Effects of Grain Growth on Molecular Abundances in Young Stellar Objects.

Recent observations suggested that the growth of dust grains may have already occurred in class 0/I young stellar objects (YSOs). Since chemical reactions on dust grain surfaces are important in determining molecular abundances, the dust size growth may affect chemical compositions in YSOs significantly. In this work, we aim to determine how grain growth affects chemical abundances. We use a time-dependent gas-grain chemical model for a star-forming core to calculate the gas-phase and grain-surface chemical abundances with variation of surface areas of grains to imitate grain growth. We also perform parameter studies in which the initial molecular abundances vary. Our results show that a smaller extent of the surface areas caused by grain growth changes the dominant form of sulfur-bearing molecules by decreasing H₂S abundances and increasing SO and/or SO₂ abundances. We also find that complex organic molecules such as CH₃CN decrease in abundances with larger grain sizes, while the abundance of other species such as CH₃OCH₃ is dependent on other parameters such as the initial conditions. Comparisons with observations of a class 0 protostar, IRAS 16293-2422, indicate that the observed abundance ratios between sulfur-bearing molecules H₂S, SO, and SO₂ can be reproduced very well when dust grains grow to a maximum grain size of a_max = 10–100 μm. [ABSTRACT FROM AUTHOR]