Neutron-capture Elements Research Articles

Observational constraints on the origin of the elements

Context. The chemical evolution history of slow neutron-capture elements in the Milky Way is still a matter of debate, especially in the metal-poor regime ([Fe/H] < −1). Aims. Based on Gaia-ESO spectroscopic data, a recent study investigated the chemical evolution of neutron-capture elements in the regime [Fe/H] > −1. Here, we aim to complement this study down to [Fe/H] = −3, and focus on Ba, Y, and Sr, along with the abundance ratios of [Ba/Y] and [Sr/Y], which give comprehensive views on s-process nucleosynthesis channels. Methods. We measured the local thermodynamic equilibrium (LTE) and non-local thermodynamic equilibrium (NLTE) abundances of Ba, Y, and Sr in 323 Galactic metal-poor stars using high-resolution optical spectra with high signal-to-noise ratios. We used the spectral fitting code TSFitPy together with 1D model atmospheres, using previously determined LTE and NLTE atmospheric parameters. Results. We find that the NLTE effects are on the order of ∼ − 0.1 to ∼0.2 dex, depending on the element. We find that stars enhanced (deficient) in [Ba/Fe] and [Y/Fe] are also enhanced (deficient) in [Sr/Fe], suggesting a common evolution channel for these three elements. We find that the ratio between heavy and light s-process elements [Ba/Y] varies weakly with [Fe/H] even in the metal-poor regime, which is consistent with the behaviour in the metal-rich regime. The [Ba/Y] scatter at a given metallicity is larger than the abundance measurement uncertainties. Homogeneous chemical evolution models with different yield prescriptions are not able to accurately reproduce the [Ba/Y] scatter in the low-[Fe/H] regime. Adopting the stochastic chemical evolution model by Cescutti & Chiappini allows us to reproduce the observed scatter in the abundance pattern of [Ba/Y] and [Ba/Sr]. Based on our observations, we have ruled out the need for an arbitrary scaling of the r-process contribution, as previously suggested by the authors behind the construction of the model. Conclusions. We show how important it is to properly include NLTE effects when measuring chemical abundances, especially in the metal-poor regime. This work demonstrates that the choice of the Galactic chemical evolution model (stochastic versus one-zone) is key when comparing models to observations. Upcoming large-scale spectroscopic surveys such as 4MOST and WEAVE are poised to deliver high-quality data for many thousands of metal-poor stars and this work gives a typical case study of what could be achieved with such surveys in the future.

Neutron-capture elements in a sample of field metal-poor N-rich dwarfs

Context. The aim of this work is to measure the abundances of n-capture elements in a sample of six metal-poor N-rich dwarfs that were formed in globular clusters, and subsequently became unbound from the cluster. These N-rich stars, HD 25329, HD 74000, HD 160617, G 24-3, G53-41, and G90-3, were previously studied in Paper I. Aims. The abundances of the n-capture elements in these stars were compared to the abundances in normal metal-poor dwarfs and in globular cluster stars in the same metallicity range in order to find evidence of an enrichment of the material from which these N-rich stars were formed, by the ejecta of massive asymptotic giant branch stars (AGB) inside the cluster. Methods. The abundances of 15 elements, from Sr to Yb, were derived line by line by comparing the observed profiles to synthetic spectra in a sample of six metal-poor N-rich dwarfs and nine classical metal-poor dwarfs. Results. We show that, generally speaking, the behaviours of the intermediate metal-poor stars here studied and the extremely metal-poor stars are very different. In particular, the scatter of the [X/Fe] ratios is much smaller since many more stars contribute to the enrichment. Among our six metal-poor N-rich stars, three stars (G24-3 and HD 74000 and maybe also HD 160617) present an enrichment in elements formed by the s-process, typical of a contribution of AGB stars, unexpected at the metallicity of these stars. This suggests that the intracluster medium from which these stars were formed was enriched by a first generation of massive AGB stars. Another N-rich star, G53-41, is also rich in s-process elements, but since it is more metal-rich this could be due to the normal galactic enrichment by low-mass AGB stars before the formation of the cluster. In contrast, two stars (G 90-3 and HD 25329) have an abundance pattern compatible with a pure r-process such as that seen in metal-poor stars with [Fe/H] < −1.5.

