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Photometric Survey to Search for Field sdO Pulsators

We present the results of a campaign to search for subdwarf O (sdO) star pulsators among bright field stars. The motivation for this project is the recent discovery by Randall et al. (2011), of four rapidly pulsating sdO stars in the globular cluster Omega Cen, with Teff near 50,000 K, 5.4 < log g < 6.0, and hydrogen-rich atmospheres. The only previously known sdO pulsator is significantly hotter at 68,500 K and log g = 6.1. All of the sdO pulsators identified so far are fainter than 17.4 in the V band and, thus, are poor candidates for an in-depth follow-up with asteroseismology. We therefore obtained high S/N light curves and spectroscopy for a number of field sdO stars to attempt to discover bright counterparts to these stars, particularly the Omega Cen pulsators. Our primary sample consisted of 19 sdO stars with hydrogen-rich atmospheres, log N(He)/N(H) < -1.0, effective temperatures in the range 40,000 K < Teff < 67,000 K, and surface gravities 5.3 < log g < 6.1. We also observed 17 additional helium-rich sdO stars with log N(He)/N(H) > -0.1 and similar temperatures and gravities. To date, we have found no detectable pulsations at amplitudes above 0.08% (4 times the mean noise level) in any of the 36 field sdO stars that we observed. The presence of pulsations in Omega Cen sdO stars and their apparent absence in seemingly comparable field sdO stars is perplexing. While very suggestive, the significance of this result is difficult to assess more completely right now due to remaining uncertainties about the temperature width and purity of the Omega Cen instability strip and the existence of any sdO pulsators with weaker amplitudes than the current detection limit in globular clusters.