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  • 1. André, Ph.
    et al.
    Hughes, A.
    Guillet, V.
    Boulanger, F.
    Bracco, Andrea
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). CEA, France; CNRS, France; Université Paris-Saclay, France; Université Paris Diderot, France; Sorbonne Paris Cité, France; ENS Paris, France.
    Ntormousi, E.
    Arzoumanian, D.
    Maury, A. J.
    Bernard, J.-Ph.
    Bontemps, S.
    Ristorcelli, I.
    Girart, J. M.
    Motte, F.
    Tassis, K.
    Pantin, E.
    Montmerle, T.
    Johnstone, D.
    Gabici, S.
    Efstathiou, A.
    Basu, S.
    Béthermin, M.
    Beuther, H.
    Braine, J.
    Di Francesco, J.
    Falgarone, E.
    Ferrière, K.
    Fletcher, A.
    Galametz, M.
    Giard, M.
    Hennebelle, P.
    Jones, A.
    Kepley, A. A.
    Kwon, J.
    Lagache, G.
    Lesaffre, P.
    Levrier, F.
    Li, D.
    Li, Z.-Y.
    Mao, S. A.
    Nakagawa, T.
    Onaka, T.
    Paladino, R.
    Peretto, N.
    Poglitsch, A.
    Revéret, V.
    Rodriguez, L.
    Sauvage, M.
    Soler, J. D.
    Spinoglio, L.
    Tabatabaei, F.
    Tritsis, A.
    van der Tak, F.
    Ward-Thompson, D.
    Wiesemeyer, H.
    Ysard, N.
    Zhang, H.
    Probing the cold magnetised Universe with SPICA-POL (B-BOP)2019In: Publications Astronomical Society of Australia, ISSN 1323-3580, E-ISSN 1448-6083, Vol. 36, article id e029Article, review/survey (Refereed)
    Abstract [en]

    Space Infrared Telescope for Cosmology and Astrophysics (SPICA), the cryogenic infrared space telescope recently pre-selected for a 'Phase A' concept study as one of the three remaining candidates for European Space Agency (ESA's) fifth medium class (M5) mission, is foreseen to include a far-infrared polarimetric imager [SPICA-POL, now called B-fields with BOlometers and Polarizers (B-BOP)], which would offer a unique opportunity to resolve major issues in our understanding of the nearby, cold magnetised Universe. This paper presents an overview of the main science drivers for B-BOP, including high dynamic range polarimetric imaging of the cold interstellar medium (ISM) in both our Milky Way and nearby galaxies. Thanks to a cooled telescope, B-BOP will deliver wide-field 100-350 mu m images of linearly polarised dust emission in Stokes Q and U with a resolution, signal-to-noise ratio, and both intensity and spatial dynamic ranges comparable to those achieved by Herschel images of the cold ISM in total intensity (Stokes I). The B-BOP 200 mu m images will also have a factor similar to 30 higher resolution than Planck polarisation data. This will make B-BOP a unique tool for characterising the statistical properties of the magnetised ISM and probing the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our Galaxy. B-BOP will also be a powerful instrument for studying the magnetism of nearby galaxies and testing Galactic dynamo models, constraining the physics of dust grain alignment, informing the problem of the interaction of cosmic rays with molecular clouds, tracing magnetic fields in the inner layers of protoplanetary disks, and monitoring accretion bursts in embedded protostars.

  • 2.
    Bracco, Andrea
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Candelaresi, S.
    Del Sordo, F.
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
    Is there a left-handed magnetic field in the solar neighborhood? Exploring helical magnetic fields in the interstellar medium through dust polarization power spectra2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A97Article in journal (Refereed)
    Abstract [en]

    Context. The analysis of the full-sky Planck polarization data at 850 mu m revealed unexpected properties of the E- and B-mode power spectra of dust emission in the interstellar medium (ISM). The positive cross-correlations over a wide range of angular scales between the total dust intensity, T, and both E and (most of all) B modes has raised new questions about the physical mechanisms that affect dust polarization, such as the Galactic magnetic field structure. This is key both to better understanding ISM dynamics and to accurately describing Galactic foregrounds to the polarization of the cosmic microwave background (CMB). In particular, in the quest to find primordial B modes of the CMB, the observed positive cross-correlation between T and B for interstellar dust requires further investigation towards parity-violating processes in the ISM.

