Polarimetry in the visible and infrared: application to CMB polarimetry

Interstellar polarization from aligned dust grains can be measured both in transmission at visible and near-infrared wavelengths and in emission at far-infrared and sub-mm wavelengths. These observations can help predict the behavior of foreground contamination of CMB polarimetry by dust in the Milky Way. Fractional polarization in emission from aligned dust grains will be at the higher range of currently observed values of 4–10%. Away from the galactic plane, fluctuations in Q and U will be dominated by fluctuations in intensity, and less influenced by fluctuations in fractional polarization and position angle.     

Microwave emission from aligned dust

Polarized microwave emission from dust is an important foreground that may contaminate polarized CMB studies unless carefully accounted for. We discuss potential difficulties associated with this foreground, namely, the existence of different grain populations with very different emission/polarization properties and variations of the polarization yield with grain temperature. In particular, we discuss observational evidence in favor of rotational emission from tiny PAH particles with dipole moments, i.e. “spinning dust”, and also consider magneto-dipole emission from strongly magnetized grains. We argue that in terms of polarization, the magneto-dipole emission may dominate even if its contribution to total emissivity is subdominant. Addressing polarized emission at frequencies larger than 100 GHz, we discuss the complications arising from the existence of dust components with different temperatures and possibly different alignment properties.     

Cassiopeia A: dust factory revealed via submillimetre polarimetry

If Type II supernovae – the evolutionary end points of short-lived, massive stars – produce a significant quantity of dust  (>0.1 M_⊙)  then they can explain the rest-frame far-infrared emission seen in galaxies and quasars in the first Gyr of the Universe. Submillimetre (submm) observations of the Galactic supernova remnant, Cas A, provided the first observational evidence for the formation of significant quantities of dust in Type II supernovae. In this paper, we present new data which show that the submm emission from Cas A is polarized at a level significantly higher than that of its synchrotron emission. The orientation is consistent with that of the magnetic field in Cas A, implying that the polarized submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarization in this way and so we attribute the excess polarized submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. This is supported by the presence of both polarized and unpolarized dust emission in the north of the remnant where there is no contamination from foreground molecular clouds. The inferred dust polarization fraction is unprecedented  ( f _pol∼ 30 per cent)  which, coupled with the brief time-scale available for grain alignment (<300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where   f _pol= 2−7  per cent) or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.     

Millimeter polarimetry of star-forming regions: magnetic field structure around low-mass stars

We have measured polarization of the 1.1 mm and 0.8 mm continuum emission for 3 pre-T Tauri stars and 2 T Tauri stars. Positive detections were made for NGC 1333 IRAS 4 and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. We have found a definite geometrical relation between the polarization and other circumstellar structure.     

Near-infrared polarimetry of the Red Rectangle

Imaging polarimetry through J and H broad-band filters and a 3.4 μm narrow-band filter is used to highlight the regions of scattered light in the Red Rectangle. We find that the scattered light identifies the circumbinary dust component of the molecular disc seen in CO emission. This region also appears to be the origin of the recently discovered Blue Luminescence. We find that the degrees of polarization are consistent with the amorphous carbon dust model invoked by Men'shchikov. Spectropolarimetry from 1.4 to 2.5 μm confirms that the degree of polarization in the central arcsecond region is very low. This suggests that the central bicone seen in the near-infrared is predominantly due to emission from hot dust and/or from stochastically heated nanoparticles, rather than due to scattering by large grains.     

CCD imaging polarimetry of Comet 2P/Encke

We present results and analysis of imaging polarimetric observations of Comet 2P/Encke. The observations were carried out at the 2-m RCC telescope of the Bulgarian National Astronomical Observatory on December 13, 1993 and on January 14, 1994, at phase angles 51.1° and 80.5°, respectively. A wide-band red filter 6940/790 Å was used. This filter is transparent for the continuum and the weak emission bands of NH₂ and H₂O⁺. There is a sunward dust fan with well-defined polarization, which peaks at≈13% in the image obtained on January 14, 1994. Along the sunward fan the degree of polarization decreases progressively. Outside of the fan the coma displays a low polarization of ≈3%. We suggest that this low polarization is caused by the NH₂ emission in the pass-band of the red wide-band filter. Assuming a spherically symmetric NH₂ coma we are able to correct the observed polarization for this effect. The correction leads to an increase of the observed polarization by 1 to 4% at distances 10,000 and 1500 km from the nucleus. A rough estimate shows that the polarization in the near nucleus region of Comet Encke is similar to that for the dusty comets. Even after correction the polarization of Comet Encke's dust fan is significantly less that the polarization observed in dusty comets. The reasons influencing the distribution of dust polarization in the coma are discussed. More polarimetric and colorimetric observations of the dust in Comet Encke on its return in 2003 are needed.     

