Other
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996pasp..108..547o&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.108, p.547
Other
1
Scientific paper
The strong stellar winds and supernovae of the most massive stars dominate the energy transfer from the stellar to the gaseous component of normal galaxies. This interaction is therefore a fundamental process determining the structure and composition of the interstellar medium (ISM), and driving star formation and galaxy evolution. Hence, the shells and bubbles that are the obvious manifestation of this phenomenon are key in understanding these processes. This work examines the stellar content and resulting dynamics of superbubbles primarily in the Large Magellanic Cloud (LMC). In Oey & Massey (1994), we study two remarkable, giant (130 pc diameter) H-alpha bubble nebulae in M33, and spectroscopically identify a single, dominant O9 star centrally located in each. We compare the nebular parameters with those predicted by the standard, analytic Weaver et al (1977) model for wind-blown bubbles, taking into account the evolution of wind power from the parent stars. We show that this evolution of wind power is important in the bubble evolution. The agreement with the model is reasonable, suggesting that these objects provide a compelling record of the effect that individual, normal OB stars have on the interstellar medium. The study of such nebulae can provide insight on the evolution of the parent star and its winds. In Oey & Massey (1995), we examine the stellar population of an OB association, LH 47/48, which is associated with a superbubble H II region, DEM 152 in the N44 nebular complex of the LMC. With CCD photometry and spectroscopy of the massive stars, we find no evidence that an unusual stellar population gave rise to the shell morphology of the gas. The slope of the initial mass function (IMF), Gamma=-1.3 +- 0.2, is consistent with that of other OB associations in the LMC, and there is no significant difference in the IMF internal or external to the supershell. The inferred stellar ionizing flux is consistent with the observed nebular H-alpha flux. We do find evidence for propagating star formation: the H-R diagram suggests an age of >~ 10 Myr for the population interior to the bubble with more recent, >~ 5 Myr, star formation on the exterior. Using the detailed data on the stellar population, we compare a numerical form of the Weaver et al. (1977) evolutionary model for wind-driven bubbles with the observed shell kinematics, finding a substantial discrepancy: the observed shell radius is too small, and/or expansion velocity too large to be explained with this version of the model. We discuss possible explanations for the inconsistency. Oey (1996a) presents $UBV$ photometry of the stellar populations associated with 7 superbubble nebulae and 5 classical H II regions in the LMC. Although the nebular morphology of the superbubbles appears to be substantially evolved compared to the classical nebulae, the color-magnitude diagrams do not reveal any noticeable correlation between the resident stellar population and nebular morphology. Oey (1996b) examines the stellar population enclosed within 6 of the superbubbles and compare these clusters with previously studied OB associations in classical H II regions. The H-R diagrams, constructed with spectral classifications of the most massive stars, do not reveal any systematic differences between OB associations resident within superbubbles and classical nebulae: the main-sequence turnoffs show stars as massive and luminous as those in classical H II regions. Assuming the superbubble structures result from the stellar winds and/or supernovae of the associations, the similarity of the stellar populations to those of classical H II regions implies that the shell formation timescale is somewhat shorter than the cluster evolutionary timescale for these objects. The stellar winds and/or supernovae of the one or two most massive stars must therefore dominate the formation of the superbubbles. The star-forming events for the superbubble associations are also no more extended in duration than that of other OB associations. Finally, the IMF slopes are not systematically different from those previously found. Since the OB associations within superbubbles appear normal, the shell structures must be the result of normal OB stellar influences. I also present a few spectrograms of interesting massive stars, including S149, a probable new B[e] supergiant. Based on the stellar populations observed within this sample of LMC superbubbles, Oey (1996c) uses the numerical version of the standard, pressure-driven bubble model (Oey & Massey 1995) to investigate the shell dynamics. The results fall into two distinct categories corresponding to a subset of objects for which the observed expansion velocity is too large for the observed shell radius ("high-velocity" superbubbles), and a subset of objects which appears more dynamically consistent with the model ("low-velocity superbubbles"). Both subsets of objects imply an overestimate in the shell growth rate equivalent to an overestimate in input power by up to an order of magnitude. The high-velocity objects exhibit X-ray evidence of supernova activity, suggesting that the dynamical discrepancy is due to acceleration by SNR impacts. (SECTION: Dissertation Summaries)
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