The radio source around η Carinae

Abstract

We present high spatial resolution radio observations of the peculiar southern star η Carinae, made with the Australia Telescope. The images, at 8 and 9 GHz with a resolution of 1″.0, show a source of dimension 10″ and total flux of 0.7 Jy dominated by a strong central peak. The radio emission is unpolarized and offers no support to models which invoke degenerate stars or more exotic objects within the core of η Car. In these data we find no evidence for more than one energy source in the core with arcsecond separations as some infrared observations have suggested. Several levels of structure are evident in the radio image, which shows symmetry on the larger scales. Conventional formulae for stellar wind radio sources give a mass loss rate of order 3 × 10 -4 M ⊙ yr -1 based on the radio flux in the central peak, which yields a wind momentum flux of order 20% of the momentum flux available from the star's radiation field. The radio emission at these frequencies is consistent with thermal emission from gas flowing away from a "luminous blue variable" star (LBV). η Car is probably the brightest thermal stellar wind radio source in the sky.