Asymptotic Giant Branch Phase Research Articles

Study of an Isolated Dust Structure Nearby the White Dwarf WD0011-399 Using IRIS, AKARI, and WISE Data

During the asymptotic giant branch (AGB) phase, a major fraction of mass is spread by the stars (0.6 - 10 M⊙) in the interstellar medium (ISM) in the form of dust. In the left phase of the post-main sequence evolution in the Hertzsprung-Russell (HR) diagram, these dust are found to be surrounding the white dwarfs (WD). Some small fraction of dust is also formed in the circumstellar shells and cavities around WD which is usually a source of infrared (IR) excess. The formation and evolution of such IR dust structures are the results of high-pressure events such as the Helium shell flash (thermal pulse) and stellar winds, and such structures are crucial in the study of interaction phenomena in ISM. This research project is focused on the study of an isolated dust structure near the white dwarf WD0011-399 located at R.A. (J2000) 00h 13m 47.48s and Dec. (J2000) -39° 370 24.2800 using Improved Reprocessing of the IRAS (IRIS), AKARI and Wide-field Infrared Survey Explorer (WISE) surveys from SkyView Virtual Observatory along with SIMBAD Astronomical Database and Gaia Archive of ESA. The size of the cavity under study is 7.43 pc×2.90 pc, 2.99pc×0.99pc, and 1.26×0.53pc with an inclination angle of 71.12°, 75.73°, and 68.48° respectively in IRIS, AKARI and WISE data suggesting the cavity is neither a face-on nor an edge-on. The relative flux density of the region has been studied through pixel extraction of Far-Infrared (FIR) images and dust color temperature along with dust mass has been calculated. Using IRIS data, the temperature of the isolated region is found between a maximum value of 36.82±4.30 K to a minimum of 22.59±2.32 K with an offset of 14.23 K. The average temperature of the region is 28.22±0.18 K. Following similar procedures for AKARI data, the temperature is found between a maximum of 26.37±3.56 K and a minimum of 16.32±1.47 K with an offset of 10.06K. The average temperature is 19.25±0.15 K. Similarly, using WISE data, the temperature is found between a maximum of 353.72±18.54 K and a minimum of 307.24±4.69 K with an offset of 46.48K. The average temperature is 316.62±0.81 K. High value of offset temperature suggests that the cavity might be evolving with disruptions from background radiative sources. Approximately Gaussian distribution of the temperature in all the surveys implies that the region might be tending towards local thermodynamic equilibrium. The total mass of the structure is estimated to be around 0.03±1.5×10−6M⊙, 1.6×10−3±1.4×10−5 M⊙, and 10−7±3×10−10 M⊙ using IRIS, AKARI, and WISE data respectively. The Jean’s mass of the structure is calculated assuming the structure is a non-degenerate gas, which is estimated around 2001.93M⊙, 178.44M⊙, and 121.47M⊙ respectively using IRIS, AKARI, and WISE data. Since Jean’s mass is much greater than the mass of the structure there seems no possibility of star formation within the region of interest. The color maps illustrate identical distribution for all wavelengths, however, no significant relation is observed between dust color temperature and dust mass.

The chemical compositions of multiple stellar populations in the globular cluster NGC 2808

ABSTRACT Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U, B, I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a sample of 70 red giant branch and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr, and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterization of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another; whereas the iron peak elements show negligible variation between the different populations (at a level of ≲0.10 dex). Our AGB stars are also characterized by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models that suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manqué star.

