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Lucky Imaging Optical Polarimetry of HL Tau and XZ

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Lucky Imaging Optical Polarimetry of HL Tau and XZ T Tauri stars Optical lucky imaging polarimetry of HL and XZ Tau Master of Science Thesis in Astrophysics 6 5 4 3 2 1 arcseconds 0 -1 -2 200 AU -3 -6 -5 -4 -3 -2 -1 0 1 2 Magnusarcseconds Persson Department of Astronomy Stockholm University 2010 Abstract Optical lucky imaging polarimetry of HL Tau and XZ Tau in the Taurus-Auriga molecular cloud was carried out with the instrument PolCor at the Nordic Opti- cal Telescope (NOT). The results show that in both the V- and R-band HL Tau show centrosymmetric structures of the polarization angle in its northeastern outflow lobe (degree of polarization∼30%). A C-shaped structure is detected which is also present at near-IR wavelengths (Murakawa et al., 2008), and higher resolution optical images (Stapelfeldt et al., 1995). The position angle of the outflow is 47.5±7.5◦, which coincides with previous measurements and the core polarization is observed to decrease with wavelength and a few scenarios are reviewed. Measuring the outflow witdh versus distance and wavelength shows that the longer wavelengths scatter deeper within the cavity wall of the outflow. In XZ Tau the binary is partially resolved, it is indicated by an elongated in- tensity distribution. The polarization of the parental cloud is detected in XZ Tau through the dichroic extinction of starlight. Lucky imaging at the NOT is a great way of increasing the resolution, shifting increases the sharpness by 000. 1 and selection the sharpest frames can increase the seeing with 000. 4, perhaps more during better conditions. About this thesis This thesis is the written part towards a Master of Science Degree in Astro- physics at Stockholm University Astronomy Department. The corresponding work was done under the supervision of Professor Göran Olofsson at Stockholm University. The work involves observations with the PolCor instrument, built by Pro- fessor Göran Olofsson and Hans-Gustav Florén, mounted on the NOT and the following reduction, calibration and analysis of the data. The observations were carried out between 26th and 30th October 2008 and are of two young stellar objects: XZ Tau and HL Tau in the Taurus-Auriga molecular cloud complex. The reduction routines are written in the Python programming language. The result of the reduction is analysed and reviewed. This work has made use of the SIMBAD database and NASA’s Astrophysics Data System. This document was typeset by the author in LATEX 2ε. Acknowledgements First I would like to thank Göran Olofsson for making all of this possible, if I never would have sent that e-mail to the wrong Olofsson this would probably never have happened. You gave me the opportunity to do everything from start to finish, and I have learned so much, thank you. Hans-Gustav Florén for answering questions about the reduction software and the company on the observation run. Matthias Maercker and Sofia Ramstedt for their help and support during the years. Ramez and Daniel, it would have been really lonely here without you around. My girlfriend Anca Mihaela Covaci for putting up with me and my childishness, I hope you can bear with me the rest of your life. Contents 1 Introduction 1 1.1 Star formation ............................ 1 1.1.1 Background - The Nebular Hypothesis........... 2 1.1.2 ISM - Structures & clouds.................. 3 1.1.3 Early evolution of Low-mass stars ............. 4 1.1.4 Feedback processes...................... 9 1.1.5 The Main-Sequence ..................... 9 1.1.6 HL Tau and XZ Tau as part of Lynds 1551 in Taurus . 10 1.2 Polarimetry.............................. 11 1.2.1 Background.......................... 11 1.2.2 Polarization in Astronomy.................. 13 1.2.3 Detecting linearly polarized light.............. 15 1.3 Diffraction limited imaging from the ground............ 17 1.3.1 Introduction ......................... 17 1.3.2 Correction methods ..................... 19 2 Observations and Data reduction 21 2.1 Observations ............................. 21 2.1.1 The PolCor instrument ................... 21 2.1.2 Observations ......................... 24 2.2 Data reduction ............................ 27 2.2.1 Overview ........................... 27 2.2.2 Dark frame and flat fielding................. 28 2.2.3 Determining the centre of reference object......... 28 2.2.4 Sharpness........................... 29 2.2.5 Shifting and adding ..................... 30 2.3 Data analysis............................. 31 2.3.1 Stokes and additional parameters.............. 31 2.3.2 Polarization standards.................... 31 3 Results 33 3.1 Results................................. 33 3.1.1 XZ Tau ............................ 34 3.1.2 HL Tau............................ 38 3.1.3 Lucky astronomy....................... 45 3.2 Summary of results.......................... 49 3.2.1 HL Tau............................ 49 3.2.2 XZ Tau ............................ 49 3.2.3 Parameters vs sharpness/psf improvement......... 49 4 Discussion 51 4.1 Discussion............................... 