Evolution Of Spots Research Articles

Keeping up with the cool stars: one TESS year in the life of AB Doradus

The long-term, high precision photometry delivered by the Transiting Exoplanet Survey Satellite (TESS) enables us to gain new insight into known and hitherto well-studied stars. In this paper, we present the result of our TESS study of the photospheric activity of the rapid rotator AB Doradus. Due to its favorable position near the southern ecliptic pole, the TESS satellite recorded almost 600 rotations of AB Doradus with high cadence, allowing us to study starspots and flares on this ultra-active star. The observed peak-to-peak variation of the rotational modulations reaches almost 11%, and we find that the starspots on AB Doradus show highly preferred longitudinal positions. Using spot modeling, we measured the positions of the active regions on AB Doradus and we find that preferred spot configurations should include large regions extending from low to high stellar latitudes. We interpret the apparent movement of spots as the result of both differential rotation and spot evolution and argue that the typical spot lifetimes should range between 10 and 20 days. We further find a connection between the flare occurrence on AB Doradus and the visibility of the active regions on its surface, and we finally recalculated the star’s rotation period using different methods and we compared it with previous determinations.

Effects of various tool plunge depths on microstructure evolution, mechanical properties and dome structure features of friction stir spot welded AA5052‐H32 similar joints

AbstractThe solid‐state nature of friction stir spot welding process provides outstanding advantages for the sound joining of aluminum alloys. Within this study, 3 mm‐thick AA5052‐H32 sheets are successfully joined by friction stir spot welding using 2344 hot‐worked steel pin to investigate the effects of various tool plunge depths on the microstructure, mechanical and metallurgical properties of similar welds. Therefore, the experiments are performed at different plunge depths in the range of 3 mm–4 mm. Accordingly, the relationships between the process parameter (tool plunge depth) and the responses (microstructure, dome structure, microhardness and lap shear tensile load) are established. Microstructure analyses demonstrate that the increase in the plunge depth leads to more grain refinement within the stir zone, which significantly affects the mechanical performance of the similar joints. This study also indicates that the tool plunge depth in friction stir spot welding process has a noteworthy influence on the characteristic features of the 5052 aluminum alloy joints, such as the dome structure. Moreover, an explicit increase in the microhardness towards the weld stir zone is observed in all specimens. It is found that the average maximum tensile‐shear force enhances with the increment in the tool plunge depth from 3 mm to 4 mm.

Reference-frame-independent quantum key distribution with random atmospheric transmission efficiency

Reference-frame-independent quantum key distribution (RFI-QKD) has been proved to be tolerant against unknown reference frame misalignment, which reserves interesting prospects in implementing global quantum communication. However, few works have been addressed on the performance and feasibility for RFI-QKD in turbulent atmospheric channels. Here, we propose to implement RFI-QKD in practical free-space links with fluctuating transmission efficiency due to beam wandering and broadening. An improved model for estimating the probability distribution of single-photon receiving efficiency has been developed and we also simulated the Gaussian beam spot evolution and secure key rate based on that. Results show that the beam wandering model of probability distribution of transmission efficiency is reasonable to improve the performance of RFI-QKD in free-space channel.

Effects of Postweld Heat Treatment on the Phase Transformation and Mechanical Behavior of NiTi Ultrasonic Spot Welded Joints With Al Interlayer

Abstract The influence of postweld heat treatments (PWHTs) on the phase transformation characteristics, microstructural evolution, and mechanical properties of ultrasonic spot welded NiTi alloy with Al interlayer was investigated. At room temperature, the as-welded joints were fully austenitic, while the microstructure of welded joint after PWHT at 450 °C consisted of a mixture of martensite and austenite. The transformation temperatures were found to increase after the PWHT. Transmission electron microscope analysis shows that Al3Ti intermetallic compounds were formed on the Al side of the Al/NiTi interface after the PWHT at 450 °C for 90 min. Furthermore, finely dispersed nanoscale Ni4Ti3 precipitates were observed in the joint. These nanoscale precipitates resulted in an increase in the transformation temperatures and improved the mechanical properties of the PWHT material. During lap shear test, the as-welded samples failed in a brittle manner in the NiTi alloy, while the PWHT samples failed in the Al interlayer and exhibited ductile-like fracture characteristics.

