Thin Dust Layer Research Articles

Vertical shear instability in two-moment radiation-hydrodynamical simulations of irradiated protoplanetary disks

Context. The vertical shear instability (VSI) is a hydrodynamical instability predicted to produce turbulence in magnetically inactive regions of protoplanetary disks. The regions in which this instability can occur and the physical phenomena leading to its saturation are a current matter of research. Aims. We explore the secondary instabilities triggered by the nonlinear evolution of the VSI and their role in its saturation. We also expand on previous investigations on stability regions by considering temperature stratifications enforced by stellar irradiation and radiative cooling, and including the effects of dust-gas collisions and molecular line emission. Methods. We modeled the gas-dust mixture in a circumstellar disk around a T Tauri star by means of high-resolution axisymmetric radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities, considering different degrees of depletion of small dust grains. Results. The flow pattern produced by the interplay of the axisymmetric VSI modes and the baroclinic torque forms bands of nearly uniform specific angular momentum. In the high-shear regions in between these bands, the Kelvin–Helmholtz instability (KHI) is triggered. A third instability mechanism, consisting of an amplification of eddies by baroclinic torques, forms meridional vortices with Mach numbers up to ∼0.4. Our stability analysis suggests that protoplanetary disks can be VSI-unstable in surface layers up to tens of au for reasonably high gas emissivities. Conclusions. The significant transfer of kinetic energy to small-scale eddies produced by the KHI and possibly even the baroclinic acceleration of eddies limit the maximum energy of the VSI modes, likely leading to the saturation of the VSI. Depending on the gas molecular composition, the VSI can operate at surface layers even in regions where the midplane is stable. This picture is consistent with current observations of disks showing thin midplane millimeter-sized dust layers while appearing vertically extended in optical and near-infrared wavelengths.

Design of Innovative High-Performance Polymer for Passive Lunar Dust Mitigation

Dust in space environments is now recognized as a major concern for successful manned and robotic exploration and colonization missions. Indeed, a thin layer of dust covers the lunar surface made of loose granular material, consisting of a broad range of shapes, sizes, and types of sediments, which mainly consists of silicate minerals in the form of micrometric, sharp, abrasive, porous, chemically reactive dust particles. The aim of this paper is to present preliminary results related to the design and development of an innovative, lightweight, high-performance polymer with an elevated strength-to-weight ratio able to mitigate particle contamination. In detail, dust mitigation or the minimization of the surface energy and consequently the adhesion forces among the external layers and the granular dust micrometric particles can be achieved through modifications of the surface properties by means of both chemical and physical methodologies. The proposed approach, to realize an innovative material and not a hydrophobic coating, is potentially applicable to a wide range of technological conditions and it relies on reproducible and controllable chemical modifications of the material’s surface properties through the design and synthesis of suitable base materials i.e., aromatic polyimides and copolyimides and incorporation of special low molecular weight additives, i.e., surface migrating agents, loaded directly within the reacting mixture during the intermediate phases of the copolymerization. The materials investigated in this work exhibit mechanical properties able to withstand the extreme space environment conditions and an elevated non-sticking behaviour of its surface layers in contact with granular, micrometric dust particles of lunar regolith i.e., abhesion capacity.

The Panchromatic Hubble Andromeda Treasury. XX. The Disk of M31 is Thick

We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height h z that is 0.14 ± 0.015 times the exponential scale length (h r ≈ 5.5 kpc). M31's RGB stars must therefore have h z = 770 ± 80 pc, which is far thicker than the Milky Way’s thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criterion for identifying “thick disk”-dominated systems, and discuss prospects for future 3D tomographic mapping of the gas and stars in M31.

