High resolution magnetic field measurements in high-mass star-forming regions [Elektronische Ressource] / Gabriele Surcis. Mathematisch-Naturwissenschaftliche Fakultät
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High resolution magnetic field measurements in high-mass star-forming regions [Elektronische Ressource] / Gabriele Surcis. Mathematisch-Naturwissenschaftliche Fakultät

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High resolution magnetic field measurements inhigh-mass star-forming regionsDissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen Fakulta¨tderRheinischen Friedrich-Wilhelms-Universit¨at Bonnvorgelegt vonGabriele SurcisausChiavari (GE), ItalienBonn (June 2011)Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakulta¨tder Rheinischen Friedrich-Wilhelms-Universit¨at Bonn1. Gutachter: Dr. Wouter Vlemmings2. Gutachter: Prof. Dr. Frank BertoldiTag der Promotion: September 12, 2011Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn unterhttp://hss.ulb.uni-bonn.de/diss onlineelektronisch publiziert. Das Erscheinungsjahr ist 2011.iiito my wife...ivAbstractA number of different formationscenarioshavebeen proposedto explain the formation of starswith masses larger than about 8 M . These include formation through the merger of less massivestars(Coalescence model)orthroughtheaccretionofunboundgasfromthemolecularcloud(Com-petitive accretion model). In the third scenario, Core accretion model, massive stars form throughgravitational collapse, which involves disc-assisted accretion to overcome radiation pressure. Thisscenario is similar to the favored picture of low-mass star formation, in which magnetic fields arethoughttoslowthecollapse,totransfertheangularmomentumandtopowerthebipolaroutflows.

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Published 01 January 2011
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High resolution magnetic field measurements in
high-mass star-forming regions
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakulta¨t
der
Rheinischen Friedrich-Wilhelms-Universit¨at Bonn
vorgelegt von
Gabriele Surcis
aus
Chiavari (GE), Italien
Bonn (June 2011)Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakulta¨t
der Rheinischen Friedrich-Wilhelms-Universit¨at Bonn
1. Gutachter: Dr. Wouter Vlemmings
2. Gutachter: Prof. Dr. Frank Bertoldi
Tag der Promotion: September 12, 2011
Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn unter
http://hss.ulb.uni-bonn.de/diss online
elektronisch publiziert. Das Erscheinungsjahr ist 2011.iii
to my wife...ivAbstract
A number of different formationscenarioshavebeen proposedto explain the formation of stars
with masses larger than about 8 M . These include formation through the merger of less massive
stars(Coalescence model)orthroughtheaccretionofunboundgasfromthemolecularcloud(Com-
petitive accretion model). In the third scenario, Core accretion model, massive stars form through
gravitational collapse, which involves disc-assisted accretion to overcome radiation pressure. This
scenario is similar to the favored picture of low-mass star formation, in which magnetic fields are
thoughttoslowthecollapse,totransfertheangularmomentumandtopowerthebipolaroutflows.
Moreover, during the formation of low-mass stars the gravitational collapse of molecular clouds
proceeds preferentially along the magnetic field lines, giving rise to large rotating disc or torus
structures orthogonal to the magnetic field. Consequently, the molecular bipolar outflows, which
originate from the protostar, are driven parallel to the magnetic field. Although the magnetic
fields play such an important and crucial role in the formation of low-mass stars, its role in the
formation of high-mass stars is still under debate.
The debate is due to the several unanswered questions about the role of the magnetic fields
in high-mass star formation. The difficulties in answering these questions is due to their fast
evolution (of order of hundreds thousands years). As a result of this, the massive star-forming
regions are rare and typically found at fairly large distance. Moreover the massive star-forming
regions consist of a large number of protostars that make the identification of individual massive
protostar very complex. Consequently it is very difficult to observe and measure the magnetic
fields during the protostellar phase of high-mass stars. Current observations of magnetic fields in
massive star-forming regions are often limited to low density regions and/or envelopes at scales of
several thousands astronomical units. Linear polarization observations of dust also only provide
information on the magnetic field in the plane of the sky, so these observations have been yet
unable to probe the strength and the full structure of the magnetic field close to the protostars
vvi
and around protostellar discs.
