The Experts below are selected from a list of 1779 Experts worldwide ranked by ideXlab platform
Javier Ballesterosparedes - One of the best experts on this subject based on the ideXlab platform.
-
six myths on the virial theorem for interstellar clouds
Monthly Notices of the Royal Astronomical Society, 2006Co-Authors: Javier BallesterosparedesAbstract:The interstellar medium is highly dynamic and turbulent. However, little or no attention has been paid in the literature to the implications that this fact has on the validity of at least six common assumptions on the virial theorem (VT), which are as follows. (i) The only role of turbulent motions within a cloud is to provide support against collapse; (ii) the surface terms are negligible compared to the volumetric ones; (iii) the gravitational term is a binding source for the clouds since it can be approximated by the gravitational energy; (iv) the sign of the second time-derivative of the moment of inertia determines whether the cloud is contracting ( ¨ I 0); (v) interstellar clouds are in virial equilibrium (VE) and (vi) Larson’s relations (mean density‐size and velocity dispersion‐size) are the observational proof that clouds are in VE. However, turbulent, supersonic interstellar clouds cannot fulfil these assumptions because turbulent fragmentation will induce flux of mass, moment and energy between the clouds and their environment, and will favour local collapse while it may also disrupt the clouds within a dynamical time-scale. It is argued that although the observational and numerical evidence suggests that interstellar clouds are not in VE, the so-called ‘virial mass’ estimations, which should actually be called ‘energy-equipartition mass’ estimations, are good order of magnitude estimations of the actual mass of the clouds just because observational surveys will tend to detect interstellar clouds appearing to be close to energy equipartition. Similarly, order of magnitude estimations of the energy content of the clouds is reasonable. However, since clouds are actually out of VE, as suggested by asymmetrical line profiles, they should be transient entities. These results are compatible with observationally based estimations for rapid star formation, and call into question the models for the star formation efficiency based on clouds being in VE.
-
six myths on the virial theorem for interstellar clouds
arXiv: Astrophysics, 2006Co-Authors: Javier BallesterosparedesAbstract:It has been paid little or no attention to the implications that turbulent fragmentation has on the validity of at least six common assumptions on the Virial Theorem (VT), which are: (i) the only role of turbulent motions within a cloud is to provide support against collapse, (ii) the surface terms are negligible compared to the volumetric ones, (iii) the gravitational term is a binding source for the clouds, (iv) the sign of the second-time derivative of the moment of inertia determines whether the cloud is contracting or expanding, (v) interstellar clouds are in Virial Equilibrium (VE), and (vi) Larson's (1981) relations are the observational proof that clouds are in VE. Interstellar clouds cannot fulfill these assumptions, however, because turbulent fragmentation will induce flux of mass, moment and energy between the clouds and their environment, and will favor local collapse while may disrupt the clouds within a dynamical timescale. It is argued that, although the observational and numerical evidence suggests that interstellar clouds are not in VE, the so-called ``Virial Mass'' estimations, which actually should be called ``energy-equipartition mass'' estimations, are good order-of magnitude estimations of the actual mass of the clouds just because observational surveys will tend to detect interstellar clouds appearing to be close to energy equipartition. However, since clouds are actually out of VE, as suggested by asymmetrical line profiles, they should be transient entities. These results are compatible with observationally-based estimations for rapid star formation. , and call into question the models for the star formation efficiency based on clouds being in VE.
Lok Ming Lui - One of the best experts on this subject based on the ideXlab platform.
-
tempo feature endowed teichmuller extremal mappings of point clouds
Siam Journal on Imaging Sciences, 2016Co-Authors: Tingwei Meng, Gary P T Choi, Lok Ming LuiAbstract:In recent decades, the use of three-dimensional point clouds has been widespread in the computer industry. The development of techniques for analyzing point clouds is increasingly important. In particular, mapping of point clouds has been a challenging problem. In this paper, we develop a discrete analogue of the Teichmuller extremal mappings, which guarantees uniform conformality distortions on point cloud surfaces. Based on the discrete analogue, we propose a novel method called TEMPO for computing Teichmuller extremal mappings between feature-endowed point clouds. Using our proposed method, the Teichmuller metric is introduced for evaluating the dissimilarity of point clouds. Consequently, our algorithm enables accurate recognition and classification of point clouds. Experimental results demonstrate the effectiveness of our proposed method.
