Asymptotic Giant Research Articles

Hybrid approach predicts a lower binding energy for benzene on water ice

ABSTRACT In this paper, we provide a highly accurate value for the binding energy of benzene to proton-ordered crystalline water ice (XIh), as a model for interstellar ices. We compare our computed value to the latest experimental data available from temperature-programmed desorption experiments and find that our binding energy value agrees well with data obtained from binding to either crystalline or amorphous ice. Importantly, our new value is lower than that used in most astrochemical networks by about nearly half its value. We explore the impact of this revised binding energy value for both an asymptotic giant branch (AGB) outflow and a protoplanetary disc. We find that the lower value of the binding energy predicted here compared with values used in the literature (4050 K versus 7587 K) leads to less depletion of gas-phase benzene in an AGB outflow, and leads to a shift outwards in the benzene snowline in the mid-plane of a protoplanetary disc. Using this new value, the AGB model predicts lower abundances of benzene in the solid phase throughout the outflow. The disc model also predicts a larger reservoir of gas-phase benzene in the inner disc, which is consistent with the recent detections of benzene for the first time in protoplanetary discs with JWST.

Volume density maps of the 862 nm DIB carrier and interstellar dust. Hints on the role of carbon-rich ejecta from AGB stars

The carbonaceous macromolecules imprinting the numerous absorptions called diffuse interstellar bands (DIBs) in astronomical spectra are omnipresent in the Galaxy and beyond. They represent a considerable reservoir of organic matter. However, their chemical formulae, formation, and destruction sites remain unknown. Their spatial distribution and the local relation to other interstellar species is key to tracing their role in the lifecycle of organic matter. Volume density maps bring local instead of line-of-sight distributed information and allow for new diagnostics to be captured. We present the first large-scale volume (3D) density map of a DIB carrier and compare it with an equivalent map of interstellar dust. The DIB carrier map was obtained through hierarchical inversion of sim 202,000 measurements of the 8621 nm DIB obtained with the Gaia -RVS instrument. It covers about 4000 pc around the Sun in the Galactic plane. We built a dedicated interstellar dust map based on the extinction towards the same target stars. At the simeq 50 pc resolution of the maps, the shape of the 3D DIB distribution is found to be remarkably similar to the 3D distribution of dust. On the other hand, the DIB-to-dust local density ratio increases in low-dust areas. It is also increasing away from the disk, however, the minimum ratio is found to be shifted above the Galactic plane to Z=simeq +50pc. Finally, the average ratio is also surprisingly found to increase away from the Galactic Center. We suggest that the three latter trends may be indications of a dominant contribution of material from the carbon-rich category of dying giant stars to the formation of the carriers. Our suggestion is based on recent catalogs of asymptotic giant branch (AGB) stars and estimates of the mass fluxes of their C-rich and O-rich ejecta.

Spatial distributions of 13CO and CS in a carbon-rich AGB star IRC+10216

IRC+10216 is the typical carbon-rich asymptotic giant branch star, and more than 100 species have been observed in its circumstellar envelope so far. The use of interferometric arrays to map molecular emission in this source has been widely reported, but to study the angular extent of molecular emission, single-dish mapping must be employed. We report here the mapping of the 13CO J = 1–0 and CS J = 2–1 lines towards IRC+10216 using the 13.7 m mm-wave radio telescope at Purple Mountain Observatory. As far as we know, these maps are the largest published 13CO J = 2–1 and CS J = 2–1 images to date. Both molecules have roughly spherically symmetric distribution, with 13CO having an emission radius of over 80″and CS extending up to ∼50″. Assuming that the two molecules are in local thermodynamic equilibrium, the column density and fractional abundance relative to H2 for 13CO and the lower limits for CS are obtained. They are 4.35 × 1016 cm−2 and > 4.72 × 1014 cm−2, and 5.25 × 10−5 and > 2.56 × 10−7 for 13CO and CS, respectively.

