Importance of source structure on complex organics emission

Abstract

Context. Complex organic molecules (COMs) are often observed toward embedded Class 0 and I protostars. However, not all Class 0 and I protostars exhibit COM emission. Aims. The aim is to study variations in methanol (CH3OH) emission and use this as an observational tracer of hot cores to test if the absence of CH3OH emission can be linked to source properties. Methods. A sample of 148 low-mass and high-mass protostars is investigated using new and archival observations with the Atacama Large Millimeter/submillimeter Array (ALMA) that contain lines of CH3OH and its isotopologues. Data for an additional 36 sources are added from the literature, giving a total of 184 different sources. The warm (T ≳ 100 K) gaseous CH3OH mass, MCH3OH, is determined for each source using primarily optically thin isotopologues and is compared to a simple toy model of a spherically symmetric infalling envelope that is passively heated by the central protostar. Results. A scatter of more than four orders of magnitude is found for MCH3OH among the low-mass protostars, with values ranging between 10−7 M⊙ and ≲10−11 M⊙. On average, Class I protostellar systems seem to have less warm MCH3OH(≲10−10 M⊙) than younger Class 0 sources (~10−7 M⊙). High-mass sources in our sample show more warm MCH3OH, up to ~10−7−10−3 M⊙. To take into account the effect of the source’s overall mass on MCH3OH, a normalized CH3OH mass is defined as MCH3OH/Mdust,0, where Mdust,0 is the cold plus warm dust mass in the disk and inner envelope within a fixed radius measured from the ALMA dust continuum. A correlation between MCH3OH/Mdust,0 and Lbol is found. Excluding upper limits, a simple power-law fit to the normalized warm CH3OH masses results in MCH3OH/Mdust,0 ∝ Lbol0.70±0.05 over an Lbol range of 10−1−106 L⊙. This is in good agreement with the toy model, which predicts that the normalized MCH3OH increases with Lbol0.70 due to the snow line moving outward. Sources for which the size of the disk is equivalent to or smaller than the estimated 100 K radius fall within the 3σ range of the best-fit power-law model, whereas sources with significantly larger disks show normalized warm CH3OH masses that are up to two orders of magnitude lower. Conclusions. The agreement between sources that are rich in CH3OH with the toy model of a spherically symmetric infalling envelope implies that the thermal structure of the envelopes in these sources is likely not strongly affected by a disk. However, based on the disagreement between the toy model and sources that show less warm CH3OH mass, we suggest that source structure such as a disk can result in colder gas and thus fewer COMs in the gas phase. Additionally, optically thick dust can hide the emission of COMs. Advanced modeling is necessary to quantify the effects of a disk and/or continuum optical depth on the presence of gaseous COMs in young protostellar systems.