ESPRESSO observations of HE 0107−5240 and other CEMP-no stars with [Fe/H] ≤ –4.5

Context. HE 0107−5240 is a hyper metal-poor star with [Fe/H] = −5.39, one of the lowest-metallicity stars known. Its stellar atmosphere is enhanced in carbon, with [C/Fe] = +4.0, without a detectable presence of neutron-capture elements. Therefore, it belongs to the carbon-enhanced metal-poor (CEMP−no) group, along with the majority of the most metal-poor stars known to date. Recent studies have revealed variations in the line-of-sight velocity of HE 0107−5240, suggesting it belongs to a binary system. CEMP-no stars are the closest descendants of the very first Pop III stars, and binarity holds important clues for the poorly known mechanism that leads to their formation. Aims. We performed high-resolution observations with the ESPRESSO spectrograph at the VLT to constrain the kinematical properties of the binary system HE 0107−5240 and to probe the binarity of the sample of the eight most metal-poor stars with [Fe/H] ≤ −4.5. Methods. Radial velocities are obtained by using a cross-correlation function in the interval 4200−4315 Å , which contains the relatively strong CH band, against a template that could be either a synthetic spectrum or a combined observed spectrum in an iterative process. A Bayesian method is applied to calculate the orbit using the ESPRESSO measurements and others from the literature. Chemical analysis has also been performed for HE 0107−5240, employing spectral synthesis with the SYNTHE and ATLAS codes. Results. Observations of HE 0107−5240 spanning more than 3 years show a monotonic decreasing trend in radial velocity at a rate of approximately 0.5 m s−1 d−1. A maximum vrad was reached between March 13, 2012, and December 8, 2014. The period is constrained at Porb = 13009−1370+1496 d. New, more stringent upper limits have been found for several elements: (a) [Sr/Fe] and [Ba/Fe] are lower than −0.76 and +0.2, respectively, confirming the star is a CEMP-no; (b) A(Li) < 0.5 is well below the plateau at A(Li) = 1.1 found in the lower red giant branch stars, suggesting Li was originally depleted; and (c) the isotopic ratio 12C/13C is 87 ± 6, showing very low 13C in contrast to what is expected from a ‘spinstar’ progenitor. Conclusions. We confirm that HE 0107−5240 is a binary star with a long period of about 13 000 d (∼36 yr). The carbon isotopic ratio excludes the possibility that the companion has gone through the asymptotic giant branch phase and transferred mass to the currently observed star. The binarity of HE 0107−5240 implies that some of the first generations of low-mass stars formed in multiple systems and indicates that the low metallicity does not preclude the formation of binaries. Finally, a solid indication of vrad variation has also been found in SMSS 1605−1443.

Nucleosynthetic yields of intermediate-mass primordial to extremely metal-poor stars

Context. Stellar models and nucleosynthetic yields of primordial to extremely metal-poor (EMP) stars are crucial to interpret the surface abundances of the most metal-poor stars observed and, ultimately, to better understand the earliest stellar populations. In addition, they are key ingredients of Galactic chemical evolution models. Aims. We aim to better characterise the evolution and fates, and determine updated nucleosynthetic yields of intermediate-mass stars between primordial and EMP metallicity (Z = 10−10, 10−8, 10−7, 10−6, and 10−5). We also probed uncertainties in the nucleosynthesis of the oldest intermediate-mass stars, namely those related to the treatment of convection and convective boundaries and those related to wind prescriptions during the asymptotic giant branch (AGB) phase. Methods. We analyse the evolution of models from their main sequence, through the thermally pulsing AGB (TP-AGB), to the latest stages of their evolution, using the Monash-Mount Stromlo stellar evolution code MONSTAR. The results were post-processed with the code MONSOON, which allowed for the determination of the nucleosynthetic yields of 77 species up to 62Ni. By comparing them to similar calculations existing in the literature, we inspected the effects of input physics on the nucleosynthesis of EMP models. Results. From the evolutionary point of view, as reported in former works, we identified proton ingestion episodes (PIEs) in our lowest-mass lowest-metallicity models. Models of Z = 10−10 and Z = 10−8 in a narrow initial mass range around 5 M⊙ experience the cessation of thermal pulses, and their final fates as type-I1/2 supernovae cannot be discarded. However, the initial mass range of models eventually leading to the formation of type-I1/2 and electron-capture supernovae is considerably reduced compared to former works. All the models of initial mass ≳6–7 M⊙ experience a corrosive second dredge-up and, analogously to those experiencing PIEs, undergo significant metal enrichment in their envelopes. The associated increase in their opacities allows them to develop a solar-like TP-AGB or TP-super-AGB, ultimately becoming white dwarfs. Except for those undergoing the cessation of thermal pulses, all of our models show the nucleosynthetic signatures of both efficient third dredge-up and hot-bottom burning, with the activation of the NeNa cycle and the MgAlSi chains. This leads to the creation of vast amounts of CNO, with typical [N/Fe] > 4), and the characteristic abundance signature [N/Fe] > [C/Fe] > [O/Fe]. Our nucleosynthetic yields present dramatic differences with respect to recent results existing in the literature for intermediate-mass models of similar metallicities. The reason for these discrepancies lay in the poorly known input physics related to stellar winds and, above all, the treatment of convection and convective boundaries.