High-resolution spectroscopy of the young open cluster M 39 (NGC 7092)

M 39 is a nearby young open cluster hardly studied in the last few decades. No giant is known among its members and its chemical composition has never been studied. In order to investigate it, we performed high-resolution spectroscopy of 20 expected cluster members with the HARPS and FIES spectrographs. By combining our observations with archival photometry and Gaia-DR3 data we searched for evolved members and studied cluster properties such as the radial velocity, extinction, and age. For the first time, we provide stellar parameters and chemical abundances for 21 species with atomic numbers up to 56. We have not found any new giant as a likely member and we notice a negligible reddening along the cluster field, which we place at 300 pc. We obtain a mean radial velocity for M 39 of −5.5 ± 0.5 km s−1 and an isochrone-fitting age of 430 ± 110 Ma, which corresponds to a MSTO mass of around 2.8 M⊙. This value is consistent with the Li content and chromospheric activity shown by its members. Based on main-sequence stars, the cluster exhibits a solar composition, [Fe/H] = +0.04 ± 0.08 dex, compatible with its Galactocentric location. However, it has a slightly subsolar abundance of Na and an enriched content of neutron-capture elements, specially Ba. In any case, the chemical composition of M 39 is fully compatible with that shown by other open clusters that populate the Galactic thin disc.

Study of the Astrophysical Origins of Neutron-captue Elements in CEMP Star HE 1005-1439

HE 1005-1439 is a carbon-enhanced metal-poor (CEMP) star with [Fe/H]∼−3.0, which is largely enhanced in elements produced by slow neutron-capture process (s-process; e.g., [Ba/Fe]=1.16±0.31, [Pb/Fe]=1.98±0.19), and mildly enhanced in elements produced by rapid neutron-capture process (r-process; e.g., [Eu/Fe]=0.46±0.22). The neutron-capture element patterns of the star can not be fitted by the s-process model or intermediate process (i-process) model. Studying the astrophysical origins of chemical elements in the star based on the method of abundance decomposition can help understanding the formation and evolution of the CEMP stars. Using the combination of s-process and r-process abundances, we fully fit the abundance patterns of the neutron-capture elements of this star. We find that the neutron-capture elements are mainly produced by the s-process of the AGB companion with low mass and low metallicity, and the r-process nucleosynthesis also contributes a little to this star.

Chemodynamic Analysis of Four r-Process Enhanced Stars Observed with GTC

Here, we delineate a comprehensive abundance analysis of four r-process enhanced metal-poor stars observed with HORuS spectrograph on a 10-m class telescope, GTC. The high signal-to-noise ratio at R≈25000 spectral resolution allowed us to detect 16 light and 20 neutron-capture elements along with Th in two stars. Four of our program stars show signatures of mixing in their atmosphere. Through detailed abundance analysis of four r-process enhanced stars together with already identified r-process-rich stars in literature, we probe the production sites of neutron-capture elements. The [Zr/Eu] ratio as a function of metallicity shows the evidence of multiple channels for the production of r-process. Thorium to first and second r-process peak elements ratios also support the similar non-universality of neutron-capture elements. An increased sample of r-process enhanced stars will enable us understand different formation channels of neutron capture elements. Using the kinematic analysis, we found the clues of accretion for two of our program stars.

Tracing Population III supernovae with extreme energies through the Sculptor dwarf spheroidal galaxy

