Modeling snowline locations in protostars: The impact of the structure of protostellar cloud cores
Authors: Murillo N. M. , Hsieh T.-H. , Walsh C. 
Published: June 2022

Context. Snowlines during star and disk formation are responsible for a range of effects during the evolution of protostars, such as setting the chemical composition of the envelope and disk. This in turn influences the formation of planets by changing the elemental compositions of solids and affecting the collisional properties and outcomes of dust grains. Snowlines can also reveal echoes of past accretion bursts, providing insight into the formation process of stars.

Aims. The objective is to identify which parameters (e.g., luminosity, gas density, and presence of disk) dictate the location of snowlines during the early, deeply embedded phase and to quantify how each parameter changes the observed snowline location.

Methods. A numerical chemical network coupled with a grid of cylindrical-symmetric physical models was used to identify what physical parameters alter the CO and H2O snowline locations. The investigated parameters are the initial molecular abundances, binding energies of CO and H2O, heating source, cloud core density, outflow cavity opening angle, and disk geometry. Simulated molecular line emission maps were used to quantify the change in the snowline location with each parameter.

Results. The snowline radius of molecules with low sublimation temperatures (≲30 K), such as CO, shift outward on the order of 103 AU with an order of magnitude increase in protostellar luminosity. An order of magnitude decrease in cloud core density also shifts the CO snowline position outward by a few 103 AU. The presence of disk(-like) structures cause inward shifts by a factor of a few, and mainly along the disk mid-plane. For molecules that sublimate at higher temperatures, such as H2O, increasing the protostellar luminosity or decreasing the cloud core density by an order of magnitude shifts the snowline position outward by a factor of a few. The presence of a disk concentrates molecules with high sublimation temperatures to compact regions (a few 10 AU) around the protostar by limiting the outward shift of snowline positions. Successful observational measurements of snowline locations are strongly dependent on spatial resolution, the presence or lack thereof of disk(-like) structures, and the inclination of the disk(-like) structure.

Conclusions. The CO and H2O snowline locations do not occur at a single, well-defined temperature as is commonly assumed. Instead, the snowline position depends on luminosity, cloud core density, and whether a disk is present or not. Inclination and spatial resolution affect the observability and successful measurement of snowline locations. We note that N2H+ and HCO+ emission serve as good observational tracers of CO and H2O snowline locations. However, constraints on whether or not a disk is present, the observation of additional molecular tracers, and estimating envelope density will help in accurately determining the cause of the observed snowline position. Plots of the N2H+ and HCO+ peak emission radius versus luminosity are provided to compare the models with observations of deeply embedded protostars aiming to measure the CO and H2O snowline locations.