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

VLA-A observations of the radio continuum at 3.6 cm and of the H92 alp ha 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 observ ed 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 H 92 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 b y similar or equal to 0''.145 are observed. Each of the clumps is spat ially unresolved (size less than or similar to 0''.13). In IC 5117 the H92 alpha line has been spatially and spectrally resolved. In this ne bula, 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-kinem atical properties suggest that the H92 alpha emission from IC 5117 ari ses in a cylindrical shell. For this planetary nebula we derive a kine matical age of similar or equal to 350 yr. By comparing the continuum and line emission data, the electron density, electron temperature, an d ionized mass have been deduced for the three nebulae. In IC 5117 var iations of the electron temperature and electron density are observed in scales of similar or equal to 0.''4. Our observations show that rad io recombination lines can be detected in very compact planetary nebul ae 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 s patio-kinematical structure and physical conditions in compact young p lanetary nebulae and to obtain information about the possible origin o f the generation of anisotropic mass ejections in these objects.