Core He-burning Phase Research Articles

Study of a Red Clump Giant, KIC 11087027, with High Rotation and Strong Infrared Excess—Evidence of Tidal Interaction for High Lithium Abundance

This Letter presents results from Kepler photometric light curves and a high-resolution spectroscopic study of a super-Li-rich giant KIC11087027. Using the light-curve analysis, we measured the star’s rotational period P rot = 30.4 ± 0.1 days, which translates to rotational velocity V rot = 19.5 ± 1.7 km s−1. The star’s location in the Hertzsprung–Russell diagram, derived values of 12C/13C = 7 ± 1 and [C/N] = −0.95 ± 0.2, and the inferred asteroseismic parameters from secondary calibration based on spectra suggest the star is a low-mass red clump giant in the He-core burning phase. Using Gaia data, we found evidence of variation in radial velocity and proper motion, indicative of presence of an unresolved binary. The large V rot is probably a result of tidal synchronization combined with the aftereffects of He flash, in which the size of the star is reduced significantly. The simultaneous presence of features like high rotation, very high Li abundance, strong dust shell, and strong flares in a single star is relatively uncommon, suggesting that the star experiencing tidal synchronization has recently undergone He flash. The results pose a question whether the binary interaction, hence the high rotation, is a prerequisite for the dredging up of the high amounts of Li from the interior to the photosphere during or immediately after the He-flash event.

Rapidly rotating Population III stellar models as a source of primary nitrogen

Context. The first stars might have been fast rotators. This would have important consequences for their radiative, mechanical, and chemical feedback. Aims. We discuss the impact of fast initial rotation on the evolution of massive Population III models and on their nitrogen and oxygen stellar yields. Methods. We explore the evolution of Population III stars with initial masses in the range of 9 M⊙ ≤ Mini ≤ 120 M⊙, starting with an initial rotation on the zero-age main sequence equal to 70% of the critical one. Results. We find that with the physics of rotation considered here, our rapidly rotating Population III stellar models do not follow a homogeneous evolution. They lose very little mass in the case in which mechanical winds are switched on when the surface rotation becomes equal to or larger than the critical velocity. The impact on the ionising flux appears to be modest when compared to moderately rotating models. Fast rotation favours, in models with initial masses above ∼20 M⊙, the appearance of a very extended intermediate convective zone around the H-burning shell during the core He-burning phase. This shell has important consequences for the sizes of the He- and CO-cores, and thus impacts the final fate of stars. Moreover, it has a strong impact on nucleosynthesis, boosting the production of primary 14N. Conclusions. Fast initial rotation significantly impacts the chemical feedback of Population III stars. Observations of extremely metal-poor stars and/or starbursting regions are essential to provide constraints on the properties of the first stars.

On the Surface Helium Abundance of B-type Hot Subdwarf Stars from the WD+MS Channel of Type Ia Supernovae

The origin of intermediate helium (He)-rich hot subdwarfs is still unclear. Previous studies have suggested that some surviving Type Ia supernovae (SNe Ia) companions from the white dwarf + main-sequence (WD+MS) channel may contribute to the intermediate He-rich hot subdwarfs. However, previous studies ignored the impact of atomic diffusion on the post-explosion evolution of surviving companion stars of SNe Ia, leading to the aspect that they could not explain the observed surface He abundance of intermediate He-rich hot subdwarfs. In this work, by taking the atomic diffusion and stellar wind into account, we trace the surviving companions of SNe Ia from the WD+MS channel using the one-dimensional stellar evolution code MESA until they evolve into hot subdwarfs. We find that the surface He-abundances of our surviving companion models during their core He-burning phases are in a range of −1≲log(NHe/NH)≲0 , which are consistent with those observed in intermediate He-rich hot subdwarfs. This seems to further support the notion that it is possible for surviving companions of SNe Ia in the WD+MS channel to form some intermediate He-rich hot subdwarfs.

