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.

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. The American Astronomical Society selected Roberto Galvan-Madrid as a prize winner for The Rodger Doxsey Travel Prize in 2010.