Shell Burning Research Articles

SN 2023ixf: An average-energy explosion with circumstellar medium and a precursor

The fortunate proximity of the Type II supernova (SN) 2023ixf has allowed astronomers to follow its evolution from almost the moment of the collapse of the progenitor's core. SN,2023ixf can be explained as an explosion of a massive star with an energy of 0.7 erg but with a greatly reduced envelope mass, probably because of binary interaction. In our radiative-transfer simulations, the SN ejecta of 6 interact with circumstellar matter (CSM) of (0.55--0.83) extending to 10^15 cm, which results in a light curve (LC) peak matching that of SN,2023ixf. The origin of this required CSM might be gravity waves originating from convective shell burning, which could enhance wind-like mass loss during the late stages of stellar evolution. The steeply rising low-luminosity flux during the first hours after observationally confirmed non-detection, however, cannot be explained by the collision of the energetic SN shock with the CSM. Instead, we consider it as a precursor that we can fit by the emission from (0.5--0.9) of matter that was ejected with an energy of ∼10^49 erg a fraction of a day before the main shock of the SN explosion reached the surface of the progenitor. The source of this energy injection into the outermost shell of the stellar envelope could also be dynamical processes related to the convective activity in the progenitor's interior or envelope. Alternatively, the early rise of the LC could point to the initial breakout of a highly non-spherical SN shock or of fast-moving asymmetrically ejected matter that was swept out well ahead of the SN shock, potentially in a low-energy, nearly relativistic jet. We also discuss that pre-SN outbursts and LC precursors can be used to study or to constrain energy deposition in the outermost stellar layers by the decay of exotic particles, such as axions, which could be produced simultaneously with neutrinos in the newly formed hot neutron star. A careful analysis of the earliest few hours of the LCs of SNe can reveal elusive precursors and provide a unique window onto the surface activity of massive stars during their core collapse. This can greatly improve our understanding of stellar physics and consequently also offer new tools for searching for exotic particles.

Finding the unusual red giant remnants of cataclysmic variable mergers

Mergers between helium white dwarfs and main-sequence stars are likely common, producing red giant-like remnants making up roughly a few percent of all low-mass ( ≲2M⊙) red giants. Through detailed modeling, we show that these merger remnants possess distinctive photometric, asteroseismic, and surface abundance signatures through which they may be identified. During hydrogen shell burning, merger remnants reach higher luminosities and possess pulsations which depart from the usual degenerate sequence on the asteroseismic Δν– ΔΠ diagram for red giant branch stars. For sufficiently massive helium white dwarfs, merger remnants undergo especially violent helium flashes which can dredge up a large amount of core material (up to ∼0.1M⊙) into the envelope. Such post-dredge-up remnants are more luminous than normal red clump stars, are surface carbon-, helium-, and possibly lithium-rich, and possess a wider range of asteroseismic g-mode period spacings and mixed-mode couplings. Recent asteroseismically determined low-mass ( ≲0.8M⊙) red clump stars may be core helium-burning remnants of mergers involving lower-mass helium white dwarfs.

Ancient and historical cooking pots and food: an eternal communion. A topical review

AbstractThis contribution provides a topical view at and review of traditional clay‐based utilitarian cooking pots that were used for millennia to prepare, serve, display, and distribute foodstuff. Key mechanical and thermal properties of ceramic cooking vessels will be discussed and strategies of property optimization outlined. In addition, some important chemical changes food constituents undergo during cooking will be explained. Mass‐produced ancient ceramic cooking pots from Neolithic Mesopotamia have revolutionized the art of cooking by allowing foodstuff to be processed in water. As an example of successfully optimizing the properties of cooking vessels, emphasis is being given to Indigenous prehistoric North American ware of the Mississippian culture (c. 800 to 1600 CE) that show impressively how ancient potters overcame the technological challenges posed by essentially unsuitable smectite‐rich clays with extreme plasticity and high swell–shrink ratio by adding copious amounts of burnt mussel shells as temper material.

