Such a chemical differentiation has been reported in other multiple systems, like the low-mass protostars IRAS16293-2422 (2004a; 2016; 2011) and IRAS4A (Santangelo et al. 2015; López-Sepulcre et al. 2017), and, recently, towards the intermediate-mass protostars NGC 2264 CMM3 (Watanabe et al. 2017). With four examples at hand, we speculate that it could be a general feature of multiple protostellar systems and not a “pathological anomaly”. There is no systematic trend between the millimetre thermal dust and molecular line emissions. Towards IRAS16293-2422 and IRAS4A, a rich content in COMs is observed towards the source with the less massive continuum source. Towards NGC 2264 CMM3, it is the most massive continuum component that displays a rich molecular content. The case of Cep E-mm appears similar to the latter one. High-mass star forming regions (HMSFRs) also present a rich chemical diversity. One of the best known examples is provided by Orion-KL. This source harbours: (i) a dichotomy between the spatial distribution of complex O-bearing and complex N-bearing species, with the latter species probing the hotter gas (see, e.g. Guélin et al. 2008; Favre et al. 2011; Friedel & Widicus Weaver 2012; Peng et al. 2013; Brouillet et al. 2013; Crockett et al. 2014, 2015) but also (ii) differences between supposed chemically related species (see, e.g. Favre et al. 2017; Pagani et al. 2017). Other examples are provided by W3(OH) (Qin et al. 2015; Nishimura et al. 2017) and SgrB2 (Belloche et al. 2008, 2013). In an observational study of four HMSFRs (Orion KL, G29.96, IRAS 23151+5912, and IRAS 05358+3543), Beuther et al. (2009) showed that the properties of CH3OH can be easily accounted for by the physical conditions (temperature) in the cores, whereas the N-bearing species appear to be more selective as they are detected only towards the sources at the (evolved) hot core stage. Recently, in an ALMA study of the filamentary HMSFR G35.20, Sánchez-Monge et al. (2014) found that only three out of the six continuum cores of the filament display COM emission typical of hot cores. Several hypotheses have been proposed to account for the observed chemical differentiation. López-Sepulcre et al. (2017) proposed that the COM-rich protostar is either more massive and/or subject to a higher accretion rate, resulting in a lower envelope mass. Watanabe et al. (2017) suggest that the less massive protostar is related to a younger evolutionary stage in which the hot corino (hot core) is not yet developed, meaning that its dimensions are still very small. In Cep E-mm, the presence of high-velocity SiO jets provides evidence of active mass ejection around both protostars. The short dynamical timescales (500–1000 yr) also indicate that these ejections began recently, meaning that both sources are still in an early evolutionary stage. Incidentally, Lykke et al. (2015) found an apparent correlation between the source luminosities and the relative abundance of complex organic molecules in a sample of sources including high-mass protostars. The authors have suggested that this could be the result of the timescale and the temperature experienced by a source during its evolution. The sample of sources with evidence for chemical differentiation should be increased in order to confirm this observational trend.
Citation: Brouillet et al. 2013
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