ABSTRACT We present a new set of models for intermediate-mass asymptotic giant branch (AGB) stars (4.0, 5.0, and 6.0 M ) at different metallicities (−2.15 ≤ Fe/H ≤ +0.15). This set integrates the ...existing models for low-mass AGB stars (1.3 ≤ M/M ≤ 3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the main sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. This effect is due to the fact that the interpulse phases are short and, therefore, thermal pulses (TPs) are weak. Moreover, the high temperature at the base of the convective envelope prevents it from deeply penetrating the underlying radiative layers. Depending on the initial stellar mass, the heavy element nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the 22Ne( ,n)25Mg reaction, which is efficiently activated during TPs. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the influence intermediate-mass models have on the carbon star luminosity function. Finally, we present the upgrade of the FRUITY web interface, which now also includes the physical quantities of the TP-AGB phase for all of the models included in the database (ph-FRUITY).
Context.
The production of neutrinos by plasma oscillations is the most important energy sink process operating in the degenerate core of low-mass red giant stars. This process counterbalances the ...release of energy induced by nuclear reactions and gravitational contraction, and determines the luminosity attained by a star at the moment of the He ignition. This occurrence coincides with the tip of the red giant branch (RGB), whose luminosity is extensively used as a calibrated standard candle in several cosmological studies.
Aims.
We aim to investigate the possible activation of additional energy sink mechanisms, as predicted by many extensions of the so-called Standard Model. In particular, our objective is to test the possible production of axions or axion-like particles, mainly through their coupling with electrons.
Methods.
By combining
Hubble
Space Telescope and ground-based optical and near-infrared photometric samples, we derived the RGB tip absolute magnitude of 22 galactic globular clusters (GGCs). The effects of varying the distance and the metallicity scales were also investigated. Then we compared the observed tip luminosities with those predicted by state-of-the-art stellar models that include the energy loss due to the axion production in the degenerate core of red giant stars.
Results.
We find that theoretical predictions including only the energy loss by plasma neutrinos are, in general, in good agreement with the observed tip bolometric magnitudes, even though the latter are ∼0.04 mag brighter on average. This small shift may be the result of systematic errors affecting the evaluation of the RGB tip bolometric magnitudes, or, alternatively, it could be ascribed to an axion-electron coupling causing a non-negligible thermal production of axions. In order to estimate the strength of this possible axion sink, we performed a cumulative likelihood analysis using the RGB tips of the whole set of 22 GGCs. All the possible sources of uncertainties affecting both the measured bolometric magnitudes and the corresponding theoretical predictions were carefully considered. As a result, we find that the value of the axion-electron coupling parameter that maximizes the likelihood probability is
g
ae
/10
−13
∼ 0.60
−0.58
+0.32
. This hint is valid, however, if the dominant energy sinks operating in the core of red giant stars are standard neutrinos and axions coupled with electrons. Any additional energy-loss process, not included in the stellar models, would reduce such a hint. Nevertheless, we find that values
g
ae
/10
−13
> 1.48 can be excluded with 95% confidence.
Conclusions.
The new bound we find represents the most stringent constraint for the axion-electron coupling available so far. The new scenario that emerges after this work represents a greater challenge for future experimental axion searches. In particular, we can exclude that the recent signal seen by the XENON1T experiment was due to solar axions.
