Chemical abundance gradients of organic molecules within a protostellar disk

Observations of low-mass protostellar systems show evidence of rich complex organic chemistry. Their low luminosity, however, makes determining abundance distributions of complex organic molecules (COMs) within the water snowline challenging. However, the excitation conditions sampled by differing molecular distributions may produce substantive changes in the resulting emission. Thus, molecular excitation may recover spatial information from spatially unresolved data. By analyzing spatially-unresolved NOrthern Extended Millimeter Array (NOEMA) observations of CH₃OH and CH₃CN, we aim to determine if CH₃OH and CH₃CN are distributed differently in the protostellar disk around HOPS-370, a highly-luminous intermediate mass protostar. Rotational diagram analysis of CH₃OH and CH₃CN yields rotational temperatures of 198 1. 2 K and 448 19 K, respectively, suggesting the two molecules have different spatial distributions. Source-specific 3D LTE radiative transfer models are used to constrain the spatial distribution of CH₃OH and CH₃CN within the disk. A uniform distribution with an abundance of 410^-8 reproduces the CH₃OH observations. In contrast, the spatial distribution of CH₃CN needs to be either more compact (within 120 au versus 240 au for CH₃OH) or exhibiting a factor of 15 increase in abundance in the inner 55 au. A possible explanation for the difference in spatial abundance distributions of CH₃OH and CH₃CN is carbon-grain sublimation.