Emerging planetary nebulae within 3D spiral patterns

We analyse the planetary nebulae (PNe) population of the Small Magellanic Cloud (SMC), based on evolutionary models of stars with metallicities in the range $10^{-3} \leq Z \leq 4\times 10^{-3}$ and mass $0.9 M\odot < M < 8M\odot$, evolved through the asymptotic giant branch (AGB) phase. The models used account for dust formation in the circumstellar envelope. To characterise the PNe sample of the SMC, we compare the observed abundances of the various species with the final chemical composition of the AGB models: this study allows us to identify the progenitors of the PNe observed, in terms of mass and chemical composition. According to our interpretation, most of the PNe descend from low-mass ($M < 2 M\odot$) stars, which become carbon rich, after experiencing repeated third dredge-up episodes, during the AGB phase. A fraction of the PNe showing the signature of advanced CNO processing are interpreted as the progeny of massive AGB stars, with mass above $\sim 6 M\odot$, undergoing strong hot bottom burning. The differences with the chemical composition of the PNe population of the Large Magellanic Cloud (LMC) is explained on the basis of the diverse star formation history and age-metallicity relation of the two galaxies. The implications of the present study for some still highly debated points regarding the AGB evolution are also commented.

Context. L2 Puppis (L2 Pup) is a nearby red giant star and an important object in late-type star research because it has a dust disc and potentially a companion. Aims. L2 Pup is often called the second-closest asymptotic giant branch (AGB) star to the sun, second only to R Doradus. However, whether the star is indeed on the AGB or the red giant branch (RGB) is questionable. We review its evolutionary state. Methods. We analysed high-resolution optical archive spectra to search for absorption lines of the third dredge-up indicator technetium (Tc) in L2 Pup. We also compared the star to a sample of well-known AGB stars in terms of luminosity and pulsation properties and placed it in a Gaia-2MASS diagram. Results. L2 Pup is found to be Tc-poor. Thus, it is not undergoing third dredge-up events. The star is fainter than the RGB tip and fainter than all Tc-rich stars in the comparison sample. L2 Pup pulsates in the fundamental mode, similar to Mira variables, but its pulsation properties do not allow us to distinguish between the RGB and AGB stages. Conclusions. In conclusion, L2 Pup could be an RGB or early AGB star, but it is more likely to be an RGB than an AGB star. Our findings are important for a better understanding of the L2 Pup system and its past and future evolution.

Although the majority of known binary Asymptotic Giant Branch (AGB) stars are symbiotic systems (i.e. with a WD as a secondary star), main-sequence companions of AGB stars can be more numerous, even though they are more difficult to find because the primary high luminosity hampers the detection of the companion at visual wavelengths. However, in the ultraviolet the flux emitted by a secondary with T eff > 5500 ~ 6000 K may prevail over that of the primary, and then it can be used to search for candidates to binary AGB stars. In this work, theoretical atmosphere models are used to calculate the UV excess in the GALEX near- and far-UV bands due to a main-sequence companion. After analysing a sample of confirmed binary AGB stars, we propose as a criterium for binarity: (1) the detection of the AGB star in the GALEX far-UV band and/or (2) a GALEX near-UV observed-to-predicted flux ratio > 20. These criteria have been applied to a volume-limited sample of AGB stars within 500 pc of the Sun; 34 out of the sample of 58 AGB stars ( ~ 60%) fulfill them, implying to have a MS companion of spectral type earlier than K0. The excess in the GALEX near- and far-UV bands cannot be attributed to a single temperature companion star, thus suggesting that the UV emission of the secondary might be absorbed by the extended atmosphere and circumstellar envelope of the primary or that UV emission is produced in accretion flows.

Low- and intermediate-mass stars evolve into asymptotic giant branch (AGB) stars near the end of their lives, losing mass through slow and massive winds. The ejected material creates a chemically-rich expanding envelope around the star, namely the circumstellar envelope (CSE). Investigating the anisotropy of the mass-loss phenomenon on the AGB is crucial in gaining a better understanding of the shaping of the CSE during the transition from AGB star to planetary nebula (PN). We investigate possible signs of deviation from spherical symmetry in the CO-emitting CSEs of 70 AGB stars by analysing their emission maps in CO J=2−1 and 3−2 observed with the Atacama Compact Array, as part of the DEATHSTAR project. We find that about one third of the sources are likely aspherical, as they exhibit large-scale asymmetries that are unlikely to have been created by a smooth wind. Further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures.

