Surface Abundances Research Articles

Do Hertzsprung‐gap stars show any chemical anomaly?

AbstractWith the aim of investigating how the surface abundances of intermediate‐mass stars off the main sequence (evolving toward the red‐giant stage) are affected by the evolution‐induced envelope mixing, we spectroscopically determined the abundances of Li, C, N, O, and Na for selected 62 late A through G subgiants, giants, and supergiants, which are often called “Hertzsprung‐gap stars,” by applying the synthetic spectrum‐fitting technique to Li i 6708, C i 5380, N i 7460, O i 6156–8, and Na i 6161 lines. A substantially large star‐to‐star dispersion (≳2 dex) was confirmed for the Li abundances, indicating that this vulnerable element can either suffer significant depletion before the red‐giant stage or almost retain the primordial composition. Regarding C, N, O, and Na possibly altered by dredge‐up of nuclear‐processed products, their abundances turned out to show considerable scatter. This suggests that these abundance results are likely to suffer appreciable uncertainties, the reason for which is not clear but might be due to some kind of inadequate modeling for the atmospheric structure. Yet, paying attention to the fact that the relative abundance ratios between C, N, and O should be more reliable (because systematic errors may be canceled as lines of similar properties are used for these species), we could confirm a positive correlation between [O/C] (ranging from ∼0 to ∼+0.5 dex) and [N/C] (showing a larger spread from ∼0 to ∼+1 dex), which is reasonably consistent with the theoretical prediction. This observational detection of C deficiency as well as N enrichment in our program stars manifestly indicates that the dredge‐up of H‐burning product can take place before entering the red‐giant stage, with its extent differing from star to star.

Surface abundances of CNO in Galactic O-stars: a pilot study with FASTWIND

Context. Rotational mixing is known to significantly affect the evolution of massive stars; however, we still lack a consensus regarding the various possible modeling approaches and mixing recipes describing this process. The empirical investigation of surface abundances of carbon, nitrogen, and oxygen (CNO) in large samples of O- and B-type stars will be essential for providing meaningful observational constraints on the different available stellar evolution models. Aims. Setting up and testing adequate tools to perform CNO surface abundance determinations for large samples of O-type stars, by means of the fast performance, NLTE, unified model atmosphere code FASTWIND. Methods. We have developed a set of semi-automatic tools for measuring and analyzing the observed equivalent widths of strategic optical C, N, and O lines from different ions. Our analysis strategy is based on a χ2 minimization of weighted differences between observed and synthetic equivalent widths, the latter computed from tailored model grids. We have paid special attention to the (significant) errors introduced by typical uncertainties in stellar parameters. In this pilot study, we describe these tools, and test their performance and reliability using a set of high quality spectra of a sample of 18 presumably single Galactic O-type stars with low projected rotational velocities (v sin i≲100 km s−1), and previously determined stellar parameters. In particular, we have compared the outcome of our analysis with results from existing studies and theoretical stellar evolution models. Results. Most of our results for carbon and nitrogen agree, within the errors, with both theoretical expectations and literature values. While many cooler dwarfs display C and N abundances close to solar, some of the early- and mid-O dwarfs – and most supergiants – show significant enrichment in N and depletion in C. Our results for oxygen in late-O dwarfs are, however, unexpectedly low, possibly indicating deficiencies in the adopted oxygen model atom. For all other objects, no systematic problems in their oxygen content have been identified. Specific stars in our sample show peculiarities in their abundances, and we suggest hypotheses regarding their origin. Conclusions. Our method is (almost) ready to be applied to large samples of late and mid O-type stars – although the oxygen model atom needs to be improved and carefully tested first. For early O-type stars (O4 and hotter), a simultaneous UV analysis seems to be inevitable, due to the scarcity and weakness of optical C and O lines. This will necessarily imply a more complex modeling, additionally accounting for the effects of X-rays from wind-embedded shocks and wind inhomogeneities.

