Context. The role of magnetic fields in the star formation process is a contentious matter of debate. In particular, no clear observational proof exists of a general influence by magnetic fields during the initial collapse of molecular clouds. Aims. Our aim is to examine magnetic fields and their influence on a wide range of spatial scales in low-mass star-forming regions. Methods. We trace the large-scale magnetic field structure on scales of 10 3 −10 5 AU in the local environment of Bok globules through optical and near-infrared polarimetry and combine these measurements with existing submillimeter measurements, thereby characterizing the small-scale magnetic field structure on scales of 10 2 −10 3 AU. Results. For the first time, we present polarimetric observations in the optical and near-infrared of the three Bok globules B335, CB68, and CB54, combined with archival observations in the submillimeter and the optical. We find a significant polarization signal (P 2%, P/σ P > 3) in the optical and near-infrared for all three globules. Additionally, we detect a connection between the structure on scales of 10 2 −10 3 AU to 10 3 −10 4 AU for both B335 and CB68. Furthermore, for CB54, we trace ordered polarization vectors on scales of ∼10 5 AU. We determine a magnetic field orientation that is aligned with the CO outflow in the case of CB54, but nearly perpendicular to the CO outflow for CB68. For B335 we find a change in the magnetic field oriented toward the outflow direction, from the inner core to the outer regions. Conclusions. We find strongly aligned polarization vectors that indicate dominant magnetic fields on a wide range of spatial scales.

In the present work, the light‐scattering properties of comet Levy 1990XX are studied through simulations using Ballistic Particle‐Cluster Aggregation (BPCA) or Ballistic Cluster–Cluster Aggregation (BCCA) aggregates of up to 128 spherical monomers of different compositions (e.g. silicates, carbonaceous materials etc.) and the best‐fitting theoretical polarization curve is generated using the Superposition T‐matrix code. The best‐fitting refractive indices coming out from the present analysis show silicate behaviour when monomer radius is am= 0.12 μm and provide excellent results on the maximum and negative degrees of linear polarization at a single wavelength λ= 0.485 μm for BCCA aggregates.

Context. The study of distant comets, which are active at large heliocentric distances, is important for better understanding of their physical properties and mechanisms of long-lasting activity. Aims. We analyze the dust environment of the distant comet C/2014 A4 (SONEAR), with a perihelion distance near 4.1 au, using comprehensive observations obtained by different methods. Methods. We present an analysis of spectroscopy, photometry, and polarimetry of comet C/2014 A4 (SONEAR), which were performed on November 5 -7, 2015, when its heliocentric distance was 4.2 au and phase angle was 4.7 • . Long-slit spectra and photometric and linear polarimetric images were obtained using the focal reducer SCORPIO-2 attached to the prime focus of the 6-m telescope BTA (SAO RAS, Russia). We simulated the behavior of color and polarization in the coma presenting the cometary dust as a set of polydisperse polyshapes rough spheroids. Results. No emissions were detected in the 3800 -7200 Å wavelength range. The continuum showed a reddening effect with the normalized gradient of reflectivity 21.6±0.2% per 1000 Å within the 4650 -6200 Å wavelength region. The fan-like structure in the sunward hemisphere was detected. The radial profiles of surface brightness differ for r-sdss and g-sdss filters, indicating predominance of submicron and micron-sized particles in cometary coma. The dust color (g-r) varies from 0.75±0.05 m to 0.45±0.06 m along the tail. For aperture radius near 20 000 km, the dust productions in various filters were estimated as A f ρ = 680±18 cm (r-sdss) and 887±16 cm (g-sdss). The polarization map showed spatial variations of polarization over the coma from about -3% near the nucleus to -8% at cometocentric distance about 150 000 km. Our simulations show that the dust particles were dominated (or covered) by ice and tholin-like organics. Spatial changes in the color and polarization can be explained by particle fragmentation.

We report the results of optical polarimetric observations of a Bok globule CB34 to study magnetic field structure on large scales (10 5 − 10 6 AU), which is combined with archival sub-mm observations to characterize the magnetic field structure of CB34 on small scales (10 4 −10 5 AU). The optical polarization measurements indicate that the magnetic field in the globule is constrained to a maximum radius of 10 5 AU around the core, out to densities not smaller than 10 4 cm −3 . Our study is mainly concentrated on two submillimeter cores C1 and C2 of CB34. The direction of magnetic field of core C2 is found to be nearly perpendicular to the CO outflow direction of the globule. The magnetic field of core C1 is almost aligned with the minor axis of the core which is typical for magnetically dominated star formation models. The mean value of offset between the minor axis of core C2 and the outflow direction is found to be 14• which suggests that the direction of the outflow is almost aligned with the minor axis of core C2. The magnetic field strength in the plane-ofsky for cores C1 and C2 is estimated to be ≈ 34µG and ≈ 70µG.

We present the polarization maps of three selected Bok globules CB56, CB60 and CB69 constructed using a V-band data from a CCD imaging polarimeter. The aim of this work is to measure the optical polarization (p v ) of background field stars in order to determine the polarization efficiency, p v /A v . We find that the local magnetic field of the cloud CB56 is almost aligned with the galactic field, but not in CB60 and CB69. A trend of decreasing polarization efficiency with increasing extinction (A v ) is observed: it can be well represented by a power law,where α = −0.56 ± 0.36, −0.59 ± 0.51 and −0.52 ± 0.49 for CB56, CB60 and CB69 respectively. This indicates that the linear polarization of the starlight due to aligned dust grains in these clouds is produced more efficiently in low extinction regions, compared with high obscured lines of sight.

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The spherical and non-spherical dust-grain characteristics of comet Levy 1990 XX are studied in this paper. Using T-matrix theory, a good theoretical fit to the polarisation data observed by Chernova and co-workers for comet Levy 1990 XX is reported in the present work. The calculated degree of linear polarisation (p) as a function of observed scattering angle (θ) produced by prolate spheroidal crystalline olivine particles with effective radius (r eff = 0.218 µm), effective variance (v eff = 0.0036), and aspect ratio (E = 0.486) are compatible with the comet's observed polarisation at λ = 0.485 µm. After comparing the above result with the Mie theory result (r eff = 0.115 µm, v eff = 0.0465), it is found that prolate grains give the better fit to the observed polarisation data. Also, the negative polarisation behaviour of comet Levy 1990 XX is discussed.

Abstract.After the last apparition of comet Halley in 1985-86, a number of other comets were observed in polarimetry with IHW-continuum filters. From the in situ dust measurements of Halley, dust size distribution functions were obtained (Lamy et al. 1987, A&A, 187, 767; Mazets et al. 1986, Nature, 321, 276), which were later used by several authors to interpret polarisation data of Halley. In the present work, polarimetric data of various comets have been analyzed, using Mie theory and assuming that the composition of dust particles does not differ from comet to comet (Delsemme 1987, ESA SP-278, 19). The individual grain size distribution functions so obtained for various comets suggest different values for the relative abundance of coarser grains. Introducing a "relative abundance of coarser grains" index g, we study any possible dependence of g on the dynamical age of a comet, where the dynamical age can be defined in terms of some of the orbital parameters of the comet. For the four non-periodic comets available for our analysis, we find a clear empirical relation g = −2.5.q 2/3 emerging from our work. This relation strengthens the concept that comets whose grains are processed more by the solar radiation (these comets may be dynamically older) contain a relatively smaller number of finer grains. The case for other periodic comets is also discussed here. Since the work has been carried out using Mie scattering theory meant for perfect compact spheres, it is also suggested to repeat the calculations with more realistic porous grains in a follow-up paper.