Fragmentation Of Clouds Research Articles

HH 175: a giant Herbig–Haro flow emanating from a multiple protostar

ABSTRACT HH 175 is an isolated Herbig–Haro object seen towards the B35 cloud in the λ Ori region. We use deep Subaru 8 m interference filter images and Spitzer images to show that HH 175 is a terminal shock in a large collimated outflow from the nearby embedded source IRAS 05417+0907. The body of the eastern outflow lobe is hidden by a dense ridge of gas. The western outflow breaks out of the front of the cometary-shaped B35 cloud, carrying cloud fragments along, which are optically visible due to photoionization by the massive λ Ori stars. The total extent of the bipolar outflow is 13.7 arcmin, which at the adopted distance of 415 pc corresponds to a projected dimension of 1.65 pc. The embedded source IRAS 05417+0907 is located on the flow axis approximately midway between the two lobes, and near-infrared images show it to be a multiple system of six sources, with a total luminosity of 31 L⊙. Millimeter maps in CO, 13CO, and C18O show that the B35 cloud is highly structured with multiple cores, of which the one that spawned IRAS 05417+0907 is located at the apex of B35. It is likely that the embedded source is the result of compression by an ionization-shock front driven by the λ Ori OB stars.

Gravitational instability with dust charge gradient and ion drag forces in unmagnetized dusty plasma

The influence of dust charge gradient force and ion drag force on the fragmentation of unmagnetized, self-gravitating dust cloud has been studied. The thermal electrons satisfy the Boltzmann relation, while inertialess ions are affected by the ion-neutral collisions. The dynamics of dusty fluid are modified by ion drag, charge gradient, and gravitational forces. The onset criterion of pinching instability and gravitational instability is derived. The pinching instability depends upon the critical ion drag coefficient and dust charge variation parameter. In the laboratory complex plasma, with finite dust charge variations, the ion drag coefficient larger than the critical value causes pinching instability. This results in the fragmentation of the dusty cloud, which is affected due to the dust charge variations. The ion drag coefficient has destabilizing, while the dust charge variation parameter has stabilizing influence on the growth rate of the linear gravitational instability. The results have been discussed to understand the dust cloud collapse in the astrophysical system.

Simulations of orbital debris clouds due to breakup events and their characterisation using the Murchison Widefield Array radio telescope

In this paper we consider the use of wide field of view radar sensors such as the Murchison Widefield Array (MWA), a low frequency radio telescope designed for astrophysics and cosmology, for rapid response observations of the debris clouds produced by collisions between objects in Earth orbit. With an increasing density of objects in Low Earth Orbit, including legacy assets used by the astronomy community over decades, the risk of new debris clouds forming is also increasing. The MWA constitutes a wide field, rapid response passive radar system and we explore its likely performance in the detection and characterisation of debris clouds. In general, astronomy facilities such as the MWA can play a role in protecting the space environment for the future. In order to undertake this work, we adapt the NASA EVOLVE 4.0 breakup model, utilising the EVOLVE outputs to produce representative dynamic debris clouds. We find that the MWA is likely to detect a large fraction (>70%) of modelled debris cloud fragments for collision masses between 100 kg and 1000 kg for orbits in the lower part of LEO, if the MWA can achieve close to optimal detection sensitivity. Useful detection fractions are still achieved for more conservative assumptions. The detection fraction of fragments decreases as a function of altitude and inversely with collision mass. Encouragingly, we find that the wide field nature of the MWA allows the full evolving debris clouds to be observed in a single observation, with only $\sim2\%$ of the debris fragments escaping the sensitive portion of the field of view after 100 seconds, for all collision masses and altitudes. These results show that the MWA is an intrinsically useful facility for the rapid characterisation of debris clouds, but that work is required to achieve the data processing within an appropriate timeframe to provide rapid alerts.

Estimating diameter at breast height using personal laser scanning data based on stem surface nodes in polar coordinates

ABSTRACT Personal laser scanning (PLS) has shown great potential in diameter of breast height (DBH) estimation. Compared with the current laser scanning technique, PLS is hardly restricted by occlusion and trafficability, and produces omnidirectional point cloud data. However, the DBH estimation accuracy using PLS data was not higher in the previous research. One of the leading sources of bias is overlapping and inaccurately co-registered point cloud fragments. Points of these fragments can be far from the stem surface, which leads to a large bias in DBH estimation. In this letter, a novel DBH estimation method using PLS data, the stem surface node method (SSN) is proposed. To reduce the impacts of the inaccurately co-registered points far from the surface, SSN attempts to use a number of stem surface nodes in polar coordinates instead of the whole stem point cloud to estimate DBH. Six plots with 247 stems under three different working conditions were selected to evaluate the method. Compared with the DBH estimation method using direct circle fitting, SSN achieved a total error reduction of 62.64% and 38.35% for bias and root mean squared error, respectively. These findings confirm that SSN is generally reliable and adaptive for different kinds of working conditions.