Compositional differences between the component stars of eclipsing close binary systems showing chemical peculiarities

A spectroscopic study was carried out on the surface chemical abundances of CNO and several heavier elements in the primary and secondary components of 5 eclipsing close binaries around A-type (AR Aur, beta Aur, YZ Cas, WW Aur, and RR Lyn), in order to investigate the nature of chemical differences between both components (being comparatively slow rotators alike due to tidal synchronization). Regarding the systems comprising similar components, beta Aur and WW Aur were confirmed to exhibit no compositional difference between the primary and secondary both showing almost the same Am anomaly, though the chemical peculiarities seen in the component stars of AR Aur show distinct differences (HgMn star and Am star). In contrast, as to the systems (YZ Cas and RR Lyn) consisting of considerably different (A and early-F) components, the surface abundances are markedly different between the primary (Am) and secondary (normal). These observational results may indicate Teff-dependent characteristics regarding the chemical anomalies of non-magnetic stars on the upper main sequence: (1) In the effective temperature range of 10000K > Teff > 7000K, rotational velocity is the most important factor for determining the extent of Am peculiarity. (2) However, the emergence of Am phenomenon seems to have a lower Teff limit at ~7000K, below which no abundance anomaly is observed regardless of stellar rotation. (3) The transition from Am anomaly (mild deficiency in CNO) to HgMn anomaly (unusually large N depletion) is likely to take place as Teff increases from ~10000K to ~11000K.

The Long-term Evolution and Appearance of Type Iax Postgenitor Stars

Abstract Type Iax supernovae may arise from failed explosions of white dwarfs (WDs) that leave behind a bound remnant (i.e., a “postgenitor” star) that could be identified in wide field surveys. To understand their observational signatures, we simulate these WD postgenitors from shortly after explosion until they move back down the WD cooling track, and we consider several possible WD masses and explosion energies. To predict the peculiar surface abundances of the WD postgenitors, our models take into account gravitational settling and radiative levitation. We find that radiative levitation is significant at temperatures above a mass-dependent critical temperature, typically in the range T eff ≈ (50–100) × 103 K, significantly increasing surface abundances of iron group elements. Due to enhanced iron group opacity compared to normal WDs, the postgenitor peak luminosity and cooling timescale depend sensitively on mass, with more massive WDs becoming brighter but cooling much faster. We discuss our results in light of recently discovered hypervelocity WDs with peculiar surface compositions, finding that our low-mass postgenitor models match many of their observational characteristics. Finally, we explore the effects of thermohaline diffusion, tentatively finding that it strongly suppresses abundance enhancements created by radiative levitation, but more realistic modeling is required to reach a firm conclusion.

On the Origin of Black Hole Spin in High-mass X-Ray Binaries

Abstract Black hole (BH) spins in low-mass X-ray binaries cover a range of values that can be explained by accretion after BH birth. In contrast, the three BH spin measurements in high-mass X-ray binaries (HMXBs) show only values near the maximum and likely have a different origin connected to the BH stellar progenitor. We explore here two possible scenarios to explain the high spins of BHs in HMXBs: formation in binaries that undergo mass transfer (MT) during the main sequence (MS; Case-A MT), and very close binaries undergoing chemically homogeneous evolution (CHE). We find that both scenarios are able to produce high-spin BHs if internal angular momentum (AM) transport in the progenitor star after its MS evolution is not too strong (i.e., weak coupling between the stellar core and its envelope). If instead efficient AM transport is assumed, we find that the resulting BH spins are always too low with respect to observations. The Case-A MT model provides a good fit for the BH spins, the masses of the two components, and the final orbital periods for two of the three BHs in HMXBs with measured spins. For one of them, the mass predicted for the BH companion is significantly lower than observed, but this depends strongly on the assumed efficiency of MT. The CHE models predict orbital periods that are too large for all three cases considered here. We expect the Case-A MT to be much more frequent at the metallicities involved, so we conclude that the Case-A MT scenario is preferred. Finally, we predict that the stellar companions of HMXBs formed through the Case-A MT have enhanced nitrogen surface abundances, which can be tested by future observations.