Long-term Photometry and Orbital Period Change of the W UMa–type Binary v0599 Aur: Evidence of about 11 yr Magnetic-activity Cycle

Abstract Combining all available photometric data from various surveys and literature with our observations, we present 10 sets of light curves for the eclipsing binary V0599 Aur covering a timescale of 20 yr. During the last two decades, the O’Connell effect continuously varied and went through at least two flips between positive and negative effects. The photometric solutions from our two sets of multicolored light curves show that V0599 Aur is a W-type shallow contact binary with an active spot on the secondary. Its absolute parameters are determined by combining the Gaia distance with the photometric solutions. The period investigation reveals a secular decrease and a cyclic variation in its orbital period. The former mainly originates from the mass transfer from the more massive secondary to the less massive primary. The latter can be preferentially explained as a result of cyclic magnetic activity of the secondary, with three observational supports: (1) the existence and evolution of an active spot suggested by the long-term photometry, (2) periodic variation in both the O’Connell effect and relative luminosity, and (3) weak correlations between O’Connell effect/relative luminosity changes and cyclic period variation. Together with the cyclic period variation and its theoretical reasonability by Applegate’s mechanism, we suggest that the secondary of V0599 Aur is a solar-type magnetic-activity star with an approximately 11 yr active cycle. Furthermore, by combining the secular period decrease with the Roche-lobe model, we infer that V0599 Aur is evolving from the marginal contact state controlled by thermal relaxation oscillation theory to the overcontact state.

Evolution of the Glorieuses seamount in the SW Indian Ocean and surrounding deep Somali Basin since the Cretaceous

Little is known about the geological history of the Glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the Somali Basin. This study focused on describing and reconstructing the long-term stratigraphic evolution of the Glorieuses seamount (SW Indian Ocean) to identify the mechanisms that have occurred through time to finally shape the emerged modern islands. Distinct terrace levels, currently submerged along the flanks of the seamount and surrounding seamounts, have already been interpreted as resulting from successive carbonate development and back-stepping episodes over the last 62 Myr. New isotopic and biostratigraphic dating on the flanks of the seamount, coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin, provide new constraints for the Late Cretaceous and Cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin. Even if starved steep slopes prevent a straightforward source-to-sink continuity between the platform and the basin domains, our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin, and discuss a long-term geological model that includes the main driving factors behind deposition (volcanic events, subsidence vs uplift phases, climate and hydro-dynamism changes) and their quantitative impact on the evolution of the isolated carbonate sedimentary system. Our results show that: (i) the Glorieuses volcanic seamount emerged from two successive Late Cretaceous magmatic pulses, firstly during the Turonian, then during the Maastrichtian (ii) at least two potential uplift phases are recognized during the Tertiary (Paleogene and/or the Eocene and Tortonian); (iii) basinal sedimentation recorded an abrupt change probably related to major regional hydro-dynamical changes in Late Eocene times in the Western Indian Ocean; (iv) the export of sediments from the platform towards the basin (numerous gravity flow processes) is strongly enhanced after the Mid Miocene, and is probably linked to the onset of the Asian monsoon winds and bipolar circulation. Finally, the Glorieuses seamount, although located in the vicinity of the Comoros islands, appears to have a much longer history and is geologically more comparable to the nearby Seychelles. This long-term study has enabled us to associate the Glorieuses seamount with the SSE-NNW Madagascar-Seychelles alignment rather than with the Comoro hot spot evolution.

Morphology and mechanisms of cavitation damage on lamellar gray iron surfaces

Engine parts in contact with liquids may suffer cavitation erosion damage. Understanding its mechanisms in realistic operating environments is necessary for improvements in the service life and durability of materials for heavy-duty diesel engines. This work illustrates the cavitation erosion behavior of a cylinder liner material with a follow-up of detailed, high-magnification SEM images of damaged sites on the surface. The cylinder liner encloses the piston and combustion chamber in the engine of large trucks and its material of choice is usually a lamellar gray cast iron, its microstructure consisting of flake-shaped graphite, a pearlitic matrix and some steadite. Testing was carried out using an ultrasonic vibratory apparatus, and the liquid of choice was a commercial engine coolant composed of water, glycol and inhibitors. Based on observations of tested surfaces, a sequence of damage patterns is proposed as an explanation of the material’s cavitation erosion behavior. Initiation consists of: chipping at graphite cluster centers, graphite flake removal, pitting along graphite flakes and direct matrix pitting. Development consists of: evolution of chipped spots into matrix-damaging pits, radial pit expansion, pit merging and surface roughening. It can be concluded that presence and morphology of graphite are critical to the cavitation erosion behavior of LGI.

