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Massive Star Cluster Evolution in Action: The Extreme Environmental Impacts of Wolf-Rayet Stars

Sokal, Kimberly
Thesis/Dissertation; Online
Sokal, Kimberly
Johnson, Kelsey
Massive stars modify their environment with ionizing photons, fast winds, and eventual supernova explosions; feedback that is enhanced when the stars are clustered. The highest concentrations of massive stars are found in massive and super star clusters, which represent some of the most intense regions of star formation and can dominate the energetics of a galaxy. Massive star clusters form in thick, dense envelopes of natal material that largely obscure the early cluster evolution. However, massive stars forming within the embedded cluster ionize the surrounding material, making the cluster detectable at radio wavelengths. These stars then blow away the surrounding material, eventually revealing the cluster at optical wavelengths. However, this transition is not well understood, despite implications for the fate of the cluster itself and its impact on the local environment and host galaxy. With my thesis, I highlight an overlooked yet potentially significant source of feed- back – Wolf-Rayet (WR) stars – that may contribute to cluster emergence. Previous works suggested that massive star clusters will have cleared out their natal mate- rial before the massive stars have evolved into the WR phase, during which huge amounts of ionization and mechanical feedback are output. Yet, I identified a mas- sive star cluster, S26 in the galaxy NGC 4449, that challenges that expectation. S26 was originally detected as a radio source and was surprisingly observed to host WR stars. Due to S26, I hypothesize the WR feedback provides the tipping point in the combined feedback processes that drive a massive star cluster to emerge. I follow up this discovery with an observational survey to investigate the role of WR stars in massive star cluster evolution. Using optical spectra to search for WR signatures in a radio-selected sample, I present a sample of 45 emerging massive star clusters, 21 with significant detections of the WR signature. Additionally, I find that the sources with highest extinctions do not host WR stars and have ages beyond the onset of the WR phase, which may indicate that clusters without WR stars stay embedded for longer periods of time. This thesis has vastly expanded the number of known “emerging WR clusters”, as well as revealed a possible acceleration of the removal of natal material in clusters hosting WRs compared to those without.
University of Virginia, Department of Astronomy, PHD (Doctor of Philosophy), 2016
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PHD (Doctor of Philosophy)
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