TheGaia-ESO Survey: impact of extra mixing on C and N abundances of giant stars

Context. TheGaia-ESO Public Spectroscopic Survey using FLAMES at the VLT has obtained high-resolution UVES spectra for a large number of giant stars, allowing a determination of the abundances of the key chemical elements carbon and nitrogen at their surface. The surface abundances of these chemical species are known to change in stars during their evolution on the red giant branch (RGB) after the first dredge-up episode, as a result of the extra mixing phenomena.Aims. We investigate the effects of thermohaline mixing on C and N abundances using the first comparison between theGaia-ESO survey [C/N] determinations with simulations of the observed fields using a model of stellar population synthesis.Methods. We explore the effects of thermohaline mixing on the chemical properties of giants through stellar evolutionary models computed with the stellar evolution code STAREVOL. We include these stellar evolution models in the Besançon Galaxy model to simulate the [C/N] distributions determined from the UVES spectra of theGaia-ESO survey and to compare them with the observations.Results. Theoretical predictions including the effect of thermohaline mixing are in good agreement with the observations. However, the field stars in theGaia-ESO survey with C and N abundance measurements have a metallicity close to solar, where the efficiency of thermohaline mixing is not very large. The C and N abundances derived by theGaia-ESO survey in open and globular clusters clearly show the impact of thermohaline mixing at low metallicity, which explains the [C/N] value observed in lower mass and older giant stars. Using independent observations of carbon isotopic ratio in clump field stars and open clusters, we also confirm that thermohaline mixing should be taken into account to explain the behaviour of12C/13C as a function of stellar age.Conclusions. Overall, the current model including thermohaline mixing is able to reproduce very well the C and N abundances over the whole metallicity range investigated by theGaia-ESO survey data.

Helium abundance in a sample of cool stars: measurements from asteroseismology

The structural stratification of a solar-type main sequence star primarily depends on its mass and chemical composition. The surface heavy element abundances of the solar-type stars are reasonably well determined using conventional spectroscopy, however the second most abundant element helium is not. This is due to the fact that the envelope temperature of such stars is not high enough to excite helium. Since the helium abundance of a star affects its structure and subsequent evolution, the uncertainty in the helium abundance of a star makes estimates of its global properties (mass, radius, age etc.) uncertain as well. The detections of the signatures of the acoustic glitches from the precisely measured stellar oscillation frequencies provide an indirect way to estimate the envelope helium content. We use the glitch signature caused by the ionization of helium to determine the envelope helium abundance of 38 stars in the {\it Kepler} seismic LEGACY sample. Our results confirm that atomic diffusion does indeed take place in solar-type stars. We use the measured surface abundances in combination with the settling predicted by the stellar models to determine the initial abundances. The initial helium and metal mass fractions have subsequently been used to get the preliminary estimates of the primordial helium abundance, $Y_p = 0.244\pm0.019$, and the galactic enrichment ratio, $\Delta Y / \Delta Z = 1.226\pm0.849$. Although the current estimates have large errorbars due to the limited sample size, this method holds great promises to determine these parameters precisely in the era of upcoming space missions.