Evolutionary population synthesis predictions for stellar systems with complex star formation histories rest on their major building blocks: single-burst population models. In this paper, we discuss how the integrated properties of intermediate-age single-burst populations, especially in the near-infrared, behave as a function of age and metallicity. Our models take into account all stellar evolutionary phases that affect the evolution of the integrated optical and near-infrared spectrum of such a population. Particular care was dedicated to the Asymptotic Giant Branch (AGB) stars, which can be dominant at near-infrared wavelengths. First, we present a new synthetic model that takes into account the relevant physical processes that control the evolution through the thermally pulsing AGB, namely (i) the mass-loss, (ii) the third dredge-up, and (iii) the envelope burning. We use this model to evaluate the AGB-termination luminosity, carbon star properties as function of initial metallicity and initial mass, and the contribution of these stars to the integrated light. In the isochrones presented in this paper the lifetime and the nature of the AGB stars (oxygen-rich or carbon-rich) are established as consequences of the interplay between the physical processes that control the AGB star evolution. The contribution of these stars to the integrated light of the population is thus obtained in a consistent way. We optimize our models by using a new stellar spectral library that explicitly takes into account the spectral features that characterize only AGB stars in comparison to other cool and luminous stars. We analyze the contribution of the upper AGB to the bolometric and the near-infrared light. Our models reproduce the contributions of luminous AGB stars to the bolometric and K-band light, and the carbon star contribution to the bolometric light as observed in the Magellanic Cloud star clusters in a satisfactory way, without ad hoc correction factors that could force agreement. Second, we describe the changes occurring in the integrated colours when AGB stars first appear. We confirm that, in contrast with the classical point of view, no sharp optical/near-infrared colour jump occurs when AGB stars start to dominate the stellar population. The envelope burning process that affects massive AGB stars, making them overluminous with respect to early standard core mass-luminosity relations, causes a smoothing of the colour evolution for stellar systems dominated by those stars. We reanalyze the observational strategy proposed by Lançon et al. ([CITE]) to identify intermediate-age stellar populations in post-starburst spectra using our new model sets. The new spectrophotometric models constitute a first step in a more extended study aimed at modelling the spectral properties of the galaxies in the near-infrared.

The bright southern star L2 Pup is a particularly prominent asymptotic giant branch (AGB) star, as its distance of 64 pc makes it the nearest of its type. We report new adaptive optics observations of L2 Pup at visible wavelengths with the SPHERE/ZIMPOL instrument of the VLT that confirm the presence of the circumstellar dust disk at high inclination discovered recently by Kervella et al. (2014b). The signature of the three-dimensional structure of the disk is clearly observed in the map of the degree of linear polarization pL. We identify the inner rim of the disk through its polarimetric signature at a radius of 6 AU from the AGB star. The ZIMPOL intensity images in the V and R bands also reveal a close-in secondary source at a projected separation of 2 AU from the primary. The identification of the spectral type of this companion is uncertain due to the strong reddening from the disk, but its photometry suggests that it is a late K giant, of comparable mass to the AGB star. We present refined physical parameters for the dust disk derived using the RADMC-3D radiative transfer code. We also interpret the pL map using a simple polarization model to infer the three-dimensional structure of the envelope. Interactions between the inner binary system and the disk apparently form spiral structures that propagate along the orthogonal axis to the disk to form streamers. Two dust plumes propagating orthogonally to the disk are also detected. They originate in the inner stellar system, and are possibly related to the interaction of the wind of the two stars with the material in the disk. Based on the morphology of the envelope of L2 Pup, we propose that this star is at an early stage of the formation of a bipolar planetary nebula.

