| Abstract
| - Context. BA-type supergiants are amongst the most optically-bright stars. They are observable in extragalactic environments, hence potential accurate distance indicators. Aims. An extensive record of emission activity in the H α line of the BA supergiants β Orionis (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 H α line-formation region in these stellar environments. Methods. High spatial- (~0.001 $\arcsec$) and spectral- ( R = 30 000) resolution observations of H α were obtained with the visible recombiner VEGA installed on the CHARA interferometer, using the S1S2 array-baseline (34 m). 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. Results. We observe a visibility decrease in H α for both Rigel and Deneb, suggesting that the line-formation region is extended (~1.5-1.75 $R_{\star}$). We observe a significant visibility decrease for Deneb in the Si ii 6371 Å line. 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 H α visibility. Based on the observed differential visibilities, we estimate that the mass-loss rate of Deneb has changed by less than 5%.
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