Study of the efficiency of detecting and recognizing drone images from a video stream

The authors have developed and experimentally tested an algorithm for processing a video stream of a stationary video camera. It consists of the stages of detecting moving objects and classifying these objects using a neural network. To detect moving objects, the methods of identifying moving objects against a stationary background and analyzing the history of motion were used. Based on the experimental data, the effectiveness of using the models of the background images of MOG, MOG2, KNN, GMG, CNT, GSOC, LSBP for solving the problem was analyzed. Recommendations for the choice of the parameters of these models were formulated. The selection criteria were as follows: high performance and low noise. Models of fully connected and convolutional neural networks were created and trained making it possible to classify 12 types of moving objects. Sets of images were created to train neural networks: drones, fragments of tree foliage, grass, clouds and insects. Based on the results of training and testing networks, recommendations are given for the number of network layers, the number of neurons in a layer, the number of convolutions to achieve maximum performance and recognition accuracy. Comparative analysis of the accuracy of drone classification using fully connected and convolutional networks when processing experimental data has proven the effectiveness of using convolutional networks. The dependence of the drone detection accuracy on the image size and, accordingly, on the distance to this drone is plotted.

Transformation of Satellite Breakup Distribution for Probabilistic Orbital Collision Hazard Analysis

Fragmentation clouds from explosions or collision of payloads and rocket bodies in space pose a threat to objects in Earth orbit. Most of the fragments are too small to be tracked and can only be accounted for statistically. Here, a framework for the fully statistical treatment of a fragmentation cloud, its evolution and ramifications, without the need of simplifying assumptions, is presented. The cloud is modeled as an uncertainty around a single fragment, which can be propagated using any of the existing, nondeterministic, nonlinear orbital uncertainty propagation methods. This work is focused on providing the initial distribution and the estimation of the statistical collision probability. The NASA standard breakup model is revisited to derive a probability distribution of the initial fragment cloud. Two density transformation methods are discussed to obtain the distribution in a subset of orbital elements, suitable for mid- to long-term evolution. The fragment spatial density and the impact rates on targets in any orbit are obtained. The method is applied to show the fragment cloud distribution of a payload collision in low Earth orbit (LEO). Its collision probability with a satellite in LEO and a rocket body in the geostationary transfer orbit are estimated. The result is compared against, and shows the limitations of, sampling and methods based on finite differences.

The links between magnetic fields and filamentary clouds – III. Field-regulated mass cumulative functions

ABSTRACT During the past decade, the dynamical importance of magnetic fields in molecular clouds has been increasingly recognized, as observational evidence has accumulated. However, how a magnetic field affects star formation is still unclear. Typical star formation models still treat a magnetic fields as an isotropic pressure, ignoring the fundamental property of dynamically important magnetic fields: their direction. This study builds on our previous work, which demonstrated how the mean magnetic field orientation relative to the global cloud elongation can affect cloud fragmentation. After the linear mass distribution reported earlier, we show here that the mass cumulative function (MCF) of a cloud is also regulated by the field orientation. A cloud elongated closer to the field direction tends to have a shallower MCF: in other words, a higher portion of the gas is at high density. The evidence is consistent with our understanding of the bimodal star formation efficiency discovered earlier, which is also correlated with the field orientation.

Deuterium Fractionation in the Oph-H-MM1 Dense Core of the L1688 Low Mass Star-Forming Region

Molecular clouds fragment to form dense cores, which are the first stage of star formation. Such objects are cold, with temperature of ~10 K and density of $${{10}^{4}}{-} {{10}^{7}}$$ cm–3, with predominance of thermal motions and high deuterium fraction. These objects give us information about the initial conditions of star formation and thus they are very important to understand this process. High abundance of deuterated species indicates that a dense core is close to the onset of star formation. In this work, we study deuterium fractionation, which occurs due to chemical reactions that take place under cold core conditions. To measure deuterium fraction, we use nitrogen-bearing species, because they stay longer in the gas phase at low temperatures. We choose the L1688 low-mass star-forming region as one of the closest ones, containing a large number of cold dense cores. We use spectral maps of two lines, N2H+(1–0) and N2D+(1–0), towards one of the dense cores in L1688, Oph-H-MM1, observed with the IRAM 30 m telescope. We measure column densities of N2D+ and N2H+ and deuterium fraction as the ratio of column densities. The map shows an increase in deuterium fraction towards the core center, which is consistent with theoretical predictions.