In this thesis we scrutinize and improve theoretical models of the two helium burning stages of low-mass (less than about 8 solar masses) stellar evolution – the core helium burning (CHeB) and asymptotic giant branch (AGB) phases. Accurate stellar models of these phases are essential for the understanding of the physics of stars, the chemical evolution of galaxies, and astrophysics more generally, where the observation of starlight is paramount. In this study we use new models and observational data to investigate two particular important uncertainties during the CHeB and AGB stages that have compounding effects on the evolution. In CHeB models, feedback from convective overshoot can lead to the development of a ‘semiconvection’ zone outside the convective core that can extend the lifetime of the phase by up to a factor of two. In the later thermally pulsing-AGB phase, accounting for the opacity increase due to in situ composition changes can increase the stellar radius, and therefore mass loss rate, truncating the evolution. We compute a grid of CHeB stellar models with four different treatments of convective boundaries using the Monash University stellar evolution code MONSTAR. Theoretical non-radial adiabatic pulsations for these models are then compared with recent asteroseismic observations from the Kepler telescope. Next, we infer the horizontal branch and AGB lifetimes of globular cluster stars from star counts in HST photometry and compare these with predictions from evolution sequences with the different mixing schemes. Finally, for models of the thermally-pulsing AGB phase we incorporate into MONSTAR new low-temperature molecular opacity data that accounts for changes in the surface abundance of carbon, nitrogen, and oxygen. The effect of this update is tested by computing a grid of low-metallicity AGB models that experience the third dredge-up. We identify two distinct CHeB structures that can simultaneously match the globular cluster and asteroseismic observations: those with (i) a slowly mixing region outside the convective core that traps oscillation modes, or (ii) the largest possible convective core, requiring a novel scheme for convective overshoot. We may be able to discriminate between these two possibilities with further work, including the analysis of biases in the observed sample and multi-dimensional simulations. The inclusion of composition-dependent low-T opacity in low-metallicity models shortens the AGB lifetime, increases the mass threshold for hot bottom burning, and radically affects chemical yields. By computing initially metal-free models, we demonstrate that if the third dredge-up occurs, there is no metallicity below which composition-dependent low-T opacity may be neglected. The reduced nitrogen yield from models with the new opacity may help explain the observed numbers of carbon- and nitrogen-enhanced metal-poor stars: this should be confirmed with binary population synthesis calculations.

We studied the dust structures in 60μm and 100μm infrared image around the two asymptotic giant branch (AGB) stars. A systematic search of dust structure in the far infrared (100 μm and 60 μm) Infrared Astronomical Satellite (IRAS) survey was performed using Sky View Observatory. In order to find the possible candidate of cavity structure not yet studied, we used SIMBAD database to locate discrete sources in the region. A new relatively symmetric spherical cavity like structure (size: 1.354 pc × 0.971 pc) for candidate I and (size: 1.424 pc × 0.925pc) for candidate II at R.A.(J2000) =04h46m13.84s, Dec.(J2000) = +32°31’39.6’’ and 05h05m59.58s; +40°40’33.4’’ respectively was found at the distance of about 280 pc and 390 pc. In this present work we have calculated the dust color temperature, mass, outflow velocity, energy and size. We also studied the flux density variation and then calculated temperature and mass profile of the dust and outflow nature of AGB wind using data reduction software’s Aladin v2.5 and Aladinv8.0. Our aim was to test how outflow can be seen in this region. We have studied two cavity-like structures. The dust color temperature is found to lie in the range 21.7 K to 35.6 K for candidate I and 18.5 K to 19.3 K, with an offset of about 14 K and 0.8 K respectively. An offset of 14 K suggests that the AGB is hot in early AGB phase now in late AGB stage. We expect deviation from symmetrical outflow, i.e., AGB wind probably prefer polytropic behaviour implies bipolar type wind whereas the offset 0.8 K for candidate II suggests that the AGB is in the symmetric AGB phase, i.e., early AGB stage. The total mass of the dust in the cavity structure is found about 5.93×1025 Kg (0.00003Mʘ) for candidate I and 1.95×1027Kg (0.001 Mʘ) for candidate II. In addition we have calculated outflow energy of C-rich AGB star along ISM using outflow velocity of our candidates. It is found that the outflow velocities are 12.96 ms-1 and 10.71 ms-1 respectively. For this we have calculated the value of speed of sound (Cs) and escape velocity (uesc). In this way we have determined outflow energy, i.e., 1.173×1025J and 2.766 × 1027J respectively.The Himalayan Physics Vol. 6 &amp; 7, April 2017 (41-47)

We use the stellar evolution code MESA–binary and follow the evolution of three exoplanets and two brown dwarfs (BDs) to determine their potential role in the future evolution of their parent star on the red giant branch (RGB) and on the asymptotic giant branch (AGB). We limit this study to exoplanets and BDs with orbits that have semimajor axis of $1 {~\rm au}\lesssim a_0 \lesssim 20 {~\rm au}$, a high eccentricity, $e_0 \gtrsim 0.25$, and having a parent star of mass M*,0 ≥ 1 M⊙. We find that the star HIP 75 458 will engulf its planet HIP 75 458b during its RGB phase. The planet will remove the envelope and terminate the RGB evolution, leaving a bare helium core of mass 0.4 M⊙ that will evolve to form a helium white dwarf. Only in one system out of five, the planet beta Pic c will enter the envelope of its parent star during the AGB phase. For that to occur, we have to reduce the wind mass-loss rate by a factor of about four from its commonly used value. This strengthens an early conclusion, which was based on exoplanets with circular orbits, which states that to have a non-negligible fraction of AGB stars that engulf planets we should consider lower wind mass-loss rates of isolated AGB stars (before they are spun-up by a companion). Such an engulfed planet might lead to the shaping of the AGB mass-loss geometry to form an elliptical planetary nebula.