    Aims. In this theoretical paper we investigate the possibility that the observed cross-correlations in the dust polarization power spectra, and specifically the one between T and B, can be related to a parity-odd quantity in the ISM such as the magnetic helicity.

    Methods. We produce synthetic dust polarization data, derived from 3D analytical toy models of density structures and helical magnetic fields, to compare with the E and B modes of observations. We present several models. The first is an ideal fully helical isotropic case, such as the Arnold-Beltrami-Childress field. Second, following the nowadays favored interpretation of the T-E signal in terms of the observed alignment between the magnetic field morphology and the filamentary density structure of the diffuse ISM, we design models for helical magnetic fields wrapped around cylindrical interstellar filaments. Lastly, focusing on the observed T-B correlation, we propose a new line of interpretation of the Planck observations advocating the presence of a large-scale helical component of the Galactic magnetic field in the solar neighborhood.

    Results. Our analysis shows that: I) the sign of magnetic helicity does not affect E and B modes for isotropic magnetic-field configurations; II) helical magnetic fields threading interstellar filaments cannot reproduce the Planck results; and III) a weak helical left-handed magnetic field structure in the solar neighborhood may explain the T-B correlation seen in the Planck data. Such a magnetic-field configuration would also account for the observed large-scale T-E correlation.

    Conclusions. This work suggests a new perspective for the interpretation of the dust polarization power spectra that supports the imprint of a large-scale structure of the Galactic magnetic field in the solar neighborhood.

  • 3.
    Brandenburg, Axel
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA; Carnegie Mellon University, USA.
    Bracco, Andrea
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kahniashvili, Tina
    Mandal, Sayan
    Pol, Alberto Roper
    Petrie, Gordon J. D.
    Singh, Nishant K.
    E and B Polarizations from Inhomogeneous and Solar Surface Turbulence2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 870, no 2, article id 87Article in journal (Refereed)
    Abstract [en]

    Gradient- and curl-type or E- and B-type polarizations have been routinely analyzed to study the physics contributing to the cosmic microwave background polarization and galactic foregrounds. They characterize the parity-even and parity-odd properties of the underlying physical mechanisms, such as, for example, hydromagnetic turbulence in the case of dust polarization. Here, we study spectral correlation functions characterizing the parity-even and parity-odd parts of linear polarization for homogeneous and inhomogeneous turbulence to show that only the inhomogeneous helical case can give rise to a parity-odd polarization signal. We also study nonhelical turbulence and suggest that a strong non-vanishing (here negative) skewness of the E polarization is responsible for an enhanced ratio of the EE to the BB (quadratic) correlation in both the helical and nonhelical cases. This could explain the enhanced EE/BB ratio observed recently for dust polarization. We close with a preliminary assessment of using the linear polarization of the Sun to characterize its helical turbulence without being subjected to the pi ambiguity that magnetic inversion techniques have to address.

  • 4. Levrier, F.
    et al.
    Neveu, J.
    Falgarone, E.
    Boulanger, F.
    Bracco, Andrea
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Université Paris-Saclay,, France; Université Paris Diderot, France.
    Ghosh, T.
    Vansyngel, F.
    Statistics of the polarized submillimetre emission maps from thermal dust in the turbulent, magnetized, diffuse ISM2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id A124Article in journal (Refereed)
    Abstract [en]