Seeding the second star – II. CEMP star formation enriched from faint supernovae

ABSTRACT Carbon-enhanced metal-poor (CEMP) stars are the living fossils holding records of chemical enrichment from early generations of stars. In this work, we perform a set of numerical simulations of the enrichment from a supernova (SN) of a first generation of metal-free (Pop III) star and the gravitational collapse of the enriched cloud, considering all relevant cooling/heating processes and chemical reactions as well as the growth of dust grains. We adopt faint SN models for the first time with progenitor masses MPopIII = 13–$80 \ {\rm M_{\bigodot }}$, which yield C-enhanced abundance patterns ([C/Fe] = 4.57–4.75) through mixing and fallback of innermost layers of the ejecta. This model also considers the formation and destruction of dust grains. We find that the metals ejected by the SN can be partly re-accreted by the same dark matter minihalo, and carbon abundance of the enriched cloud A(C) = 3.80–5.06 is lower than the abundance range of observed CEMP stars (A(C) ≳ 6) because the mass of the metals ejected by faint SNe is smaller than normal core-collapse SNe due to extensive fallback. We also find that cloud fragmentation is induced by gas cooling from carbonaceous grains for $M_{\rm Pop III}= 13 \ {\rm M_{\bigodot }}$ even with the lowest iron abundance [Fe/H] ∼ −9. This leads to the formation of low-mass stars, and these ‘giga metal-poor’ stars can survive until the present-day Universe and may be found by future observations.

Random fragmentation of turbulent molecular clouds lying in the central region of giant galaxies

A stochastic model of fragmentation of molecular clouds has been developed for studying the resulting Initial Mass Function (IMF) where the number of fragments, inter-occurrence time of fragmentation, masses and velocities of the fragments are random variables. Here two turbulent patterns of the velocities of the fragments have been considered, namely, Gaussian and Gamma distributions. It is found that for Gaussian distribution of the turbulent velocity, the IMFs are shallower in general compared to Salpeter mass function. On the contrary, a skewed distribution for turbulent velocity leads to an IMF which is much closer to Salpeter mass function. The above result might be due to the fact that strong driving mechanisms e.g. shocks, arising out of a big explosion occurring at the centre of the galaxy or due to big number of supernova explosions occurring simultaneously in massive parent clouds during the evolution of star clusters embedded into them are responsible for stripping out most of the gas from the clouds. This inhibits formation of massive stars in large numbers making the mass function a steeper one.

On models of meteoroid disruption into the cloud of fragments

The meteoroid disruption into a large number of fragments when they move with a common shock wave before dispersion to a sufficient distance to move independently is considered. Disrupted meteoroid under the action of pressure forces is deformed: it is compressed in a flight direction and expands in a lateral one. Four models of the meteoroid disruption into a cloud of fragments moving as a single body are considered: two models developed by the authors and two models used in the literature (Grigoryan, 1979; Hills and Goda, 1993). The principal differences between the proposed and accepted models are shown. Numerical solutions of the meteor physics equations are obtained using the above models to simulate the interaction of the Chelyabinsk meteoroid with the atmosphere. To model the meteoroid ablation, an approximate formula is used for the radiative heat transfer coefficient as a function of the meteoroid velocity and size and the atmospheric density. The influence of the heat transfer coefficient uncertainty (including its constant value) on the meteoroid mass loss, energy deposition and initial mass estimate is studied. Solutions using different fragmentation models are compared with each other and with observational data and the results of comparisons are discussed.

