1999A&A...351..533L

We searched for the lines of Tc in the spectra of Semiregular variables (SRVs) in the wavelength region from 4180 to 4300 Å using high resolution spectroscopy. Tc as an s-process element is produced on the thermally pulsing AGB and is therefore a good indicator for the evolutionary status of Semiregular variables. Combining our results with previous investigations we get a database large enough for a statistical study. Tc is not found in SRVs with periods below 100 days, spectral types earlier than M5 and photospheric IRAS colours. These objects are `blue' SRVs in the classification system of Kerschbaum & Hron (1994A&AS..106..397K). Among the `red' SRVs (periods longer than 100 days) the fraction of stars showing Tc in their spectra is about 15% with a probably lower fraction among the stars with periods above 150 days. This is significantly lower than for the typical Miras. Taking into account the probable conditions for the occurence of the third dredge-up and the expected behavior of the Tc abundance along an evolutionary track on the AGB, our results support an evolutionary scenario from `blue' SRVs (early AGB) to `red' SRVs (early TP-AGB) and on to long period Miras. Only the most massive (masses above 2M_☉) stars show Tc during the SRV stage. The luminosities of the Tc-rich SRVs and Miras are compatible with theoretical estimates of the minimum core mass required for the third dredge-up.