AG Carinae: A Luminous Blue Variable with a High Rotational Velocity

Abstract

We report the detection of broad absorptions due to Si IV λλ4088-4116 in the luminous blue variable (LBV) AG Carinae during its last hot phase (2001-2003). Our non-LTE spectral analysis, with the radiative transfer code CMFGEN, revealed the photospheric nature of these lines, predicting, however, much narrower and deeper absorption profiles than observed. Using a recently developed code to compute synthetic spectra in two-dimensional geometry allowing for the effects of rotation, we were able to match the broad absorptions with a high projected rotational velocity of 190+/-30 km s-1 in 2001 April. Analysis of spectra obtained in 2002 March and 2003 January, when the star was cooling, yielded projected rotational velocities of 110+/-10 and 85+/-10 km s-1, respectively. The derived rotational velocities are proportional to R-1*, as expected from angular momentum conservation. We discuss the effects of such high rotation on the spectral analysis of AG Car and on the wind terminal velocity. Our results show direct spectroscopic evidence, for the first time, that an LBV may rotate at a significant fraction of its breakup velocity. Thus, AG Car (and possibly other LBVs) is indeed close to the ΓΩ limit, as predicted by theoretical studies of LBVs. Based on observations made with the 1.52 m ESO telescope at La Silla, Chile, and the 8 m ESO Very Large Telescope (VLT) at Paranal, Chile.