Aims: We report the discovery that Mira variables with and without absorption
lines of the element technetium (Tc) occupy two different regions in a diagram
of near- to mid-infrared colour versus pulsation period. Tc is an indicator of
a recent or ongoing mixing event called the third dredge-up (3DUP), and the
near- to mid-IR colour, such as the (K-[22]) colour where [22] is the the 22
micron band of the WISE space observatory, is an indicator of the dust
mass-loss rate of a star. Methods: We collected data from the literature about
the Tc content, pulsation period, and near- and mid-infrared magnitudes of more
than 190 variable stars on the asymptotic giant branch (AGB) to which Miras
belong. The sample is naturally biased towards optical AGB stars, which have
low to intermediate (dust) mass-loss rates. Results: We show that a clear
relation between dust mass-loss rate and pulsation period exists if a
distinction is made between Tc-poor and Tc-rich Miras. Surprisingly, at a given
period, Tc-poor Miras are redder in (K-[22]) than are Tc-rich Miras; i.e. they
have higher mass-loss rates than the Tc-rich Miras. A few stars deviate from
this trend; physical explanations are given for these exceptions, such as
binarity or high mass. Conclusions: We put forward two hypotheses to explain
this dichotomy and conclude that the two sequences formed by Tc-poor and
Tc-rich Miras are probably due to the different masses of the two groups. The
pulsation period has a strong correlation with the dust-mass loss rate,
indicating that the pulsations are indeed triggering a dust-driven wind. The
location in the (K-[22]) vs. period diagram can be used to distinguish between
pre- and post-3DUP Miras, which we apply to a sample of Galactic bulge AGB
stars. We find that 3DUP is probably not common in AGB stars in the inner
bulge.Comment: Accepted for publication in A&A (5 pages, 2 figures, 1 on-line
table); final version (language corrected