Fault classification using deep learning based model and impact of dust accumulation on solar photovoltaic modules

ABSTRACT Solar panel performance is affected by ambient temperature, sunlight, module surface temperature, dust, and shadows. Dust inhibits sunlight from reaching photovoltaic modules, reducing power generation. Dust is aerosol pollution from natural or human-made sources. Dust on photovoltaic (PV) panels reduces power generation and raises the surface temperature, shortening panel life. This work uses a Fluke TiS60 Thermal Imager for detection of hotspot created by the accumulation of dust and further classification of the faulty and healthy images is done. The image classifiers used in this paper are SqueezeNet and AlexNet transfer learning methods, the accuracy of both methods is compared, and AlexNet performs best with an accuracy of 99.3%. Further, the effect of dust on PV modules is studied, and hotspots are created intentionally with the help of dust for study purposes. Initially, 50% of a solar module is covered with dust and then 100% of the solar module is covered with dust particles to find the power loss, when a thin layer of dust was spread over 50% of a module power loss is of 4.7% and when a thick layer of dust is spread over 50% the power loss is 5.9%, when thin layer of dust was applied over 100% of a module power loss is of 6.49% and when thick layer of dust is spread over 100% the power loss is 10.17%. This analysis for a healthy and faulty panel is done that helps PV module inspection by facilitating a more precise and cost-effective identification of PV defects.

Razor-thin dust layers in protoplanetary disks: Limits on the vertical shear instability

Context. Recent observations with the Atacama Large Millimeter Array (ALMA) have shown that the large dust aggregates observed at millimeter wavelengths settle to the midplane into a remarkably thin layer. This sets strong limits on the strength of the turbulence and other gas motions in these disks. Aims. We intend to find out if the geometric thinness of these layers is evidence against the vertical shear instability (VSI) operating in these disks. We aim to verify if a dust layer consisting of large enough dust aggregates could remain geometrically thin enough to be consistent with the latest observations of these dust layers, even if the disk is unstable to the VSI. If this is falsified, then the observed flatness of these dust layers proves that these disks are stable against the VSI, even out to the large radii at which these dust layers are observed. Methods. We performed hydrodynamic simulations of a protoplanetary disk with a locally isothermal equation of state, and let the VSI fully develop. We sprinkled dust particles with a given grain size at random positions near the midplane and followed their motion as they got stirred up by the VSI, assuming no feedback onto the gas. We repeated the experiment for different grain sizes and determined for which grain size the layer becomes thin enough to be consistent with ALMA observations. We then verified if, with these grain sizes, it is still possible (given the constraints of dust opacity and gravitational stability) to generate a moderately optically thick layer at millimeter wavelengths, as observations appear to indicate. Results. We found that even very large dust aggregates with Stokes numbers close to unity get stirred up to relatively large heights above the midplane by the VSI, which is in conflict with the observed geometric thinness. For grains so large that the Stokes number exceeds unity, the layer can be made to remain thin, but we show that it is hard to make dust layers optically thick at ALMA wavelengths (e.g., τ1.3mm ≳ 1) with such large dust aggregates. Conclusions. We conclude that protoplanetary disks with geometrically thin midplane dust layers cannot be VSI unstable, at least not down to the disk midplane. Explanations for the inhibition of the VSI out to several hundreds of au include a high dust-to-gas ratio of the midplane layer, a modest background turbulence, and/or a reduced dust-to-gas ratio of the small dust grains that are responsible for the radiative cooling of the disk. A reduction of small grains by a factor of between 10 and 100 is sufficient to quench the VSI. Such a reduction is plausible in dust growth models, and still consistent with observations at optical and infrared wavelengths.

Plasma dust deposition in low-pressure parallel plates reactor

Thin dust layers have been elaborated by plasma enhanced chemical vapor deposition (PECVD) in a parallel plates reactor using hexamethyldisiloxane (HMDSO) vapors. The chemical and physical properties of the obtained coating were discussed using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The performed characterizations show a significant impact of the deposition time on powders (NPs) composition and the organic groups concentration in particles such as Si(CH3)2, Si(CH3)3 and Si(CH3)x increases when increasing the time deposition.