Oversized Gas Clumps in an Extremely Metal-poor Molecular Cloud Revealed by ALMA’s Parsec-scale Maps

Abstract Metals are thought to have profound effects on the internal structures of the molecular clouds in which stars are born. The absence of metals is expected to prevent gas from efficient cooling and fragmentation in theory. However, this effect has not yet been observed in low-metallicity environments, such as in the early universe and local dwarf galaxies, because of the lack of high-spatial-resolution maps of gas. We carried out Atacama Large Millimeter/submillimeter Array observations of the carbon monoxide (CO) J = 2–1 emission line at 1.4 parsec resolutions of a molecular cloud in DDO 70 at 7% solar metallicity, the most metal-poor galaxy currently known with a CO detection. In total, five clumps have been identified and they are found to more or less follow Larson’s law. Since the CO emission exists in regions with visual extinction A V around 1.0, we converted this A V to the gas-mass surface density using a gas-to-dust ratio of 4594 ± 2848 for DDO 70. We found that the CO clumps in DDO 70 exhibit significantly larger (on average four times) sizes than those at the same gas-mass surface densities in the massive star formation regions of the Milky Way. The existence of such large clumps appears to be consistent with theoretical expectations that gas fragmentation in low-metallicity clouds is suppressed. While our observation is only for one cloud in the galaxy, if it is representative, the above result implies suppressed gas fragmentation during the cloud collapse and star formation in the early universe.

Annular Neighboring Points Distribution Analysis: A Novel PLS Stem Point Cloud Preprocessing Algorithm for DBH Estimation

Personal laser scanning (PLS) has significant potential for estimating the in-situ diameter of breast height (DBH) with high efficiency and precision, which improves the understanding of forest structure and aids in building carbon cycle models in the big data era. PLS collects more complete stem point cloud data compared with the present laser scanning technology. However, there is still no significant advantage of DBH estimation accuracy. Because the error caused by merging different point cloud fragments has not yet been eliminated, overlapping and inaccurate co-registered point cloud fragments are often inevitable, which are usually the leading error sources of PLS-based DBH estimation. In this study, a novel pre-processing algorithm named annular neighboring points distribution analysis (ANPDA) was developed to improve PLS-based DBH estimation accuracy. To reduce the impact of inaccurately co-registered point cloud fragments, ANPDA identified outliers through iterative removal of outermost points and analyzing the distribution of annular neighboring points. Six plots containing 247 trees under different forest conditions were selected to evaluate the ANPDA. Results showed that in the six plots, error reductions of 53.80–87.13% for bias, 38.82–57.30% for mean absolute error (MAE), and 27.17–56.02% for root mean squared error (RMSE) were achieved after applying ANPDA. These results confirmed that ANPDA was generally effective for improving PLS-based DBH estimation accuracy. It appeared that ANPDA could be conveniently fused with an automatic PLS-based DBH estimation process as a preprocessing algorithm. Furthermore, it has the potential to predict and warn operators of potential large errors during hierarchical semi-automatic DBH estimation.

A Rotation Rate for the Planetary-mass Companion DH Tau b

Abstract DH Tau b is a young planetary-mass companion orbiting at a projected separation of 320 au from its ∼2 Myr old host star DH Tau. With an estimated mass of 8–22 this object straddles the deuterium-burning limit, and might have formed via core or pebble accretion, disk instability, or molecular cloud fragmentation. To shed light on the formation history of DH Tau b, we obtain the first measurement of rotational line broadening for this object using high-resolution (R ∼ 25,000) near-infrared spectroscopy from Keck/NIRSPEC. We measure a projected rotational velocity (v sin i) of 9.6 ± 0.7 km s−1, corresponding to a rotation rate that is between 9% and 15% of DH Tau b’s predicted breakup speed. This low rotation rate is in good agreement with scenarios in which magnetic coupling between the companion and its circumplanetary disk during the late stages of accretion reduces angular momentum and regulates spin. We compare the rotation rate of DH Tau b to published values for other planetary-mass objects with masses between 0.3 and 20 and find no evidence of a correlation between mass and rotation rate in this mass regime. Finally, we search for evidence of individual molecules in DH Tau b’s spectrum and find that it is dominated by CO and H2O, with no evidence of the presence of CH4. This agrees with expectations given DH Tau b’s relatively high effective temperature (∼2300 K).

Cloud fragmentation cascades and feedback: on reconciling an unfettered inertial range with a low star formation rate

ABSTRACT Molecular cloud complexes exhibit both (i) an unfettered Larson-type spectrum over much of their dynamic range, whilst (ii) still producing a much lower star formation rate than were this cascade to remain unfettered all the way down to star-forming scales. Here we explain the compatibility of these attributes with minimalist considerations of a mass-conserving fragmentation cascade, combined with estimates of stellar feedback. Of importance is that the amount of feedback needed to abate fragmentation and truncate the complex decreases with decreasing scale. The scale at which the feedback momentum matches the free-fall momentum marks a transition scale below most of the cascade is truncated and the molecular cloud complex dissipated. For a 106 M⊙ giant molecular cloud (GMC) complex starting with radius of ∼50 pc, the combined feedback from young stellar objects, supernovae, radiation, and stellar winds for a GMC cloud complex can truncate the cascade within an outer free-fall time but only after the cascade reaches parsec scales.