Modelling predicts a molecule-rich disc around the AGB star L2 Puppis

ABSTRACT The nearby oxygen-rich AGB star L$_2$ Pup hosts a well-studied nearly edge-on disc. To date, discs around AGB stars have not been chemically studied in detail. By combining a parametrization commonly used for protoplanetary discs and archival ALMA observations, we retrieved an updated density and temperature structure of this disc. This physical model was then used as input to the first chemical model of an AGB disc. The model shows that the physical structure of the disc has a large impact on its chemistry, with certain species showing large changes in column density relative to a radial outflow, indicating that chemistry could be used as a tracer of discs that cannot be directly imaged. Despite its oxygen-rich nature, the daughter species formed within the disc are surprisingly carbon-rich. Two chemical regimes can be distinguished: cosmic-ray induced chemistry in the midplane and photochemistry induced by the interstellar radiation field in the outer regions. Certain complex organic molecules are formed in the midplane. This occurs via gas-phase chemistry only, as the disc is too warm for dust-gas chemistry. The photochemistry in the outer regions leads to the efficient formation of (long) carbon-chains. The predictions of the model allow us to tentatively put the disc’s age $\lesssim 10^5$ yr. Additional observations are necessary to better constrain the physical structure of L$_2$ Pup’s disc and are essential to test the predictions made by the chemical model. Our exploratory work paves the way for a more general study of the chemistry of AGB discs.

Imaging of I Zw 18 by JWST. I. Detecting Dusty Stellar Populations

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.

Mid-infrared Period–Luminosity Relations of Gaia DR3 Long Period Variables

Long period variable (LPV) stars are very promising distance indicators in the infrared bands. We selected asymptotic giant branch (AGB) stars in the Large and Small Magellanic Cloud (LMC and SMC) from the Gaia Data Release 3 LPV catalog, and classified them into oxygen-rich (O-rich) and carbon-rich (C-rich) AGB stars. Using the Wide-field Infrared Survey Explorer database, we determined the W1- and W2-band period–luminosity relations (PLRs) for each pulsation-mode sequence of AGB stars. The dispersion of the PLRs of O-rich AGB stars in sequences C′ and C is relatively small, around 0.14 mag. The PLRs of LMC and SMC are consistent in each sequence. In the W2 band, the PLR of large-amplitude C-rich AGB stars is steeper than that of small-amplitude C-rich AGB stars, due to their more circumstellar dust. By two methods, we find that some PLR sequences of O-rich AGB stars in the LMC are dependent on metallicity. The coefficients of the metallicity effect are β = –0.533 ± 0.213 mag dex1 and β = –0.767 ± 0.158 mag dex1 for sequence C in W1 and W2 bands, respectively. The significance of the metallicity effect in W1 band for the four sequences is 2.2−3.5σ. Both of these imply that distance measurements using O-rich Mira may need to take the metallicity effect into account.

He-enriched starevol models for globular cluster multiple populations. Self-consistent isochrones from ZAMS to the TP-AGB phase

A common property of globular clusters (GCs) is to host multiple populations characterized by peculiar chemical abundances. Recent photometric studies suggest that the He content could vary between the populations of a GC by up to Delta He sim 0.13, in mass fraction. The initial He content impacts the evolution of low-mass stars by ultimately modifying their lifetimes, luminosity, temperatures, and, more generally, the morphology of post-red giant branch (RGB) evolutionary tracks in the Hertzsprung-Russell diagram. We present new physically accurate isochrones with different initial He enrichments and metallicities, with a focus on the methods implemented to deal with the post-RGB phases. The isochrones are based on tracks computed with the stellar evolution code starevol for different metallicities (Z = 0.0002, 0.0009, 0.002, and 0.008) and with a different He enrichment (from 0.25 to 0.6 in mass fraction). We describe the effect of He enrichment on the morphology of the isochrones, and we tested these by comparing the predicted number counts of horizontal branch and asymptotic giant branch stars with those of selected GCs. Comparing the number ratios, we find that our new theoretical ones agree with the observed values within $1 in most cases. The work presented here sets the ground for future studies on stellar populations in GCs, in which the abundances of light elements in He-enhanced models will rely on different assumptions for the causes of this enrichment. The developed methodology permits the computation of isochrones from new stellar tracks with noncanonical stellar processes. The checked number counts ensure that, at least in this reference set, the contribution of the luminous late stages of stellar evolution to the integrated light of a GC is represented adequately.

