Zooming on the emerging ionized regions of pPNe with ALMA

We report on recent results from our successful and pioneering observational program with ALMA to study emerging ultracompact HII regions of pre-planetary nebulae (pPNe) using mm-wavelength recombination lines (mRRLs) as new optimal tracers. We focus on our study of two poster-child pPNe, namely, M 2-9 and CRL 618. We reveal the structure and kinematics of the enigmatic inner nebular regions of these objects with an unprecedented angular resolution down to 20-30 mas (∼15-30 AU linear scales). For both targets, the ionized central regions are elongated along the main symmetry axis of the large-scale nebulae, consistent with bipolar winds, and show notable axial velocity gradients with expansion velocities of up to ∼100 kms. The intensity and width of the H30α profiles are found to be time variable, denoting changes on scales of a few years of the physical properties and kinematics of the present-day post-AGB ejections. Our ongoing analysis involves 3D, non-LTE radiative transfer modeling of the mRRLs and free-free continuum emission. This approach allows us to provide an exceptionally detailed description of the physical conditions in the innermost layers of these well known pPNe.