    Context. The interstellar medium (ISM) is now widely acknowledged to display features ascribable to magnetized turbulence. With the public release of Planck data and the current balloon-borne and ground-based experiments, the growing amount of data tracing the polarized thermal emission from Galactic dust in the submillimetre provides choice diagnostics to constrain the properties of this magnetized turbulence. Aims. We aim to constrain these properties in a statistical way, focussing in particular on the power spectral index beta(B) of the turbulent component of the interstellar magnetic field in a diffuse molecular cloud, the Polaris Flare. Methods. We present an analysis framework based on simulating polarized thermal dust emission maps using model dust density (proportional to gas density n(H)) and magnetic field cubes, integrated along the line of sight (LOS), and comparing these statistically to actual data. The model fields are derived from fractional Brownian motion (fBm) processes, which allows a precise control of their one-and two-point statistics. The parameters controlling the model are (1)-(2) the spectral indices of the density and magnetic field cubes, (3)-(4) the RMS-to-mean ratios for both fields, (5) the mean gas density, (6) the orientation of the mean magnetic field in the plane of the sky (POS), (7) the dust temperature, (8) the dust polarization fraction, and (9) the depth of the simulated cubes. We explore the nine-dimensional parameter space through a Markov chain Monte Carlo analysis, which yields best-fitting parameters and associated uncertainties. Results. We find that the power spectrum of the turbulent component of the magnetic field in the Polaris Flare molecular cloud scales with wavenumber as k(-beta B) with a spectral index beta(B) = 2.8 +/- 0.2. It complements a uniform field whose norm in the POS is approximately twice the norm of the fluctuations of the turbulent component, and whose position angle with respect to the north-south direction is chi(0) approximate to -69 degrees. The density field n(H) is well represented by a log-normally distributed field with a mean gas density < n(H)> approximate to 40 cm(-3), a fluctuation ratio sigma(nH )/ < n(H >) approximate to 1.6, and a power spectrum with an index beta(n) = 1.7(-0.3)(+0.4). We also constrain the depth of the cloud to be d approximate to 13 pc, and the polarization fraction p(0) approximate to 0.12. The agreement between the Planck data and the simulated maps for these best-fitting parameters is quantified by chi(2) value that is only slightly larger than unity. Conclusions. We conclude that our fBm-based model is a reasonable description of the diffuse, turbulent, magnetized ISM in the Polaris Flare molecular cloud, and that our analysis framework is able to yield quantitative estimates of the statistical properties of the dust density and magnetic field in this cloud.

  • 5. Ritacco, A.
    et al.
    Macias-Perez, J. F.
    Ponthieu, N.
    Adam, R.
    Ade, P.
    Andre, P.
    Aumont, J.
    Beelen, A.
    Benoit, A.
    Bideaud, A.
    Billot, N.
    Bourrion, O.
    Bracco, Andrea
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Université Paris Diderot, France.
    Calvo, M.
    Catalano, A.
    Coiffard, G.
    Comis, B.
    D'Addabbo, A.
    De Petris, M.
    Desert, F. -X.
    Doyle, S.
    Goupy, J.
    Kramer, C.
    Lagache, G.
    Leclercq, S.
    Lestrade, J. -F.
    Mauskopf, P.
    Mayet, F.
    Maury, A.
    Monfardini, A.
    Pajot, F.
    Pascale, E.
    Perotto, L.
    Pisano, G.
    Rebolo-Iglesias, M.
    Reveret, V.
    Rodriguez, L.
    Romero, C.
    Roussel, H.
    Ruppin, F.
    Schuster, K.
    Sievers, A.
    Siringo, G.
    Thum, C.
    Triqueneaux, S.
    Tucker, C.
    Wiesemeyer, H.
    Zylka, R.
    NIKA 150 GHz polarization observations of the Crab nebula and its spectral energy distribution2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, article id A35Article in journal (Refereed)
    Abstract [en]

    The Crab nebula is a supernova remnant exhibiting a highly polarized synchrotron radiation at radio and millimetre wavelengths. It is the brightest source in the microwave sky with an extension of 7 by 5 arcmin, and is commonly used as a standard candle for any experiment which aims to measure the polarization of the sky. Though its spectral energy distribution has been well characterized in total intensity, polarization data are still lacking at millimetre wavelengths. We report in this paper high resolution observations (18 00 FWHM) of the Crab nebula in total intensity and linear polarization at 150 GHz with the NIKA camera. NIKA, operated at the IRAM 30 m telescope from 2012 to 2015, is a camera made of Lumped Element Kinetic Inductance Detectors (LEKIDs) observing the sky at 150 and 260 GHz. From these observations we are able to reconstruct the spatial distribution of the polarization degree and angle of the Crab nebula, which is found to be compatible with previous observations at lower and higher frequencies. Averaging across the source and using other existing data sets we find that the Crab nebula polarization angle is consistent with being constant over a wide range of frequencies with a value of -87.7 degrees +/- 0.3 in Galactic coordinates. We also present the first estimation of the Crab nebula spectral energy distribution polarized flux in a wide frequency range: 30-353 GHz. Assuming a single power law emission model we find that the polarization spectral index beta(pol) = -0.347 +/- 0.026 is compatible with the intensity spectral index beta = -0.323 +/- 0.001.

