Red Giant Branch Research Articles

Digging into the Galactic Bulge: Stellar Population and Structure of the Poorly Studied Cluster NGC 6316

High-resolution Hubble Space Telescope optical observations have been used to analyze the stellar population and the structure of the poorly investigated bulge globular cluster NGC 6316. We constructed the first high-resolution reddening map in the cluster direction, which allowed us to correct the evolutionary sequences in the color–magnitude diagram (CMD) for the effects of differential reddening. A comparison between the CMDs of NGC 6316 and 47 Tucanae revealed strikingly similar stellar populations, with the two systems basically sharing the same turnoff, subgiant branch, and horizontal branch morphologies, indicating comparable ages. The red giant branch in NGC 6316 appears slightly bluer than in 47 Tucanae, suggesting a lower metal content. This has been confirmed by the isochrone fitting of the observed CMD, which provided us with updated values of the cluster age, distance, average color excess, and metallicity. We estimated an absolute age of 13.1 ± 0.5 Gyr, consistent with the age of 47 Tucanae, an average color excess E(B − V) = 0.64 ± 0.01, and a true distance modulus (m − M)0 = 15.27 ± 0.03 that sets the cluster distance at 11.3 kpc from the Sun. In addition, the photometric estimate of the cluster metallicity suggests [Fe/H] ≈ −0.9, which is ∼0.2 dex smaller than that of 47 Tucanae. We also determined the gravitational center and the density profile of the system from resolved stars. The latter is well reproduced by a King model. Our results confirm that NGC 6316 is another extremely old relic of the assembly history of the Galaxy.

The search for transiting planets around hot subdwarfs

The main goal of this project is to investigate star-planet interactions during the Red Giant Branch (RGB) phase. In particular, we focus on what happens to planetary systems once their host stars leave the main sequence and engulf close-in planets, presumably, perturbing the dy-namical architecture of the system. In this context, we wonder if close-in planets may survive this evolutionary phase, evolving towards a new stable configuration, or on the contrary, if these planets are destroyed and the entire system collapses. Whatever the outcome, how will this interaction alter the star and affect its subsequent evolution? Here, we search for observational evidence of transiting planets orbiting hot subdwarfs (sdBs) stars, which are direct post-RGB objects that have lost most of their envelopes. To this end, we analyze light curves from the space missions Kepler/K2, TESS, and CHEOPS to search for shallow periodic transits of close-in, previously engulfed planets or their remnants, such as the rocky cores of giant gas planets. By determining the occurrence of planets or remnants around sdB stars, we will place strong constraints on the evolution of planetary systems after the RGB phase of their host stars. This summary describes the methods, tools, and strategies used to explore the available photometric data in the search for transiting planets and establish detection limits that allow us to build planet occurrence rates around these stars.

DDO68 C: The Actual Appearance of a Ghost Satellite Dwarf through Adaptive Optics at the Large Binocular Telescope

Through adaptive optics (AO) imaging with the SOUL+LUCI instrument at the Large Binocular Telescope we were able to resolve, for the first time, individual stars in the gas-rich galaxy DDO68 C. This system was already suggested to be interacting with the extremely metal-poor dwarf DDO68, but its nature has remained elusive so far because of the presence of a bright foreground star close to its line of sight that hampers a detailed study of its stellar population and distance. In our study, we turned this interloper star into an opportunity to have a deeper insight on DDO68 C, using it as a guide star for the AO correction. Although the new data do not allow for a direct distance measurement through the red giant branch tip method, the combined analysis of the resolved-star color–magnitude diagram, of archival GALEX far-UV and near-UV photometry, and of Hα data provides a self-consistent picture in which DDO68 C is at the same ∼13 Mpc distance as its candidate companion DDO68. These results indicate that DDO68 is a unique case of a low-mass dwarf, less massive than the Magellanic Clouds, interacting with three satellites (DDO68 C and two previously confirmed accreting systems), providing useful constraints on cosmological models and a potential explanation for its anomalous extremely low metallicity.