The Sculptor dwarf spheroidal galaxy is old and metal-poor, making it ideal to study the earliest chemical enrichment in the Local Group. We followed up on the most metal-poor star known in this (or any external) galaxy, AS0039, with high-resolution ESO VLT/UVES spectra. Our new analysis confirmed its low metallicity, [Fe/H]LTE = −3.90 ± 0.15, and that it is extremely C-poor, with A(C) = + 3.60, which corresponds to [C/Fe]LTE = −0.33 ± 0.17 (accounting for internal mixing). This adds to the evidence of Sculptor being intrinsically C-poor at low [Fe/H] ≲ −3. However, here we also report a new discovery of a carbon-enhanced metal-poor (CEMP-no) star in Sculptor, DR20080, with no enhancement of Ba, indicative of enrichment by zero-metallicity low-energy supernovae, ESN < 1 × 1051. This is the first piece of evidence of a dual population of CEMP-no and C-normal stars in Sculptor at [Fe/H] ≤ −3. The fraction of CEMP-no stars is still low, fCEMPScl = 9−8+11% at −4 ≤ [Fe/H] ≤ −3, compared to the significantly higher fraction in the Milky Way halo, fCEMPMW ≈ 40%. To further investigate the early chemical enrichment of Sculptor, we re-derived chemical abundances of light, α, iron-peak, and neutron-capture elements in all Sculptor stars at [Fe/H] ≤ −2.8, with available high-resolution spectra. Our results show that at these low [Fe/H], Sculptor is deficient in light elements (e.g. C, Na, Al, Mg) relative to both the Milky Way halo, and ultra-faint dwarf galaxies, pointing towards a significant contribution from high-energy supernovae. Furthermore, the abundance pattern of the star AS0039 is best fitted with a zero-metallicity hypernova progenitor, ESN = 10 × 1051, with a mass of M = 20 M⊙. Our results in Sculptor, at [Fe/H] ≤ −3, therefore suggest significant enrichment by both very low-energy supernovae and hypernovae, solidifying this galaxy as one of the benchmarks for understanding the energy distribution of the first supernova in the Universe.

Formation Scenarios of Ba Stars: New Evidence from the Masses of the Companion AGBs

The abundances of the slow neutron-capture (s-) process elements observed in barium stars can be explained by considering mass transfer in a binary system from an asymptotic giant branch (AGB) star onto a smaller mass and less evolved binary companion star. Based on the abundances of neutron-capture elements, the barium stars are broadly divided into two categories: “strong” and “mild” barium stars. However, an understanding of the role of the characteristic properties of the companion AGBs on the formation of the different types of barium stars is still lacking. This work focuses on investigating the role of the masses of the companion AGB stars in this context. In a recent high-resolution spectroscopic study, we studied in detail the chemical composition of a few barium stars that we have identified. In order to understand the nature and the mass of the companion stars, we first estimated the masses of these objects and then used a parametric model-based analysis to derive the masses of the primary companion stars. The calculation is extended to 205 barium stars taken from literature. An analysis of the mass distribution revealed that both the strong and the mild barium stars occupy the same range of masses. However, their companion AGB stars’ mass distributions peak at two different values of 2.5M⊙ and 3.7M⊙, for the strong and the mild barium stars, respectively. This provides clear evidence that the formation of mild and strong Ba stars is greatly influenced by the initial masses of the companion AGB stars. However, the possible impact of other factors, such as orbital period and metallicity, on the formation scenarios of barium stars remains to be seen.

Peculiar Abundances of the Cool Carbon Star HE 1104-0957

We present, for the first time, a detailed abundance analysis of the carbon star HE 1104−0957 based on high resolution (R ≃ 50 000) spectra. Our analysis shows that the object is an extremely metal-poor star with [Fe/H] ≃ −2.96. We find that the object shows enhancement of carbon with [C/Fe] ≃ 1.82. However, it does not fall into any of the sub-groups of carbon-enhanced metal-poor (CEMP) stars based on the characteristic elemental abundance ratios used for the classification of various CEMP sub-groups. HE 1104−0957 is also found to exhibit an enhancement of oxygen and nitrogen with [O/Fe], and [N/Fe] ≃ 1.54, and 2.54 respectively. In HE 1104−0957, α-elements are found to be slightly enhanced with [α/Fe] ≃ 0.46. Fe-peak elements are also moderately enhanced in HE 1104−0957 with a value 0.63 with respect to Fe. Our analysis shows that HE 1104−0957 exhibits enhancement of neutron-capture elements, particularly r-process elements. The low-resolution spectra of this object shows the spectral features characteristics of a typical C-R star. However, we find that the surface chemical compositions of this object is contradictory to that expected for a C-R star. It requires a detailed analysis to better understand the abundance anomalies exhibited by this object.