S stars and s-process in the Gaia era

Context. S stars are transition objects between M-type giants and carbon stars on the asymptotic giant branch (AGB). They are characterized by overabundances of s-process elements. Roughly half of them are enhanced in technetium (Tc), an s-process element with no stable isotope, while the other half lack technetium. This dichotomy arises from the fact that Tc-rich S stars are intrinsically producing s-process elements and have undergone third dredge-up (TDU) events, while Tc-poor S stars owe their s-process overabundances to a past pollution by a former AGB companion which is now an undetected white dwarf, and since the epoch of the mass transfer, technetium has totally decayed. Aims. Our aim is to analyse the abundances of S stars and gain insights into their evolutionary status and on the nucleosynthesis of heavy s-process elements taking place in their interior. In particular, the location of extrinsic and intrinsic S stars in the HR diagram will be compared with the theoretical onset of the TDU on the thermally pulsing AGB. Methods. A sample of 19 S-type stars was analysed by combining HERMES high-resolution spectra, accurate Gaia Data Release 2 (GDR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Various stellar parameters impact the atmospheric structure of S stars, not only effective temperature, gravity, metallicity and microturbulence but also C/O and [s/Fe]. We show that photometric data alone are not sufficient to disentangle these parameters. We present a new automatic spectral-fitting method that allows one to constrain the range of possible atmospheric parameters. Results. Combining the derived parameters with GDR2 parallaxes allows a joint analysis of the location of the stars in the Hertzsprung–Russell diagram and of their surface abundances. For all 19 stars, Zr and Nb abundances are derived, complemented by abundances of other s-process elements for the three Tc-rich S stars. These abundances agree within the uncertainties with nucleosynthesis predictions for stars of corresponding mass, metallicity and evolutionary stage. The Tc dichotomy between extrinsic and intrinsic S stars is seen as well in the Nb abundances: intrinsic, Tc-rich S stars are Nb-poor, whereas extrinsic, Tc-poor S stars are Nb-rich. Most extrinsic S stars lie close to the tip of the red giant branch (RGB), and a few are located along the early AGB. All appear to be the cooler analogues of barium stars. Barium stars with masses smaller than 2.5 M⊙ turn into extrinsic S stars on the RGB, because only for those masses does the RGB tip extend to temperatures lower than ~4200 K, which allows the ZrO bands distinctive of S-type stars to develop. On the contrary, barium stars with masses in excess of ~2.5 M⊙ can only turn into extrinsic S stars on the E-AGB, but those are short-lived, and thus rare. The location of intrinsic S stars in the HR diagram is compatible with them being thermally-pulsing AGB stars. Although nucleosynthetic model predictions give a satisfactory distribution of s-process elements, fitting at the same time the carbon and heavy s-element enrichments still remains difficult. Finally, the Tc-rich star V915 Aql is challenging as it points at the occurrence of TDU episodes in stars with masses as low as M ~ 1 M⊙.

Oxygen and silicon abundances in Cygnus OB2

Context. Cygnus OB2 is a rich OB association in the Galaxy located at only ∼1.4 kpc from us which has experienced intense star formation in the last 20–25 Myr. Its stellar population shows a correlation between age and Galactic longitude. Exploring the chemical composition of its stellar content we will be able to check the degree of homogeneity of the natal molecular cloud and possible effects of self-enrichment processes. Aims. Our aim is to determine silicon and oxygen abundances for a sample of eight early-type slow rotators (with rotational velocities below 80 km s−1) in Cygnus OB2 in order to check possible inhomogeneities across the whole association and whether there exists a correlation of chemical composition with Galactic longitude. Methods. We have performed a spectroscopic analysis of a sample of late O and early B stars with low rotational velocity in Cygnus OB2, which have been chosen so as to cover the whole association area. We have carried out an analysis based on equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND stellar atmosphere models to determine their stellar fundamental parameters (effective temperature and surface gravity) as well as the silicon and oxygen surface abundances. Results. We derive a rather homogeneous distribution of silicon and oxygen abundances across the region, with average values of 12 + log(Si/H) = 7.53 ± 0.08 dex and 12 + log(O/H) = 8.65 ± 0.12 dex. Conclusions. We find a homogeneous chemical composition in Cygnus OB2 with no clear evidence for significant chemical self-enrichment, despite indications of strong stellar winds and possible supernovae during the history of the region. Comparison with different scenarios of chemical enrichment by stellar winds and supernovae point to star forming efficiencies not significantly above 10%. The degree of homogeneity that we find is consistent with the observed Milky Way oxygen gradient based on H II regions. We also find that the oxygen scatter within Cygnus OB2 is at least of the same order than among H II regions at similar Galactocentric distance.