Coping with loss

Context. The stability of mass transfer is critical in determining pathways towards various kinds of compact binaries, such as compact main-sequence white-dwarf binaries, and transients, such as double white-dwarf mergers and luminous red novae. Despite its importance, very few systematic studies of the stability of mass transfer exist. Aims. We study the behaviour of mass-losing donor stars in binary systems in a systematic way. We focus on identifying and understanding the parameter space for stable mass transfer in low- and intermediate-mass binaries with post-main-sequence donor stars as well as the properties of ultimately unstable binary systems at the onset of the instabilities. Methods. We employed the 1D stellar evolution code MESA to simulate the mass-transfer evolution of 1404 binary systems with donor-star masses between 1 M⊙ and 8 M⊙. We studied the behaviour of the binaries during mass transfer, without assuming that the donor star responds adiabatically to mass loss. We treated the accretor as a point mass, which we do not evolve, and assumed the mass transfer is conservative. Results. We considered several criteria to define when unstable mass transfer occurs. We find that the criterion that best predicts the onset of runaway mass transfer is based on the transition to an effectively adiabatic donor response to mass loss. Using this quasi-adiabatic criterion, we determine the location of the stability boundary to within a relative uncertainty of five per cent in the mass ratio at the onset of mass transfer. Defining this critical mass ratio (qqad) in terms of accretor mass over donor mass, we find that qqad ∼ 0.25 for stars with radiative envelopes that cross the Hertzsprung gap, while for convective giants qqad decreases from ∼1 at the base of the red giant branch to ∼0.1 at the onset of thermal pulses on the asymptotic giant branch. Compared with recent similar studies, we find increased stability of mass transfer from convective giants. This is because an effectively adiabatic response of the donor star only occurs at a very high critical mass-transfer rate due to the short local thermal timescale in the outermost layers of a red giant. Furthermore, we find that for q > qqad mass transfer is self-regulated, but that for evolved giants the resulting mass-transfer rates can be so high that the evolution becomes dynamical and/or the donor can overflow its outer lobe. Conclusions. Our results indicate that mass transfer is stable for a wider range of binary parameter space than typically assumed in rapid binary population synthesis. Moreover, we find a systematic dependence of the critical mass ratio on the donor star mass and radius, which may have significant consequences for predictions of post-mass-transfer populations.

Catalogue of solar-like oscillators observed by TESS in 120-s and 20-s cadence

Context. The Transiting Exoplanet Survey Satellite (TESS) mission has provided photometric light curves for stars across nearly the entire sky. This allows for the application of asteroseismology to a pool of potential solar-like oscillators that is unprecedented in size. Aims. We aim to produce a catalogue of solar-like oscillators observed by TESS in the 120-s and 20-s cadence modes. The catalogue is intended to highlight stars oscillating at frequencies above the TESS 30-min cadence Nyquist frequency with the purpose of encompassing the main-sequence and subgiant evolutionary phases. We aim to provide estimates for the global asteroseismic parameters vmax and ∆v. Methods. We applied a new probabilistic detection algorithm to the 120-s and 20-s light curves of over 250 000 stars. This algorithm flags targets that show characteristic signatures of solar-like oscillations. We manually vetted the resulting list of targets to confirm the presence of solar-like oscillations. Using the probability densities computed by the algorithm, we measured the global asteroseismic parameters vmax and ∆v. Results. We produce a catalogue of 4177 solar-like oscillators, reporting ∆v and vmax for 98% of the total star count. The asteroseismic data reveal a vast coverage of the Hertzsprung-Russell diagram, populating the red giant branch, the subgiant regime, and extending towards the main sequence. Conclusions. A crossmatch with external catalogues shows that 25 of the detected solar-like oscillators are a component of a spectroscopic binary, and 28 are confirmed planet host stars. These results provide the potential for precise, independent asteroseismic constraints on these and any additional TESS targets of interest.

Magnetic and non-magnetic AGB mixing for s-processing

I outline a few features of recent models for the formation of the neutron source 13C(α,n)16O in low mass stars (1 ≲ M/M⊙ ≲ 3, LMS ) ascendingfor the second time the Red Giant Branch, generally called Asymptotic Giant Branch, or AGB stars. I also briefly outline the nucleosynthesis results obtained trough them. The mentioned models consider the physical structure below the frequent downward extensions of the convective envelope into the He-intershell (the so-called third dredge-up or TDU episodes). There, the conditions are such that the occurrence of further mixing is strongly facilitated, due to the minimal temperature gradient. A way to induce proton mixing from the envelope (certainly not the only one) arises whenever the ambient magnetic fields expected for LMS promote the buoyancy of strongly magnetized flux tubes. I review some characteristics of the ensuing mixing episodes, mentioning how different hydrodynamical processes might yield similar effects, thus encouraging stellar physicists to verify in more detail this possibility.

