Rotational Evolution of Massive Stars

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This project is motivated by the need to better understand the evolution of massive starsas shapers of the interstellar medium through ionization and winds, sources of heavyelements 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 asrapid rotation can trigger strong mixing which can in turn extend their core-hydrogenburning lifetimes, significantly increase their luminosities, and change the chemicalcompositions of their surfaces. Indeed, some massive stars may even experience a shortepisode of faster rotation during their approach to the terminal main sequence (caused bychanges in internal structure) which may account for the occurrence of the rapidlyrotating Be stars, B-type stars with Balmer line emission and an infrared excess due to acircumstellar 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 arespinning so fast that the centrifugal and gravitational accelerations nearly balance eachother at the stellar equator. This work will center on new observations with the GeorgiaState University Center for High Angular Resolution Astronomy Array (CHARA) atMount Wilson Observatory. This six-element, optical long baseline interferometer willresolve the shapes and sizes of these ultrafast rotators, and numerical models will beconstructed 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 toeject matter out into a circumstellar disk of the kind found in Be stars. CHARA Arrayobservations of Be disks will help determine their size, orientation, and mass outflowrate.Through this award, a postdoctoral research fellow will be supported and trained. Thepostdoctoral fellow will also participate in instrument support and development at theCHARA Array, developing expertise in electro-optics and computation and supportingthe optical interferometer. Graduate students are also expected to be involved andeducated through the work. The results of this work will be included in introductoryastronomy course lectures and GSU and disseminated to the public through press releasesand public talks.