Polarimetry toward theIRASVela Shell. II. Extinction and Magnetic Fields

Abstract

We explore correlations between visual extinction and polarization along the western side of the IRAS Vela Shell using a published polarimetric catalog of several hundreds of objects. Our extinction maps along this ionization front (I-front) find evidence of clumpy structure with typical masses between 1.5 and 6 M ⊙ and a mean length scale L ∼0.47 pc. The polarimetric data allowed us to investigate the distribution of the local magnetic field in small (∼pc) scales across the I-front. Using the dispersion of polarization position angles, we find variations in the kinetic-to-magnetic energy density ratio of, at least, one order of magnitude along the I-front, with the magnetic pressure generally dominating over the turbulent motions. These findings suggest that the magnetic component has a significant contribution to the dynamical balance of this region. Along the I-front, the mean magnetic field projected on the sky is [0.018 ± 0.013] mG. The polarization efficiency seems to change along the I-front. We attribute high polarization efficiencies in regions of relatively low extinction to an optimum degree of grain alignment. Analysis of the mass-to-magnetic flux ratio shows that this quantity is consistent with the subcritical regime (λ < 1), showing that magnetic support is indeed important in the region. Our data extend the overall λ−N(H 2 ) relation toward lower density values and show that such trend continues smoothly toward low N(H 2 ) values. This provides general support for the evolution of initially subcritical clouds to an eventual supercritical stage.

CHARTING THE INTERSTELLAR MAGNETIC FIELD CAUSING THEINTERSTELLAR BOUNDARY EXPLORER(IBEX) RIBBON OF ENERGETIC NEUTRAL ATOMS

“…The 20th century data sets used in this analysis included the discovery of the ISMF within 40 pc of the Sun in the fourth galactic quadrant (ℓ = 270°-360°, Piirola 1977;Tinbergen 1982). More recently, high sensitivity polarimeters capable of 3σ detections of polarization strengths <0.01% have become available (Pereyra & Magalhães 2007;Wisniewski et al 2007;Bailey et al 2010Bailey et al , 2015Berdyugin et al 2014;Piirola et al 2014;.…”

“…In this paper we utilize new polarization measurements that have been acquired with the DiPol2 polarimeter at the KVA telescope in La Palma, Canary Islands , the IAGPOL polarimeter at the LNA at Picos dos Dios in Brazil (Pereyra & Magalhães 2007;Wisniewski et al 2007), and the POLISH2 polarimeter at the Lick Observatory in California . These data include measurements with 3σ sensitivities of 0.01% or better.…”

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

“…There is an extensive literature on this topic based on observations of starlight polarization, which have been interpreted from two different viewpoints, i.e., grain alignement and magnetic field structure, without achieving a clear understanding of the respective roles of these processes in accounting for variations of polarization across the sky. A number of papers (e.g., Pereyra & Magalhães 2007;Alves et al 2008;Marchwinski et al 2012) use the data to infer the magnetic field strength using the Chandreskar-Fermi method (Chandrasekhar & Fermi 1953). Other papers focus on the observed decrease of polarization fraction p with N H to interpret the data as a decrease of the dust alignment efficiency in dense clouds (Lazarian et al 1997;Whittet et al 2008;Chapman et al 2011).…”

Open problems and perspectives offered by current and future instruments

“…Submillimeter images reveal that the interstellar matter is organized in a network of filaments whose orientation is correlated with the direction of the magnetic field [21,42,47]. Indeed, interstellar turbulence must be understood in the framework of the magnetohydrodynamics as the magnetic field is dynamically important.…”