Core Helium Burning Phase Research Articles

Testing Angular Momentum Transport and Wind Loss in Intermediate-mass Core-helium Burning Stars

Abstract Stars between 2 and 3 M ☉ rotate rapidly on the main sequence, and their rotation rates in the core-helium burning (secondary-clump) phase can therefore be used to test models of angular momentum loss used for gyrochronology in a new regime. Because both their core and surface rotation rates can be measured, these stars can also be used to set strong constraints on angular momentum transport inside stars. We find that they are rotating slower than angular momentum conservation and rigid rotation would predict. Our results are insensitive to the degree of core–envelope coupling because of the small moment of inertia of the radiative core. We discuss two possible mechanisms for slowing down the surfaces of these stars: (1) substantial angular momentum loss and (2) radial differential rotation in the surface convection zone. Modern angular momentum loss prescriptions used for solar-type stars predict secondary-clump surface rotation rates in much better agreement with the data than prior variants used in the literature, and we argue that such enhanced loss is required to understand the combination of core and surface rotation rates. However, we find that the assumed radial differential rotation profile in convective regions has a strong impact on the predicted surface rotation rates, and that a combination of enhanced loss and radial differential rotation in the surface convection zone is also consistent with the data. We discuss future tests that can quantify the impact of both phenomena. Current data tentatively suggest that some combination of the two processes fits the data better than either one alone.

Angular Momentum Transport in Stellar Interiors

Stars lose a significant amount of angular momentum between birth and death, implying that efficient processes transporting it from the core to the surface are active. Space asteroseismology delivered the interior rotation rates of more than a thousand low- and intermediate-mass stars, revealing the following: ▪ Single stars rotate nearly uniformly during the core-hydrogen and core-helium burning phases. ▪ Stellar cores spin up to a factor of 10 faster than the envelope during the red giant phase. ▪ The angular momentum of the helium-burning core of stars is in agreement with the angular momentum of white dwarfs. Observations reveal a strong decrease of core angular momentum when stars have a convective core. Current theory of angular momentum transport fails to explain this. We propose improving the theory with a data-driven approach, whereby angular momentum prescriptions derived frommultidimensional (magneto)hydrodynamical simulations and theoretical considerations are continuously tested against modern observations. The TESS and PLATO space missions have the potential to derive the interior rotation of large samples of stars, including high-mass and metal-poor stars in binaries and clusters. This will provide the powerful observational constraints needed to improve theory and simulations.

Overshooting calibration and age determination from evolved binary systems

Aims. The capability of grid-based techniques to estimate the age together with the convective core overshooting efficiency of stars in detached eclipsing binary systems for main sequence stars has previously been investigated. We have extended this investigation to later evolutionary stages and have evaluated the bias and variability on the recovered age and convective core overshooting parameter accounting for both observational and internal uncertainties. Methods. We considered synthetic binary systems, whose age and overshooting efficiency should be recovered by applying the SCEPtER pipeline to the same grid of models used to build the mock stars. We focus our attention on a binary system composed of a 2.50 M⊙ primary star coupled with a 2.38 M⊙ secondary. To explore different evolutionary scenarios, we performed the estimation at three different times: when the primary is at the end of the central helium burning, when it is at the bottom of the RGB, and when it is in the helium core burning phase. The Monte Carlo simulations have been carried out for two typical values of accuracy on the mass determination, that is, 1% and 0.1%. Results. Adopting typical observational uncertainties, we found that the recovered age and overshooting efficiency are biased towards low values in all three scenarios. For an uncertainty on the masses of 1%, the underestimation is particularly relevant for a primary in the central helium burning stage, reaching − 8.5% in age and − 0.04 (− 25% relative error) in the overshooting parameter β. In the other scenarios, an undervaluation of the age by about 4% occurs. A large variability in the fitted values between Monte Carlo simulations was found: for an individual system calibration, the value of the overshooting parameter can vary from β = 0.0 to β = 0.26. When adopting a 0.1% error on the masses, the biases remain nearly unchanged but the global variability is suppressed by a factor of about two. We also explored the effect of a systematic discrepancy between the artificial systems and the model grid by accounting for an offset in the effective temperature of the stars by ± 150 K. For a mass error of 1% the overshooting parameter is largely biased towards the edges of the explored range, while for the lower mass uncertainty it is basically unconstrained from 0.0 to 0.2. We also evaluate the possibility of individually recovering the β value for both binary stars. We found that this is impossible for a primary near to central hydrogen exhaustion owing to huge biases for the primary star of + 0.14 (90% relative error), while in the other cases the fitted β are consistent, but always biased by about − 0.04 (− 25% relative error). Finally, the possibility to distinguish between models computed with mild overshooting from models with no overshooting was evaluated, resulting in a reassuring power of distinction greater than 80%. However, the scenario with a primary in the central helium burning was a notable exception, showing a power of distinction lower than 5%.