We used a new generation of asymptotic giant branch (AGB) stellar models that include dust formation in the stellar winds to find the links between evolutionary models and the observed properties of a homogeneous sample of Large Magellanic Cloud (LMC) planetary nebulae (PNe). Comparison between the evolutionary yields of elements such as CNO and the corresponding observed chemical abundances is a powerful tool to shed light on evolutionary processes such as hot bottom burning (HBB) and third dredge-up (TDU). We found that the occurrence of HBB is needed to interpret the nitrogen-enriched (log(N/H)+12>8) PNe. In particular, N-rich PNe with the lowest carbon content are nicely reproduced by AGB models of mass M >=6 Mo, whose surface chemistry reflects the pure effects of HBB. PNe with log(N/H)+12<7.5 correspond to ejecta of stars that have not experienced HBB, with initial mass below about 3 Mo. Some of these stars show very large carbon abundances, owing to the many TDU episodes experienced. We found from our LMC PN sample that there is a threshold to the amount of carbon accumulated at AGB surfaces, log(C/H)+12<9. Confirmation of this constraint would indicate that, after the C-star stage is reached,AGBs experience only a few thermal pulses, which suggests a rapid loss of the external mantle, probably owing to the effects of radiation pressure on carbonaceous dust particles present in the circumstellar envelope. The implications of these findings for AGB evolution theories and the need to extend the PN sample currently available are discussed.

Two types of models for the formation of semiperiodic concentric multiple arcs (M-arcs) around asymptotic giant branch (AGB) stars and in planetary nebulae are compared against observations. Models that attribute the M-arcs to processes in an extended wind acceleration zone around AGB stars result in an optically thick acceleration zone, which reduces the acceleration efficiency in outer parts of the extended acceleration zone. This makes such models an unlikely explanation for the formation of M-arcs. Models which attribute the M-arcs to semiperiodic variation in one or more stellar properties are most compatible with observations. The only stellar variation models on timescales of ~50-1500 yr that have been suggested are based on an assumed solar-like magnetic cycle. Although ad hoc, the magnetic cycle assumption fits naturally into the increasingly popular view that magnetic activity plays a role in shaping the wind from upper AGB stars.

I propose that the mechanism behind the formation of concentric semi-periodic shells found in several planetary nebulae (PNs) and proto-PNs, and around one asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in the progenitor AGB stars. The time intervals between consecutive ejection events is about 200-1,000 years, which is assumed to be the cycle period (the full magnetic cycle can be twice as long, as is the 22-year period in the sun). The magnetic field has no dynamical effects; it regulates the mass loss rate by the formation of magnetic cool spots. The enhanced magnetic activity at the cycle maximum results in more magnetic cool spots, which facilitate the formation of dust, hence increasing the mass loss rate. The strong magnetic activity implies that the AGB star is spun up by a companion, via a tidal or common envelope interaction. The strong interaction with a stellar companion explains the observations that the concentric semi-periodic shells are found mainly in bipolar PNs.

