The chemical structure of the Class 0 protostellar envelope NGC 1333
  IRAS 4A
  It is not well known what drives the chemistry of a protostellar envelope, in
particular the role of the stellar mass and the outflows on its chemical
enrichment. We study the chemical structure of NGC 1333 IRAS 4A in order to (i)
investigate the influence of the outflows on the chemistry, (ii) constrain the
age of our object, (iii) compare it with a typical high-mass protostellar
envelope. In our analysis we use JCMT line mapping and HIFI pointed spectra. To
study the influence of the outflow on the degree of deuteration, we compare
JCMT maps of HCO+ and DCO+ with non-LTE (RADEX) models in a region that
spatially covers the outflow activity of IRAS 4A. To study the envelope
chemistry, we derive empirical molecular abundance profiles for the observed
species using the radiative transfer code (RATRAN) and adopting a 1D dust
density/temperature profile from the literature. We compare our best-fit
observed abundance profiles with the predictions from the time dependent gas
grain chemical code (ALCHEMIC). The CO, HCN, HNC and CN abundance require an
enhanced UV field which points towards an outflow cavity. The abundances (wrt
H2) are 1 to 2 orders of magnitude lower than those observed in the high mass
protostellar envelope (AFGL 2591), while they are found to be similar within
factors of a few with respect to CO. Differences in UV radiation may be
responsible for such chemical differentiation, but temperature differences seem
a more plausible explanation. The CH3OH modeled abundance profile points
towards an age of > 4x10^4 yrs for IRAS 4A. The spatial distribution of H2D+
differs from that of other deuterated species, indicating an origin from a
foreground colder layer (<20 K). The observed abundances can be explained by
passive heating towards the high mass protostellar envelope, while the presence
of UV cavity channels become more important toward the low mass protostellar
envelope.