In this paper, we analyze the effects induced by rotation on low-mass asymptotic giant branch stars. We compute two sets of models, M = 2.0 M sub(middot in circle) at Fe/H = 0 and M = 1.5 M ...sub(middot in circle) at Fe/H = -1.7, by adopting main-sequence rotation velocities in the range 0-120 km s super(-1). At high metallicity, we find that the Goldreich-Schubert-Fricke instability, active at the interface between the convective envelope and the rapid rotating core, contaminates the super(13)C-pocket (the major neutron source) with super(14)N (the major neutron poison), thus reducing the neutron flux available for the synthesis of heavy elements. As a consequence, the yields of heavy-s elements (Ba, La, Nd, Sm) and, to a lesser extent, those of light-s elements (Sr, Y, Zr) decrease with increasing rotation velocities up to 60 km s super(-1). However, for larger initial rotation velocities, the production of light-s and, to a lesser extent, that of heavy-s, begins again to increase, due to mixing induced by meridional circulations. At low metallicity, the effects of meridional circulations are important even at rather low rotation velocity. The combined effect of the Goldreich-Schubert-Fricke instability and meridional circulations determines an increase of light-s and, to a lesser extent, heavy-s elements, while lead is strongly reduced. For both metallicities, the rotation-induced instabilities active during the interpulse phase reduce the neutron-to-seed ratio, so that the spectroscopic indexes hs/ls and Pb/hs decrease by increasing the initial rotation velocity. Our analysis suggests that rotation could explain the spread in the s-process indexes, as observed in s-process enriched stars at different metallicities.
Recent observations of heavy elements in globular clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common ...feature of halo stars, s-process enhancements are found in a few globular clusters only. We show that the combined pollution of asymptotic giant branch (AGB) stars with a mass ranging between 3 to 6 M sub(middot in circle) may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two very different neutron-capture nucleosynthesis episodes. The first is due to the super(13)C( alpha , n) super(16)O reaction and takes place during the interpulse periods. The second is due to the super(22)Ne( alpha , n) super(25)Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Rb, Sr, Y, Zr). The first mainly operates in the less massive AGB stars, while the second dominates in the more massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for 150 + or - 50 Myr, a period short enough compared to the formation timescale of the globular cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.
By using updated stellar low-mass stars models, we systematically investigate the nucleosynthesis processes occurring in asymptotic giant branch (AGB) stars. In this paper, we present a database ...dedicated to the nucleosynthesis of AGB stars: FRANEC Repository of Updated Isotopic Tables & Yields (FRUITY). An interactive Web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up (TDU) episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 <=M/M <= 3.0 and metallicities 1 X 10--3 <= Z <= 2 X 10--2, is discussed. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parameterization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical hs/ls and Pb/hs ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the TDU efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find good agreement with observations.
High-resolution spectroscopic observations of 100 metal-poor carbon and s-rich stars (CEMP-s) collected from the literature are compared with the theoretical nucleosynthesis models of the asymptotic ...giant branch (AGB) presented in Paper I (M
AGB
ini= 1.3, 1.4, 1.5, 2 M⊙, − 3.6 ≲ Fe/H ≲− 1.5). The s-process enhancement detected in these objects is associated with binary systems: the more massive companion evolved faster through the thermally pulsing AGB phase (TP-AGB), synthesizing s-elements in the inner He intershell, which are partly dredged up to the surface during the third dredge-up (TDU) episode. The secondary observed low-mass companion became CEMP-s by the mass transfer of C- and s-rich material from the primary AGB.
We analyse the light elements C, N, O, Na and Mg, as well as the two s-process indicators, hs/ls (where ls =〈Y, Zr〉 is the the light-s peak at N = 50 and hs =〈La, Nd, Sm〉 the heavy-s peak at N = 82) and Pb/hs. We distinguish between CEMP-s with high s-process enhancement, hs/Fe >rsim 1.5 (CEMP-sII), and mild s-process enhanced stars, hs/Fe < 1.5 (CEMP-sI). To interpret the observations, a range of s-process efficiencies at any given metallicity is necessary. This is confirmed by the high spread observed in Pb/hs (∼2 dex). A degeneration of solutions is found with some exceptions: most main-sequence CEMP-sII stars with low Na/Fe can only be interpreted with M
AGB
ini= 1.3-1.4 M⊙. Giants having suffered the first dredge-up (FDU) need a dilution >rsim1 dex (dil is defined as the mass of the convective envelope of the observed star, M
obs
★, over the material transferred from the AGB to the companion, M
trans
AGB). Then AGB models with higher AGB initial masses (M
AGB
ini= 1.5-2 M⊙) are adopted to interpret CEMP-sII giants. In general, solutions with AGB models in the mass range M
AGB
ini= 1.3-2 M⊙ and different dilution factors are found for CEMP-sI stars.
