Resolving the asymmetric inner wind region of the yellow hypergiant IRC +10420 with VLTI/AMBER in low and high spectral resolution mode

International audienceContext: IRC +10420 is a massive evolved star belonging to the group of yellow hypergiants. Currently, this star is rapidly evolving through the Hertzprung-Russell diagram, crossing the so-called yellow void. IRC +10420 is suffering from intensive mass loss which led to the formation of an extended dust shell. Moreover, the dense stellar wind of IRC +10420 is subject to strong line emission. Aims: Our goal was to probe the photosphere and the innermost circumstellar environment of IRC +10420, to measure the size of its continuum- as well as the Brγ line-emitting region on milliarcsecond scales, and to search for evidence of an asymmetric distribution of IRC +10420's dense, circumstellar gas. Methods: We obtained near-infrared long-baseline interferometry of IRC +10420 with the AMBER instrument of ESO's Very Large Telescope Interferometer (VLTI). The measurements were carried out in May/June 2007 and May 2008 in low-spectral resolution mode in the JHK bands using three auxillary telescopes (ATs) at projected baselines ranging from 30 to 96 m, and in October 2008 in high-spectral resolution mode in the K band around the Brγ emission line using three unit telescopes (UTs) with projected baselines between 54 and 129 m. The high-spectral resolution mode observations were analyzed by means of radiative transfer modeling using CMFGEN and the 2D Busche & Hillier codes. Results: For the first time, we have been able to absolutely calibrate the H- and K-band data and, thus, to determine the angular size of IRC+10420's continuum- and Brγ line-emitting regions. We found that both the low resolution differential and closure phases are zero within the uncertainty limits across all three bands. In the high-spectral resolution observations, the visibilities show a noticeable drop across the Brγ line on all three baselines. We found differential phases up to -25° in the redshifted part of the Brγ line and a non-zero closure phase close to the line center. The calibrated visibilities were corrected for AMBER's limited field-of-view to appropriately account for the flux contribution of IRC +10420's extended dust shell. From our low-spectral resolution AMBER data we derived FWHM Gaussian sizes of 1.05±0.07 and 0.98±0.10 mas for IRC +10420's continuum-emitting region in the H and K bands, respectively. From the high-spectral resolution data, we obtained a FWHM Gaussian size of 1.014±0.010 mas in the K-band continuum. The Brγ -emitting region can be fitted with a geometric ring model with a diameter of 4.18^+0.19-0.09 mas, which is approximately 4 times the stellar size. The geometric model also provides some evidence that the Brγ line-emitting region is elongated towards a position angle of 36°, well aligned with the symmetry axis of the outer reflection nebula. Assuming an unclumped wind and a luminosity of 6×10^5{ {L}ȯ}, the spherical radiative transfer modeling with CMGFEN yields a current mass-loss rate of 1.5-2.0×10-5{ {M}ȯ yr-1} based on the Brγ equivalent width. However, the spherical CMFGEN model poorly reproduces the observed line shape, blueshift, and extension, definitively showing that the IRC +10420 outflow is asymmetric. Our 2D radiative transfer modeling shows that the blueshifted Brγ emission and the shape of the visibility across the emission line can be explained with an asymmetric bipolar outflow with a high density contrast from pole to equator (8-16), where the redshifted light is substantially diminished. The low-spectral resolution data have been obtained as part of the Guaranteed Time Programme for VLTI/AMBER (program ID: 079.D-0356(B)), while the high-spectral resolution data were obtained in the context of science verification observations (program ID: 60.A-9053(D))