Keywords

We have chosen the Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code to calculate the coalescence of silicon carbide (SiC), silicon oxide dust (SiO) in the AGB stellar wind. LAMMPS is a classical molecular dynamics simulation code. At the same time, we consider the effect of temperature on the evolution of molecular dynamics. We also calculated the temperature change of non-spherical SiC, SiO dust coalescence. The condensation temperature range of SiC dust in the AGB stellar wind is [300–500]k and [900–1100]k for SiO. Finally, the infrared spectrum of SiC was calculated using Gaussian 16 software. The 77SiC, 70Si₃C₃, and 121Si₃C₃ models have clear characteristic peaks of infrared spectra responding at 5, 8.6, 11.3, 15, 19, and 37 μm.

New analyses of earlier ALMA observations of oxygen-rich AGB star EP Aquarii are presented, which complete a previously published analysis and offer a different interpretation of the morpho-kinematics of the circumstellar envelope. The birth of the equatorial density enhancement (EDE) is shown to occur very close to the star where evidence for rotation has been obtained. Close to the star and where outflows have been observed: their interaction with the gas of the nascent EDE is seen to play an important role in the development of the wind and the evolution of its radial velocity from 8 to 10 km s⁻¹ on the polar symmetry axis to ∼2 km s⁻¹ at the equator. It implies complex morpho-kinematics: making reliable interpretations with reasonable confidence is difficult. In particular, it questions an earlier interpretation implying the presence of a white dwarf companion orbiting the star at an angular distance of ∼04 from its center. It proposes instead an interpretation in terms of a standard mass ejection associated with a shock wave leaving a void of emission in its wake. High Doppler velocity wings are seen to consist of two components, the upper velocity end of the global wind, reaching above ±12 km s⁻¹, and an effective line broadening, confined within 200 mas from the center of the star, reaching above ±20 km s⁻¹ and interpreted as caused by the pattern of shock waves resulting from the interaction between stellar pulsation and convective cell partition.

We carry out an optical morphological and infrared spectral study for two young planetary nebulae (PNs) Hen 2-158 and Pe 1-1 to understand their complex shapes and dust properties. Hubble Space Telescope optical images reveal that these nebulae have several bipolar-lobed structures and a faint arc with a clear boundary is located at the northwestern side of Pe 1-1. The presence of this arc-shaped structure suggests that the object interacts with its nearby interstellar medium. Spitzer IRS spectroscopic observations of these young nebulae clearly show prominent unidentified infrared emission features and a weak silicate band in Pe 1-1, indicating that Hen 2-158 is a carbon-rich nebula and Pe 1-1 has a mixed chemistry dust environment. Furthermore, we construct two three-dimensional models for these PNs to realize their intrinsic structures. The simulated models of the nebulae suggest that multipolar nebulae may be more numerous than we thought. Our analyses of spectral energy distributions for Hen 2-158 and Pe 1-1 show that they have low luminosities and low stellar effective temperatures, suggesting that these nebulae are young PNs. A possible correlation between typical multipolar young PNs and nested nebulae is also discussed.

We measure the significance of thermally pulsing asymptotic giant branch (TP-AGB) stars via the spectral energy distributions (SEDs) of a sample of post-starburst (PSB) galaxies at z = 0.2–0.7. Using ground- and space-based photometry from the 3D-HST catalog, as well as associated near-infrared (NIR) Hubble Space Telescope (HST) slitless grism spectroscopy, we evaluate the importance of TP-AGB stars in the SEDs of 177 PSB galaxies by fitting simple stellar populations with different levels of TP-AGB contributions. The grism spectra, despite their low resolution of R ∼ 100, enable the detection of molecular features specific to TP-AGB stars and thus improve constraints on their contribution. A majority (∼70%) of galaxies in the PSB sample show features indicative of TP-AGB stars, while the remainder does not and they are well fit by Bruzual & Charlot TP-AGB light models. Stacked spectra of sources classified to be the best fit by TP-AGB heavy/mild models reveal strong detections of NIR molecular features associated with TP-AGB stars. Additionally, we observe a tentative trend with redshift where more TP-AGB heavy galaxies are observed in the higher redshift PSB galaxy population. Finally, neglecting the contribution of TP-AGB stars can yield an over-prediction of stellar masses measured in the K-band ranging from 0.13–0.23 dex.

We use archival ALMA observations of the CO(2–1) and SiO(5–4) molecular line emissions of AGB star R Hya to illustrate the relative contributions of rotation, expansion and line broadening to the morphology and kinematics of the circumstellar envelope (CSE) within some ∼70 au (∼05) from the centre of the star. We give evidence for rotation and important line broadening to dominate the inner region, within ∼14 au (∼100 mas) from the centre of the star. The former is about an axis that projects a few degrees west of north and has a projected rotation velocity of a few km s⁻¹. The latter occurs within some 7–14 au (50–100 mas) from the centre of the star, with the line width reaching two to three times its value outside this region. We suggest that it is caused by shocks induced by stellar pulsations and convective cell granulation. We show the importance of properly accounting for the observed line broadening when discussing rotation and evaluating the radial dependence of the rotation velocity.

We describe a revised procedure for the numerical simulation of planetary nebulae luminosity functions (PNLFs), improving on previous work. The procedure is now based on new H-burning post-asymptotic giant branch (AGB) evolutionary tracks. For a given stellar mass, the new central stars are more luminous and evolve faster. We have slightly changed the distribution of the [O iii] 5007 intensities relative to those of ${\rm{H}}\beta$ and the generation of absorbing factors, while still basing their numerical modeling on empirical information extracted from studies of galactic planetary nebulae (PNs) and their central stars. We argue that the assumption of PNs being completely optically thick to H-ionizing photons leads to conflicts with observations and show that to account for optically thin PNs is necessary. We then use the new simulations to estimate a maximum final mass, clarifying its meaning, and discuss the effect of internal dust extinction as a possible way of explaining the persistent discrepancy between PNLFs and surface brightness fluctuation distances. By adjusting the range of minimum to maximum final mass, it is also possible to explain the observed variety of PNLF shapes at intermediate magnitudes. The new PN formation rates are calculated to be slightly lower than suggested by previous simulations based on older post-AGB evolutionary tracks.

