Wind-shearing in gaseous protoplanetary disks and the evolution of binary planetesimals

One of the first stages of planet formation is the growth of small planetesimals. This early stage occurs much before the dispersal of most of the gas from the protoplanetary disk. Due to their different aerodynamic properties, planetesimals of different sizes and shapes experience different drag forces from the gas during this time. Such differential forces produce a wind-shearing (WISH) effect between close by, different size planetesimals. For any two planetesimals, a WISH radius can be considered, at which the differential acceleration due to the wind becomes greater than the mutual gravitational pull between the planetesimals. We find that the WISH radius could be much smaller than the Hill radius, i.e. WISH could play a more important role than tidal perturbations by the star. Here we study the WISH radii for planetesimal pairs of different sizes and compare the effects of wind and gravitational shearing (drag force vs. gravitational tidal force). We then discuss the role of WISH for the stability and survival of binary planetesimals. Binaries are sheared apart by the wind if they are wider than their WISH radius. WISH-stable binaries can inspiral and possibly coalesce due to gas drag. Here, we calculate the WISH radius and the gas drag-induced merger timescale, providing stability and survival criteria for gas-embedded binary planetesimals. Our results suggest that even WISH-stable binaries may merge in times shorter than the lifetime of the gaseous disk. This may constrain currently observed binary planetesimals to have formed far from the star or at a late stage after the dispersal of most of the disk gas. We note that the WISH radius may also be important for other processes such as planetesimal erosion and planetesimal encounters and collisions in a gaseous environment.Comment: ApJ, in pres

On some aspects of teaching pop and jazz singing. The significance and the degree of importance of the empirical method for constructing the vocal technique of the students

Singing, resp. verbal communication, is performed through one instrument – our voice. Messages and emotions are entirely personal and relate to the personality of the performer (the emitter of the message), which requires a variety and uniqueness of models and layouts for creating vocal and verbal specimens. As such, this report focuses on some aspects and issues arising from my work with students in pop and jazz singing. The first and main problem is related to the level of expectations and potentialities of the young performers. The professor should identify the most effective and efficient methodological principles for each specific case. The evolution of pop and jazz music is extremely dynamic, quick, the variety of styles and forms is immense. Therefore, the vocal pedagogue should systematize the most popular and dominant types of vocal and performing techniques in the particular music environment, some of which are outlined in this report. The teacher should determine the level of technical and theoretical preparation that each pupil has. The main problem is to define properly the individual vocal abilities and provide the correct orientation for the student to a suitable style, repertoire and realization as a vocal performer

Cover – from See how it’s done to You get a pat on the shoulder (titles of popular Bulgarian songs)

Cover is a common and valuable musical phenomenon that needs to be studied, well-defined, and classified. This report is an initial attempt of the author in this direction. Certain songs and artists provide arguments for cover’s significance at a micro and macro-structural level. Cover is the main carrier of models and stylistic features that are modified and presented in different musical environments. The interpreted works influence the processes of development and evolution in popular music, unite them and promote the creation of new styles such as rock‘n‘roll. Covers of some types of music can be perceived more easily by a wider range of listeners. This is essential for the upgrading and cultivation of musical taste

The influence of high speed collisions and gas flow on the formation of planetesimals

Ein zentrales Problem der Planetologie ist die Frage nach der Entstehung der Planeten. Als Vorläufer der Planeten werden die so genannten Planetesimale betrachtet. Diese km-großen Körper wachsen in protoplanetaren Scheiben aus kleinen Staubteilchen. Verantwortlich für das Wachstum sind unter anderem die zahlreichen Stöße zwischen den Staubteilchen, die dabei entstehende Staubaggregate, und die Wechselwirkung dieser Körper mit dem Gas in der Scheibe. Wie genau die Entstehungsprozesse ablaufen und ob größere Körper in Stößen wachsen können, ist noch umstritten. In gezielten Laborexperimenten haben wir versucht diese Prozesse wiederzugeben. In Stoßexperimenten haben wir den Ausgang von Hochgeschwindigkeitskollisionen zwischen cm-großen Staubaggregaten mit verschiedener Struktur untersucht. Zusätzlich haben wir in einer Serie von Windtunnelexperimenten die Stabilität dieser Aggregate in starker Windströmung erforscht. Unsere Ergebnisse zeigen, dass bei bestimmten Bedingungen ein Wachstum von Planetesimalen möglich ist.thesi

Dust Ejection from Planetary Bodies by Temperature Gradients: Laboratory Experiments

Laboratory experiments show that dusty bodies in a gaseous environment eject dust particles if they are illuminated. We find that even more intense dust eruptions occur when the light source is turned off. We attribute this to a compression of gas by thermal creep in response to the changing temperature gradients in the top dust layers. The effect is studied at a light flux of 13 kW/(m*m) and 1 mbar ambient pressure. The effect is applicable to protoplanetary disks and Mars. In the inner part of protoplanetary disks, planetesimals can be eroded especially at the terminator of a rotating body. This leads to the production of dust which can then be transported towards the disk edges or the outer disk regions. The generated dust might constitute a significant fraction of the warm dust observed in extrasolar protoplanetary disks. We estimate erosion rates of about 1 kg/s for 100 m parent bodies. The dust might also contribute to subsequent planetary growth in different locations or on existing protoplanets which are large enough not to be susceptible to particle loss by light induced ejection. Due to the ejections, planetesimals and smaller bodies will be accelerated or decelerated and drift outward or inward, respectively. The effect might also explain the entrainment of dust in dust devils on Mars, especially at high altitudes where gas drag alone might not be sufficient.Comment: 7 pages, 10 figure