The probes of magnetic fields in the high density regions close to the massive protostars cur-
rentlyavailablearemasers. Theirbrightandnarrowspectrallineemissionsareidealformeasuring
theZeeman-splittingaswellasfordeterminingtheorientationofthemagneticfieldin3-dimension.
So far the most investigated sources have been H O and OH masers that revealed ordered mag-2
netic fields and field strengths between 10 and 600 mG and few mG, respectively. Even though
CH OH is the most abundant maser species in the massive star formation, the very first 6.7-GHz3
CH OH maser linear and circular polarization observations have only recently been made. This3
maser emission can significantly improve the understanding of the role of the magnetic fields in
massive star-forming regions.
The aim of this Ph.D. thesis is to investigatethe roleof magnetic fields in massivestar-forming
regions at milliarcsecond resolution by observing the polarization emission of H O and in particu-2
lar 6.7-GHz CH OH masers. Six massive star-forming regions were studied using European VLBI3
Network observationsat 6.7-GHz and one using the MERLIN telescope. Furthermore two of them
(W75N and NGC7538) were also observed at 22-GHz in order to determine the magnetic fields
using the H O maser emission. These observations allowed us to observe magnetic fields with2
hourglass morphology, magnetic fields along outflows and on the surfaces of torus and disc and in
some cases also to determine the strength of the field. Moreover several important characteristics
of masers were also determined.Contents
Introduction 7
1 Star formation: low-mass stars vs high-mass stars 9
1.1 Low-mass star formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 High-mass star formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.1 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.2 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Maser theory 19
2.1 Foundations of Maser Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.1 Population inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.2 Unsaturated and saturated maser emission . . . . . . . . . . . . . . . . . . 22
2.1.3 Line narrowing and rebroadening . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2 Polarization of maser radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.2 Model of Nedoluha & Watson (1992) for water maser radiation . . . . . . . 26
2.3 Maser Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.1 Water maser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.2 Methanol maser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3 Instruments, data reduction and analysis 31
3.1 Polarization in radio-interferometry. . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 “Software tools” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2.1 EVN data reduction procedure in AIPS . . . . . . . . . . . . . . . . . . . . 35
3.2.2 Maser identification process . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
12 CONTENTS
3.2.3 Full radiative transfer method code . . . . . . . . . . . . . . . . . . . . . . . 37
Results 41
4 Cepheus A 43
4.1 The source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2 Observations and data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.2.1 6.7-GHz MERLIN data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3.1 Maser distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3.2 Maser polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4.1 Methanol maser and the discs of CepheusA HW2. . . . . . . . . . . . . . . 51
4.4.2 Magnetic field in CepheusA HW2 . . . . . . . . . . . . . . . . . . . . . . . 51
4.5 Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.5.1 Cepheus A as a prototype CH OH maser source? . . . . . . . . . . . . . . . 553
4.5.2 Further e-MERLIN observations . . . . . . . . . . . . . . . . . . . . . . . . 56
4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5 W75N 57
5.1 The source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.2 Observations and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.1 22-GHz VLBA data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.2 6.7-GHz EVN data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.3.1 Maser distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.3.2 Linear and circular polarization . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.4.1 H O maser properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722
5.4.2 Magnetic field in W75N(B) . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.4.3 J- or C-shocks? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80CONTENTS 3
6 NGC7538-IRS1 81
6.1 The source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.2 Observations and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.2.1 22-GHz VLBA data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.2.2 6.7-GHz MERLIN data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.2.3 6.7-GHz EVN data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.3.1 H O masers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882
6.3.2 CH OH masers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903
6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.4.1 Comparing CH OH spectra at different resolution . . . . . . . . . . . . . . 953
6.4.2 H O and CH OH maser properties . . . . . . . . . . . . . . . . . . . . . . . 972 3
6.4.3 Magnetic field in NGC7538-IRS1 . . . . . . . . . . . . . . . . . . . . . . . . 99
6.4.4 Structure of NGC7538-IRS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7 First EVN Sample 107
7.1 The sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.1.1 W51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.1.2 W48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.1.3 IRAS18556+0138 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7.1.4 W3(OH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.2 Observations and data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.3.1 W51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.3.2 W48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
7.3.3 IRAS18556+0318 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.3.4 W3(OH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
7.4.1 CH OH maser properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293
7.4.2 Individual sources and magnetic fields . . . . . . . . . . . . . . . . . . . . . 131
7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137