PI: Kathryn F. Neugent, Lowell Observatory, KNeugent@lowell.edu
Address: 1400 W Mars Hill Road, Flagstaff, AZ 86001, USA
CoI: Philip Massey, Lowell Observatory
CoI: Georges Meynet, Geneva Observatory
Title: Determining the Binary Frequency of Wolf-Rayet Stars as a Function of Metallicity
Abstract: Wolf-Rayet (WR) stars are evolved massive stars, and the relative number of WC-type and WN-type WRs should vary with the metallicity of the host galaxy, providing a sensitive test of stellar evolutionary theory. Our recent studies determined unbiased censuses for WR stars in M31 and M33, resulting in the first accurate determination of WC to WN ratios at metallicities higher than that of the Magellanic Clouds. Current evolutionary models correctly predict the WC to WN ratios at lower metallicities, but fail to predict sufficiently large ratios in higher metallicity environments, such as in M31 and the center of M33. While the majority of WRs are created by strong stellar winds peeling away the stars' outer layers, it has been suggested that the models' deficiency is due to their single-star nature. However, this would require an increased number of WR binaries formed through Roche-love overflow (RLOF) in these high metallicity regions. Here we aim to observe the known WRs in both M31 and M33 in order to compare the binary frequencies as a function of metallicity, using the middle and outer regions of M33 (where the models do work) as our ``control". Our result will allow us to compare the frequency across different metallicities as well as help refine future evolutionary models by determining if the current single-star models are adequate or if Roche-lobe overflow plays a significant role in WR-star formation.
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