Angular spectra of polarized Galactic foregrounds

It is believed that magnetic field lines are twisted and bend by turbulent motions in the Galaxy. Therefore, both Galactic synchrotron emission and thermal emission from dust reflects statistics of Galactic turbulence. Our simple model of Galactic turbulence, motivated by results of our simulations, predicts that Galactic disk and halo exhibit different angular power spectra. We show that observed angular spectra of synchrotron emission are compatible with our model. We also show that our model is compatible with the angular spectra of star-light polarization for the Galactic disk. Finally, we discuss how one can estimate polarized microwave emission from dust in the Galactic halo using star-light polarimetry.     

A search for optical polarization from the narrow-line Seyfert 1 galaxy RE J1034+396

We present spectropolarimetry data on RE J1034+396, an ultra-soft X-ray narrow-line Seyfert 1 galaxy, and find an upper limit of ∼ 0.4 to the linear polarization in the optical band. This suggests that there is no synchrotron emission in this AGN, and thus it is unlikely that RE J1034+396 is related to BL Lac objects. Furthermore, any other polarization arising from transmission through dust, or reflection from dust and electrons, must be cancelled out by geometrical effects or diluted to a high degree by unpolarized radiation.     

Milestones in the Observations of Cosmic Magnetic Fields

Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from anal     

BIMA and JCMT Spectropolarimetric Observations of the CO J = 2 − 1 Line Towards Orion KL/IRc2

We present results of JCMT and BIMA CO J = 2 ? 1 polarization observations towards the Orion KL/IRc2 high mass star forming region. The linear polarization fraction of the JCMT CO J = 2 ? 1 spectra presents a clear decrease towards the center of the line, as expected, due to the increase of the optical depth. The position angle remains constant along the spectral line, except at the line center, where the highest optical depth and lower fractional polarization are measured. The combined BIMA and JCMT maps of the redshifted and blueshifted CO emission show a uniform polarization pattern that does not coincide with previous dust continuum observations at similar angular resolution. This suggests that the CO and dust are tracing different spatial components along the line of sight.     

polarimetric and photometric observations of split comet C/2001 A2 (LINEAR)

We present the results of polarimetric and photometricobservations of split Comet C/2001 A2 (LINEAR), which wereobtained at the 70-cm telescope of the Astronomical Observatoryof Kharkiv National University between 30 June and 31 July 2001.The brightest fragment of the comet, nucleus B, was observed.Eight narrowband cometary filters in the continuum and in emissionbands and a wideband red filter have been used. The comet wasobserved through apertures of 88, 33, and 19 arcsec. Polarizationphase curves were obtained for the continuum and for the firsttime, for NH₂(0, 7, 0) emission. The degree of polarization ofthe light scattered by the dust decreases with the increase ofaperture size. An important temporal variation of the polarizationwith a rotation of the polarization plane was observed at twophase angles (26.5° and 36.2°). Molecular column densities and production rates of CN, C₂,C₃, and NH₂ species are calculated in the framework of theHaser model. A comparative analysis of the temporal variations ofthe visual magnitudes, gas and dust production rates, dust colorand polarization are presented.     

On some observations of the thermal emission of iron flakes produced in a grinding wheel

A report is given of some laboratory experiments on the thermal emission of glowing iron flakes. Clear effects of polarization are found sometimes in flakes of small size, indicating polarization of a kind similar to that appearing in the thermal emission from narrow metallic filaments. Sudden flashes of light appear in the thermal emission from flakes produced in a grinding wheel. At the same time the flake splits into two parts. It is suggested that the flash is due to tribo-thermoluminescence. It seems possible that the infrared radiation of the solar corona may contain a faintly polarized component due to thermal emission from dust particles.Paper dedicated to Professor Hannes. Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Non-emission-line young stars of intermediate mass

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.     

Modeling the linear polarization of optical emission from red giants

We present the results of solving the radiative transfer equation for the Stokes vector in the case of light scattering by spherical forsterite dust particles in an axisymmetric circumstellar envelope of a red giant. We have assumed that the surfaces of constant scattering-particle density are prolate or oblate spheroids, the particle density decreases with radius as N _d ∝ r ⁻², and the dust particles at the inner boundary of the envelope are in thermal equilibrium with the stellar emission at solid-phase evaporation temperature T _ev = 800 K. In the wavelength range 0.27 μm ≤ λ ≤ 1 μm, particles with radii 0.03 μm ≲ a ≲ 0.2 μm make a major contribution to the linear polarization of the stellar emission. The increase in scattering efficiency factor with decreasing wavelength λ is mainly responsible for the growth of polarization toward the short wavelengths known from observations. However, at a mean number of scatterings 1.2 ≤ N _sca ≤ 1.6, the polarization ceases to grow due to depolarization effects and decreases rapidly as the wavelength decreases further. The wavelength of the polarization maximum is determined mainly by two quantities: the particle radius and the mass loss rate. The upper limits for the degree of linear polarization in the case of light scattering in circumstellar dust envelopes with the geometries of prolate and oblate spheroids are p ≈ 3 and 5%, respectively. The polarization for light scattering by enstatite particles is higher than that for light scattering by forsterite particles approximately by 0.3%. Original Russian Text ? Yu.A. Fadeyev, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33, No. 2, pp. 123–133.     