Tracing the origin of neutron-capture elements in the metalpoor star HE 1244-3036

Tracing the origin of neutron-capture elements in the metalpoor star HE 1244-3036

Metal-poor stars observed with the Magellan Telescope – IV. Neutron-capture element signatures in 27 main-sequence stars

ABSTRACT We present a chemo-dynamical analysis for 27 near main-sequence turnoff metal-poor stars, including 20 stars analysed for the first time. The sample spans a range in [Fe/H] from −2.5 to −3.6, with 44 per cent having [Fe/H]<−2.9. We derived chemical abundances for 17 elements, including strontium and barium. We derive Li abundances for the sample, which are in good agreement with the ‘Spite Plateau’ value. The lighter elements (Z < 30) generally agree well with those of other low-metallicity halo stars. This broadly indicates chemically homogeneous gas at the earliest times. We used the [Sr/Ba] versus [Ba/Fe] diagram to classify metal-poor stars into five populations based on their observed ratios. We find HE 0232 − 3755 to be a likely main r-process star, and HE 2214 − 6127 and HE 2332 − 3039 to be limited-r stars. CS30302-145, HE 2045 − 5057, and CD −24°17504 plausibly originated in long-disrupted early dwarf galaxies. We also find that the derived [Sr/H] and [Ba/H] values for CD −24°17504 are not inconsistent with the predicted yields of the s-process in massive rotating low-metallicity stars models. Further theoretical explorations will be helpful to better understand the earliest mechanisms and time scales of heavy element production for comparison with these and other observational abundance data. Finally, we investigate the orbital histories of our stars. Most display halo-like kinematics although three stars (CS 29504-018, HE 0223 − 2814, and HE 2133 − 0421) appear to be disc-like in nature. This confirms the extragalactic origin for CS 30302-145, HE 2045 − 5057, and, in particular, CD −24°17504 which likely originated from a small accreted stellar system as one of the oldest stars.

Observational Studies of r- and s-Process Elements for Milky Way Stars

Elements heavier than iron-group are produced by rapid and slow neutron-capture processes (r-process and s-process) in the universe. The site of the processes and history of the enrichment in the Milky Way Galaxy have been studied by spectroscopic observations of stars with different ages and population (Galactic disks, halo etc.). The abundance studies of neutron-capture elements for Milky Way stars, including our long-term observational efforts using the spectroscopic survey telescope LAMOST and the 8.2m Subaru Telescope are reviewed. This article covers the observational results on the abundance distributions of heavy elements (Sr, Ba and Eu), and elemental abundance patterns of r- and s-process enhanced stars.

Chemical Analysis of the Brightest Star of the Cetus II Ultrafaint Dwarf Galaxy Candidate* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

We present a detailed chemical abundance analysis of the brightest star in the ultrafaint dwarf (UFD) galaxy candidate Cetus II from high-resolution Magellan/MIKE spectra. For this star, DES J011740.53-173053, abundances or upper limits of 18 elements from carbon to europium are derived. Its chemical abundances generally follow those of other UFD galaxy stars, with a slight enhancement of the α-elements (Mg, Si, and Ca) and low neutron-capture element (Sr, Ba, and Eu) abundances supporting the classification of Cetus II as a likely UFD. The star exhibits lower Sc, Ti, and V abundances than Milky Way (MW) halo stars with similar metallicity. This signature is consistent with yields from a supernova originating from a star with a mass of ∼11.2 M ⊙. In addition, the star has a potassium abundance of [K/Fe] = 0.81, which is somewhat higher than the K abundances of MW halo stars with similar metallicity, a signature that is also present in a number of UFD galaxies. A comparison including globular clusters and stellar stream stars suggests that high K is a specific characteristic of some UFD galaxy stars and can thus be used to help classify objects as UFD galaxies.

