2015A&A...584A..28L

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 conditions of some molecular species can also be used to trace these physical conditions, making it possible to constrain for instance sources of heating and excitation. We study the range of influence of an intermediate-mass Herbig Be protostar. We also study the effect of feedback from the environment on the chemical and physical properties of embedded protostars. We followed up on an earlier line survey of the Class 0/I source R CrA IRS7B in the 0.8mm window with an unbiased line survey of the same source in the 1.3mm window using the Atacama Pathfinder Experiment (APEX) telescope. We also studied 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 were employed to establish abundances of the molecular species. 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 (the latter two are only tentatively detected). CH₃CN and several other complex organic molecules are significantly under-abundant in comparison with what is found towards hot corino protostars. The H₂CO rotational 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. The high H₂CO temperatures observed towards objects close to R CrA suggest that this star has a sphere of influence of several 10000AU in which it increases the temperature of the molecular gas to 30-50K 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.