Bruce Bohannan

Tucson Nighttime Scientific Staff

Areas of Interest

Stellar Spectroscopy, Structure and Evolution of Massive Stars, Astrophysical Instrumentation and Data Reduction

Recent Research Results

Bohannan's research centers on observational studies of the evolution of massive stars. Massive stars, through radiation, mass loss, and supernovae explosions, are significant to the chemical element evolution and kinetic energy of their parent galaxies and, because of their high luminosity, are readily observed to great distance in the Milky Way and in other galaxies. The evolution of these stars is poorly understood because models are not readily connected to simple spectral morphology. Bohannan and his colleagues have used the measurement of basic stellar properties (temperature, gravity, mass, mass loss rate, and surface element abundances) to define the evolutionary path of massive stars and to make connections between various stages of massive star evolution, which are most readily observed as morphological differences. For example, recently P.A. Crowther (University College, London) and Bohannan (1996, submitted to Astronomy & Astrophysics) concluded that a direct evolutionary connection may exist between certain hot, massive stars (those classified as O8 Iafpe) and a later stage of stellar evolution (low excitation WN stars) without an intermediate stage of rapid and unstable evolution (commonly known as Luminous Blue Variables), a conclusion which is contrary to current evolutionary theory and suggestive that an additional process (e.g., mixing) brings core processed material to the surface.

Future Research Plans

Similar investigations of stars in the Galactic Center and in other galaxies provide critical diagnostics of stellar evolution because they can examine the effect of different stellar environments on stellar evolution. Such studies depend on infrared observations because of extreme interstellar absorption (the Galactic Center) and severe crowding of stellar images (other galaxies). The IR diagnostics are being developed with Crowther and tested through a set of spectral line profiles recently obtained at CTIO and analyzed with Hillier model atmospheres. The next stage will be to analyze a set of Of and WN stars in the Large Magellanic Cloud, to be followed by observations of stars in the Galactic Center thought to be of similar spectral morphology. The final step would be to undertake a similar study of stars in M33, research which requires a moderate resolution IR spectrometer combined with an image compensation system to provide high spatial resolution (e.g., tip-tilt and fast focus). In work with E. L. Fitzpatrick, Bohannan will be completing abundance analyses of B supergiants in the Large Magellanic Cloud. These stars, somewhat less massive than the O-type stars studied with Crowther, exhibit a range of CNO abundances, an observation indicating that a significant fraction of these stars have evolved through the red supergiant stage and have returned to the blue supergiant region with core processed material now in their atmospheres. This latter conclusion is also not consistent with models of massive star evolution, and mixing may need to be invoked to bring the processed material to the surface; mixing has not yet been included in model calculations in sufficient detail to enable a comparison with observations. The results of these studies will contribute significantly to defining future theoretical modeling.


As Assistant Director for Operations and Projects, Bohannan has day-to-day and long-range supervisory responsibility for all KPNO mountain-based activities, including electronic and computer maintenance, and instrument and observing support for astronomers using KPNO telescopes. Personnel reporting to him also provide electronic maintenance for the National Solar Observatory site on Kitt Peak. He works with Tucson-based engineering personnel on telescope and instrument improvement projects. Major projects in the past year include thermal control of the 4-meter primary mirror, a number of "re-engineering projects" to address the management and operation of NOAO/KPNO as it adjusts to level or declining budgets, and training of NOAO personnel to operate and maintain the WIYN telescope. Major activities for the coming year include continuing work on improving the optical performance of the 4-meter telescope and coordination of work on the thermal control system of the 4-meter dome.

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NOAO is operated by the Association of U niversities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation
Posted: 06Dec1996