-
tempo feature endowed teichm uller extremal mappings of point clouds
arXiv: Computational Geometry, 2015Co-Authors: Tingwei Meng, Gary P T Choi, Lok Ming LuiAbstract:In recent decades, the use of 3D point clouds has been widespread in computer industry. The development of techniques in analyzing point clouds is increasingly important. In particular, mapping of point clouds has been a challenging problem. In this paper, we develop a discrete analogue of the Teichm\"{u}ller extremal mappings, which guarantee uniform conformality distortions, on point cloud surfaces. Based on the discrete analogue, we propose a novel method called TEMPO for computing Teichm\"{u}ller extremal mappings between feature-endowed point clouds. Using our proposed method, the Teichm\"{u}ller metric is introduced for evaluating the dissimilarity of point clouds. Consequently, our algorithm enables accurate recognition and classification of point clouds. Experimental results demonstrate the effectiveness of our proposed method.
Aleš Blinc - One of the best experts on this subject based on the ideXlab platform.
-
Biochemical and biophysical conditions for blood clot lysis
Pflügers Archiv - European Journal of Physiology, 2000Co-Authors: Mišo Šabovič, Aleš BlincAbstract:We have studied how pharmacological dissolution of blood clots was affected by clot retraction, the mode of transport of fibrinolytic agents into the clot and the thickness of the composite fibrin fibers. Retracted clots were resistant to fibrinolysis in a milieu without dissolved plasminogen, because the amount of fibrin-bound plasminogen in retracted clots was insufficient for successful clot lysis. In plasma containing plasminogen, retracted clots were successfully lysed with fibrin-specific plasminogen activators, but not with non-fibrin-specific activators. Preincubation of retracted clots in plasma increased their plasminogen content as well as their sensitivity to fibrinolysis. The rate of lysis was increased up to 100-times when plasminogen activator and plasminogen were introduced into cylindrical clots by pressure-induced bulk flow in comparison with diffusion alone. The magnitude of the increase was similar in retracted and nonretracted clots, but the absolute rate of lysis was faster in non-retracted clots. The influence of fibrin fiber thickness on fibrinolysis was studied by atomic force microscopy. The time to complete lateral section of fibers did not differ between thick and thin composite fibers, and the rate of diameter reduction was faster in thick fibers than in thin ones. Taken together our results suggest that lysis of retracted clots proceeds in circular stages: (a) activation of bound plasminogen followed by partial degradation of fibrin, (b) opening of new plasminogen-binding sites on partly degraded fibrin, (c) binding of plasminogen to the new binding sites which enhances the susceptibility of clots to lysis. Lysis is accelerated by bulk flow of plasminogen activator and plasminogen into clots in comparison to diffusion alone. Fibrinolysis of thick composite fibrin fibers proceeds more efficiently than lysis of thin fibers.
Daniel Proga - One of the best experts on this subject based on the ideXlab platform.
-
cloud coalescence a dynamical instability affecting multiphase environments
The Astrophysical Journal, 2019Co-Authors: Tim Waters, Daniel ProgaAbstract:Mass and size distributions are the key characteristics of any astrophysical object, including the densest clumps comprising the cold phase of multiphase environments. In our recent papers, we showed how individual clouds of various sizes form and evolve in active galactic nuclei. In particular, we showed that large clouds undergo damped oscillations as a response to their formation process. Here we follow up this investigation, addressing how different size clouds interact. We find that smaller clouds become trapped in the advective flows generated by larger clouds. The explanation for this behavior leads to a rather remarkable conclusion: even in the absence of gravity, complexes of clouds are dynamically unstable. In an idealized environment (e.g., one free of turbulence and magnetic fields) a perfectly symmetric arrangement of static clouds will remain static, but any small spatial perturbation will lead to all clouds coalescing into a single, large cloud, given enough time. Using numerical simulations, we investigate the main factors that determine the rate of coalescence. In addition to the cloud separation distance, we find that the transient response of clouds to a disturbance is the primary factor. Turbulent motions in the flow can easily suppress this tendency for spatially well-separated clouds to coalesce, so it is as yet unclear if this phenomenon can occur in nature. Nevertheless, this Letter casts strong doubts on a recent hypothesis that large clouds are prone to fragmentation.
