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Durham E-Theses Durham E-Theses Optical polarimetry of star forming regions Gledhill, Timothy Michael How to cite: Gledhill, Timothy Michael (1987) Optical polarimetry of star forming regions, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/6757/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk Optical Polarimetry of Star Forming Regions Timothy Michael Gledhill, B.Sc. A thesis submitted to the University of Durham for the degree of Doctor of Philosophy The copyright of this thesis rests with the author. No quotation from it should be published without his prior written consent and information derived from it should be acknowledged. Physics Department December 1987 \ -~ ' ("''"". Optical Polarimetry of Star Forming Regions T.M. Gledhill The work presented in this thesis details the polarimetric investigation of nebulosity associated with low mass pre-mainsequence (PMS) stellar ob­ jects. Three regions of on-going star formation are considered, specifi­ cally, the Haro 6-5 and the HL/XZ Tau systems-both associated with dark clouds in the Taurus complex-and the PV Cephei nebulosity near NGC7023. The work is based on CCD observations at optical wavelengths using the Durham Imaging polarimeter. Theories of PMS stellar evolution have developed rapidly over the last three years in tandem with observational discoveries and so a review of current ideas, necessary for the interpretation of the data, is given. The viability of the Davis-Greenstein grain alignment mechanism in regions of star formation is also briefly examined. In each region, the imaging observations suggest bipolarity in the optical structure of the nebulosity and the polarimetric data are used to determine the locations of the illuminating sources. Evidence is found for the associa­ tion of circumstellar discs of obscuration with the PMS objects Haro 6-5A (FS Tau), Haro 6-5B, HL Tau and PV Cephei. In each case the polarimet­ ric data suggest that the local magnetic field has played an important role in the evolution of the star and the circumstellar material. Examination of the source region polarisation maps suggests that at least one of the objects considered is surrounded by a dust grain-aligning magnetic field which has a predominantly toroidal geometry in the plane of the circumstellar disc and the implications for current theories of outflow acceleration and cloud evolution are discussed. 11 Contents Page Title Page Abstract List of Contents 11 List of Figures Vl List of Tables V111 Declaration and Copyright lX Chapter 1 :Star Formation in the Galaxy 1 1.1 The Distribution of Molecular Material 1 1.1.1 The Conversion from CO to H2 2 1.1.2 The Radial Distribution 3 1.1.3 Spiral Structure 7 1.1.4 CO Observations of M51 8 1.2 Molecular Clouds 9 1.2.1 The Cloud Size and Mass Distributions 9 1.2.2 The Formation of Molecular Clouds 10 1.2.2.1 Formation by Coagulation 10 1.2.2.2 Formation in Parker Instabilities 12 1.2.3 Summary of Arguments 12 1.3 Star Formation in Molecular Clouds 14 1.3.1 Isotropic Cloud Collapse 14 1.3.2 The Importance of the Magnetic Field 15 1.3.3 Cloud Collapse with a Magnetic Field 16 1.3.4 Collapse of a Rotating Cloud 18 1.3.5 Magnetic Braking 19 1.3.6 Flux Loss and Ambipolar Diffusion 20 1.3. 7 Simple Scenarios 21 1.3.8 Fragmentation and Numerical Simulation 23 Chapter 2:Evidence for Energetic Outflow 25 2.1 T Tauri Stars 27 2.2 FU Orionis Stars 28 2.3 Molecular Outflow 28 lll 2.3.1 Determination of Outflow Mass 29 2.3.2 Outflow Energetics 31 2.3.3 Angular Structure 32 2.3.4 Spatial Structure 34 2.3.4.1 Two Empirical Models 35 2.3.5 Frequency of Occmrence 38 2.3.6 Other Observations 38 2.3. 