Barium stars as tracers of s -process nucleosynthesis in AGB stars III. Systematic deviations from the AGB models

Barium (Ba) stars help to verify asymptotic giant branch (AGB) star nucleosynthesis models since they experienced pollution from an AGB binary companion and thus their spectra carry the signatures of the slow neutron capture process ($s$ process). For a large number (180) of Ba stars, we searched for AGB stellar models that match the observed abundance patterns. We aim to uncover any systematic deviations of the sample abundances from the predictions of the nucleosynthesis models. We employed three machine learning algorithms as classifiers: a Random Forest method, developed for this work, and the two classifiers used in our previous study. Compared to that work, we also expanded our observational sample with 11 Ba stars available in the supersolar metallicity range. We studied the statistical behaviour of the different $s$-process elements in the observational sample to investigate if the AGB models systematically under- or overpredict the abundances observed in the Ba stars and show the results in the form of violin plots of the residuals between spectroscopic abundances and model predictions. We inspected the correlations between the observed Fe/H the $s$-process elemental abundances, and the residuals. We employed the Zr/Fe and Nb/Fe abundances as a thermometer to constrain the operational temperature that rules the production of these elements in the sample stars, assuming a steady-state $s$ process. We also investigated the mass distribution of the identified polluter AGB stars and the behaviour of the delta parameter, which describes the fraction of accreted AGB material relative to the Ba star envelope. We find a significant trend in the residuals that implies an underproduction of the elements just after the first $s$-process peak (Nb, Mo, and Ru) in the models relative to the observations. This may originate from a neutron-capture process (e.g. the intermediate neutron-capture process, $i$ process) not yet included in the AGB models of metallicity from solar to roughly 1/5 solar, corresponding to the range of the Ba stars. Correlations are found between the residuals of these peculiar elements, suggesting a common origin for the deviations from the models. In addition, there is a weak metallicity dependence of the residuals of these elements. The $s$-process temperatures derived with the Zr/Fe Nb/Fe thermometer have an unrealistic value for the majority of our stars. The most likely explanation is that at least a fraction of these elements are not produced in a steady-state $s$ process, and instead may be due to processes not included in the AGB models. The mass distribution of the identified models confirms that our sample of Ba stars was polluted by low-mass AGB stars (< 4 Most of the matching AGB models require low accreted mass, but a few systems with high accreted mass are needed to explain the observations.

Evolving past instabilities on the thermally pulsing-(super)asymptotic giant branch

ABSTRACT We address the challenge of running thermally pulsing-(super)asymptotic giant branch [TP-(S)AGB] models, with a 1D hydrostatic stellar evolution code, without suffering instabilities that terminate the evolution. We investigate two instabilities that usually occur during the luminosity peak following a thermal pulse: the hydrogen recombination instability and the Fe-peak instability. Both instabilities occur when the stellar mass is significantly reduced ($M \lesssim M_\mathrm{i}/2$) at the end of the TP-(S)AGB in our models with initial mass $M_\mathrm{i}\gtrsim 2~\mathrm{M}_\odot$. The hydrogen recombination instability occurs due to the difficulty of modelling a thermally and dynamically unstable envelope in a 1D hydrostatic code, and is prevented by damping the energy released by hydrogen recombination in the outer envelope. The Fe-peak instability occurs when the radiation pressure drops at the base of the convective envelope and is prevented by boosting the convective energy transport in this region. We provide custom routines to prevent these instabilities in the stellar evolution code mesa. The impact of these routines on the stellar structure is minimized so as to not affect the efficiency of third dredge-up, hot-bottom burning, or the wind mass-loss rate. We find only a modest reduction in third dredge-up efficiency at small envelope masses ($M_\mathrm{env}\lesssim 1.0~\mathrm{M}_\odot$). Consequently, our $M_\mathrm{i}=5~\mathrm{M}_\odot$ star, with hot-bottom burning, becomes a carbon star for the last $\sim 10~{{\ \rm per\ cent}}$ of its thermally pulsing lifetime. The largest stellar radii are reached during the final thermal pulses, which may have important consequences for binary–star interactions.

