Distinct Formation Mechanisms Research Articles

Formation mechanism of Fe oxyhydroxides and behavior of metals during the oxidation of submarine sulfides at the Wocan-1 hydrothermal field, Carlsberg Ridge

Hydrothermal sulfides undergo complex oxidation process under seawater conditions, which has the potential to impact resource values and marine environments. However, the oxidation process is not fully explored, particularly regarding the transformation of minerals and the behavior of metals. In this paper, we conducted detailed mineralogical and geochemical analyses on a series of samples with varying degrees of oxidation, collected from the Wocan-1 hydrothermal field, Carlsberg Ridge, Northwest Indian Ocean. The principal purpose is to illuminate the formation and transformation of Fe (oxyhydr)oxides, and further to reveal the migration and redistribution of key metals throughout the oxidation process. We identified four types of Fe(−Si) (oxyhydr)oxides with two distinct formation mechanisms. Three Fe (oxyhydr)oxides are the direct oxidation products of pyrite and marcasite, while Fe-Si (oxyhydr)oxide precipitates from low-temperature hydrothermal fluids. Thin Fe (oxyhydr)oxide forms as the secondary product of euhedral pyrite at the early stage of oxidation. Pseudomorphic Fe (oxyhydr)oxide with low Fe contents, occurs as the replacement of euhedral marcasite. Filamentous Fe (oxyhydr)oxide, enriched in trace metals, is attributed to the uniform oxidation of subhedral pyrite-marcasite intergrowth. Moreover, major and trace elements occur multiple migrations and redistributions among primary sulfides, secondary products, and seawater. Notably, Fe (oxyhydr)oxides, through the sequestration mechanism, not only retain Cu and Zn released by the oxidation of primary sulfide minerals, but also scavenge Cu from seawater. However, the dissolution of Pb, As, Mo, and Co exceeds the amount retained during the oxidation, indicating that the potential release of toxic metals into the environment could pose a threat to the local ecosystem. These new insights can provide an initial foundation for the effect of submarine oxidation on economic values of sulfides and ecological environments of deep sea.

Microstructural Stability and High-Temperature Oxidation Behavior of Al0.25CoCrCuFeNi High Entropy Alloy

Al0.25CoCrCuFeNi is a high-entropy alloy composed of transition metals, specifically designed for high-temperature applications owing to its favorable mechanical properties, high melting point, and excellent high-temperature resistance. This alloy has been identified as a promising material for space exploration, particularly in the fabrication of combustion chambers and rocket nozzles by the National Aeronautics and Space Agency. Ongoing alloy development involves modifying the elemental composition. This study reduced aluminum content in the equiatomic AlCoCrCuFeNi alloy to Al0.25CoCrCuFeNi, followed by isothermal oxidation treatments at 800, 900, and 1000℃. A series of experiments were conducted to investigate the microstructure stability and oxidation behavior of the Al0.25CoCrCuFeNi alloy. The alloying elements were melted using a single DC electric arc furnace, followed by homogenization at 1100°C for 10 hours in an inert atmosphere. Subsequently, samples were cut into coupons for isothermal oxidation testing at the desired temperatures for 2, 16, 40, and 168 hours. The oxidized samples were characterized using XRD (x-ray diffraction), SEM (scanning electron microscopy) equipped with EDS (energy-dispersive X-ray spectroscopy), optical microscopy, and Vickers hardness testing. The as-homogenized alloy consisted of two constituent phases: an FCC (face-centered cubic) phase in the dendritic region and a copper-rich FCC phase in the inter-dendritic region. The oxides formed during the oxidation process included Al2O3, Cr2O3, Fe3O4, CoO, CuO, NiO, and spinel oxides (Co,Ni,Cu)(Al,Cr,Fe)2O4), with distinct formation mechanisms at each temperature.

