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Analysis of high spatial resolution VLA images shows that the free-freeemission from NGC7538 IRS1 is dominated by a collimated ionized wind. We havere-analyzed high angular resolution VLA archive data from 6 cm to 7 mm, andmeasured separately the flux density from the compact bipolar core and theextended (1.5" - 3") lobes. We find that the flux density of the core isproportional to the frequency to the power of alpha, with alpha being about0.7. The frequency dependence of the total flux density is slightly steeperwith alpha = 0.8. A massive optically thick hypercompact core with a steepdensity gradient can explain this frequency dependence, but it cannot explainthe extremely broad recombination line velocities observed in this source.Neither can it explain why the core is bipolar rather than spherical, nor theobserved decrease of 4% in the flux density in less than 10 years. An ionizedwind modulated by accretion is expected to vary, because the accretion flowfrom the surrounding cloud will vary over time. BIMA and CARMA continuumobservations at 3 mm show that the free-free emission still dominates at 3 mm.HCO+ J = 1 - 0 observations combined with FCRAO single dish data show a clearinverse P Cygni profile towards IRS1. These observations confirm that IRS1 isheavily accreting with an accretion rate of about 2 times 10(-4) solar massesper year.

NGC7538 IRS1 - an ionized jet powered by accretion