Evolved stars in the Local Group galaxies – II. AGB, RSG stars, and dust production in IC10

We study the evolved stellar population of the Local Group galaxy IC10, with the aim of characterizing the individual sources observed and to derive global information on the galaxy, primarily the star formation history and the dust production rate. To this aim, we use evolutionary sequences of low- and intermediate-mass ($M < 8~M_{\odot}$) stars, evolved through the asymptotic giant branch phase, with the inclusion of the description of dust formation. We also use models of higher mass stars. From the analysis of the distribution of stars in the observational planes obtained with IR bands, we find that the reddening and distance of IC10 are $E(B-V)=1.85$ mag and $d=0.77$ Mpc, respectively. The evolved stellar population is dominated by carbon stars, that account for $40\%$ of the sources brighter than the tip of the red giant branch. Most of these stars descend from $\sim 1.1-1.3~M_{\odot}$ progenitors, formed during the major epoch of star formation, which occurred $\sim 2.5$ Gyr ago. The presence of a significant number of bright stars indicates that IC10 has been site of significant star formation in recent epochs and currently hosts a group of massive stars in the core helium-burning phase. Dust production in this galaxy is largely dominated by carbon stars; the overall dust production rate estimated is $7\times 10^{-6}~M_{\odot}$/yr.

Pulsational instabilities driven by the ∈ mechanism in hot pre-horizontal branch stars

Context. The ∈ mechanism is a self-excitation mechanism of stellar pulsations that acts in regions inside the star where nuclear burning takes place. It has been shown that the ∈ mechanism can excite pulsations in models of hot pre-horizontal branch stars before they settle into the stable helium core-burning phase. Moreover, it has been shown that this mechanism could explain the shortest periods of LS IV-14°116, a mild He-sdBV star. Aims. We aim to study the ∈ mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties and the instability domain in the log g − log Teff plane. Methods. We performed detailed computations of non-adiabatic non-radial pulsations on stellar models during the helium subflashes just before the helium-core burning phase. Computations were carried out for different values of initial helium composition, metallicity, and envelope mass at the moment of helium flash. Results. We find an instability domain of long-period gravity modes due to the ∈ mechanism in the log g − log Teff plane at roughly 22 000 K ≲ Teff ≲ 50 000 K and 4.67 ≲ log g ≲ 6.15. Consequently, we find instabilities due to the ∈ mechanism on pre-extreme horizontal branch stellar models (Teff ≳ 22 000 K), but not on pre-blue horizontal branch stellar models (Teff ≲ 21 000 K). The periods of excited modes range between ~200 and ~2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that ∈ mechanism can excite gravity modes in stars with similar surface properties (He abundances, log g, log Teff), but in our models it is only able to excite modes in the range of the shortest observed periods. Conclusions. We predict a new instability strip for hot-subdwarf stars of which LS IV-14°116 could be the first inhabitant. Based on simple estimates we expect 1 to 10 stars in the current samples of hot-subdwarf stars to be pulsating by the ∈ mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.