Mining the GALAH Data. I. Study of Five Super Lithium-rich Metal-poor Giants

Abstract The presence of a large amount of Li in giants is still a mystery. Most of the super Li-rich (SLR) giants reported in recent studies are in the solar metallicity regime. Here, we study the five metal-poor SLRs from the Galactic Archeology with HERMES Data Release 3, with their [Fe/H] ranging from −1.35 to −2.38 with lithium abundance of A(Li) ≥ 3.4 dex. The asteroseismic analysis reveals that none are on the red giant branch. The average period spacing (ΔP ) values indicate giants are in the core He-burning phase. All of them are low-mass giants (M < 1.5 M ⊙). Their location in the Hertzsprung–Russell diagram suggests one of them is in the red clump (RC) phase, and interestingly, the other four are much brighter and coincide with the early asymptotic giant branch phase. The analysis of the abundance reveals that C, O, Na, Ba, and Eu are normal in giants of respective metallicities and evolutionary phases. Further, we did not find any strong evidence of the presence of dust in the form of infrared excess or binarity from the available radial velocity data. We discuss a few scenarios for the existence of SLRs at higher luminosity, including past merger events. Our findings will help in understanding the production and evolution of Li among giants, in particular, during the RC phase and the post-RC phase.

The impact of convective criteria on the properties of massive stars

Context. Libraries of stellar models computed with either the Ledoux or the Schwarzschild criterion to determine the sizes of convective regions are available in the literature. It is still not clear, however, which of these two criteria should be used, although many works have been devoted to that question in the past. Aims. In the framework of the evolution of single rotating stars, we study the differences between models computed with Ledoux and Schwarzschild criteria on the internal structure, evolutionary track in the Hertzsprung–Russell diagram (HRD), lifetimes, evolution of the surface abundances and velocities, and masses of the He and CO cores. We investigate the consequences on the nature of the supernova (SN) progenitors and the type of SN events, as well as on the stellar yields of light elements. We also study the impact on the outputs of population synthesis models. Methods. Models with initial masses between 7 and 120 M⊙ at solar metallicity (Z = 0.014) and with an initial rotation equal to 0 or 0.4 times the critical velocity at the zero-age main sequence were computed with either the Schwarzschild or the Ledoux criterion until the end of the C-burning phase. Results. Models with initial masses between 15 and 32 M⊙ computed with the Schwarzschild criterion show larger intermediate convective zones attached to the H-burning shell than models computed with the Ledoux criterion. Their CO cores and outer convective zones in the red supergiant (RSG) phase are also smaller. This impacts many outputs of stars during the core He-burning phase. Schwarzschild models have smaller CO cores and outer convective zones in the RSG phase, and their blue-to-red supergiant ratio is much higher than for Ledoux models. They also produce longer crossings of the Hertzsprung gap and favour blue loops. The upper luminosity of RSGs is little affected by the change in the convective criterion. The maximum luminosity of RSG progenitors for type II-P SN events is lowered from 5.2 to 4.95 when the Ledoux criterion is used instead of the Schwarzschild criterion in non-rotating models. The Schwarzschild criterion predicts longer-lasting, less nitrogen-enriched, and faster-rotating Cepheids. Rotational mixing tends to decrease the differences between Schwarzschild and Ledoux models. Conclusions. The results of this paper can be used as first guidelines to set up observational programs that may help to distinguish between these two model families.

Grids of Wolf–Rayet Stars Using MESA with the k − ω Model: From 25 to 120 M ⊙ at Z = 0.02

To explore overshoot mixing and rotational mixing beyond the convective core during the core He-burning phase in massive stars, we computed a grid of stellar models, both rotating and nonrotating, with the k − ω model at Z = 0.02, covering a mass range of 25–120 M ⊙. The rotating models start with a rotation rate of v ini/v crit = 0.4 at the zero-age main sequence, and the evolution is computed until the end of the central carbon-burning phase. Models with the k − ω model provide larger convective cores and a broadening of the main-sequence width. The diffusive-overshoot models with f ov = 0.027 are, on average, closer to the k − ω models for massive stars at Z = 0.02, particularly for the stars with masses greater than 40 M ⊙. The final masses of the Wolf–Rayet (WR) stars range from 9.5–17.5 M ⊙ and 10–23 M ⊙ for the rotating and nonrotating models, respectively. In the rotating models, the C/N ratio decreases slowly below 0.1 outside the convective core, resulting in a flatter element transition region. In addition, the lifetimes of the WNC phase are 1–4 × 104 yr, which is about 1 order of magnitude longer than that in the nonrotating models. The masses of the WNC stars are dominated by internal mixing processes and the maximum masses of the He-burning convective cores during the core He-burning phase are in the range of 15–35 M ⊙. The expected WNC/WR ratios are 0.059 and 0.004 for the rotating and nonrotating models, respectively.