Two Shell-burning White Dwarfs in Symbiotics of the Small Magellanic Cloud

SMC3 and Lin358 are super-soft X-ray sources (SSS) in symbiotics of the Small Magellanic Cloud. They have been observed as SSS for over 30 yr. We present new observations done in 2014, 2021 and in 2023 with XMM-Newton, NICER, and Chandra. Lin358 is modeled with an effective temperature around 200,000 K, and a maximum flux value around 8.25 × 10−13 erg cm−2 s−1 in 2021. An observation close to periastron revealed hardening of the source. SMC3 is known to undergo an X-ray obscuration approximately every 4.5 yr; we measured the peak X-ray flux of 4.12 × 10−13 in 2014 with the Chandra Low Energy Transmission Grating. Models indicate an effective temperature of about 500,000 K, consistent with previous findings, and increasing luminosity excursion between maxima and minima. We suggest that with such high effective temperature, indicative of steady shell burning, the possibility these systems are on the path to type Ia supernova is worth considering.

PENELLOPE. VI. Searching the PENELLOPE/UVES sample with spectro-astrometry: Two new microjets of Sz 103 and XXCha

The physical mechanism leading to the formation of the blue loop in the Hertzsprung-Russell (HR) diagram is not satisfactorily explained by the evolutionary track of single stars. Rapid rotation and low metallicity drastically modify the internal structures and surface compositions of stars. Therefore, they provide a very significant pattern to investigate the evolutionary properties of the blue loop. In this paper, we mainly explore how rapid rotation and low metallicity have an important impact on the occurrence and extension of the blue loop. To this end, we implemented the rotating stellar evolution model, including the angular momentum transportation and chemical element mixing. We incorporated several initial rotational velocities and two characteristic metallicities in various models to explore the blue loop extension. The blue loop can occur when the hydrogen burning shell merges with the hydrogen--helium abundance discontinuity. We find that the blue loop extension strongly depends on the amplitude and gradient of the hydrogen--helium discontinuity. The hydrogen--helium discontinuity is created by the intermediate convective region or the convective dredge-up. A steeper hydrogen gradient in association with a greater amplitude of the hydrogen abundance discontinuity may favour a hotter star. Both the low metallicity and rapid rotation tend to restrain the development of the outer convective envelope and thus disfavour the occurrence and extension of the blue loop. There are three main reasons for this occurrence. Firstly, the helium core and its core potential can be enlarged by rotational mixing or low metallicity. Secondly, rapid rotation reduces the convective dredge-up depth in the star with $ Z=0.014$ and the mass extension of the intermediate convective region in the star with $ Z=0.0008$. Both of these phenomena lead to a reduction of the amplitude of the hydrogen abundance gradient. Thirdly, strong rotational mixing in the model (i.e. $ ini =350$ Km/s) with $ Z=0.0008$ reduces the energy generation rate from the hydrogen burning shell. Without bending towards higher effective temperature in the HR diagram, the additional helium brought near the H-burning shell associated with the larger He core can cause the star to expand towards becoming a red giant star directly after the core hydrogen burning. Rapid rotation and low metallicity tend to produce surface enrichment of the ratio of nitrogen to carbon and reduce the $ C$ left in the core; this has an important influence on the stellar compactness of the supernovae progenitor.

HD 214220 (BD+56 2813): An eclipsing binary with its primary component at the end of the main sequence

Context. Analysis of the light curves of an eclipsing binary allows one to derive the absolute dimensions of the system. This in turn yields information on the radii of the components, which allows the stars to be accurately placed on the Hertzsprung-Russell diagram and their evolutionary phase to be interpreted via comparisons to tracks of stellar evolution models. Aims. I aim to derive the stellar and system parameters of HD 214220. Methods. I measured the epochs of three primary and three secondary minima of the eclipsing binary HD 214220 from 2019 to 2022 from photometric fluxes obtained by the TESS satellite. I modeled the light curve and the velocity amplitudes, which were obtained by the Gaia satellite, with the software PHOEBE. Results. HD 214220 is an eclipsing binary system with an orbital period of P = 43.14 d, eclipse depths of 17% and 13%, and masses of 2.49 M⊙ and 2.42 M⊙. The sum of the radii is R1 + R2 ≈ 8.5 R⊙, and the temperatures of the components are similar, with a ratio of T2/T1 ≈ 1.03. Conclusions. By consulting stellar evolution models, I find that the primary component has ended core hydrogen burning and is potentially in the contraction phase, prior to shell burning.

