Importance of source structure on complex organics emission. I. Observations of CH${}_3$OH from low-mass to high-mass protostars

Abstract

Complex organic molecules (COMs) are often observed toward embedded Class 0 and I protostars. However, not all Class 0 and I protostars exhibit COMs emission. In this work, variations in methanol (CH${}_3$OH) emission are studied to test if absence of CH${}_3$OH emission can be linked to source properties. Combining both new and archival observations with ALMA and sources from the literature, a sample of 184 low-mass and high-mass protostars is investigated. The warm (T > 100 K) gaseous CH${}_3$OH mass, $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$, is determined for each source using primarily optically thin isotopologues. On average, Class I protostellar systems seem to have less warm $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$ ($<10^{-10}$ M${}_{\odot }$) than younger Class 0 sources ($\sim 10^{-7}$ M${}_{\odot }$). High-mass sources in our sample show higher warm $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$ up to $10^{-7}-10^{-3}$ M${}_{\odot }$. To take into account the effect of the source's overall mass on $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$, a normalized CH${}_3$OH mass is defined as $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}/M_{\mathrm{dust},0}$, where $M_{\mathrm{dust},0}$ is the cold + warm dust mass within a fixed radius. Excluding upper limits, a simple power-law fit to the normalized warm CH${}_3$OH masses results in $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}/M_{\mathrm{dust},0}\propto L_{\mathrm{bol}}^{0.70\pm 0.05}$. This is in good agreement with a simple hot core toy model which predicts that the normalized $M_{\mathrm{C}{\mathrm{H}}_3\mathrm{OH}}$ increases with $L_{\mathrm{bol}}^{0.75}$ due to the snowline moving outward. Sources for which the size of the disk is equivalent or smaller than the estimated 100 K radius agree well with the best-fit power-law model, whereas sources with significantly larger disks show up to two orders of magnitude lower normalized warm CH${}_3$OH masses. Based on the latter results, 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.

22 pages, 6 figures, accepted for publication in A&A

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