Thermodynamics of dust condensation around the dimming Betelgeuse

ABSTRACT Betelgeuse is one of the brightest red supergiant (RSG) stars because of its proximity to the Solar system. This makes it important when deducing the features and evolutionary phases of RSG stars. Betelgeuse has always been a well-observed target but especially during the past year, because of the reduction in its brightness. It has been speculated that the star is in its last evolutionary stage(s), and that it is soon going to explode. However, in recent work, it has been proposed that the episodic mass loss and dust condensation around the star are major reasons for the reduction in its brightness. In this work, we have performed detailed thermodynamical equilibrium and non-equilibrium calculations of the condensation of dust grains around the cooling envelope of Betelgeuse. Based on the deduced chemical composition, we have ventured to determine the nature of dust that could condense in the stellar winds. The dust grains are essentially found to be oxides of Al, Ca and Ti, and silicates of Al, Ca, Mg and Fe-metal. Further, we have determined the normalized masses of the dust grains of various compositions that could be present around the star and could be causing the reduction in its brightness.

Dust condensation in evolving discs and the composition of planetary building blocks

ABSTRACT Partial condensation of dust from the Solar nebula is likely responsible for the diverse chemical compositions of chondrites and rocky planets/planetesimals in the inner Solar system. We present a forward physical–chemical model of a protoplanetary disc to predict the chemical compositions of planetary building blocks that may form from such a disc. Our model includes the physical evolution of the disc and the condensation, partial advection, and decoupling of the dust within it. The chemical composition of the condensate changes with time and radius. We compare the results of two dust condensation models: one where an element condenses when the mid-plane temperature in the disc is lower than the 50 per cent condensation temperature ($\rm T_{50}$) of that element and the other where the condensation of the dust is calculated by a Gibbs free energy minimization technique assuming chemical equilibrium at local disc temperature and pressure. The results of two models are generally consistent with some systematic differences of ∼10 per cent depending upon the radial distance and an element’s condensation temperature. Both models predict compositions similar to CM, CO, and CV chondrites provided that the decoupling time-scale of the dust is of the order of the evolution time-scale of the disc or longer. If the decoupling time-scale is too short, the composition deviates significantly from the measured values. These models may contribute to our understanding of the chemical compositions of chondrites, and ultimately the terrestrial planets in the Solar system, and may constrain the potential chemical compositions of rocky exoplanets.

Thermodynamics of the condensation of dust grains in Wolf–Rayet stellar winds

ABSTRACT Wolf–Rayet (WR) stars are the evolutionary phases of very massive stars prior to the final supernova explosion stage. These stars lose substantial mass during the WN and WC stages. The mass losses are associated with diverse elemental and isotopic signatures that represent distinct stellar evolutionary processes. WR strong winds can host environments favourable for the condensation of dust grains with diverse compositions. The condensation of dust in the outflows of massive stars is supported by a number of observations. The present work is an attempt to develop a theoretical framework for the thermodynamics associated with the condensation of dust grains in the winds of the WN and WC phases. A novel numerical code has been developed for dust condensation. In addition to the equilibrium dust condensation calculations, we have attempted, perhaps for the first time, a set of non-equilibrium scenarios for dust condensation in various WR stages. These scenarios differ in terms of the magnitude of the non-equilibrium state, defined in terms of a simulation non-equilibrium parameter. Here, we attempt to understand the effect of the simulation non-equilibrium parameter on the condensation sequence of dust grains. In general, we found that mostly C (graphite), TiC, SiC, AlN, CaS and Fe-metal are condensed in WR winds. The extent of non-equilibrium influences the relative proportions of the earliest dust condensate compared with the condensates formed at later stages subsequent to the cooling of the gas. The results indicate that dust grains that are condensed in the WC phase may make a substantial contribution of carbon-rich dust grains to the interstellar medium.

Features of the geochemistry of the Moon and the Earth, determined by the mechanism of formation of the Earth – Moon system (report at the 81st international meteorite conference, Moscow, july 2018)