Theory of ignition and burn propagation in inertial fusion implosions

A detailed analytic model is presented here to investigate the physics of burn propagation in inertially confined plasmas. The onset of ignition and burn propagation occurs when alpha heating of the hot spot causes rapid ablation of shell mass into the hot spot. This allows large energy gains to be achieved since most of the fuel mass is located in the shell. Here, we first present a comprehensive review of previous analytic models that have been used to describe the physics of hot-spot evolution and ignition; we then show that a proper description of a propagating burn wave requires a comprehensive model of hot spot and shell evolution that includes proper mass conservation in the shell, fusion reactivity, and fuel depletion. The analytic theory is in good agreement with detailed radiation-hydrodynamic simulations that predict the onset of burn propagation as occurring when the yield enhancement caused by alpha heating is between 15- and 25-fold, fα ∼ 1.4, where fα = alpha energy deposited/hot-spot energy at bang time, and the hot-spot burnup fraction is approximately 2%. We show that the definition of ignition is not sensitive to the alpha-particle stopping power nor asymmetries provided that the absorbed fraction of alpha particles θα is correctly accounted for. Finally, we use the results of 2-D simulations to show that even when θα is small and unknown (as is true in hot spots with mid modes that have significant leakage of alpha particles into the surrounding cold bubbles), one can still relate the experimentally measureable parameter χα53 to the yield amplification and the burning-plasma parameter Qαhs = alpha energy deposited/total input work delivered to the hot spot.

A bibliometric study for DEA applied to energy efficiency: Trends and future challenges

Bibliometrics refers to the statistical analysis of publications, which mainly include journal papers, books, and conference proceedings. It is an effective method for organizing and analyzing available information on a given research topic and has been commonly used in various disciplines. This paper provides a comprehensive overview of all publications about the researches on energy efficiency based on data envelopment analysis (DEA) retrieved from the Web of Science database. A total of 1206 documents in this field, published until 2018, are retrieved from the Web of Science. This study pays special attention to several key issues such as the general citation structure, the most cited publications, the productive journals, institutions and countries/ territories in the area. The cooperation model and cooperative network between countries and research institutes are presented. The key nodes documents in this field are analyzed through the study of literature co-citation. The evolution of research hot spots is explored by analyzing the keywords based on text mining techniques. Three different knowledge diffusion paths such as forward local main path, global main path and key-route main path are presented to identify the knowledge diffusion path of this field. The main advantage of this study is it provides a general picture of this domain. The achievements of this study will undoubtedly be valuable for future research in energy efficiency and will have great reference value to other disciplines.

Temporal Evolution of Spatially Resolved Individual Star Spots on a Planet-hosting Solar-type Star: Kepler-17

Abstract Star spot evolution is visible evidence of the emergence/decay of the magnetic field on a stellar surface, and it is therefore important for the understanding of the underlying stellar dynamo and consequential stellar flares. In this paper, we report the temporal evolution of individual star spot areas on the hot-Jupiter-hosting, active solar-type star Kepler-17, whose transits occur every 1.5 days. The spot longitude and area evolution are estimated (1) from the stellar rotational modulations of Kepler data and (2) from the brightness enhancements during the exoplanet transits caused by existence of large star spots. As a result of the comparison, the number of spots, spot locations, and the temporal evolution derived from the rotational modulations are largely different from those of in-transit spots. We confirm that, although only two light-curve minima appear per rotation, there are clearly many spots present on the star. We find that the observed differential intensity changes are sometimes consistent with the spot pattern detected by transits, but at other times they do not match with each other. Although the temporal evolution derived from the rotational modulation differs from those of in-transit spots to a certain degree, the emergence/decay rates of in-transit spots are within an order of magnitude of those derived for sunspots as well as our previous research based only on rotational modulations. This supports the hypothesis that the emergence/decay of sunspots and extremely large star spots on solar-type stars occur through the same underlying processes.

