Magnetic fields in Bok globules: Multi-wavelength polarimetry as tracer across large spatial scales

[abridged] The role of magnetic fields in the process of star formation is a matter of continuous debate. Clear observational proof of the general influence of magnetic fields on the early phase of cloud collapse is still pending. First results on Bok globules with simple structures indicate dominant magnetic fields across large spatial scales (Bertrang+2014). The aim of this study is to test the magnetic field influence across Bok globules with more complex density structures. We apply near-infrared polarimetry to trace the magnetic field structure on scales of 10^4-10^5au in selected Bok globules. The combination of these measurements with archival data in the optical and sub-mm wavelength range allows us to characterize the magnetic field on scales of 10^3-10^6au. We present polarimetric data in the near-infrared wavelength range for the three Bok globules CB34, CB56, and [OMK2002]18, combined with archival polarimetric data in the optical wavelength range for CB34 and CB56, and in the sub-millimeter wavelength range for CB34 and [OMK2002]18. We find a strong polarization signal (P>2%) in the near-infrared and strongly aligned polarization segments on large scales (10^4-10^6au) for all three globules. This indicates dominant magnetic fields across Bok globules with complex density structures. To reconcile our findings in globules, the lowest mass clouds known, and the results on intermediate (e.g., Taurus) and more massive (e.g., Orion) clouds, we postulate a mass dependent role of magnetic fields, whereby magnetic fields appear to be dominant on low and high mass but rather sub-dominant on intermediate mass clouds.Comment: 7 pages, 6 figures; Accepted by A&

