We have carried out a search for optically visible post-AGB candidates in the Small Magellanic Cloud ( SMC ) .
Firstly , we used mid-IR observations from the Spitzer Space Telescope to select optically visible candidates with excess mid-IR flux and then we obtained low-resolution optical spectra for 801 of the candidates .
After removing poor quality spectra and contaminants such as M-stars , C-stars , planetary nebulae , quasi-stellar objects and background galaxies , we ended up with a final sample of 63 high probability post-AGB/RGB candidates of A - F spectral type .
From the spectral observations , we estimated the stellar parameters : effective temperature ( T _ { eff } ) , surface gravity ( \log g ) , and metallicity ( [ Fe/H ] ) .
We also estimated the reddening and deduced the luminosity using the stellar parameters combined with photometry .
For the post-AGB/RGB candidates , we found that the metallicity distribution peaks at [ Fe/H ] \approx -1.00 dex .
Based on a luminosity criterion , 42 of these 63 sources were classified as post-RGB candidates and the remaining 21 as post-AGB candidates .
From the spectral energy distributions we were able to infer that 6 of the 63 post-AGB/RGB candidates have a surrounding circumstellar shell suggesting that they are single stars , while 27 of the post-AGB/RGB candidates have a surrounding disc , suggesting that they lie in binary systems .
For the remaining 30 post-AGB/RGB candidates the nature of the circumstellar environment was unclear .
Variability is displayed by 38 of the 63 post-AGB/RGB candidates with the most common variability types being the Population II Cepheids ( including RV-Tauri stars ) and semi-regular variables .
This study has also revealed a new RV Tauri star in the SMC , J005107.19-734133.3 , which shows signs of s -process enrichment .
From the numbers of post-AGB/RGB stars in the SMC , we were able to estimate evolutionary rates .
We find that the number of post-AGB and post-RGB candidates that we have identified are in good agreement with the stellar evolution models with some mass loss in the post-AGB phase and a small amount of re-accretion in the lower luminosity post-RGB phase .
This study also resulted in a new sample of 40 young stellar objects ( YSOs ) of A - F spectral type .
The 40 YSO candidates for which we could estimate stellar parameters are luminous and of high mass ( \sim 3 - 10 \mbox { $ M _ { \odot } $ } ) .
They lie on the cool side of the usually adopted birthline in the HR-diagram .
This line separates visually obscured protostars from optically visible pre-main sequence stars , meaning that our YSO candidates have become optically visible in the region of the HR-diagram usually reserved for obscured protostars .
Additionally , we also identified a group of 63 objects whose spectra are dominated by emission lines and in some cases , a significant UV continuum .
These objects are very likely to be either hot post-AGB/RGB candidates or luminous YSOs .