Asymmetry Revisited: The Effect of Dust Attenuation and Galaxy Inclination

Abstract Dust attenuation of an inclined galaxy can cause additional asymmetries in observations, even if the galaxy has a perfectly symmetric structure. Taking advantage of the integral field spectroscopic data observed by the Sloan Digital Sky Survey-IV Mapping Nearby Galaxies at the Apache Point Observatory survey, we investigate the asymmetries of the emission-line and continuum maps of star-forming disk galaxies. We define new parameters, A a and A b , to estimate the asymmetries of a galaxy about its major and minor axes, respectively. Comparing A a and A b in different inclination bins, we attempt to detect the asymmetries caused by dust. For the continuum images, we find that A a increases with the inclination, while A b is a constant as inclination changes. Similar trends are found for g − r, g − i, and r − i color images. The dependence of the asymmetry on inclination suggests a thin dust layer with a scale height smaller than the stellar populations. For the Hα and Hβ images, neither A a nor A b shows a significant correlation with inclination. Also, we do not find any significant dependence of the asymmetry of E(B − V) g on inclination, implying that the dust in the thick disk component is not significant. Compared to the SKIRT simulation, the results suggest that the thin dust disk has an optical depth of τ V ∼ 0.2. This is the first time that the asymmetries caused by the dust attenuation and the inclination are probed statistically with a large sample. Our results indicate that the combination of the dust attenuation and the inclination effects is a potential indicator of the 3D disk orientation.

Lower Bounds on the Thickness and Dust Content of Layers within the North Polar Layered Deposits of Mars from Radar Forward Modeling

Abstract The Mars Reconnaissance Orbiter's Shallow Radar (SHARAD) emits radar signals and records their reflections from layer boundaries within the Martian north polar ice cap. Previous studies have suggested that the ice cap is composed of thin dust-rich layers between thicker layers of nearly pure water ice. The prevailing hypotheses suggest that each dust-rich layer represents either a period of ice sublimation at the poles or a period of reduced ice deposition relative to dust deposition. To test whether thin dust beds are a plausible hypothesis for the observed SHARAD reflectors, we use RadSPy (radar sounding simulator in Python, https://github.com/scourvil/RadSPy.git), an open-source N-layer radar sounder forward-modeling software that we have developed and describe herein. We forward model radar data from thin dust-rich beds interspersing pure ice, and compare them to observed radar reflection data over Gemina Lingula in the north polar layered deposits (NPLD). We consider two end-member cases: (1) thin beds composed entirely of dust, but with thickness varying from 0.05 m to 0.4 m; and (2) dust beds all with the same thickness, but with varying dust content. We find that the observed reflections can be explained by either scenario, i.e., varying thickness or varying dust content, and we conclude that a combination of both is likely. More importantly, our results provide lower bounds on the layer thickness and dust fraction for the flat-lying reflectors of Gemina Lingula in the NPLD. Our findings support the thin dust layer hypothesis, providing new constraints on layer composition and geometry for Mars climate researchers.

Large-Scale Explosion Propagation Testing of Treated and Non-treated Rock Dust When Overlain by a Thin Layer of Coal Dust

To prevent coal dust explosion propagations, rock dust needs to be lifted and suspended in the air with the coal dust during an explosion. The addition of anti-caking agents prevents caking of rock dust in the presence of water. Mining and rock dusting processes can frequently create alternating layers of rock dust and float coal dust on mine surfaces. For this test series, a thin layer of coal dust was distributed on top of a layer of either treated or non-treated rock dust in the Experimental Mine Barbara, Poland. The experimental results compare the effectiveness of treated and non-treated rock dusts to attenuate a propagating coal dust explosion initiated with either strong or weak methane explosions. Experimental results indicate that the treated rock dust performs better than non-treated rock dust in arresting a propagating explosion, especially in the presence of moisture.

Simulating dust grain-radiation coupling on a moving mesh

ABSTRACT We present a model for the interaction between dust and radiation fields in the radiation hydrodynamic code arepo-rt, which solves the moment-based radiative transfer equations on an unstructured moving mesh. Dust is directly treated using live simulation particles, each of which represent a population of grains that are coupled to hydrodynamic motion through a drag force. We introduce methods to calculate radiation pressure on and photon absorption by dust grains. By including a direct treatment of dust, we are able to calculate dust opacities and update radiation fields self-consistently based on the local dust distribution. This hybrid scheme coupling dust particles to an unstructured mesh for radiation is validated using several test problems with known analytic solutions, including dust driven via spherically symmetric flux from a constant luminosity source and photon absorption from radiation incident on a thin layer of dust. Our methods are compatible with the multifrequency scheme in arepo-rt, which treats UV, optical photons as single scattered and IR photons as multi scattered. At IR wavelengths, we model heating of and thermal emission from dust. Dust and gas are not assumed to be in local thermodynamic equilibrium but transfer energy through collisional exchange. We estimate dust temperatures by balancing these dust-radiation and dust-gas energy exchange rates. This framework for coupling dust and radiation can be applied in future radiation hydrodynamic simulations of galaxy formation.