The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. V. The Star Formation History of Sagittarius Dwarf Irregular Galaxy Derived from Long-period Variable Stars

We conducted an optical monitoring survey of the Sagittarius dwarf irregular galaxy (SagDIG) during the period of 2016 June–2017 October, using the 2.5 m Isaac Newton Telescopeat La Palama. Our goal was to identify long-period variable stars (LPVs), namely, asymptotic giant branch stars (AGBs) and red supergiant stars, to obtain the star formation history of isolated, metal-poor SagDIG. For our purpose, we used a method that relies on evaluating the relation between luminosity and the birth mass of these most evolved stars. We found 27 LPV candidates within 2 half-light radii of SagDIG. 10 LPV candidates were in common with previous studies, including one extreme-AGB (x-AGB). By adopting the metallicity Z = 0.0002 for older populations and Z = 0.0004 for younger ages, we estimated that the star formation rate changes from 0.0005 ± 0.0002 M ⊙ yr−1 kpc−2 (13 Gyr ago) to 0.0021 ± 0.0010 M ⊙ yr−1 kpc−2 (0.06 Gyr ago). Like many dwarf irregular galaxies, SagDIG has had continuous star formation activity across its lifetime, though with different rates, and experiences an enhancement of star formation since z ≃ 1. We also evaluated the total stellar mass within 2 half-light radii of SagDIG for three choices of metallicities. For metallicity Z = 0.0002 and 0.0004, we estimated the stellar mass M* = (5.4 ± 2.3) × 106 and (3.0 ± 1.3) × 106 M ⊙, respectively. Additionally, we determined a distance modulus of μ = 25.27 ± 0.05 mag, using the tip of the red giant branch.

The Origin of Cosmic Structures Part 5— Resolution of the Hubble Tension Problem

The two principal contributors to the Hubble tension problem are the predictions of the baryonic acoustic oscillation model and the H0 parameter fit of the “Tip of the Red Giant Branch” collaboration. In this paper, we show that the former is neither necessary nor possible and that the latter yields a value in agreement with the supernovae results when adjustments are made for errors in the peculiar velocity model used to isolate the recession velocities of galaxies. We also make comparisons between the predictions of our new model of cosmology and the curve fits of the standard model. For values of redshift ≤ 1 we find that, with a Hubble constant of H0 = 73, the two agree almost exactly. We resolve the Hubble constant problem and validate the new model predictions for small redshifts.

A Red Giants’ Toy Story. II. Understanding the Red-giant Branch Bump

The Red-Giant Branch Bump (RGBB) is one of the most noteworthy features in the red-giant luminosity function of stellar clusters. It is caused by the passage of the hydrogen-burning shell through the composition discontinuity left at the point of the deepest penetration by the convective envelope. When crossing the discontinuity the usual trend in increasing luminosity reverses for a short time before it increases again, causing a zig-zag in the evolutionary track. In spite of its apparent simplicity the actual physical reason behind the decrease in luminosity is not well understood and several different explanations have been offered. Here we use a recently proposed simple toy model for the structure of low-mass RGs, together with previous results, to show beyond reasonable doubt that the change in luminosity at the RGBB can be traced to the change in the mean molecular weight of the layers on top of the burning shell. And that these changes happen on a nuclear timescale. The change in the effective mean molecular weight, as the burning shell approaches the discontinuity, causes a drop in the temperature of the burning shell which is attenuated by the consequent feedback contraction of the layers immediately below the burning shell. Our work shows that, when applied correctly, including the feedback on the structure of the core together with the increase in the mass of the core, shell-source homology relations do a great quantitative job in explaining the properties of full evolutionary models at the RGBB.

Detailed analysis of the variable star population in the globular cluster NGC 7006

ABSTRACT A 5-yr CCD photometric VI time series of NGC 7006 is employed to perform a detailed analysis of the known population of variable stars. In the process, we have corrected inconsistent classifications, sky coordinates and found 10 new cluster member variables. An independent reddening estimate with a value E(B−V) = 0.08 ± 0.05 is made. Using Fourier decompositions of RR Lyrae light curves and well-established calibrations, the cluster mean metallicity and distance [Fe/H]ZW = −1.53 ± 0.15 and 41.2 ± 1.4 kpc are estimated based on an extended sample of cluster member RRab stars. Using the Gaia DR3 data, we performed an extensive membership analysis that leads to a clean colour–magnitude diagram, and hence to the identification of variables that are likely field stars, and to considerations on the variable distribution in the horizontal branch (HB). A double-mode RR Lyrae and three CW stars are discussed. The origin of CW stars from precursors in the blue tail of the HB with very thin (∼0.06 ± 0.01 M⊙) envelopes is argued. Our models indicate that the main-sequence predecessor of RR Lyrae stars had a mass of 0.82–0.85 M⊙ and lost about 25–35 per cent of its mass during the red giant branch events before settling in the HB some 12–13.5 Gyr later.

