Wavelength dependence of polarization. X. Interstellar polarization.

Coyne, G. V.; Gehrels, T.

doi:10.1086/110357

Cited by 54 publications

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confidence: 90%

“…About 40 h of integration time were invested per region, in six filters ranging in effective wavenumber from 1.21 to 3.03 per micron. The color data were generally in good agreement with previous results, but polarizations in NGC 2068 and NGC 7023 were found to be systematically larger than those reported by Elvius and Hall (1966).The observations were compared with single-scattering Mie calculations for a wide variety of complex refractive indices m* = rri -im". Two types of broad size distribu tions were used, namely the Oort-van de Hulst distribution as modified by Greenberg (OHG distribution) (Greenberg, 1966), n(a) = n 0 exp -5[a/a 0 ] 3 , and the exponential distribution, n(a) = n 0 exp -5\a/a 0 ].…”

supporting

confidence: 85%

“…Details of the observations and of the theoretical treatment are given elsewhere (Zellner, 1970(Zellner, , 1973. Briefly, observations of color and polarization were made in a few selected regions of NGC 2068, NGC 7023, and IC 5067 with the two-channel digital polarimeter described by Coyne and Gehrels (1967). Most of the data were taken at the 154-cm Catalina reflector of the Lunar and Planetary Laboratory.…”

Section: Introductionmentioning

confidence: 99%

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Dust Grains in Reflection Nebulae

Zellner

1973

Symp. - Int. Astron. Union

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Abstract.Observations and Mie theory calculations of colors and polarization in several reflection nebulae are compared.A new study of light scattering in reflection nebulae was begun in 1968. Details of the observations and of the theoretical treatment are given elsewhere (Zellner, 1970(Zellner, , 1973. Briefly, observations of color and polarization were made in a few selected regions of NGC 2068, NGC 7023, and IC 5067 with the two-channel digital polarimeter described by Coyne and Gehrels (1967). Most of the data were taken at the 154-cm Catalina reflector of the Lunar and Planetary Laboratory. About 40 h of integration time were invested per region, in six filters ranging in effective wavenumber from 1.21 to 3.03 per micron. The color data were generally in good agreement with previous results, but polarizations in NGC 2068 and NGC 7023 were found to be systematically larger than those reported by Elvius and Hall (1966).The observations were compared with single-scattering Mie calculations for a wide variety of complex refractive indices m* = rri -im". Two types of broad size distribu tions were used, namely the Oort-van de Hulst distribution as modified by Greenberg (OHG distribution) (Greenberg, 1966), n(a) = n 0 exp -5[a/a 0 ] 3 , and the exponential distribution, n(a) = n 0 exp -5\a/a 0 ]. The factor of 5 is introduced in order to make the scale radius a 0 roughly comparable with the equivalent single radius a in the absence of a size distribution. Wavelength-independent refractive indices were assumed, so that a 0 could be left as a free parameter to be determined by the agreement between theory and observation. In most theoretical studies of reflection nebulae, the dust cloud has been assumed to be homogeneous and bounded by a simple geometrical figure such as a sphere or a plane-parallel slab. As illustrated in Figure 1, my models are free of geometrical constraints; in each separate nebular region, the extreme scat tering angles 6 t and 6 2 are independently adjusted to best match the observations. The computed polarization is very sensitive both to the scattering geometry and to the scale radius. The color data, being more perturbed by internal and foreground extinc tion, are only indicative.Reflection nebula polarizations usually decrease linearly with wavenumber from the infrared into the ultraviolet. This behavior is characteristic of forward-scattering dielectric grains, but can be produced by particles with a large m" only under contrived conditions of internal attenuation. Thus pure graphite or metallic grains are imme diately ruled out. It has been stated (Hanner, 1971) that silicate grains (m' = 1.5 to 1.7) would give negative polarization at short wavelengths, contrary to all observations. Figure 2, however, shows this statement to be a halftruth. With an appropriate shift Greenberg and Van de Hulst (eds.)

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supporting

confidence: 90%

supporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

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Dust Grains in Reflection Nebulae

Zellner

1973

Symp. - Int. Astron. Union

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“…The results of our observations concerning the Be-type stars (see Tables I and 2 [3]. In these stars, the intrinsic polarization is too small to perturb the results given by interstellar matter; namely, observed values 0.83 < Pulpv < 0.95.…”

Section: Resultsmentioning

confidence: 58%

Polarization in be‐type stars

Martel

1971

Astron Nachr

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At I/A = 2.79 pm-1, the comparison of a Be star with a nearby normal R star shows that the interstellar polarization of the Be star is diminished by a factor 0.7.The variable polarization of Be-type stars was detected by SHAKOVSKOJ [IO] and by COYNE and GEHRELS [3].In the emission-lines stars, the observed polarization is of intrinsic as well as of interstellar origin. The intrinsic polarization diminishes in the ultra-violet more rapidly than the interstellar polarizationRecently, COYNE and KRUZEWSKI [4] found that the average polarization for eight Be-type stars is large when the hydrogen absorption is small and vice versa. They observed in seven spectral bands from I/A = 3.03 pm-l to I / A = 1.06 pm-l. ObservationsIn the UBVR system, polarimetric observations of eleven emission-lines stars are presented here at I / A = 2.79, 2.38, 1.87, and 1.61 pm-l. The observation and reduction techniques are described by, except that the analyser of the electro-polarimeter is now a Rochon. ResultsThe results of our observations concerning the Be-type stars (see Tables I and 2 We show results in Table 2 for which the observed polarization diminishes more rapidly in the ultraviolet than the normal interstellar polarization, namely 0.54 < Pulpv < 0.74. We

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“…A computer program kindly supplied by N. C. Wickramasinghe was appropriately modified to include computations of Q p . In Figure 1 some results are shown for models chosen to match the mean observed linear polarization curve tabulated by Coyne and Gehrels (1967). No attempt is made at this point to use a size distribution or oblique incidence since we are interested in isolating the effects of changing the complex re- What is important is that these different grains which produce similar linear polariz ation curves in Figure 1 do not have the same circular polarization, as is clear from Figure 2.…”

Section: Wavelength Dependence Of Circular Polarizationmentioning

confidence: 99%

Interstellar Circular Polarization and the Composition of Interstellar Dust

Martin¹

1973

Symp. - Int. Astron. Union

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Abstract. This paper shows that optical observations of circular polarization produced by aligned interstellar grains could yield valuable information about the grain material. The interstellar medium is known to be linearly dichroic from observations of interstellar linear polarization; many different grain models using a large variety of compositions can be found to reproduce these observations. Since the same aligned grains make the medium linearly birefringent, a small component of circular polarization can result from incident linearly polarized light if the position angle of the linear polariza tion does not coincide with either principal axis of the medium. Here calculations are presented to demonstrate that the wavelength of the circular polarization is sensitive to the imaginary part of the complex refractive index of the grain material. This provides an opportunity of investigating whether the grains are characteristically dielectric or metallic. Some possible observations are suggested.

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Wavelength dependence of polarization. X. Interstellar polarization.

Cited by 54 publications

References 0 publications

Dust Grains in Reflection Nebulae

Dust Grains in Reflection Nebulae

Polarization in be‐type stars

Interstellar Circular Polarization and the Composition of Interstellar Dust

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