$\mathit{Chempy}$: A flexible chemical evolution model for abundance
  fitting - Do the Sun's abundances alone constrain chemical evolution models?
  Elemental abundances of stars are the result of the complex enrichment
history of their galaxy. Interpretation of observed abundances requires
flexible modeling tools to explore and quantify the information about Galactic
chemical evolution (GCE) stored in such data. Here we present Chempy, a newly
developed code for GCE modeling, representing a parametrized open one-zone
model within a Bayesian framework. A Chempy model is specified by a set of 5-10
parameters that describe the effective galaxy evolution along with the stellar
and star-formation physics: e.g. the star-formation history, the feedback
efficiency, the stellar initial mass function (IMF) and the incidence of
supernova type Ia (SN Ia). Unlike established approaches, Chempy can sample the
posterior probability distribution in the full model parameter space and test
data-model matches for different nucleosynthetic yield sets. We extend Chempy
to a multi-zone scheme. As an illustrative application, we show that
interesting parameter constraints result from only the ages and elemental
abundances of Sun, Arcturus and the present-day interstellar medium (ISM). For
the first time, we use such information to infer IMF parameter via GCE
modeling, where we properly marginalize over nuisance parameters and account
for different yield sets. We find that of the IMF $11.6_{-1.6}^{+2.1}$ %
explodes as core-collapse SN, compatible with Salpeter 1955. We also constrain
the incidence of SN Ia per 10^3 Msun to 0.5-1.4. At the same time, this Chempy
application shows persistent discrepancies between predicted and observed
abundances for some elements, irrespective of the chosen yield set. These
cannot be remedied by any variations of Chempy's parameters and could be an
indication for missing nucleosynthetic channels. Chempy should be a powerful
tool to confront predictions from stellar nucleosynthesis with far more complex
abundance data sets.