An enzymatic hydrolysis-based platform technology for the efficient high-yield production of cellulose nanospheres

This study evaluates the feasibility of using enzymatic technology to produce novel nanostructures of cellulose nanomaterials, specifically cellulose nanospheres (CNS), through enzymatic hydrolysis with endoglucanase and xylanase of pre-treated cellulose fibers. A statistical experimental design facilitated a comprehensive understanding of the process parameters, which enabled high yields of up to 82.7 %, while maintaining a uniform diameter of 54 nm and slightly improved crystallinity and thermal stability. Atomic force microscopy analyses revealed a distinct CNS formation mechanism, where initial fragmentation of rod-like nanoparticles and subsequent self-assembly of shorter rod-shaped nanoparticles led to CNS formation. Additionally, adjustments in process parameters allowed precise control over the CNS diameter, ranging from 20 to 100 nm, highlighting the potential for customization in high-performance applications. Furthermore, this study demonstrates how the process framework, originally developed for cellulose nanocrystals (CNC) production, was successfully adapted and optimized for CNS production, ensuring scalability and efficiency. In conclusion, this study emphasizes the versatility and efficiency of the enzyme-based platform for producing high-quality CNS, providing valuable insights into energy consumption for large-scale economic and environmental assessments.

Binary Cation Matrix Electrolyte and Its Effect on Solid Electrolyte Interphase Suppression and Evolution of Si Anode.

An unstable solid electrolyte interphase (SEI) has been recognized as one of the biggest challenges to commercializing silicon (Si) anodes for high-energy-density batteries. This work thoroughly investigates a binary cation matrix of Mg2++Li+ electrolyte and its role in SEI development, suppression, and evolution of a Si anode. Findings demonstrate that introducing Mg ions dramatically reduces the SEI growth before lithiation occurs, primarily due to the suppression of solvent reduction, particularly ethylene carbonate (EC) reduction. The Mg2+ alters the Li+ cation solvation environment as EC preferably participates in the oxophyllic Mg2+ solvation sheath, thereby altering the solvent reduction process, resulting in a distinct SEI formation mechanism. The initial SEI formation before lithiation is reduced by 70% in the electrolyte with the presence of Mg2+ cations. While the SEI continues to develop in the postlithiation, the inclusion of Mg ions results in an approximately 80% reduction in the postlithiation SEI growth. Continuous electrochemical cycling reveals that Mg2+ plays a crucial role in stabilizing the deep-lithiated Si phases, which effectively mitigates side reactions, resulting in controlled SEI growth and stable interphase while eliminating complex LixSiy formation. Mg ions promote the development of a notably more rigid and homogeneous SEI, characterized by a reduced dissipation (ΔD) in the Mg2++Li+ ion matrix compared to the solely Li+ system. This report reveals how the Mg2++Li+ ion matrix affects the SEI evolution, viscoelastic properties, and electrochemical behavior at the Si interface in real time, laying the groundwork for devising strategies to enhance the performance and longevity of Si-based next-generation battery systems.

Geology and in-situ S-Pb isotopes of the Luoboshan carbonate-hosted Pb-Zn deposit in Yunnan Province, SW China

The Luoboshan Pb-Zn deposit (0.2 Mt Pb + Zn metal reserves, @ 0.94 % Pb and 5.33 % Zn) is located in the Lanping-Simao Basin in the southern part of the Sanjiang Tethys metallogenic domain in Yunnan Province, SW China. The deposit comprises three ore bodies hosted in the lower Permian bioclastic and micritic limestone. Ore body I is stratiform (mineralization is small in scale and ores have been mined out), with average grades of 0.02–1.44 % Pb and 1.5–14.88 % Zn. Ore bodies II and III are lens-like, the average grades of them are 0.10–5.07 % Pb and 1.68–12.39 % Zn, and 0.10–1.71 % Pb and 1.14–11.79 % Zn, respectively. Two hydrothermal stages were identified in the Luoboshan deposit: Pyrite-I-sphalerite-galena-calcite (stage I) and Pyrite-II-calcite (stage II). The in-situ δ34S values of pyrite and sphalerite reveals a medium range of −8.80 ‰ to +10.1 ‰. Furthermore, Py-I and sphalerite exhibit low δ34S values from −8.80 ‰ to −3.04 ‰, but Py-II displays high δ34S values (+2.37 ‰ to +10.0 ‰). These results suggest distinct formation mechanism for reduced sulfur during different stages of Pb-Zn mineralization. A reasonable explanation is that a transition from bacterial sulfate reduction (BSR) in the early stage to thermochemical sulfate reduction (TSR) in the late stage as the likely mechanism for the formation of reduced sulfur. In addition, the in-situ Pb isotopes exhibit a narrow range (206Pb/204Pb = 18.660–18.672, 207Pb/204Pb = 15.665–15.677 and 208Pb/204Pb = 38.940–38.980), indicating that the metals originated from either a single source or a uniform mixture of different sources. Comparison of geological charateristics and isotopic geochemical data (S and Pb) indicates that the basin sedimentary strata (wall rocks) is the only and most probable source. Hence, we propose that the formation of the Luoboshan deposit involved biogenic in-situ acid generation of marine sulfates, which subsequently mixed with Pb-and-Zn-rich hydrothermal fluids, initiating TSR and resulting in Pb-Zn mineralization.

