Aims. In this study, we determine the morphology and mass-loss rate of jets emanating from the companion in post-asymptotic giant branch (post-AGB) binary stars with a circumbinary disk. In doing so, we also determine the mass accretion rates on to the companion and investigate the source feeding the circum-companion accretion disk. Methods. We perform a spatio-kinematic modelling of the jet of two well-sampled post-AGB binaries, BD+46442 and IRAS19135+3937, by fitting the orbital phased time series of H-alpha spectra. Once the jet geometry, velocity and scaled density structure are computed, we carry out radiative transfer modelling of the jet for the first four Balmer lines to determine the jet densities, thus allowing us to compute the jet mass-loss rates and mass accretion rates. Results. The spatio-kinematic model of the jet reproduces the observed absorption feature in the H-alpha lines. In both objects, the jets have an inner region with extremely low density. Using our radiative transfer model, we find the full three-dimensional density structure of both jets. From these results, we can compute mass-loss rates of the jets, which are of the order of 10^-7 - 10^-5 M_sol/yr. We estimate mass accretion rates onto the companion of 10^-6 - 10^-4 M_sol/yr. Conclusions. Based on the mass accretion rates found for these two objects, we conclude that the circumbinary disk is most likely the source feeding the circum-companion accretion disk. This is in agreement with the observed depletion patterns in post-AGB binaries. The high accretion rates from the circumbinary disk imply that the lifetime of the disk will be short. Mass-transfer from the post-AGB star cannot be excluded in these systems, but it is unlikely to provide a sufficient mass-transfer rate to sustain the observed jet mass-loss rates.
An original method of estimating the projected rotational velocity, $vsini$, and the macroturbulent velocity, $v_{\rm mac}$, of evolved M giant stars is presented. It is based on the use of spectrum synthesis and multi-line analysis tools. The goal is to fit the mean line profile of observations with that of synthetic spectra. The method is applied to the red giant star RZ Ari and the results $v\sin i$ = 6.0 $\pm$ 0.5 km/s and $v_{\rm mac}$ = 2.0 $\pm$ 1.0 km/s are obtained.
The determination of Li and proton-capture element abundances in globular cluster (GC) giants allows us to constrain several key questions on the multiple population scenarios in GCs, from formation and early evolution, to pollution and dilution mechanisms. In this Letter, we present our results on Li abundances for a large sample of giants in the intermediate-metallicity GC NGC 6121 (M4), for which Na and O have been already determined by Marino et al. The stars analyzed are both below and above the red giant branch bump luminosity. We found that the first and second generation stars share th…
A method for determining the effective temperature Teff of G- and K-type giants and supergiants is proposed. The method is based on the use of two photometric indices free from the interstellar absorption influence: the Q index in the UBV photometric system and the [c1] index in the uvby system. Empirical relations between the Teff values found for nearby and bright G- and K-type giants and supergiants with the use of the infrared fluxes method (IRFM), on the one hand, and the observed Q and [c1] indices for these stars, on the other hand, are plotted. A systematic discrepancy between the depe…
The distinguishing feature of the evolution of close binary stars is the role played by the mass exchange between the component stars. Whether the mass transfer is dynamically stable is one of the essential questions in binary evolution. In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We use the adiabatic mass loss model to systematically survey the thresholds for dynamical timescale mass transfer over the entire span of possible donor star evolutionary states. We also simulate mass loss pro…
We performed a detailed study of the evolution of the luminosity of He-ignition stage and of the red giant branch bump luminosity during the red giant branch phase transition for various metallicities. To this purpose we calculated a grid of stellar models that sample the mass range of the transition with a fine mass step equal to ${\rm 0.01M_\odot}$. We find that for a stellar population with a given initial chemical composition, there is a critical age (of 1.1-1.2~Gyr) around which a decrease in age of just 20-30 million years causes a drastic drop in the red giant branch tip brightness. We …
We present new measurements of the magnitude of the main sequence turn off and the red giant branch bump in the luminosity function of a sample of Galactic globular clusters with updated estimates of [Fe/H] and [$α$/Fe], employing photometric data collected with the Advanced Camera for Survey on board the HST. We compare measured and predicted values of the magnitude difference between these two features, a rarely employed diagnostic of the internal structure of low-mass stars at the beginning of their red giant evolution. Our analysis discloses a clear discrepancy between theory and observati…
This paper presents a summary of four invited and twelve contributed presentations on asymptotic giant branch stars and red supergiants, given over the course of two afternoon splinter sessions at the 19th Cool Stars Workshop. It highlights both recent observations and recent theory, with some emphasis on high spatial resolution, over a wide range of wavelengths. Topics covered include 3D models, convection, binary interactions, mass loss, dust formation and magnetic fields.