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.

99 oscillating red-giant stars in binary systems with NASA TESS and NASAKepleridentified from the SB9-Catalogue

Oscillating red-giant stars in binary systems are an ideal testbed for investigating the structure and evolution of stars in the advanced phases of evolution. With 83 known red giants in binary systems, of which only ∼40 have determined global seismic parameters and orbital parameters, the sample is small compared to the numerous known oscillating stars. The detection of red-giant binary systems is typically obtained from the signature of stellar binarity in space photometry. The time base of such data biases the detection towards systems with shorter periods and orbits of insufficient size to allow a red giant to fully extend as it evolves up the red-giant branch. Consequently, the sample shows an excess of H-shell burning giants while containing very few stars in the He-core burning phase. From the ninth catalogue of spectroscopic binary orbits (SB9), we identified candidate systems hosting a red-giant primary component. Searching space photometry from the NASA missionsKepler, K2, and TESS (Transiting Exoplanet Survey Satellite) as well as the BRITE (BRIght Target Explorer) constellation mission, we find 99 systems, which were previously unknown to host an oscillating giant component. The revised search strategy allowed us to extend the range of orbital periods of systems hosting oscillating giants up to 26 000 days. Such wide orbits allow a rich population of He-core burning primaries, which are required for a complete view of stellar evolution from binary studies. Tripling the size of the sample of known oscillating red-giant stars in binary systems is an important step towards an ensemble approach for seismology and tidal studies. While for non-eclipsing binaries the inclination is unknown, such a seismically well-characterized sample will be a treasure trove in combination withGaiaastrometric orbits for binary systems.

Lithium Evolution of Giant Stars Observed by LAMOST and Kepler

Abstract Mapping lithium evolution for evolved stars will provide restrictions and constraints on the fundamental stellar interior physical processes, which will shed further light on our understanding of the theory of stellar structure and evolution. Based on a sample of 1848 giants with known evolutionary phases and lithium abundances from the LAMOST-Kepler and LAMOST-K2 fields, we construct mass–radius diagrams to characterize the evolutionary features of lithium. The stars at red giant branch (RGB) phase show natural depletion along with their stellar evolution; particularly, there are no obvious crowd stars with anomalously high Li abundances near the bump. Most of the low-mass stars reaching their zero-age sequence of core helium burning (ZAHeB) have Li abundances around ∼1.0 dex, which shows an increase of Li abundance by ∼0.6 dex compared to the stars above the RGB bump. This suggests that helium flash may be responsible for moderate Li production, while for super Li-rich stars, some special mechanisms should be considered during helium flash. Other scenarios, such as mergers, could also be sources given that Li-rich stars can be found at any time during the steady-state phase of core He burning. During the core He-burning (HeB) phase, there is no indication of obvious lithium depletion.

Core–Envelope Coupling in Intermediate-mass Core-helium Burning Stars

Abstract Stars between two and three solar masses rotate rapidly on the main sequence, and the detection of slow core and surface rotation in the core-helium burning phase for these stars places strong constraints on their angular momentum transport and loss. From a detailed asteroseismic study of the mixed-dipole mode pattern in a carefully selected, representative sample of stars, we find that slow core rotation rates in the range reported by prior studies are a general phenomenon and not a selection effect. We show that the core rotation rates of these stars decline strongly with decreasing surface gravity during the core He-burning phase. We argue that this is a model-independent indication of significant rapid angular momentum transport between the cores and envelopes of these stars. We see a significant range in core rotation rates at all surface gravities, with little evidence for a convergence toward a uniform value. We demonstrate using evolutionary models that measured surface rotation periods are a biased tracer of the true surface rotation distribution, and we argue for using stellar models for interpreting the contrast between core and surface rotation rates. The core rotation rates we measure do not have a strong mass or metallicity dependence. We argue that the emerging data strongly favor a model where angular momentum transport is much more efficient during the core He-burning phase than in the shell-burning phases that precede and follow it.