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.

A high-resolution spectroscopic search for multiple populations in the 2 Gyr old cluster NGC 1846

ABSTRACT We present detailed C, O, Na, Mg, Si, Ca, Ti, V, Fe, Zr, Ba, and Eu abundance measurements for 20 red giant branch (RGB) stars in the Large Magellanic Cloud (LMC) star cluster NGC 1846 ([Fe/H] = −0.59). This cluster is 1.95 Gyr old and lies just below the supposed lower age limit (2 Gyr) for the presence of multiple populations in massive star clusters. Our measurements are based on high- and low-resolution VLT/FLAMES spectra combined with photometric data from Hubble Space Telescope (HST). Corrections for non-local thermodynamic equilibrium effects are also included for O, Na, Mg, Si, Ca, Fe, and Ba. Our results show that there is no evidence for multiple populations in this cluster based on the lack of any intrinsic star-to-star spread in the abundances of Na and O: We place 95 per cent confidence limits on the intrinsic dispersion for these elements of ≤0.07 and ≤0.09 dex, respectively. However, we do detect a significant spread in the carbon abundances, indicating varying evolutionary mixing occurring on the RGB that increases with luminosity. Overall, the general abundance patterns for NGC 1846 are similar to those seen in previous studies of intermediate-age LMC star clusters and field stars.

Modelling of the post-asymptotic giant branch phase as a tool to understand asymptotic giant branch evolution and nucleosynthesis

ABSTRACT We study a sample of single Galactic post-asymptotic giant branch (post-AGB) stars with known surface chemical composition. Gaia EDR3 and Gaia DR3 parallaxes have enabled accurate determinations of the luminosities of these objects, thus making it possible to characterize them in terms of their initial masses, chemical compositions, and progenitor ages. We used extant evolutionary sequences of asymptotic giant branch (AGB) stars produced using the ATON stellar evolutionary code extended to the post-AGB phase, specifically for this study. The luminosity and surface carbon abundance together prove to be the most valuable indicator of the previous evolution and nucleosynthetic history of the star, particularly regarding the efficiency of mixing mechanisms and the growth of the core. This analysis allows us to place the post-AGB sample into classes based on their evolutionary history on the AGB. This includes low-mass objects that evolved as M-type stars without third dredge-up, carbon stars, and intermediate-mass AGB stars with hot bottom burning (HBB). Additionally, our analysis reveals that AGB nucleosynthesis is not homogeneous, even for stars with similar initial masses and metallicities. The approach of using observations of post-AGB stars to constrain AGB and post-AGB models shed new light on still debated issues related to the AGB evolution, such as the threshold mass required to become a C-star, the minimum mass for HBB and the amount of carbon that can be accumulated in the surface regions of the star during the AGB lifetime, as well as deep mixing experienced during the red giant branch phase.

Volume-limited sample of low-mass red giant stars, the progenitors of hot subdwarf stars