Population II Distance Indicators: RR Lyrae Variables, Tip of the Red Giant Branch (TRGB) Stars and J-Branch Asymptotic Giant Branch (JAGB/Carbon) Stars
Barium (Ba) stars form via mass-transfer in binary systems, and can subsequently interact with their white dwarf companion in a second stage of binary interaction. We used observations of main-sequence Ba systems as input for our evolutionary models, and try to reproduce the orbits of the Ba giants. We show that to explain short and sometimes eccentric orbits, additional interaction mechanisms are needed along the RGB.
Binary stars can interact via mass transfer when one member (the primary) ascends onto a giant branch. The amount of gas ejected by the binary and the amount of gas accreted by the secondary over the lifetime of the primary influence the subsequent binary phenomenology. Some of the gas ejected by the binary will remain gravitationally bound and its distribution will be closely related to the formation of planetary nebulae. We investigate the nature of mass transfer in binary systems containing an AGB star by adding radiative transfer to the AstroBEAR AMR Hydro/MHD code.
Asteroseismology allows us to probe stellar interiors. Mixed modes can be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for the red giant core rotation. Thus large-scale measurements of the red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. This work aims at identifying the components of …
The space-borne missions CoRoT and Kepler opened up a new opportunity for better understanding stellar evolution by probing stellar interiors with unrivalled high-precision photometric data. Kepler has observed stellar oscillation for four years, which gave access to excellent frequency resolution that enables deciphering the oscillation spectrum of evolved red giant branch and asymptotic giant branch stars. The internal structure of stars in the upper parts of the red and asymptotic giant branches is poorly constrained, which makes the distinction between red and asymptotic giants difficult. …
We performed hydrodynamic computations of nonlinear stellar pulsations of population I stars at the evolutionary stages of the ascending red giant branch and the following luminosity drop due to the core helium flash. Red giants populating this region of the Hertzsprung--Russel diagram were found to be the fundamental mode pulsators. The pulsation period is the largest at the tip of the red giant branch and for stars with initial masses from 1.1M_\odot to 1.9M_\odot ranges from 254 day to 33 day, respectively. The rate of period change during the core helium flash is comparable with rates of s…
We have determined stellar parameters and abundances for 221 giant branch stars in the globular cluster ω Centauri. A combination of photometry and lower-resolution spectroscopy was used to determine temperature, gravity, metallicity, [C/Fe], [N/Fe] and [Ba/Fe]. These abundances agree well with those found by previous researchers and expand the analysed sample of the cluster. k-means clustering analysis was used to group the stars into four homogeneous groups based upon these abundances. These stars show the expected anticorrelation in [C/Fe] to [N/Fe]. We investigated the distribution of CN-w…
We herein determine the average integrated mass loss from stars stars belonging to the dominant metal-poor population ([Fe/H] ~ --1.7) of the Galactic globular cluster omega Centauri (NGC 5139) during their red giant and horizontal branch evolution. Masses are empirically calculated from spectroscopic measurements of surface gravity and photometric measurements of temperature and luminosity. Systematic uncertainties prevent an absolute measurement of masses at a phase of evolution. However, the relative masses of early asymptotic giant branch stars and central red giant branch stars can be mea…
We present additional evidence that dust is really forming along the red giant branch (RGB) of 47 Tuc at luminosities ranging from above the horizontal branch to the RGB-tip (Origlia et al. 2007). The presence of dust had been inferred from an infrared excess in the (K-8) color, with K measured from high spatial resolution ground based near-IR photometry and "8" referring to Spitzer-IRAC 8 micron photometry. We show how (K-8) is a far more sensitive diagnostic for detecting tiny circumstellar envelopes around warm giants than colors using only the Spitzer-IRAC bands, for example the (3.6-8) co…
