Modern yields per stellar generation: the effect of the IMF
  Gaseous and stellar metallicities in galaxies are nowadays routinely used to
constrain the evolutionary processes in galaxies. This requires the knowledge
of the average yield per stellar generation, $y_{\text{Z}}$, i.e. the quantity
of metals that a stellar population releases into the interstellar medium
(ISM), which is generally assumed to be a fixed fiducial value. Deviations of
the observed metallicity from the expected value of $y_{\text{Z}}$ are used to
quantify the effect of outflows or inflows of gas, or even as evidence for
biased metallicity calibrations or inaccurate metallicity diagnostics. Here we
show that $\rm y_{\text{Z}}$ depends significantly on the Initial Mass Function
(IMF), varying by up to a factor larger than three, for the range of IMFs
typically adopted in various studies. Varying the upper mass cutoff of the IMF
implies a further variation of $y_{\text{Z}}$ by an additional factor that can
be larger than two. These effects, along with the variation of the gas mass
fraction restored into the ISM by supernovae ($R$, which also depends on the
IMF), may yield to deceiving results, if not properly taken into account. In
particular, metallicities that are often considered unusually high can actually
be explained in terms of yield associated with commonly adopted IMFs such as
the Kroupa (2001) or Chabrier (2003). We provide our results for two different
sets of stellar yields (both affected by specific limitations) finding that the
uncertainty introduced by this assumption can be as large as $\sim0.2$ dex.
Finally, we show that $y_{\text{Z}}$ is not substantially affected by the
initial stellar metallicity as long as $\text{Z}> 10^{-3}~\text{Z}_{\odot}$.
