Dejan Vinković

OUTFLOWS FROM EVOLVED STARS: THE RAPIDLY CHANGING FINGERS OF CRL 618

Our ultimate goal is to probe the nature of the collimator of the outflows in the pre-planetary nebula CRLxa0618. CRLxa0618 is uniquely suited for this purpose owing to its multiple, bright, and carefully studied finger-shaped outflows east and west of its nucleus. We compare new Hubble Space Telescope images to images in the same filters observed as much as 11xa0yr ago to uncover large proper motions and surface brightness changes in its multiple finger-shaped outflows. The expansion age of the ensemble of fingers is close to 100xa0yr. We find strong brightness variations at the fingertips during the past decade. Deep IR images reveal a multiple ring-like structure of the surrounding medium into which the outflows propagate and interact. Tightly constrained three-dimensional hydrodynamic models link the properties of the fingers to their possible formation histories. We incorporate previously published complementary information to discern whether each of the fingers of CRLxa0618 are the results of steady, collimated outflows or a brief ejection event that launched a set of bullets about a century ago. Finally, we argue on various physical grounds that fingers of CRLxa0618 are likely to be the result of a spray of clumps ejected at the nucleus of CRLxa0618 since any mechanism that form a sustained set of unaligned jets is unprecedented.

Morphological Evolution of Spiders Predicted by Pendulum Mechanics

Background Animals have been hypothesized to benefit from pendulum mechanics during suspensory locomotion, in which the potential energy of gravity is converted into kinetic energy according to the energy-conservation principle. However, no convincing evidence has been found so far. Demonstrating that morphological evolution follows pendulum mechanics is important from a biomechanical point of view because during suspensory locomotion some morphological traits could be decoupled from gravity, thus allowing independent adaptive morphological evolution of these two traits when compared to animals that move standing on their legs; i.e., as inverted pendulums. If the evolution of body shape matches simple pendulum mechanics, animals that move suspending their bodies should evolve relatively longer legs which must confer high moving capabilities. Methodology/Principal Findings We tested this hypothesis in spiders, a group of diverse terrestrial generalist predators in which suspensory locomotion has been lost and gained a few times independently during their evolutionary history. In spiders that hang upside-down from their webs, their legs have evolved disproportionately longer relative to their body sizes when compared to spiders that move standing on their legs. In addition, we show how disproportionately longer legs allow spiders to run faster during suspensory locomotion and how these same spiders run at a slower speed on the ground (i.e., as inverted pendulums). Finally, when suspensory spiders are induced to run on the ground, there is a clear trend in which larger suspensory spiders tend to run much more slowly than similar-size spiders that normally move as inverted pendulums (i.e., wandering spiders). Conclusions/Significance Several lines of evidence support the hypothesis that spiders have evolved according to the predictions of pendulum mechanics. These findings have potentially important ecological and evolutionary implications since they could partially explain the occurrence of foraging plasticity and dispersal constraints as well as the evolution of sexual size dimorphism and sociality.

Radiation-pressure mixing of large dust grains in protoplanetary disks

Dusty disks around young stars are formed out of interstellar dust that consists of amorphous, submicrometre grains. Yet the grains found in comets and meteorites, and traced in the spectra of young stars, include large crystalline grains that must have undergone annealing or condensation at temperatures in excess of 1,000u2009K, even though they are mixed with surrounding material that never experienced temperatures as high as that. This prompted theories of large-scale mixing capable of transporting thermally altered grains from the inner, hot part of accretion disks to outer, colder disk regions, but all have assumptions that may be problematic. Here I report that infrared radiation arising from the dusty disk can loft grains bigger than one micrometre out of the inner disk, whereupon they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation-pressure support for a given grain size and solid density. Properties of the observed disks suggest that this process might be active in almost all young stellar objects and young brown dwarfs.

THE ILLUMINATION AND GROWTH OF CRL 2688: AN ANALYSIS OF NEW AND ARCHIVAL HUBBLE SPACE TELESCOPE OBSERVATIONS

We present four-color images of CRL 2688 obtained in 2009 using the Wide-Field Camera 3 on Hubble Space Telescope. The F606W image is compared with archival images in very similar filters to monitor the proper motions of nebular structure. We find that the bright N-S lobes have expanded uniformly by 2.5% and that the ensemble of rings has translated radially by 007 in 6.65?yr. The rings were ejected every 100?yr for ~4 millennia until the lobes formed 250?yr ago. Starlight scattered from the edges of the dark E-W dust lane is coincident with extant H2 images and leading tips of eight pairs of CO outflows. We interpret this as evidence that fingers lie within geometrically opposite cones of opening angles 30? like those in CRL618. By combining our results of the rings with 12CO?absorption from the extended asymptotic giant branch (AGB) wind we ascertain that the rings were ejected at ~18?km?s?1 with very little variation and that the distance to CRL 2688, , is 300-350?pc. Our 2009 imaging program included filters that span 0.6-1.6 ?m. We constructed a two-dimensional dust scattering model of stellar radiation through CRL 2688 that successfully reproduces the details of the nebular geometry, its integrated spectral energy distribution, and nearly all of its color variations. The model implies that the optical opacity of the lobes 1, the dust particle density in the rings decreases as radius?3, and that the mass and momentum of the AGB?winds and their rings have increased over time.

