A multitude of phenomena-such as the chemical enrichment of the Universe, the mass spectrum of planetary nebulae, white dwarfs and gravitational wave progenitors, the frequency distribution of ...supernovae, the fate of exoplanets, etc.-are highly regulated by the amounts of mass that stars expel through a powerful wind. For more than half a century, these winds of cool aging stars have been interpreted within the common interpretive framework of 1D models. I here discuss how that framework now appears to be highly problematic.
Current 1D mass-loss rate formulae differ by orders of magnitude, rendering contemporary stellar evolution predictions highly uncertain.
These stellar winds harbor 3D complexities that bridge 23 orders of magnitude in scale, ranging from the nanometer up to thousands of astronomical units. We need to embrace and understand these 3D spatial realities if we aim to quantify mass loss and assess its effect on stellar evolution. We therefore need to gauge the following:
The 3D life of molecules and solid-state aggregates: The gas-phase clusters that form the first dust seeds are not yet identified. This limits our ability to predict mass-loss rates using a self-consistent approach.
The emergence of 3D clumps: They contribute in a nonnegligible way to the mass loss, although they seem of limited importance for the wind-driving mechanism.
The 3D lasting impact of a (hidden) companion: Unrecognized binary interaction has biased previous mass-loss rate estimates toward values that are too large.
Only then will it be possible to drastically improve our predictive power of the evolutionary path in 4D (classical) spacetime of any star.
Context. While the existence of more than 1800 exoplanets have been confirmed, there is evidence of a wide variety of elemental chemical composition, that is to say different metallicities and ...C/N/O/H ratios. Atmospheres with a high C/O ratio (above 1) are expected to contain a high quantity of hydrocarbons, including heavy molecules (with more than two carbon atoms). To correctly study these C-rich atmospheres, a chemical scheme adapted to this composition is necessary. Aims. We have implemented a chemical scheme that can describe the kinetics of species with up to six carbon atoms (C0-C6 scheme). This chemical scheme has been developed with combustion specialists and validated by experiments that were conducted on a wide range of temperatures (300−2500 K) and pressures (0.01−100 bar). Methods. To determine for which type of studies this enhanced chemical scheme is mandatory, we created a grid of 12 models to explore different thermal profiles and C/O ratios. For each of them, we compared the chemical composition determined with a C0-C2 chemical scheme (species with up to two carbon atoms) and with the C0-C6 scheme. We also computed synthetic spectra corresponding to these 12 models. Results. We found no difference in the results obtained with the two schemes when photolyses were excluded from the model, regardless of the temperature of the atmosphere. In contrast, differences can appear in the upper atmosphere (P> ~ 1−10 mbar) when there is photochemistry. These differences are found for all the tested pressure-temperature profiles if the C/O ratio is above 1. When the C/O ratio of the atmosphere is solar, differences are only found at temperatures lower than 1000 K. The differences linked to the use of different chemical schemes have no strong influence on the synthetic spectra. However, with this study, we have confirmed C2H2 and HCN as possible tracers of warm C-rich atmospheres. Conclusions. The use of this new chemical scheme (instead of the C0-C2) is mandatory for modelling atmospheres with a high C/O ratio and, in particular, for studying the photochemistry in detail. If the focus is on the synthetic spectra, a smaller scheme may be sufficient, because it will be faster in terms of computation time.
We observed Betelgeuse using ALMA’s extended configuration in band 7 (f ≈ 340 GHz, λ ≈ 0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the ...28SiO(ν= 2, J = 8−7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of νeqsini = 5.47 ± 0.25 km s-1 at the equivalent continuum angular radius Rstar = 29.50 ± 0.14 mas. This corresponds to an angular rotation velocity of ω sini = (5.6 ± 1.3) × 10-9 rad s-1. The position angle of its north pole is PA = 48.0 ± 3.5°. The rotation period of Betelgeuse is estimated to P/ sini = 36 ± 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ≈ 10 au (45 mas or 1.5 × the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong “rogue” convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants.
ABSTRACT More than three thousand exoplanets have been detected so far, and more and more spectroscopic observations of exoplanets are performed. Future instruments (James Webb Space Telescope ...(JWST), E-ELT, PLATO, Ariel, etc.,) are eagerly awaited, as they will be able to provide spectroscopic data with greater accuracy and sensitivity than what is currently available. This will allow more accurate conclusions to be drawn regarding the chemistry and dynamics of exoplanetary atmospheres, provided that the observational data are carefully processed. One important aspect to consider is temporal stellar atmospheric disturbances that can influence the planetary composition, and hence spectra, and potentially can lead to incorrect assumptions about the steady-state atmospheric composition of the planet. In this paper, we focus on perturbations coming from the host star in the form of flare events that significantly increase photon flux impingement on the exoplanets atmosphere. In some cases, particularly for M stars, this sudden increase may last for several hours. We aim to discover to what extent a stellar flare is able to modify the chemical composition of the planetary atmosphere and, therefore, influence the resulting spectra. We use a one-dimensional thermo-photochemical model to study the neutral atmospheric composition of two hypothetical planets located around the star AD Leo. We place the two planets at different distances from the star, which results in effective atmospheric temperatures of 412 and 1303 K. AD Leo is an active star that has already been observed during a flare. Therefore, we use the spectroscopic data from this flare event to simulate the evolution of the chemical composition of the atmospheres of the two hypothetical planets. We compute synthetic spectra to evaluate the implications for observations. The increase in the incoming photon flux affects the chemical abundances of some important species (such as H and NH3), down to altitudes associated with an atmospheric pressure of 1 bar, which can lead to variations in planetary spectra (up to 150 ppm) if performed during transit. We find that each exoplanet has a post-flare steady-state composition that is significantly different from the pre-flare steady-state. We predict that these variations could be detectable with both current and future spectroscopic instruments, if sufficiently high signal-to-noise spectra are obtained.
