Nucleosynthesis In Massive Stars Research Articles

Magnesium isotope ratios in Milky Way and dwarf galaxy stars

ABSTRACT Under the assumption of hierarchical galaxy formation, dwarf galaxies are the closest existing analogues to the high-redshift protogalaxies that merged to form the Milky Way. These low-mass systems serve as unique laboratories for studying nucleosynthetic channels given that the chemical compositions of their stars play a pivotal role in constraining their chemical enrichment history. To date, stellar abundances in dwarf galaxies have focused almost exclusively on elemental abundance ratios. While important, elemental abundances omit critical information about the isotopic composition. Here, we compute the Mg isotopic ratios of six accreted dwarf galaxy stars (low $\alpha$) and seven Milky Way stars (high $\alpha$) using a set of high-resolution (65 000 < R < 160 000) and high signal-to-noise ratio ($\rm {S/N} \gt 250$) optical spectra. We show, for the first time, that at a given [Fe/H] stars born in a dwarf galaxy differ in their Mg isotopic ratios from stars born in the Milky Way. However, when comparing isotopic ratios at a given [Mg/H] rather than [Fe/H], a powerful diagnostic emerges that suggests nucleosynthesis processes are consistent across different stellar environments. This universality of Mg isotopic abundances provides additional dimensionality for chemical evolution models and helps to constrain massive star nucleosynthesis across cosmic time.

Galactic population synthesis of radioactive nucleosynthesis ejecta

Diffuse γ-ray line emission traces freshly produced radioisotopes in the interstellar gas, providing a unique perspective on the entire Galactic cycle of matter from nucleosynthesis in massive stars to their ejection and mixing in the interstellar medium (ISM). We aim to construct a model of nucleosynthesis ejecta on a galactic scale that is specifically tailored to complement the physically most important and empirically accessible features of γ-ray measurements in the MeV range, in particular for decay γ rays such as 26Al, 60Fe, or 44Ti. Based on properties of massive star groups, we developed a Population SYnthesis COde (PSYCO), which can instantiate galaxy models quickly and based on many different parameter configurations, such as the star formation rate (SFR), density profiles, or stellar evolution models. As a result, we obtain model maps of nucleosynthesis ejecta in the Galaxy which incorporate the population synthesis calculations of individual massive star groups. Based on a variety of stellar evolution models, supernova (SN) explodabilities, and density distributions, we find that the measured 26Al distribution from INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with a SFR of 4–8 M⊙ yr−1 and a spiral-arm-dominated density profile with a scale height of at least 700 pc. Our model requires that most massive stars indeed undergo a SN explosion. This corresponds to a SN rate in the Milky Way of 1.8–2.8 per century, with quasi-persistent 26Al and 60Fe masses of 1.2–2.4 M⊙ and 1–6 M⊙, respectively. Comparing the simulated morphologies to SPI data suggests that a frequent merging of superbubbles may take place in the Galaxy, and that an unknown yet strong foreground emission at 1.8 MeV could be present.

<sup>22</sup>Ne(α, n)<sup>25</sup>Mg: The key neutron source in massive stars

<p indent="0mm">The origin of heavy elements from iron to uranium in the universe is an important frontier direction for nuclear astrophysics. It is considered to be one of the 11 greatest unanswered questions of physics in this century. The neutron capture reaction can overcome the Coulomb barrier and is the key way to generate heavy elements in the universe. The average life of free neutrons is only about <sc>15 min.</sc> Therefore, the neutron source is a hot topic for understanding the origin of heavy elements. There are two main neutron sources in the s-process nucleosynthesis, one is the ¹³C(α, n)¹⁶O reaction, the other is the ²²Ne(α, n)²⁵Mg reaction. The ¹³C(α, n)¹⁶O reaction mainly provides neutrons for the s-process nucleosynthesis of small or medium mass stars, and affects the nucleosynthesis of nuclides with the mass numbers from 90 to 209; while the ²²Ne(α, n)²⁵Mg reaction mainly provides neutrons for the s-process nucleosynthesis of massive stars and affects the nucleosynthesis of nuclides with the mass number less than 90. The ²²Ne(α, n)²⁵Mg reaction takes place in the core helium burning phase and the subsequent shell carbon burning phase of massive stars. Due to the difficulty of experiment measurement, the data of the ²²Ne(α, n)²⁵Mg reaction rate is still lacking, which leads to the problem of understanding effective neutron supply in the nucleosynthesis of the heavy elements in massive stars. Limited by cosmic ray backgrounds, a large number of key reactions in hydrostatic nuclear burning phases cannot be accurately measured. Therefore, direct measurement of these reactions has gradually moved from the earth-surface to underground laboratories. The Laboratory for Underground Nuclear Astrophysics (LUNA) experimental platform in Italy is a mature underground nuclear astrophysics laboratory in the world. Since the 1990s, direct measurement of many nuclear reactions has been carried out. Jinping Underground Nuclear Astrophysics Laboratory (JUNA) is a new experimental platform. JUNA is located in the world’s deepest underground laboratory—China JinPing Underground Laboratory (CJPL). JUNA team has successfully developed a <sc>10 mA</sc> low-energy accelerator and experimental terminal, which aims to conduct experimental research on a number of key nuclear reactions in hydrostatic hydrogen and helium burnings of stars in an underground environment with very low background. Since its operation in the underground laboratory at the end of 2020, JUNA has completed the first phase of experimental tasks and successfully studied several key nuclear reactions including the neutron source reaction¹³C(α, n)¹⁶O. In this paper, we review the motivation of the key neutron source ²²Ne(α, n)²⁵Mg in the origin of heavy elements and in the massive stars, the latest research progress of this neutron source and future research plan, especially the underground direct measurement experimental plan. As the key neutron source reaction of the s-process, the ²²Ne(α, n)²⁵Mg reaction is of great significance for understanding the origin of heavy elements in the universe. As the first experiment in the second phase of JUNA, the ²²Ne(α, n)²⁵Mg reaction is expected to achieve world-class research results and provides the crucial nuclear input for understanding the nucleosynthesis in weak s-process.

