We present post process neutron capture computations for Asymptotic Giant Branch stars of 1.5 to 3 Mo and metallicities -1.3 to 0.1. The reference stellar models are computed with the FRANEC code, using the Schwarzschild's criterion for convection. Motivations for this choice are outlined. We assume that MHD processes induce the penetration of protons below the convective boundary, when the third dredge up occurs. There, the 13C(alpha,n)16O neutron source can subsequently operate, merging its effects with those of the 22Ne(alpha,n)25Mg reaction, activated at the temperature peaks characterizing AGB stages. This work has three main scopes. i) We provide a grid of abundance yields, as produced through our MHD mixing scheme, uniformly sampled in mass and metallicity. From it, we deduce that the solar s process distribution, as well as the abundances in recent stellar populations, can be accounted for, without the need of the extra primary like contributions suggested in the past. ii) We formulate analytical expressions for the mass of the 13C pockets generated, in order to allow easy verification of our findings. iii) We compare our results with observations of evolved stars and with isotopic ratios in presolar SiC grains, also noticing how some flux tubes should survive turbulent disruption, carrying C rich materials into the winds even when the envelope is O rich. This wind phase is approximated through the G component of AGB s processing. We conclude that MHD induced mixing is adequate to drive slow neutron capture phenomena accounting for observations. Our prescriptions should permit its inclusion into current stellar evolutionary codes.
Asymptotic giant branch (AGB) stars are considered to be among the most significant contributors to the fluorine budget in our Galaxy. While at close-to-solar metallicity observations and theory agree, at lower metallicities stellar models overestimate the fluorine production with respect to heavy elements. We present ${}^{19}$F nucleosynthesis results for a set of AGB models with different masses and metallicities in which magnetic buoyancy acts as the driving process for the formation of the ${}^{13}$C neutron source (the so-called ${}^{13}$C pocket). We find that ${}^{19}$F is mainly produc…
We present a new set of models for intermediate mass AGB stars (4.0, 5.0 and, 6.0 Msun) at different metallicities (-2.15<=Fe/H]<=+0.15). This integrates the existing set of 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. The latter is due to the fact that the interpulse phases are short and, then, Thermal Pulses are weak…
Alongside the slow (s) and rapid (r) neutron capture processes, an intermediate neutron capture process (i-process) is thought to exist. It happens when protons are mixed in a convective helium-burning zone, and is referred to as proton ingestion event (PIE). A possible astrophysical site is the asymptotic giant branch (AGB) phase of low-mass low-metallicity stars. We provide i-process yields of a grid of AGB stars experiencing PIEs. We computed 12 models with initial masses of 1, 2, and 3 $M_{\odot}$ and metallicities of [Fe/H] $=-3.0$, $-2.5$ $-2.3,$ and $-2.0, $ with the stellar evolution c…
The production of the neutron-capture isotopes beyond iron that we observe today in the solar system is the result of the combined contribution of the r-process, the s- process and possibly the i-process. Low-mass AGB (2 < M/Msun < 3) and massive (M >10 Msun ) stars have been identified as the sites of the s-process. In this work we consider the evolution and nucleosynthesis of low-mass AGB stars. We provide an update of the NuGrid Set models, adopting the same general physics assumptions but using an updated convective-boundary mixing model accounting for the contribution from internal gravit…
Revised spectroscopic parameters for the HF molecule and a new CN line list in the 2.3 mu region have been recently available, allowing a revision of the F content in AGB stars. AGB carbon stars are the only observationally confirmed sources of fluorine. Nowadays there is not a consensus on the relevance of AGB stars in its Galactic chemical evolution. The aim of this article is to better constrain the contribution of these stars with a more accurate estimate of their fluorine abundances. Using new spectroscopic tools and LTE spectral synthesis, we redetermine fluorine abundances from several …
