K-band spectra of selected post-AGB candidates

TL;DR

This study presents new K-band spectra of 12 post-AGB star candidates, revealing emission lines indicating photo-ionization and molecular excitation, and suggests rapid evolution towards planetary nebulae for several objects.

Contribution

First-time medium resolution K-band spectra of 12 post-AGB candidates, providing insights into their ionization and molecular excitation states and evolutionary status.

Findings

Brγ emission detected in seven objects indicating photo-ionization.

H2 emission lines suggest UV radiative excitation from hot central stars.

Some objects show spectral variability and rapid evolution towards planetary nebula phase.

Abstract

We present medium resolution (1000) K-band spectra of 12 post-AGB candidates and related stars. For several objects in our sample, these spectra were obtained for the first time. The Br$\gamma$ line in emission is detected in seven objects indicating the onset of photo-ionization in these objects. Four objects show the presence of He i line. We detect H2 emission line in the spectra of IRAS 06556+1623, IRAS 22023+5249, IRAS 18062+2410 and IRAS 20462+3416. H2 emission line ratio 1-0 S(1)/2-1 S(1) indicate that H2 is radiatively excited due to the UV radiation of hot post-AGB central stars. When compared with the recent observations by other investigators, the Br$\gamma$ and H2 emission fluxes varied in some of the objects. The hot post-AGB stars IRAS 22495+5134, IRAS 22023+5249, IRAS 18062+2410 and IRAS 20462+3416 seem to be evolving rapidly to young low excitation planetary nebula phase. The spectra of the objects presented in this paper may be useful for future observers as some of these stars show spectral variation and since the post-AGB evolution of some of these stars is relatively rapid.