Resolving the Hα-emitting Region in the Wind of η Carinae

The massive evolved star η Carinae is the most luminous star in the Milky Way and has the highest steady wind mass-loss rate of any known star. Radiative transfer models of the spectrum by Hillier et al. predict that Hα is mostly emitted in regions of the wind at radii of 6-60 au from the star (2.5-25 mas at 2.35 kpc). We present diffraction-limited images (FWHM ∼ 25 mas) with Magellan adaptive optics in two epochs, showing that η Carinae consistently appears ∼2.5-3 mas wider in Hα emission compared to the adjacent 643 nm continuum. This implies that the Hα line-forming region may have a characteristic emitting radius of 12 mas or ∼30 au, in very good agreement with the Hillier stellar-wind model. This provides direct confirmation that the physical wind parameters of that model are roughly correct, including the mass-loss rate of , plus the clumping factor, and the terminal velocity. Comparison of the Hα images (ellipticity and PA) to the continuum images reveals no significant asymmetries at Hα. Hence, any asymmetry induced by a companion or by the primary's rotation do not strongly influence the global Hα emission in the outer wind.