Abundances relative to iron for carbon, nitrogen, strontium and barium are presented for 33 stars on the red giant branch of the globular cluster omega Centauri. They are based on intermediate-resolution spectroscopic data covering the blue spectral region analyzed using spectrum synthesis techniques. The data reveal the existence of a broad range in the abundances of these elements, and a comparison with similar data for main sequence stars enables insight into the evolutionary history of the cluster. The majority of the red giant branch stars were found to be depleted in carbon, i.e. [C/Fe]<…
Asteroseismic Modeling of 1,153 Kepler Red Giant Branch Stars: Improved Stellar Parameters with Gravity-Mode Period Spacings and Luminosity Constraints
Barium (Ba), CH, and extrinsic or Tc-poor S-type stars are evolved low- and intermediate-mass stars that show enhancement of slow-neutron-capture-process elements on their surface, an indication of mass accretion from a former asymptotic giant branch (AGB) companion, which is now a white dwarf (WD). Ba and CH stars can be found in the main-sequence (MS), the sub-giant, and the giant phase, while extrinsic S-type stars populate the giant branches only. As these polluted stars evolve, they might be involved in a second phase of interaction with their now white dwarf companion. In this paper, we …
Using Spitzer IRAC observations from the SAGE-SMC Legacy program and archived Spitzer IRAC data, we investigate dust production in 47 Tuc, a nearby massive Galactic globular cluster. A previous study detected infrared excess, indicative of circumstellar dust, in a large population of stars in 47 Tuc, spanning the entire Red Giant Branch (RGB). We show that those results suffered from effects caused by stellar blending and imaging artifacts and that it is likely that no stars below about 1 mag from the tip of the RGB are producing dust. The only stars that appear to harbor dust are variable sta…
The red giant branch (RGB) of globular clusters (GCs) is home to some exotic stars, which may provide clues on the formation of multiple stellar populations in GCs. It is well known that binary interactions are responsible for many exotic stars. Thus, it is important to understand what fraction of stars on the RGB of GCs is the result of binary interactions. In this paper, we performed a binary population synthesis study to track the number of post-binary-interaction (post-BI) stars that appear on the RGB, with particular emphasis on the evolved blue straggler stars (E-BSSs). Assuming an initi…
The mechanism behind the Long Secondary Period (LSP) observed in pulsating red giants still remains unknown. In this work, I investigate the connection between the Red Giant Branch and Asymptotic Giant Branch evolution and the appearance of the LSP - the phenomenon observed in a large fraction the red giants. I use the OGLE-III sample of the OSARG variables in the Large Magellanic Cloud. I construct the density maps in the period-luminosity as well as color-magnitude planes for the stars showing LSP and compare them to the remaining giants. I also fit the spectral energy distribution to test w…
The Gaia-ESO survey recently reported on a large sample of lithium (Li) abundance determinations for evolved stars in the rich open cluster Trumpler 20. They argue for a scenario where virtually all stars experience post main sequence mixing and Li is preserved in only two objects. We present an alternate explanation, where Li is normal in the vast majority of cluster stars and anomalously high in these two cases. We demonstrate that the Li upper limits in the red giants can be explained with a combination of main sequence depletion and standard dredge-up, and that they are close to the detect…
Thermally pulsing asymptotic giant branch models of globular cluster stars are calculated using a synthetic model with the goal of reproducing the chemical composition, core masses and other observational parameters of the four known globular cluster planetary nebulae as well as roughly matching the overall cluster properties. The evolution of stars with an enhanced helium abundance ($Y$) and blue stragglers are modeled. New pre-thermally pulsing asymptotic giant branch mass-losses for red giant branch and early asymptotic giant branch stars are calculated from the Padova stellar evolution mod…
