On the Formation of the Most Massive Stars in the Galaxy DOCTORAL THESIS ACCEPTED BY UNIVERSITAD NACIONAL AUTONOMA DE MEXICO

The most massive stars in the galaxy - those with more than 15 to 20 solar masses - are lilkely to ionize their surroundings before they reach their final mass. How can they accrete in spite of the presence of over-pressurized gas?

This thesis presents results of Submillimeter Array (SMA) and Very Large Array (VLA) studies of massive star formation regions in the early stages of ionization, as well as an analysis of numerical simulations of the evolution of these young HII regions. The results favor a picture in which very massive stars form in accretion flows that are partially ionized and that keep accreting material from their environment.

Foreword by Prof. Luis F. Rodriguez (CRyA-UNAM) and Dr. Qizhou Zhang (Harvard-Smithsonian Center for Astrophysics).-Chapter 1: Introduction to the Scientific Problem.- Chapter 2: A MSFR at the Onset of Ionization: W33A.- Chapter 3: A MSFR with young UC and HC HII Regions: G20.08N.- Chapter 4: Time Variability of HII Regions: A Signature of Accretion?.- Chapter 5: Time Variability of HII Regions in Numerical Simulations of MSFR.- Chapter 6: Conclusions.- Bibliography.- Appendix I: Radio and (Sub)millimeter Interferometers.- Appendix II: Molecular-line Emission.- Appendix III: Ionized-Gas Emission.- Index.

Dr. Roberto Galván-Madrid was born in 1982 in Chetumal, Mexico, where he had his basic and high-school education. He obtained a bachelor degree in Physics from the Autonomous University of Nuevo Leon (UANL) in 2005, and a Masters degree in Astronomy from the National Autonomous University of Mexico (UNAM) in 2007.

The thesis published here is a compilation of the work presented by RGM in August 2011 to obtain his Ph.D. degree in Astronomy from UNAM, under the supervision of Prof. Luis F. Rodríguez. During most of his thesis work, RGM was based at the Harvard-Smithsonian Center for Astrophysics (CfA), working as a Pre-doctoral fellow of the Submillimeter Array Project (SMA) under the supervision of Dr. Qizhou Zhang and Dr. Paul T. P. Ho.

RGM's scientific interests are the processes of star formation, with emphasis on the formation of high-mass stars and clusters. Starting September 2011, RGM works as a postdoctoral fellow for the European Organization for Astronomical Research in the Southern Hemisphere (ESO).