Perley, R. A. ; Zijlstra, A. ; van Hoof, P.

published in Bulletin of the American Astronomical Society, 38, pp. 92.02 (2006)

Abstract: NGC 7027 is the planetary nebula with the highest flux density in the radio spectrum, and is widely used as a flux density calibrator. We present the results of 23 years of precise monitoring of its flux density, using the VLA, at seven frequency bands between 1.4 and 43 GHz. At 1.4 GHz, we find the flux density is uniformly increasing at a rate of 0.24 percent/year. This directly measures the expansion of the nebula. The expansion parallax gives a distance of 800 pc. The optically thin flux density is decreasing at a rate of 0.15 percent/year. This is attributed to a decreasing number of ionizing photons as the star increases in temperature. Based on Rauch models, the temperature increase is found to be 280 +/25 K/year, assuming constant luminosity. This increase is very sensitive to stellar mass -we determine a value of 0.675 +/.002 solar masses. This is the first measurement of the remnant mass of a carbon star.

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