Synthesis, protection performance, and failure analysis of α-Al2O3/Mo(Si,Al)2 coatings

To prepare the advanced cooling structure of hollow turbine blades, molybdenum metal core (MMC) has been proposed as a substitute for ceramic core substrate, but the protective coating needs to be redesigned to resist ablation during the shell burning process and erosion during the casting process. To this end, in this study, an α-Al2O3/Mo(Si,Al)2 barrier coating is fabricated on a molybdenum surface utilizing a two-step pack cementation process, which includes first siliconizing and then aluminizing, combined with pre-oxidation treatments. The pre-oxidation behavior of the new Mo(Si,Al)2 coating under isothermal oxidation (at 1300 °C) and step oxidation and its protection performance during the superalloy casting process are comprehensively examined. The results indicate that the as-deposited coating consists of an outer Mo(Si,Al)2 layer and an inner Al8Mo3 layer. After pre-oxidation, an admixture (SiO2▪Al2O3) monolayer is first formed on the surface of MMC. Then, internal Al2O3 emerges in the lower part of (SiO2▪Al2O3) mixture. Finally, the SiO2 → Al2O3 displacement reaction encapsulates MMC with an exclusive α-Al2O3 layer. Besides, the step oxidation process can obviously enhance the formation of external α-Al2O3 compared to the isothermal oxidation process. Unfortunately, after pre-oxidation treatment, a bulge arises at the interface of Mo(Si,Al)2 and Al8Mo3. The buckled coating induces partially destruction of the MMC coating by the alloy melt, whereas the intact coating can protect MMC during the casting process. Furthermore, the interfacial gap of MMC varies with the addition of alumina.

TCP J18224935-2408280: a symbiotic star identified during outburst

ABSTRACT TCP J18224935-2408280 was reported to be in outburst on 2021 May 19. Follow-up spectroscopic observations confirmed that the system was a symbiotic star. We present optical spectra obtained from the Himalayan Chandra Telescope during 2021–22. The early spectra were dominated by Balmer lines, He i lines and high ionization lines such as He ii. In the later observations, Raman scattered O vi was also identified. Outburst in the system started as a disc instability, and later the signature of enhanced shell burning and expansion of photospheric radius of the white dwarf was identified. Hence we suggest this outburst is of combination nova type. The post-outburst temperature of the hot component remains above 1.5 × 105 K indicating a stable shell burning in the system for a prolonged time after the outburst. Based on our analysis of archival multiband photometric data, we find that the system contains a cool giant of M1-2 III spectral type with a temperature of ∼3600 K and a radius of ∼69 R⊙. The pre- and post-outburst light curve shows a periodicity of 631.25 ± 2.93 d; we consider this as the orbital period.

3D simulations of magnetoconvection in a rapidly rotating supernova progenitor

ABSTRACT We present a first 3D magnetohydrodynamic (MHD) simulation of oxygen, neon, and carbon shell burning in a rapidly rotating $16\hbox{-}\mathrm{M}_\odot$ core-collapse supernova progenitor. We also run a purely hydrodynamic simulation for comparison. After $\mathord \approx 180\mathrm{s}$ ($\mathord \approx$ 15 and 7 convective turnovers, respectively), the magnetic fields in the oxygen and neon shells achieve saturation at 1011 and 5 × 1010 G. The strong Maxwell stresses become comparable to the radial Reynolds stresses and eventually suppress convection. The suppression of mixing by convection and shear instabilities results in the depletion of fuel at the base of the burning regions, so that the burning shell eventually move outward to cooler regions, thus reducing the energy generation rate. The strong magnetic fields efficiently transport angular momentum outwards, quickly spinning down the rapidly rotating convective oxygen and neon shells and forcing them into rigid rotation. The hydrodynamic model shows complicated redistribution of angular momentum and develops regions of retrograde rotation at the base of the convective shells. We discuss implications of our results for stellar evolution and for the subsequent core-collapse supernova. The rapid redistribution of angular momentum in the MHD model casts some doubt on the possibility of retaining significant core angular momentum for explosions driven by millisecond magnetars. However, findings from multidimensional models remain tentative until stellar evolution calculations can provide more consistent rotation profiles and estimates of magnetic field strengths to initialize multidimensional simulations without substantial numerical transients. We also stress the need for longer simulations, resolution studies, and an investigation of non-ideal effects.