The surface abundances of red supergiants at core collapse

In the first weeks-to-months of a type II-P supernova (SN), the spectrum formation region is within the hydrogen-rich envelope of the exploding star. Optical spectra taken within a few days of the SN explosion, when the photosphere is hot, show features of ionised carbon, nitrogen and oxygen, as well as hydrogen and helium. Quantitative analysis of this very early phase may therefore constrain the chemical abundances of the stellar envelope at the point of core-collapse. Using existing and new evolutionary calculations for Red Supergiants (RSGs), we show that the predictions for the terminal surface [C/N] ratio is correlated with the initial mass of the progenitor star. Specifically, a star with an initial mass above 20M$_{\odot}$ exploding in the RSG phase should have an unequivocal signal of a low [C/N] abundance. Furthermore, we show that the model predictions are relatively insensitive to uncertainties in the treatment of convective mixing. Although there is a dependence on initial rotation, this can be dealt with in a probabilistic sense by convolving the model predictions with the observed distribution of stellar rotation rates. Using numerical experiments, we present a strategy for using very early-time spectroscopy to determine the upper limit to the progenitor mass distribution for type II-P SNe.

The GALAH survey: verifying abundance trends in the open cluster M67 using non-LTE modelling

Open cluster members are coeval and share the same initial bulk chemical compositions. Consequently, differences in surface abundances between members of a cluster that are at different evolutionary stages can be used to study the effects of mixing and internal chemical processing. We carry out an abundance analysis of seven elements (Li, O, Na, Mg, Al, Si, Fe) in 66 stars belonging to the open cluster M67, based on high resolution GALAH spectra, 1D MARCS model atmospheres, and, for the first time for a large sample of M67 stars, non-local thermodynamic equilibrium (non-LTE) radiative transfer. From the non-LTE analysis, we find a typical star-to-star scatter in the abundance ratios of around 0.05 dex; this scatter is slightly but systematically larger when LTE is assumed instead. We find trends in the abundance ratios with effective temperature, indicating systematic differences in the surface abundances between turn-off and giant stars; these trends are more pronounced when LTE is assumed. However, in the non-LTE analysis, most of the element trends have been flattened. Two species are exceptions to this behaviour, namely Al and Si, which both clearly display remaining trends in the non-LTE analysis. We comment on the possible origin of these trends, by comparing them with recent stellar models that include atomic diffusion.

A chemical study of M67 candidate blue stragglers and evolved blue stragglers observed with APOGEE DR14

Within the variety of objects populating stellar clusters, blue straggler stars (BSSs) are among the most puzzling ones. BSSs are commonly found in globular clusters, but they are also known to populate open clusters of the Milky Way. Two main theoretical scenarios (collisions and mass transfer) have been suggested to explain their formation, although finding observational evidence in support of either scenario represents a challenging task. Among the APOGEE observations of the old open cluster M67, we found 8 BSS candidates known from the literature and two known evolved BSSs. We carried out a chemical analysis of 3 BSS candidates and of the 2 evolved BSSs out of the sample and found that the BSS candidates have surface abundances similar to those of stars on the main-sequence turn-off of M67. Especially the absence of any anomaly in their carbon abundances seems to support a collisional formation scenario for these stars. Furthermore, we note that the abundances of the evolved BSSs S1040 and S1237 are consistent with the abundances of the red clump stars of M67. In particular, they show a depletion in carbon by $\sim0.25$ dex, which could be either interpreted as the signature of mass transfer or as the product of stellar evolutionary processes. Finally, we summarise the properties of the individual BSS stars observed by APOGEE, as derived from their APOGEE spectra and/or from information available in the literature.

Star Cluster Ages in the Gaia Era

Abstract We use the framework developed as part of the MESA Isochrones and Stellar Tracks (MIST) project to assess the utility of several types of observables in jointly measuring the age and 1D stellar model parameters in star clusters. We begin with a pedagogical overview summarizing the effects of stellar model parameters, such as the helium abundance, mass-loss efficiency, and mixing-length parameter, on observational diagnostics such as the color–magnitude diagram, mass–radius relation, and surface abundances, among others. We find that these parameters and the stellar age influence observables in qualitatively distinctive, degeneracy-breaking ways. To assess the current state of affairs, we use the recent Gaia Data Release 2 (DR2) along with data from the literature to investigate three well-studied old open clusters—NGC 6819, M67, NGC 6791—as case studies. Although there is no obvious tension between the existing observations and the MIST models for NGC 6819, there are interesting discrepancies in the cases of M67 and NGC 6791. At this time, parallax zero-point uncertainties in Gaia DR2 remain one of the limiting factors in the analysis of these clusters. With a combination of exquisite photometry, parallax distances, and cluster memberships from Gaia at the end of its mission, we anticipate precise and accurate ages for these and other star clusters in the Galaxy.