Photopolarimetric studies of comet Austin

Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope (VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the 1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990. The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of 0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C₂ bands are quite strong; C₃ emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C₂ emission. Molecular band polarization for CN, C₃, C₂ and H₂ O+ have been calculated. It has been found that emission polarization in CN, C₂ and C₃ is between 1–7% (phase angle between 107.4–109 degrees). For CN and C₂ the polarization values are close to the theoretically predicted values, but for C₃ the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley.     

Foreground removal from WMAP 7 yr polarization maps using an MLP neural network

One of the fundamental problems in extracting the cosmic microwave background signal (CMB) from millimeter/submillimeter observations is the pollution by emission from the Milky Way: synchrotron, free-free, and thermal dust emission. To extract the fundamental cosmological parameters from CMB signal, it is mandatory to minimize this pollution since it will create systematic errors in the CMB power spectra. In previous investigations, it has been demonstrated that the neural network method provide high quality CMB maps from temperature data. Here the analysis is extended to polarization maps. As a concrete example, the WMAP 7-year polarization data, the most reliable determination of the polarization properties of the CMB, has been analyzed. The analysis has adopted the frequency maps, noise models, window functions and the foreground models as provided by the WMAP Team, and no auxiliary data is included. Within this framework it is demonstrated that the network can extract the CMB polarization signal with no sign of pollution by the polarized foregrounds. The errors in the derived polarization power spectra are improved compared to the errors derived by the WMAP Team.     

The Infrared Spectral Energy Distribution And Polarization Of Comet C/1995 O1 (Hale–Bopp) During 1997

Comets, such as C/1995 O1 (Hale-Bopp), are important to studies of the origins of the solar system because they are believed to be frozen reservoirs of the most primitive pre-solar dust grains and ices. Here, we report 1.2–18.5 μm infrared (IR) spectrophotometric and polarimetric observations of comet Hale-Bopp. Our measurements of the spectral energy distribution (SED) and IR polarization near perhelion passage suggest that emission from the coma was dominated by scattering and thermal emission from sub-micron sized dust grains. Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather than the size of its nucleus. The thermal emission continuum from the grains had a superheat of S = T_color/T_BB ≥ 1.7, the peak of the 10 μm silicate emission feature was 1.7 mags above the carbon grain continuum, and the albedo (reflectivity) of the grains was ≥ 0.4 at a scattering angles, θ ≥ 135° This revised version was published online in July 2006 with corrections to the Cover Date.     

Broad-band optical polarimetric study of IC 1805

We present the BVR broad-band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature, we have modelled and subtracted the foreground dust contribution from the maximum polarization ( P _max) and colour excess [ E ( B − V )]. The mean value of the P _max for intracluster medium and the foreground are found to be 5.008 ± 0.005 and 4.865 ± 0.022 per cent, respectively. Moreover, the mean value of the wavelength of maximum polarization (λ_max) for intracluster medium is 0.541 ± 0.003 μ m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.     

星际尘埃研究现状与进展

由于星际尘埃的广泛存在和其在恒星与行星系统的形成、星系以及整个宇宙演化中的重要作用,星际尘埃的研究成为当今天体物理领域的热点前沿课题。该文从尘埃与电磁场相互作用的观测证据出发,系统地介绍了星际消光(包括吸收和散射)、星际红外辐射、星际偏振等的研究现状,讨论了星际元素减损,以及行星际尘埃和陨石中的前太阳尘埃等问题。从相应的观测证据中,可以得到关于星际尘埃的丰度、化学组成、尺寸和形状的信息。该文还对当前比较流行的三种尘埃模型(硅酸盐-石墨-PAHs模型、硅酸盐核-碳有机耐熔质壳层模型和多孔尘埃模型)进行了讨论与比较,对该研究领域中待解决的问题也作了简要的概括。     

Dust coma of comet C/2009 P1 (Garradd) by imaging polarimetry

Comet C/2009 P1 (Garradd) was observed by imaging polarimetry for nearly 5 months from October 2011 to March 2012, over an intermediate phase angle range (28°–35°). Two months before perihelion and one month after, dust particles seem to be ejected all around the optocenter and jets extend to distances greater than 40,000 km. An increase of activity is noticed in intensity and polarization after perihelion. Two months before perihelion and one month after, the dust emission seems to be all around the optocenter. Two and three months after perihelion the jets are mainly toward the solar direction with an extension of more than 20,000 km projected on the sky. The values of the aperture polarization are comparable to those of other comets. On the polarization maps in October 2011 and January 2012 the higher polarization zones extend in large regions perpendicularly to the solar direction where jets are also observed. In February and March 2012, the polarization in the jets is larger in the solar direction than in the surrounding coma. By its activity visible on intensity images and polarization maps at large distances from the nucleus, comet Garradd probably belongs to the high‐P_max class of comets.