We present an analysis and comparison of the 30 micron dust features seen in the Spitzer Space Telescope spectra of 207 carbon-rich asymptotic giant branch (AGB) stars, post-AGB objects, and planetary nebulae located in the Milky Way, the Magellanic Clouds (MCs), or the Sagittarius dwarf spheroidal galaxy (Sgr dSph), which are characterised by different average metallicities. We investigated whether the formation of the 30 micron feature carrier may be a function of the metallicity. Through this study we expect to better understand the late stages of stellar evolution of carbon-rich stars in t…
Recent spectropolarimetric observations of otherwise ordinary G, K, and M giants revealed localized magnetic strips in the HRD coincident with the regions where the first dredge-up and core He-burning occur. We seek to understand the origin of magnetic fields in such late-type giant stars. In analogy with late-type dwarf stars, we focus primarily on parameters known to influence the generation of magnetic fields in the outer convective envelope. We compute the classical dynamo parameters along the evolutionary tracks of low- and intermediate-mass stars at various metallicities using stellar mo…
A recent study reported a strong apparent depression of Fe I, relative to Fe II, in the AGB stars of NGC 6752. This depression is much greater than that expected from the neglect of non-local thermodynamic equilibrium effects, in particular the dominant effect of overionisation. Here we attempt to reproduce the apparent Fe discrepancy, and investigate differences in reported sodium abundances. We compare in detail the methods and results of the recent study with those of an earlier study of NGC 6752 AGB stars. Iron and sodium abundances are derived using Fe I, Fe II, and Na I lines. Various un…
Luminous Galactic OH/IR stars have been identified as massive (>4-5 M_s) AGB stars experiencing HBB and Li production. Their Rb abundances and [Rb/Zr] ratios derived from hydrostatic model atmospheres, are significantly higher than predictions from AGB nucleosynthesis models, posing a problem to our understanding of AGB evolution and nucleosynthesis. We report new Rb and Zr abundances in the full sample of massive Galactic AGB stars, previously studied with hydrostatic models, by using more realistic extended model atmospheres. We use a modified version of the spectral synthesis code Turbospec…
Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
We study the effects of a reduced mass-loss rate on the evolution of low metallicity Jsolated stars, following our earlier classification for angular momentum (J) isolated stars. By using the stellar evolution code MESA we study the evolution with different mass-loss rate efficiencies for stars with low metallicities of Z=0.001 and Z=0.004, and compare with the evolution with solar metallicity, Z=0.02. We further study the possibility for late asymptomatic giant branch (AGB)-planet interaction and its possible effects on the properties of the planetary nebula (PN). We find for all metallicitie…
We introduce a new generation of PARSEC-COLIBRI stellar isochrones that include a detailed treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase, and covering a wide range of initial metallicities (0.0001<Zi<0.06). Compared to previous releases, the main novelties and improvements are: use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with…
Slow neutron captures are responsible for the production of about $50\%$ of elements heavier than iron, mainly, occurring during the asymptotic giant branch phase of low-mass stars ($1$ $\lesssim M$/M$_{\odot}$ $\lesssim$ $3$), where the main neutron source is the $^{13}$C($α$,n)$^{16}$O reaction. This last is activated from locally-produced $^{13}$C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt at describing a physical mechanism for the formation of the $^{13}$C reservoir, studying the mass circulation induced by magnetic buoyancy and without…
The red-giant branch bump provides valuable information for the investigation of the internal structure of low-mass stars. Because current models are unable to accurately predict the occurrence and efficiency of mixing processes beyond convective boundaries, one can use the luminosity of the bump --- a diagnostic of the maximum extension of the convective envelope during the first-dredge up --- as a calibrator for such processes. By combining asteroseismic and spectroscopic constraints, we expand the analysis of the bump to masses and metallicities beyond those previously accessible using glob…