Polarisierte Strahlung als Indikator für Magnetfelder in der Sternentstehung

Magnetic fields are observed on a wide range of astronomical scales: from the entire cosmos, galaxies and giant molecular clouds, to smaller molecular clouds with low-mass star formation and protostellar disks, to protoplanetary disks. There is an ongoing debate about whether magnetic fields play an important role in star and planet formation. For a better understanding of the influence of magnetic fields during this process, it is necessary to study the magnetic fields on different evolutionary stages of star and planet formation with both observations and simulations. This is the goal of this thesis, which is therefore split into two parts. The first part addresses the earliest phase of star formation and presents an observational multi-wavelength polarimetry study of low-mass star-forming regions. Until now, there exists no observational evidence of a general influence of magnetic fields during the initial collapse of molecular clouds. This study examines magnetic fields and their influence during the initial collapse of low-mass star-forming regions on a wide range of spatial scales. The large-scale magnetic field structure was traced on scales of 103 − 105 AU in the local environment of the three Bok globules B335, CB68, and CB54, by optical and near-infrared polarimetry. These measurements are combined with existing sub-millimeter measurements characterizing the smallscale magnetic field structure on 102 − 103 AU. For the first time, magnetic fields in three Bok globules with different complex density structures were found to be well-ordered and dominant on large scales. The second part deals with a more evolved phase of star and planet formation, with protoplanetary disks. (Sub-)Millimeter observations of polarized radiation from protoplanetary disks enable us to study the magnetic fields within the disk. In the context of this thesis, the 3D radiative transfer code MC3D was extended by including the treatment of thermal polarization and dust grain alignment of nonspherical grains. These pioneering simulations achieve the characterization and theoretical explanation of observations of polarized radiation towards protoplanetary disks, performed for instance with the Atacama Large Millimeter/submillimeter Array (ALMA). In addition to these extensions of the radiative transfer simulations, this code can be combined with magnetohydrodynamic simulations. With this polarized radiative transfer code, it is possible to model polarization maps, intensity maps, as well as polarized and unpolarized spectral energy distributions of thermal dust emission and dichroic extinction that can be fitted to or constrained by observations. These simulations demonstrate the feasibility of polarimetry on protoplanetary disks with ALMA. With simulations and observations in combination, it is possible to distinguish between different models of protoplanetary disks and magnetic fields therein.Magnetfelder lassen sich auf allen astronomischen Größenskalen finden: von kosmischen Skalen, Galaxien und Riesenmolekülwolken, über kleinere, massearme Molekülwolken, bis hin zu protoplanetaren Scheiben. Die Frage, ob Magnetfelder eine wichtige Rolle in der Stern- und Planetenentstehung spielen, ist Gegenstand anhaltender Debatte. Um das Verständnis über den Einfluss von Magnetfeldern auf die Entstehung von Sternen und Planeten zu erweitern, ist es wichtig Magnetfelder auf den verschiedenen evolutionären Stufen der Stern- und Planetenentstehung mit Beobachtungen und Simulationen zu erforschen. Dies ist das Ziel der vorliegenden Arbeit, welche aus diesem Grund in zwei Teile gegliedert ist. Der erste Teil behandelt die früheste Phase der Sternentstehung, und präsentiert eine Multiwellenlängenpolarisationsstudie, durchgeführt an massearmen Sternentstehungsregionen. Bis heute gibt es keinen Hinweis aus Beobachtungen für den generellen Einfluss von Magnetfeldern auf die initiale Phase des Kollapses der Molekülwolke. Ziel dieser Studie ist es, die Magnetfelder in diesen massearmen Molekülwolken zu untersuchen und ihren Einfluss auf den initialen Kollaps weitskalig zu erforschen. Hierzu wurde die großskalige Magnetfeldstruktur auf Skalen von 103 − 105 AE in der lokalen Umgebung dreier Bok Globulen, B335, CB68 und CB54, mittels optischer und Nah-Infrarotpolarimetrie bestimmt, und kombiniert mit archivierten Daten aus dem Submillimeterbereich, welche das kleinskalige Magnetfeld auf Skalen von 102 − 103 AE charakterisieren. Auf diese Weise wurden erstmalig geordnete und damit dominierende Magnetfeldstrukturen in drei verschieden komplexen Bok Globulen über weite Skalen hinweg gefunden. Der zweite Teil beschäftigt sich mit einer fortgeschrittenen Phase der Sternund Planetenentstehung, mit protoplanetaren Scheiben. Im Rahmen dieser Arbeit ist der 3D Strahlungstransportcode MC3D um die Effekte von thermischer Polarisation und Ausrichtung von asphärischen Staubkörnern an Magnetfeldern erweitert worden. Diese Simulationen gehören zu den ersten ihrer Art und dienen dazu Beobachtungen von polarisierter Strahlung von protoplanetaren Scheiben zu charakterisieren und theoretisch zu fundieren, welche etwa mit dem Atacama Large Millimeter/submillimeter Array (ALMA) durchgeführt werden können. Zusätzlich wurde MC3D angepasst, um die aus Magnetohydrodynamiksimulationen resultierenden Staubdichteverteilungen und Magnetfelder zu verarbeiten. Mit diesem Strahlungstransportcode ist es nun möglich Polarisationskarten, Intensitätskarten, sowie polarisierte und unpolarisierte spektrale Energieverteilungen thermischer Staubemission und dichroitischer Absorption zu modellieren, welche zur Analyse und Vorhersage von Beobachtungsdaten dienen. Diese Simulationen demonstrieren die Machbarkeit polarimetrischer Beobachtungen protoplanetarer Scheiben mit ALMA. Mit beidem in Kombination gelingt es unterschiedliche Modelle von protoplanetaren Scheiben und ihren Magnetfeldern zu differenziere

Intrinsic polarisation of elongated porous dust grains

ALMA observations revealed recently polarised radiation of several protoplanetary disks in the (sub-)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarisation signal. We calculate the wavelength dependent absorption and intrinsic polarisation of spheroidally shaped, micrometre and sub-millimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity which appears to be present in disks when small grains coagulate to form larger aggregates. For the first time our results show that (a) the intrinsic polarisation decreases for increasing grain porosity and (b) the polarisation orientation flips by 90 degree for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary disks using multi-wavelength polarisation observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal{P}\lesssim0.7$) potentially explain the observed polarisation fraction in the system HD 142527 while highly porous grains ($\mathcal{P}>0.7$) fail unless the grain's axis ratio is extraordinarily large.Comment: 10 pages, 10 figure

