Silicate grain growth due to ion trapping in supernova remnants

Figure 1: Net yield, Ynet, that gives the change of number of atoms in the grain per incident oxygen ion as a function of energy E of the incident oxygen ion. A positive yield corresponds to dust destruction; a negative yield corresponds to dust growth. When the energy of the incident oxygen ion is higher than the sputtering threshold energy, dust atoms can be sputtered. Left: trapping conditions are not fulfilled (oxygen particle escaping after the sputtering event) so the resulting net yield is equal to the regular sputtering yield, Ynet = Ysp (red line). Right: the oxygen ion is trapped so the net yield amounts to Ynet = Ysp – 1 (black line). For both cases, the incident oxygen atom is accreted at lower energies (green shaded region).

Dust material in the clumpy ejecta of supernova remnants is exposed to high gas temperatures and shock velocities. On the one hand, the energetic conditions can cause a significant destruction of the dust grains due to sputtering or grain-grain collisions. On the other hand, the energetic gas ions can penetrate deep into the dust grains. For grain temperatures below ~500 K, the diffusion rate of oxygen and other heavy ions in silicates is very low and they are trapped once they have intruded into the grain. This process, named as ion trapping, has not been considered so far as a measure to counteract grain destruction by sputtering.

Figure 2: Surviving dust mass, M, as a function of time, taking into account grain-grain collisions (GG) and sputtering (SP) with or without trapping or accretion of oxygen ions (different colors and line types). KeV oxygen ion trapping and gas accretion reduce the dust destruction significantly.

To study this phenomenon we considered a clump impacted by a shock-wave in the oxygen-rich supernova remnant Cassiopeia A. DiRAC HPC Facilities were used to conduct hydrodynamics simulations which were further post-processed with the code Paperboats (Kirchschlager et al. 2019). This allowed us to follow the dust mass and grain size evolution in the shocked ejecta material.

In our new study (Kirchschlager et al. 2020) we showed that penetration and trapping within silicate grains of oxygen, silicon, and magnesium, the same impinging ions that are responsible for grain surface sputtering, can significantly reduce the net loss of grain material (Fig. 1). We found for a pre-shock gas density contrast between clump and ambient medium of χ = 100 that ion trapping increases the surviving masses of silicate dust by factors of up to two to four, compared to cases where the effect is neglected (Fig. 2). The formation of grains larger than those that had originally condensed is facilitated and allows the presence of micrometre-sized grains in the post-shock medium. For higher density contrasts (χ ≥ 180), we found that the effect of gas accretion on the surface of dust grains surpasses ion trapping, and the survival rate increases to ∼55% of the initial dust mass for χ = 256.

References:
Kirchschlager, Barlow, M. J., &

Schmidt, F. D. 2020, ApJ, 893, p.70-78