Observations of Magnetic Fields and Clustering Functions in Prestellar Cores by David Nutter A thesis submitted to the University of Wales for the degree of Doctor of Philosophy January 2004 UMI Number: U584640 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI U584640 Published by ProQuest LLC 2013. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 DECLARATION This work has not previously been accepted in substance for any degree and is not being concurrently submitted in candidature for any degree. Signed ................. Date ............................ STATEMENT 1 The work presented in this thesis is all my own work carried out under the su­ pervision of Dr. D. Ward-Thompson with the exception of the following: I did not take the SCUBA scan-map observations at the telescope, although I carried out all of the data reduction, calibration and analysis. I did not take the original SCUBA polarimetry observations of L1544, L43 and L183, taken in 1999. These data have previously been published (Ward-Thompson et al., 2000). I did take the subsequent observations of LI83 in 2002, and I carried out the data reduction and analysis for all of the L183 polarimetry data. I undertook the analysis of the LI544 and L43 data, based on the previously published data. The SCUBA scan-map work has been carried out in collaboration with Dr. Philippe Andre at the CEA Service d’Astrophysique, Saclay. The polarimetry work has been car­ ried in collaboration with Prof. R. M. Crutcher at the University of Illinois. A bibliography is appended. ] Signed . fA av ., Date ...A.. kS./.k STATEMENT 2 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. ACKNOWLEDGMENTS First and foremost, I would like to thank Hannah for her love, support and friend­ ship over the last three years. I thank Dr. Derek Ward-Thompson for giving me the opportunity to take part in this exciting research, and for all his help and support over the course of the PhD. I would also like to thank the members past and present, of the Star Formation research group here at Cardiff, especially Dr. Simon Goodwin, Dr. Jason Kirk, and Prof. Ant Whitworth. Many thanks must also go out to the friends I have made here over the years, especially Iain Brown and Emma Pearce, and all the guys in the terminal room who have kept me sane. An extra note of thanks to Tim Waskett for his help over the years. I would like to acknowledge and thank the collaborators I have worked with during my PhD, including Prof. Dick Crutcher, Dr. Philippe Andre, Dr. Jonathan Rawlings and Dr. Matt Redman. A special note of thanks to Dr. Jane Greaves for putting up with many emails on Friday afternoons asking ‘Exactly how does polarimetry work again?’ A big thank-you to the computing support staff here at Cardiff, especially Dr. Rodney Smith, and also the support staff of the JCMT, for support during and after many observing runs. Finally, a thank-you to my parents, for their love and support both before and during the PhD, without which, I would never have made it here in the first place. Figures 4.2 and 4.10 include photographic data obtained using The UK Schmidt Telescope. The UK Schmidt Telescope was operated by the Royal Observatory Edin­ burgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright © the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permis­ sion. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166. IRAS 100 /im data in Figures 3.3, 4.1, 4.2 and 4.10 courtesy of NASA IPAC/Jet Propulsion Laboratory. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. ABSTRACT In this thesis, I present SCUBA polarimetry data of three prestellar cores, L1544, L43 and L183. I also present SCUBA scan-map observations of three molecular clouds, Orion B, L1689, and RCrA. I use the Chandrasekhar-Fermi technique to measure the magnetic field strength in LI544, L43 and LI83, and find that each core is approximately magnetically critical or slightly supercritical. I measure the core mass function for prestellar cores in the Orion B molecular cloud and find that above 1 M0 , the data are best fitted by a power-law of the form diV/dlog M oc M ~15. The mass function has a peak at approximately 1 M©. This core mass function is comparable with the Salpeter slope of A/-1,35. I also determine the mean surface density of companions £ c(0) for the Orion B region, and find that it is best fitted by a power law of the form £ c(0) oc 0~0-3. This is smaller than the slope of 0-0 6 that is typically found for young stars in most molecular clouds. However, this slope is consistent with my data at the extreme limits of my lcr error bars. My value of T,c(0) oc 0~° 3 is comparable with the slope of 9~0 2 that is found for the nearby Orion Trapezium association. I find that the LI689 molecular cloud has a comparable rate of star formation to the neighbouring and more massive LI688. In addition, the star formation in LI689 occurs in a number of filaments that are parallel both to each other, and to similar filaments in L1688. I conclude that these observations indicate that the star formation is being influenced by the nearby Sco OB2 association. I compare the RCrA data to previous surveys of the region, and discover a previously unknown Class 0 source. Contents 1 An Introduction to Star Formation 1 1.1 Introduction ................................................................................................. 1 1.2 Protostellar Evolution .............................................................................. 3 1.2.1 The Birthplace of Stars ............................................ 3 1.2.2 The Youngest S ta rs ....................................................................... 4 1.2.3 Class 0 Protostars ....................................................................... 5 1.2.4 Protostellar Evolution ................................................................ 8 1.2.5 Decreasing Accretion Rate with T im e ....................................... 11 1.2.6 Protostellar L ife tim e .................................................................... 11 1.3 Prestellar c o r e s .......................................................................................... 12 1.3.1 History ............................................................................................. 13 1.3.2 Density Profile ............................................................................. 14 1.3.3 Core Temperature ....................................................................... 17 1.3.4 Prestellar Core L ife tim e ............................................................. 18 1.3.5 Magnetic F ie ld s ............................................................................. 19 1.4 Theories of Prestellar Evolution .............................................................. 20 i ii_________________________________________________________________________ CONTENTS 1.4.1 Similarity Solutions ....................................................................... 21 1.4.2 Additional Cloud Support Mechanisms .................................... 24 1.4.3 Magnetic Fields and Ambipolar Diffusion ................................. 24 1.4.4 Turbulent Cloud Support .......................................................... 27 1.4.5 Molecular Cloud Lifetimes .......................................................... 29 1.5 Measuring Interstellar Magnetic F ield s ................................................ 29 1.5.1 Polarimetry ..................................................................................... 30 1.5.2 Grain A lig n m e n t ........................................................................... 31 1.5.3 Zeeman e ffe c t .................................................................................. 34 1.5.4 Chandrasekhar-Fermi m e th o d .................................................... 36 1.6 Mass Estimates of Molecular Clouds and Cores ................................ 39 1.6.1 Determining Masses from Submillimetre Observations .... 40 1.6.2 Virial M a s s e s ................................................................................. 44 1.7 The Initial Mass F u n c tio n ....................................................................... 46 1.8 Summary and Thesis Overview ............................................................. 49 2 Data Acquisition and Reduction 51 2.1 Instrumentation .........................................................................................