Abstract

Aims. We present the first NIR spectro-interferometry of the LBV eta Carinae. The observations were performed with the AMBER instrument of the ESO Very Large Telescope Interferometer (VLTI) using baselines from 42 to 89 m. The aim of this work is to study the wavelength dependence of eta Car's optically thick wind region with a high spatial resolution of 5 mas (11 AU) and high spectral resolution. Methods. The observations were carried out with three 8.2 m Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1500 (MR-K mode) and 12 000 (HR-K mode). The MR-K observations were performed in the wavelength range around both the He I 2.059 mu m and the Br gamma 2.166 mu m emission lines, the HR-K observations only in the Br gamma line region. Results. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of eta Car. In the K-band continuum, a diameter of 4.0 +/- 0.2 mas (Gaussian FWHM, fit range 28-89 m baseline length) was measured for eta Car's optically thick wind region. If we fit Hillier et al. (2001, ApJ, 553, 837) model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17 mu m continuum, the He I, and the Br gamma emission lines, respectively. In the continuum near the Br gamma line, an elongation along a position angle of 120 degrees +/- 15 degrees was found, consistent with previous VINCI/VLTI measurements by van Boekel et al. (2003, A, 410, L37). We compare the measured visibilities with predictions of the radiative transfer model of Hillier et al. ( 2001), finding good agreement. Furthermore, we discuss the detectability of the hypothetical hot binary companion. For the interpretation of the non-zero differential and closure phases measured within the Br gamma line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.