MOST Finds No Coherent Oscillations in the Hot Carbon-rich Wolf-Rayet Star HD 165763 (WR 111)

Abstract

We have photometrically monitored the V = 8 mag Galactic Population I WC5 star WR 111 for 3 weeks nonstop using the MOST microsatellite. Each of the ~27,000 data points has a precision of ~3 mmag. We find no coherent Fourier components above the 50 part per million level over the whole interval for frequencies f>10 cd-1 (periods P < 2.4 hr). This limit is nearly 2 orders of magnitude below recent predictions for early-type WR stars based on strange-mode pulsation simulations, with expected periods in the range 10-30 minutes. Simultaneous spectroscopic observations of WR 111 reveal a normal level of stochastic clumps propagating in the wind, which possibly manifest themselves in the slow 1/f rise in the MOST power spectrum below f~10 cd-1. Time-frequency analysis of the MOST data shows no obvious short-lived frequencies above the 1 mmag level, in stark contrast to the highly variable cool WR stars WR 123 (WN8) and WR 103 (WC9d), monitored previously by MOST. Radiation pressure therefore appears to be the main, if not sole, driver of WR 111's strong wind. Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Dynacon, Inc., the University of Toronto Institute of Aerospace Studies, and the University of British Columbia with the assistance of the University of Vienna.