2D chemical evolution models

Context. According to observations and numerical simulations, the Milky Way could exhibit several spiral-arm modes of various pattern speeds, with the slower patterns located at larger galactocentric distances. Aims. Our aim is to quantify the effects of the spiral arms on the azimuthal variations in the chemical abundances of oxygen and iron and for the first time of neutron-capture elements (europium and barium) in the Galactic disc. We assume a model based on multiple spiral-arm modes with different pattern speeds. The resulting model is an updated version of previous 2D chemical evolution models. Methods. We apply new analytical prescriptions for the spiral arms in a 2D Galactic disc chemical evolution model, exploring the possibility that the spiral structure is formed by the overlap of chunks with different pattern speeds and spatial extent. Results. The predicted azimuthal variations in abundance gradients are dependent on the considered chemical element. Elements synthesised on short timescales (i.e. oxygen and europium in this study) exhibit larger abundance fluctuations. Moreover, for progenitors with short lifetimes, the chemical elements returned to the ISM perfectly trace the star formation perturbed by the passage of the spiral arms. The map of the star formation rate (SFR) predicted by our chemical evolution model with multiple patterns of spiral arms presents arcs and arms compatible with those revealed by multiple tracers (young upper-main sequence stars, Cepheids, and the distribution of stars with low radial actions). Finally, our model predictions are in good agreement with the azimuthal variations that emerged from the analysis of Gaia DR3 GSP-Spec [M/H] abundance ratios, if at most recent times the pattern speeds match the Galactic rotational curve at all radii. Conclusions. We provide an updated version of a 2D chemical evolution model capable of tracing the azimuthal density variations created by the presence of multiple spiral patterns. We show that elements synthesised on short timescales exhibit larger abundance fluctuations.

Chemodynamical study of two CEMP-no stars from the Hamburg/ESO Survey

ABSTRACT The Carbon-Enhanced Metal-Poor (CEMP) stars with no enhancement of neutron-capture elements, the so-called CEMP-no stars are believed to be the direct descendants of first-generation stars and provide a unique opportunity to probe the early Galactic nucleosynthesis. We present a detailed chemical and kinematic analysis for two extremely metal-poor stars HE 1243 − 2408 and HE 0038 − 0345 using high-resolution (R∼86 000) HERMES spectra. For the object HE 1243 − 2408, we could make a detailed comparison with the available literature values; however, only limited information is available for the other object HE 0038 − 0345. Our estimated metallicity for these two objects are −3.05 and −2.92, respectively. With estimated [C/Fe] (1.03 and 1.05) and [Ba/Fe] (−0.18 and −0.11), respectively, the objects are found to be bonafide CEMP-no stars. From the observed abundances of C, Na, Mg, and Ba (i.e. A(C), A(Na), A(Mg), A(Ba)), the objects are found to belong to Group II CEMP-no stars. A detailed abundance profile analysis indicates that the objects are accreted from dSph satellite galaxies that support hierarchical galaxy assembly. Further, our analysis shows that the progenitors of the stars are likely Pop II Core-Collapse Supernovae. The object HE 0038 − 0345 is found to be a high-energy, prograde, outer-halo object, and HE 1243 − 2408 is found to be a high-energy, retrograde, inner-halo object. Our detailed chemodynamical analysis shows that HE 1243 − 2408 is related to I’itoi structure, where as HE 0038 − 0345 is likely related to Sgr or GSE events. The mass of the progenitor galaxies of the programme stars inferred from their dynamics is at par with their likely origin in massive dSph galaxies.

R-process Abundance Patterns in the Globular Cluster M92

Whereas light-element abundance variations are a hallmark of globular clusters, there is little evidence for variations in neutron-capture elements. A significant exception is M15, which shows a star-to-star dispersion in neutron-capture abundances of at least one order of magnitude. The literature contains evidence both for and against a neutron-capture dispersion in M92. We conducted an analysis of archival Keck/HIRES spectra of 35 stars in M92, 29 of which are giants, which we use exclusively for our conclusions. M92 conforms to the abundance variations typical of massive clusters. Like other globular clusters, its neutron-capture abundances were generated by the r-process. We confirm a star-to-star dispersion in r-process abundances. Unlike M15, the dispersion is limited to “first-generation” (low-Na, high-Mg) stars, and the dispersion is smaller for Sr, Y, and Zr than for Ba and the lanthanides. This is the first detection of a relation between light-element and neutron-capture abundances in a globular cluster. We propose that a source of the main r-process polluted the cluster shortly before or concurrently with the first generation of star formation. The heavier r-process abundances were inhomogeneously distributed while the first-generation stars were forming. The second-generation stars formed after several crossing times (∼0.8 Myr); hence, the second generation shows no r-process dispersion. This scenario imposes a minimum temporal separation of 0.8 Myr between the first and second generations.

