Time, spatial, and spectral resolution of the H α line-formation region of Deneb and Rigel with the VEGA/CHARA interferometer

Abstract

Context. BA-type supergiants are amongst the most optically-brightstars. They are observable in extragalactic environments, hencepotential accurate distance indicators. Aims: An extensive recordof emission activity in the H{α} line of the BA supergiants {β}Orionis (Rigel, B8Ia) and {α} Cygni (Deneb, A2Ia) is indicative oflocalized time-dependent mass ejections. However, little is known aboutthe spatial distribution of these apparent structures. Here, we employoptical interferometry to study the H{α} line-formation region inthese stellar environments. Methods: High spatial- (0.001^{''}) and spectral- (R = 30 000) resolution observationsof H{α} were obtained with the visible recombiner VEGA installed onthe CHARA interferometer, using the S1S2 array-baseline (34 m). Sixindependent observations were done on Deneb during the years 2008 and2009, and two of Rigel in 2009. We analyze this dataset with the 1Dnon-LTE radiative-transfer code cmfgen, and assess the impact of thewind on the visible and near-IR interferometric signatures, using bothBalmer-line and continuum photons. Results: We observe avisibility decrease in H{α} for both Rigel and Deneb, suggestingthat the line-formation region is extended ( 1.5-1.75 R_{star}).We observe a significant visibility decrease for Deneb in the Siii 6371{Å} line. We witness time variations in the differential phase forDeneb, 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-windmass loss, accounts fairly well for the observed decrease in theH{α} visibility. Based on the observed differential visibilities, weestimate that the mass-loss rate of Deneb has changed by less than 5%.Based on observations made with the CHARA array.