Structure and evolution of ultra-massive white dwarfs in general relativity

Context. Ultra-massive white dwarfs (M⋆ ≳ 1.05 M⊙) are of utmost importance in view of the role they play in type Ia supernovae explosions, merger events, the existence of high-magnetic -field white dwarfs, and the physical processes in the super asymptotic giant branch phase. Aims. We aim to present the first set of constant rest-mass ultra-massive oxygen-neon white dwarf cooling tracks with masses of M⋆ > 1.29 M⊙ which fully take into account the effects of general relativity on their structural and evolutionary properties. Methods. We computed the full evolution sequences of 1.29, 1.31, 1.33, 1.35, and 1.369 M⊙ white dwarfs with the La Plata stellar evolution code, LPCODE. For this work, the standard equations of stellar structure and evolution have been modified to include the effects of general relativity. Specifically, the fully general relativistic partial differential equations governing the evolution of a spherically symmetric star are solved in a way so that they resemble the standard Newtonian equations of stellar structure. For comparison purposes, the same sequences have been computed for the Newtonian case. Results. According to our calculations, the evolutionary properties of the most massive white dwarfs are strongly modified by general relativity effects. In particular, the resulting stellar radius is markedly smaller in the general relativistic case, being up to 25% smaller than predicted by the Newtonian treatment for the more massive ones. We find that oxygen-neon white dwarfs more massive than 1.369 M⊙ become gravitationally unstable with respect to general relativity effects. When core chemical distribution due to phase separation on crystallization is considered, such instability occurs at somewhat lower stellar masses, ≳1.36 M⊙. In addition, cooling times for the most massive white dwarf sequences are about a factor of two smaller than in the Newtonian case at advanced stages of evolution. Finally, a sample of white dwarfs have been identified as ideal candidates to test these general relativistic effects. Conclusions. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural and evolutionary properties of the most massive white dwarfs. These new ultra-massive white dwarf models constitute a considerable improvement over those computed in the framework of the standard Newtonian theory of stellar interiors.

Understanding dust production and mass loss in the AGB phase using post-AGB stars in the Magellanic Clouds

Context. The asymptotic giant branch (AGB) phase of evolution in low- and intermediate-mass stars is governed by poorly understood physical mechanisms, such as convection, mixing, dust production and mass loss, which play a crucial role in determining the internal structure and the evolution of these stars. The spectra of post-asymptotic giant branch (post-AGB) stars hold critical chemical fingerprints that serve as exquisite tracers of the evolution, nucleosynthesis, and dust production during the AGB phase. Aims. We aim to understand the variation in the surface chemistry that occurs during the AGB phase by analysing results from observations of single post-AGB stars in the Magellanic Clouds. We also aim to reconstruct dust-formation processes, which are active in the circumstellar envelope of AGB stars, occurring towards the end of the AGB phase and during the subsequent course of evolution when contraction to the post-AGB has begun. Methods. We study likely single post-AGB sources in the Magellanic Clouds that exhibit a double-peaked (shell-type) spectral energy distribution (SED). We interpret their SED by comparing with results from radiative transfer calculations to derive the luminosity and the dust content of the individual sources. Additionally, we compare the observationally derived stellar parameters and the photospheric chemical abundances of the target sample with results from stellar evolution modelling of AGB and post-AGB stars. This allows for the characterization of the individual sources in terms of the initial mass and formation epoch of the progenitors. The theoretically derived dust mineralogy and optical depth is used to assess when dust formation ceases and to determine the propagation velocity of the dust-gas system during post-AGB evolution. Results. We find that amongst our target sample of 13 likely single post-AGB stars with shell-type SED, eight objects are carbon stars descending from ∼1−2.5 M⊙ progenitors. Five of the 13 objects are of lower mass, descending from M < 1 M⊙ stars. Based on the dust mineralogy, we find that these five stars are surrounded by silicate dust, and thus failed to become carbon stars. The dust optical depth and the luminosity of the stars are correlated, owing to the faster evolutionary timescale of brighter stars, which makes the dusty layer closer to the central object. From our detailed analysis of the SEDs, we deduce that the dust currently observed around post-AGB stars was released after the onset of the central star contraction and an increase in the effective temperature to ∼3500−4000 K.