  • 6. Shimajiri, Y.
    et al.
    André, Ph.
    Palmeirim, P.
    Arzoumanian, D.
    Bracco, Andrea
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Könyves, V.
    Ntormousi, E.
    Ladjelate, B.
    Probing accretion of ambient cloud material into the Taurus B211/B213 filament2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A16Article in journal (Refereed)
    Abstract [en]

    Context. Herschel observations have emphasized the role of molecular filaments in star formation. However, the origin and evolution of these filaments are not yet well understood, partly because of the lack of kinematic information.

    Aims. We confirm from a kinematic viewpoint that the Taurus B211/B213 filament is accreting background cloud material, and we investigate the potential influence of large-scale external effects on the formation of the filament.

    Methods. To examine whether the B211/B213 filament is accreting background gas because of its gravitational potential, we produced a toy accretion model and compared its predictions to the velocity patterns observed in (CO)-C-12 (1-0) and (CO)-C-13 (1-0). We also examined the spatial distributions of H alpha, Planck 857 GHz dust continuum, and HI emission to search for evidence of large-scale external effects.

    Results. We estimate that the depth of the Taurus cloud around the B211/B213 filament is similar to 0.3-0.7 pc under the assumption that the density of the gas is the same as the critical density of (CO)-C-13 (1-0). Compared to a linear extent of >10 pc in the plane of the sky, this suggests that the 3D morphology of the cloud surrounding the B211/B213 filament is sheet-like. Position-velocity (PV) diagrams observed in (CO)-C-12 (1-0) and (CO)-C-13 (1-0) perpendicular to the filament axis show that the emission from the gas surrounding B211/B213 is redshifted to the northeast of the filament and blueshifted to the southwest, and that the velocities of both components approach the velocity of the B211/B213 filament as the line of sight approaches the crest of the filament. The PV diagrams predicted by our accretion model are in good agreement with the observed (CO)-C-12 (1-0) and (CO)-C-13 (1-0) PV diagrams, supporting the previously proposed scenario of mass accretion into the filament. Moreover, inspection of the spatial distribution of the H alpha and Planck 857 GHz emission in the Taurus-California-Perseus region on scales up to >200 pc suggests that the B211/B213 filament may have formed as a result of an expanding supershell generated by the Per OB2 association.

    Conclusions. Based on these results, we propose a scenario in which the B211/B213 filament was initially formed by large-scale compression of HI gas and is now growing in mass by gravitationally accreting molecular gas of the ambient cloud.

  • 7. Soler, J. D.
    et al.
    Bracco, Andrea
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Université Paris Diderot, France; CNRS, France; CEA/IRFU/SAp, France.
    Pon, A.
    The magnetic environment of the Orion-Eridanus superbubble as revealed by Planck2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 609, article id L3Article in journal (Refereed)
    Abstract [en]

    Using the 353-GHz polarization observations by the Planck satellite we characterize the magnetic field in the Orion-Eridanus super-bubble, a nearby expanding structure that spans more than 1600 square degrees in the sky. We identify a region of both low dispersion of polarization orientations and high polarization fraction associated with the outer wall of the superbubble identified in the most recent models of the large-scale shape of the region. We use the Davis-Chandrasekhar-Fermi method to derive plane-of-the-sky magnetic field strengths of tens of mu G toward the southern edge of the bubble. The comparison of these values with existing Zeeman splitting observations of HI in emission suggests that the large-scale magnetic field in the region was primarily shaped by the expanding superbubble.

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