Gaia DR3 Data of Very Young Open Cluster NGC 6530: Distance and Evolutionary Stage of Selected Stars

Gaia EDR3 and Gaia DR3 data of selected stars in the very young (1.5 million years old) open cluster NGC 6530 are presented. All the selected stars have very accurate Gaia DR3 distances. Most of the selected stars are very young B stars and several of them are young stellar objects. From the analysis of selected stars, the distance to NGC 6530 is found to be 1318 pc. The two very red stars VAJ 304 and 338 have little bit less proper motion than other members of the NGC 6530 and their Mv values are found to be −3.57 and −3.68 respectively. They are much brighter than K5 III RGB stars and less luminous than massive K5 red supergiants. It is likely VAJ 304 and VAJ 338 are not members of NGC 6530 they may be field K5 type AGB or post-AGB stars. Further study of these two stars is needed.

Water vapour masers in long-period variable stars

Context. Water maser emission is often found in the circumstellar envelopes of evolved stars, that is, asymptotic giant branch stars and red supergiants with oxygen-rich chemistry. The H2O emission shows strong variability in evolved stars of both of these types. Aims. We wish to understand the reasons for the strong variability of water masers emitted at 22 GHz. In this paper, we study U Her and RR Aql as representatives of Mira variable stars. Methods. We monitored U Her and RR Aql in the 22 GHz maser line of water vapour with single-dish telescopes. The monitoring period covered about two decades between 1990 and 2011, with a gap between 1997 and 2000 in the case of RR Aql. Observations were also made in 1987 and 2015 before and after the period of contiguous monitoring. In addition, maps of U Her were obtained in the period 1990–1992 with the Very Large Array. Results. We find that the strongest emission in U Her is located in a shell with boundaries of 11–25 AU. The gas-crossing time is 8.5 yr. We derive lifetimes for individual maser clouds of ≤4 yr based on the absence of detectable line-of-sight velocity drifts of the maser emission. The shell is not evenly filled, and its structure is maintained over much longer timescales than those of individual maser clouds. Both stars show brightness variability on several timescales. The prevalent variation is periodic, following the optical variability of the stars with a lag of 2–3 months. Superposed are irregular fluctuations of a few months in duration, with increased or decreased excitation at particular locations, and long-term systematic variations on timescales of a decade or more. Conclusions. The properties of the maser emission are governed by those of the stellar wind while traversing the H2O maser shell. Inhomogeneities in the wind affecting the excitation conditions and prevalent beaming directions likely cause the variations seen on timescales of longer than the stellar pulsation period. We propose the existence of long-living regions in the shells, which maintain favourable excitation conditions on timescales of the wind-crossing times through the shells or orbital periods of (sub)stellar companions. The H2O maser properties in these two Mira variables are remarkably similar to those in the semiregular variables studied in our previous papers regarding shell location, outflow velocity, and lifetime. The only difference is the regular brightness variations of the Mira variables caused by the periodic pulsation of the stars.

Molecular line polarisation from the circumstellar envelopes of asymptotic giant branch stars