Orbital Characteristics of the Subdwarf-B and F V Star Binary EC 20117-4014 (=V4640 Sgr)

Abstract Among the competing evolution theories for subdwarf-B (sdB) stars is the binary evolution scenario. EC 20117-4014 (=V4640 Sgr) is a spectroscopic binary system consisting of a pulsating sdB star and a late F main-sequence companion; however, the period and the orbit semimajor axes have not been precisely determined. This paper presents orbital characteristics of the EC 20117-4014 binary system using 20 years of photometric data. Periodic observed minus calculated (O–C) variations were detected in the two highest-amplitude pulsations identified in the EC 20117-4014 power spectrum, indicating the binary system’s precise orbital period (P = 792.3 days) and the light-travel-time amplitude (A = 468.9 s). This binary shows no significant orbital eccentricity, and the upper limit of the eccentricity is 0.025 (using 3σ as an upper limit). This upper limit of the eccentricity is the lowest among all wide sdB binaries with known orbital parameters. This analysis indicated that the sdB is likely to have lost its hydrogen envelope through stable Roche lobe overflow, thus supporting hypotheses for the origin of sdB stars. In addition to those results, the underlying pulsation period change obtained from the photometric data was P ˙ = 5.4 (±0.7) × 10−14 d d−1, which shows that the sdB is just before the end of the core helium-burning phase.

Antisolar differential rotation with surface lithium enrichment on the single K-giant V1192 Orionis

Context: Stars with about 1$-$2 solar masses at the red giant branch (RGB) represent an intriguing period of stellar evolution, i.e. when the convective envelope interacts with the fast-rotating core. During these mixing episodes freshly synthesized lithium can come up to the stellar surface along with high angular momentum material. This high angular momentum may alter the surface rotation pattern. Aims: The single rapidly rotating K-giant V1192 Ori is revisited to determine its surface differential rotation, lithium abundance, and basic stellar properties such as a precise rotation period. The aim is to independently verify the antisolar differential rotation of the star and possibly find a connection to the surface lithium abundance. Methods: We applied time-series Doppler imaging to a new multi-epoch data set. Altogether we reconstructed 11 Doppler images from spectroscopic data collected with the STELLA robotic telescope between 2007--2016. We used our inversion code iMap to reconstruct all stellar surface maps. We extracted the differential rotation from these images by tracing systematic spot migration as a function of stellar latitude from consecutive image cross-correlations. Results: The position of V1192 Ori in the Hertzsprung-Russell diagram suggests that the star is in the helium core-burning phase just leaving the RGB bump. We measure $A({\rm Li})_{\rm NLTE}=1.27$, i.e. a value close to the anticipated transition value of 1.5 from Li-normal to Li-rich giants. Doppler images reveal extended dark areas arranged quasi-evenly along an equatorial belt. No cool polar spot is found during the investigated epoch. Spot displacements clearly suggest antisolar surface differential rotation with $\alpha=-0.11\pm0.02$ shear coefficient. Conclusions: The surface Li enrichment and the peculiar surface rotation pattern may indicate a common origin.

The treatment of mixing in core helium-burning models – III. Suppressing core breathing pulses with a new constraint on overshoot

Theoretical predictions for the core helium burning phase of stellar evolution are highly sensitive to the uncertain treatment of mixing at convective boundaries. In the last few years, interest in constraining the uncertain structure of their deep interiors has been renewed by insights from asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of growth of helium-burning convective cores based on the higher buoyancy of material ingested from outside the convective core. In this paper we test the implications of such a limit for stellar models with a range of initial mass and metallicity. We find that the constraint on mixing beyond the Schwarzschild boundary has a significant effect on the evolution late in core helium burning, when core breathing pulses occur and the ingestion rate of helium is fastest. Ordinarily, core breathing pulses prolong the core helium burning lifetime to such an extent that models are at odds with observations of globular cluster populations. Across a wide range of initial stellar masses ($0.83 \leq M/\text{M}_\odot \leq 5$), applying the Spruit constraint reduces the core helium burning lifetime because core breathing pulses are either avoided or their number and severity reduced. The constraint suggested by Spruit therefore helps to resolve significant discrepancies between observations and theoretical predictions. Specifically, we find improved agreement for $R_2$, the observed ratio of asymptotic giant branch to horizontal branch stars in globular clusters; the luminosity difference between these two groups; and in asteroseismology, the mixed-mode period spacing detected in red clump stars in the \textit{Kepler} field.

