Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks

Planetesimals in gaseous protoplanetary disks may grow by collecting dust particles. Hydrodynamical studies show that small particles generally avoid collisions with the planetesimals because they are entrained by the flow around them. This occurs when $St$, the Stokes number, defined as the ratio of the dust stopping time to the planetesimal crossing time, becomes much smaller than unity. However, these studies have been limited to the laminar case, whereas these disks are believed to be turbulent. We want to estimate the influence of gas turbulence on the dust-planetesimal collision rate and on the impact speeds. We used three-dimensional direct numerical simulations of a fixed sphere (planetesimal) facing a laminar and turbulent flow seeded with small inertial particles (dust) subject to a Stokes drag. A no-slip boundary condition on the planetesimal surface is modeled via a penalty method. We find that turbulence can significantly increase the collision rate of dust particles with planetesimals. For a high turbulence case (when the amplitude of turbulent fluctuations is similar to the headwind velocity), we find that the collision probability remains equal to the geometrical rate or even higher for $St\geq 0.1$, i.e., for dust sizes an order of magnitude smaller than in the laminar case. We derive expressions to calculate impact probabilities as a function of dust and planetesimal size and turbulent intensity

Student Showcase Concert

Ida GotkovskyRobert PlanelEdith Piaf & Louis GuglielmiJohnny MandelKen ThomsonBob Becke

HPA-axis activity and externalizing behavior problems in early adolescents from the general population:the role of comorbidity and gender The TRAILS study

Contradictory findings on the relationship between hypothalamus-pituitary-adrenal (HPA)-axis activity and externalizing behavior problems could be due to studies not accounting for issues of comorbidity and gender. In a population-based cohort of 1768 (10- to 12-year-old) early adolescents, we used a person-oriented approach and a variable-oriented approach to investigate whether comorbidity with internalizing behavior problems and gender moderate the relationship between HPA-axis activity (cortisol awakening response and evening cortisol levels) and externalizing behavior problems. We found that: (1) in early adolescents with pure externalizing behavior problems, there was a particularly strong effect of gender, in that girls showed significantly higher total cortisol levels after awakening (AUC(G) levels) and a significantly higher cortisol awakening response (AUC(I) levels) than boys. (2) Girls with pure externalizing behavior problems showed a significantly higher cortisol awakening response (AUC(I) levels) than girls without behavior problems or girls with comorbid internalizing behavior problems. This effect was absent in boys. (3) Externalizing behavior problems, in contrast to internalizing behavior problems, were associated with higher evening cortisol levels. This effect might, however, result from girls with externalizing behavior problems showing the highest evening cortisol levels. Overall, we were unable to find the expected relationships between comorbidity and HPA-axis activity, and found girls with pure externalizing behavior problems to form a distinct group with regard to their HPA-axis activity. There is need for prospective longitudinal studies of externalizing behavior problems in boys and girls in relation to their HPA-axis activity. It would be useful to consider how other risk factors such as life events and family and parenting factors as well as genetic risks affect the complex relationship between externalizing behavior problems and HPA-axis activity

Determining the Parameters of Massive Protostellar Clouds via Radiative Transfer Modeling

A one-dimensional method for reconstructing the structure of prestellar and protostellar clouds is presented. The method is based on radiative transfer computations and a comparison of theoretical and observed intensity distributions at both millimeter and infrared wavelengths. The radiative transfer of dust emission is modeled for specified parameters of the density distribution, central star, and external background, and the theoretical distribution of the dust temperature inside the cloud is determined. The intensity distributions at millimeter and IR wavelengths are computed and quantitatively compared with observational data. The best-fit model parameters are determined using a genetic minimization algorithm, which makes it possible to reveal the ranges of parameter degeneracy as well. The method is illustrated by modeling the structure of the two infrared dark clouds IRDC-320.27+029 (P2) and IRDC-321.73+005 (P2). The derived density and temperature distributions can be used to model the chemical structure and spectral maps in molecular lines.Comment: Accepted for publication in Astronomy Report