arXiv subjects

Masafumi Matsumura

Publications and source records attributed to Masafumi Matsumura.

21 records · Page 2Linked to original sources

arXiv2017

First results from BISTRO -- a SCUBA-2 polarimeter survey of the Gould Belt

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. The most important of these is the role of magnetic fields in the star formation process on the scale of individual filaments and cores in dense regions. We describe the data acquisition and reduction processes for POL-2, demonstrating both repeatability and consistency with previous data. We present a first-look analysis of the first results from the BISTRO survey in the OMC 1 region. We see that the magnetic field lies approximately perpendicular to the famous 'integral filament' in the densest regions of that filament. Furthermore, we see an 'hour-glass' magnetic field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discuss possible causes for this. We also discuss the more complex morphology seen along the Orion Bar region. We examine the morphology of the field along the lower-density north-eastern filament. We find consistency with previous theoretical models that predict magnetic fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.

astro-ph.GA↗

arXiv2011

Correlation between Interstellar Polarization and Dust Temperature: Alignment of Grains by Radiative Torques is Ubiquitous?

We investigate the efficiency of interstellar polarization $p_λ/A_λ$ where $p_λ$ is the fractional linear polarization and $A_λ$ is extinction, in 16 lines of sight as a function of wavelength $λ$. We have used the data obtained with the low-dispersion spectropolarimeter HBS as well as those in literature. It is found that the polarization efficiency $p_λ/A_λ$ is proportional to $\exp(-β/λ)$ in wavelength $λ\approx 0.4-0.8 \micron$, where $β$ is a parameter which varies from 0.5 to 1.2 $\micron$. We find that $β$ is negatively correlated with the dust temperature deduced from infrared data by Schlegel et al., suggesting that the polarization efficiency is higher in short wavelength for higher temperature. According to the alignment theory by radiative torques (RATs), if the radiation is stronger and bluer, RATs will make small grains align better, and the polarization efficiency will increase in short wavelength. Our finding of the correlation between $β$ and the temperature is consistent with what is expected with the alignment mechanism by RATs.

astro-ph.GA↗

arXiv2009

Grain Alignment in OMC1 as Deduced from Observed Large Circular Polarization

The properties of polarization in scattered light by aligned ellipsoidal grains are investigated with the Fredholm integral equation method (FIM) and the T-matrix method (Tmat), and the results are applied to the observed circular polarization in OMC1. We assume that the grains are composed of silicates and ellipsoidal (oblate, prolate, or tri-axial ellipsoid) in shape with a typical axial ratio of 2:1. The angular dependence of circular polarization p_c on directions of incident and scattered light is investigated with spherical harmonics and associated Legendre polynomials. The degree of circular polarization p_c also depends on the Rayleigh reduction factor R which is a measure of imperfect alignment. We find that p_c is approximately proportional to R for grains with |m|x_{eq} < 3 - 5, where x_{eq} is the dimensionless size parameter and m is the refractive index of the grain. Models that include those grains can explain the observed large circular polarization in the near infrared, ~15%, in the south-east region of the BN object (SEBN) in OMC1, if the directions of incidence and scattering of light is optimal, and if grain alignment is strong, i.e. R > 0.5. Such a strong alignment cannot be explained by the Davis-Greenstein mechanism; we prefer instead an alternative mechanism driven by radiative torques. If the grains are mixed with silicates and ice, the degree of circular polarization p_c decreases in the 3 micron ice feature, while that of linear polarization increases. This wavelength dependence is different from that predicted in a process of dichroic extinction.

astro-ph.GA↗