The [Y/Mg] clock works for evolved solar metallicity stars

Previously [Y/Mg] has been proven to be an age indicator for solar twins. Here, we investigate if this relation also holds for helium-core-burning stars of solar metallicity. High resolution and high signal-to-noise ratio (S/N) spectroscopic data of stars in the helium-core-burning phase have been obtained with the FIES spectrograph on the NOT 2.56m telescope and the HIRES spectrograph on the Keck I 10 m telescope. They have been analyzed to determine the chemical abundances of four open clusters with close to solar metallicity; NGC 6811, NGC 6819, M67 and NGC 188. The abundances are derived from equivalent widths of spectral lines using ATLAS9 model atmospheres with parameters determined from the excitation and ionization balance of Fe lines. Results from asteroseismology and binary studies were used as priors on the atmospheric parameters, where especially the $\log g$ is determined to much higher precision than what is possible with spectroscopy. It is confirmed that the four open clusters are close to solar metallicity and they follow the [Y/Mg] vs. age trend previously found for solar twins. The [Y/Mg] vs. age clock also works for giant stars in the helium-core burning phase, which vastly increases the possibilities to estimate the age of stars not only in the solar neighborhood, but in large parts of the Galaxy, due to the brighter nature of evolved stars compared to dwarfs.

HD 16771: A lithium-rich giant in the red-clump stage

Aims: We report the discovery of a young lithium rich giant, HD 16771, in the core-helium burning phase that does not seem to fit existing proposals of Li synthesis near the luminosity function bump or during He-core flash. We aim to understand the nature of Li enrichment in the atmosphere of HD 16771 by exploring various Li enhancement scenarios. Methods: We have collected high-resolution echelle spectra of HD 16771 and derived stellar parameters and chemical abundances for 27 elements by either line equivalent widths or synthetic spectrum analyses. Results: HD 16771 is a Li-rich (log(n(Li))=+2.67+/-0.10 dex) intermediate mass giant star (M=2.4+/-0.1 Msun) with age=0.76+/-0.13 Gyr and located at the red giant clump. Kinematics and chemical compositions are consistent with HD 16771 being a member of the Galactic thin disk population. The non-detection of 6Li(< 3%), a low carbon isotopic ratio (12C/13C=12+/-2), and the slow rotation (vsini=2.8 km/s) all suggest that lithium might have been synthesized in this star. On the contrary, HD 16771 with a mass of 2.4 Msun has no chance of encountering luminosity function bump and He-core flash where the possibility of fast deep-mixing for Li enrichment in K giants has been suggested previously. Conclusions: Based of the evolutionary status of this star, we discuss the possibility that 7Li synthesis in HD 16771 is triggered by the engulfment of close-in planet(s) during the RGB phase.

WEIGHING STARS: THE IDENTIFICATION OF AN EVOLVED BLUE STRAGGLER STAR IN THE GLOBULAR CLUSTER 47 TUCANAE*

ABSTRACT Globular clusters are known to host peculiar objects named blue straggler stars (BSSs), significantly heavier than the normal stellar population. While these stars can be easily identified during their core hydrogen-burning phase, they are photometrically indistinguishable from their low-mass sisters in advanced stages of the subsequent evolution. A clear-cut identification of these objects would require the direct measurement of the stellar mass. We used the detailed comparison between chemical abundances derived from neutral and from ionized spectral lines as a powerful stellar “weighing device” to measure stellar mass and to identify an evolved BSS in 47 Tucanae. In particular, high-resolution spectra of three bright stars, located slightly above the level of the “canonical” horizontal branch (HB) sequence in the color–magnitude diagram of 47 Tucanae, have been obtained with the UVES spectrograph. The measurements of iron and titanium abundances performed separately from neutral and ionized lines reveal that two targets have stellar parameters fully consistent with those expected for low-mass post-HB objects, while for the other target the elemental ionization balance is obtained only by assuming a mass of &sim; 1.4 M &CircleDot; ?> , which is significantly larger than the main sequence turn-off mass of the cluster ( &sim; 0.85 M &CircleDot; ?> ). The comparison with theoretical stellar tracks suggests that this is a BSS descendant possibly experiencing its core helium-burning phase. The large applicability of the proposed method to most of the globular clusters in our Galaxy opens the possibility to initiate systematic searches for evolved BSSs, thus giving access to still unexplored phases of their evolution.

