This project is motivated by the need to better understand the evolution of massive stars as shapers of the interstellar medium through ionization and winds, sources of heavy elements throughout Galactic history, and as the progenitors of supernovae. In particular, the rotation properties of massive early-type OB stars play a key role in their evolution as rapid rotation can trigger strong mixing which can in turn extend their core-hydrogen burning lifetimes, significantly increase their luminosities, and change the chemical compositions of their surfaces. Indeed, some massive stars may even experience a short episode of faster rotation during their approach to the terminal main sequence (caused by changes in internal structure) which may account for the occurrence of the rapidly rotating Be stars, B-type stars with Balmer line emission and an infrared excess due to a circumstellar disk.
Observational studies of stellar rotation are problematic because in most cases it is difficult to determine the inclination of the spin axis relative to our line of sight. However, this shortcoming can be overcome in statistical analyses of large samples of targets. The purpose of this investigation is to obtain such data on the projected rotational velocities of a significant sample of massive B-type stars in young open clusters. Model line synthesis codes will be used to determine fundamental stellar parameters such as effective temperature and gravity in order to show how the rotational properties vary from the pre-main sequence phase, through the duration of the main sequence lifetime, and up to the terminal age main sequence. The spectra will also be analyzed to determine the influence of rotation on mixing of CNO processed gas into the surface layers. The results will provide the crucial means to test modern evolutionary codes and to aid in our interpretation of the properties of young stellar clusters and associations.
A second aspect of the program is a study of the physical processes in stars that are spinning so fast that the centrifugal and gravitational accelerations nearly balance each other at the stellar equator. This work will center on new observations with the Georgia State University Center for High Angular Resolution Astronomy Array (CHARA) at Mount Wilson Observatory. This six-element, optical long baseline interferometer will resolve the shapes and sizes of these ultrafast rotators, and numerical models will be constructed to match both the interferometric fringe data and complementary high signalto- noise spectroscopy. The goal is to determine how fast a star must rotate in order to eject matter out into a circumstellar disk of the kind found in Be stars. CHARA Array observations of Be disks will help determine their size, orientation, and mass outflow rate.
Through this award, a postdoctoral research fellow will be supported and trained. The postdoctoral fellow will also participate in instrument support and development at the CHARA Array, developing expertise in electro-optics and computation and supporting the optical interferometer. Graduate students are also expected to be involved and educated through the work. The results of this work will be included in introductory astronomy course lectures and GSU and disseminated to the public through press releases and public talks.
