Methanol along the path from envelope to protoplanetary disc

Abstract

Interstellar methanol is considered to be a parent species of larger, more complex organic molecules. A physicochemical simulation of infalling parcels of matter is performed for a low-mass star-forming system to trace the chemical evolution from cloud to disc. An axisymmetric 2D semi-analytic model generates the time-dependent density and velocity distributions, and full continuum radiative transfer is performed to calculate the dust temperature and the UV radiation field at each position as a function of time. A comprehensive gas-grain chemical network is employed to compute the chemical abundances along infall trajectories. Two physical scenarios are studied, one in which the dominant disc growth mechanism is viscous spreading, and another in which continuous infall of matter prevails. The results show that the infall path influences the abundance of methanol entering each type of disc, ranging from complete loss of methanol to an enhancement by a factor of ą 1 relative to the prestellar phase. Critical chemical processes and parameters for the methanol chemistry under different physical conditions are identified. The exact abundance and distribution of methanol is important for the budget of complex organic molecules in discs, which will be incorporated into forming planetary system objects such as protoplanets and comets. These simulations show that the comet-forming zone contains less methanol than in the precollapse phase, which is dominantly of prestellar origin, but also with additional layers built up in the envelope during infall. Such intriguing links will soon be tested by upcoming data from the Rosetta mission.

Water delivery from cores to disks: Deuteration as a probe of the prestellar inheritance of H2O

“…At t = 0.5t acc , the CH 3 OH ice abundance and the total abundance of icy organics are similar to their precollapse values. At t = t acc , the CH 3 OH ice abundance ranges from 1 to 10 % of water ice, which is lower than its precollapse value (see also Drozdovskaya et al, 2014). CH 3 OH ice is selectively lost with respect to H 2 O ice, since CH 3 OH is mostly present in the upper ice layers in contrast to H 2 O which is mostly present in the lower layers (Figure 2).…”

“…Fluid parcels are traced in the physical model, and a gas-ice chemical simulation is performed along the streamlines (e.g., Drozdovskaya et al, 2014). The chemical evolution along a total of ∼35,000 streamlines is calculated for our fiducial model in order to investigate the envelope-scale and disk-scale chemical structures with their time evolution.…”

“…The model corresponds to case 3 in Visser et al (2009b) and 'spread-dominated disk' case in Drozdovskaya et al (2014) lapse as mentioned in Section 4.2 for the fiducial model. Because of the lower rotation rate, the envelope material which is exposed to the stellar UV radiation mostly accretes at small spatial scales (i.e., becomes part of the star) or enters into the outflow cavity and does not significantly contribute to the disk formation.…”

On the origin of O2 and other volatile species in comets

“…As shown by dynamical models of protoplanetary disk formation, volatile species that evaporated during their journey from dark clouds to upper disk layers can subsequently freeze-out onto ices again once they reach the colder disk midplane (see Drozdovskaya et al 2014). N 2 is known to be slightly more volatile than CO with a binding energy lower by ∼ 150 K for H 2 O ice and by ∼ 60 K for pure ices (Bisschop et al 2006, Fayolle et al 2016.…”

A novel approach to measure photodesorption rates of interstellar ice analogues

“…Later on in the star and planet formation sequence, light from the young stellar object also becomes important, determining for example the position of photo-induced snowlines (Öberg et al 2015). Therefore, accurate photodesorption rates are needed to quantify the role that (inter)stellar radiation has on abundances in the solid state and gas phase in different astronomical environments, varying from molecular clouds (Hollenbach et al 2009) to protoplanetary disks (Willacy & Langer 2000;Drozdovskaya et al 2014;Walsh et al 2015).…”