7 Theoretical models 40 2.3.7.1 Magnetic Twist Acceleration 40 2.3.7.2 Centrifugal Acceleration 42 2.4 Herbig-Haro Objects 45 2.4.1 Emission Spectra 46 2.4.2 Velocities of HH Objects 4 7 2.4.3 Theoretical Models of HH Objects .50 2.4.3.1 Shocked Cloudlet Models 50 2.4.3.2 Interstellar Bullets 52 2.4.3.3 Focussed Wind Models 53 2.5 Evidence for Discs around Young Stellar Objects 55 2.5.1 Indirect Evidence 56 2.5.2 Direct Evidence 57 Chapter 3:Polarisation and Polarimetry 60 3.1 Polarisation by Extinction 61 3.2 Imaging Polarimetry 63 Chapter 4:The Haro 6-5 System 66 4.1 Previous Observations 66 4.2 New Observations 67 4.2.1 Wise Observatory Data 68 4.2.2 INT Data 71 4.3 Interpretation 76 4.3.1 Evidence for Circumstellar Discs 76 4.3.1.1 The Polarisation Map 77 4.3.1.2 Total Intensity Image 79 4.3.1.3 Polarised Intensity Image 82 4.3.1.4 R-I Colour Image 83 4.3.2 The illuminating Sources 86 IV 4.3.3 The Proposed Geometry 88 4.3.3.1 The Haro 6-5B Bicone 88 4.3.3.2 The Haro 6-5A Bicone 91 4.4 Polarimetry of the Jet Region 92 4.5 Discussion 97 Chapter 5:The HL/XZ Tau System 101 5.1 Previous Observations 101 5.2 New Observations 103 5.2.1 Imaging 103 5.2.2 Polarisation Mapping 105 5.2.2.1 The HL Ta1.1 Region 110 5.2.2.2 The LkHa358 Region 111 5.2.2.3 The XZ Tau Region 113 5.3 Interpretation and Discussion 115 5.3.1 Evidence for a Disc/Outflow Nature for HL Tau 117 5.3.2 The Jet Region 121 5.3.3 XZ Tau and LkHa358 126 5.4 Discussion and Summary 127 Chapter 6:The PV Cephei System 130 6.1 Present Observations 132 6.1.1 Imaging Observations 132 6.1.2 Polarimetry 135 6.2 Interpretation 142 6.3 Summary 147 Chapter 7:General Discussion 151 7.1 Polarisation of the Source Regions 153 7.1.1 The Observational Evidence 153 7.1.2 Polarisation by Scattering 154 7.1.3 Polarisation by Aligned Grains 157 7.2 Scattering or Aligned Grains? 160 7.3 Models Involving Magnetic Fields 161 7.4 Implications for Outflow Acceleration 164 v Concluding Remarks 169 Abbricviations 173 Ad.'ilowledgements. 173 References 174 Vl Figures Page 1.1 Radial Distribution of Galactic CO Intensity 5 1.2 A Magnetic Braking Geometry 20 2.1 Asymmetric Line Profiles 30 2.2 Collimation Factors of Molecular Outflows 35 2.3 The Expanding Shell Model 37 2.4 HH Object Velocities (tangential) 48 2.5 HH Object Velocities (radial) 49 2.6 The Shocked Cloudlet Model 51 2. 7 The Focussed Wind Model 54 4.1 Polarisation map of Haro 6-5 (R band-Wise 0 bs. data) 70 4.2 Polarisation map of Haro 6-5 (R band-INT data) -·),_ 4.3 Higher spatial resolution map of Haro 6-5 (INT) 74 4.4 Polarisation subtraction (Haro 6-5B) 80 4.5 Polarisation subtraction (Haro 6-5A) 81 4.6 Contoured Polarised Intensity Image (Haro 6-5) 84 4. 7 R-1 Colour Index (Haro 6-5) 87 4.8 The Geometry Proposed for Haro 6-5 89 4.9 The Polarising Influences in Haro 6-5 90 4.10 Polarisation in the Jet Region of Haro 6-5 93 4.11 Polarisation in the Jet Region of Haro 6-5 95 5.1 Contour Map of the HL/XZ Tau Region 106 5.2 Polarisation Map of the HL/XZ Tau Region 108 5.3 Polarisation Map of the HL Tau Region 112 5.4 Polarisation Map of the LkHa358 Region 114 5.5 Polarisation Map of the XZ Tau Region 116 5.6 Radial Map of the HL Tau Region 119 5. 7 Contoured Polarised Intensity Image of HL Tau Region 122 5.8 Polarisation in the Jet Region of HL Tau 124 6.1 1-Band Image of the PV Cephei Region 132a 6.2 V, Rand I Images of PV Cephei 133a 6.3 R-band images of PV Cephei 1981-1986 134a 6.4 Unfiltered Polarisation Map of PV Cephei 138 6.5 1-Band Polarisation Map of PV Cephei 139 Vll 6.6 R-I Colour Index Image of PV Cephei 143 6. 7 The Geometry Proposed for PV Cephei 150 7.1 Elsasser and Staude Model 156 7.2 The Distribution of Field and Outflow Angles 168 Vlll Tables Page 1.1 The Conversion from CO to H2 3 1.2 Mass of Hydrogen in the Galaxy 6 2.1 Parameters of Molecular Outflows 33 3.1 The Polarimeter Filter System 65 4.1 The Haro 6-5 Observations 68 4.2 Polarisation of the Haro 6-5 Sources 76 5.1 Polarisation of the HL Tau Region Sources 109 5.2 Polarisation of the HL Tau Region Sources 109 6.1 The PV Cephei Observations 132 6.2 Changes in the PV Cephei System 1981-1986 136 6.3 Polarisation of PV Cephei 142 7.1 Polarimetry of Optical Nebulae 154 7.2 Field and Flow Directions 167 IX With the exception of the 1984 Jun and 1984 Aug PV Cephei observations, all the data presented in this thesis were recorded by the au thor at the \Vise Observatory, Israel and the Isaac Newton Telescope, La Palma between 1984 Nov and 1987 Jan.
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