviolet wavelength ∆τ = 2P ….……..2 ranges. Observations of the wavelength dependence of interstellar polarization in the Ultraviolet and its or expressed in magnitudes , using equation: behavior across the 2175°A feature can play an A=1.086τ , we have important role .in distinguishing among the Amp = 2.172P …3 completing models for interstellar dust. where Amp denotes the differences of polarization . The wavelength dependence of interstellar linear 181 Am. J. Sci. Ind. Res., 2010, 1(2): 180-189 Observation and Astrometry data 1- HD 142250: Constellation: Scorpius Right ascension: 15h54m30.00s Declination: -27°20'19.0" Apparent magnitude: 6.14 Distance: 162.866 parsecs Proper motion RA: -14 Proper motion Dec: -26.3 B-T magnitude: 6.056 V-T magnitude: 6.128 2- HD 34078: Constellation: Auriga Right ascension: 05h16m18.20s Declination: +34°18'43.0" Apparent 5.96 magnitude: Distance: 446.429 parsecs Proper motion RA: -4.3 Proper motion Dec: 44 B-T magnitude: 6.237 V-T magnitude: 6.025 182 Am. J. Sci. Ind. Res., 2010, 1(2): 180-189 3- HD 46202: Constellation: Monoceros Right ascension: 06h32m10.47s Declination: +04°57'59.8" Apparent magnitude: 8.271 Proper motion RA: -3.3 Proper motion Dec: 1.4 B-T magnitude: 8.354 V-T magnitude: 8.278 4- HD 48099: Constellation: Monoceros Right ascension: 06h41m59.30s Declination: +06°20'42.0" Apparent magnitude: 6.37 Distance: 10000000 parsecs Proper motion RA: 1.6 Proper motion Dec: 3.8 B-T magnitude: 6.254 V-T magnitude: 6.341 183 Am. J. Sci. Ind. Res., 2010, 1(2): 180-189 5- HD 147889: Constellation: Ophiucus Right ascension: 16h25m24.32s Declination: -24°27'56.6" Apparent magnitude: 7.957 Distance: 135.87 parsecs Proper motion RA: -1.4 Proper motion Dec: -25 B-T magnitude: 8.877 V-T magnitude: 8.033 6- HD 154445: Constellation: Ophiucus Right ascension: 17h05m32.30s Declination: -00°53'31.0" Apparent magnitude: 5.64 Distance: 234.742 parsecs Proper motion RA: 4 Proper motion Dec: -2 B-T magnitude: 5.753 V-T magnitude: 5.648 Table 1: Polarization characteristics of 6 individual stars HD P K P A . 0 max 197770 3.81 0.857 131.1 37903 2.04 1.139 120.6 2905 1.53 0.89 86.1 25443 5.17 0.823 137.1 161056 4.03 0.989 68.4 7252 3.49 0.873 95.2 184 Am. J. Sci. Ind. Res., 2010, 1(2): 180-189 RESULT AND DISCUSSION while the imaginary parts of the bacterial refractive Interstellar grains cause a partial linear polarization of indices are k= 0.025, 0.04, 0.03, 0.008, 0.02, 0.02 for star light because components of them are aligned the stars in figure (3) to (8) in sequence. with regard to the magnetic field of the galaxy. The The modified Serkowski law fit, equation (4), with the existence of polarization of star lights requires a values of K, equation (5) for value Pmax and λmax degree of anisotropy in the aligned dust grains. The tabulated in table (1). It can be seen that the current understanding of alignment requires Serkowski relation and hollow cylindrical bacteria polarization to be produced by aligned, non spherical model are in good agreement with the observed dust grains. polarization data .The fitting of the models in the UV Historically, the use of the infinite cylinder has region are not good .
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