Decimetre dust aggregates in protoplanetary discs

The growth of planetesimals is an essential step in planet formation. Decimetre-size dust agglomerates mark a transition point in this growth process. In laboratory experiments we simulated the formation, evolution, and properties of decimetre-scale dusty bodies in protoplanetary discs. Small sub-mm size dust aggregates consisting of micron-size SiO$_2$ particles randomly interacted with dust targets of varying initial conditions in a continuous sequence of independent collisions. Impact velocities were 7.7 m/s on average and in the range expected for collisions with decimetre bodies in protoplanetary discs. The targets all evolved by forming dust \emph{crusts} with up to several cm thickness and a unique filling factor of 31% $\pm$3%. A part of the projectiles sticks directly. In addition, some projectile fragments slowly return to the target by gravity. All initially porous parts of the surface, i.e. built from the slowly returning fragments, are compacted and firmly attached to the underlying dust layers by the subsequent impacts. Growth is possible at impact angles from 0$^{\circ}$ (central collision) to 70$^{\circ}$. No growth occurs at steeper dust surfaces. We measured the velocity, angle, and size distribution of collision fragments. The average restitution coefficient is 3.8% or 0.29 m/s ejection velocity. Ejecta sizes are comparable to the projectile sizes. The high filling factor is close to the most compact configuration of dust aggregates by local compression ($\sim 33$%). This implies that the history of the surface formation and target growth is completely erased. In view of this, the filling factor of 31% seems to be a universal value in the collision experiments of all self-consistently evolving targets at the given impact velocities. We suggest that decimetre and probably larger bodies can simply be characterised by one single filling factor.Comment: 10 pages, 9 figure

#thatswhatshesaid Gender Partisanship: Differences in Senatorial Twitter Behavior

In the space of 140 characters or less, and with a push of a tweet button, you can give the world access to your thoughts and feelings regarding any topic. The social media platform Twitter allows users to interact with one another and address their followers with whatever is on their mind. The power of Twitter is that information can be spread instantly and at any time, while also providing an accessible medium that promotes conversation and expression between users. This can be very useful for the leaders of our country as they can communicate their policies, build relationships and interact with their followers, and respond quickly and efficiently to any dire situations or crises that arise. However, when we look to our leaders on Twitter, specifically the members of the Senate, is there a difference in Twitter behavior based on gender? Governmen

Process Mining as a Strategy of Inquiry: Understanding Design Interventions and the Development of Business Processes

Process (re-)design and improvement are important aspectsof the Business Process Management (BPM) life-cycle. Yet, there is lit-tle empirical evidence on how design interventions materialize in actualprocess execution, leading to repeated failure of such initiatives. In thisdissertation I use the emerging affordances of process mining algorithmsto address this important limitation. In particular, I devise a methodthat combines process mining and grounded theory to study processualphenomena. Consequently, this method is applied to investigate changein business processes. This thesis contributes to the body of knowledgein BPM and bordering disciplines by demonstrating how process min-ing can be used as a method to study processual phenomena. Furtherthis research sheds light on the impact of design interventions on actualprocess execution and vica versa

Kleene Algebras, Regular Languages and Substructural Logics

We introduce the two substructural propositional logics KL, KL+, which use disjunction, fusion and a unary, (quasi-)exponential connective. For both we prove strong completeness with respect to the interpretation in Kleene algebras and a variant thereof. We also prove strong completeness for language models, where each logic comes with a different interpretation. We show that for both logics the cut rule is admissible and both have a decidable consequence relation.Comment: In Proceedings GandALF 2014, arXiv:1408.556

Synchronous Subsequentiality and Approximations to Undecidable Problems

We introduce the class of synchronous subsequential relations, a subclass of the synchronous relations which embodies some properties of subsequential relations. If we take relations of this class as forming the possible transitions of an infinite automaton, then most decision problems (apart from membership) still remain undecidable (as they are for synchronous and subsequential rational relations), but on the positive side, they can be approximated in a meaningful way we make precise in this paper. This might make the class useful for some applications, and might serve to establish an intermediate position in the trade-off between issues of expressivity and (un)decidability.Comment: In Proceedings GandALF 2015, arXiv:1509.0685

High Velocity Dust Collisions: Forming Planetesimals in a Fragmentation Cascade with Final Accretion

In laboratory experiments we determine the mass gain and loss in central collisions between cm to dm-size SiO2 dust targets and sub-mm to cm-size SiO2 dust projectiles of varying mass, size, shape, and at different collision velocities up to ~56.5 m/s. Dust projectiles much larger than 1 mm lead to a small amount of erosion of the target but decimetre targets do not break up. Collisions produce ejecta which are smaller than the incoming projectile. Projectiles smaller than 1 mm are accreted by a target even at the highest collision velocities. This implies that net accretion of decimetre and larger bodies is possible. Independent of the original size of a projectile considered, after several collisions all fragments will be of sub-mm size which might then be (re)-accreted in the next collision with a larger body. The experimental data suggest that collisional growth through fragmentation and reaccretion is a viable mechanism to form planetesimals

Influence of plastic cover on fruit-quality and monilia laxa infestations with organically produced apricots

In the years 2005 to 2007 a plastic cover was tested in a testing area planted with apricots and operated according to the regulations for organic production. Compared to the untreated control, fruit below the plastic cover showed decreased coloration. Monilia laxa infestations below the plastic cover could not be fully prevented, but were reduced considerably also by copper applications during flowering in 2007. Yield of the variant treated with copper, however, remained below average. Losses due to rot during storage were smallest with the covered variant