Supernovae from yellow, blue supergiants: origin and consequences for stellar evolution

AbstractA few core collapse supernovae progenitors have been found to be yellow or blue supergiants. Weshall discuss possible scenarios involving single and close binary evolution allowing to explain this kind of corecollapse supernova progenitors. According to stellar models for both single and close binaries, blue supergiants, at theend of their nuclear lifetimes and thus progenitors of core collapse supernovae, present very different characteristicsfor what concerns their surface compositions, rotational surface velocities and pulsational properties with respect toblue supergiants in their core helium burning phase. We discuss how the small observed scatter of the flux-weightedgravity-luminosity (FWGL) relation of blue supergiants constrains the evolution of massive stars after the Main-Sequence phase and the nature of the progenitors of supernovae in the mass range between 12 and 40 solarmasses. The present day observed surface abundances of blue supergiants, of their pulsational properties, as well asthe small scatter of the FWGL relation provide strong constraints on both internal mixing and mass loss in massivestars and therefore on the end point of their evolution.

Pulsations in B-type supergiants with masses $\boldsymbol {M&lt;20}\,\mathbf {M}_{\odot }$ before and after core helium ignition

Evolutionary tracks and pulsational analysis of models with masses of 13-18 $M_\odot$ are presented. We address two important questions. The first one deals with one of the most unresolved problems in astrophysics, i.e., the existence of a blue loop after core helium ignition; the so called "to loop or not to loop" problem. We show that inward overshooting from the outer convective zone in the red giant phase is prerequisite for the development of the blue loop. Our second question concerns pulsational instability of models in the core helium burning phase. We present for the first time that models on the blue loop can have unstable modes driven by the $\kappa$ mechanism operating in the $Z-$bump. Contrary to post-main sequence models in the shell hydrogen burning phases, pulsational instability of the blue loop models depends mainly on effective temperature and metallicity is of secondary importance. Finally, we try to interpret the oscillation spectrum of the blue supergiant HD 163899, the only member of the SPBsg class, and to get some clue on the evolutionary status of the star.

LUMINOUS BLUE VARIABLES AND SUPERLUMINOUS SUPERNOVAE FROM BINARY MERGERS

Evidence suggests that the direct progenitor stars of some core-collapse supernovae (CCSNe) are luminous blue variables (LBVs), perhaps including some `superluminous supernovae' (SLSNe). We examine models in which massive stars gain mass soon after the end of core hydrogen burning. These are mainly intended to represent mergers following a brief contact phase during early Case B mass transfer, but may also represent stars which gain mass in the Hertzsprung Gap or extremely late during the main-sequence phase for other reasons. The post-accretion stars spend their core helium-burning phase as blue supergiants (BSGs), and many examples are consistent with being LBVs at the time of core collapse. Other examples are yellow supergiants at explosion. We also investigate whether such post-accretion stars may explode successfully after core collapse. The final core properties of post-accretion models are broadly similar to those of single stars with the same initial mass as the pre-merger primary star. More surprisingly, when early Case B accretion does affect the final core properties, the effect appears likely to favour a successful SN explosion, i.e., to make the core properties more like those of a lower-mass single star. However, the detailed structures of these cores sometimes display qualitative differences to any single-star model we have calculated. The rate of appropriate binary mergers may match the rate of SNe with immediate LBV progenitors; for moderately optimistic assumptions we estimate that the progenitor birthrate is ~1% of the CCSN rate.

