The ALMA-PILS survey: First tentative detection of 3-hydroxypropenal
  (HOCHCHCHO) in the interstellar medium and chemical modeling of the
  C$_3$H$_4$O$_2$ isomers
  Characterizing the molecular composition of solar-type protostars is useful
for improving our understanding of the physico-chemical conditions under which
the Sun and its planets formed. In this work, we analyzed the Atacama Large
Millimeter/submillimeter Array (ALMA) data of the Protostellar Interferometric
Line Survey (PILS), an unbiased spectral survey of the solar-type protostar
IRAS~16293--2422, and we tentatively detected 3-hydroxypropenal (HOCHCHCHO) for
the first time in the interstellar medium towards source B. Based on the
observed line intensities and assuming local thermodynamic equilibrium, its
column density is constrained to be $\sim$10$^{15}$ cm$^{-2}$, corresponding to
an abundance of 10$^{-4}$ relative to methanol, CH$_3$OH. Additional
spectroscopic studies are needed to constrain the excitation temperature of
this molecule. We included HOCHCHCHO and five of its isomers in the chemical
network presented in Manigand et al. (2021) and we predicted their chemical
evolution with the Nautilus code. The model reproduces the abundance of
HOCHCHCHO within the uncertainties. This species is mainly formed through the
grain surface reaction CH$_2$CHO + HCO $\rightarrow$ HCOCH$_2$CHO, followed by
the tautomerization of HCOCH$_2$CHO into HOCHCHCHO. Two isomers, CH$_3$COCHO
and CH$_2$COHCHO, are predicted to be even more abundant than HOCHCHCHO.
Spectroscopic studies of these molecules are essential in searching for them in
IRAS~16293--2422 and other astrophysical sources.