Dynamical He Flashes in Double White Dwarf Binaries

The detonation of an overlying helium layer on a 0.8–1.1 M ⊙ carbon–oxygen (CO) white dwarf (WD) can detonate the CO WD and create a thermonuclear supernova (SN). Many authors have recently shown that when the mass of the He layer is low (≲0.03 M ⊙), the ashes from its detonation minimally impact the spectra and light curve from the CO detonation, allowing the explosion to appear remarkably similar to Type Ia SNe. These new insights motivate our investigation of dynamical He shell burning and our search for a binary scenario that stably accumulates thermally unstable He shells in the 0.01–0.08 M ⊙ range, thick enough to detonate, but also often thin enough for minimal impact on the observables. We first show that our improved nonadiabatic evolution of convective He shell burning in this range of shell mass leads to conditions ripe for a He detonation. We also find that a stable mass transfer scenario with a high-entropy He WD donor of mass 0.15–0.25 M ⊙ yields the He shell masses needed to achieve the double detonations. This scenario also predicts that the surviving He donor leaves with a spatial velocity consistent with the unusual runaway object, D6-2. We find that hot He WD donors originate in common-envelope events when a 1.3–2.0 M ⊙ star fills its Roche lobe at the base of the red giant branch at orbital periods of 1–10 days with the CO WD.

Carbon Abundance of Globular Cluster M22 (NGC 6656) and the Surface Carbon Depletion Rates of the Milky Way Globular Clusters

It is well known that metal-poor red giant branch (RGB) stars show variations in some elemental abundances, including carbon, due to the internal mixing accompanied by their own in situ CN cycle in the hydrogen burning shell. With our new photometric carbon abundance measurements of RGB stars in M22 and other globular clusters (GCs) in our previous studies, M5, M3, and M92, we derive the carbon depletion rates against the V magnitude, d[C/Fe]/M V , for individual populations in each GC. We find the metallicity dependence of the carbon depletion rates, d[C/Fe]/M V ∝ −0.25[Fe/H]. Our results also suggest that the carbon depletion rates of the second generation (SG) of stars are larger than those of the first generation (FG) of stars in our sample GCs, most likely due to different internal temperature profiles with different initial helium abundances between the FG and SG. Our results can provide critical constraints both on understanding the mixing efficiency in the theoretical models, which is largely unknown, and on interpretation of the observational carbon abundance evolution of the bright halo RGB stars.

Taking a break: Paused accretion in the symbiotic binary RT Cru

Symbiotic binaries sometimes hide their symbiotic nature for significant periods of time. There is mounting observational evidence that, in symbiotics that are powered solely by the accretion of the red giant’s wind material onto a white dwarf, without any quasi-steady shell burning on the surface of the white dwarf, the characteristic emission lines in the optical spectrum can vanish, leaving the semblance of an isolated red giant spectrum. Here we present compelling evidence that this disappearance of optical emission lines from the spectrum of RT Cru in 2019 was due to a decrease in the accretion rate, which we derived by modeling the X-ray spectrum. This drop in accretion rate leads to a lower flux of ionizing photons and thus to faint or absent photoionization emission lines in the optical spectrum. We observed the white dwarf symbiotic RT Cru with XMM-Newton and Swift in X-rays and UV and collected ground-based optical spectra and photometry obtained over the last 33 yr. This long-term coverage shows that, during most of the year 2019, the accretion rate onto the white dwarf was so low, Ṁ = (3.2 ± 0.06) × 10−11 M⊙ yr−1 (d/2.52 kpc)2, that the historically detected hard X-ray emission almost vanished, the UV flux faded by roughly 5 mag, the U, B, and V flickering amplitude decreased, and the Balmer lines virtually disappeared from 2019 January through March. Long-lasting low-accretion episodes such as the one reported here may hamper the chances of RT Cru experiencing a nova-type outburst despite the high mass of the accreting white dwarf.

A Red Giants’ Toy Story. II. Understanding the Red-giant Branch Bump

The Red-Giant Branch Bump (RGBB) is one of the most noteworthy features in the red-giant luminosity function of stellar clusters. It is caused by the passage of the hydrogen-burning shell through the composition discontinuity left at the point of the deepest penetration by the convective envelope. When crossing the discontinuity the usual trend in increasing luminosity reverses for a short time before it increases again, causing a zig-zag in the evolutionary track. In spite of its apparent simplicity the actual physical reason behind the decrease in luminosity is not well understood and several different explanations have been offered. Here we use a recently proposed simple toy model for the structure of low-mass RGs, together with previous results, to show beyond reasonable doubt that the change in luminosity at the RGBB can be traced to the change in the mean molecular weight of the layers on top of the burning shell. And that these changes happen on a nuclear timescale. The change in the effective mean molecular weight, as the burning shell approaches the discontinuity, causes a drop in the temperature of the burning shell which is attenuated by the consequent feedback contraction of the layers immediately below the burning shell. Our work shows that, when applied correctly, including the feedback on the structure of the core together with the increase in the mass of the core, shell-source homology relations do a great quantitative job in explaining the properties of full evolutionary models at the RGBB.