Magnetic Field Structure and Activity of the He-burning Giant 37 Comae

AbstractWe present the first magnetic map of the late-type giant 37 Com. The Least Squares Deconvolution (LSD) method and Zeeman Doppler Imaging (ZDI) inversion technique were applied. The chromospheric activity indicators Hα, S-index, Ca ii IRT and the radial velocity were also measured. The evolutionary status of the star has been studied on the basis of state-of-the-art stellar evolutionary models and chemical abundance analysis. 37 Com appears to be in the core Helium-burning phase.

KIC 8410637: a 408-day period eclipsing binary containing a pulsating giant star

Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement of masses and radii of ther component stars. If at least one of the stars has evolved off the main sequence (MS), the masses and radii give a strict constraint on the age of the stars. Several dEBs containing a bright K giant and a fainter MS star have been discovered by the Kepler satellite. The mass and radius of a red giant (RG) star can also be derived from its asteroseismic signal. The parameters determined in this way depend on stellar models and may contain systematic errors. It is important to validate the asteroseismically determined mass and radius with independent methods. This can be done when stars are members of stellar clusters or members of dEBs. KIC 8410637 consists of an RG and an MS star. The aim is to derive accurate masses and radii for both components and provide the foundation for a strong test of the asteroseismic method and the accuracy of the deduced mass, radius and age. We analyse high-resolution spectra from three different spectrographs. We also calculate a fit to the Kepler light curve and use ground-based photometry to determine the flux ratios between the component stars in the BVRI passbands. We measured the masses and radii of the stars in the dEB, and the classical parameters Teff, log g and [Fe/H] from the spectra and ground-based photometry. The RG component of KIC 8410637 is most likely in the core helium-burning red clump phase of evolution and has an age and composition very similar to the stars in the open cluster NGC 6819. The mass of the RG in KIC 8410637 should therefore be similar to the mass of RGs in NGC 6819, thus lending support to the most up-to-date version of the asteroseismic scaling relations. This is the first direct measurement of both mass and radius for an RG to be compared with values for RGs from asteroseismic scaling relations.

Blue supergiant progenitor models of type II supernovae

We show that within all the uncertainties that govern the process of Roche-lobe overflow in case Br-type massive binaries, it cannot be excluded that a significant fraction of them merge and become single stars. We demonstrate that at least some of them will spend most of their core helium-burning phase as hydrogen-rich blue stars, populating the massive blue supergiant region and/or the massive Be-type star population. The evolutionary simulations lead us to suspect that these mergers will explode as luminous blue hydrogen-rich stars, and it is tempting to link them to at least some superluminous supernovae.

On the evidence of extra mixing in models of 8 M⊙computed with the new solar abundances

Stars more massive than about 3Mare known to experience loops in the HR diagram during core helium burning phase. Except for very massive stars the extent of loops increases with the stellar mass. We show that a stellar evolution track for a 8Mstar computed with the new solar abundances (2) shows only a very tiny loop located near the red giant branch. An overshooting below the convective envelope is required to obtain a H-discontinuity located deep enough in the -gradient region and thus to allow the development of a loop in the HR diagram. When the central temperature reaches about 10 8 K, intermediate and massive stars ignite helium in a non-degenerate core at the tip of the RGB, reversing the upward climbing along the RGB. The star is composed of a He burning core and a H burning shell, both these regions producing the total luminosity of the star. The presence of a H shell allows the core to grow, and this phase is thus characterized by a long lifetime (about 20% of the core H burning phase while the star is almost two orders of magnitude brighter). Therefore the star has a large probability to be observed in that phase. The relative importance of one nuclear burning region to another plays an important role in the formation of a loop in the HRD (e.g. (5)). 2. WHAT ABOUT THE NEW SOLAR ABUNDANCES? We have compared the core He burning phase for 8Mmodels computed either with GN93 mixture (3) or with the new solar chemical composition, AGSS09 (2), each with their solar metallicity. The models have been computed with the CLES evolutionary code (7) with the OPAL opacities and with the Ledoux's criterion for the appearance of a convective zone. For the sake of clarity we will refer to the models in the following as GN1 for models computed with GN93, Z = 0.02, X = 0.70, and as AG1 for models computed with AGSS09, Z = 0.014, X = 0.72 (both models computed without overshooting). The blue or red evolution of He-burning stars has been widely investigated (e.g. (4) and references therein). As expected, the track computed with GN1 presents a loop during core He-burning, however, this loop is absent for AG1 track. It is well known that the location of the H discontinuity at the maximum extent of the convective envelope plays an important role in the formation of a loop during the late phases of core helium burning (e.g. (1, 6, 8)). This is indeed the encounter of the H-shell with this discontinuity that suddenly increases the contribution of the H-shell to the total luminosity with a consecutive heating of the envelope and the formation of a loop. If the discontinuity is too superficial, the whole core He- burning takes place before such an encounter and no loop can be formed.