Deep secrets of intermediate-mass giants and supergiants

Context. Recent observational results have demonstrated an increase in the surface Na abundance that correlates with stellar mass for red giants between 2 and 3M⊙. This trend supports evolutionary mixing processes as the explanation for Na overabundances seen in some red giants. In this same mass range, the surface Al abundance was shown to be constant.Aims. Our main aim was to extend the investigation of the Na and Al surface abundances to giants more massive than 3M⊙. We sought to establish accurately whether the Na abundances keep increasing with stellar mass or a plateau is reached. In addition, we investigated whether mixing can affect the surface abundance of Al in giants more massive than 3M⊙.Methods. We obtained new high-resolution spectra of 20 giants in the field of 10 open clusters; 17 of these stars were found to be members of 9 clusters. The giants have masses between 2.5M⊙and 5.6M⊙. A model atmosphere analysis was performed and abundances of up to 22 elements were derived using equivalent widths. Additionally, abundances of C, N, and O were determined using spectrum synthesis. The abundances of Na and Al were corrected for non-local thermodynamic equilibrium (non-LTE) effects. Moreover, to extend the mass range of our sample, we collected from the literature high-quality C, N, O, and Na abundances of 32 Galactic Cepheids with accurate masses in the range between 3M⊙and 14M⊙.Results. The surface abundances of C, N, O, Na, and Al were compared to predictions of stellar evolution models with and without the inclusion of rotation-induced mixing. The surface abundances of most giants and Cepheids of the sample can be explained by models without rotation. For giants above Ȉ2.5M⊙, the Na abundances reach a plateau level of about [Na/Fe] ~ 0.20–0.25 dex (in non-LTE). This is true for both Cepheids and giants in open clusters. Regarding Al, the non-LTE [Al/Fe] ratios are mostly close to solar and suggest that Al is not affected by the first dredge-up up to ~5.0M⊙. Our results support previous works that found models with rotation to overestimate the mixing effects in intermediate-mass stars.

Orbital parameters and evolutionary status of the highly peculiar binary system HD 66051

Context. The spectroscopic binary system HD 66051 (V414 Pup) consists of a highly peculiar CP3 (HgMn) star and an A-type component. It also shows out-of-eclipse variability that is due to chemical spots. This combination allows the derivation of tight constraints for the testing of time-dependent diffusion models. Aims. We aim at deriving astrophysical parameters, information on age, and an orbital solution of the system. Methods. We analysed radial velocity and photometric data using two different methods to determine astrophysical parameters and the orbit of the system. Appropriate isochrones were used to derive the age of the system. Results. The orbital solution and the estimates from the isochrones are in excellent agreement with the estimates from a prior spectroscopic study. The system is very close to the zero-age main sequence and younger than 120 Myr. Conclusions. HD 66051 is a most important spectroscopic binary system that can be used to test the predictions of the diffusion theory explaining the peculiar surface abundances of CP3 stars.

Triton’s surface ices: Distribution, temperature and mixing state from VLT/SINFONI observations