The Metallicity Variations Along the Chromosome Maps: The Globular Cluster 47 Tucanae* *Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program 105.20NB, and on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

The “chromosome maps” (ChMs) of globular clusters (GCs) have revealed that these ancient structures are inhomogeneous in metallicity in various ways and in different natures. Type II GCs generally display larger variations, sometimes coupled with slow neutron-capture (s) element enrichments on the ChMs redder sequences, which have been interpreted as due to multiple generations of stars. On the other hand, most GCs have inhomogeneous first populations (1P) in the form of large ranges in the ΔF275W,F814W values, pointing toward a not fully mixed pristine molecular cloud. We analyze the chemical composition of GC 47 Tucanae, which shows both inhomogeneous 1P stars and, although not formally a Type II GC, hosts a small number of stars distributed on the red side of the main stream of ChM stars. Our results suggest that 1P stars are inhomogeneous in the overall metallicity, with variations on the order of ∼0.10 dex in all the chemical species. The anomalous stars distributed on a redder sequence of the ChM are further enriched in metals but have no evidence for a significant enrichment in the s elements. Our three second population stars located on the normal component of the map have metallicities similar to those of the metal-richer 1P oup, suggesting that this population formed from these stars. Although three stars is a too-small sample to draw strong conclusions, the low spread in metals of these objects might point toward formation in a fully mixed medium, possibly after a cooling flow phase.

The 12C/13C isotopic ratio at the dawn of chemical evolution

Context. The known mega metal-poor (MMP) and hyper metal-poor (HMP) stars, with [Fe/H] < −6.0 and < −5.0, respectively, likely belong to the CEMP-no class, namely, carbon-enhanced stars with little or no second peak neutron-capture elements. They are likely second-generation stars, and the few elements measurable in their atmospheres are used to infer the properties of a single or very few progenitors. Aims. The high carbon abundance in the CEMP-no stars offers a unique opportunity to measure the carbon isotopic ratio, which directly indicates the presence of mixing between the He- and H-burning layers either within the star or in the progenitor(s). By means of high-resolution spectra acquired with the ESPRESSO spectrograph at the VLT, we aim to derive values for the 12C/13C ratio at the lowest metallicities. Methods. We used a spectral synthesis technique based on the SYNTHE code and on ATLAS models within a Markov chain Monte Carlo methodology to derive 12C/13C in the stellar atmospheres of four of the most metal-poor stars known: the MMP giant SMSS J0313–6708 ([Fe/H] < −7.1), the HMP dwarf HE 1327–2326 ([Fe/H] = −5.8), the HMP giant SDSS J1313–0019 ([Fe/H] = −5.0), and the ultra metal-poor subgiant HE0233 -0343 ([Fe/H] = −4.7). We also revised a previous value for the MMP giant SMSS J1605–1443 ([Fe/H] = −6.2). Results. In four stars we derive an isotopic value while for HE 1327–2326 we provide a lower limit. All measurements are in the range 39 < 12C/13C < 100, showing that the He- and H-burning layers underwent partial mixing either in the stars or, more likely, in their progenitors. This provides evidence of a primary production of 13C at the dawn of chemical evolution. CEMP-no dwarf stars with slightly higher metallicities show lower isotopic values, <30 and even approaching the CNO cycle equilibrium value. Thus, extant data suggest the presence of a discontinuity in the 12C/13C ratio at around [Fe/H] ≈ − 4, which could mark a real difference between the progenitor pollution captured by stars with different metallicities. We also note that some MMP and HMP stars with high 12C/13C show low 7Li values, providing an indication that mixing in the CEMP-no progenitors is not responsible for the observed Li depletion.

