Dust destruction in supernova remnants

It is well established that (sub-)micrometre sized dust grains can form in over-dense gas clumps in the expanding ejecta of supernovae 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 is heated up to billions of Kelvin and is accelerated to a few hundreds of kilometre per seconds which causes thermal and kinematic sputtering of the dust grains. Moreover, the dust grains can collide among each other with high velocities and get fragmented or even vaporized. Previous predictions for the dust survival rate 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.

Figure 1: Hydrodynamical simulation of the temporal evolution of the spatial gas density when the reverse shock impacts the clump. Our simulations were performed on the DiRAC @ Cambridge Service and show that the clump is disrupted within ∼60 years. (Adapted from Kirchschlager et al. 2019).

In order to model a shock wave interacting with an ejecta clump we performed hydrodynamics simulations using the grid-based code AstroBEAR (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 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.

Figure 2: Surviving silicate mass as a function of the initial grain size a peak and density contrast χ between clump and ambient medium (Kirchschlager et al. 2019).

We find that up to 40 % of the silicate (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 show 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.

References:

Kirchschlager, F., Schmidt, F. D., Barlow, M. J., Fogerty, E. L., Bevan, A., & Priestley, F. D. 2019, MNRAS, 489, p.4465-4496