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&

Large-scale magnetic fields in Bok globules

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. Method: 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/sigma(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. Conclusion: We find strongly aligned polarization vectors that indicate dominant magnetic fields on a wide range of spatial scales.Comment: 9 pages, 7 figures, accepted by A&

Self-scattering of non-spherical dust grains: The limitations of perfect compact spheres

Context. The understanding of (sub-)millimetre polarisation has made a leap forward since high-resolution imaging with the Atacama Large (sub-)Mm Array (ALMA) became available. Amongst other effects, self-scattering (i.e. the scattering of thermal dust emission on other grains) is thought to be the origin of millimetre polarisation. This opens the first window to a direct measurement of dust grain sizes in regions of optically thick continuum emission as it can be found in protoplanetary discs and star-forming regions. However, the newly derived values of grain sizes are usually around ~100 μm and thus one order of magnitude smaller than those obtained from more indirect measurements, as well as those expected from theory (~1 mm). / Aims. We see the origin of this contradiction in the applied dust model of current self-scattering simulations: a perfect compact sphere. The aim of this study is to test our hypothesis by investigating the impact of non-spherical grain shapes on the self-scattering signal. / Methods. We applied discrete dipole approximation simulations to investigate the influence of the grain shape on self-scattering polarisation in three scenarios: an unpolarised and polarised incoming wave under a fixed and a varying incident polarisation angle. / Results. We find significant deviations of the resulting self-scattering polarisation when comparing non-spherical to spherical grains. In particular, tremendous deviations are found for the polarisation signal of grains when observed outside the Rayleigh regime, that is for >100 μm sized grains observed at the 870 μm wavelength. Self-scattering by oblate grains produces higher polarisation degrees compared to spheres, which challenges the interpretation of the origin of observed millimetre polarisation. A (nearly) perfect alignment of the non-spherical grains is required to account for the observed millimetre polarisation in protoplanetary discs. Furthermore, we find conditions under which the emerging scattering polarisation of non-spherical grains is flipped in orientation by 90°. / Conclusions. These results show clearly that the perfect compact sphere is an oversimplified model, which has reached its limit. Our findings point towards a necessary re-evaluation of the dust grain sizes derived from (sub-)millimetre polarisation

An Extreme-AO Search for Giant Planets around a White Dwarf --VLT/SPHERE performance on a faint target GD 50

CONTEXT. Little is known about the planetary systems around single white dwarfs although there is strong evidence that they do exist. AIMS. We performed a pilot study with the extreme-AO system on the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) on the Very Large Telescopes (VLT) to look for giant planets around a young white dwarf, GD 50. METHODS. We were awarded science verification time on the new ESO instrument SPHERE. Observations were made with the InfraRed Dual-band Imager and Spectrograph in classical imaging mode in H band. RESULTS. Despite the faintness of the target (14.2 mag in R band), the AO loop was closed and a strehl of 37\% was reached in H band. No objects were detected around GD 50. We achieved a 5-sigma contrast of 6.2, 8.0 and 8.25 mags at 0{\farcs}2, 0{\farcs}4 and 0{\farcs}6 and beyond, respectively. We exclude any substellar objects more massive than 4.0 M$_\textrm{J}$ at 6.2 AU, 2.9 M$_\textrm{J}$ at 12.4 AU and 2.8 M$_\textrm{J}$ at 18.6 AU and beyond. This rivals the previous upper limit set by Spitzer. We further show that SPHERE is the most promising instrument available to search for close-in substellar objects around nearby white dwarfs.Comment: A&A letters, accepte

HD 169142 in the eyes of ZIMPOL/SPHERE

We present new data of the protoplanetary disc surrounding the Herbig Ae/Be star HD 169142 obtained in the very broad-band (VBB) with the Zurich imaging polarimeter (ZIMPOL), a sub-system of the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) at the Very Large Telescope (VLT). Our Polarimetric Differential Imaging (PDI) observations probe the disc as close as 0.03" (3.5au) to the star and are able to trace the disc out to ~1.08" (~126au). We find an inner hole, a bright ring bearing substructures around 0.18" (21au), and an elliptically shaped gap stretching from 0.25" to 0.47" (29-55au). Outside of 0.47", the surface brightness drops off, discontinued only by a narrow annular brightness minimum at ~0.63"-0.74" (74-87au). These observations confirm features found in less-well resolved data as well as reveal yet undetected indications for planet-disc interactions, such as small-scale structures, star-disk offsets, and potentially moving shadows.Comment: Accepted for publication in MNRA

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

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