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We present the results of a 25-year program to monitor the radio fluxevolution of the planetary nebula NGC7027. We find significant evolution of thespectral flux densities. The flux density at 1465 MHz, where the nebula isoptically thick, is increasing at a rate of 0.251+-0.015 % per year, caused bythe expansion of the ionized nebula. At frequencies where the emission isoptically thin, the spectral flux density is changing at a rate of-0.145+-0.005 % per year, caused by a decrease in the number of ionizingphotons coming from the central star. A distance of 980+-100 pc is derived. Byfitting interpolated models of post-AGB evolution to the observed changes, wefind that over the 25-yr monitoring period, the stellar temperature hasincreased by 3900+-900 K and the stellar bolometric luminosity has decreased by1.75+-0.38 %. We derive a distance-independent stellar mass of 0.655+-0.01solar masses adopting the Bloecker stellar evolution models, or about 0.04solar masses higher when using models of Vassiliadis & Wood which may provide abetter fit. A Cloudy photoionization model is used to fit all epochs at allfrequencies simultaneously. The differences between the radio flux densitypredictions and the observed values show some time-independent residuals oftypically 1 %. A possible explanation is inaccuracies in the radio flux scaleof Baars et al. We propose an adjustment to the flux density scale of theprimary radio flux calibrator 3C286, based on the Cloudy model of NGC7027. Wealso calculate precise flux densities for NGC7027 for all standard continuumbands used at the VLA, as well as for some new 30GHz experiments.

The Evolution of NGC 7027 at Radio Frequencies: A New Determination of the Distance and Core Mass