A Study of Elemental Abundance Pattern of the r-II Star HD 222925

HD 222925 is a rare r-process enhanced star in the Milky Way because of its metal abundance ([Fe/H] =−1.46 ± 0.10) and Eu abundance ([Eu/Fe] = 1.32 ± 0.08). Based on the very complete set of elemental abundances of HD 222925, we use the abundance decomposition method to fit the observed abundances of 58 elements in the sample star, which is also the largest number of elemental abundances fitted at the same time for a fixed star. We analyze the astrophysical origins of elements in HD 222925 by its abundance ratios and component ratios. It is found that the light elements and the iron group elements in HD 222925 mainly originate from the primary process of the Type II supernovae (SNe II) with the progenitor mass M > 10 M ⊙ and have no contribution from SNe Ia and the first generation of very massive stars. The contribution of the weak r-process to Ga, Ge, and As is superior to that of the other processes, and its contribution decreases linearly with increasing atomic number. The main r-process that is likely derived from a neutron star merger plays a key role in the formation of neutron-capture elements (Z ≥ 38) in HD 222925.

A comparative study of three modes of s-process nucleosynthesis in extremely metal-poor AGB stars

Abstract Carbon-enhanced metal-poor (CEMP) stars in the Galactic halo have a wide range of neutron-capture element abundance patterns. To identify their origin, we investigate three modes of s-process nucleosynthesis that have been proposed to operate in extremely metal-poor asymptotic giant branch (AGB) stars: convective 13C burning, which occurs when hydrogen is engulfed by helium-flash convection in low-mass AGB stars; convective 22Ne burning, which occurs in helium-flash convection of intermediate-mass AGB stars; and radiative 13C burning, which occurs in the 13C pocket that is formed during inter-pulse periods. We show that the production of s-process elements per iron seed (s-process efficiency) does not depend on metallicity below [Fe$/$H] = −2, because 16O in the helium zone dominates the neutron poison. The convective 13C mode can produce a variety of s-process efficiencies for Sr, Ba, and Pb, including the maxima observed among CEMP stars. The 22Ne mode only produces the lowest end of s-process efficiencies among CEMP models. We show that the combination of these two modes can explain the full range of observed enrichment of s-process elements in CEMP stars. In contrast, the 13C pocket mode can hardly explain the high level of enrichment observed in some CEMP stars, even if considering star-to-star variations of the mass of the 13C pocket. These results provide a basis for discussing the binary mass transfer origin of CEMP stars and their subgroups.

The complex stellar system M 22: constraining the chemical enrichment from AGB stars using magnesium isotope ratios

ABSTRACT The complex star cluster M 22 (NGC 6656) provides a unique opportunity for studying the slow neutron capture (s-)process nucleosynthesis at low metallicity due to its two stellar groups with distinct iron-peak and neutron capture element abundances. Previous studies attribute these abundance differences to pollution from $3-6 \ \rm {M}_{\odot }$ asymptotic giant branch (AGB) stars which produce significant quantities of the neutron-rich Mg isotopes 25Mg and 26Mg. We report the first-ever measurements of Mg isotopic abundance ratios at $\rm {[Fe/H]} \ \sim -2$ in a globular cluster-like system using very high-resolution and signal-to-noise spectra (R = 110 000, S/N = 300 per pixel at 514 nm) from the VLT/UVES spectrograph for six stars; three in each s-process group. Despite the presence of star-to-star variations in 24Mg, 25Mg, and 26Mg, we find no correlation with heavy element abundances, implying that the nucleosynthetic source of s-process enrichment must not influence Mg isotope ratios. Instead, a key result of this work is that we identify correlations between 26Mg/24Mg and some light elements. Using a custom suite of AGB nucleosynthesis yields tailored to the metallicity of M 22, we find that low mass ($\sim 1 \rm {-} 3 \ \rm {M}_{\odot }$) AGB stars are capable of reproducing the observed s-process abundances of M 22 and that the absence of any difference in Mg isotope ratios between the two s-process groups precludes AGBs with masses above $\sim 3 \ \rm {M}_{\odot }$. This places tighter constraints on possible formation scenarios and suggests an age difference of at least $\sim 280 \rm {-} 480 \ \rm {Myr}$ between the two populations which is independent of isochrone fitting.