Context. Polarisation observations of masers in the circumstellar envelopes (CSEs) around asymptotic giant branch (AGB) stars have revealed strong magnetic fields. However, masers probe only specific lines of sight through the CSE. Non-masing molecular line polarisation observations can more directly reveal the large-scale magnetic field morphology and hence probe the effect of the magnetic field on AGB mass loss and the shaping of the AGB wind. Aims. Observations and models of CSE molecular line polarisation can now be used to describe the magnetic field morphology and estimate its strength throughout the entire CSE. Methods. We used observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) of molecular line polarisation in the envelope of two AGB stars: CW Leo and R Leo. We modelled the observations using the multi-dimensional polarised radiative transfer tool PORTAL. Results. We found linearly polarised emission, with maximum fractional polarisation on the order of a few percent, in several molecular lines towards both stars. Towards R Leo, we also found a high level of linear polarisation (up to ∼35%) for one of the SiO v = 1 maser transitions. We can explain the observed differences in polarisation structure between the different molecular lines by alignment of the molecules through a combination of the Goldreich-Kylafis effect and radiative alignment effects. We specifically show that the polarisation of CO traces the morphology of the magnetic field. Competition between the alignment mechanisms allowed us to describe the behaviour of the magnetic field strength as a function of the radius throughout the circumstellar envelope of CW Leo. The magnetic field strength derived using this method is inconsistent with the magnetic field strength derived using a structure-function analysis of the CO polarisation and the strength previously derived using CN Zeeman observations. In contrast with CW Leo, the magnetic field in the outer envelope of R Leo appears to be advected outwards by the stellar wind. Conclusions. The ALMA observations and our polarised radiative transfer models show the power of using multiple molecular species to trace the magnetic field behaviour throughout the circumstellar envelope. While the observations appear to confirm the existence of a large-scale magnetic field, further observations and modelling are needed to understand the apparent inconsistency of the magnetic field strength derived with different methods in the envelope of CW Leo.

Formation of long-period post-common envelope binaries

Context. It has been claimed for more than a decade that energies other than orbital and thermodynamic internal are required to explain post-common envelope (CE) binaries with sufficiently long orbital periods (≳1 d) hosting AFGK-type main-sequence stars (∼0.5 − 2.0 M⊙) paired with oxygen-neon white dwarfs (≳1.1 M⊙). This would imply a completely different energy budget during CE evolution for these post-CE binaries in comparison to the remaining systems hosting M dwarfs and/or less massive white dwarfs. Aims. In this first in a series of papers related to long-period post-CE binaries, we investigated whether extra energy is required to explain the currently known post-CE binaries with sufficiently long orbital periods consisting of oxygen-neon white dwarfs with AFGK-type main-sequence star companions. Methods. We carried out binary population simulations with the BSE code adopting empirically derived inter-correlated main-sequence binary distributions for the initial binary population and assuming that the only energy, in addition to orbital, that help to unbind the CE is thermal energy. We also searched for the formation pathways of the currently known systems from the zero-age main-sequence binary to their present-day observed properties. Results. Unlike what has been claimed for a long time, we show that all such post-CE binaries can be explained by assuming inefficient CE evolution, which is consistent with results achieved for the remaining post-CE binaries. There is therefore no need for an extra energy source. We also found that for CE efficiency close to 100%, post-CE binaries hosting oxygen-neon white dwarfs with orbital periods as long as one thousand days can be explained. For all known systems we found formation pathways consisting of CE evolution triggered when a highly evolved (i.e. when the envelope mass is comparable to the core mass), thermally pulsing, asymptotic giant branch star fills its Roche lobe at an orbital period of several thousand days. Due to the sufficiently low envelope mass and sufficiently long orbital period, the resulting post-CE orbital period can easily be several tens of days. Conclusions. We conclude that the known post-CE binaries with oxygen-neon white dwarfs and AFGK-type main-sequence stars can be explained without invoking any energy source other than orbital and thermal energy. Our results strengthen the idea that the most common formation pathway of the overall population of post-CE binaries hosting white dwarfs is through inefficient CE evolution.