About half of the CEMP-s stars with europium measurements show a high r-process enhancement (CEMP-s/r). The scenario for the origin of CEMP-s/r stars is a debated issue. We propose that the molecular cloud from which the binary system formed was previously enriched in r-process elements, most likely by local SN II pollution. This initial r-enrichment does not affect the s-process nucleosynthesis. However, for a high r-process enrichment (r/Feini= 2) the r-process contributions to solar La, Nd and Sm (30, 40 and 70 per cent) have to be considered. This increases the maximum hs/ls up to ∼0.3 dex. CEMP-s/r stars reflect this behaviour, showing higher hs/ls than observed in CEMP-s on average.
Detailed analyses for individual stars will be provided in Paper III.
Context.
Stars evolving along the asymptotic giant branch (AGB) can become carbon rich in the final part of their evolution. The detailed description of their spectra has led to the definition of ...several spectral types: N, SC, J, and R. To date, differences among them have been partially established only on the basis of their chemical properties.
Aims.
An accurate determination of the luminosity function (LF) and kinematics together with their chemical properties is extremely important for testing the reliability of theoretical models and establishing on a solid basis the stellar population membership of the different carbon star types.
Methods.
Using
Gaia
Data Release 2 (
Gaia
DR2) astrometry, we determine the LF and kinematic properties of a sample of 210 carbon stars with different spectral types in the solar neighbourhood with measured parallaxes better than 20%. Their spatial distribution and velocity components are also derived. Furthermore, the use of the infrared Wesenheit function allows us to identify the different spectral types in a
Gaia
-2MASS diagram.
Results.
We find that the combined LF of N- and SC-type stars are consistent with a Gaussian distribution peaking at
M
bol
∼ −5.2 mag. The resulting LF, however, shows two tails at lower and higher luminosities more extended than those previously found, indicating that AGB carbon stars with solar metallicity may reach
M
bol
∼ −6.0 mag. This contrasts with the narrower LF derived in Galactic carbon Miras from previous studies. We find that J-type stars are about half a magnitude fainter on average than N- and SC-type stars, while R-hot stars are half a magnitude brighter than previously found, although fainter in any case by several magnitudes than other carbon types. Part of these differences are due to systematically lower parallaxes measured by
Gaia
DR2 with respect to H
IPPARCOS
values, in particular for sources with parallax
ϖ
< 1 mas. The Galactic spatial distribution and velocity components of the N-, SC-, and J-type stars are very similar, while about 30% of the R-hot stars in the sample are located at distances greater than ∼500 pc from the Galactic plane, and show a significant drift with respect to the local standard of rest.
Conclusions.
The LF derived for N- and SC-type in the solar neighbourhood fully agrees with the expected luminosity of stars of 1.5−3
M
⊙
on the AGB. On a theoretical basis, the existence of an extended low-luminosity tail would require a contribution of extrinsic low-mass carbon stars, while the high-luminosity tail would imply that stars with mass values up to ∼5
M
⊙
may become carbon stars on the AGB. J-type stars differ significantly not only in their chemical composition with respect to the N- and SC-types, but also in their LF, which reinforces the idea that these carbon stars belong to a different type whose origin is still unknown. The derived luminosities of R-hot stars means that it is unlikely that these stars are in the red-clump, as previously claimed. On the other hand, the derived spatial distribution and kinematic properties, together with their metallicity values, indicate that most of the N-, SC-, and J-type stars belong to the thin disc population, while a significant fraction of R-hot stars show characteristics compatible with the thick disc.
