The planetary nebula nature and properties of IRAS 18197−1118★†

Abstract

IRAS18197-1118 is a stellar-like object that has been classified as a planetary nebula from its radio continuum emission and high [SIII]9532 to Paschen9 line intensity ratio, as derived from direct images. We present intermediate- and high-resolution, optical spectroscopy, VLA 8.46 GHz radio continuum data, and narrow-band optical images of IRAS18197-1118 aimed at confirming its planetary nebula nature, and analyzing its properties. The optical spectrum shows that IRAS18197-1118 is a medium-excitation planetary nebula suffering a high extinction (c(H_beta) ~3.37). The optical images do not resolve the object but the 8.46 GHz image reveals an elliptical shell of ~2.7x1.6 arcsec^2 in size, a compact central nebular region, and possible bipolar jet-like features, indicating several ejection events. The existence of a compact central nebula makes IRAS18197-1118 singular because this kind of structure is observed in a few PNe only. An expansion velocity ~20 km/s and a systemic velocity (LSR) ~+95 km/s are obtained for the object. An electron density of ~3.4x10^4 cm-3 and an ionized mass of ~2.1x10^-2 M_sun are deduced from the 8.46 GHz radio continuum data for an estimated statistical distance of 6 kpc. Helium abundance is high but nitrogen is not enriched, which is not consistently reproduced by evolutionary models, suggesting different abundances in the elliptical shell and central region. The properties of IRAS18197-1118 indicate a relatively young planetary nebula, favor a distance of >~6 kpc, and strongly suggest that it is an inner-disc planetary nebula.