Triton, the largest Solar System satellite beyond Saturn, was probably captured from the Transneptunian population by Neptune. It is mainly covered by N2, CO, CO2, CH4 and H2O in solid state and, except for H2O and CO2, these species are also present in gas phase. Sublimation and recondensation of the volatile species may lead to geographical and temporal variation on the surface composition, and could participate to the formation of complex chemical compounds formed from photochemistry occurring in the atmosphere (Krasnopolsky and Cruikshank, 1999) or from irradiation of N2 : CH4 : CO layers (Moore and Hudson, 2003).We present new near-IR observations performed at the VLT-ESO with SINFONI in 2010, 2011 and 2013, from which band depths, areas and positions of the main ice features are determined at different longitudes. We also re-reduce and re-analyze earlier data obtained during the last 20 years. Models based on the Hapke theory (Hapke, 1993) are developed to constrain the abundance, grain size, temperature, and state of mixing of the different ices (N2, CH4, CO, CO2, H2O) as well as attempt to identify other species. For this purpose, we present and use new optical constants of CO2 measured at 35 and 54 K.Our analyses confirm the longitudinal variation of the N2 and CO surface abundances previously evidenced, and suggest additional latitudinal and/or temporal variability of these two species. We confirm the presence of deep N2 layers in which CO and CH4 are diluted. In particular, we demonstrate that CO is present in diluted (as opposed to pure ice) form. In contrast, our models support the presence of small and longitudinally variable amounts of pure CH4 ice, providing an explanation to the enhanced atmospheric CH4/N2 ratio over expectations based on an ideal N2-CH4-CO mixture. They also suggest significant smaller particles of H2O and CO2 than reported previously in Quirico et al. (1999), with CO2 being probably distributed over a large area of the surface. We find that the 2.40 µm band is primarily due to 13CO ice, with a telluric value of the 13CO/12CO ratio, and not to ethane. We infer a N2 ice temperature of 37.5 ± 1K, suggesting that the atmospheric pressure over 2010–2013 was similar to that during the Voyager 1989 epoch.

The shape of convective core overshooting from gravity-mode period spacings

Context.The evolution of stars born with a convective core is highly dependent on the efficiency and extent of near core mixing processes, which effectively increases both the core mass and main-sequence lifetime. These mixing processes remain poorly constrained and therefore result in large uncertainties in the stellar structure and evolution models of such stars.Aims.We investigate to what extent gravity-mode period spacings in slowly pulsating B-type stars observed by theKeplermission can be used to constrain both the shape and extent of convective core overshoot and additional mixing in the radiative envelope.Methods.We compute grids of 1D stellar structure and evolution models for two different shapes of convective core overshooting and three shapes of radiative envelope mixing. The models in these grids are compared to a set of benchmark models to evaluate their capability of mimicking the dipole progradeg-modes of the benchmark models.Results.Through our model comparisons we find that at a central hydrogen content ofXc = 0.5, dipole progradeg-modes in the period range 0.8−3 d are capable of differentiating between step and exponential diffusive overshooting. This ability disappears towards the terminal age main sequence atXc = 0.1. Furthermore, theg-modes behave the same for the three different shapes of radiative envelope mixing considered. However, a constant envelope mixing requires a diffusion coefficient near the convective core five times higher than chemical mixing from internal gravity waves to obtain a surface nitrogen excess of ~ 0.5 dex within the main-sequence lifetime.Conclusions.Within the estimated frequency errors of theKeplermission, the ability ofg-modes to distinguish between step and exponential diffusive overshooting depends on the evolutionary stage. Combining information from the average period spacing and observed surface abundances, notably nitrogen, could potentially be used to constrain the shape of mixing in the radiative envelope of massive stars.

The Importance of the 13C(α,n)16O Reaction in Asymptotic Giant Branch Stars

Abstract Low-mass asymptotic giant branch stars are among the most important polluters of the interstellar medium. In their interiors, the main component (A ≳ 90) of the slow neutron capture process (the s-process) is synthesized, the most important neutron source being the 13C(α,n)16O reaction. In this paper, we review its current experimental status, discussing possible future synergies between some experiments currently focused on the determination of its rate. Moreover, in order to determine the level of precision needed to fully characterize this reaction, we present a theoretical sensitivity study, carried out with the FUNS evolutionary stellar code and the NEWTON post-process code. We modify the rate up to a factor of 2 with respect to a reference case. We find that variations of the 13C(α,n)16O rate do not appreciably affect s-process distributions for masses above 3 M ⊙ at any metallicity. Apart from a few isotopes, in fact, the differences are always below 5%. The situation is completely different if some 13C burns in a convective environment: this occurs in FUNS models with M < 3 M ⊙ at solar-like metallicities. In this case, a change of the 13C(α,n)16O reaction rate leads to nonnegligible variations of the element surface distribution (10% on average), with larger peaks for some elements (such as rubidium) and neutron-rich isotopes (such as 86Kr and 96Zr). Larger variations are found in low-mass, low-metallicity models if protons are mixed and burned at very high temperatures. In this case, the surface abundances of the heavier elements may vary by more than a factor of 50.