Abstract
Efforts over 40 yr still leave the source of astronomical infrared emission bands largely unidentified. Here, we report the first laboratory infrared (6–25
μ
m) spectra of gas-phase ...fullerene-metal complexes, C
60
-Metal
+
(Metal = Fe, V) and show with density functional theory calculations that complexes of C
60
with cosmically abundant metals, including Li, Na, K, Mg, Ca, Al, V, and Fe, all have similar spectral patterns. Comparison with observational infrared spectra from several fullerene-rich planetary nebulae demonstrates a strong positive linear cross-correlation. The infrared features of C
60
-Metal
+
coincide with four bands attributed earlier to neutral C
60
bands and in addition also with several bands unexplained to date. Abundance and collision theory estimates indicate that C
60
-Metal
+
could plausibly form and survive in astrophysical environments. Hence, C
60
-Metal
+
are proposed as promising carriers, in supplement to C
60
, of observational bands, potentially representing the largest molecular species in space other than C
60
, C
60
+
, and C
70
.
While models of grain formation in the outflows of carbon-rich stars have been relatively successful, models of outflows from oxygen-rich, asymptotic giant branch stars have been less fortunate. ...Under current modeling, it is difficult to produce sufficient amounts of silicate grains with high enough opacity to form a dust-driven wind from these stars. To investigate the cause of this difference, this work is a comparison between typical outflow model results and a model using input from Atacama Large Millimeter/submillimeter Array observations of L2 Puppis. The temperatures from these observations are much lower than would typically be used in the standard outflow model. In addition, the observed gas densities are much higher than predicted from typical outflow models. Both of these differences make the formation of silicate grains much more favorable than predicted in current outflow models. We then explore the effects of other possible nonideal conditions including the efficiency of cluster growth prior to nucleation, the efficiency of grain growth following nucleation and the variation of grain coupling to stellar radiation during grain growth. Finally, we calculate the potential enhancement in grain production based on possible increased refractory abundances resulting from the vaporization of millimeter-scale and larger particles left over from the presence of a former planetary system.
Abstract We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only ∼3% Z ⊙ , it is among the ...lowest-metallicity systems known in the local Universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early Universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf SF galaxies in the local Universe.
Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or ...its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigated the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They included clusters of magnesia (MgO)n, silicon monoxide (SiO)n, alumina (Al2O3)n, and titania (TiO2)n with stoichiometric sizes of n = 1–8. The magnesia, alumina, and titania clusters showed relatively little variation in their ionization energies with respect to the cluster size n: 7.1–8.2 eV for (MgO)n, from 8.9–10.0 eV for (Al2O3)n, and 9.3–10.5 eV for (TiO2)n. In contrast, the (SiO)n ionization energies decrease with size n, starting from 11.5 eV for n = 1, and decreasing to 6.6 eV for n = 8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral–neutral reactions.
The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density ...structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch (AGB) star π1Gruis, which has a known companion at ∼440 au and is thought to harbour a second, closer-by (< 10 au) companion, was observed with the Atacama Large Millimeter/submillimeter Array as part of the ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCN molecular line transitions are analysed. The continuum map shows two maxima, separated by 0.04″ (6 au). The CO data unambiguously reveal that π1Gru’s circumstellar environment harbours an inclined, radially outflowing, equatorial density enhancement. It contains a spiral structure at an angle of ∼38 ± 3° with the line-of-sight. The HCN emission in the inner wind reveals a clockwise spiral, with a dynamical crossing time of the spiral arms consistent with a companion at a distance of 0.04″ from the AGB star, which is in agreement with the position of the secondary continuum peak. The inner wind dynamics imply a large acceleration region, consistent with a beta-law power of ∼6. The CO emission suggests that the spiral is approximately Archimedean within 5″, beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.5″ exhibits signatures of gas in rotation, which is found to fit the expected behaviour of gas in the wind-companion interaction zone. An investigation of SiO maser emission reveals what could be a stream of gas accelerating from the surface of the AGB star to the companion. Using these dynamics, we have tentatively derived an upper limit on the companion mass to be ∼1.1 M⊙.
The modern era of highly sensitive telescopes is enabling the detection of more and more molecular species in various astronomical environments. Many of these are now being carefully examined for the ...first time. However, to move beyond detection to more detailed analysis such as radiative transfer modelling, certain molecular properties need to be properly measured and calculated. The importance of contributions from vibrationally excited states or collisional (de-)excitations can vary greatly, depending on the specific molecule and the environment being studied. Here, we discuss the present molecular data needs for detailed radiative transfer modelling of observations of molecular rotational transitions, primarily in the (sub-)millimetre and adjacent regimes, and with a focus on the stellar winds of AGB stars.
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