Analysis of Interstellar Dust Using Linear Polarization

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: [email protected] 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

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0.4

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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

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Observation and Astrometry data 1- HD 142250:

Constellation: Scorpius

Right ascension: 15h54m30.00s

Declination: -27Â°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: +34Â°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

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3- HD 46202:

Constellation: Monoceros Right ascension: 06h32m10.47s Declination: +04Â°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: +06Â°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

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5- HD 147889:

Constellation: Ophiucus Right ascension: 16h25m24.32s

Declination: -24Â°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: -00Â°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

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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 . It is generally accepted that the dominated polarization models. The small - particle grains are aligned such that the short axis of the approximation ( 2πa/λ<<1,where a is the grain size ) grain profile, i.e., the position angle of the polarization has been applied successfully , for the interpretation (see last column of table (1) is in the direction of of interstellar polarization .The infinite cylinder magnetic field .The orientation of spinning interstellar calculation , in addition to having a fair degree of grains (end- over- end) involve two aspects qualitative success, is extremely fast for electromagnetic scattering geometry: The inclination computations. Fig(2) shows the calculation of of the spin axis to the direction of the incident = polarization cross - section [Qpol QextE – QextH] for radiation field (line of sight) and degree of alignment. infinite cylinder, where Q represent the extinction It is for this reason, some vibration in the data points efficiency factor for the direction of the incident can be observed, since all the grains particles are not electric - field vector parallel (E) and perpendicular equally symmetrically aligned. (H) to the major axis of the spheroid . It appears from fig. (2) that the use of infinite cylinder to represent CONCLUSIONS elongated , finite grains may , in fact , be valid Dust grain model has been used to fit the quantitatively as well as qualitatively . However, this polarization from Infrared to visible have been apparent success in representing finite particles, reexamined in the light of new interstellar lines of encourage us to consider the infinite cylinder sight. The model considered a Serkowski law fit, approximation to be adequate for the purpose of the from the analysis presented in this work, the study of bacterial grains in the form of long hollow following conclusions ca made: needles. 1- The hollow needle shape cylindrical bacterial Figures (3) to (8) shows plot of observed percent grains give reasonable fit to the polarization linear polarization of six regions; HD142250 , in the infrared and bump region for the stars HD34078 , HD46202, HD48099, HD147889 and in these regions. HD154445. The observed data (Solid Line) are fitted 2- The Serkowski law fit of wavelength with the linear polarization data produced by hollow dependence of linear polarization still remain bar shape bacteria. The mean real part of the as excellent interpretation of polarization star bacterial refractive index is n=1.16 for all the stars, lights.

1.20 1.12 1.04 0.96 0.88 0.80

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0.64

0.56

0.48 0.40 pol 0.32

Q 0.24 0.16 0.08 0.00 -0.08 -0.16 -0.24 -0.32 -0.40 0.000.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 3.60 4.00 4.40 4.80 5.20 5.60 6.00 6.40 6.80 7.20 7.60 8.00 Size Parameter, x=2πa/ λ = Fig 2 : Calculation Of The Polarization Cross-Section(Qpol Qexte – Qexth )For Normally Illuminated, Static Infinite Cylinder (M=1.6+0.05i).The Data Are Normalized To Its Maximum Polarization.

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1.60 1.52 Serkowski Empirical Law Fit 1.44 Observed Microbial Model 1.36 1.28

1.20 1.12 1.04 max 0.96 0.88 P/P

0.80 0.72 0.64 0.56 0.48

0.40 0.32 0.24 0.16 0.08

0.00 -0.08 -0.16

0.000.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 3.60 4.00 4.40 4.80 5.20 5.60 6.00 1/λ (µm-1) Fig 3 : Best Microbial Model and Modified Serkowski Law Fit to the Observed Interstellar Liner Polarization of HD 142250 .

1.36 Serkowski Empirical Law Fit 1.28 Observed Microbial Model 1.20

1.12

1.04 0.96

0.88 max 0.80 P/P

0.72

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0.56 0.48 0.40 0.32 0.24

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0.08

0.00 0.00 0.40 0.80 1.20 1.602.004.006.00 2.40 2.80 3.20 3.60 4.40 4.80 5.20 5.60 1/λ (µm-1)

Fig 4 : Best Microbial Model and Modified Serkowski Law Fit to the Observed Interstellar Liner Polarization of HD 34078 .