Dynamic Magma Storage at Near‐Ridge Hot Spots: Evidence From New Galápagos Gravity Data

AbstractA morphological dichotomy exists between the active western and older (>1–2.5 Ma) eastern volcanoes of the Galápagos Archipelago. All of the young shield volcanoes in the west have calderas, but none are present on the older volcanoes in the east. This striking difference suggests that there has been a change in volcanic construction and magmatic supply processes, a finding dissimilar to the prevailing Hawaiian model of hot spot evolution. Bouguer anomaly highs (30–50 mGal) consistent with dense cumulate bodies are measured over these calderas. In contrast, we present new gravity data from Santa Cruz and San Cristóbal islands that lack comparable gravity highs. We propose that formation of shallow magma reservoirs and their associated Bouguer anomaly highs were inhibited during evolution of the eastern volcanoes because the proximal Galápagos Spreading Center diverted plume material more efficiently ~1–2 Ma when the spreading center was ~100 km closer to the plume. The difference in caldera expression between the western and eastern islands may be a surface expression of this process. These results suggest that plume‐ridge interaction may play a first‐order role in the evolution of magmatic plumbing systems of near‐ridge ocean islands, which account for one third of hot spot systems.

Evidence for a Multipolar Magnetic Field in SGR J1745-2900 from X-Ray Light-curve Analysis

Abstract SGR J1745-2900 was detected from its outburst activity in 2013 April and it was the first soft gamma repeater (SGR) detected near the center of the Galaxy (Sagittarius A*). We use 3.5 yr Chandra X-ray light-curve data to constrain some neutron star (NS) geometric parameters. We assume that the flux modulation comes from hot spots on the stellar surface. Our model includes the NS mass, radius, a maximum of three spots of any size, temperature and positions, and general relativistic effects. We find that the light curve of SGR J1745-2900 could be described by either two or three hot spots. The ambiguity is due to the small amount of data, but our analysis suggests that one should not disregard the possibility of multi-spots (due to a multipolar magnetic field) in highly magnetized stars. For the case of three hot spots, we find that they should be large and have angular semiapertures ranging from 16° to 67°. The large size found for the spots points to a magnetic field with a nontrivial poloidal and toroidal structure (in accordance with magnetohydrodynamics investigations and Neutron Star Interior Composition Explorer’s (NICER) recent findings for PSR J0030+0451) and is consistent with the small characteristic age of the star. Finally, we also discuss possible constraints on the mass and radius of SGR J1745-2900 and briefly envisage possible scenarios accounting for the 3.5 yr evolution of SGR J1745-290 hot spots.

Off-axis pumped Hermite-Gaussian mode solid-state laser

To study the modes’ pattern and the modes’ competition behavior of an off-axis pumped solid-state laser, a small signal approximation method is derived, which simplifies the multiple-mode differential equations into liner algebraic equations. When the pump beam radius is small, the higher-order Hermite-Gaussian modes emerge successively with the off-axis displacement increasing, while the pattern evolution shows some complexity when the pump radius is larger. The percentage of the modes with a small pump power near the threshold, calculated with the small signal method, is close to that calculated at a higher pump power by directly solving the rate equations numerically. This indicates that we can estimate the modes’ pattern of an actual high power laser by using the small signal method. For a multiple Hermite-Gaussian modes off-axis pumped solid state laser, as the pump power increases, the photon number of the mode increases linearly as its net gain becomes positive, while that of the second mode with a smaller net gain does not increase immediately as it becomes positive successively. Larger pump power is required until the photon number begins to increase. The increasing slope of first mode decreases as the second mode begins to grow. The dynamics of the modes’ competition presents cross spiking and cross relaxation process before they become stable. Moreover, the outputs of the modes HG<sub>00</sub>-HG<sub>50</sub> are experimentally demonstrated, and the spot evolution with the off-axis displacement agrees very well with the calculated result.

