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

We report the detection of broad absorptions due to Si IV 4088-4116 A in theLuminous Blue Variable (LBV) AG Carinae during its last hot phase (2001-2003).Our NLTE spectral analysis, with the radiative transfer code CMFGEN, revealedthe photospheric nature of these lines predicting, however, much narrower anddeeper absorption profiles than observed. Using a recently-developed code tocompute synthetic spectra in 2D geometry allowing for the effects of rotation,we could match the broad absorptions with a high projected rotational velocityof 190 +/- 30 km/s on 2001 April. Analysis of spectra obtained on 2002 Marchand 2003 January, when the star was cooling, yielded to a projected rotationalvelocity of 110 +/- 10 km/s and 85 +/- 10 km/s, respectively. The derivedrotational velocities are proportional to R^-1, as expected from angularmomentum conservation. We discuss the effects of such high rotation on thespectral analysis of AG Car, and on the wind terminal velocity. Our resultsshow direct spectroscopic evidence, for the first time, that a LBV may rotateat a significant fraction of its break-up velocity. Thus, AG Car (and possiblyother LBVs) is indeed close to the Gamma-Omega limit, as predicted bytheoretical studies of LBVs.Comment: 15 pages, 4 figures, submitted to ApJ Letter