Constraints on the height of the inner disk rim in pre-main-sequence stars

The structure of inner region of protoplanetary disks around young pre-main-sequence stars is still poorly understood. This part of the disk is shaped by various forces that influence dust and gas dynamics, and by dust sublimation, which creates abrupt drops in the dust density. This region also emits strong near-infrared excess that cannot be explained by classical accretion disk models, which suggests the existence of some unusual dust distribution or disk shape. The most prevalent explanation to date is the puffed-up inner disk rim model, where the disk exhibits an optically thin cavity around the star up to the distance of dust sublimation. The critical parameter in this model is the inner disk rim height z max relative to the rim distance from the star R in . Observations often require z max / R in ≳ 0.2 to reproduce the near-infrared excess in the spectra. We compile a comprehensive list of processes that can shape the inner disk rim and combine them into a self-consistent model. Two of them, radiation pressure force and the gas velocity profile, have never been applied in this context before. The aim was to find the most plausible theoretical values of z max / R in . The results show that this value is ≲0.13 for Herbig Ae stars, ≲0.11 for T Tau stars, and ≲0.10 for young brown dwarfs. This is lower than the observational requirements for Herbig Ae stars. We argue that the same problem exists in T Tau stars as well. We conclude that the puffed-up inner rim model cannot be the sole explanation for the near-infrared excess in young pre-main-sequence stars.

Inner dusty regions of protoplanetary discs – I. High‐resolution temperature structure

Our current understanding of the physical conditions in the inner regions of protoplanetary discs is being increasingly challenged by more detailed observational and theoretical explorations. The calculation of the dust temperature is one of the key features that we strive to understand and this is a necessary step in image and flux reconstruction. Here, we explore the coexistence of small (0.1-µm radius) and large (2-µm radius) dust grains, which can coexist at distances from the star where small grains would not survive without large grains shielding them from the direct starlight. Our study required a high-resolution radiative transfer calculation, which is capable of resolving the large temperature gradients and disc-surface curvatures caused by dust sublimation. This method of calculation was also capable of resolving the temperature inversion effect in large grains, where the maximum dust temperature is at a visual optical depth of τ V ∼ 1.5. We also show disc images and spectra, with disentangled contributions from small and large grains. Large grains dominate the near-infrared flux, mainly because of the bright hot inner disc rim. Small grains populate almost the entire interior of the inner disc, but they appear at the disc’s surface at distances 2.2 times larger than the closest distance of the large grains from the star. Nevertheless, small grains can contribute to the image surface brightness at smaller radii because they are visible below the optically thin surface defined by stellar heating. Our calculations demonstrate that the sublimation temperature does not provide a unique boundary condition for radiative transfer models of optically thick discs. The source of this problem is the temperature inversion effect, which allows the survival of optically thin configurations of large grains closer to the star than the inner radius of the optically thick disc. Future attempts to derive more realistic multigrain inner disc models will need the numerical resolution shown in our study, especially if the dust dynamics is considered where grains can travel through zones of local temperature maxima.

Proton-induced halo formation in charged meteors

Despite a very long history of meteor science, our understanding of meteor ablation and its shocked plasma physics is still far from satisfactory as we are still missing the microphysics of meteor shock formation and its plasma dynamics. Here we argue that electrons and ions in the meteor plasma above

Test-field for evaluation of laboratory craters using a Crater Shape-based interpolation crater detection algorithm and comparison with Martian and Lunar impact craters

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Test-Field for Evaluation of Laboratory Craters Using Interpolation-Based Crater Detection Algorithm and Comparison with Martian Impact Craters

100 km altitude undergo spatial separation due to electrons being trapped by gyration in the Earths magnetic field, while the ions are carried by the meteor as their dynamics is dictated by collisions. This separation process charges the meteor and creates a strong local electric field. We show how acceleration of protons in this field leads to the collisional excitation of ionospheric N

3D Topography of Laboratory Craters and their Comparison with Martian Impact Craters

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