Planetary nebulae of the Large Magellanic Cloud I. A multiwavelength analysis

Planetary nebulae (PNe) have three main components: a central star (CS), ionized gas, and dust in the nebula. Each contains critical chemical fingerprints of the PN's evolution, which serve as tracers of the evolution, nucleosynthesis, and dust production that occurred during the preceding asymptotic giant branch (AGB) phase. We aim to build a bridge to link the PN phase to the evolution of progenitors, to better understand the dust production and mass-loss mechanism during the final AGB phase. Here we present a comprehensive study of nine Large Magellanic Cloud spherical or elliptical PNe whose observations from the UV through the IR are available in the literature. We characterize nebulae and CSs, finding information necessary to reconstruct the evolutionary history of mass-loss and dust production, such as as the amount of gas that makes up the nebula and the dust that surrounds the CS. We compared the observed energy distribution of the selected PNe to that obtained from photoionization modeling, taking the presence of dust into account. The physical and chemical parameters of the CSs were then compared with predictions from the evolutionary tracks. We characterize the source, assigning a progenitor, early-AGB mass to each CS. We estimate the mass of the nebula and the dust-to-gas ratio. For five objects, we find evidence for the presence of a near-IR bump, which would indicate the presence of hot dust.

Spectroscopic Survey of Faint Planetary-nebula Nuclei. IV. The A35-type Central Star of Pa 27* *Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universität München, and Georg-August Universität Göttingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert

We present optical spectroscopy of the 12th-magnitude central star of the planetary nebula (PN) Patchick 27 (Pa 27), obtained during a survey of faint PN nuclei (PNNs) with the Low-Resolution Spectrograph of the Hobby–Eberly Telescope. The optical spectrum of Pa 27 is that of a K0 III red giant with rotationally broadened lines. However, the star is detected in the near-ultraviolet (near-UV) with GALEX, showing that a hot binary component is also present. The spectral-energy distribution from the near-UV to the mid-infrared can be fitted with a combination of the K0 III giant and a hot PNN with an effective temperature of about 50,000 K. Photometric observations of Pa 27, both ground-based and from TESS, show a low-amplitude sinusoidal variation with a period of 7.36 days, probably due to starspots on a rotating and magnetically active cool giant. Pa 27 is a new member of the rare class of “A35-type central stars,” which are binary PNNs consisting of a spotted late-type star and a hot pre–white dwarf. They are likely the result of a situation where an asymptotic-giant-branch (AGB) star ejects its outer layers in a dense wind, part of which is captured by a distant companion, spinning up its rotation by accretion of material and angular momentum. We suggest several useful follow-up observations.

Uncovering the Ghostly Remains of an Extremely Diffuse Satellite in the Remote Halo of NGC 253* *This research is based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

We present the discovery of NGC253-SNFC-dw1, a new satellite galaxy in the remote stellar halo of the Sculptor Group spiral, NGC 253. The system was revealed using deep, resolved star photometry obtained as part of the Subaru Near-Field Cosmology Survey that uses the Hyper Suprime-Cam on the Subaru Telescope. Although rather luminous (M V = −11.7 ± 0.2) and massive (M * ∼ 1.25 × 107 M ⊙), the system is one of the most diffuse satellites yet known, with a half-light radius of R h = 3.37 ± 0.36 kpc and an average surface brightness of ∼30.1 mag arcmin−2 within the R h . The color–magnitude diagram shows a dominant, old (∼10 Gyr), and metal-poor ([M/H] = −1.5 ± 0.1 dex) stellar population, as well as several candidate thermally pulsing asymptotic giant branch stars. The distribution of red giant branch stars is asymmetrical and displays two elongated tidal extensions pointing toward NGC 253, suggestive of a highly disrupted system being observed at apocenter. NGC253-SNFC-dw1 has a size comparable to that of the puzzling Local Group dwarfs Andromeda XIX and Antlia 2 but is 2 magnitudes brighter. While unambiguous evidence of tidal disruption in these systems has not yet been demonstrated, the morphology of NGC253-SNFC-dw1 clearly shows that this is a natural path to produce such diffuse and extended galaxies. The surprising discovery of this system in a previously well-searched region of the sky emphasizes the importance of surface-brightness limiting depth in satellite searches.