Mapping of Ice Storage Processes on the Moon with Time-dependent Temperatures

Abstract Lunar cold traps are defined by extremely low time-integrated sublimation loss, so that water ice is expected to accumulate in them. Due to the strong dependence of the sublimation rate on temperature, they have heretofore been delineated by the peak rather than the time-averaged sublimation rate. Here, time-averaged sublimation rates are calculated for the south polar region of the Moon based on 11 yr of Diviner surface temperature measurements. Data for each spatial pixel are binned according to subsolar (diurnal) and ecliptic (seasonal) longitude. Frequency-domain filtering of temperature time series is applied to interpolate and smooth the data set. The cold trap area is 17,000 km2 from 80°S to the south pole, about 24% larger than defined by peak temperature. In addition, subsurface ice stability is mapped based on solutions of the heat equation with Diviner surface temperatures as the upper boundary condition. Even a thin layer of dust reduces the sublimation loss dramatically. A third potential mechanism for ice storage, vapor pumping by temperature cycles, is also mapped, based on a model for the time-variable population of adsorbed water molecules on the lunar surface. The area of significant pumping is estimated to be 96,000 km2 poleward of 80°S. In summary, a new processing technique has been developed that exploits information about the time variability of lunar surface temperatures measured by Diviner, and it results in new maps for the geographic distribution of potential water-ice deposits in the south polar region of the Moon.

A water droplet-cleaning of a dusty hydrophobic surface: influence of dust layer thickness on droplet dynamics

Water droplet cleaning of a dusty hydrophobic surface is examined. Environmental dust are used in the experiments and cloaking velocity of a dust layer by a droplet fluid is measured and hemi-wicking conditions for the dust layer are analyzed adopting the pores media wick structure approach. A droplet motion on dusty and inclined hydrophobic surface is analyzed using a high speed digital imaging system. Influences of dust layer thickness, droplet volume, and surface inclination angle on the mechanisms of dust removal by a rolling droplet are evaluated. The findings revealed that dust cloaking velocity decreases exponentially with time. The droplet fluid can cloak the dust layer during its transition on the dusty surface. The transition period of droplet wetted length on the dusty surface remains longer than the cloaking time of the dust layer by the droplet fluid. Translational velocity of rolling droplet is affected by the dust layer thickness, which becomes apparent for small volume droplets. Small volume droplet (20 µL) terminates on the thick dust layer (150 µm) at low surface inclination angle (1°). The quantity of dust picked up by the rolling droplet increases as the surface inclination angle increases. The amount of dust residues remaining on the rolling droplet path is relatively larger for the thick dust layer (150 µm) as compared to its counterpart of thin dust layer (50 µm).

The Streaming Instability in Two-dimensional Protoplanetary Disks

Abstract We conduct a two-dimensional (2D) stability analysis for the aerodynamically coupled system of gas and dust in protoplanetary disks using the WKB approximation. The 2D analysis may apply to a vertically integrated disk or a thin dust layer at the disk midplane. We show that the streaming instability exists in 2D and examine the growth rate and its dependence on the mean dust-to-gas ratio, ϵ ¯ , and the Stokes number, St. For small ϵ ¯ , the dust back reaction may be viewed as a perturbation to the wave system, and analytical results are successfully derived by the method of perturbation with respect to ϵ ¯ . At a given azimuthal wavenumber, m, the growth rate as a function of the radial wavenumber, kr, shows a peak corresponding to the resonance between the relative drift velocity and the Rossby/trivial waves in the gas. For small ϵ ¯ , the peak growth rate at resonant modes increases as m 1/2 when the Rossby wave effect is negligible and the gas wave becomes trivial. With increasing St, the peak growth rate increases as St 1/2 at St ≪ 1, peaks at St ∼ 1, and finally decreases as St −1/2 for St ≫ 1. We find that the growth rate scales with ϵ ¯ as ϵ ¯ 1 / 2 at ϵ ¯ ≪ 1 and peaks at ϵ ¯ ∼ 1 . The instability is physically interpreted as a runaway process of dust trapping in relative drift-induced anticyclonic vortices. The effects of the compressibility and viscosity of the gas, the applicability of the WKB application, and the effects of the Keplerian shear are discussed.

