Technetium-rich M stars: Prime diagnostics of recent third dredge-up events on the asymptotic giant branch

Context. Technetium (Tc)-rich M stars have been known for more than 45 years. However, the origin of Tc in these stars, particularly its detection without the concomitant detection of other s -process elements, which are typically produced during asymptotic giant branch (AGB) s -process nucleosynthesis, remains poorly understood. Technetium was first identified in the spectra of S stars (which exhibit prominent ZrO bands) in 1952. The simultaneous enrichment of both Zr and Tc is well understood within the framework of s -process nucleosynthesis, which occurs during the AGB phase. However, Tc-rich M stars remain an enigma, as M stars are typically not enriched in heavy elements. Aims. This study aims to analyse high-resolution spectra of a large sample of M stars to examine their spectral characteristics, and to compare their spectral properties with those of Tc-rich S stars in an attempt to understand the origin of their difference. Methods. We defined a robust classification scheme to assign M stars to the Tc-rich or Tc-poor class. We computed nucleosynthesis models to trace the evolution of Zr and Tc abundances across successive thermal pulses during the AGB phase. We further analysed spectral indices measuring the depth of the TiO and ZrO bands, as well as the wavelength of the Tc blend, on both synthetic and observed spectra. Results. The Tc lines in Tc-rich M stars are similar to those in S stars. However, Tc-rich M stars exhibit stronger TiO bands than S stars, while displaying similarly strong ZrO bands. Spectral synthesis, together with location in the Hertzsprung–Russell (HR) diagram and spatial properties, suggests that Tc-rich M stars may have slightly lower metallicity and lower masses than Tc-rich S stars.