The intermediate neutron capture process

Context. Alongside the slow (s) and rapid (r) neutron capture processes, an intermediate neutron capture process (i-process) is thought to exist. It happens when protons are mixed in a convective helium-burning zone, and is referred to as proton ingestion event (PIE); however, the astrophysical site of the i-process is still a matter of debate. The asymptotic giant branch (AGB) phase of low-mass low-metallicity stars is among the promising sites in this regard. Aims. For the first time, we provide i-process yields of a grid of AGB stars experiencing PIEs. Methods. We computed 12 models with initial masses of 1, 2, and 3 M⊙ and metallicities of [Fe/H] = −3.0, −2.5 −2.3, and −2.0, with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species at maximum, coupled to the chemical transport equations. These simulations do not include any extra mixing process. Results. Proton ingestion takes place preferentially in low-mass and low-metallicity models, arising in six out of our 12 AGB models: the 1 M⊙ models with [Fe/H] = −3, −3 and α-enhancement, −2.5, −2.3, and the 2 M⊙ models with [Fe/H] = −3 and −2.5. These models experience i-process nucleosynthesis characterized by neutron densities of ≃1014 − 1015 cm−3. Depending on the PIE properties two different evolution paths follow: either the stellar envelope is quickly lost and no more thermal pulses develop or the AGB phase resumes with additional thermal pulses. This behaviour critically depends on the pulse number when the PIE occurs, the mass of the ingested protons, and the extent to which the pulse material is diluted in the convective envelope. We show that the surface enrichment after a PIE is a robust feature of our models and it persists under various convective assumptions. In our i-process models, elements above iodine (Z = 53) are the most overproduced, particularly Xe, Yb, Ta, Pb, and Bi. Our 3 M⊙ models do not experience any i-process, but instead go through a convective s-process in the thermal pulse with a clear signature on their yields. Conclusions. Thus, AGB stars at low-mass and low-metallicity are expected to contribute to the chemical evolution of heavy elements through the s- and i-processes. Our models can synthesise heavy elements up to Pb without any parametrized extra mixing process such as overshoot or inclusion of a 13C-pocket. Nevertheless, it remains to be explored how the i-process depends on mixing processes, such as overshoot, thermohaline, or rotation.

Variability in Protoplanetary Nebulae. IX. Evidence for Evolution in a Decade

We have carried out a new photometric V,R C study of 12 protoplanetary nebulae, objects in the short-lived transition between the asymptotic giant branch and planetary nebula phases of stellar evolution. These had been the subjects of an earlier study, using data from 1994−2007, that found that all 12 varied periodically, with pulsation periods in the range of ∼38 to ∼150 days. They are all carbon-rich, with F−G spectral types. We combined our new (2008–2018) data with publicly available All-Sky Automated Survey for Supernovae data and determined new periods for their variability. The older and newer period values were compared to investigate evidence of period change, for which there is theoretical support that it might be detectable in a decade or two in some cases. Such a detection is challenging since the light curves are complicated, with multiple periods, changing amplitudes, and evidence of shocks. Nevertheless, we found one, and possibly two, such cases, which are associated with the higher-temperature stars in the sample (7250 and 8000 K). These results are most consistent with the evolution of stars at the lower end of the mass range of carbon stars, ∼1.5–2 M ☉. Several of the stars show longer-term trends of increasing (six cases) or decreasing (one case) brightness, which we think is most likely due to changes in the circumstellar dust opacity. There is one case of a possible ∼1.8 yr period in addition to the shorter pulsation. This is interpreted as possible evidence of an orbiting companion.

The Yarkovsky Effect in REBOUNDx

To more thoroughly study the effects of radiative forces on the orbits of small, astronomical bodies, we introduce the Yarkovsky effect into reboundx, an extensional library for the N-body integrator rebound. Two different versions of the Yarkovsky effect (the “Full Version” and the “Simple Version”) are available for use, depending on the needs of the user. We provide demonstrations for both versions of the effect and compare their computational efficiency with another previously implemented radiative force. In addition, we show how this effect can be used in tandem with other features in reboundx by simulating the orbits of asteroids during the asymptotic giant branch phase of a 2 M ☉ star. This effect is made freely available for use with the latest release of reboundx.