Concerning the Li-rich status of KIC 9821622: a Kepler field RGB star reported as a Li-rich giant

ABSTRACT Given the implications for the origin of Li enhancement in red giants, we have reviewed the Li-rich classification of KIC 9821622, the only bonafide red giant branch (RGB) giant with a He inert core to date, reported as a Li-rich giant by reanalysing the high-resolution spectra. We have obtained A(Li)LTE = 1.42 ± 0.05 dex. After correcting for non-local thermodynamic equilibrium (NLTE), we have A(Li)NLTE = 1.57 ± 0.05 dex, which is significantly less than the reported A(Li) = 1.80 ± 0.2 dex. We found that the subordinate line at 6103 Å is too weak or absent to measure Li abundance. The derived abundance is normal for red giants undergoing dilution during the first dredge-up. Since all known Kepler field Li-rich giants belong to the red clump region, this clarification removes the anomaly and strengthens the evidence that Li enhancement in low-mass giants may be associated only with the He-core burning phase. The origin of Li excess probably lies during the He flash at the RGB tip, a phase immediately preceding the red clump.

Survey of Li-rich Giants among Kepler and LAMOST Fields: Determination of Li-rich Giants’ Evolutionary Phase

Abstract In this Letter, we report the discovery of 24 new super Li-rich (A(Li) ≥ 3.2) giants of He-core burning phase at the red clump region. Results are based on a systematic search of a large sample of about 12,500 giants common to the LAMOST spectroscopic and Kepler time-resolved photometric surveys. The two key parameters derived from Kepler data are an average period spacing (Δp) between l = 1 mixed gravity-dominated g-modes and average large frequency-separation (Δν) l = 0 acoustic p-modes, which suggest all the Li-rich giants are in the He-core burning phase. This is the first unbiased survey subjected to a robust technique of asteroseismic analysis to unambiguously determine the evolutionary phase of Li-rich giants. The results provide strong evidence that the Li enhancement phenomenon is associated with giants in the He-core burning phase post He-flash, rather than any other phase on the red giant branch with an inert He-core surrounded by a H-burning shell.

Self-enrichment in globular clusters: the extreme He-rich populationof NGC 2808

Almost several decades after the discovery of the first multiple populations in galactic globular clusters (GC) the debate on their formation is still extremely current and NGC2808 remains one of the best benchmark to test any scenario for their origin and the evolution. In this work we focus on the chemical composition of stars belonging to the extreme He-rich population populated by stars with the most extreme abundance of Mg, Al, Na, O and Si. We checked whether the most recent measures are consistent with the AGB yields of stars of $6.5-8~M_{\odot}$. These stars evolve on time scales of the order of 40-60 Myr and eject matter strongly enriched in helium, owing to a deep penetration of the surface convective zone down to regions touched by CNO nucleosynthesis occurring after the core He-burning phase. Since the big unknown of the AGB phase of massive stars is the mass loss, we propose a new approch that takes into account the effects of the radiation pressure on dust particles. We show that this more realistic description is able to reproduce the observed abundances of Mg, Al, Na and Si in these extreme stars. The large spread in the oxygen abundances is explained by invoking deep mixing during the RGB phase. It will be possible to check this work hypothesis as soon as the oxygen measurements of the main sequence stars of NGC2808 will be available.

Rotational broadening and conservation of angular momentum in post-extreme horizontal branch stars

We show that the recent realization that isolated post-extreme horizontal branch (post-EHB) stars are generally characterized by rotational broadening with values of V rot sini between 25 and 30 km s−1 can be explained as a natural consequence of the conservation of angular momentum from the previous He-core burning phase on the EHB. The progenitors of these evolved objects, the EHB stars, are known to be slow rotators with an average value of V rot sini of ~7.7 km s−1. This implies significant spin-up between the EHB and post-EHB phases. Using representative evolutionary models of hot subdwarf stars, we demonstrate that angular momentum conservation in uniformly rotating structures (rigid-body rotation) boosts that value of the projected equatorial rotation speed by a factor ~3.6 by the time the model has reached the region of the surface gravity-effective temperature plane where the newly-studied post-EHB objects are found. This is exactly what is needed to account for their observed atmospheric broadening. We note that the decrease of the moment of inertia causing the spin-up is mostly due to the redistribution of matter that produces more centrally-condensed structures in the post-EHB phase of evolution, not to the decrease of the radius per se.

