In search of binary Wolf-Rayet central stars of planetary nebulae

Master Thesis


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University of Cape Town

Planetary Nebulae (PNe) are circumstellar gas envelopes ejected during Asymptotic Giant Branch (AGB) phase of the stars between 1M࿉ to 8M࿉. In around 3000 galactic PNe, 10 to 20 % are known to host a Wolf-Rayet type central star ([WR] CSPN). These [WR] CSPNe are known to be very hydrogen deficient (H-deficient), the mechanism behind which is not clearly understood. A binary central star (CS) or a merger scenario is very likely to be responsible for the H-deficiency. As an attempt to investigate the former, a radial velocity (RV) monitoring program of 7 [WR] CSPNe was conducted, which is the first systematic study of its kind. Spectroscopic observations were made at the SAAO 1.9m Telescope over 3 separate weeks in May 2012, June 2012 and July 2013. The spectra were reduced and analyzed with a cross-correlation method to determine RV shifts in the stellar and nebular lines. Together with a review of the major mechanisms involved in the formation and evolution [WR] CSPNe, the main results obtained in this work are discussed. An offset is found between the mean stellar and nebular lines, which might be due to the fact that most of the stellar emission lines are formed in the wind and are likely to be blue-shifted as in several Of-type central stars. No significant amplitude or periodic variations are seen in the stellar lines, as we would expect from a close binary system. However, small (~ 10-30 km s-¹) stellar RV variations are seen and these are most likely due to wind variability. A Χ² probability test reveals that the objects are only 30 - 40 % variable and this variability is not high enough to be attributed to a binary CS. The main conclusions drawn are that these [WR] CSPNe might contain wide binaries with longer periods that would produce RV variations below our detection limit. Another feasible explanation might be that [WR] CSPNe are actually the result of post-CE mergers. We absolutely need RV monitoring using high resolution spectra (e.g. High Resolution Spectrograph on the South African Large Telescope) in the future to confirm the presented results.

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