With an aim to examine whether the surface oxygen composition suffers any appreciable change due to evolution-induced mixing of nuclear-processed material in the envelope of red giants, abundance determinations for O/Fe/Ni based on the synthetic spectrum-fitting method were performed by using the moderate-dispersion spectra in the 7770-7792A region (comprising O I 7771-5, Fe I 7780, and Ni I 7788 lines) for 16 stars of the old open cluster M 67 in various evolutionary stages from the turn-off point through the red giant branch. We could not find any meaningful difference in the oxygen abundanc…
HYDRA spectra of 287 stars in the field of NGC 2506 from the turnoff through the giant branch are analyzed. With previous data, 22 are identified as probable binaries; 90 more are classified as potential non-members. Spectroscopic analyses of ~60 red giants and slowly rotating turnoff stars using line equivalent widths and a neural network approach lead to [Fe/H] = -0.27 +/- 0.07 (s.d.) and [Fe/H] = -0.27 +/- 0.06 (s.d.), respectively. Li abundances are derived for 145 probable single-star members, 44 being upper limits. Among turnoff stars outside the Li-dip, A(Li) = 3.04 +/- 0.16 (s.d.), wit…
Magneto-Archeology of White Dwarfs. Revisiting the fossil field scenario with observational constraints during the red giant branch
Institute of Science and Technology Austria (ISTA)（奥地利科学技术研究所）；Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天文学与地球物理科学学院恒星演化与脉动研究组）；Instituto de Astrofísica La Plata, CONICET-UNLP（拉普拉塔天文研究所（CONICET-UNLP））；Center for Astrophysics | Harvard & Smithsonian（哈佛-史密森天体物理中心）
We investigate the relationship between the mass of a globular cluster core and the sizes of its various stellar populations in a sample of 56 globular clusters. The number of core red giant branch stars is found to scale sub-linearly with core mass at the 3-$σ$ confidence level, whereas the relation is linear to within one standard deviation for main-sequence turn-off and sub-giant branch stars. We interpret our results as evidence for a surplus of red giant branch stars in the least massive cluster cores which is not seen for main-sequence turn-off and sub-giant branch stars. We explore vari…
We obtain stringent constraints on the actual efficiency of mass loss for red giant branch stars in the Galactic globular cluster 47 Tuc, by comparing synthetic modeling based on stellar evolution tracks with the observed distribution of stars along the horizontal branch in the colour-magnitude-diagram. We confirm that the observed, wedge-shaped distribution of the horizontal branch can be reproduced only by accounting for a range of initial He abundances --in agreement with inferences from the analysis of the main sequence-- and a red giant branch mass loss with a small dispersion. We have ca…
Evolutionary tracks from the zero age main sequence to the asymptotic giant branch were computed for stars with initial masses 2M_\odot <= Mzams <= 5M_\odot and metallicity Z=0.02. Some models of evolutionary sequences were used as initial conditions for equations of radiation hydrodynamics and turbulent convection describing radial stellar pulsations. The early asymptotic giant branch stars are shown to pulsate in the fundamental mode with periods from 30 to 400 day. The rate of period change gradually increases as the star evolves but is too small to be detected (d ln P/dt < 1.e-5 yr^-1). Pu…
The globular cluster NGC 288 was previously reported to exhibit two distinct red giant branches (RGBs) in the narrow-band Calcium (HK) and Strömgren b and y band passes. In order to investigate this phenomenon further, we obtained moderate resolution (R$\sim$18,000) spectra of 27 RGB stars in NGC 288 with the Hydra multifiber spectrograph on the Blanco 4m telescope at Cerro Tololo Inter-American Observatory. From these data we derive iron ($\langle$[Fe/H]$\rangle$=-1.19; $σ$=0.12), oxygen ($\langle$[O/Fe]$\rangle$=$+$0.25; $σ$=0.13), and sodium ($\langle$[Na/Fe]$\rangle$=$+$0.15; $σ$=0.26) abu…
The onset of cool massive winds in evolved giants is correlated with an evolutionary feature on the red giant branch known as the bump. Also at the bump, shear instability in the star leads to magnetic fields that occur preferentially on small length scales. Pneuman (1983) has suggested that the emergence of small scale flux tubes in the Sun can give rise to enhanced acceleration of the solar wind as a result of plasmoid acceleration (the melon seed mechanism). In this paper, we examine the Pneuman formalism to determine if it may shed some light on the process that drives mass loss from stars…
