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

¹ Nicolaus Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
e-mail: szczerba@ncac.torun.pl
² Space Radio-Diagnostics Research Centre, University of Warmia and Mazury in Olsztyn, 10-719 Olsztyn, Poland
³ Main Astronomical Observatory, NASU, Zabolotnoho 27, 03680 Kyiv, Ukraine
⁴ Department of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383, USA
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁷ Laser Center, Faculty of Physics and Mathematics, University of Latvia, Raina bulvaris 19, LV-1586, Riga, Latvia
⁸ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland

Received: 4 March 2019
Accepted: 22 June 2020

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.