JWST MIRI and NIRCam unveil previously unseen infrared stellar populations in NGC 6822

ABSTRACT NGC 6822 is a nearby (∼490 kpc) non-interacting low-metallicity (0.2 Z⊙) dwarf galaxy which hosts several prominent H ii regions, including sites of highly embedded active star formation. In this work, we present an imaging survey of NGC 6822 conducted with the Near Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) onboard JWST. We describe the data reduction, source extraction, and stellar population identifications from combined near- and mid-infrared (IR) photometry. Our NIRCam observations reach 7 mag deeper than previous JHKs surveys of this galaxy, which were sensitive to just below the tip of the red giant branch (TRGB). These JWST observations thus reveal for the first time in the near-IR the red clump stellar population and extend nearly 3 mag deeper. In the mid-IR, we observe roughly 2 mag below the TRGB with the MIRI F770W and F1000W filters. With these improvements in sensitivity, we produce a catalogue of ∼900 000 point sources over an area of ∼6.0 × 4.3 arcmin2. We present several NIRCam and MIRI colour–magnitude diagrams and discuss which colour combinations provide useful separations of various stellar populations to aid in future JWST observation planning. Finally, we find populations of carbon- and oxygen-rich asymptotic giant branch stars which will assist in improving our understanding of dust production in low-metallicity, early Universe analogue galaxies.

On the nascent wind of oxygen-rich AGB stars: scrutiny of a sample of nearby stars

On the nascent wind of oxygen-rich AGB stars: scrutiny of a sample of nearby stars

Pinwheel Outflow Induced by Stellar Mass Loss in a Coplanar Triple System

We develop a physical framework for interpreting complex circumstellar patterns whorled around asymptotic giant branch (AGB) stars by investigating stable, coplanar triple systems using hydrodynamic and particle simulations. The introduction of a close tertiary body causes an additional periodic variation in the orbital velocity and trajectory of the AGB star. As a result, the circumstellar outflow builds a fine non-Archimedean spiral pattern superimposed upon the Archimedean spiral produced by the outer binary alone. This fine spiral can be approximated by off-centered circular rings that become tangent to each other at the location of the Archimedean spiral. The superimposed fine pattern fades out relatively quickly as a function of distance from the center of the system, in contrast to the dominant Archimedean spiral pattern, which presents a much slower fractional density decrease with radius. The different rates of radial decrease of the density contrast in the two superimposed patterns, coupled with their different time and spatial scales, lead to an apparent, but illusory radial change in the observed pattern interval, as has been reported, for example, in CW Leo. The function describing the detailed radial dependence of the expansion velocity is different in the two patterns, which may be used to distinguish them. The shape of the circumstellar whorled pattern is further explored as a function of the orbital eccentricity and the inner companion’s mass. Although this study is confined to stable, coplanar triple systems, the results are likely applicable to moderately noncoplanar systems and open interesting avenues for studying noncoplanar systems.

Infrared Properties of Carbon Stars in Our Galaxy

We explore the characteristics of carbon stars within our Galaxy through a comprehensive analysis of observational data spanning visual and infrared (IR) bands. Leveraging data sets from IRAS, the Infrared Space Observatory, Akari, the Midcourse Space Experiment, the Two Micron All Sky Survey, the Wide-field Infrared Survey Explorer (WISE), Gaia DR3, AAVSO, and the SIMBAD object database, we conduct a detailed comparison between the observational data and theoretical models. To facilitate this comparison, we introduce various IR two-color diagrams (2CDs), IR color–magnitude diagrams (CMDs), and spectral energy distributions (SEDs). We find that the CMDs, which utilize the latest distance and extinction data from Gaia DR3 for a substantial number of carbon stars, are very useful for distinguishing carbon-rich asymptotic giant branch (CAGB) stars from extrinsic carbon stars that are not in the AGB phase. To enhance the accuracy of our analysis, we employ theoretical radiative transfer models for dust shells around CAGB stars. These theoretical dust shell models demonstrate a commendable ability to approximate the observations of CAGB stars across various SEDs, 2CDs, and CMDs. We present the infrared properties of known pulsating variables and explore the infrared variability of the sample stars by analyzing WISE photometric data spanning the last 14 years. Additionally, we present a novel catalog of CAGB stars, offering enhanced reliability and a wealth of additional information.