Evolution of Thermally Pulsing Asymptotic Giant Branch Stars III. Dust production at supersolar metallicities
我们扩展的形式主义，在我们最近的计算尘埃喷出物的热脉冲渐近巨星分支（TP-AGB）阶段，超太阳的金属丰度恒星的情况下。TP-AGB演化模型是使用COLIBRI代码计算的。我们采用我们的首选方案来控制粉尘的增长。对于M-巨星，我们忽略了H $_2$分子的化学溅射，对于C-恒星，我们假设一个均匀的生长方案，主要是由碳超过氧过剩控制。在超太阳的金属丰度下，尘埃形成的效率更高，硅酸盐倾向于在更接近光球层的地方（r~1.5 R $_*$）--因此温度和密度更高--比在太阳和亚太阳的金属丰度下（r~2-3 R $_*$）更容易凝结。在这种情况下，气体和尘埃之间的热解耦的假设变得可疑，而尘埃加热由于碰撞起着重要的作用。加热机制延迟了尘埃凝结到星周包层稍靠外的区域。我们发现，同样的机制是不显着的太阳和亚太阳的金属丰度。超太阳金属丰度的主要尘埃产物是硅酸盐。我们计算了总的尘埃喷出物和尘埃气体喷出物，恒星的初始质量和初始金属丰度Z=0.04，0.06的各种值。将这些新的计算与低金属丰度的计算相结合，结果表明，与通常假设的相反，中等质量恒星的尘埃到气体的总喷出物对初始金属含量的依赖性很弱。
This paper is part of a larger project in which we systematically study the chemical abundances of Galactic and extragalactic post-asymptotic giant branch (post-AGB) stars. Lead (Pb) is the final product of the s-process nucleosynthesis and is predicted to have large overabundances with respect to other s-process elements in AGB stars of low metallicities. However, Pb abundance studies of enriched post-AGB stars in the Magellanic Clouds show the Pb overabundance is not observed. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of 14 Galactic…
The atmospheres and winds of C-type AGB stars are modeled with the 1D spherically symmetric radiation-hydrodynamical code DARWIN. To explore the metallicity-dependence of mass loss we calculate model grids at three different chemical abundances. Since carbon may be dredged up during the thermal pulses as AGB stars evolve, we keep the carbon abundance as a free parameter. The models in these three different grids all have a current mass of one solar mass; effective temperatures of 2600K, 2800K, 3000K, or 3200K; and stellar luminosities equal to log(L/Lsun)=3.70, 3.85, or 4.00. The models show t…
The most metal-rich stars in the universe: chemical contributions of low and intermediate mass asymptotic giant branch stars with metallicities between $0.04 \leq Z \leq 0.10.$
We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero age main sequence until near the end of the thermally pulsing phase. These super-AGB and massive AGB star models are in the mass range between 5.0 and 10.0 Msun for metallicities spanning the range Z=0.02-0.0001. We probe the mass limits M_up, M_n and M_mass, the minimum masses for the onset of carbon burning, the formation of a neutron star, and the iron core-collapse supernovae respectively, to constrain the white dwarf/electron-capture supernova boundary. We provide a theoretical…
The origin of fluorine is a longstanding problem in nuclear astrophysics. It is widely recognized that Asymptotic Giant Branch (AGB) stars are among the most important contributors to the Galactic fluorine production. In general, extant nucleosynthesis models overestimate the fluorine production by AGB stars with respect to observations. In this paper we review the relevant nuclear reaction rates involved in the fluorine production/destruction. We perform this analysis on a model with initial mass M=2 M$_\odot$ and Z=0.001. We found that the major uncertainties are due to the $^{13}$C($α$,n)$^…
We study the evolution of extremely metal-poor AGB stars, with metallicities down to [Fe/H]=-5, to understand the main evolutionary properties, the efficiency of the processes able to alter their surface chemical composition and to determine the gas and dust yields. We calculate two sets of evolutionary sequences of stars in the 1-7.5Msun mass range, evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries we adopted the metallicities Z=3x10^{-5} and Z=3x10^{-7} which correspond, respectively to [Fe/H]=-3 and [Fe/H]=-5. The results from st…
Chemical Signatures of AGB Mass Transfer in Gaia White Dwarf Companions
California Institute of Technology（加州理工学院）；Gemini Observatory, NSF’s NOIRLab（双子座天文台，美国国家光学-红外天文研究实验室）；Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）
(Abridged) The chemical behaviour of an ample sample of PNe in NGC6822 is analyzed. Spectrophotometric data of 11 PNe and two H II regions were obtained with the OSIRIS spectrograph attached to the Gran Telescopio Canarias. Data for other 13 PNe and three H II regions were retrieved from the literature. Physical conditions and chemical abundances of O, N, Ne, Ar and S were derived for 19 PNe and 4 H II regions. Abundances in the PNe sample are widely distributed showing 12+log(O/H) from 7.4 to 8.2 and 12+log(Ar/H) from 4.97 to 5.80. Two groups of PNe can be differentiated: one old, with low me…
Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy
本文介绍了用\it斯皮策太空望远镜对人马座矮球状星系（Sgr dSph）中六颗富碳AGB星和两颗前景银河系碳星的光谱观测结果。观测到的C $_2$ H $_2$和SiC特征的能带强度与在银河系AGB星中观测到的非常相似。的金属度估计从乙炔光学深度和SiC功能的强度之间的经验关系。金属丰度高于大麦哲伦星云，接近银河系。虽然高金属丰度可能意味着大约1 Gyr的年龄，但对于尘埃AGB恒星，脉动周期表明年龄超过2或3 Gyr。我们用DUSTY辐射转移模型拟合了观测到的恒星的光谱，并确定它们的尘埃质量损失率在1.0- 3.3 $\times 10^{-8} $ M $_{\odot}$ yr $^{-1}$的范围内。银河系前景的两颗富碳AGB恒星位于太阳圈的远端，在银河系中心之外。这两颗星中的一颗在我们目前的本星系群样本中显示出最强的SiC特征。
Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) stars are relatively short lived (less than a few Myr), yet their cool effective temperatures, high luminosities, efficient mass-loss and dust production can dramatically effect the chemical enrichment histories and the spectral energy distributions of their host galaxies. The ability to accurately model TP-AGB stars is critical to the interpretation of the integrated light of distant galaxies, especially in redder wavelengths. We continue previous efforts to constrain the evolution and lifetimes of TP-AGB stars by modeling their underlying st…
The NuGrid AGB Evolution and Nucleosynthesis Data Set
University of Hull（赫尔大学）；Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH)（孔科利天文台，匈牙利科学院地球科学研究中心，厄特沃什·罗兰研究网络）；Joint Institute for Nuclear Astrophysics, Center for the Evolution of the Elements, Michigan State University（密歇根州立大学核天体物理联合研究所元素演化中心）；University of Edinburgh（爱丁堡大学）；University of Victoria（维多利亚大学）
In the last ten years three main facts about the thermally pulsing asymptotic giant branch (TP-AGB) have become evident: 1) the modelling of the TP-AGB phase is critical for the derivation of basic galaxy properties (e.g. mass and age) up to high redshift, with consequent cosmological implications; 2) current TP-AGB calibrations based on Magellanic Cloud (MC) clusters come out not to work properly for other external galaxies, yielding a likely TP-AGB overestimation; 3) the significance of the TP-AGB contribution in galaxies, hence their derived properties, are strongly debated, with conflictin…
We discuss the self-enrichment scenario by AGB stars for the formation of multiple populations in globular clusters (GCs) by analyzing data set of giant stars observed in 9 Galactic GCs, covering a wide range of metallicities and for which the simultaneous measurements of C, N, O, Mg, Al, Si are available. To this aim we calculated 6 sets of AGB models, with the same chemical composition as the stars belonging to the first generation of each GC. We find that the AGB yields can reproduce the set of observations available, not only in terms of the degree of contamination shown by stars in each G…
We study the effects of neutron captures in AGB stars on \oq Fe-group\cqb elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in $^{54}$Cr, $^{58}$Fe, and $^{64}$Ni in solar-system materials, which are commonly attributed to SNe. However, as large fractions of the interstellar medium (ISM) were reprocessed in AGB stars, these elements were reprocessed, too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through 1.5\msb and 3\msb AGB models, adopting solar and 1/3 solar metallicities. All cases produce excesses of $^{54}$Cr, $^{58}$Fe, and $^{64}$Ni, whi…
Context: Stars evolving through the asymptotic giant branch (AGB) phase provide significant feedback to their host system, in form of both gas enriched in nuclear-burning products and dust formed in their winds, which they eject into the interstellar medium. Therefore AGB stars are an essential ingredient for the chemical evolution of the Milky Way and other galaxies. Aims: We study AGB models with super-solar metallicity, to complete our large database, so far extending from metal-poor to solar chemical compositions. We provide chemical yields for masses in the range 1-8 Msun and metallicitie…
We present asymptotic giant branch (AGB) models of metallicity $Z=10^{-4}$ and $Z=3\times 10^{-4}$, with the aim of understanding how the gas enrichment and the dust production change in very metal-poor environments and to assess the general contribution of AGB stars to the cosmic dust yield. The stellar yields and the dust produced are determined by the change in the surface chemical composition, with a transition occurring at $\sim 2.5~M_{\odot}$. Stars of mass $M < 2.5~M_{\odot}$ reach the carbon stage and produce carbon dust, whereas their higher mass counterparts produce mainly silicates …