Context. The abundance ratios of the main isotopes of carbon, nitrogen and oxygen are modified by the CNO-cycle in the stellar interiors. When the different dredge-up events mix the burning material ...with the envelope, valuable information on the nucleosynthesis and mixing processes can be extracted by measuring these isotope ratios. Aims. Previous determinations of the oxygen isotopic ratios in asymptotic giant branch (AGB) carbon stars were at odds with the existing theoretical predictions. We aim to redetermine the oxygen ratios in these stars using new spectral analysis tools and further develop discussions on the carbon and nitrogen isotopic ratios in order to elucidate this problem. Methods. Oxygen isotopic ratios were derived from spectra in the K-band in a sample of galactic AGB carbon stars of different spectral types and near solar metallicity. Synthetic spectra calculated in local thermodynamic equillibrium (LTE) with spherical carbon-rich atmosphere models and updated molecular line lists were used. The CNO isotope ratios derived in a homogeneous way, were compared with theoretical predictions for low-mass (1.5–3 M⊙) AGB stars computed with the FUNS code assuming extra mixing both during the RGB and AGB phases. Results. For most of the stars the 16O/17O/18O ratios derived are in good agreement with theoretical predictions confirming that, for AGB stars, are established using the values reached after the first dredge-up (FDU) according to the initial stellar mass. This fact, as far as the oxygen isotopic ratios are concerned, leaves little space for the operation of any extra mixing mechanism during the AGB phase. Nevertheless, for a few stars with large 16O/17O/18O, the operation of such a mechanism might be required, although their observed 12C/13C and 14N/15N ratios would be difficult to reconcile within this scenario. Furthermore, J-type stars tend to have lower 16O/17O ratios than the normal carbon stars, as already indicated in previous studies. Excluding these peculiar stars, AGB carbon stars occupy the same region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram, showing little spread. This reinforces the idea that these grains were probably formed in low-mass stars during the previous O-rich phases.
The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex ...internal nucleosynthesis involving p, a, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M ), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin 13C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this 13C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the 13C(a, n)16O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ~ 1.0 X 108 K, with typical densities of (106-107) neutrons cm-3. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the 13C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T ~ 1.5 X 108 K, thus producing larger neutron densities (up to 1011 neutrons cm-3). An additional neutron exposure, caused by the 22Ne(a, n)25Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the 22Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed hs/ls index observed in intrinsic C stars at different Fe/H is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like 60Fe, as a consequence of the anomalous first 13C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.
We provide an individual analysis of 94 carbon-enhanced metal-poor stars showing an s-process enrichment (CEMP-s) collected from the literature. The s-process enhancement observed in these stars is ...ascribed to mass transfer by stellar winds in a binary system from a more massive companion evolving faster towards the asymptotic giant branch (AGB) phase. The theoretical AGB nucleosynthesis models have been presented in Bisterzo et al. (Paper I of this series). Several CEMP-s show an enhancement in both s- and r-process elements (CEMP-s/r). In order to explain the peculiar abundances observed in CEMP-s/r, we assume that the molecular cloud from which CEMP-s formed was previously enriched in r-elements by supernova pollution.
A general discussion and the method adopted in order to interpret the observations have been provided in Bisterzo et al. (Paper II of this series). We present in this paper a detailed study of spectroscopic observations of individual stars. We consider all elements from carbon to bismuth, with particular attention to the three s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios hs/ls and Pb/hs. The presence of an initial r-process contribution may be typically evaluated by La/Eu. We found possible agreements between theoretical predictions and spectroscopic data. In general, the observed Na/Fe (and Mg/Fe) provides information on the AGB initial mass, while hs/ls and Pb/hs are mainly indicators of the s-process efficiency. A range of 13C-pocket strengths are required to interpret the observations. However, major discrepancies between models and observations exist. We highlight star by star the agreements and the main problems encountered and, when possible, we suggest potential indications for further studies. These discrepancies provide starting points of debate for unsolved problems in which spectroscopic and theoretical studies may intervene.
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