330-MHz radio continuum observations of the H II regions M42 and M43

Abstract

We present a radio continuum image of the Orion Nebula (M42 and M43) made with a resolution of 57 arcsec at a frequency of 330 MHz. At this low frequency, the H II region M42 appears to be asymmetric on all scales exceeding an arcminute, whereas M43 appears to have circular symmetry. The intensity distribution in M42 is flat topped and has a sharp cut-off to the north-east, whereas diffuse emission with a low surface brightness extends to the south and west. Towards the continuum peak of M42 the optical depth is large (τ ≫ 1) and the imaging directly measures the electron temperature to be T_e = 7865 ± 360 K, where the uncertainty is the rms error expected.