Millimeter Gap Contrast as a Probe for Turbulence Level in Protoplanetary Disks

Turbulent motions are believed to regulate angular momentum transport and influence dust evolution in protoplanetary disks. Measuring the strength of turbulence is challenging through gas line observations because of the requirement for high spatial and spectral resolution data, and an exquisite determination of the temperature. In this work, taking the well-known HD 163296 disk as an example, we investigated the contrast of gaps identified in high angular resolution continuum images as a probe for the level of turbulence. With self-consistent radiative transfer models, we simultaneously analyzed the radial brightness profiles along the disk major and minor axes, and the azimuthal brightness profiles of the B67 and B100 rings. By fitting all the gap contrasts measured from these profiles, we constrained the gas-to-dust scale height ratio $\Lambda$ to be $3.0_{-0.8}^{+0.3}$, $1.2_{-0.1}^{+0.1}$ and ${\ge}\,6.5$ for the D48, B67 and B100 regions, respectively. The varying gas-to-dust scale height ratios indicate that the degree of dust settling changes with radius. The inferred values for $\Lambda$ translate into a turbulence level of $\alpha_{\rm turb}\,{<}\,3\times10^{-3}$ in the D48 and B100 regions, which is consistent with previous upper limits set by gas line observations. However, turbulent motions in the B67 ring are strong with $\alpha_{\rm turb}\,{\sim}1.2\,{\times}\,10^{-2}$. Due to the degeneracy between $\Lambda$ and the depth of dust surface density drops, the turbulence strength in the D86 gap region is not constrained.Comment: 13 pages, 7 figures, accepted for publication in Science China Physics, Mechanics & Astronom

ALMA observations of Elias 2–24: a protoplanetary disk with multiple gaps in the Ophiuchus molecular cloud

We present ALMA 1.3 mm continuum observations at 0. 2 (25 au) resolution of Elias 2–24, one of the largest and brightest protoplanetary disks in the Ophiuchus Molecular Cloud, and we report the presence of three partially resolved concentric gaps located at ∼20, 52, and 87 au from the star. We perform radiative transfer modeling of the disk to constrain its surface density and temperature radial profile and place the disk structure in the context of mechanisms capable of forming narrow gaps such as condensation fronts and dynamical clearing by actively forming planets. In particular, we estimate the disk temperature at the locations of the gaps to be 23, 15, and 12 K (at 20, 52, and 87 au, respectively), very close to the expected snowlines of CO (23–28 K) and N2 (12–15 K). Similarly, by assuming that the widths of the gaps correspond to 4–8× the Hill radii of forming planets (as suggested by numerical simulations), we estimate planet masses in the range of 0.2 1.5 – MJup, 1.0 8.0 – MJup, and 0.02 0.15 – MJup for the inner, middle, and outer gap, respectively. Given the surface density profile of the disk, the amount of “missing mass” at the location of each one of these gaps (between 4 and 20 MJup) is more than sufficient to account for the formation of such planets.Fil: Cieza, Lucas A.. Universidad Diego Portales; ChileFil: Casassus, Simon. Universidad de Chile; ChileFil: Pérez, Sebastian. Universidad de Chile; ChileFil: Hales, Antonio. Alma Observatory; ChileFil: Cárcamo, Miguel. Universidad de Chile; ChileFil: Ansdell, Megan. University of California at Berkeley; Estados UnidosFil: Avenhaus, Henning. Universitat Zurich; SuizaFil: Bayo, Amelia. Universidad de Valparaiso; ChileFil: Bertrang, Gesa H.-M.. Universidad Diego Portales; ChileFil: Cánovas, Hector. Agencia Espacial Europea; EspañaFil: Christiaens, Valentin. Universidad de Chile; ChileFil: Dent, William. Alma Observatory; ChileFil: Ferrero, Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Gamen, Roberto Claudio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Olofsson, Johan. Universidad de Valparaiso; ChileFil: Orcajo, Santiago. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Osses, Axel. Universidad de Chile; ChileFil: Peña Ramirez, Karla. Universidad de Antofagasta; ChileFil: Principe, David. Massachusetts Institute of Technology; Estados UnidosFil: Ruíz Rodríguez, Dary. Rochester Institute Of Technology; Estados UnidosFil: Schreiber, Matthias R.. Universidad de Valparaiso; ChileFil: Plas, Gerrit van der. Univ. Grenoble Alpes; SuizaFil: Williams, Jonathan P.. Institute For Astronomy, University Of Hawaii; Estados UnidosFil: Zurlo, Alice. Universidad Diego Portales; Chil

