The ALMA-PILS survey: the sulphur connection between protostars and comets: IRAS 16293–2422 B and 67P/Churyumov–Gerasimenko

Abstract

The evolutionary past of our Solar System can be pieced together by comparing analogous low-mass protostars with remnants of our Protosolar Nebula - comets. Sulphur-bearing molecules may be unique tracers of the joint evolution of the volatile and refractory components. ALMA Band 7 data from the large unbiased Protostellar Interferometric Line Survey (PILS) are used to search for S-bearing molecules in the outer disc-like structure, 60 au from IRAS 16293-2422 B, and are compared with data on 67P/C-G stemming from the ROSINA instrument aboard Rosetta. Species such as SO$_{2}$, SO, OCS, CS, H$_{2}$CS, H$_{2}$S and CH$_{3}$SH are detected via at least one of their isotopologues towards IRAS 16293-2422 B. The search reveals a first-time detection of OC$^{33}$S towards this source and a tentative first-time detection of C$^{36}$S towards a low-mass protostar. The data show that IRAS 16293-2422 B contains much more OCS than H$_{2}$S in comparison to 67P/C-G; meanwhile, the SO/SO$_{2}$ ratio is in close agreement between the two targets. IRAS 16293-2422 B has a CH$_{3}$SH/H$_{2}$CS ratio in range of that of our Solar System (differences by a factor of 0.7-5.3). It is suggested that the levels of UV radiation during the initial collapse of the systems may have varied and have potentially been higher for IRAS 16293-2422 B due to its binary nature; thereby, converting more H$_{2}$S into OCS. It remains to be conclusively tested if this also promotes the formation of S-bearing complex organics. Elevated UV levels of IRAS 16293-2422 B and a warmer birth cloud of our Solar System may jointly explain the variations between the two low-mass systems.