High-resolution infrared spectroscopy of field Red Horizontal Branch stars

We obtained the surface abundances of more than 20 elements from H- and K-band observations of three Red Horizontal Branch (RHB) stars. High-resolution (R ≃ 45,000) infrared spectra were obtained using IGRINS (the Immersion Grating Infrared Spectrograph) on the 2.7 m Smith telescope at McDonald Observatory. IGRINS has the capacity of making simultaneous observations with a wavelength coverage of essentially the entire H and K-bands (1.5–2.4 μm). High S/N (>200) IGRINS data serve as a laboratory of molecular bands. A large number of OH, CO and CN molecular bands provided us invaluable information on the abundances of C, N and O, which can be altered as stars evolve from birth to death. OH bands confirmed and greatly strengthened the oxygen abundances previously measured from visible [O I] lines. Carbon and nitrogen abundances were determined from CO and CN molecular bands, respectively. IGRINS also made it possible to achieve more robust abundances of light elements such as Na, Mg, Al, and Si than obtained from their visible counterparts, and allowed S, P and K abundance determination mostly for the first time. Most importantly, we were able to detect 12C/13C isotopic ratios from the CO (2–0) and CO (3–1) band-heads present in the K-band, which give us important clues about the evolutionary status of the stars and mixing processes that take place in the interiors of these RHB He core-burning stars.

Spectroscopic and physical parameters of Galactic O-type stars

Context.Massive stars play a key role in the evolution of galaxies and our Universe.Aims.Our goal is to compare observed and predicted properties of single Galactic O stars to identify and constrain uncertain physical parameters and processes in stellar evolution and atmosphere models.Methods.We used a sample of 53 objects of all luminosity classes and with spectral types from O3 to O9.7. For 30 of these, we determined the main photospheric and wind parameters, including projected rotational rates accounting for macroturbulence, and He and N surface abundances, using optical spectroscopy and applying the model atmosphere code FASTWIND. For the remaining objects, similar data from the literature, based on analyses by means of the CMFGEN code, were used instead. The properties of our sample were then compared to published predictions based on two grids of single massive star evolution models that include rotationally induced mixing.Results.Any of the considered model grids face problem in simultaneously reproducing the stellar masses, equatorial gravities, surface abundances, and rotation rates of our sample stars. The spectroscopic masses derived for objects below ~30M⊙tend to be smaller than the evolutionary ones, no matter which of the two grids have been used as a reference. While this result may indicate the need to improve the model atmosphere calculations (e.g. regarding the treatment of turbulent pressure), our analysis shows that the established mass problem cannot be fully explained in terms of inaccurate parameters obtained by quantitative spectroscopy or inadequate model values ofVroton the zero age main sequence. Within each luminosity class, we find a close correlation of N surface abundance and luminosity, and a stronger N enrichment in more massive and evolved O stars. Additionally, we also find a correlation of the surface nitrogen and helium abundances. The large number of nitrogen-enriched stars above ~30M⊙argues for rotationally induced mixing as the most likely explanation. However, none of the considered models can match the observed trends correctly, especially in the high mass regime.Conclusions.We confirm mass discrepancy for objects in the low mass O-star regime. We conclude that the rotationally induced mixing of helium to the stellar surface is too strong in some of the models. We also suggest that present inadequacies of the models to represent the N enrichment in more massive stars with relatively slow rotation might be related (among other issues) to problematic efficiencies of rotational mixing. We are left with a picture in which invoking binarity and magnetic fields is required to achieve a more complete agreement of the observed surface properties of a population of massive main-sequence stars with corresponding evolutionary models.