We examine the effects of thermohaline mixing on the composition of the envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have evolved models of 1, 1.5 and 2 solar masses from the pre-main sequence to the end of the thermally pulsing asymptotic giant branch with thermohaline mixing applied throughout the simulations. In agreement with other authors, we find that thermohaline mixing substantially reduces the abundance of helium-3 on the upper part of the red giant branch in our lowest mass model. However, the small amount of helium-3 that remains is enough to drive thermohali…
Stellar mass $M$, radius $R$, and gravity $g$ are important basic parameters in stellar physics. Accurate values for these parameters can be obtained from the gravitational interaction between stars in multiple systems or from asteroseismology. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. The cluster members are therefore expected to have some properties in common. These common properties strengthen our ability to constrain stellar models and asteroseismically derived $M$, $R$ and $g$ when tested against an ensemble of cluster stars. He…
In the last few years many globular clusters (GCs) have revealed complex color-magnitude diagrams, with the presence of multiple main sequences (MSs), broaden or multiple sub-giant branches (SGBs) and MS turn offs, and broad or split red giant branches (RGBs). After a careful correction for differential reddening, high accuracy photometry with the Hubble Space Telescope presented in this paper reveals a broadened or even split SGB in five additional Milky Way GCs: NGC 362, NGC 5286, NGC 6656, NGC 6715, and NGC 7089. In addition, we confirm (with new and archival HST data) the presence of a spl…
We find that single-star mechanisms for Intermediate Luminosity Optical Transients (ILOTs; Red Transients; Red Novae) which are powered by energy release in the core of asymptotic giant branch (AGB) stars are likely to eject the entire envelope, and hence cannot explain ILOTs in AGB and similar stars. There are single-star and binary models for the powering of ILOTs, which are eruptive stars with peak luminosities between those of novae and supernovae. In single-star models the ejection of gas at velocities of ~500-1000 km/s and a possible bright ionizing flash, require a shock to propagate fr…
The Magellanic Clouds (MCs) offer a unique opportunity to study the stellar evolution and nucleosynthesis of massive Asymptotic Giant Branch (AGB) stars in low metallicity environments where distances are known. Rubidium is a key element to distinguish between high mass AGB stars and low mass AGBs or other type of astronomical objects such as massive red supergiant stars. Theoretically, high mass AGBs are predicted to produce a lot of Rb. We present the discovery of massive Rb-rich AGB stars in the MCs, confirming for the first time that these stars also exist in other galaxies. Our findings s…
Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and h…
Every Sun-like star will eventually evolve into a red giant, a transition which can profoundly affect the evolution of a surrounding planetary system. The timescale of dynamical planet evolution and orbital decay has important implications for planetary habitability, as well as post-main sequence star and planet interaction, evolution and internal structure. Here, we investigate these effects by estimating planet occurrence around 2476 low-luminosity red giant branch (LLRGB) stars observed by the NASA K2 mission. We measure stellar masses and radii using asteroseismology, with median random un…
We present new $BVI$ photometry containing 40,000 stars of the Galactic globular cluster M15 (NGC 7078) covering a $25 \times 25$ arcminute area centered on the cluster with a magnitude range from the tip of the Red Giant Branch to three magnitudes below the main sequence turn-off. Using $α$-enhanced Dartmouth Stellar Evolution Program models, we find an age of $13.0 \ \pm \ 1.0$ Gyr and distance modulus of $(m-M)_V = 15.4 \pm 0.1$ through isochrone fitting. Unlike previous works, we find good agreement between the observed completeness-corrected stellar luminosity function and models.