Hazard assessment of seismic-collapsed loess landslides on the Loess Plateau as exemplified by the M6.2 earthquake in Jishishan County, China

The Loess Plateau is marked by intense neotectonic activity and frequent earthquakes. Its unique physico-mechanical properties, combined with the granular overhead pore structure of loess, render it prone to seismic landslides triggered by strong earthquakes. Different types of loess seismic landslides have distinct formation mechanisms, disaster-causing characteristics, and risk assessment programs. In this study, the risk of seismic-collapsed loess landslides as one of the types of loess seismic landslides was evaluated on the Loess Plateau. A risk zoning map for seismic-collapsed loess landslides on the Loess Plateau, considering various exceedance probabilities, was compiled by assessing eight factors. These factors include peak ground acceleration, microstructure of loess, and were evaluated using both the minimum disaster-causing seismic peak ground acceleration zoning method and the analytic hierarchy process. The following conclusions were obtained: (1) Earthquakes are the primary inducing factor for seismic-collapsed loess landslides, with other factors serving as influencers, among which the microstructure of loess carries the highest weight; (2) Across various exceedance probabilities, the likelihood of seismic-collapsed loess landslides occurring at 63% of the 50-year exceedance probability is low. Moreover, as the minimum hazard-causing seismic peak ground acceleration increases, the risk of occurrence of seismic-collapsed loess landslides rises, leading to a gradual expansion of the area share in moderate and high-risk zones; (3) Hazard evaluation results align well with existing data on seismic-collapsed loess landslides and findings from field investigations. The case of seismic-collapsed loess landslides induced by the M6.2 magnitude earthquake in Jishishan County, China, is presented as an illustration. The combined use of the minimum hazard-causing seismic peak ground acceleration zoning method and the analytic hierarchy process method offers a reference for geohazard hazard assessment, with earthquakes as the primary inducing factor and other factors as influencers.

Co-existing two distinct formation mechanisms of micro-scale ooid-like manganese carbonates hosted in Cryogenian organic-rich black shales in South China

Manganese-rich deposits in the lower member of the Datangpo Formation (DTP) (ca. 663–654 Ma) in South China formed in the aftermath of the Cryogenian Sturtian glaciation. The Mn in the DTP occurs dominantly as rhodochrosite and Ca-rhodochrosite. A hydrothermal origin of the Mn2+ is shown by the rare earth element distribution and significantly high Mn/Fe ratios (3–19, average = 10.1). Previous studies suggested a microbially-mediated process for controlling the DTP black-shale hosted Mn carbonate deposits. However, detailed reports on the formation mechanisms of micro-scale (<2–5 μm) ooid-like Mn carbonates in the DTP have rarely been published. Systematic petrography and geochemical analyses in this study demonstrate the coexistence of two types of micro-scale ooidal-like Mn carbonates formed through two distinct mechanisms, either dominated by microbially-mediated or physiochemically-forced pathways. The Type I Mn carbonate has relatively larger grain size of 2–5 μm and exhibits a radial-concentric microfabric that shows signs of growth banding in the form of alternating light and dark laminae, which mainly express variation in Ca and Mn concentrations. The initial precipitation phase of the Type I Mn carbonate is interpreted to be Mn oxide/hydroxide, based on positive Ce anomalies and selective enrichments of particular trace elements. Novel evidence indicates that the capture of Mn as a carbonate phase directly from the water column by primarily precipitated calcite, which is referred to as the Type II Mn carbonate, has also contributed to the DTP Mn-rich deposits. Multiple roles of organic matter in Mn carbonate formation have been established: (1) catalysed Mn-redox cycling; (2) trapping and transportation of initial mineral precipitates to sediments; (3) serving as a carbon source; (4) regulating the morphology of the Mn carbonate. As a key link for understanding Cryogenian carbon and Mn cycling, specific formation pathways for the DTP Mn-carbonates are likely to have been controlled by given atmospheric-oceanic compositions (including oxygen level, pCO2, and redox conditions) in response to major geological and biological events during the interglacial period. In turn, massive storage of inorganic carbon and phosphorous in Mn carbonate phases would have had a substantial influence on biogeochemical carbon cycling during the Cryogenian.