Slowly Cooling White Dwarfs in NGC 6752

Recently, a new class of white dwarfs (“slowly cooling WDs”) has been identified in the globular cluster M13. The cooling time of these stars is increased by stable thermonuclear hydrogen burning in their residual envelope. These WDs are thought to be originated by horizontal branch (HB) stars populating the HB blue tail that skipped the asymptotic giant branch phase. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in NGC 6752, another Galactic globular cluster with a metallicity, age, and HB morphology similar to M13. The normalized WD luminosity function derived in NGC 6752 turns out to be impressively similar to that observed in M13, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. As in the case of M13, the comparison with theoretical predictions is consistent with ∼70% of the investigated WDs evolving at slower rates than standard, purely cooling WDs. Thanks to its relatively short distance from Earth, NGC 6752 photometry reaches a luminosity 1 order of a magnitude fainter than the case of M13, allowing us to sample a regime where the cooling time delay, with respect to standard WD models, reaches ∼300 Myr. The results presented in this paper provide new evidence for the existence of slowly cooling WDs and further support to the scenario proposing a direct causal connection between this phenomenon and the HB morphology of the host stellar cluster.

S-type Stars: LaO Line List for the B2Σ+–X2Σ+ Band System

LaO bands are a characteristic feature in the spectrum of cool S-type stars. La is made primarily by the s-process during the asymptotic giant branch phase of stellar evolution. The B2Σ+–X2Σ+ and A2Π–X2Σ+ band systems can be used to determine the La abundances in cool S stars. The bands of the B2Σ+–X2Σ+ with and have been rotationally analyzed from an emission spectrum from a carbon furnace. Line strengths are calculated using an ab initio transition dipole function, corrected using experimental lifetimes. We provide a line list for the B2Σ+–X2Σ+ band system that can be used to determine La abundances.

The relic galaxy NGC 1277 rules out intermediate-age stellar populations origin of CO-strong absorptions in massive early-type galaxies

ABSTRACT Massive early-type galaxies (ETGs) show several strong CO absorption features in their H- and K-band spectra that cannot be explained by state-of-the-art stellar population models. For many years, the disagreement has been attributed to the presence of intermediate-age stellar components that are dominated by stars in the asymptotic giant branch phase. However, no robust evidence of this scenario has been provided so far. One way to test this claim is by comparison of CO indices for ETGs and for relic galaxies. Lacking the intermediate-age stellar populations, relic galaxies provide us with a unique opportunity to address the origin of strong CO absorptions in ETGs. Here, we utilize the prototype relic galaxy NGC 1277 and compare the CO absorption features of this galaxy with the ones of a representative sample of massive ETGs. We show that the CO lines in both systems have similar strengths, significantly stronger than the predictions of stellar population synthesis models. We conclude that intermediate-age stellar populations in massive ETGs are not the culprit of the strong CO absorptions.

The intermediate neutron capture process

Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. At the beginning of the AGB phase, during the third thermal pulse, the helium driven convection zone is able to penetrate the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of $\approx 4.3 \times 10^{14}$ cm$^{-3}$ at the origin of the synthesis of i-process elements. The nuclear energy released by proton burning in the helium-burning convective shell strongly affects the internal structure: the thermal pulse splits and after approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in the opacity, which triggers a strong mass loss and prevents any further thermal pulse. We show that specific isotopic ratios of Ba, Nd, Sm, and Eu can represent good tracers of i-process nucleosynthesis. Finally, an extended comparison with 14 selected r/s-stars show that the observed composition patterns can be well reproduced by our i-process AGB model.

Planetary Nebulae as Sources of Chemical Enrichment of the Galaxy

During the late asymptotic giant branch (AGB) phase of stellar evolution, the element carbon is created through the triple-α nuclear reaction. This is followed by the synthesis of other heavy elements via neutron capture in the core and the dredge-up of these elements to the surface by convection. Simple molecules and solid-state minerals begin to form in the upper photosphere. These molecules and solids are ejected into the interstellar medium by a strong stellar wind. During the subsequent post-AGB evolution, complex organics with aromatic and aliphatic structures are synthesized in the circumstellar envelope. Planetary nebulae, formed by swept-up circumstellar material, are found to show strong spectral signatures of ions, atoms, molecules, and inorganic and organic solids. These ejected materials of planetary nebulae serve as primordial ingredients of formation of new stars and planetary systems. Stellar synthesized organic solids may survive their journeys through the interstellar medium and evidence for remnants of such organics can be found in our own Solar System. In this paper, we summarize the recent observations of circumstellar synthesis of molecules and solids and discuss the implications of these ejected circumstellar materials on the chemical enrichment of the Galaxy and planetary systems.