Polarization in Disks

This white paper discusses how disk polarization observations can be used to study disk and grain properties during the planet formation process. Such studies require very sensitive and high resolution multi-wavelength observations.Fil: Stephens, Ian W.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Li, Zhi Yun. University of Virginia; Estados UnidosFil: Yang, Haifeng. Tsinghua University; ChinaFil: Kataoka, Akimasa. National Astronomical Observatory of Japan; JapónFil: Looney, Leslie. University of Illinois at Urbana; Estados UnidosFil: Hull, Charles L. H.. National Astronomical Observatory Of Japan; JapónFil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Sadavoy, Sarah. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Kwon, Woojin. Korea Astronomy And Space Science Institute; Corea del SurFil: Satoshi, Ohashi. Riken Cluster For Pioneering Research; JapónFil: Tazaki, Ryo. Tohoku University; JapónFil: Li, Dan. National Optical Astronomy Observatory; Estados UnidosFil: Hoang, Thiem. Korea Astronomy And Space Science Institute; Corea del SurFil: Bertrang, Gesa H. M.. Max Planck Institute For Extraterrestrial Physics; AlemaniaFil: Carrasco Gonzalez, Carlos Eugenio. Instituto de Radioastronomía y Astrofísica; MéxicoFil: Dent, William. Atacama Large (sub)millimeter Array; ChileFil: Takahashi, Satoko. National Institutes Of Natural Sciences - National Astronomical Observatory Of Japan; JapónFil: Bacciotti, Francesca. Istituto Nazionale di Astrofisica; ItaliaFil: Alves, Felipe O.. Max Planck Institute For Extraterrestrial Physics; AlemaniaFil: Girart, Josep M.. Instituto de Ciencias del Espacio; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Zhang, Qizhou. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Rao, Ramprasad. Academia Sinica; ChinaFil: Pohl, Adriana. Max Planck Institute For Extraterrestrial Physics; AlemaniaFil: Padovani, Marco. Istituto Nazionale di Astrofisica; ItaliaFil: Galli, Daniele. Istituto Nazionale di Astrofisica; ItaliaFil: Lee, Chin Fei. Academia Sinica; ChinaFil: Segura Cox, Dominique. Max Planck Institute For Extraterrestrial Physics; AlemaniaAstro2020: APC Science White PapersEstados UnidosAmerica Astronomical Societ

How to interpret observations of Magnetic Fields in Protoplanetary Disks

<p>(Sub-)Millimeter observations of the polarized emission of aligned aspherical dust grains enable us to study magnetic fields in protoplanetary disks. Yet, the interpretation of these observations is complex and must include various effects that alter the measured polarized signal. We will present our 3D radiative transfer code capable of simulating ALMA observations of the polarized dust emission.</p

3D radiative transfer of intrinsically polarized dust emission based on aligned aspherical grains

(Sub-)millimetre observations of the polarized emission of aligned aspherical dust grains enable us to study the magnetic fields within protoplanetary disc. However, the interpretation of these observations is complex. One must consider the various effects that alter the measured polarized signal, such as the shape of dust grains, the efficiency of grain alignment, the magnetic field properties and the projection of the signal along the line of sight. We aim at analysing observations of the polarized dust emission by disentangling the effects on the polarization signal in the context of 3D radiative transfer simulations. For this purpose, we developed a code capable of simulating dust grain alignment of aspherical grains and intrinsical polarization of thermal dust emission. We find that the influence of thermal polarization and dust grain alignment on the polarized emission displayed as spatially resolved polarization map or as spectral energy distribution trace disc properties that are not traced in total (unpolarized) emission such as the magnetic field topology. The radiative transfer simulations presented in this work enable the 3D analysis of intrinsically polarized dust emission - observed with e.g. Atacama Large Millimeter/submillimeter Array (ALMA) - which is essential to constrain magnetic field properties.DFG WO857/11-1 1573 Millennium Science Initiative (Chilean Ministry of Economy) through grant Nucleus RC1300