2002MNRAS.336.1139G

We present high angular resolution (0.1 arcsec) VLA observations at 7-mm wavelength towards the bipolar planetary nebula IC 4997. The 7-mm emission in IC 4997 is dominated by a bright, compact core, the flux density of which at this wavelength is about six times brighter than at 2.0 and 3.6 cm. We model the centimetre and millimetre continuum spectrum in terms of free-free emission from an ionized core-halo source. In this model a dense (2.6x10⁶ cm ^–3), optically thick inner core dominates the millimetre emission, while a low-density (1x10⁴ cm ^–3), optically thin halo dominates the centimetre emission. The spectral index of the core (between 2 cm and 7 mm) is α≃ 1.6±0.1(S_ν∝ν ^α), which is consistent with optically thick free-free emission. An upper limit for the mass-loss rate that created this inner core is ∼ 3x10^–6 M_☉yr ^–1, suggesting that this core may have been formed at the end of the superwind phase.