We present a study of Midcourse Space Experiment (MSX) point sources in the Galactic bulge (|l| < 3°, 1° < |b| < 5°), observed in the A, C, D and E bands (8–21μm), with a total area ∼48 deg2 and more than 7000 detected sources in the MSXD band (15 μm). We discuss the nature of the MSX sources [mostly asymptotic giant branch (AGB) stars], their luminosities, the interstellar extinction, the mass-loss rate distribution and the total mass-loss rate in the bulge. The mid-infrared data of MSX point sources have been combined with the near-infrared (J, H and Ks) data of Two Micron All Sky Survey. The cross-identification was restricted to Ks-band detected sources with Ks≤ 11 mag. However, for those bright MSXD-band sources ([D] < 4.0 mag), which do not satisfy this criterion, we have set no Ks-band magnitude cut-off. The bolometric magnitudes and the corresponding luminosities of the MSX sources were derived by fitting blackbody curves. The relation between and (Ks−[15])0 was used to derive the mass-loss rate of each MSX source in the bulge fields. Except for very few post-AGB stars, planetary nebulae and OH/IR stars, a large fraction of the detected sources at 15 μm (MSXD band) are AGB stars well above the red giant branch tip. A number of them show an excess in ([A]−[D])0 and (Ks−[D])0 colours, characteristic of mass-loss. These colours, especially (Ks−[D])0, enable estimation of the mass-loss rates of the sources in the bulge fields which range from 10−7 to 10−4 M⊙ yr−1. Taking into consideration the completeness of the mass-loss rate bins, we find that the contribution to the integrated mass-loss is probably dominated by mass-loss rates larger than 3 × 10−7 M⊙ yr−1 and is about 1.96 × 10−4 M⊙ yr−1 deg−2 in the ‘intermediate’ and ‘outer’ bulge fields of sources with mass-loss rates, . The corresponding integrated mass-loss rate per unit stellar mass is 0.48 × 10−11 yr−1. Apart from this, the various mid- and near-infrared colour–colour and colour–magnitude diagrams are discussed in the paper to study the nature of the stellar population in the MSX bulge fields.

We surveyed a sample of compact Galactic planetary nebulae (PNe) with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST) to determine their gas-phase carbon abundances. Carbon abundances in PNe constrain the nature of their asymptotic giant branch (AGB) progenitors, as well as cosmic recycling. We measured the carbon abundances, or the limits thereof, of 11 compact Galactic PNe, notably increasing the sample of Galactic PNe whose carbon abundance based on HST ultraviolet spectra is available. The dust content of most targets has been studied elsewhere from Spitzer spectroscopy; given the compact nature of the nebulae, both UV and IR spectra can be directly compared to study gas- and dust-phase carbon. We found that carbon-poor (C/O < 1) compact Galactic PNe have an oxygen-rich dust type, while their carbon-enhanced counterparts (C/O > 1) have carbon-rich dust, confirming the correlation between gas- and dust-phase carbon content that was known for Magellanic Cloud PNe. Based on models of expected final yields from AGB evolution, we interpret the majority of the carbon-poor PNe in this study as the progeny of ∼1.1–1.2 M ⊙ stars that experienced some extra mixing on the red giant branch. They went through the AGB but did not go through the carbon star phase. Most PNe in this group have a bipolar morphology, possibly due to the presence of a subsolar companion. The carbon-enhanced PNe in our sample could be the progeny of stars in the ∼1.5–2.5 M ⊙ range, depending on their original metallicity.

We conduct a detailed study of the planetary nebula (PN) DdDm 1 in the Galactic halo field. DdDm 1 is a metal-deficient and the most carbon-poor PN (${\rm C/O} =0.11 \pm 0.02$) identified in the Galaxy. We aim to verify whether it evolved into a PN without experiencing the third dredge-up (TDU) during the thermal pulse asymptotic giant branch (AGB) phase and to investigate its origin and evolution through accurate measurements of the physical parameters of the nebula and its central star. We perform a comprehensive investigation of DdDm 1 using multiwavelength spectra. The KOOLS-IFU emission-line images achieve $\sim$${0^{\prime \prime }_{.}9}$ resolution, resolving the elliptical nebula and revealing a compact spatial distribution of the [Fe iii] line compared to the [O iii] line, despite their similar volume emissivities. This indicates that iron, with its higher condensation temperature than oxygen, is easily incorporated into dust grains such as silicate, making the iron abundance prone to underestimation. Using a fully data-driven approach, we directly derive 10 elemental abundances, the gas-to-dust mass ratio, and the gas and dust masses based on our own heliocentric distance scale (19.4 kpc) and the emitting volumes of gas and dust. Our analysis reveals that DdDm 1 is a unique PN evolved from a single star with an initial mass of $\sim 1.0\, M_{\odot }$ and a metallicity $Z$ of 0.18$\, Z_{\odot }$. Thus, DdDm 1 is the only known PN that is confirmed to have evolved without experiencing TDUs. The photoionization model reproduces all observed quantities in excellent agreement with predictions from AGB nucleosynthesis, post-AGB evolution, and AGB dust production models. Our study provides new insights into the internal evolution of low-mass and metal-deficient stars like DdDm 1 and highlights the role of PN progenitors in the chemical enrichment of the Galaxy.