South-South cross-border marriage between Chinese men and Ethiopian women

This article develops an ideal type of South-South cross-border marriage. Based on an 8-month multi-sited ethnography in Ethiopia and China, I identified an unusual conjuncture of global forces, connections, and imaginations that facilitated cross-border marriages between Chinese men and local women in Ethiopia, which should be considered a novel ideal type. Its theoretical novelty is not only defined by the unique dynamics among Sino-Ethiopian spouses vis-à-vis the ‘segregated’ Chinese documented in existing studies but also by these marriages’ distinct formation mechanisms. Sino-Ethiopian marriage is not formed due to China being an attractive destination but associated with China’s incompatible hard and soft power as forces, Chinese factories and accumulated Sino-Ethiopian social networks in local communities as connections, and localized imaginations. Furthermore, this study calls for a paradigm shift in examining cross-border marriages between a developing South and a rising South in this dramatically changing global capitalist world system.

Square and rhombic lattices of magnetic skyrmions in a centrosymmetric binary compound

Magnetic skyrmions are topologically stable swirling spin textures with particle-like character, and have been intensively studied as a candidate of high-density information bit. While magnetic skyrmions were originally discovered in noncentrosymmetric systems with Dzyaloshinskii-Moriya interaction, recently a nanometric skyrmion lattice has also been reported for centrosymmetric rare-earth compounds, such as Gd2PdSi3 and GdRu2Si2. For the latter systems, a distinct skyrmion formation mechanism mediated by itinerant electrons has been proposed, and the search of a simpler model system allowing for a better understanding of their intricate magnetic phase diagram is highly demanded. Here, we report the discovery of square and rhombic lattices of nanometric skyrmions in a centrosymmetric binary compound EuAl4, by performing small-angle neutron and resonant elastic X-ray scattering experiments. Unlike previously reported centrosymmetric skyrmion-hosting materials, EuAl4 shows multiple-step reorientation of the fundamental magnetic modulation vector as a function of magnetic field, probably reflecting a delicate balance of associated itinerant-electron-mediated interactions. The present results demonstrate that a variety of distinctive skyrmion orders can be derived even in a simple centrosymmetric binary compound, which highlights rare-earth intermetallic systems as a promising platform to realize/control the competition of multiple topological magnetic phases in a single material.

Nanoscale transformations of amphiboles within human alveolar epithelial cells

Amphibole asbestos is related to lung fibrosis and several types of lung tumors. The disease-triggering mechanisms still challenge our diagnostic capabilities and are still far from being fully understood. The literature focuses primarily on the role and formation of asbestos bodies in lung tissues, but there is a distinct lack of studies on amphibole particles that have been internalized by alveolar epithelial cells (AECs). These internalized particles may directly interact with the cell nucleus and the organelles, exerting a synergistic action with asbestos bodies (AB) from a different location. Here we document the near-atomic- to nano-scale transformations induced by, and taking place within, AECs of three distinct amphiboles (anthophyllite, grunerite, “amosite”) with different Fe-content and morphologic features. We show that: (i) an Fe-rich layer is formed on the internalized particles, (ii) particle grain boundaries are transformed abiotically by the internal chemical environment of AECs and/or by a biologically induced mineralization mechanism, (iii) the Fe-rich material produced on the particle surface does not contain large amounts of P, in stark contrast to extracellular ABs, and (iv) the iron in the Fe-rich layer is derived from the particle itself. Internalized particles and ABs follow two distinct formation mechanisms reaching different physicochemical end-states.