Light-induced disassembly of dusty bodies in inner protoplanetary discs: implications for the formation of planets

Laboratory experiments show that a solid-state greenhouse effect in combination with thermophoresis can efficiently erode a dust bed in a low-pressure gaseous environment. The surface of an illuminated, light absorbing dusty body is cooler than the dust below the surface (solidstate greenhouse effect). This temperature gradient leads to a directed momentum transfer between gas and dust particles and the dust particles are subject to a force towards the surface(thermophoresis). If the thermophoretic force is stronger than gravity and cohesion, dust particles are ejected. Applied to protoplanetary discs, dusty bodies smaller than several kilometres in size which are closer to a star than about 0.4 au are subject to a rapid and complete disassembly to submillimetre size dust aggregates by this process. While an inward-drifting dusty body is destroyed, the generated dust is not lost for the disc by sublimation or subsequent accretion on to the star but can be reprocessed by photophoresis or radiation pressure. Planetesimals cannot originate through aggregation of dust inside the erosion zone. If objects larger than several kilometres already exist, they prevail and further grow by collecting dust from disassembled smaller bodies. The pile-up of solids in a confined inner region of the disc, in general, boosts the formation of planets. Erosion is possible in even strongly gas-depleted inner regions as observed for TW Hya. Reprocessing of dust through light-induced erosion offers one possible explanation for growth of large cores of gas-poor giant planets in a gas-starved region as recently found around HD 149026b

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

The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments

The growth processes from protoplanetary dust to planetesimals are not fully understood. Laboratory experiments and theoretical models have shown that collisions among the dust aggregates can lead to sticking, bouncing, and fragmentation. However, no systematic study on the collisional outcome of protoplanetary dust has been performed so far so that a physical model of the dust evolution in protoplanetary disks is still missing. We intend to map the parameter space for the collisional interaction of arbitrarily porous dust aggregates. This parameter space encompasses the dust-aggregate masses, their porosities and the collision velocity. With such a complete mapping of the collisional outcomes of protoplanetary dust aggregates, it will be possible to follow the collisional evolution of dust in a protoplanetary disk environment. We use literature data, perform own laboratory experiments, and apply simple physical models to get a complete picture of the collisional interaction of protoplanetary dust aggregates. In our study, we found four different types of sticking, two types of bouncing, and three types of fragmentation as possible outcomes in collisions among protoplanetary dust aggregates. We distinguish between eight combinations of porosity and mass ratio. For each of these cases, we present a complete collision model for dust-aggregate masses between 10^-12 and 10^2 g and collision velocities in the range 10^-4 to 10^4 cm/s for arbitrary porosities. This model comprises the collisional outcome, the mass(es) of the resulting aggregate(s) and their porosities. We present the first complete collision model for protoplanetary dust. This collision model can be used for the determination of the dust-growth rate in protoplanetary disks.Comment: accepted by Astronomy and Astrophysic

Planet formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks

As accretion in protoplanetary disks is enabled by turbulent viscosity, the border between active and inactive (dead) zones constitutes a location where there is an abrupt change in the accretion flow. The gas accumulation that ensues triggers the Rossby wave instability, that in turn saturates into anticyclonic vortices. It was suggested that the trapping of solids within them leads to a burst of planet formation on very short timescales. We perform two-dimensional global simulations of the dynamics of gas and solids in a non-magnetized thin protoplanetary disk with the Pencil Code. We use multiple particle species of radius 1, 10, 30, and 100 cm, solving for the particles' gravitational interaction by a particle-mesh method. The dead zone is modeled as a region of low viscosity. Adiabatic and locally isothermal equations of state are used. We find that the Rossby wave instability is triggered under a variety of conditions, thus making vortex formation a robust process. Inside the vortices, fast accumulation of solids occurs and the particles collapse into objects of planetary mass in timescales as short as five orbits. Because the drag force is size-dependent, aerodynamical sorting ensues within the vortical motion, and the first bound structures formed are composed primarily of similarly-sized particles. In addition to erosion due to ram pressure, we identify gas tides from the massive vortices as a disrupting agent of formed protoplanetary embryos. We also estimate the collisional velocity history of the particles that compose the most massive embryo by the end of the simulation, finding that the vast majority of them never experienced a collision with another particle at speeds faster than 1 m/s.Comment: 19 pages, 15 figures + Appendices. Accepted by A&A. Nature of replacement: included a missing referenc

A Mechanism to Produce the Small Dust Observed in Protoplanetary Disks

Small (sub)-micron dust is present over the entire lifetime of protoplanetary disks. As aggregation readily depletes small particles, one explanation might be that dust is continuously generated by larger bodies in the midplane and transported to the surface of the disks. In general, in a first step of this scenario, the larger bodies have to be destroyed again and different mechanisms exist with the potential to accomplish this. Possible destructive mechanisms are fragmentation in collisions, erosion by gas drag or light induced erosion. In laboratory experiments we find that the latter, light induced erosion by Knudsen compression and photophoresis, can provide small particles. It might be a preferred candidate as the dust is released into a low particle density region. The working principle of this mechanism prevents or decreases the likelihood for instant re-accretion or re-growth of large dense aggregates. Provided that there is a particle lift, e.g. turbulence, these particles might readily reach the surface of the disk.Comment: 7 pages, 6 figure