Context. Current theory predicts that hot subdwarf binaries are produced from evolved low-mass binaries that have undergone mass transfer and drastic mass loss during either a common-envelope phase or a stable Roche-lobe overflow while on the red giant branch (RGB). Aims. We perform a spectroscopic survey to find binary systems that include low-mass red giants near the tip of the RGB, which are predicted to be the direct progenitors of subdwarf B (sdB) stars. We aim to obtain a homogeneous sample to search for the observational evidence of correlations between the key parameters governing the formation of sdB stars and constrain the physics of stable mass transfer. Methods. Based on data from the Gaia mission and several ground-based, multiband photometry surveys, we compiled a sample of low-mass red giant branch (RGB) candidates. The candidates were selected according to their Gaia data release 2 (DR2) color, absolute magnitude, and proper motion cuts. In this work, we concentrated on the southern hemisphere targets and conducted a spectroscopic survey of 88 red giant stars to search for the long-period RGB plus main-sequence binary systems within 200 pc. Combining radial velocity (RV) measurements from ground-based observations with CORALIE and RV measurements from Gaia DR2 and from the early data release 3 (eDR3) as well as the astrometric excess noise and renormalized unit weight error measurements from Gaia DR3, we defined a robust binary classification method. In addition, we searched for known binary systems in the literature and in Gaia DR3. Results. We select a total of 211 RGB candidates in the southern hemisphere within 200 pc based on the Gaia DR2 color-magnitude diagram. Among them, a total of 33 red giants were reported as binary systems with orbital periods between 100 and 900 days, some of which are expected to be the direct progenitors of wide binary sdB stars. In addition, we classified 37 new main-sequence plus RGB binary candidates, whose orbital parameters will be measured with future spectroscopic follow-up. Conclusions. Using high-quality astrometric measurements provided by the Gaia mission coupled with high-resolution spectroscopy from the ground, we provide a powerful method for searching for low-mass red giant stars in long-period binary systems.

Establishing the accuracy of asteroseismic mass and radius estimates of giant stars

Context. Eclipsing binary stars with an oscillating giant component allow accurate stellar parameters to be derived and asteroseismic methods to be tested and calibrated. To this aim, suitable systems need to be firstly identified and secondly measured precisely and accurately. KIC 4054905 is one such system, which has been identified, but with measurements of a relatively low precision and with some confusion regarding its parameters and evolutionary state. Aims. Our aim is to provide a detailed and precise characterisation of the system and to test asteroseismic scaling relations. Methods. Dynamical and asteroseismic parameters of KIC 4054905 were determined from Kepler time-series photometry and multi-epoch high-resolution spectra from FIES at the Nordic Optical Telescope. Results. KIC 4054905 was found to belong to the thick disk and consist of two lower red giant branch (RGB) components with nearly identical masses of 0.95 M⊙ and an age of 9.9 ± 0.6 Gyr. The most evolved star with R ≃ 8.4 R⊙ displays solar-like oscillations. These oscillations suggest that the star belongs to the RGB, supported also by the radius, which is significantly smaller than the red clump phase for this mass and metallicity. Masses and radii from corrected asteroseismic scaling relations can be brought into full agreement with the dynamical values if the RGB phase is assumed, but a best scaling method could not be identified. Conclusions. The dynamical masses and radii were measured with a precision better than 1.0%. We firmly establish the evolutionary nature of the system to be that of two early RGB stars with an age close to 10 Gyr, unlike previous findings. The metallicity and Galactic velocity suggest that the system belongs to the thick disk of the Milky Way. We investigate the agreement between dynamical and asteroseismic parameters for KIC 4054905 measured in various ways. This suggests that consistent solutions exist, but the need to analyse more of these systems continues in order to establish the accuracy of asteroseismic methods.

KIC 7955301: A hierarchical triple system with eclipse timing variations and an oscillating red giant

KIC 7955301 is a hierarchical triple system with clear eclipse timing and depth variations that was discovered by the Kepler satellite during its original mission. It is composed of a non-eclipsing primary star at the bottom of the red giant branch (RGB) on a 209-day orbit with a K/G-type main-sequence (MS) inner eclipsing binary (EB), orbiting in 15.3 days. This system was noted for the large amplitude of its eclipse timing variations (ETVs, over 4 h), and the detection of clear solar-like oscillations of the red-giant (RG) component, including p-modes of degree up to l = 3 and mixed l = 1 modes. The system is a single-lined spectroscopic triple, meaning that only spectral lines from the RG are detected. We performed a dynamical model by combining the 4-year-long Kepler photometric data, ETVs, and radial-velocity data obtained with the high-resolution spectrometers ARCES, of the 3.5 m ARC telescope at Apache Point observatory, and SOPHIE, of the 1.93 m telescope at Haute-Provence Observatory. The “dynamical” mass of the RG component was determined with a 2% precision at 1.30−0.02+0.03 M⊙. We performed asteroseismic modeling based on the global seismic parameters and on the individual frequencies. Both methods provide an estimate of the mass of the RG that matches the dynamical mass within the uncertainties. Asteroseismology also revealed the rotation rate of the core (≈15 days), the envelope (∼150 days), and the inclination (∼75°) of the RG. Three different approaches led to an estimation of the age between 3.3 and 5.8 Gyr, which highlights the difficulty of determining stellar ages despite the exceptional wealth of information available for this system. On short timescales, the inner binary exhibits eclipses with varying depths during a 7.3-year long interval, and no eclipses during the consecutive 11.9 years. This is why Kepler could detect its eclipses but TESS cannot, and the future ESA PLATO mission should detect these. In the long term, the system appears to be stable and owes its evolution to the evolution of its individual components. This triple system could end its current smooth evolution by merging by the end of the RGB of the primary star because the periastron distance is ≈142 R⊙, which is close to the expected radius of the RG at the tip of the RGB.