Effect of subatmospheric pressures on heat transfer, vapor bubbles and dry spots evolution during water boiling

The present paper reports the results of the comprehensive experimental investigation of an influence of subatmospheric pressures on multiscale heat transfer characteristics during liquid pool boiling. Experiments were carried out in the pressure range of 8.8–103 kPa at saturated water boiling using high-speed IR thermography, high-speed visualization from different sides and the specially designed transparent ITO heater. This made it possible to obtain simultaneously extensive data set on the effect of reduced pressure on main characteristics of boiling, including heat transfer coefficients, nucleation site density, growth rate and departure diameter of vapor bubbles. High-speed visualization from a bottom side of transparent heater allowed to investigate an evolution of dry spots bounded by triple contact line depending on pressure for the first time. It was demonstrated that the growth rate of dry spots is constant in time and has a non-monotonic dependence on pressure.

Spot evolution on LQ Hya from 2006–2017: temperature maps based on SOFIN and FIES data

Context. LQ Hya is one of the most frequently studied young solar analogue stars. Recently, it has been observed to show intriguing behaviour when analysing long-term photometry. For instance, from 2003–2009, a coherent spot structure migrating in the rotational frame was reported by various authors. However, ever since, the star has entered a chaotic state where coherent structures seem to have disappeared and rapid phase jumps of the photometric minima occur irregularly over time. Aims. LQ Hya is one of the stars included in the SOFIN/FIES long-term monitoring campaign extending over 25 yr. Here, we publish new temperature maps for the star during 2006–2017, covering the chaotic state of the star. Methods. We used a Doppler imaging technique to derive surface temperature maps from high-resolution spectra. Results. From the mean temperatures of the Doppler maps, we see a weak but systematic increase in the surface temperature of the star. This is consistent with the simultaneously increasing photometric magnitude. During nearly all observing seasons, we see a high-latitude spot structure which is clearly non-axisymmetric. The phase behaviour of this structure is very chaotic but agrees reasonably well with the photometry. Equatorial spots are also frequently seen, but we interpret many of them to be artefacts due to the poor to moderate phase coverage. Conclusions. Even during the chaotic phase of the star, the spot topology has remained very similar to the higher activity epochs with more coherent and long-lived spot structures. In particular, we see high-latitude and equatorial spot activity, the mid latitude range still being most often void of spots. We interpret the erratic jumps and drifts in phase of the photometric minima to be caused by changes in the high-latitude spot structure rather than the equatorial spots.

Temporal evolution and correlations of optical activity indicators measured in Sun-as-a-star observations

Context. Understanding stellar activity in solar-type stars is crucial for the physics of stellar atmospheres as well as for ongoing exoplanet programmes. Aims. We aim to test how well we understand stellar activity using our own star, the Sun, as a test case. Methods. We performed a detailed study of the main optical activity indicators (Ca II H & K, Balmer lines, Na I D1 D2, and He I D3) measured for the Sun using the data provided by the HARPS-N solar-telescope feed at the Telescopio Nazionale Galileo. We made use of periodogram analyses to study solar rotation, and we used the pool variance technique to study the temporal evolution of active regions. The correlations between the different activity indicators as well as the correlations between activity indexes and the derived parameters from the cross-correlation technique are analysed. We also study the temporal evolution of these correlations and their possible relationship with indicators of inhomogeneities in the solar photosphere like sunspot number or radio flux values. Results. The value of the solar rotation period is found in all the activity indicators, with the only exception being Hδ. The derived values vary from 26.29 days (Hγ line) to 31.23 days (He I). From an analysis of sliding periodograms we find that in most of the activity indicators the spectral power is split into several “bands” of periods around 26 and 30 days. They might be explained by the migration of active regions between the equator and a latitude of ∼30°, spot evolution, or a combination of both effects. A typical lifetime of active regions of approximately ten rotation periods is inferred from the pooled variance diagrams, which is in agreement with previous works. We find that Hα, Hβ, Hγ, Hϵ, and He I show a significant correlation with the S index. Significant correlations between the contrast, bisector span, and the heliocentric radial velocity with the activity indexes are also found. We show that the full width at half maximum, the bisector, and the disc-integrated magnetic field correlate with the radial velocity variations. The correlation of the S index and Hα changes with time, increasing with larger sun spot numbers and solar irradiance. A similar tendency with the S index and radial velocity correlation is also present in the data. Conclusions. Our results are consistent with a scenario in which higher activity favours the correlation between the S index and the Hα activity indicators and between the S index and radial velocity variations.

