2010A&A...521A...5C

BA-type supergiants are amongst the most optically-bright stars. They are observable in extragalactic environments, hence potential accurate distance indicators. An extensive record of emission activity in the Hα line of the BA supergiants βOrionis (Rigel, B8Ia) and α Cygni (Deneb, A2Ia) is indicative of localized time-dependent mass ejections. However, little is known about the spatial distribution of these apparent structures. Here, we employ optical interferometry to study the Hα line-formation region in these stellar environments. High spatial- (∼0.001'') and spectral- (R=30000) resolution observations of Hα were obtained with the visible recombiner VEGA installed on the CHARA interferometer, using the S1S2 array-baseline (34m). Six independent observations were done on Deneb during the years 2008 and 2009, and two of Rigel in 2009. We analyze this dataset with the 1D non-LTE radiative-transfer code cmfgen, and assess the impact of the wind on the visible and near-IR interferometric signatures, using both Balmer-line and continuum photons. We observe a visibility decrease in Hα for both Rigel and Deneb, suggesting that the line-formation region is extended (∼1.5-1.75R_*). We observe a significant visibility decrease for Deneb in the SiII6371Åline. We witness time variations in the differential phase for Deneb, implying an inhomogeneous and unsteady circumstellar environment, while no such variability is seen in differential visibilities. Radiative-transfer modeling of Deneb, with allowance for stellar-wind mass loss, accounts fairly well for the observed decrease in the Hα visibility. Based on the observed differential visibilities, we estimate that the mass-loss rate of Deneb has changed by less than 5%.