A pilot search for mm-wavelength recombination lines from emerging ionized winds in pre-planetary nebulae candidates

We report the results from a pilot search for radio recombination line (RRL) emission at millimeter wavelengths in a small sample of pre-planetary nebulae (pPNe) and young PNe (yPNe) with emerging central ionized regions. Observations of the H30 alpha, H31 alpha, H39 alpha, H41 alpha, H48 beta, H49 beta, H51 beta, and H55 gamma lines at similar to 1 and similar to 3mm have been performed with the IRAM 30m radio telescope. These lines are excellent probes of the dense inner (less than or similar to 150 au) and heavily obscured regions of these objects, where the yet unknown agents for PN-shaping originate. We detected mm-RRLs in three objects: CRL 618, MWC922, and M2-9. For CRL 618, the only pPN with previous published detections of H41 alpha, H35 alpha, and H30 alpha emission, we find significant changes in the line profiles indicating that current observations are probing regions of the ionized wind with larger expansion velocities and mass-loss rate than similar to 29 yr ago. In the case of MWC922, we observe a drastic transition from single-peaked profiles at 3mm (H39 alpha and H41 alpha) to double-peaked profiles at 1mm (H31 alpha and H30 alpha), which is consistent with maser amplification of the highest frequency lines; the observed line profiles are compatible with rotation and expansion of the ionized gas, probably arranged in a disk + wind system around a similar to 5-10 M-circle dot central mass. In M2-9, the mm-RRL emission appears to be tracing a recent mass outburst by one of the stars of the central binary system. We present the results from non-LTE line and continuum radiative transfer models, which enables us to constrain the structure, kinematics, and physical conditions (electron temperature and density) of the ionized cores of our sample. We find temperatures T-e similar to 6000-17 000 K, mean densities n(e) similar to 10(5)-10(8) cm(-3), radial density gradients n(e) proportional to r(-alpha n) with alpha(n) similar to 2-3.5, and motions with velocities of similar to 10-30 km s(-1) in the ionized wind regions traced by these mm-wavelength observations. We deduce mass-loss rates of (M) over dot(pAGB) similar to 10(-6)-10(-7) M(circle dot)yr(-1), which are significantly higher than the values adopted by stellar evolution models currently in use and would result in a transition from the asymptotic giant branch to the PN phase faster than hitherto assumed.