The Initial-Final Mass Relation of White Dwarfs: A Tool to Calibrate the Third Dredge-Up

The initial mass-final mass relationship (IFMR) of white dwarfs (WD) represents a crucial benchmark for stellar evolution models, especially for the efficiency of mixing episodes and mass loss during the asymptotic giant branch (AGB) phase. In this study, we argue that this relation offers the opportunity to constrain the third dredge-up (3DU), with important consequences for chemical yields. The results are discussed in light of recent studies that have identified a kink in the IFMR for initial masses close to 2M⊙. Adopting a physically-sound approach in which the efficiency λ of the 3DU varies as a function of core and envelope masses, we calibrate λ in solar-metallicity TP-AGB models in order to reproduce the final masses of their WD progeny, over the range of initial masses 0.9≤Mi/M⊙≤6. In particular, we find that in low-mass stars with 1.4≲Mi/M⊙≲2.0 the efficiency is small, λ≤0.3, it steeply rises to about λ≃0.65 in intermediate-mass stars with 2.0≤Mi/M⊙≤4.0, and then it drops in massive TP-AGB stars with 4.0≲Mi/M⊙≲6.0. Our study also suggests that a second kink may show up in the IFMR at the transition between the most massive carbon stars and those that are dominated by hot-bottom burning.

The VVV survey: Long-period variable stars

Context. Long-period variable stars (LPVs) are pulsating red giants, primarily in the asymptotic giant branch phase, and they include both Miras and semi-regular variables (SRVs). Their period-age and period-luminosity relations enable us to trace different stellar populations, as they are intrinsically very bright and cover a wide range in distances and ages. Aims. The purpose of this study is to establish a census of LPV stars in a region close to the Galactic center, using the six-year database of the Vista Variables in the Vía Láctea (VVV) ESO Public Survey, as well as to describe the methodology that was employed to search for and characterize LPVs using VVV data. Near-IR surveys such as VVV provide a unique opportunity to probe the high-extinction innermost regions of the Milky Way. The detection and analysis of the intrinsically bright Miras in this region could provide us with an excellent probe of the properties of the Milky Way far behind its bulge. Methods. We used point-spread function photometry for all available Ks-band images in ten VVV tiles, covering 16.4 deg2 in total, overlapping fields observed in the course of the Optical Gravitational Lensing Experiment (OGLE)-III survey. We designed a method to select LPV candidates, and we used the known variables from OGLE-III and other known variables from the literature to test our approach. The reduced χ2 statistic, along with the flux-independent index K(fi), were used in our analysis. The Lomb-Scargle period search method, Fourier analysis, template fitting, and visual inspection were then performed to refine our sample and characterize the properties of the stars included in our catalog. Results. A final sample of 130 Mira candidates, of which 129 are new discoveries, was thus obtained, with periods in the range between about 80 and 1400 days. Moreover, a sample of 1013 LPV candidates is also presented, whose periods are however not sufficiently constrained by the available data. A fraction of the latter may eventually turn out to be SRVs. Ages are measured for these stars based on a reassessment of the period-age relations available in the literature. The Miras in our catalog include 18 stars satisfying the requirements to serve as reliable distance indicators and which are not saturated in the VVV Ks-band images. Their distances are accordingly derived and discussed. A number of objects that are seemingly placed far behind the Milky Way’s bulge was detected.