Winds from asymptotic giant branch (AGB) stars not only provide mass and energy return, but also produce dust grains in massive elliptical galaxies. Due to the fast stellar velocity, the wind is thought to form a comet-like tail, similar to Mira in the Local Bubble. Many massive elliptical galaxies and cluster centrals host extended dusty cold filaments. We carry out both analytical and numerical studies of the interaction between an AGB wind and the surrounding hot gas. We find that the cooling time of the tail is inversely proportional to the ambient pressure. In the absence of cooling, or in low-pressure environments (e.g., the outskirts of elliptical galaxies), AGB winds are quickly mixed into the hot gas, and all the AGB winds have a similar appearance and head-to-tail ratio. In high-pressure environments, such as the Local Bubble and the central regions of massive elliptical galaxies, some of the gas in the mixing layer between the stellar wind and the surrounding hot gas can cool efficiently and cause the tail to become longer. Our simulated tail of Mira itself has a similar length and velocity to that observed, and appears similar to the simulated AGB tail in the central regions of massive galaxies. While confirmation awaits future studies, we speculate that instead of thermal instability, the induced condensation at the mixing layer of AGB winds may be the origin of cold filaments in massive galaxies and galaxy clusters. This naturally explains the existence of dust and polycyclic aromatic hydrocarbon in the filaments.

Temporal variation of millimeter lines is a new direction of research for evolved stars. It has the potential to probe the dynamical wind-launching processes from a time dimension. We report here the first Atacama Large Millimeter Array (ALMA) results that cover 817 days of ongoing monitoring of 1.1 mm lines in the archetypal carbon star IRC +10216. The monitoring is done with the compact 7 m array and in infrared with a 1.25 m telescope in Crimea. High sensitivity of the cumulative spectra covering a total of ∼7.2 GHz between 250 and 270 GHz has allowed us to detect about 148 known transitions of 20 molecules, together with more of their isotopologues, and 81 unidentified lines. An overview of the variabilities of all detected line features is presented in spectral plots. Although a handful of lines are found to be very possibly stable in time, most other lines are varying either roughly in phase or in anticorrelation with the near-infrared light. Several lines have their variations in the ALMA data coincident with existing single-dish monitoring results, while several others do not, which requires a yet-unknown mechanism in the circumstellar envelope to explain.

We have conducted laboratory experiments with analog crystalline silicon carbide (SiC) grains using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The 3C polytype of SiC was used—the type commonly produced in the envelopes of asymptotic giant branch (AGB) stars. We rapidly heated small (∼50 nm) synthetic SiC crystals under vacuum to ∼1300 K and bombarded them with 150 keV Xe ions. TEM imaging and EELS spectroscopic mapping show that such heating and bombardment leaches silicon from the SiC surface, creating layered graphitic sheets. Surface defects in the crystals were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C₆₀. Such nonplanar features require the formation of five-membered rings. We also identified a circumstellar grain, preserved inside the Murchison meteorite, that contains the remnant of an SiC core almost fully encased by graphite, contradicting long-standing thermodynamic predictions of material condensation. Our combined laboratory data suggest that C₆₀ can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains. Such heating/bombardment could occur in the protoplanetary nebula phase, accounting for the observation of C₆₀ in these objects, in planetary nebulae (PNs) and other interstellar sources receiving PN ejecta. The synthesis of C₆₀ in astronomical sources poses challenges, as the assembly of 60 pure carbon atoms in an H-rich environment is difficult. The formation of C₆₀ from the surface decomposition of SiC grains is a viable mechanism that could readily occur in the heterogeneous, hydrogen-dominated gas of evolved circumstellar shells.

Molecules and dust produced by the atmospheres of cool evolved stars contribute to a significant amount of the total material found in the interstellar medium. To understand the mechanism behind the mass loss of these stars, it is of pivotal importance to investigate the structure and dynamics of their atmospheres. Our goal is to verify if the extended molecular and dust layers of the carbon-rich asymptotic giant branch (AGB) star V Oph, and their time variations, can be explained by dust-driven winds triggered by stellar pulsation alone, or if other mechanisms are in play. We model V Oph mid-infrared interferometric VLTI-MIDI data (8–13 μm), at phases 0.18, 0.49, and 0.65, together with literature photometric data, using the latest-generation self-consistent dynamic atmosphere models for carbon-rich stars: DARWIN. We determine the fundamental stellar parameters: T_eff = 2600 K, L_bol = 3585 L_⊙, M = 1.5 M_⊙, C/O = 1.35, $\dot{M}=2.50\times {10}^{-6}$ M_⊙ yr⁻¹. We calculate the stellar photospheric radii at the three phases: 479, 494, 448 R_⊙; and the dust radii: 780, 853, 787 R_⊙. The dynamic models can fairly explain the observed N-band visibility and spectra, although there is some discrepancy between the data and the models, which is discussed in the text. We discuss the possible causes of the temporal variations of the outer atmosphere, deriving an estimate of the magnetic field strength, and computing upper limits for the Alfvén waves velocity. In addition, using period–luminosity sequences, and interferometric modeling, we suggest V Oph as a candidate to be reclassified as a semi-regular star.