Durham E-Theses A polarimetric study of ETA carinae and the carina nebula Carty, T. F. How to cite: Carty, T. F. (1979) A polarimetric study of ETA carinae and the carina nebula, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/8206/ 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 .A POLARIMETRIC STUDY OF ETA CARINAE AND THE CARINA NEBULA BY T.F. CARTY 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. SECTION A thesis submitted to the University of Durham for the degree of Doctor of Philosophy April 1979 ABSTRACT This thesis contains an account of some of the work undertaken by the author while a member of the Astronomy Group at the University of Durham. The work of the group involves the measurement and ..interpretation of the linear polarization in nebular astronomical objects. The initial chapters briefly describe the theory of polarized light and the. method of observing linear polarization, along with an indication of the techniques involved when analyzing the data. Chapter three is a review of recent work on interstellar grains with a siammary of the present ideas on the nature of the grains. The next chapter summarizes the observations of the peculiar object n Carinae and in the following chapter, a polarization map of the nebulosity around n Carinae is presented and a model determined to account for the observed linear polarization. The model involves the assumption of a bipolar type geometry for n Carinae with a size distribution of silicate grains of the form p 0.85 n(a) = exp -[f/0.022_ and it is found that grains of .15y contribute most to the scattering at optical wavelengths. Using the same distribution it is found that maximum emission at lOy can be expected from grains of size .ly . The final chapter contains observations of the linear polarization in the Carina Nebula in the region of n Carinae, and the results are interpreted in the light of present ideas concerning n Carinae and its relationship with the Carinae Nebula. It seems that the light from n Carinae is being reflected by clouds in the Carina Nebula. CONTENTS page INTRODUCTICN CHAPTER 1 POLARIZED LIGHT 1.1 Wave Polarization 3 1.2 The Stokes' Parameters 3 1.3 The Polarization Ellipse 5 1.4 Determination of the Stokes' Parameters 7 References 10 CHAPTER 2 1HE NEBDLAR POLARIMETER AND REDUCTION TECHNIQUE 2.1 Intxoduction 11 2.2 The Nebular Polarimeter 11 2.3 TSie Electronographic Camera 11 2.4 Reduction of the Data 13 References 18 CHAPTER 3 INTERSTELLAR DUST GRAINS 3.1 Introduction 19 3.2 Interstellar Extinction 19 3.3 Interstellar Polarization 26 3.4 Far Infrared Observations 29 3.5 Ccanetary Dust 30 3.6 Summary 31 References 33 CHAPTER 4 ETA CARINAE 4.1 Ecirly History 35 4.2 The Structure of Eta Carinae 39 4.3 Modern Light Curve Measurements 42 4.4 Optical Spectrum 43 4.5 Infrared Measurements 46 4.6 The Nature of Eta Carinae 47 References 51 SECTION - 2 - Page CHAPTER 5 A DUST SCATTERING MODEL OF THE ETA CARINAE NEBULA 5.1 The Data 53 5.2 The Polarization Map 54 5.3 The Model 55 5.4 Method of Computation 56 5.5 Results 60 5.6 Emission at lOy 62 5.7 Calculation of R 62 5.8 Discussion 64 5.9 Conclusions 65 References 67 CHAP1ER 6 A POLARIMETRIC INVESTIGATION OF THE - . CARINA NEBULA NEAR T\ CARINAE 6.1 Introduction 68 6.2 Observations of the Carina Nebula 70 6.3 7\nalysis of the Data 78 6.4 The Polarization Map 81 6.5 Discussion 82 6.6 Conclusions 84 References 86 GENERAL CONCLUSIONS 88 Appendix 89 ACKNOWLEDGEMENTS 93 INTRODUCTION ' ' ' ' "Spiarred by discoveries of elliptical poleirization, by the combination of photometry and polarimetry, by the introduction of new computers and instruments, including those on spacecraft and by the growing realization of the usefulness of these techniques when supported by well- developed interpretive theories " (Gehrels, 1974) there has, in recent years, been a wave of activity in polarimetry. With most astronomical problems, there, are many parameters to solve. For example, the geometry and aspect of a reflecting object with respect to the light source must be determined or assumed and this may be complicated by'a dependence on wavelength as in the case of planetary atmospheres where penetration depends on the wavelength of the incident and emergent light. Also dependent on the wavelength is the complex refractive index and the number of scatterers or optical depth of atmosphere,. Distribution of sizes and shapes of particles must also be determined and