Effect of temperature on the structure of porous dust aggregates formed by coagulation

Abstract

The source of high redshift dust is currently under debate. One possibility are the ejecta of pair-instability and core collapse supernovae. However, it is uncertain how much newly formed dust can survive the supernova reverse shock and be injected into the interstellar medium. We anticipate the structure of the pre-shocked dust to affect how much of it survives. Yet, the structure of dust formed in supernova is not well understood. We present three-dimensional soft-sphere, dust coagulation simulations, using sequential collisions, aimed at studying the impact of temperature and monomer size distribution on the structure of growing dust aggregates. Due to the qualitative nature of the concept of structure, there are many ways to define and quantify it, especially for an irregular aggregate. Thus, we test eight metrics commonly used in the literature in order to compare the aggregate properties as well as the strengths and weaknesses of the metrics themselves. Our findings show that higher temperatures result in denser, more compact structures for all metrics tested. Additionally, we find that structures that coagulate from a distribution of monomer sizes are denser and more compact than structures formed from identical monomers under similar conditions. The latter finding, however, is true for all of the metrics except for the average number of contact points, which has proven to be the least reliable of the eight considered metrics.

Figures (9)

• Figure 1: Example aggregates of sizes $N = 30$, $100$, and $300$ monomers formed at temperatures $3$, $10$, $30$, $100$, $300$, and $1000$ K. Note that for all sizes, but especially for larger sizes, higher temperatures produce more compact aggregates. The top panel shows aggregates with a constant monomer-size distribution, while the bottom panel shows aggregates with a lognormal monomer-size distribution.
• Figure 2: Visualizations of the porosities used in this paper. The hulls represent the volume occupied by the aggregate as defined by each porosity metric. Note that panel (e) shows a convex hull generated using 32 points per sphere for visualization purposes (as opposed to 64 used in calculations). Panel (f) shows the geometric cross section for three different directions, as well as the resulting sphere representing the volume the aggregate occupies. The grid shown in panel (f) is much larger than the one used for actual calculations.
• Figure 3: Structure metrics for the constant monomer-size aggregates at various temperatures and for various sizes. In order from (a)-(h) the panels show the average number of contacts, fractal dimension, $\mathcal{P}_{abc}$, $\mathcal{P}_{KBM}$, $\mathcal{P}_{fee}$, $\mathcal{P}_{fes}$, $\mathcal{P}_{ch}$, $\mathcal{P}_{gcs}$. Values for the same size aggregate at different temperatures are joined with a line (see the legend on top of panels (a) and (b)). Each symbol shows the average of 30 statistically independent realizations. The error bars represent the standard error of these sub-samples.
• Figure 4: Same as Figure \ref{['fig:const_measures']} but for aggregates with a lognormal monomer-size distribution.
• Figure 5: Absolute value of the slope of a line fit to the data from Figures \ref{['fig:const_measures']} and \ref{['fig:lognorm_measures']} as a way to quantify the sensitivity of an aggregate's structure to temperature, for a constant (top panel) and lognormal (bottom panel) monomer radii distribution. Vertical error bars represent uncertainty in the slope. In almost all cases a statistically significant temperature dependence is identified.