High-dispersion spectroscopy of two A supergiant systems in the Small Magellanic Cloud with novel properties

Abstract

We present the results of a spectroscopic investigation of two novel variable bright blue stars in the SMC, OGLE004336.91-732637.7 (SMC-SC3) and the periodically occulted star OGLE004633.76-731204.3 (SMC-SC4), whose photometric properties were reported by Mennickent et al. (2010). High-resolution spectra in the optical and far-UV show that both objects are actually A + B type binaries. Three spectra of SMC-SC4 show radial velocity variations, consistent with the photometric period of 184.26 days found in Mennickent et al. 2010. The optical spectra of the metallic lines in both systems show combined absorption and emission components that imply that they are formed in a flattened envelope. A comparison of the radial velocity variations in SMC-SC4 and the separation of the V and R emission components in the Halpha emission profile indicate that this envelope, and probably also the envelope around SMC-SC3, is a circumbinary disk with a characteristic orbital radius some three times the radius of the binary system. The optical spectra of SMC-SC3 and SMC-SC4 show, respectively, HeI emission lines and discrete Blue Absorption Components (BACs) in metallic lines. The high excitations of the HeI lines in the SMC-SC3 spectrum and the complicated variations of FeII emission and absorption components with orbital phase in the spectrum of SMC-SC4 suggests that shocks occur between the winds and various static regions of the stars' co-rotating binary-disk complexes. We suggest that BACs arise from wind shocks from the A star impacting the circumbinary disk and a stream of former wind-efflux from the B star accreting onto the A star. We dub these objects prototype of a small group of Magellanic Cloud wind-interacting A + B binaries.