whether or not they are preferentially aligned. In order to solve for all these parameters many independant phenomena must be observed. Since the object is extremely remote, solution of the astronomical problem requires that observations of the light emitted or reflected by the body be made in the fullest detail, and hence the need for light scattering theories is obvious. From the beginning the development of light.scattering theories has been linked with astronomy. Computations on the radiation pressure exerted on perfectly conducting spheres were made before 1900 in order to explain the theory of comet tails.. Since 1930 much attention has been peiid to the optical properties viz scattering and/or extinction of inter• stellar grains.. Later work on scattering by cylinders was aimed at explaining interstellar polarization, • • V ' • • • . • . ' • • Laboratory work in formulating physical and chemical theories of the properties of such grains in the ambient gas and radiation field have complemented the basic data which should be obtained over as many parameters as possible. Therefore, results of radioastronomy, spectroscopy, photometry, radial velocity studies, and direct photographs and visual observations should be added to those of polarimetry over as large a : range of wavelengths and phases as possible. This is especially important in objects such as the Carina Nebula which present strikingly different appearances at different wavelengths. Also in n Carinae, vrtiere previous observations have suggested a particular geometry and grain type as the nature of the scattering particle, present poiarimetric observations may allow the determination of a size distribution of grains, but fut\are poiarimetric observations along with techniques previously mentioned, at different wavelengths, will be very important in completely determining the nature of this peculiar object. CHAPTER 1 POLARIZED LIGHT 1.1 WAVE POLARIZATION Lig^t is a superposition of many electromagnetic waves, each exhibiting transverse, sinusoidal vibrations, and satisfying Mcixwell's Equations. The direction and amplitude of each wave is chciracterized by its electric field vector E_ and its magnetic field vector H. C.G .Stokes (1852) was the first to investigate in detail the phenomenon of 'polarized', as opposed to 'natureil' light. He defined natural li^t as :- 'Light which is incapable of exhibiting rings of any kind when examined by a crystal of Icelcind spar and an analyzer or by some equivalent combination'. This is equivalent to saying that, on average, natural light has no preferred orientation for the direction of its E vector. Polarized light, on the other hand, arises when the direction of vibration of the light is restricted in some manner from its natural random• ness. Ploine polarization arises when the vibrations are confined to one plane. When the plane of polarization of polarized light rotates with time, eind the an^jlitude of vibration varies, elliptical polarization occurs, if the amplitude of vibration of elliptically polarized light remains the same, the light is said to be circularly polarized. Mixtures of these different, polarizations are found in nature. 1.2 THE STOKES' PARAMETERS The polarization of a beam of light may be completely chciracterized by four vectors : I, Q, U and V, which were introduced by C.G.Stokes (1852) as follows :- 'When euay number of independent polarized streams of given refrangibility cire mixed together, the nature of the mixture is completely determined by the values of four constants which . are certain functions of the intensities of the streams, and of the azimuths and eccentricities of the ellipses by which they are respectively characterized, so that any two groups of polarized streams which furnish the same values for each of these four constants are optically eqtiivalent'. An extension of this is the 'Principle of Equivalence' which states that it is iu^jossible by means of any instriunents to distinguish between "various incoherent sums of electromagnetic waves that may together form a beam with the Stokes' Parameters (I, Q, U, V). Hence, there is only one kind of naturai light with (I, Q, U, V) = (1, 0, 0, 0) wliich theoretically may be composed of waves in an infinite vzuriety of ways. The essential property of the Stokes' Parameters is their additivity in the superposition of two independent beams of light, corresponding to the absence of any interference.