It is well established that (sub-)micron sized dust grains can form in over-dense gas clumps in the expanding ejecta of supernova remnants. However, highly energetic shock waves occur in the ejecta which can potentially destroy a large fraction of the newly formed dust grains. The gas in the ejecta can be heated up to billions of Kelvins and is accelerated to a few hundred kilometere per second, which causes thermal and kinematic sputtering of the dust grains. Moreover, dust grains can collide with each other at high velocities and get fragmented or even vaporized. Previous predictions for dust survival rates depend strongly on initial parameters and range from less than 0.1% to 99%. The net dust survival rate is crucial for determining whether or not supernovae significantly contribute to the dust budget in the interstellar medium.
In order to model a shock wave interacting with an ejecta clump we have performed hydrodynamics simulations using the grid-based code AstroBEAR (Carroll-Nellenback et al. 2013), see Fig. 1. Afterwards, dust motions and dust destruction rates are computed using our newly developed external post-processing code, Paperboats, which includes gas and plasma drag, grain charging, kinematic and thermal sputtering as well as grain-grain collisions. We have used DiRAC HPC Facilities to determine the dust survival rates for the oxygen-rich supernova remnant Cassiopeia A for a huge range of parameters, including initial grain sizes, dust materials and clump gas densities.
We find that up to 40% of the silicate dust (Fig. 2) and up to 30% of the carbon dust mass is able to survive the passage of the reverse shock. The survival rates depend strongly on the initial grain size distribution, with ∼ 10−50 nm and ∼ 0.5−1.5 µm as the grain radii that show the highest surviving dust masses. The dust processing causes a rearranging of the initial grain size distribution. Our results showed that grain-grain collisions and sputtering are synergistic and that grain-grain collisions can play a vital role in determining the surviving dust budget in supernova remnants. These results were presented by Kirchschlager et al. (2019).
Due to the high gas temperatures and shock velocities in supernova remnants, heavy energetic ions can penetrate deep into 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, called ion trapping, had not been considered so far as a measure to counteract grain destruction by sputtering.
Kirchschlager et al. (2020) have shown that in oxygen-rich supernova remnants such as Cassiopeia A, the penetration and trapping within silicate grains of the same impinging ions of oxygen, silicon, and magnesium that are responsible for grain surface sputtering can significantly reduce the net loss of grain material (Fig. 3). We have used Paperboats to follow the dust mass and grain size evolution in a shocked clump. We find 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 and depending on initial grain radii (Fig. 4). The formation of grains larger than those that had originally condensed is facilitated and enables the presence of micron-sized grains in the post-shock medium. For higher density contrasts (χ ≥ 180), we find that the effect of gas accretion on the surface of dust grains can surpass ion trapping, and for χ = 256 the survival rate can increase to ∼55% of the initial dust mass.
last updated 06 July 2022
