Interstellar detection and chemical modeling of iso-propanol and its
  normal isomer
  The detection of a branched alkyl molecule in the high-mass star forming
protocluster Sgr B2(N) permitted by the advent of ALMA revealed a new dimension
of interstellar chemistry. Astrochemical simulations subsequently predicted
that beyond a certain degree of molecular complexity, branched molecules could
even dominate over their straight-chain isomers. More generally, we aim at
probing further the presence in the ISM of complex organic molecules with the
capacity to exhibit both a normal and iso form, via the attachment of a
functional group to either a primary or secondary carbon atom. We used the
imaging spectral line survey survey ReMoCA performed with ALMA and the results
of a recent spectroscopic study of propanol to search for the iso and normal
isomers of this molecule in the hot molecular core Sgr B2(N2). We expanded the
network of the astrochemical model MAGICKAL to explore the formation routes of
propanol. We report the first interstellar detection of iso-propanol toward a
position of Sgr B2(N2) that shows narrow linewidths. We also report the first
secure detection of normal-propanol in a hot core. i-Propanol is found to be
nearly as abundant as n-propanol, with an abundance ratio of 0.6 similar to the
ratio of 0.4 that we obtained previously for i- and n-propyl cyanide in Sgr
B2(N2). The results are in good agreement with the outcomes of our
astrochemical models, which indicate that OH-radical addition to propylene in
dust-grain ice mantles, driven by water photodissociation, can produce
appropriate quantities of n- and i-propanol. The n-to-i ratio in Sgr B2(N2) may
be a direct inheritance of the branching ratio of this reaction process. The
detection of n- and i-propanol and their ratio indicate that the modest
preference toward the normal form of propyl cyanide determined previously may
be a more general feature among similarly sized interstellar molecules.
[abridged]