Hot subdwarfs formed from the merger of two He white dwarfs

We perform stellar evolution calculations of the remnant of the merger of two He white dwarfs (WDs). Our initial conditions are taken from hydrodynamic simulations of double WD mergers and the viscous disc phase that follows. We evolve these objects from shortly after the merger into their core He-burning phase, when they appear as hot subdwarf stars. We use our models to quantify the amount of H that survives the merger, finding that it is difficult for $\gtrsim 10^{-4}\;{\rm M}_\odot$ of H to survive, with even less being concentrated in the surface layers of the object. We also study the rotational evolution of these merger remnants. We find that mass loss over the $\sim 10^4\;\rm yr$ following the merger can significantly reduce the angular momentum of these objects. As hot subdwarfs, our models have moderate surface rotation velocities of $30-100\; {\rm km\,s^{-1}}$. The properties of our models are not representative of many apparently-isolated hot subdwarfs, suggesting that those objects may form via other channels or that our modelling is incomplete. However, a sub-population of hot subdwarfs are moderate-to-rapid rotators and/or have He-rich atmospheres. Our models help to connect the observed properties of these objects to their progenitor systems.

The evolution of massive stars and their spectra

For the first time, the interior and spectroscopic evolution of a massive star is analyzed from the zero-age main sequence (ZAMS) to the pre-supernova (SN) stage. For this purpose, we combined stellar evolution models using the Geneva code and atmospheric models using CMFGEN. With our approach, we were able to produce observables, such as a synthetic high-resolution spectrum and photometry, aiding the comparison between evolution models and observed data. Here we analyze the evolution of a non-rotating 60 Msun star and its spectrum throughout its lifetime. Interestingly, the star has a supergiant appearance (luminosity class I) even at the ZAMS. We find the following evolutionary sequence of spectral types: O3 I (at the ZAMS), O4 I (middle of the H-core burning phase), B supergiant (BSG), B hypergiant (BHG), hot luminous blue variable (LBV; end of H-core burning), cool LBV (H-shell burning through the beginning of the He-core burning phase), rapid evolution through late WN and early WN, early WC (middle of He-core burning), and WO (end of He-core burning until core collapse). We find the following spectroscopic phase lifetimes: 3.22e6 yr for the O-type, 0.34e5 yr (BSG), 0.79e5 yr (BHG), 2.35e5 yr (LBV), 1.05e5 yr (WN), 2.57e4 yr (WC), and 3.80e4 yr (WO). Compared to previous studies, we find a much longer (shorter) duration for the early WN (late WN) phase, as well as a long-lived LBV phase. We show that LBVs arise naturally in single-star evolution models at the end of the MS when the mass-loss rate increases as a consequence of crossing the bistability limit. We discuss the evolution of the spectra, magnitudes, colors, and ionizing flux across the star's lifetime, and the way they are related to the evolution of the interior. [abridged]

THE INSIDIOUS BOOSTING OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS IN INTERMEDIATE-AGE MAGELLANIC CLOUD CLUSTERS

(Abridged) In the recent controversy about the role of TP-AGB stars in evolutionary population synthesis (EPS) models of galaxies, one particular aspect is puzzling: TP-AGB models aimed at reproducing the lifetimes and integrated fluxes of the TP-AGB phase in Magellanic Cloud (MC) clusters, when incorporated into EPS models, are found to overestimate the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect that in all probability is the main cause of this conundrum. As soon as stellar populations intercept the ages at which RGB stars first appear, a sudden change in the lifetime of the core He-burning phase causes a temporary boost in the production rate of subsequent evolutionary phases, including the TP-AGB. For a timespan of about 0.1 Gyr, triple TP-AGB branches develop at slightly different initial masses, causing their frequency and contribution to the integrated luminosity of the stellar population to increase by a factor of 2. The boost occurs just in the proximity of the expected peak in the TP-AGB lifetimes, and for ages of 1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host most of the TP-AGB stars used to constrain stellar evolution and EPS models. This concomitance makes the AGB-boosting particularly insidious in the context of present EPS models. The effect brings about three main consequences. (1) Present estimates of the TP-AGB contribution to the integrated light of galaxies derived from MC clusters, are biased towards too large values. (2) The relative TP-AGB contribution of single-burst populations falling in this critical age range cannot be accurately derived by the fuel consumption theorem. (3) A careful revision of AGB star populations in intermediate-age MC clusters is urgently demanded.