Application of Boiler Fly Ash for Oil Palm Kernel Separation in Claybath

The burning of shells and fiber as boiler fuel in palm oil mills produces waste in the form of ash which is not utilized and managed optimally, resulting in environmental damage. Based on the chemical compounds contained in fly ash, its abundant availability, its cheap price and easy-to-obtain, fly ash can be used as a cheap raw material in claybaths as a substitute for clay and calcium carbonate. This study aims to determine the effective weight of boiler ash in reducing kernel and shell production losses at the kernel processing station. This study used variations in the weight of boiler fly ash, namely 3000 g, 3500 g, 4000 g, 4500 g, and 5000 g which were tested first in the laboratory. The application of boiler ash variations that were close to the norm was tested directly in the claybath. The best weight parameter for using fly ash is found in the weight variation of B5 (5000 g) with a loss of production in the sample of 3.98% or 0.159% of the oil palm fresh fruit bunches. At the time of application, boiler fly ash should be mixed with water at a ratio of 1:2 (fly ash to water) to result the best effect. Key words : Boilers, Clay bath, Fly Ash, Kernel, Shell

Mass Distribution of Black Holes with Effects of Convective Carbon Shell Burning on Pair-instability Pulsation and Fe Core Collapse

Motivated by the determination of black hole masses with gravitational-wave observations, we calculate the evolution of massive stars through presupernova stages and obtain the mass distribution of black holes. In the first part, we calculate the evolution of He stars with masses of 30–120 M ⊙. We study in detail how convective carbon shell burning controls pair-instability pulsations before and during oxygen burning and determine their final fates. In the second part, we calculate the evolution of H-rich stars with initial masses of 13–80 M ⊙ until Fe core collapse and obtain the possible black hole mass range by applying the criterion of the compactness parameters. From these models, we predict the mass distribution of black holes for stars that undergo Fe core collapse and pair-instability pulsation. The predicted masses for black holes range from 4.2 to 46 M ⊙, which are consistent with the gravitational-wave observations.

A Red Giants’ Toy Story

In spite of the spectacular progress accomplished by stellar evolution theory, some simple questions remain unanswered. One of these questions is “Why do stars become red giants?”. Here we present a relatively simple analytical answer to this question. We validate our analysis by constructing a quantitative toy model of a red giant and comparing its predictions to full stellar evolutionary models. We find that the envelope forces the value of at, and above, the burning shell into a very narrow range of possible values. Together with the fact that the stellar material at the burning shell both provides and transports most of the stellar luminosity, this leads to tight relations between the thermodynamic variables at the burning shell and the mass and radius of the core—T s (M c , R s ), P s (M c , R s ), and ρ s (M c , R s ). When complemented by typical mass–radius relations of the helium cores, this implies that for all stellar masses the evolution of the core dictates the values of T s , P s , and ρ s . We show that for all stellar masses evolution leads to an increase in the pressure and density contrasts between the shell and the core, forcing a huge expansion of the layers on top of the burning shell. Besides explaining why stars become red giants our analysis also offers a mathematical demonstration of the so-called shell homology relations, and provides simple quantitative answers to some properties of low-mass red giants.

Early settlement construction in Southeast Asia: lime mortar floor sequences at Loc Giang, southern Vietnam

Research on prehistoric mainland Southeast Asia is dominated by mortuary contexts, leaving processes such as the transition to sedentism relatively understudied. Recent excavations in southern Vietnam, however, have recovered new evidence for settlement. The authors report on investigations at the neolithic site of Loc Giang (3980–3270 cal BP) in southern Vietnam, where excavation revealed a vertical sequence of more than 30 surfaces. Microarchaeological analyses indicate that these features are carefully prepared lime mortar floors; the lime was probably produced from burnt shell. The floors date to between 3510 and 3150 cal BP, providing the earliest-known evidence for the use of lime mortar, and for durable settlement construction, in this region.

