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 d 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 Hα emission profile indicate that this envelope, and probably also the envelope around SMC-SC3, is a circumbinary disc 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' corotating binary-disc complexes. We suggest that BACs arise from wind shocks from the A star impacting the circumbinary disc and a stream of former wind-efflux from the B star accreting on to the A star. The latter picture is broadly similar to mass transfer occurring in the more evolved (but less massive) ALGOL (B/A + K) systems, except that we envision transfer occurring in the other direction and not through the inner Lagrangian point. Accordingly, we dub these objects prototype of a small group of Magellanic Cloud wind-interacting A + B binaries.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2010
- DOI:
- 10.1111/j.1365-2966.2010.16960.x
- arXiv:
- arXiv:1005.1230
- Bibcode:
- 2010MNRAS.407..734M
- Keywords:
-
- stars: early-type;
- stars: evolution;
- stars: mass-loss;
- stars: winds;
- outflows;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- To be published in MNRAS