We analyze a set of published elemental abundances from a sample of CH stars which are based on high resolution spectral analysis of ELODIE and SUBARU/HDS spectra. All the elemental abundances were derived from local thermodynamic equilibrium analysis usingmodel atmospheres, and thus, they represent the largest homogeneous abundance data available for CH stars up to date. For this reason, we can use the set to constrain the physics and the nucleosynthesis occurring in low mass AGB stars. CH stars have been polluted in the past from an already extinct AGB companion and thus show s-process enric…
To understand the formation and evolution of the different stellar populations within our Galaxy it is essential to combine detailed kinematical and chemical information for large samples of stars. We derive chemical abundances of Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd and Eu for a large sample of more than 1000 FGK dwarf stars with high-resolution ($R \sim$\,115000) and high-quality spectra from the HARPS-GTO program. The abundances are derived by a standard Local Thermodinamyc Equilibrium (LTE) analysis using measured Equivalent Widths (EWs) injected to the code MOOG and a grid of Kurucz ATLAS9 atmos…
Post-AGB stars are exquisite probes of AGB nucleosynthesis. However, the previous lack of accurate distances jeopardised comparison with theoretical AGB models. The $Gaia$ Early Data Release 3 ($Gaia$ EDR3) has now allowed for a breakthrough in this research landscape. In this study, we focus on a sample of single Galactic post-AGBs for which chemical abundance studies were completed. We combined photometry with geometric distances to carry out a spectral energy distribution (SED) analysis and derive accurate luminosities. We subsequently determined their positions on the HR-diagram and compar…
在中红外波段，渐近巨星分支变星的周期-光度关系与K_S（2.14微米）相似，即使在24微米处，星周尘埃的辐射占主导地位。LMC和NGC 6522的M，logP图上的位点基本相同，尽管年龄和金属丰度不同。斜率随波长的变化没有系统的趋势。视星等与logP关系的偏移量意味着两个场之间的距离模数差为3.8。变量的颜色证实log P > 1.75的主周期是可检测质量损失的必要条件。在观测到的最长波长24微米处，许多半规则变星的尘埃壳层的光度与Miras周围的尘埃壳层相当。在涉及24微米星等的LMC颜色星等图中有一个明显的分歧。
We present a near-complete catalog of the metal-rich population of Thermally-Pulsing Asymptotic Giant Branch stars in the northwest quadrant of M31. This metal-rich sample complements the equally complete metal-poor Magellanic Cloud AGB catalogs produced by the SAGE program. Our catalog includes HST wide-band photometry from the Panchromatic Hubble Andromeda Treasury survey, HST medium-band photometry used to chemically classify a subset of the sample, and Spitzer mid- and far-IR photometry that we have used to isolate dust-producing AGB stars. We have detected 346,623 AGB stars; these include…
Context: This paper is part of a larger project in which we study the chemical abundances of extra-galactic post-AGB stars with the ultimate goal of improving our knowledge of the poorly understood AGB third dredge-up mixing processes and s-process nucleosynthesis. Aims: In this paper, we study two carefully selected post-AGB stars in the LMC. The combination of favourable atmospheric parameters for detailed abundance studies and their known distances make these objects ideal probes of the internal AGB third dredge-up and s-process nucleosynthesis in that they provide observational constraints…
We have performed detailed high-resolution spectroscopic analysis on seven metal-poor stars (BD+75 348, BD+09 3019, HD238020, HE0319-0215, HE0507-1653, HE0930-0018, HE1023-1504) and derived their atmospheric parameters T$_{eff}$, log$g$, [Fe/H], and mictroturbulent velocity ($ξ$). The metallicity range is found to be -2.57$<$[Fe/H]$<$-0.42. The elemental abundances of 17 light elements and 12 heavy elements are estimated. We have classified BD+75 348 and BD+09 3019 as strong Ba stars, HD238020 as a mild Ba star, and the remaining four objects as CEMP-s stars. We have estimated the masses of th…
On the alumina dust production in the winds of O-rich Asymptotic Giant Branch stars
富氧渐近巨星分支（AGB）恒星经历了强烈的质量损失和有效的尘埃凝聚，它们是星际介质中尘埃的主要来源。氧化铝尘埃（Al $_2$ O $_3$）是富氧拱壳层中的一种重要尘埃成分，在质量较大且富氧的AGB星的星风中含量相当丰富。通过耦合AGB恒星核合成和尘埃的形成，我们提出了一个自洽的探索AGB星的风与祖质量之间的$\sim$ 3和7 M $_{\odot}$和金属丰度范围内的0.0003 $\le$ Z $\le$ 0.018的Al $_2$ O $_3$生产。我们发现，Al $_2$ O $_3$粒子形成在径向距离中心之间的$\sim2$和4 R $_*$（取决于金属丰度），这是在最近的银河系富O AGB星的干涉观测。铝$_2$ O $_3$尘埃的质量几乎与金属丰度成线性关系，太阳金属丰度AGB产生的氧化铝尘埃量最高（约10 $^{-3}$ M $_{\odot}$）。Al $_2$ O $_3$晶粒尺寸随着金属丰度（和初始恒星质量）的减小而减小，对于太阳金属丰度模型，Al $_2$ O $_3$晶粒的最大尺寸为$\sim$ 0.075 $μm$。有趣的是，在低金属丰度的HBB AGB恒星HV 2576中观察到的气态Al的强烈耗尽似乎与我们的模型预测的Al $_2$ O $_3$尘埃的形成一致。我们建议，铝的含量可以作为一个质量（和演化阶段）的指标，在AGB星经历HBB。