Spontaneous Formation and Fusion of Raspberry Vesicle Self-Assembled from Star Block Terpolymers in Aqueous Solution.

The spontaneous formation and fusion of raspberry vesicles was studied using the dissipative particle dynamics (DPD) method. The vesicles were formed through the self-assembly of amphiphilic E12O6F2 star terpolymers in selective solvent. E and F blocks are solvophobic and the O block is solvophilic. The shortest F block plays a major role in the formation of raspberry vesicles. Distinct vesicle formation mechanisms were observed at different polymer concentrations. At higher concentrations, vesicles form via the bending and closure of an oblate F-bump-E bilayer. At lower concentrations, the formation pathway contains: the initial formation of a vesicle with a core, the combination of such vesicles into cylindrical micelles, and the bending of the cylindrical micelles to form a hollow vesicle. In addition, raspberry vesicle fusion is regulated by F bumps through the continuous coalescence of them from apposed vesicle membranes. The contact area bends, followed by the formation of a fusion pore and a tilted inner layer. As the pore sealed, the hemifusion structure appears, which further restructures to form a vesicle. Our results provide guidance on understanding the dynamic processes of complex vesicles and biological membrane fusion.

Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

Similar carbonate fabrics may result from different pathways of precipitation and diagenetic replacement. Distinguishing the underlying mechanisms leading to a given carbonate fabric is relevant, both in terms of an environmental and diagenetic interpretation. Prominent among carbonate fabrics are aragonite botryoids and spherulites, typically interpreted as direct seawater precipitates and used as proxies for fluid properties and depositional environments. This study investigated μm to mm-scale Holocene botryoidal and spherulitic aragonite from marine and non-marine carbonate settings associated with microbial mats, and reports two distinct formation mechanisms: 1) early diagenetic replacement, and 2) primary precipitation via nanocrystal aggregation. In the intertidal microbial mats of Khawr Qantur (Abu Dhabi), botryoidal and spherulitic aragonite are replacement products of heavily micritized bioclasts. To form the botryoidal and spherulitic aragonite, skeletal rods and needles, resulting from disintegration of micritized bioclasts, recrystallize into nanocrystals during early marine diagenesis. These nanocrystals then grow into fibrous crystals, forming botryoidal and spherulitic aragonite. In the lacustrine microbial bioherms of the hypersaline Great Salt Lake (United States) and in the hydrothermal travertines of Bagni San Filippo (Italy), botryoidal and spherulitic aragonite evolve from nanocrystals via precipitation. The nanocrystals are closely associated with extracellular polymeric substances in microbial biofilms and aggregate to form fibrous crystals of botryoidal and spherulitic aragonite. The studied fabrics form a portion of the bulk sediment and show differences in terms of their formation processes and petrological features compared to the often larger (few mm to over 1 m) botryoidal and spherulitic aragonite described from open-marine reefal cavities. Features shown here may represent modern analogues for ancient examples of carbonate depositional environments associated with microbialites. The implication of this research is that botryoidal and spherulitic aragonite associated with microbial mats are relevant in paleoenvironmental interpretations, but must be combined with a detailed evaluation of their formation process. Care must be taken as the term “botryoidal and spherulitic aragonite” may in fact include, from the viewpoint of their nucleation and formation mechanism, similar fabrics originated from different pathways. At present, it seems unclear to which degree the μm to mm-scale botryoids and spherulites described here are comparable to their cm-to dm-size counterparts precipitated as cements in the open pore space of reefal environments. However, it is clear that the investigation of ancient botryoidal and spherulitic aragonite must consider the possibility of an early diagenetic replacement origin of these precipitates.