Features of vapor bubbles evolution at liquid boiling at subatmospheric pressures

The article presents the results of an experimental study of the dynamics of vaporization at water boiling at subatmospheric pressures up to 8.8 kPa. The usage of a transparent heater, high-speed video and IR thermography allowed analyzing the effect of pressure on the dynamics of vapor bubbles, the evolution of dry spots as well as the surface superheating corresponding to the onset of boiling. It is shown that the pressure reduction at a given heat flux density leads to a significant increase in the Jacob number at the activation of the nucleation site and the growth rate of vapor bubbles. It has been found that the curve of dry spots growth rate has a nonlinear dependence on the pressure in the range of 8.8-103 kPa. The minimum value of the growth rate of dry spots is observed at a pressure of 42 kPa.

Fusion zone characterization of resistance spot welded maraging steels via selective laser melting

High-performance maraging steels fabricated by selective laser melting has received a great deal of attention in automotive applications. This paper reports on the microstructure evolution of resistance spot welded maraging steels via selective laser melting and their relation to mechanical performance. Four regions of inner fusion zone, outer fusion zone, heat-affected zone and base metal are found in the spot welds of maraging steels. The inner fusion zone is observed in the form of column dendritic structure in directional solidification, whereas the outer fusion zone forms coarse equiaxial structure with various crystal orientations. Grain refinement and the existence of texture would increase the hardness in the inner fusion zone with comparison to the value of outer fusion zone. Moreover, the relative softened outer fusion zone would make the crack initial in the zone and propagate along the sheet thickness, which results in the occurrence of partial interfacial failure.

Influence of dwell time on microstructure evolution and mechanical properties of dissimilar friction stir spot welded aluminum–copper metals

1060 aluminum–T2 copper dissimilar lap joints were produced by friction stir spot welding (FSSW) with various dwell time. All the joints possess a Cu hook extruded upward from the lower Cu plate into the upper Al plate with intermetallic compounds (IMCs) developed on its interface. Increasing the dwell time produced an increase in the heat input during welding and promoted IMC growth. At short dwell time, the interface was characterized by the interruptedly distributed CuAl2 layer partially mingled with CuAl phase. However, continuous CuAl2–CuAl–Al4Cu9 laminated layer developed at the interface at longer dwell time. IMC formation sequence for CuAl2, CuAl and Al4Cu9 were determined by thermodynamic principles. Microhardness was quite different in different zones. Hardness values in the stir zone (SZ) were much higher due to the dispersively distributed IMC particles and the refined grains. Joints with better tensile properties have a higher penetration depth of the hook into the upper Al plate as well as a continuous IMC layer at the hook interface. Results of fracture path analysis indicated that all the fracture initiated at the CuAl2–CuAl or CuAl2–Al interface and then extended along the IMC layer at the hook interface. The dispersed IMC particles in the SZ provided an alternative path for crack extension when strong metallurgical bonding was achieved at the hook interface.

Starspot activity of HD 199178

Context. Studying the spots of late-type stars is crucial for distinguishing between the various proposed dynamo mechanisms believed to be the main cause of starspot activity. For this research it is important to collect observation time series that are long enough to unravel both long- and short-term spot evolution. Doppler imaging is a very efficient method for studying spots of stars that cannot be angularly resolved.Aims. High-resolution spectral observations during 1994–2017 are analysed in order to reveal long- and short-term changes in the spot activity of the FK Comae-type subgiant HD 199178.Methods. Most of the observations were collected with the Nordic Optical Telescope. The Doppler imaging temperature maps were calculated using an inversion technique based on Tikhonov regularisation and utilising multiple spectral lines.Results. We present a unique series of 41 temperature maps spanning more than 23 years. All reliable images show a large cool spot region centred near the visible rotation pole. Some lower latitude cool features are also recovered, although the reliability of these is questionable. There is an expected anti-correlation between the mean surface temperature and the spot coverage. Using the Doppler images, we construct the equivalent of a solar butterfly diagram for HD 199178.Conclusions. HD 199178 clearly has a long-term large and cool spot structure at the rotational pole. This spot structure dominated the spot activity during the years 1994–2017. The size and position of the structure has evolved with time, with a gradual increase during the last years. The lack of lower latitude features prevents the determination of a possible differential rotation.