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1.36 Serkowski Empirical Law Fit 1.28 Observed 1.20 Microbial Model 1.12 1.04 0.96

max 0.88

0.80 P/P

0.72 0.64 0.56 0.48

0.40 0.32 0.24 0.16

0.08 0.00 -0.08

-0.16 -0.80 -0.400.00 0.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 3.60 4.00 4.40 4.80 5.20 5.60 6.00 1/λ (µm-1)

Fig 5 : Best Microbial Model and Modified Serkowski Law Fit to the Observed Interstellar Liner Polarization of HD 46202 .

1.36 Serkowski Empirical Law Fit 1.28 Observed Microbial Model 1.20 1.12 1.04

0.96 max 0.88

P/P

0.80 0.72 0.64 0.56 0.48 0.40 0.32 0.24 0.16 0.08 0.00 -0.08 -0.16 -0.400.00 0.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 3.60 4.00 4.40 4.80 5.20 5.60 6.00 -1 1/λ (µm ) Fig 6 : Best Microbial Model and Modified Serkowski Law Fit to the Observed Interstellar Liner Polarization of HD 48099 .

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1.60 1.52 Serkowski Empirical Law Fit 1.44 Observed

1.36 Microbial Model 1.28 1.20 1.12 1.04 max 0.96

P/P 0.88

0.80 0.72 0.64

0.56 0.48 0.40 0.32 0.24

0.16 0.08 0.00 -0.08 -0.16

0.000.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 3.60 4.00 4.40 4.80 5.20 5.60 6.00 6.40 6.80 1/λ (µm-1)

Fig 7: Best Microbial Model and Modified Serkowski Law Fit to the Observed Interstellar Liner Polarization of HD 147889 .

1.60

1.52 Serkowski Empirical Law Fit Observed 1.44 Microbial Model 1.36 1.28 1.20 1.12 1.04 max 0.96 P/P

0.88 0.80 0.72 0.64 0.56 0.48 0.40 0.32 0.24 0.16 0.08 0.00 -0.08 -0.16 0.000.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 3.60 4.00 4.40 4.80 5.20 5.60 6.00 1/λ (µm-1) Fig 8 : Best Microbial Model and Modified Serkowski Law Fit to the Observed Interstellar Liner Polarization of HD 154445 . 188

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REFERENCES [6] Clayton. G.C, et. al. (1992): the first spectropolarimetric study of the wavelength [1] Clayton, G.C. and Martin, P G. (1985): interstellar dependence of interstellar polarization in the dust in the large magellanic cloud. The ultraviolet .The Astrophysical Journal, 385, 1.53 - astrophysical journal. 288. 558 - 568. 1.57. [2] Clatyton, G.C. and Andmartin, P.G. (1983): the wavelength dependence of interstellar [7] Wickramasinghe, C. (1967): interstellar grains .the polarizationin the large magellanic cloud. The international astrophysics series. Volume nine, astrophysical journal, 265, 194 - 201. Butler and Tanner Ltd., U.K.

[3] Clayton, G.C. and Mathis, I.S. (1988): On the [8] Massa.D. et. al. (1983): Peculiar Ultraviolet Interstellar Extinction. The Astrophysical Journal, relationship between optical polarization and 266.662 - 683. extinction. The Astrophysical Journal. 327, 911- [9] Szomoru, U. And Guhatiiakutta, P. (1999): 919. Extinction Curves, Distances, and Clumpiness of [4] Serkowsk I, K., Mathewson, D.S., and Ford, V.I. Diffuse Interstellar Dust Clouds.-The Astrophysical (1975): wavelength dependence of interstellar Journal, 117, 2226 - 2243. polarization and ratio of total to selective extinction. The Astrophysical Journal. 196, 261- [10] Bradely, .J. P. et. al. (1999): An Infrared Spectral 290. Match between GEMS and Interstellar Grains. Science, 285, 1716 - 1718. [5] Kutner, M.L. (1987): astronomy, a physical perspective, chapter 14: contents of the interstellar medium. p 305 - 334 John Wiley and Sons.

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