CRL 618 is an object that exhibits characteristics of both AGB and post-AGB star. It also displays a spectacular array of bipolar lobes with a dense equatorial region, which makes it an excellent object to study the development of asymmetries in evolved stars. In the recent decades, an elliptical compact HII region located in the center of the nebula has been seen to be increasing in size and flux. This seems to be due to the ionization of the circumstellar envelope by the central star, and it would be indicating the beginning of the planetary nebula phase for CRL 618. We analyzed interferometric radio continuum data at ~5 and 22 GHz from observations carried out at seven epochs with the VLA. We traced the increase of the flux of the ionized region over a period of ~26 years. We measured the dimensions of the HII region directly from the brightness distribution images to determine the increase of its size over time. For one of the epochs we analyzed observations at six frequencies from which we estimated the electron density distribution. We carried out model calculations of the spectral energy distribution at two different epochs to corroborate our observational results. We found that the radio continuum flux and the size of the ionized region have been increasing monotonically in the last three decades. The size of the major axis of the HII region shows a dependance with frequency, which has been interpreted as a result of a gradient of the electron density in this direction. The growth of the HII region is due to the expansion of an ionized wind whose mass-loss rate increased continuously for a period of ~100 years until a few decades ago, when the mass-loss rate experienced a sudden decline. Our results indicate that the beginning of the ionization of the circumstellar envelope began around 1971, which marks the start of the planetary nebula phase of CRL 618.
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