Evolution, nucleosynthesis and yields of AGB stars at different metallicities (III): intermediate mass models, revised low mass models and the ph-FRUITY interface

We present a new set of models for intermediate mass AGB stars (4.0, 5.0 and, 6.0 Msun) at different metallicities (-2.15<=Fe/H]<=+0.15). This integrates the existing set of models for low mass AGB stars (1.3<=M/M<=3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the Main Sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. The latter is due to the fact that the interpulse phases are short and, then, Thermal Pulses are weak. Moreover, the high temperature at the base of the convective envelope prevents it to deeply penetrate the radiative underlying layers. Depending on the initial stellar mass, the heavy elements nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the \nean~reaction, which is efficiently activated during Thermal Pulses. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the weight that intermediate mass models have on the carbon stars luminosity function. Finally, we present the upgrade of the FRUITY web interface, now also including the physical quantities of the TP-AGB phase of all the models included in the database (ph-FRUITY).Comment: Accepted for publication on ApJ

Nucleation of small silicon carbide dust clusters in AGB stars

Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB stars. The formation pathways of these grains are, however, not fully understood.\ We calculate ground states and energetically low-lying structures of (SiC)$_n$, $n=1,16$ clusters by means of simulated annealing (SA) and Monte Carlo simulations of seed structures and subsequent quantum-mechanical calculations on the density functional level of theory. We derive the infrared (IR) spectra of these clusters and compare the IR signatures to observational and laboratory data.\ According to energetic considerations, we evaluate the viability of SiC cluster growth at several densities and temperatures, characterising various locations and evolutionary states in circumstellar envelopes.\ We discover new, energetically low-lying structures for Si$_{4}$C$_{4}$, Si$_{5}$C$_{5}$, Si$_{15}$C$_{15}$ and Si$_{16}$C$_{16}$, and new ground states for Si$_{10}$C$_{10}$ and Si$_{15}$C$_{15}$. The clusters with carbon-segregated substructures tend to be more stable by 4-9 eV than their bulk-like isomers with alternating Si-C bonds. However, we find ground states with cage ("bucky"-like) geometries for Si$_{12}$C$_{12}$ and Si$_{16}$C$_{16}$ and low-lying, stable cage structures for n $\ge$ 12. The latter findings indicate thus a regime of clusters sizes that differs from small clusters as well as from large-scale crystals. Thus, and owing to their stability and geometry, the latter clusters may mark a transition from a quantum-confined cluster regime to crystalline, solid bulk-material. The calculated vibrational IR spectra of the ground-state SiC clusters shows significant emission. They include the 10-13 $\mu$m wavelength range and the 11.3 $\mu$m feature inferred from laboratory measurements and observations, respectively, though the overall intensities are rather low.Comment: 16 pages, 25 figures, 3 tables, accepted for publication in Ap

Montreat Aquifer Recharge: Implementing Low Impact Design Patterns in a Sustainable Community to Achieve Optimum Aquifer Recharge

This study was conducted over the period of several months in response to a problem presented to the author by Administrators of a town named Montreat located in the Blue Ridge Mountains in North Carolina. The problem this study attempts to address is decreasing aquifer recharge within Montreat, which in the past has led to strains on the local well system during times of drought. Anytime this happens it is a problem for the town of Montreat because the community has to then draw its potable water from nearby Asheville. This is an expense that a town with a limited budget like Montreat would like to avoid. In order to address the issue of aquifer recharge in this area research of storm water management techniques known as low impact development (LID) was conducted. Research was focused specifically on those LID strategies that slow down or stop stormwater runoff and allow it to infiltrate the soil and replenish ground water supplies. An in-depth, site-specific analysis helped to locate where in the town these strategies should be implemented to be most effective. Based on this research a site-specific plan involving several different LID design solutions was developed to better manage stormwater runoff in Montreat and help recharge the community\u27s aquifer

Interplay between pulsation, mass loss, and third dredge-up: More about Miras with and without technetium

We follow-up on a previous finding that AGB Mira variables containing the 3DUP indicator technetium (Tc) in their atmosphere form a different sequence of K-[22] colour as a function of pulsation period than Miras without Tc. A near- to mid-infrared colour such as K-[22] is a good probe for the dust mass-loss rate of the stars. Contrary to what might be expected, Tc-poor Miras show redder K-[22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at a given period. Here, the previous sample is extended and the analysis is expanded towards other colours and dust spectra. The most important aim is to investigate if the same two sequences can be revealed in the gas mass-loss rate. We analysed new optical spectra and expanded the sample by including more stars from the literature. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated. Literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed. Our results show that Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR, suggesting that the previous finding based on the K-[22] colour is not due to a specific dust feature in the 22 micron band. We establish a linear relation between K-[22] and the gas mass-loss rate. We also find that the 13 micron feature disappears above K-[22]~2.17 mag, corresponding to \dot{M}_{\rm g}\sim2.6\times10^{-7}M_{\sun}yr^{-1}. No similar sequences of Tc-poor and Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most probably owing to limitations in the available data. Different hypotheses to explain the observation of two sequences in the P vs. K-[22] explain the observation of two sequences in the P vs. K-[22 diagram are discussed and tested, but so far none of them convincingly explains the observations. Nevertheless, we might have found an hitherto unknown but potentially important process influencing mass loss on the TP-AGB.Comment: 16 pages, 15 figures, 2 online tables, accepted for publication in A&

