Probing the effects of external irradiation on low-mass protostars
  through unbiased line surveys
  (abridged) Context: The envelopes of molecular gas around embedded low-mass
protostars show different chemistries, which can be used to trace their
formation history and physical conditions. The excitation of some molecular
species can also be used to trace these physical conditions, making it possible
to constrain e.g. sources of heating and excitation.
  Aims: To study the range of influence of an intermediate-mass Herbig Be
protostar, and to find what chemical and physical impact feedback effects from
the environment may have on embedded protostars.
  Methods: We follow up on an earlier line survey of the Class 0/I source R CrA
IRS7B in the 0.8 mm window with an unbiased line survey of the same source in
the 1.3 mm window using the APEX telescope. We also study the excitation of the
key species H2CO, CH3OH, and c-C3H2 in a complete sample of the 18 embedded
protostars in the Corona Australis star-forming region. Radiative transfer
models are used to establish abundances of the molecular species.
  Results: We detect line emission from 20 molecular species (32 including
isotopologues) in the two surveys. The most complex species detected are CH3OH,
CH3CCH, CH3CHO, and CH3CN. Several complex organics are significantly
under-abundant in comparison with "hot corino" protostars. The H2CO
temperatures of the sources in the region decrease with the distance to the
Herbig Be star R CrA, whereas the c-C3H2 temperatures remain constant across
the star-forming region.
  Conclusions: The high H2CO temperatures observed towards objects close to R
CrA suggest that this star has a sphere of influence of several 10000 AU in
which it increases the temperature of the molecular gas to 30-50 K through
irradiation. The chemistry in the IRS7B envelope differs significantly from
many other embedded protostars, which could be an effect of the external
irradiation from R CrA.
