Discovery of extended radio emission in the young cluster Wd1

Clark, J. S.; Fender, R. P.; Waters, L. B. F. M.; Dougherty, S. M.; Koornneef, J.; Steele, I. A. and van Blokland, A. (1998). Discovery of extended radio emission in the young cluster Wd1. Monthly Notices of the Royal Astronomical Society, 299(4) L43-L47.



We present 10 μm, ISO-SWS and Australia Telescope Compact Array observations of the region in the cluster Wd1 in Ara centred on the B[e] star Ara C. An ISO-SWS spectrum reveals emission from highly ionised species in the vicinity of the star, suggesting a secondary source of excitation in the region. We find strong radio emission at both 3.5cm and 6.3cm, with a total spatial extent of over 20 arcsec. The emission is found to be concentrated in two discrete structures, separated by ~14 arcsec. The westerly source is resolved, with a spectral index indicative of thermal emission. The easterly source is clearly extended and nonthermal (synchrotron) in nature. Positionally, the B[e] star is found to coincide with the more compact radio source, while the southerly lobe of the extended source is coincident with Ara A, an M2 I star. Observation of the region at 10?m reveals strong emission with an almost identical spatial distribution to the radio emission. Ara C is found to have an extreme radio luminosity in comparison to prior radio observations of hot stars such as O and B supergiants and Wolf-Rayet stars, given the estimated distance to the cluster. An origin in a detatched shell of material around the central star is therefore suggested; however given the spatial extent of the emission, such a shell must be relatively young (τ ∼ 103yr). The extended non thermal emission associated with the M star Ara A is unexpected; to the best of our knowledge this is a unique phenomenon. SAX (2-10 keV) observations show no evidence of X-ray emission, which might be expected if a compact companion were present.

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