THEMIS‐VIS Investigations of Sand at Gale Crater

AbstractGale crater is the landing site of the Mars Science Laboratory (MSL) rover, Curiosity. Here we present Thermal Emission Imaging System Visible Imaging Subsystem (THEMIS‐VIS) mosaics in gray scale and in false color of Gale crater. We use these data products, in addition to THEMIS thermal infrared decorrelation stretch (DCS) mosaics, thermal inertia derivations, and near‐infrared spectra to investigate the MSL traverse area and sand across Gale crater. We identified several THEMIS‐VIS color units in the MSL traverse area that may correlate to the amount of sand cover that is present on each unit. This suggests that THEMIS‐VIS color is extremely useful in identifying dark basaltic sand, which appears as blue in THEMIS‐VIS false color images. We test this hypothesis by identifying small (~several pixels) blue patches on the southeast side of the central mound in Gale crater and confirming that they are patches of sand in High Resolution Imaging Science Experiment (HiRISE) images. Sand dunes on the crater floor exhibit variations in all data sets utilized in this study, with the Bagnold dunes and the western sand sea exhibiting the most notable differences. We propose that these differences are the result of a thin layer of dust that covers the western sand sea, although we do not rule out possible effects from grain size sorting.

Polarization as a Probe of Thick Dust Disks in Edge-on Galaxies: Application to NGC 891

Abstract Radiative transfer models were developed to understand the optical polarizations in edge-on galaxies, which are observed to occur even outside the geometrically thin dust disk, with a scale height of ≈0.2 kpc. In order to reproduce the vertically extended polarization structure, we find that it is essential to include a geometrically thick dust layer in the radiative transfer model, in addition to the commonly known thin dust layer. The models include polarizations due to both dust scattering and dichroic extinction, which are responsible for the observed interstellar polarization in the Milky Way. We also find that the polarization level is enhanced if the clumpiness of the interstellar medium, and the dichroic extinction by vertical magnetic fields in the outer regions of the dust lane are included in the radiative transfer model. The predicted degree of polarization outside the dust lane was found to be consistent with that (ranging from 1% to 4%) observed in NGC 891.

Exposed bright features on the comet 67P/Churyumov–Gerasimenko: distribution and evolution

Context. Since its arrival at the comet 67P/Churyumov–Gerasimenko in August 2014, the Rosetta spacecraft followed the comet as it went past the perihelion and beyond until September 2016. During this time there were many scientific instruments operating on board Rosetta to study the comet and its evolution in unprecedented detail. In this context, our study focusses on the distribution and evolution of exposed bright features that have been observed by OSIRIS, which is the scientific imaging instrument aboard Rosetta. Aims. We envisage investigating various morphologies of exposed bright features and the mechanisms that triggered their appearance. Methods. We co-registered multi-filter observations of OSIRIS images that are available in reflectance. The Lommel–Seeliger disk function was used to correct for the illumination conditions and the resulting colour cubes were used to perform spectrophotometric analyses on regions of interest. Results. We present a catalogue of 57 exposed bright features observed on the nucleus of the comet, all of which are attributed to the presence of H2O ice on the comet. Furthermore, we categorise these patches under four different morphologies and present geometric albedos for each category. Conclusions. Although the nucleus of 67P/Churyumov–Gerasimenko appears to be dark in general, there are localised H2O ice sources on the comet. Cometary activity escalates towards the perihelion passage and reveals such volatile ices. We propose that isolated H2O ice patches found in smooth terrains in regions, such as Imhotep, Bes, and Hapi, result from frost as an aftermath of the cessation of the diurnal water cycle on the comet as it recedes from perihelion. Upon the comet’s return to perihelion, such patches are revealed when sublimation-driven erosion removes the thin dust layers that got deposited earlier. More powerful activity sources such as cometary outbursts are capable of revealing much fresher, less contaminated H2O ice that is preserved with consolidated cometary material, as observed on exposed patches resting on boulders. This is corroborated by our albedo calculations that attribute higher albedos for bright features with formations related to outbursts.