A turbulent-entropic instability and the fragmentation of star-forming clouds

ABSTRACT The kinetic energy of supersonic turbulence within interstellar clouds is subject to cooling by dissipation in shocks and subsequent line radiation. The clouds are therefore susceptible to a condensation process controlled by the specific entropy. In a form analogous to the thermodynamic entropy, the entropy for supersonic turbulence is proportional to the log of the product of the mean turbulent velocity and the size scale. We derive a dispersion relation for the growth of entropic instabilities in a spherical self-gravitating cloud and find that there is a critical maximum dissipation time-scale, about equal to the crossing time, that allows for fragmentation and subsequent star formation. However, the time-scale for the loss of turbulent energy may be shorter or longer, for example, with rapid thermal cooling or the injection of mechanical energy. Differences in the time-scale for energy loss in different star-forming regions may result in differences in the outcome, for example, in the initial mass function.

On limiting the lateral expansion of the cloud of fragments of a destructed meteoroid

On limiting the lateral expansion of the cloud of fragments of a destructed meteoroid

Improved cloud detection for Landsat 8 images using a combined neural network model

ABSTRACTHigh-precision cloud detection is a key step in the processing of remote sensing imagery. However, the existing cloud detection methods struggle to extract high-accuracy cloud pixels, especially for images of thin and fragmented clouds or those over high-brightness surfaces. In this study, we developed a new model by combining the existing models of Fully Convolutional Network-8 sample (FCN-8s) and U-network (U-net) (based on the three visible bands) to take full advantage of spectral and spatial information. In the proposed Fully Convolutional Network Ensembling Learning (FCNEL) model, U-net and FCN-8s initially conduct separate classifications based on their relative strengths, and their outputs are fused by the voting strategy to integrate multi-scale features from both the models. Different surface and cloud types in Landsat 8 Operational Land Imager (OLI) data were used to test the model, which showed an average overall accuracy of 91.68% and an average producer accuracy of 98.52%. Thus, the proposed FCNEL model was superior to FCN-8s or U-net as well as the widely used function of mask algorithm. The proposed method has good adaptability to various cloud types and diverse underlying surface environments.

An Improved Model to Increase Retrieval Time and Security by Data Fragmentation and Replication Process in Cloud

Cloud computing provides several features to users as well as to the organizations. Even though, there are some issues faced by the user while usingthe cloud. Security is a major concern that is always considered. Likewise Data replication is a significant technique to be consideredfor retrieval time. Replication helps to fetch the data from remote which is a high-time consuming process.To overcome the security issue along with data replication a novel approach is proposed in this paper.Dynamic fragmentation is utilized for the division of a file into fragments. Each cloud nodes has a different fragment to enhance the data security of the system. Blowfish technique is used for encrypt the files before storing in cloudthatdivides messages into 64 bits blocks then encrypts them separately. The result of experimental evaluation shows that this schemes increase the overall performance.

Cloudlet capture by transitional disk and FU Orionis stars

After its formation, a young star spends some time traversing the molecular cloud complex in which it was born. It is therefore not unlikely that, well after the initial cloud collapse event which produced the star, it will encounter one or more low mass cloud fragments, which we call “cloudlets” to distinguish them from full-fledged molecular clouds. Some of this cloudlet material may accrete onto the star+disk system, while other material may fly by in a hyperbolic orbit. In contrast to the original cloud collapse event, this process will be a “cloudlet flyby” and/or “cloudlet capture” event: A Bondi–Hoyle–Lyttleton type accretion event, driven by the relative velocity between the star and the cloudlet. As we will show in this paper, if the cloudlet is small enough and has an impact parameter similar or less than GM*/v∞2 (with v∞ being the approach velocity), such a flyby and/or capture event would lead to arc-shaped or tail-shaped reflection nebulosity near the star. Those shapes of reflection nebulosity can be seen around several transitional disks and FU Orionis stars. Although the masses in the those arcs appears to be much less than the disk masses in these sources, we speculate that higher-mass cloudlet capture events may also happen occasionally. If so, they may lead to the tilting of the outer disk, because the newly infalling matter will have an angular momentum orientation entirely unrelated to that of the disk. This may be one possible explanation for the highly warped/tilted inner/outer disk geometries found in several transitional disks. We also speculate that such events, if massive enough, may lead to FU Orionis outbursts.