This article discusses some features of geochemistry of the Earth and the Moon, which manifests the specificity of the mechanism of their formation by fragmentation of protoplanetary gas-dust condensation (Galimov & Krivtsov, 2012). The principal difference between this model and other hypotheses of the Earth-Moon system formation, including the megaimpact hypothesis, is that it assumes the existence of a long stage of the dispersed state of matter, starting with the formation of protoplanetary gas-dust condensation, its compression and fragmentation and ending with the final accretion to the formed high-temperature embryos of the Earth and the Moon. The presence of the dispersed state allows a certain way to interpret the observed properties of the Earth-Moon system. Partial evaporation of solid particles due to adiabatic heating of the compressing condensation leads to the loss of volatiles including FeO. Computer simulations show that the final accretion is mainly performed on a larger fragment (the Earth’s embryo) and only slightly increases the mass of the smaller fragment (the Moon embryo).This explains the relative depletion of the Moon in iron and volatile and the increased concentration of refractory components compared to the Earth. The reversible nature of evaporation into the dispersed space, in contrast to the kinetic regime, and the removal of volatiles in the hydrodynamic flow beyond the gas-dust condensation determines the loss of volatiles without the effect of isotopes fractionation. The reversible nature of volatile evaporation also provides, in contrast to the kinetic regime, the preservation of part of the high-volatile components, such as water, in the planetary body, including the Moon. It follows from the essence of the model that at least a significant part of the Earth’s core is formed not by segregation of iron in the silicate-metal melt, but by evaporation and reduction of FeO in a dispersed medium, followed by deposition of clusters of elemental iron to the center of mass. This mechanism of formation of the core explains the observed excess of siderophilic elements in the Earth’s mantle. It also provides a plausible explanation for the observed character of iron isotopes fractionation (in terms of δ57Fe‰) on Earth and on the Moon. It solves the problem of the formation of iron core from initially oxide (FeO) form. The dispersed state of the substance during the period of accretion suggests that the loss of volatiles occurred during the time of accretion. Using the fact that isotopic systems: U–Pb, Rb–Sr, 129J–129Xe, 244Pu–136Xe, contain volatile components, it is possible to estimate the chronology of events in the evolution of the protoplanetary state. As a result, agreed estimates of the time of fragmentation of the primary protoplanetary condensation and formation of the embryos of the Earth and the Moon are obtained: from 10 to 40 million years, and the time of completion of the earth’s accretion and its birth as a planetary body: 110 – 130 million years after the emergence of the solar system. The presented interpretation is consistent with the fact that solid minerals on the Moon have already appeared at least 60 million years after the birth of the solar system (Barboni et al., 2017), and the metal core in the Earth and in the Moon could not have formed before 50 million years from the start of the solar system, as follows from the analysis of the Hf-W system (Kleine et al., 2009). It is shown that the hypothesis of megaimpact does not satisfy many constraints and does not create a basis for the explanation of the geochemistry of the Earth and the Moon.

The dust content of the Crab Nebula

ABSTRACT We have modelled the near-infrared to radio images of the Crab Nebula with a Bayesian SED model to simultaneously fit its synchrotron, interstellar (IS), and supernova dust emission. We infer an IS dust extinction map with an average AV = 1.08 ± 0.38 mag, consistent with a small contribution (${\lesssim }22{{\ \rm per\ cent}}$) to the Crab’s overall infrared emission. The Crab’s supernova dust mass is estimated to be between 0.032 and 0.049 M⊙ (for amorphous carbon grains) with an average dust temperature Tdust = 41 ± 3 K, corresponding to a dust condensation efficiency of 8–12 ${{\ \rm per\ cent}}$. This revised dust mass is up to an order of magnitude lower than some previous estimates, which can be attributed to our different IS dust corrections, lower SPIRE flux densities, and higher dust temperatures than were used in previous studies. The dust within the Crab is predominantly found in dense filaments south of the pulsar, with an average V-band dust extinction of AV = 0.20–0.39 mag, consistent with recent optical dust extinction studies. The modelled synchrotron power-law spectrum is consistent with a radio spectral index αradio = 0.297 ± 0.009 and an infrared spectral index αIR = 0.429 ± 0.021. We have identified a millimetre excess emission in the Crab’s central regions, and argue that it most likely results from two distinct populations of synchrotron emitting particles. We conclude that the Crab’s efficient dust condensation (8–12 ${{\ \rm per\ cent}}$) provides further evidence for a scenario where supernovae can provide substantial contributions to the IS dust budgets in galaxies.

Dust formation and evolution around carbon stars

Evolved intermediate-mass stars with carbon-to-oxygen ratios (C/O) above unity are known as carbon stars. Carbon stars are surrounded by dust shells dominated by carbon (C) and silicon carbide (SiC) grains. These SiC grains have a diagnostic spectral feature at [about]11 [mu]m. We have selected a sample of 9 carbon stars with low mass-loss rates such that their dust shells are sufficiently optically thin to allow abundance analysis of the stars' photospheres. This allows the study of how atomic abundances affect dust formation around carbons stars. We present the result of radiative transfer modeling for these stars, and compare the resulting dust shell parameters to published abundance measurements. To constrain model parameters, we use published mass-loss rates, expansion velocities, and theoretical dust condensation models to estimate the dust condensation temperature, and spectral types to constrain stellar effective temperatures. We found significant correlations for the single-shell modeling with graphite/iron grains and amorphous carbon (AmC)/iron grains: (1) [subscript]0.55[mu]m and gas-to-dust ratio, (2) iron grains and graphite or AmC grains, (3) graphite or AmC grains and Fe/H, and (4) iron grains and Fe/H. For the collated data the significant correlations we found were: (1) for dust formation temperature and the change of temperature from the formation of graphite grains to the formation of SiC grains (2) C/O and the change of temperature from the formation of graphite grains to the formation of SiC grains. Lastly, between the abundance of SiC grains when compared to the abundances of SiC grains in graphite, AmC, graphite and iron and AmC and iron grains models. This shows that there is no sensitivity in the continuum when choosing the type of carbon to model with.