Grids of stellar models with rotation

(shortened) We provide a grid of single star models covering a mass range from 0.8 to 120 Msun with an initial metallicity Z = 0.002 with and without rotation. We discuss the impact of a change in the metallicity by comparing the current tracks with models computed with exactly the same physical ingredients but with a metallicity Z = 0.014 (solar). We show that the width of the main-sequence (MS) band in the upper part of the Hertzsprung-Russell diagram (HRD), for luminosity above log(L/Lsun) > 5.5, is very sensitive to rotational mixing. Strong mixing significantly reduces the MS width. We confirm, but here for the first time on the whole mass range, that surface enrichments are stronger at low metallicity provided that comparisons are made for equivalent initial mass, rotation and evolutionary stage. We show that the enhancement factor due to a lowering of the metallicity (all other factors kept constant) increases when the initial mass decreases. Present models predict an upper luminosity for the red supergiants (RSG) of log (L/Lsun) around 5.5 at Z = 0.002 in agreement with the observed upper limit of RSG in the Small Magellanic Cloud. We show that models using shear diffusion coefficient calibrated to reproduce the surface enrichments observed for MS B-type stars at Z = 0.014 can also reproduce the stronger enrichments observed at low metallicity. In the framework of the present models, we discuss the factors governing the timescale of the first crossing of the Hertzsprung gap after the MS phase. We show that any process favouring a deep localisation of the H-burning shell (steep gradient at the border of the H-burning convective core, low CNO content) and/or the low opacity of the H-rich envelope favour a blue position in the HRD for the whole or at least a significant fraction of the core He-burning phase.

TESTING CONVECTIVE-CORE OVERSHOOTING USING PERIOD SPACINGS OF DIPOLE MODES IN RED GIANTS

Uncertainties on central mixing in main sequence (MS) and core He-burning (He-B) phases affect key predictions of stellar evolution such as late evolutionary phases, chemical enrichment, ages etc. We propose a test of the extension of extra-mixing in two relevant evolutionary phases based on period spacing Delta_P of solar-like oscillating giants. From stellar models and their corresponding adiabatic frequencies (respectively computed with ATON and LOSC codes) we provide the first predictions of the observable Delta_P for stars in the red giant branch (RGB) and in the red clump (RC). We find: i) a clear correlation between Delta_P and the mass of the helium core (M_He); the latter in intermediate-mass stars depends on the MS overshooting, hence it can be used to set constraints on extra mixing during MS when coupled with chemical composition; ii) a linear dependence of the average value of the asymptotic period spacing (<Delta_P>_a) during the He-B phase on the size of the convective core. A first comparison with the inferred asymptotic period spacing for Kepler RC stars suggests the need for extra mixing also during this phase, as evinced from other observational facts.

Can R Coronae Borealis stars form from the merger of two helium white dwarfs?

Abstract Due to orbital decay by gravitational wave radiation, some close binary helium white dwarfs are expected to merge within a Hubble time. The immediate merger products are believed to be He-rich subdwarf O (sdO) stars, essentially helium main-sequence stars. We present new evolution calculations for these post-merger stars beyond the core He-burning phase. The most massive He-rich sdO stars develop a strong He-burning shell and evolve to become He-rich giants. We include nucleosynthesis calculations following the merger of 0.4 M⊙ He white dwarf pairs with metallicities Z= 0.0001, 0.004, 0.008 and 0.02. The surface chemistries of the resulting giants are in partial agreement with the observed abundances of R Coronae Borealis (R CrB) and extreme He stars. Such stars might represent a third, albeit rare, evolution channel for the latter, in addition to the CO+He white dwarf merger and the very late thermal pulse channels proposed previously. We confirm a recent suggestion that Li seen in R CrB stars could form naturally during the hot phase of a merger in the presence of 3He from the donor white dwarf.