Controllable Vacancy Redistributions in Embedded Thin Films Under Concentrated Electric Fields

The redistribution and accumulation of oxygen vacancies under electric field underpins both the functionality and degradation of electrical and electrochemical devices comprising oxide layers. Interfacial accumulations of oxygen vacancies form defect dipoles, significantly reduce the breakdown strength, increase the breakdown path area, yet facilitates charge transfer reactions. Confinement of both oxygen vacancy redistributions and their mobility remains critical to preserving function in oxide systems.Here we study one of the most important, yet random transport processes: the redistribution of oxygen vacancies in binary oxides under high local electric fields. The ability to identify or predefine the shape, size, and location of vacancy distributions is critical to controlling interfacial properties of functional oxides. We achieved superior uniformity in HfO2 films by embedding highly-ordered metal nanoisland (NI) arrays to act as electric field concentrators. Embedded films exhibit significant reduction in operating voltages while displaying enhanced uniformity of resistance states. This behavior is attributed to the concentration of electric fields along Pt and Ti NIs and their interactions with the surrounding oxide film matrix environment, which induce separate and distinct filamentary formation mechanisms that affect the stability. A comparison of the density and distribution of the oxygen vacancies responsible driving filament dynamics is made via combined electrostatic force microscopy (EFM) and conductive atomic force microscopy (c-AFM) studies. Finally, the morphological evolution of conducting vacancy filaments is enabled by three-dimensional (3-D) c-AFM nanotomography and cross-sectional scanning transmission electron microscopy – energy dispersive spectroscopy (STEM-EDS) to provide direct correlations of nanoisland-oxide interactions with overall switching performance. The application of these exciting results to other vacancy-mediated electronic and electrochemical processes will also be discussed.

Grain boundary serration in nickel alloy inconel 600: Quantification and mechanisms

The serration of grain boundaries in Inconel 600 caused by heat treatment is studied systematically. A new method based on Fourier transforms is used to analyse the multiple wave-like character of the serrated grain boundaries. A new metric – the serration index – is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between serrated and non-serrated boundaries. By considering the variation of the serration index with processing parameters, a causal relationship between degree of serration and solution treatment/cooling rate is elucidated. Processing maps for the degree of serration are presented. Two distinct formation mechanisms arise which rely upon grain boundary interaction with carbides: (i) Zener-type dragging which hinders grain boundary migration and (ii) a faceted carbide growth-induced serration.

Are submarine and subaerial drainages morphologically distinct?

AbstractThe qualitative resemblance between terrestrial and submarine branched valley networks has led to speculation that common underlying processes control their formation. However, quantitative comparisons have been impeded by methodological limitations and coarse resolution in marine systems. We analyze channel concavity and steepness indices of 23 terrestrial and 29 submarine catchments to determine whether their profile morphologies are distinct. Statistical comparisons of these quantities demonstrate that concavity indices in submarine systems are, in general, lower than in subaerial systems, and that submarine tributaries are steeper than their associated mainstem. These differences may reflect distinct drainage formation mechanisms and dynamics of submarine sediment gravity flows as compared to overland flow processes.

Highly Uniform Resistive Switching in HfO2 Films Embedded with Ordered Metal Nanoisland Arrays

AbstractMemristors enter a critical developmental stage where emerging large‐scale integration methods face major challenges with severe switching instabilities in the oxide layer. Here, the superior uniformity is achieved within HfO2 films by embedding highly ordered metal nanoisland (NI) arrays. Embedded films exhibit a significant reduction in both SET and RESET while displaying enhanced uniformity in operating voltages and resistance states. This behavior is attributed to the concentration of electric fields along Pt and Ti NIs and their interactions with the surrounding oxide film matrix environment, which induce separate and distinct filamentary formation mechanisms that affect the stability. A method is reported to further optimize the uniformity of the SET voltage by translating the NI array position down the film‐thickness dimension towards the bottom electrode. A comparison of the density and distribution of the oxygen vacancies responsible for the formation/dissolution of conducting filaments is made via combined electrostatic force microscopy and conductive atomic force microscopy (c‐AFM) studies. Finally, complete observation of the morphological evolution of conducting filaments produced by Pt and Ti is enabled by 3D c‐AFM nanotomography and cross‐sectional scanning transmission electron microscopy–energy dispersive spectroscopy to provide direct correlations between NI‐oxide interactions and overall switching performance.

