A radio continuum study of the `Engraved Hourglass' nebula, MyCn 18

We present the first radio continuum images of the `Engraved Hourglass' planetary nebula (PN) MyCn 18 from multifrequency observations with the Australia Telescope. The radio emission is strongly core-dominated at all frequencies investigated. At the three higher observing frequencies, the radio emission is seen to trace the optically-visible hourglass lobes. In the highest resolution image, the position of peak radio brightness is found in a region which we show to be geometrically off-centre in the same sense as the offset of the `central' star seen in the WFPC2 HST images of Sahai et al. The brightness temperatures measured are surprisingly low and this is interpreted as an effect of beam dilution and nebular clumpiness. We have attempted to separate the contributions to the integrated flux density from the lobes and the core and we present individual plots of the radio spectrum for these regions and also for the total nebular emission. These plots are used to obtain (lower, upper) limits to the turnover frequency of the core and the lobes, which in turn are used to derive (lower, upper) limits to the emission measure, density and ionized mass of the respective regions. Assuming the PN is at a distance of 2.4 kpc, we derive (lower, upper) limits to the total ionized mass of the PN of (0.2, 0.8) M_solar.