Characterising the AGB bump and its potential to constrain mixing processes in stellar interiors

Context. In the 1990s, theoretical studies motivated the use of the asymptotic giant branch bump (AGBb) as a standard candle given the weak dependence between its luminosity and stellar metallicity. Because of the small size of observed asymptotic giant branch (AGB) samples, detecting the AGBb is not an easy task. However, this has now been made possible thanks to the wealth of data collected by the CoRoT, Kepler, and TESS space-borne missions. Aims. It is well-known that the AGB bump provides valuable information on the internal structure of low-mass stars, particularly on mixing processes such as core overshooting during the core He-burning phase. Here, we investigate the dependence of the AGBb position on stellar parameters such as the stellar mass and metallicity based on the calibration of stellar models to observations. Methods. In this context, we analysed ∼4000 evolved giants observed by Kepler and TESS, including red giant branch (RGB) stars and AGB stars, for which asteroseismic and spectrometric data are available. By using statistical mixture models, we detected the AGBb both in frequency at maximum oscillation power, νmax, and in effective temperature, Teff. Then, we used the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to model AGB stars and match the AGBb occurrence with observations. Results. From the observations, we were able to derive the AGBb location in 15 bins of mass and metallicity. We noted that the higher the mass, the later the AGBb occurs in the evolutionary track, which agrees with theoretical works. Moreover, we found a slight increase in the luminosity at the AGBb when the metallicity increases. By fitting those observations with stellar models, we noticed that low-mass stars (M ≤ 1.0 M⊙) require a small core overshooting region during the core He-burning phase. This core overshooting extent increases toward high mass; however, above M ≥ 1.5 M⊙, we found that the AGBb location cannot be reproduced with a realistic He-core overshooting alone. Thus, additional mixing processes have to be invoked instead. Conclusions. The observed dependence on metallicity complicates the application of the AGBb as a standard candle. Moreover, different mixing processes may occur according to stellar mass. At low mass (M ≤ 1.5 M⊙), the AGBb location can be used to constrain the He-core overshooting. At high mass (M ≥ 1.5 M⊙), an additional mixing induced, for instance, by rotation is needed to reproduce what is seen in observations.

The Tip of the Red Giant Branch as a Cosmological Tool

AbstractThe luminosity of the brightest stars (tip) of the red giant branch (TRGB) in the color–magnitude diagrams of old stars was used early on to introduce the ‘multiple stellar populations’ concept, in 1944, by Walter Baade. However, the precision and accuracy of the TRGB for distance estimation has not been known well for long. In the modern era, equipped with high spatial resolution imaging telescopes, the TRGB is considered an excellent standard candle for any type of resolved galaxies, thus representing a powerful probe for cosmology. The TRGB has several advantages over the classical Cepheids. I review how we can apply the TRGB in cosmology. Four science cases, from large to small scales, are presented: (1) the Hubble flow with Type Ia supernovae; (2) Virgo Cluster infall and dark matter; (3) dark galaxies; and (4) dark matter-free galaxies.

What the JWST can tell us about the Origin of Multiple Stellar Populations in Globular Clusters

AbstractI present the first results of multiple stellar populations in globular clusters from James Webb Space Telescope (JWST) data. We obtained combined NIRSpec and NIRCam data of the GC 47 Tucanae to investigate the properties of the different stellar populations in the low-mass regime, down to ∼0.5 M⊙. Our analysis of both the photometric data and the spectra suggests that the multiple stellar populations among M dwarfs share similar properties of those extensively studied on bright red giant branch stars. The comparison between stellar populations’ properties at different stellar mass, e.g. at the red giants and M dwarfs’ phases, can potentially provide strong constraints to the formation scenarios of the still eluding phenomenon of the multiple populations in star clusters.

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.