Abstract
We present a new gas-grain chemical model to constrain the effect of grain size distribution on molecular abundances in physical conditions corresponding to starless and pre-stellar cores. We simultaneously introduce grain-size dependence for desorption efficiency induced by cosmic rays (CRs) and for grain equilibrium temperatures. The latter were calculated with a radiative transfer code via custom dust models built for the present work. We explicitly tracked of ice abundances on a set of grain populations. We find that the size-dependent CR desorption efficiency affects ice abundances in a highly nontrivial way that depends on the molecule. Species that originate in the gas phase, such as CO, follow a simple pattern in which the ice abundance is highest on the smallest grains and these are the most abundant in the distribution. Some molecules, such as HCN, are instead concentrated on large grains throughout the time evolution; others, such as N2, are initially concentrated on large grains, but at late times on small grains because of grain-size-dependent competition between desorption and hydrogenation. Most of the water ice is on small grains at high medium density (n(H2) ≳ 106 cm−3), where the water ice fraction, with respect to the total water ice reservoir, can be as low as ~10−3 on large (>0.1 μm) grains. Allowing the grain equilibrium temperature to vary with grain size induces strong variations in relative ice abundances in low-density conditions in which the interstellar radiation field and in particular its ultraviolet component are not attenuated. Our study implies consequences not only for the initial formation of ices preceding the starless core stage, but also for the relative ice abundances on the grain populations going into the protostellar stage. In particular, if the smallest grains can lose their mantles owing to grain-grain collisions as the core is collapsing, the ice composition in the beginning of the protostellar stage could be very different than in the pre-collapse phase because the ice composition depends strongly on the grain size.
Highlights
Interstellar dust grains play a large role in the chemical and physical evolution of molecular clouds that host starless and prestellar cores – the seeds of low-mass star formation
We present a new gas-grain chemical model to constrain the effect of grain size distribution on molecular abundances in physical conditions corresponding to starless and pre-stellar cores
We find that the size-dependent cosmic rays (CRs) desorption efficiency affects ice abundances in a highly nontrivial way that depends on the molecule
Summary
Interstellar dust grains play a large role in the chemical and physical evolution of molecular clouds that host starless and prestellar cores – the seeds of low-mass star formation. In gas-grain chemical models used to predict molecular abundances in interstellar matter (ISM), it almost ubiquitously assumed that the grains are monodisperse; that is, they all have the same radii, which is usually taken to be around 0.1 μm (e.g., Hasegawa et al 1992; Garrod & Herbst 2006; Taquet et al 2012; Sipilä et al 2015; Vasyunin et al 2017). This assumption greatly simplifies the chemical modeling, but has negative consequences for the accuracy of the simulations. Considering that the efficiencies of chemical reactions on grain surfaces are strongly dependent on the grain temperature (e.g., Hasegawa et al 1992), coupled with the fact that nonthermal desorption allows the surface material to interact with the gas phase, it is highly desirable to investigate the effect that size distribution may have on the chemical evolution of a star-forming object as a whole
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