DETECTION OF THE H92-ALPHA RECOMBINATION LINE TOWARD THE VERY COMPACT PLANETARY-NEBULAE IC-5117 AND HB-12

VLA-A observations of the radio continuum at 3.6 cm and of the H92 alpha emission line in the compact (angular size similar or equal to 1'') young planetary nebulae Vy 2-2, Hb 12, and IC 5117 are presented. The radio continuum maps at 3.6 cm are similar to those previously observed at other wavelengths. H92 alpha emission has been detected from Hb 12 and IC 5117 at a level of 1 and 1.2 mJy beam(-1) respectively. No H92 alpha emission from Vy 2-2 has been detected with an upper limit of 0.6 mJy beam(-1). In Hb 12, two H92 alpha emission clumps separated by similar or equal to 0''.145 are observed. Each of the clumps is spatially unresolved (size less than or similar to 0''.13). In IC 5117 the H92 alpha line has been spatially and spectrally resolved. In this nebula, the radio continuum and H92 alpha maps are rather different from each other. Strong variations of the radial velocity are observed in angular scales of similar or equal to 0.''3. The observed spatio-kinematical properties suggest that the H92 alpha emission from IC 5117 arises in a cylindrical shell. For this planetary nebula we derive a kinematical age of similar or equal to 350 yr. By comparing the continuum and line emission data, the electron density, electron temperature, and ionized mass have been deduced for the three nebulae. In IC 5117 variations of the electron temperature and electron density are observed in scales of similar or equal to 0.''4. Our observations show that radio recombination lines can be detected in very compact planetary nebulae with high spatial resolution using the VLA in its A configuration. We think that this is a powerful method to study in great detail the spatio-kinematical structure and physical conditions in compact young planetary nebulae and to obtain information about the possible origin of the generation of anisotropic mass ejections in these objects.