Seimei KOOLS-IFU mapping of the gas and dust distributions in Galactic planetary nebulae: the case of IC 2165

ABSTRACT We investigated the physical and chemical properties of the gas and dust components in a carbon-rich planetary nebula (PN) IC 2165 using two-dimensional emission-line maps with superior resolution. The extinction map is generated in a self-consistent and assumption-free manner. The circumstellar gas-to-dust mass ratio (GDR) map ranges radially from 1210 in the central nebula filled with hot gas plasma to 120 near the ionization front. The determined GDR is comparable to ∼400, which is commonly adopted for carbon-rich asymptotic giant branch (AGB) stars, and ∼100 for interstellar medium. Except for the inner regions, the GDR in IC 2165 is nearly the same as in such AGB stars, indicating that most dust grains withstand the harsh radiation field without being destroyed. The gas and dust mass distributions concentrated in the equatorial plane may be related to the non-isotropic mass loss during the AGB phase and nebula shaping. The spatial distributions of electron densities/temperatures and ionic/elemental abundances were investigated herein. We determined 13 elemental abundances using point-spread-function-matched spatially integrated multiwavelength spectra extracted from the same aperture. Their values are consistent with values predicted by a theoretical model for stars of initially 1.75 M⊙ and Z = 0.003. Finally, we constructed the photoionization model using our distance measurement to be consistent with all derived quantities, including the GDR, gas and dust masses, and post-AGB evolution. Thus, we demonstrate the capability of the Kyoto Okayama Optical Low-dispersion Spectrograph with optical-fibre Integral Field Unit (KOOLS-IFU) and how the spatial variation of the gas and dust components in PNe derived from IFU observations can help understand the evolution of the circumstellar/interstellar medium.

Evolutionary map of the Universe (EMU): 18-cm OH-maser discovery in ASKAP continuum images of the SCORPIO field

ABSTRACT Low- and intermediate-mass stars end their life dispersing their outer layers into the circumstellar medium, during the asymptotic and post-asymptotic giant branch phases. OH masers at 18 cm offer an effective way to probe their circumstellar environment. In this work, we present the discovery of seven OH maser sources likely associated with such evolved stars from the visual inspection of Australian SKA Pathfinder (ASKAP) continuum images. These seven sources do not emit real continuum emission, but the high sensitivity of our images allows us to detect their maser emission, resembling continuum sources. To confirm their nature, we carried out spectral-line observations with ATCA. All the sources showed the double-peaked spectra at 1612 MHz, typical of evolved stars. The detection of maser emission in continuum images can be a complementary and easy-to-use method to discover new maser sources with the large-area deep surveys conducted with the Square Kilometre Array (SKA) precursors. The implication for radio stars studies are remarkable since pure OH maser sources (i.e. with no continuum associated) represent, at a sensitivity of $100\,\mu \mathrm{Jy\,beam}^{-1}$, about 4 per cent of all Galactic sources and by far the most numerous stellar population.

Discovery of SiO Masers in the “Water Fountain” Source IRAS 16552−3050

Abstract In this paper, we report new detections of SiO v = 1 and v = 2 J = 1 → 0 masers in the “water fountain” source IRAS 16552−3050, which was observed with the Nobeyama 45 m telescope during 2021 March–April. Water fountains are evolved stars whose H2O maser spectra trace high-velocity outflows of >100 km s−1. This is the second known case of SiO masers in a water fountain, after their prototypical source, W 43A. These SiO masers should shed light on the evolutionary status of this category of evolved stars, which are likely to be at the end of the asymptotic giant branch phase, when the star exhibits the most copious stellar mass loss, followed by development of the complicated morphologies of planetary nebulae. The origin of a large (up to 25 km s−1) velocity offset of the SiO masers with respect to the systemic velocity derived from the spectrum of CO J = 2 → 1 line is discussed here.

The Abundance of S-Process Elements: Temporal and Spatial Trends from Open Cluster Observations

Spectroscopic observations of stars belonging to open clusters, with well-determined ages and distances, are a unique tool for constraining stellar evolution, nucleosynthesis, mixing processes, and, ultimately, Galactic chemical evolution. Abundances of slow (s) process neutron capture elements in stars that retain their initial surface composition open a window into the processes that generated them. In particular, they give us information on their main site of production, i.e., the low- and intermediate-mass Asymptotic Giant Branch (AGB) stars. In the present work, we review some observational results obtained during the last decade that contributed to a better understanding of the AGB phase: the growth of s-process abundances at recent epochs, i.e., in the youngest stellar populations; the different relations between age and [s/Fe] in distinct regions of the disc; and finally the use of s-process abundances combined with those of α elements, [s/α], to estimate stellar ages. We revise some implications that these observations had both on stellar and Galactic evolution, and on our ability to infer stellar ages.