51 4.1.1 HL Tau............................ 51 4.1.2 XZ Tau ............................ 54 4.1.3 Other ............................. 54 4.2 Summary ............................... 55 List of Figures 57 List of Tables 59 Bibliography 61 Appendix 69 A. Data with three valid angles observed................. 69 B. Python code description ........................ 70 B.1 Introduction ........................... 70 B.2 Help Functions.......................... 70 B.3 Classes, Attributes and Methods................ 71 B.4 Example Usage.......................... 72 “Space is the place.” Sun Ra 1 Introduction This thesis consists of work in the areas low-mass star formation, lucky imaging and polarimetry. This chapter gives an introduction to these areas. The first part is dedicated to the star formation process, the main topic of this thesis. Second part is an introduction to polarimetry, and lastly an introduction to lucky astronomy; a technique to obtain diffraction limited images from ground based telescopes. Section 1.1 consists of a brief history of the star formation process, a de- scription of interstellar clouds, the early evolution of low-mass1 stars followed by feedback processes, the main sequence (MS), and lastly a description of the star forming cloud were the sources in this thesis are located - the Lynds 1551 (L1551) nebula in the Taurus Molecular cloud. In order to understand how the observations where made an introduction to polarimetry is given in section 1.2. The section give a background to polarimetry and a review of polarization in astronomy, were the technique to detect circum- stellar dust is described and lastly the theory behind the detection technique is explained. The last section (1.3) gives and introduction to various techniques to obtain (near) diffraction limited images. 1.1 Star formation Stars form out of the gravitational instability in a turbulent density enhance- ment, called molecular cloud (MC) in the interstellar medium (ISM), the col- lapse form a protostar. The protostar starts to accrete matter thus forming an accretion disk and a bipolar outflow. When it has accreted enough a pre-Main Sequence (pre-MS) star emerges, the circumstellar envelope starts to dissipate and by the time the core has ignited its nuclear burning of Hydrogen it has settled on the Main Sequence (MS) and there is just a small debris disk left. Somewhere on the journey planets are formed through collisions and coagula- tion. The protostellar and pre-MS phases are sometimes grouped together and the object is then be referred to as a Young Stellar Object (YSO), also the phases of star formation are classified from the Spectral Energy Distribution (SED) of the unresolved object, i.e. the classification systems origin depended on whether the object was resolved or not. 1 Low-mass stars M≤ 2 M , intermediate-mass 2 <M≤ 8 M and high-mass M>8 M . 1 CHAPTER 1. INTRODUCTION A lot of important and energetic chemistry takes place in forming a star, the neutral Hydrogen in the MC goes from neutral H2 in the cold MCs to ionized H+ in the core of stars, complex chemistry on dust grains in the cloud takes place and creates complex molecules. The enormous density contrast between typical cloud densities and the hydrogen-burning centres of the final stars is typically about 24 orders of magnitude. The following is a description of the current star formation paradigm, which applies to low- and possibly intermediate-mass stars that form in isolation. Al- though most stars seem to be formed in clusters (Lada and Lada, 2003); groups of several stars of different masses, from brown dwarfs to O and B stars. By having other, possibly more massive, stars forming in the vicinity could effect the protostar in serious ways. The outflow could trigger other gravitationally unstable clouds to collapse into stars, but also if the other star has a strong radiation field it could photoevaporate the circumstellar cloud of the protostar and limit the growth of the star during its accretion phase. For a recent review in star formation see McKee and Ostriker(2007) and for a recent review of the advances in numerical studies of star formation see Klessen et al.(2009). 1.1.1 Background - The Nebular Hypothesis The idea that stars are formed out of interstellar clouds have been present for several centuries. The initial idea, that the Sun and Planets formed out of a rotating cloud or disk of material was called the Nebular Hypothesis, which was formulated by Emanuel Swedenborg in 1734. It sprung from the realisation that the orbital planes of the Planets around the Sun are all, to a good approximation in the same plane and direction, due to the formation process. Although the theory was successful in explaining the motion of the Planets, it could not explain why the Sun has the low, much lower than expected from the theory, angular momentum. Thus other theories were worked out during the years. In 1945 Alfred Joy analysed 11 irregular variable stars that shared photomet- ric and spectral properties (Joy, 1945). The characteristics included variability in the optical lightcurve and emission lines including that from Hydrogen (Hα) and Calcium (Ca II).
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