Structure of protoplanetary discs with magnetically driven winds

We present a new set of analytical solutions to model the steady state structure of a protoplanetary disc with a magnetically-driven wind. Our model implements a parametrization of the stresses involved and the wind launching mechanism in terms of the plasma parameter at the disc midplane, as suggested by the results of recent, local MHD simulations. When wind mass-loss is accounted for, we find that its rate significantly reduces the disc surface density, particularly in the inner disc region. We also find that models that include wind mass-loss lead to thinner dust layers. As an astrophysical application of our models, we address the case of HL Tau, whose disc exhibits a high accretion rate and efficient dust settling at its midplane. These two observational features are not easy to reconcile with conventional accretion disc theory, where the level of turbulence needed to explain the high accretion rate would prevent a thin dust layer. Our disc model that incorporates both mass-loss and angular momentum removal by a wind is able to account for HL Tau observational constraints concerning its high accretion rate and dust layer thinness.

A Thermodynamic View of Dusty Protoplanetary Disks

Abstract Small solids embedded in gaseous protoplanetary disks are subject to strong dust–gas friction. Consequently, tightly coupled dust particles almost follow the gas flow. This near conservation of the dust-to-gas ratio along streamlines is analogous to the near conservation of entropy along flows of (dust-free) gas with weak heating and cooling. We develop this thermodynamic analogy into a framework to study dusty gas dynamics in protoplanetary disks. We show that an isothermal dusty gas behaves like an adiabatic pure gas, and that finite dust–gas coupling may be regarded as effective heating/cooling. We exploit this correspondence to deduce that (1) perfectly coupled, thin dust layers cannot cause axisymmetric instabilities; (2) radial dust edges are unstable if the dust is vertically well-mixed; (3) the streaming instability necessarily involves a gas pressure response that lags behind dust density; and (4) dust-loading introduces buoyancy forces that generally stabilize the vertical shear instability associated with global radial temperature gradients. We also discuss dusty analogs of other hydrodynamic processes (e.g., Rossby wave instability, convective overstability, and zombie vortices) and how to simulate dusty protoplanetary disks with minor tweaks to existing codes for pure gas dynamics.

Laboratory measurements of dusty surface charging in plasma.

A novel method is developed to study the charging of a conducting surface covered by a thin dust layer in plasma. The potential profile in the dust layer and the floating potential of the surface underneath are measured directly by embedding conducting wires in the dust and connecting the wires to a measurement plate outside the vacuum chamber, where a Trek non-contacting electrostatic voltmeter measures the floating potential of the measurement plate. Laboratory experiments are carried out to study plasma charging of a conducting plate covered by lunar dust simulant, JSC-1A. The results show that the plate potential is dependent on both the ambient plasma condition and the dust layer thickness. The current balance condition controls the floating potential of the dust surface while the dust layer acts as a capacitor and controls the potential of the plate with respect to the dust surface. Hence, a dust covered conducting plate will be charged more negatively than a clean plate.

Global Dust Transport as Observed by A-Train Satellites

This paper presented a new height-resolved view of global dust transport based on 2007-2010 Atrain observations. First, a new dust identification methodology was developed to improve optically thin dust layer detection. Second a new dust partition methodology was developed and applied to CALIPSO lidar measurements to derive dust partitions in external mixed aerosols. These new approaches allow a new view of global dust distribution from dense dust layers near the strong source regions to the optically thin, but significant dust layers from the point of view of aerosol– cloud interactions, over the weak source regions, the transport areas, and the upper troposphere. The results will not only help us to better understand global dust transport and dust-cloud interactions, but also provide critical information for model evaluations and improvements.