Validating post-AGB candidates in the LMC and SMC using SALT spectra

TL;DR

This study used SALT spectra to validate post-AGB candidates in the Magellanic Clouds, revealing that many candidates are misclassified and emphasizing the need for additional criteria for accurate identification.

Contribution

The paper demonstrates that low-resolution optical spectra can effectively identify genuine post-AGB stars among candidates selected by infrared colours, highlighting the limitations of colour-based selection alone.

Findings

Only 27% of observed candidates are genuine post-AGB stars.

Colour-colour selection overlaps with young stellar objects and planetary nebulae.

Optical spectra are sufficient for initial post-AGB classification.

Abstract

We selected a sample of post-AGB candidates in the Magellanic Clouds on the basis of their near- and mid-infrared colour characteristics. Fifteen of the most optically bright post-AGB candidates were observed with the South African Large Telescope in order to determine their stellar parameters and thus to validate or discriminate their nature as post-AGB objects in the Magellanic Clouds. The spectral types of absorption-line objects were estimated according to the MK classification, and effective temperatures were obtained by means of stellar atmosphere modelling. Emission-line objects were classified on the basis of the fluxes of the emission lines and the presence of the continuum. Out of 15 observed objects, only 4 appear to be genuine post-AGB stars (27\%). In the SMC, 1 out of 4 is post-AGB, and in the LMC, 3 out 11 are post-AGB objects. Thus, we can conclude that the selected region in the colour-colour diagram, while selecting the genuine post-AGB objects, overlaps severely with other types of objects, in particular young stellar objects and planetary nebulae. Additional classification criteria are required to distinguish between post-AGB stars and other types of objects. In particular, photometry at far-IR wavelengths would greatly assist in distinguishing young stellar objects from evolved ones. On the other hand, we showed that the low-resolution optical spectra appear to be sufficient to determine whether the candidates are post-AGB objects.