Probing the effects of external irradiation on low-mass protostars through unbiased line surveys

TL;DR

This study investigates how external irradiation from a nearby Herbig Be star influences the chemistry and physical conditions of low-mass protostars, revealing temperature gradients and chemical differences caused by environmental feedback.

Contribution

It provides the first unbiased line survey of a low-mass protostar in the 1.3 mm window and analyzes the impact of external irradiation on protostellar chemistry and temperature.

Findings

H2CO temperatures decrease with distance from R CrA

External irradiation extends influence to several 10,000 AU

Protostellar chemistry shows significant differences due to irradiation

Abstract

(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.