Macroscopic Dust in Protoplanetary Disks—from Growth to Destruction

Abstract

The collision dynamics of dusty bodies are crucial for planetesimal formation. Decimeter agglomerates are especially important in the different formation models. Therefore, in continuation of our experiments on mutual decimeter collisions, we investigate collisions of centimeter onto decimeter dust agglomerates in a small drop tower under vacuum conditions (p ∼ < 5·10 −1 mbar) at a mean collision velocity of 6.68±0.67 m s −1 . We use quartz dust with irregularly shaped micrometer grains. Centimeter projectiles with different diameters, masses and heights are used, their typical volume filling factor is Φ p,m = 0.466 ± 0.02. The decimeter agglomerates have a mass of about 1.5 kg, a diameter and height of 12 cm and a mean filling factor of Φ t,m = 0.44 ± 0.004. At lower collision energies only the projectile gets destroyed and mass is transferred to the target. The accretion efficiency decreases with increasing obliquity and increasing difference in filling factor, if the projectile is more compact than the target. The accretion efficiency increases with increasing collision energy for collision energies under a certain threshold. Beyond this threshold at 298 ± 25 mJ catastrophic disruption of the target can be observed. This corresponds to a critical fragmentation strength Q * = 190 ± 16 mJ kg −1 , which is a factor of four larger than expected. Analyses of the projectile fragments show a power law size distribution with average exponent of −3.8 ± 0.3. The mass distributions suggest that the fraction of smallest fragments increases for higher collision energies. This is interesting for impacts of small particle on large target bodies within protoplanetary disks, as smaller fragments couple better to the surrounding gas and re-accretion by gas drag is more likely.Subject headings: planets and satellites: formation -protoplanetary disks 1 johannes.deckers@uni-due.de

The Role of Pebble Fragmentation in Planetesimal Formation. I. Experimental Study

“…Technically the largest fragment in such collisions is actually the remnant of the original target aggregate. Deckers and Teiser (2014) showed that the mass fraction of the largest fragment decreases with increasing impact energy, in agreement with our findings (see Figure 9) and discussions in Section 4.2.…”

“…Fragment size distributions of colliding dust aggregates have previously been observed to be composed of two parts, with a high count of smaller fragments following a power law in size-frequency distribution and fewer counts of the largest ones (see, e.g., Blum and Münch (1993); Deckers and Teiser (2014)). Technically the largest fragment in such collisions is actually the remnant of the original target aggregate.…”

“…Two competing models of planetesimal formation in the presence of the above-mentioned obstacles have been developed in the past years. Based upon an extensive body of laboratory work on mass transfer in high-velocity collisions between dust aggregates of dissimilar masses (Wurm et al 2005a;Teiser and Wurm 2009a;Güttler et al 2010;Kothe et al 2010;Deckers and Teiser 2014), Windmark et al (2012a), Windmark et al (2012b) and Garaud (2013) describe the direct collisional formation of planetesimals, ignoring particle transport by radial drift. Although mass transfer in the process of fragmentation of the smaller projectile aggregate during an impact into the larger target aggregate has been clearly proven to exist, the formation of planetesimals of kilometre sizes or larger by this process faces severe problems, such as the rather large time scales required , the role of counter-acting erosion (Schräpler and Blum 2011), and fragmentation in collisions between similar-sized planetesimals.…”

Dust Evolution in Protoplanetary Discs and the Formation of Planetesimals

“…Above the fragmentation limit of the small projectile aggregate, part of its mass is permanently transferred to the target aggregate so that the latter gains mass. In the past years, many experimental investigations have studied mass transfer (Wurm et al 2005b;Teiser and Wurm 2009a,b;Beitz et al 2011;Meisner et al 2013;Deckers and Teiser 2014;. Typical efficiencies of the mass-transfer process are between a few and ∼ 50 per cent and the deposited mass is compressed to a volume filling factor of typically 0.3-0.4 Meisner et al 2013).…”

Gas and multispecies dust dynamics in viscous protoplanetary discs: the importance of the dust back-reaction

“…26), adopting different values of the power law index, q, and the minimum and maximum values of the grain-size distribution smin and smax, corresponding to the minimum and maximum values of Stokes number Stmin and Stmax. Experimental studies of fragmentation over a single target found that q can range from ∼ 1.9 for low-velocity collisions to ∼ 4 for catastrophic impacts (Davis & Ryan 1990;Deckers & Teiser 2014). Importantly, there is no specific reason to assume that in protoplanetary discs, q should take the same value as in the ISM (qISM = 3.5).…”