TOWARDS A COHERENT PERSPECTIVE: A LITERATURE REVIEW ON THE INTERPLAY OF THE INTERNET OF THINGS AND ECOSYSTEMS

The success of the IoT is inextricably linked to the balanced interplay of technological, organisational and economical interdependencies that occur in the cyber physical IoT ecosystem. De-spite growing scientific interest, recent literature lags behind cumulativeness and focuses on individual constitutions of the IoT rather than on a comprehensive and coherent understanding. To better position the IoT as a whole - that is more than the sum of its parts - we take an ecosystemic perspective on the phenome and systematically identify and analyse a collection of 70 relevant publications on the IoT ecosystem. We intend to give the first comprehensive overview of the ecosystem construct in the IoT, including a systematization of the existing literature and thus contribute to the synthesis of prior fragmented literature. Drawing on an ecosystem view we aim to reveal research avenues for the IS community that none of the IoT’ s individual constitutions can provid

The effect of thermal non-equilibrium on kinetic nucleation

Nucleation is considered to be the first step in dust and cloud formation in the atmospheres of asymptotic giant branch (AGB) stars, exoplanets, and brown dwarfs. In these environments dust and cloud particles grow to macroscopic sizes when gas phase species condense onto cloud condensation nuclei (CCNs). Understanding the formation processes of CCNs and dust in AGB stars is important because the species that formed in their outflows enrich the interstellar medium. Although widely used, the validity of chemical and thermal equilibrium conditions is debatable in some of these highly dynamical astrophysical environments. We aim to derive a kinetic nucleation model that includes the effects of thermal non-equilibrium by adopting different temperatures for nucleating species, and to quantify the impact of thermal non-equilibrium on kinetic nucleation. Forward and backward rate coefficients are derived as part of a collisional kinetic nucleation theory ansatz. The endothermic backward rates are derived from the law of mass action in thermal non-equilibrium. We consider elastic collisions as thermal equilibrium drivers. For homogeneous TiO2 nucleation and a gas temperature of 1250 K, we find that differences in the kinetic cluster temperatures as small as 20 K increase the formation of larger TiO2 clusters by over an order of magnitude. An increase in cluster temperature of around 20 K at gas temperatures of 1000 K can reduce the formation of a larger TiO2 cluster by over an order of magnitude. Our results confirm and quantify the prediction of previous thermal non-equilibrium studies. Small thermal non-equilibria can cause a significant change in the synthesis of larger clusters. Therefore, it is important to use kinetic nucleation models that include thermal non-equilibrium to describe the formation of clusters in environments where even small thermal non-equilibria can be present.Comment: 13 pages, 4 figure

Nanotechnology and Model Catalysis: The Use of Photolithography for Creating Active Surfaces

New and very stable model catalysts have been developed. Two types of samples on oxidized 4-inch wafers were produced using processes that are generally employed in semiconductor device technology. A single wafer exhibits 109 to 1010 active sites on an otherwise flat silicon oxide surface. Sputter etching of a number of bilayers (Pd/SiO2), stacked on an oxidized Si wafer surface resulted in billions of isolated towers, consisting of disks of active metal layers, separated by inert substrate material. A second system was produced by etching pits into a heavily oxidized 4-inch Si wafer. Active material was deposited into the pits by e-beam evaporation or spin-coating of precursor solutions. The topography and chemical composition, and the changes induced by the reaction conditions applied, including stability and chemical behavior of the nanostructured systems, were investigated by means of AFM, SEM, temperature-programmed methods and XPS

Molecule and dust synthesis in the inner winds of oxygen-rich AGB stars

This thesis aims to explain the masses and compositions of prevalent molecules, dust clusters, and dust grains in the inner winds of oxygen-rich AGB stars. In this context, models have been developed, which account for various stellar conditions, reflecting all the evolutionary stages of AGB stars, as well as different metallicities. Moreover, we aim to gain insight on the nature of dust grains, synthesised by inorganic and metallic clusters with associated structures, energetics, reaction mechanisms, and finally possible formation routes. We model the circumstellar envelopes of AGB stars, covering several C/O ratios below unity and pulsation periods of 100 - 500 days, by employing a chemical-kinetic approach. Periodic shocks, induced by pulsation, with speeds of 10 - 32 km/s enable a non-equilibrium chemistry to take place between 1 and 10 R* above the photosphere. The various models include the well-studied, galactic Mira variables like IK Tau and TX Cam, galactic S-stars, semi-regular variables of type SRa and SRb, as well as Mira stars of lower metallicity in the Magellanic clouds. In addition, we perform quantum-chemical calculations on the Density Functional Theory (DFT) level for several alumina and silicate clusters, in order to obtain structures, electronic properties, and infrared (IR) spectra of the potential dust components. The results for the gas phase agree well with the most recent observational data for IK Tau and VY CMa. Major parent molecules form in the shocked gas under non-equilibrium conditions and include CO, H2O, SiO, SiS, SO and SO2, as well as the unexpected carbon-bearing species HCN, CS and CO2, and the recently detected phosphorous species PO and PN. In the galactic models, small alumina clusters form and condense efficiently close to the star. In the case of galactic Miras, silicate clusters with forsterite mineralogy form and coalesce around 4 R*. In the lower metallicity and semi-regular models, the dust formation is hampered by the unavailability of the critical elements (Si and Al), low densities, and high temperatures. The dust/gas mass ratio ranges from 10^(-9) to 10^(-5) for alumina, and from 10^(-6) to 10^(-3) for forsterite, and agrees with the dust-to-gas mass ratio derived for oxygen-rich AGB stars. For the first time, a complete non-equilibrium model - including gas phase chemistry, cluster growth and dust formation - is built up self-consistently, and explaining successfully the most recent observations