Impact of the new 12C+12C reaction rate on presupernova nucleosynthesis* *Supported by the National Natural Science Foundation of China (11988101, 11890694), and the National Key R&D Program of China (2019YFA0405502). K. Nomoto is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan, and the Japan Society for the Promotion of Science (JSPS) KAKENHI grant (JP17K05382, JP20K04024, JP21H04499)

The 12C+12C reaction rate plays an essential role in stellar evolution and nucleosynthesis. Nevertheless, the uncertainties of this reaction rate are still large. We calculate a series of stellar evolution models with the near solar abundance from the zero-age main-sequence through presupernova stages for initial masses of 20 M to 40 M . The 12C+12C reaction rates from two different studies are used in our investigation. One is the rate obtained using the Trojan Horse Method (THM) by Tumino et al. [Nature 557(7707), 687 (2018)], and the other was obtained by Mukhamedzhanov et al. [Physical Review C 99(6), 064618 (2019)] (Muk19). Then, comparisons of the nucleosynthesis and presupernova isotopic abundances are conducted. In particular, we find that in the C burning shell, models with the THM produce a smaller amount of 23Na and some neutron-rich isotopes than Muk19. The difference in the abundance ratios of Na/Mg, S/Mg, Ar/Mg, and K/Mg between the two models are apparent. We compare Na/Mg obtained from our theoretical presupernovae models with Na/Mg in stellar atmospheres observed with high-resolution spectra as well as from the latest galactic chemical evolution model. Although Na/Mg obtained using the THM is within 2σ of the observed stellar ratio, the theoretical uncertainty on Na/Mg introduced by the uncertainty of the 12C+12C reaction rate is almost equivalent to the standard deviation of astronomical observations. Therefore, a more accurate 12C+12C reaction rate is crucial.

Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

Many core-collapse supernovae (SNe) with hydrogen-poor and low-mass ejecta, such as ultra-stripped SNe and type Ibn SNe, are observed to interact with dense circumstellar material (CSM). These events likely arise from the core collapse of helium stars that have been heavily stripped by a binary companion and have ejected significant mass during the last weeks to years of their lives. In helium star models run to days before core collapse we identify a range of helium core masses ≈2.5–3 M ⊙ whose envelopes expand substantially due to the helium shell burning while the core undergoes neon and oxygen burning. When modeled in binary systems, the rapid expansion of these helium stars induces extremely high rates of late-stage mass transfer () beginning weeks to decades before core collapse. We consider two scenarios for producing CSM in these systems: either mass transfer remains stable and mass loss is driven from the system in the vicinity of the accreting companion, or mass transfer becomes unstable and causes a common envelope event (CEE) through which the helium envelope is unbound. The ensuing CSM properties are consistent with the CSM masses (∼10−2–1 M ⊙) and radii (∼1013–1016 cm) inferred for ultra-stripped SNe and several type Ibn SNe. Furthermore, systems that undergo a CEE could produce short-period neutron star binaries that merge in less than 100 Myr.

Persistent nuclear burning in Nova Sgr 2016 N.4 (=V5856 Sgr = ASASSN-16ma) six years past its outburst

We report on the fast Nova Sgr 2016 N.4 being surprisingly trapped in a long-lasting and bright plateau (ΔI≥10 mag above quiescence) six years past the nova eruption. Very few other novae experience a similar occurrence. We carried out an intensive observing campaign collecting dailyBVRIphotometry and monthly high-resolution optical spectroscopy, and observed the nova in ultraviolet and X-rays withSwiftat five distinct epochs. The bolometric luminosity radiated during the plateau is ∼4200L⊙(scaled to the distance of the Galactic Bulge), corresponding to stable nuclear burning on a 0.6M⊙white dwarf. A stable wind is blown off at full width at zero intensity (FWZI) ∼ 1600 km s−1, with episodic reinforcement of a faster FWZI ∼ 3400 km s−1mass loss, probably oriented along the polar directions. The collision of these winds could power the emission detected in X-rays. The burning shell has an outer radius of ∼25R⊙at which the effective temperature is ∼7600 K, values similar to those of a F0 II/Ib bright giant. The Δm < 1 mag variability displayed during the plateau is best described as chaotic, with the irregular appearance of quasi-periodic oscillations with a periodicity of 15–17 days. A limited amount of dust (≈3 × 10−11M⊙) continuously condenses atTdust ∼ 1200 K in the outflowing wind, radiatingLdust ∼ 52L⊙.