We investigate the effects of alpha-element enhancement and the thermally pulsing-asymptotic giant branch (TP-AGB) stars on the surface brightness fluctuation (SBF) magnitudes and broadband colors of simple stellar populations and compare to the empirical calibrations. We consider a broad range of ages and metallicities using the recently updated Teramo BaSTI isochrones. We find that the alpha-element enhanced I-band SBF magnitudes are about 0.35 mag brighter and their integrated V - I colors are about 0.02 mag redder, mostly because of oxygen enhancement effects on the upper red giant branch …
A possibility of applying 2MASS J, H, Ks, IPHAS r, i and MegaCam u, g photometry of red giants for determining distances to dark clouds is investigated. Red clump giants with a small admixture of G5-K1 and M2-M3 stars of the giant branch can be isolated and used in determining distances to separate clouds or spiral arms. The method is applied to an area of the North America and Pelican nebulae complex. Interstellar extinctions of background red giants can be also used for mapping dust surface density in the cloud.
We derived chemical abundances for C, N, O, Na, Mg and Al in 20 asymptotic giant branch (AGB) stars in the globular cluster NGC 6752. All these elements (but Mg) show intrinsic star-to-star variations and statistically significant correlations or anticorrelations analogous to those commonly observed in red giant stars of globular clusters hosting multiple populations. This demonstrates that, at odds with previous findings, both first and second generation stars populate the AGB of NGC 6752. The comparison with the Na abundances of red giant branch stars in the same cluster reveals that second …
We study the hydrodynamical behavior of the gas expelled by moving Asymptotic Giant Branch Stars interacting with the ISM. Our models follow the wind modulations prescribed by stellar evolution calculations, and we cover a range of expected relative velocities (10 to 100 km/s), ISM densities (between 0.01 and 1 cm-3), and stellar progenitor masses (1 and 3.5 Msun). We show how and when bow-shocks, and cometary-like structures form, and in which regime the shells are subject to instabilities. Finally, we analyze the results of the simulations in terms of the different kinematical stellar popula…
Based on multicolor photometry from the 2MASS and Tycho-2 catalogues, we have produced a sample of 38 368 branch red giants that has less than 1\% of admixtures and is complete within 500 pc of the Sun. The sample includes 30 671 K giants, 7544 M giants, 49 C giants, and 104 suspected supergiants or S stars. The photometric distances have been calculated for K, M, and C stars with an accuracy of 40\%. Tycho-2 proper motions and PCRV radial velocities are used to analyze the stellar kinematics. The decrease in the stellar distribution density with distance from the Galactic equator approximated…
Understanding the fate of planetary systems through white dwarfs which accrete debris crucially relies on tracing the orbital and physical properties of exo-asteroids during the giant branch phase of stellar evolution. Giant branch luminosities exceed the Sun's by over three orders of magnitude, leading to significantly enhanced Yarkovsky and YORP effects on minor planets. Here, we place bounds on Yarkovsky-induced differential migration between asteroids and planets during giant branch mass loss by modelling one exo-Neptune with inner and outer exo-Kuiper belts. In our bounding models, the as…
Multiple stellar populations (MPs) representing star-to-star light-element abundance variations are common in nearly all ancient Galactic globular clusters. Here we provide the strongest evidence yet that the populous, ~ 1.7 Gyr-old Large Magellanic Cloud cluster NGC 2173 also exhibits light-element abundance variations. Thus, our results suggest that NGC 2173 is the youngest cluster for which MPs have been confirmed to date. Our conclusion is based on the distinct bifurcation at the tip of its red-giant branch in high-quality color--magnitude diagrams generated from Hubble Space Telescope ima…
Synthetic RGBB magnitudes are generated with the most recent theoretical stellar evolution models computed with the Dartmouth Stellar Evolution Program (DSEP) code. They are compared to the observational work of Nataf et al., who present RGBB magnitudes for 72 globular clusters. A DSEP model using a chemical composition with enhanced $α$ capture [$α$/Fe] $ =+0.4$ and an age of 13 Gyr shows agreement with observations over metallicities ranging from [Fe/H] = $0$ to [Fe/H] $\approx-1.5$, with discrepancy emerging at lower metallicities.