THE EFFECT OF THE14N(p, γ)15O REACTION ON THE BLUE LOOPS IN INTERMEDIATE-MASS STARS

We present stellar evolutionary sequences of stars in the mass range 5-12 M ☉, having solar-like initial composition. The stellar models are obtained using updated input physics, including recent rates of thermonuclear reactions. We investigate the effects of a modification of the 14N(p, γ)15O reaction rate, as suggested by recent evaluations, on the formation and extension of the blue loops encountered during the evolution of the stars in the above mass range. We find that a reduced 14N(p, γ)15O rate, as described in the text, has a striking impact on the physical conditions of burning and mixing during shell hydrogen burning when the blue loops are formed. In particular, we find that the efficiency of shell hydrogen burning is crucial for the formation of an extended blue loop. We show that a significantly reduced 14N(p, γ)15O rate affects severely the extension of the blue loops and the time spent by the star in the blue part of the Hertzsprung-Russell diagram in the mass range 5-7 M ☉ if the treatment of convection is based on the Schwarzschild criterion only. In this case, envelope overshooting helps to restore well-extended blue loops as supported by the observations of the Cepheid stars. If core overshooting is included during the core hydrogen and core helium burning phases, the loop formation and its properties depend on how this overshooting is treated for a given stellar mass range, as well as on its efficiency.

Weak G-band stars on the H-R diagram: clues to the origin of the Li anomaly

Weak G-band (WGB) stars are a rare class of cool luminous stars that present a strong depletion in carbon, but also lithium abundance anomalies that have been little explored in the literature since the first discovery of these peculiar objects in the early 50's. Here we focus on the Li-rich WGB stars and report on their evolutionary status. We explore different paths to propose a tentative explanation for the lithium anomaly. Using archive data, we derive the fundamental parameters of WGB (Teff, log g, log(L/Lsun)) using Hipparcos parallaxes and recent temperature scales. From the equivalent widths of Li resonance line at 6707 {\AA}, we uniformly derive the lithium abundances and apply when possible NLTE corrections following the procedure described by Lind et al. (2009). We also compute dedicated stellar evolution models in the mass range 3.0 to 4.5 Msun, exploring the effects of rotation-induced and thermohaline mixing. These models are used to locate the WGB stars in the H-R diagram and to explore the origin of the abundance anomalies. The location of WGB stars in the H-R diagram shows that these are intermediate mass stars of masses ranging from 3.0 to 4.5 Msun located at the clump, which implies a degeneracy of their evolutionary status between subgiant/red giant branch and core helium burning phases. The atmospheres of a large proportion of WGB stars (more than 50%) exhibit lithium abundances A(Li) \geq 1.4 dex similar to Li-rich K giants. The position of WGB stars along with the Li-rich K giants in the H-R diagram however indicates that both are well separated groups. The combined and tentatively consistent analysis of the abundance pattern for lithium, carbon and nitrogen of WGB stars seems to indicate that carbon underabundance could be decorrelated from the lithium and nitrogen overabundances.