SMASHing the LMC: Mapping a Ring-like Stellar Overdensity in the LMC Disk

Abstract We explore the stellar structure of the Large Magellanic Cloud (LMC) disk using data from the Survey of the MAgellanic Stellar History and the Dark Energy Survey. We detect a ring-like stellar overdensity in the red clump star count map at a radius of ∼6° (∼5.2 kpc at the LMC distance) that is continuous over ∼270° in position angle and is only limited by the current data coverage. The overdensity shows an amplitude up to 2.5 times higher than that of the underlying smooth disk. This structure might be related to the multiple arms found by de Vaucouleurs. We find that the overdensity shows spatial correlation with intermediate-age star clusters, but not with young (&lt;1 Gyr) main-sequence stars, indicating the stellar populations associated with the overdensity are intermediate in age or older. Our findings on the LMC overdensity can be explained by either of two distinct formation mechanisms of a ring-like overdensity: (1) the overdensity formed out of an asymmetric one-armed spiral wrapping around the LMC main body, which is induced by repeated encounters with the Small Magellanic Cloud (SMC) over the last Gyr, or (2) the overdensity formed very recently as a tidal response to a direct collision with the SMC. Although the measured properties of the overdensity alone cannot distinguish between the two candidate scenarios, the consistency with both scenarios suggests that the ring-like overdensity is likely a product of tidal interaction with the SMC, but not with the Milky Way halo.

HD 104860 and HD 192758: Two Debris Disks Newly Imaged in Scattered Light with the Hubble Space Telescope

Abstract We present the first scattered-light images of two debris disks around the F8 star HD 104860 and the F0V star HD 192758, respectively ∼45 and ∼67 pc away. We detected these systems in the F110W and F160W filters through our reanalysis of archival Hubble Space Telescope (HST) NICMOS data with modern starlight-subtraction techniques. Our image of HD 104860 confirms the morphology previously observed by Herschel in thermal emission with a well-defined ring at a radius of ∼114 au inclined by ∼58°. Although the outer edge profile is consistent with dynamical evolution models, the sharp inner edge suggests sculpting by unseen perturbers. Our images of HD 192758 reveal a disk at radius ∼95 au inclined by ∼59°, never resolved so far. These disks have low scattering albedos of 10% and 13%, respectively, inconsistent with water ice grain compositions. They are reminiscent of several other disks with similar inclination and scattering albedos: Fomalhaut, HD 92945, HD 202628, and HD 207129. They are also very distinct from brighter disks in the same inclination bin, which point to different compositions between these two populations. Varying scattering albedo values can be explained by different grain porosities, chemical compositions, or grain size distributions, which may indicate distinct formation mechanisms or dynamical processes at work in these systems. Finally, these faint disks with large infrared excesses may be representative of an underlying population of systems with low albedo values. Searches with more sensitive instruments on HST or on the James Webb Space Telescope and using state-of-the art starlight-subtraction methods may help discover more of such faint systems.

Impact of aerosols on ice crystal size.

The interactions between aerosols and ice clouds represent one of the largest uncertainties in global radiative forcing from pre-industrial time to the present. In particular, the impact of aerosols on ice crystal effective radius (Rei), which is a key parameter determining ice clouds’ net radiative effect, is highly uncertain due to limited and conflicting observational evidence. Here we investigate the effects of aerosols on Rei under different meteorological conditions using 9-year satellite observations. We find that the responses of Rei to aerosol loadings are modulated by water vapor amount in conjunction with several other meteorological parameters. While there is a significant negative correlation between Rei and aerosol loading in moist conditions, consistent with the “Twomey effect” for liquid clouds, a strong positive correlation between the two occurs in dry conditions. Simulations based on a cloud parcel model suggest that water vapor modulates the relative importance of different ice nucleation modes, leading to the opposite aerosol impacts between moist and dry conditions. When ice clouds are decomposed into those generated from deep convection and formed in situ, the water vapor modulation remains in effect for both ice cloud types, although the sensitivities of Rei to aerosols differ noticeably between them due to distinct formation mechanisms. The water vapor modulation can largely explain the difference in the responses of Rei to aerosol loadings in various seasons. A proper representation of the water vapor modulation is essential for an accurate estimate of aerosol-cloud radiative forcing produced by ice clouds.

Near field development of artificially generated high Reynolds number turbulent boundary layers

Using particle image velocimetry, the flow in the near field of two different arrays of wall-mounted obstacles is investigated. Two clearly distinct formation mechanisms seem to be generated: wall-driven mechanism, associated with vertical slender cylinders, and a wake-driven mechanism associated with longer adaptation regions.