The globular cluster 47 Tuc exhibits a complex sub-giant branch (SGB) with a faint-SGB comprising only about the 10% of the cluster mass and a bright-SGB hosting at least two distinct populations.We present a spectroscopic analysis of 62 SGB stars including 21 faint-SGB stars. We thus provide the first chemical analysis of the intriguing faint-SGB population and compare its abundances with those of the dominant populations. We have inferred abundances of Fe, representative light elements C, N, Na, and Al, α elements Mg and Si for individual stars. Oxygen has been obtained by co-adding spectra …
We combine ground and space-based photometry of the Galactic globular cluster 47 Tuc to measure four independent lines of evidence for a helium gradient in the cluster, whereby stars in the cluster outskirts would have a lower initial helium abundance than stars in and near the cluster core. First and second, we show that the red giant branch bump (RGBB) stars exhibit gradients in their number counts and brightness. With increased separation from the cluster center, they become more numerous relative to the other red giant (RG) stars. They also become fainter. For our third and fourth lines of…
Owing to their simplicity and ease of application, seismic scaling relations are widely used to determine the properties of stars exhibiting solar-like oscillations, such as solar twins and red giants. So far, no seismic scaling relations for determining the ages of red giant stars have been developed. Such relations would be desirable for galactic archaeology, which uses stellar ages to map the history of the Milky Way. The ages of red giants must instead be estimated with reference to grids of theoretical stellar models, which can be computationally intensive. Here I present an exhaustive se…
We report an unbiased radio line survey towards the circumstellar envelopes of evolved stars at the frequency range from 20 to 25 GHz, aiming to obtain a more complete unbiased picture of the chemical evolution in the final stages of stellar evolution. The observation sample includes the asymptotic giant branch (AGB) star IRC+10216, the proto-planetary nebulae (PPNs) CRL\,2688 and CRL\,618, and the young planetary nebula (PN) NGC\,7027, representing an evolutionary sequence spanning about 10,000 years. Rotational transitions from cyanopolyyne chains and inversion lines from ammonia are detecte…
Large-amplitude asymptotic giant branch variables potentially rival Cepheid variables as fundamental calibrators of the distance scale, particularly if observations are made in the infrared, or where there is substantial interstellar obscuration. They are particularly useful for probing somewhat older populations, such as those found in dwarf spheroidal galaxies, elliptical galaxies or in the halos of spirals. Calibration data from the Galaxy and new observations of various Local Group galaxies are described and the outlook for the future, with a calibration from Gaia and observations from the…
Lithium is one of the few elements produced during the Big Bang Nucleosynthesis in the early universe. Moreover, its fragility makes it useful as a proxy for stellar environmental conditions. As such, the lithium abundance in old systems is at the core of different astrophysical problems. Stars in the lower red giant branch allow studying globular clusters where main sequence stars are too faint to be observed. We use these stars to analyze the initial Li content of the clusters and compare it to cosmological predictions, to measure spreads in Li between different stellar populations, and to s…
We present the first absolute calibration for the yellow post-asymptotic-giant-branch (PAGB) stars in the g- and r-band based on time-series observations from the Zwicky Transient Facility. These absolute magnitudes were calibrated using four yellow PAGB stars (one non-varying star and three Type II Cepheids) located in the globular clusters. We provide two calibrations of the gr-band absolute magnitudes for the yellow PAGB stars, by using an arithmetic mean and a linear regression. We demonstrate that the linear regression provides a better fit to the g-band absolute magnitudes for the yellow…