-
cloud coalescence a dynamical instability affecting multiphase environments
arXiv: Astrophysics of Galaxies, 2019Co-Authors: Tim Waters, Daniel ProgaAbstract:The mass and size distributions are the key characteristics of any astrophysical objects, including the densest clumps comprising the cold phase of multiphase environments. In our recent papers, we showed how individual clouds of various sizes form and evolve in AGN. In particular, we showed that large clouds undergo damped oscillations as a response to their formation process. Here we followup this investigation, addressing how different size clouds interact. We find that smaller clouds become trapped in the advective flows generated by larger clouds. The explanation for this behavior leads to a rather remarkable conclusion: even in the absence of gravity, complexes of clouds are dynamically unstable. In an idealized environment (e.g., one free of turbulence and magnetic fields) a perfectly symmetric arrangement of static clouds will remain static, but any small spatial perturbation will lead to all clouds coalescing into a single, large cloud, given enough time. Using numerical simulations, we investigate the main factors that determine the rate of coalescence. Besides the cloud separation distance, we find that the transient response of clouds to a disturbance is the primary factor. Turbulent motions in the flow can easily suppress this tendency for spatially well-separated clouds to coalesce, so it is as yet unclear if this phenomenon can occur in nature. Nevertheless, this work casts strong doubts on a recent hypothesis that large clouds are prone to fragmentation.
Yihong Yang - One of the best experts on this subject based on the ideXlab platform.
-
the distances to molecular clouds in the fourth galactic quadrant
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Bingqiu Chen, Shu Wang, L G Hou, Yihong Yang, He Zhao, B W JiangAbstract:Distance measurements to molecular clouds are essential and important. We present directly measured distances to 169 molecular clouds in the fourth quadrant of the Milky Way. Based on the near-infrared photometry from the Two Micron All Sky Survey and the Vista Variables in the Via Lactea Survey, we select red clump stars in the overlapping directions of the individual molecular clouds and infer the bin averaged extinction values and distances to these stars. We track the extinction versus distance profiles of the sightlines toward the clouds and fit them with Gaussian dust distribution models to find the distances to the clouds. We have obtained distances to 169 molecular clouds selected from Rice et al. The clouds range in distances between 2 and 11 kpc from the Sun. The typical internal uncertainties in the distances are less than 5 per cent and the systematic uncertainty is about 7 per cent. The catalogue presented in this work is one of the largest homogeneous catalogues of distant molecular clouds with the direct measurement of distances. Based on the catalogue, we have tested different spiral arm models from the literature.
-
The distances to molecular clouds in the fourth Galactic quadrant
'Oxford University Press (OUP)', 2020Co-Authors: Bingqiu Chen, Shu Wang, Yihong Yang, He Zhao, Ligang Hou, Li Zhiwen, Jiang BiweiAbstract:Distance measurements to molecular clouds are essential and important. We present directly measured distances to 169 molecular clouds in the fourth quadrant of the Milky Way. Based on the near-infrared photometry from the Two Micron All Sky Survey and the Vista Variables in the Via Lactea Survey, we select red clump stars in the overlapping directions of the individual molecular clouds and infer the bin averaged extinction values and distances to these stars. We track the extinction versus distance profiles of the sightlines toward the clouds and fit them with Gaussian dust distribution models to find the distances to the clouds. We have obtained distances to 169 molecular clouds selected from Rice et al. The clouds range in distances between 2 and 11 kpc from the Sun. The typical internal uncertainties in the distances are less than 5 per cent and the systematic uncertainty is about 7 per cent. The catalogue presented in this work is one of the largest homogeneous catalogues of distant molecular clouds with the direct measurement of distances. Based on the catalogue, we have tested different spiral arm models from the literature.Comment: 9 pages, 7 figures, accepted for publication in MNRA