Spectroscopic surveys of massive AGB and super-AGB stars
  It is now about 30 years ago that photometric and spectroscopic surveys of
asymptotic giant branch (AGB) stars in the Magellanic Clouds (MCs) uncovered
the first examples of truly massive (> 3-4 M_s) O-rich AGB stars experiencing
hot bottom burning (HBB). Massive (Li-rich) HBB AGB stars were later identified
in our own Galaxy and they pertain to the Galactic population of obscured OH/IR
stars. High-resolution optical spectroscopic surveys have revealed the massive
Galactic AGB stars to be strongly enriched in Rb compared to other nearby
s-process elements like Zr, confirming that Ne22 is the dominant neutron source
in these stars. Similar surveys of OH/IR stars in the MCs disclosed their
Rb-rich low-metallicity counterparts, showing that these stars are usually
brighter (because of HBB flux excess) than the standard adopted luminosity
limit for AGB stars (Mbol~-7.1) and that they might have stellar masses of at
least ~6-7 M_s. The chemical composition and photometric variability are
efficient separating the massive AGB stars from massive red supergiants (RSG)
but the main difficulty is to distinguish between massive AGB and super-AGB
stars because the present theoretical nucleosynthesis models predict both stars
to be chemically identical. Here I review the available multiwavelength (from
the optical to the far-IR) observations on massive AGB and super-AGB stars as
well as the current caveats and limitations in our undestanding of these stars.
Finally, I underline the expected observations on massive AGB and super-AGB
stars from on-going massive surveys like Gaia and SDSS-IV/APOGEE-2 and future
facilities such as the James Webb Space Telescope.
