An ALMA zoomed-in journey to explore the emerging ionized regions of pre-Planetary Nebulae

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).  By utilizing mm-wavelength recombination lines (mm-RRLs) as novel tracers, we are able to delve deeper than ever before into the inner workings of these fascinating objects. I will focus on our study of two poster-child pPNe, namely, M 2-9 and CRL 618. We unveil the structure and kinematics of the elusive inner nebular regions of these objects with an unprecedented angular resolution of 20-30 mas (i.e., down to ~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 ~200 km/s. The intensity and width of the H30alpha profiles are found to be time variable, denoting changes on scales of a few years of the physical properties and kinematics of the on-going post-AGB ejections. We have modelled our observations using the 3D non-LTE radiative transfer code co3RaL (by D. Tafoya). This has allowed us to describe with unparalleled detail the physical conditions in the inner layers of these iconic pPNe, which are key to understand the development of multi-scale asymmetries in these stages.