A search for very young Planetary Nebulae

Abstract

Abstract. Despite numerous efforts, the transition from Asymptotic Giant Branch (AGB) stars to Planetary Nebulae (PN) is a poorly understood phase of stellar evolution. We have therefore carried out interferometric (VLA) radio observations of a sample of hot post-AGB stars, selected on the basis of their optical and infrared properties. Ten sources out of the 16 observed were detected. This indicates that most of our targets are surrounded by a nebula where the ionization has already started. This definitively determines the evolutionary status of the selected sources and provides us with a unique sample of very young Planetary Nebulae (yPNe). A comparison with another sample of yPNe confirms our working hypothesis that our targets are indeed very young, probably just in the transition toward PN. Finding transition objects is extremely important as they can provide unique clues for a better understanding of this important phase of stellar evolution.

Chemical abundances of hot post-AGB stars

“…Using lowresolution UV(IUE) spectra, Gauba & Parthasarathy (2003) estimated the effective temperature and surface gravity as being 17 000 K and 3.4 dex, respectively. In their search for young PNe, Umana et al (2004) did not detect signs of nebular radio emission for IRAS 17074-1845, indicating that the surrounding nebula was not ionized by the central star yet. Suarez et al (2006) classified LSE 3 as a transition object based on its optical low-resolution spectra.…”

“…Parthasarathy et al (2000) detected H β and H γ in emission and classified LSE 63 as a hot post-AGB star based on far-infrared colours similar to PNe, high galactic latitude and B supergiant spectral type (B1Iabe). Umana et al (2004) the presence of a low-excitation nebula. Gauba & Parthasarathy (2003) detected a low circumstellar extinction and, considering a spectral type of B1Iabe, they estimated the effective temperature and surface gravity to be 20 800 K and 2.9 dex, respectively.…”

“…In their paper, Suarez et al (2006) classified this source as a post-AGB star. Umana et al (2004) have detected no nebular radio emission surrounding the central star in their search for very young PNe.…”

“…Crampton (1971) classified it as a O7.5 star associated to the HII region Sharpless15 based on UBV photometry. Ratag et al (1990) and Umana et al (2004) detected nebular radio emission typical of PNe using the VLA telescopes. Based on IRAS fluxes and spectral type, Parthasarathy et al (2000) classified it as O8III hot post-AGB star.…”

Imaging the transition between pre-planetary and planetary nebulae: integral field spectroscopy of hot post-AGB stars with NIFS

“…These authors also present a DUSTY model of the continuum and SED, using a silicate-only dust composition, but a hotter stellar temperature (T * =24 000 K) than Gauba & Parthasarathy (2004) (T * =19 000 K). Interestingly the object is not detected in the radio at 3.6 cm (Umana et al 2004). …”