Probing Massive Star Nucleosynthesis with Data on Metal-Poor Stars and the Solar System

Metal-poor stars were formed during the early epochs when only massive stars had time to evolve and contribute to the chemical enrichment. Low-mass metal-poor stars survive until the present and provide fossil records of the nucleosynthesis of early massive stars. On the other hand, short-lived radionuclides (SLRs) in the early solar system (ESS) reflect the nucleosynthesis of sources that occurred close to the proto-solar cloud in both space and time. Both the ubiquity of Sr and Ba and the diversity of heavy-element abundance patterns observed in single metal-poor stars suggest that some neutron-capture mechanisms other than the r-process might have operated in early massive stars. Three such mechanisms are discussed: the weak s-process in non-rotating models with initial carbon enhancement, a new s-process induced by rapid rotation in models with normal initial composition, and neutron-capture processes induced by proton ingestion in non-rotating models. In addition, meteoritic data are discussed to constrain the core-collapse supernova (CCSN) that might have triggered the formation of the solar system and provided some of the SLRs in the ESS. If there was a CCSN trigger, the data point to a low-mass CCSN as the most likely candidate. An 11.8 M⊙ CCSN trigger is discussed. Its nucleosynthesis, the evolution of its remnant, and the interaction of the remnant with the proto-solar cloud appear to satisfy the meteoritic constraints and can account for the abundances of the SLRs 41Ca, 53Mn, and 60Fe in the ESS.

Studying the heavy-ion fusion reactions at stellar energies using Time Projection Chamber

Heavy-ion fusion reactions, such as 12C+12C, play crucial roles in stellar evolution and nucleosynthesis in massive stars. However, limited by the low cross section and large background, it is very challenging to study the reactions at stellar energies. We have developed a 1024-channel prototype TPC named MATE (Multi-purpose time projection chamber for nuclear AsTrophysical and Exotic beam experiments) to study these reactions at stellar energies. The design of our prototype TPC and its performance are described, and the preliminary results of a commissioning experiment for measuring the 12C(12C, α0)20Ne reaction at Ecm = 3.0 MeV are presented. Perspectives of future experiments for the 12C+12C fusion cross section measurement are also provided.

Magnesium isotopes: a tool to understand self-enrichment in globular clusters

A critical issue in the asymptotic giant branch (AGB) self-enrichment scenario for the formation of multiple populations in Globular Clusters (GCs) is the inability to reproduce the magnesium isotopic ratios, despite the model in principle can account for the depletion of magnesium. In this work we analyze how the uncertainties on the various p-capture cross sections affect the results related to the magnesium content of the ejecta of AGB stars. The observed distribution of the magnesium isotopes and of the overall Mg-Al trend in M13 and NGC 6752 are successfully reproduced when the proton-capture rate by 25Mg at the temperatures 100 MK, in particular the 25Mg(p, gamma)26Alm channel, is enhanced by a factor 3 with respect to the most recent experimental determinations. This assumption also allows to reproduce the full extent of the Mg spread and the Mg-Si anticorrelation observed in NGC 2419. The uncertainties in the rate of the 25Mg(p,gamma)26Alm reaction at the temperatures of interest here leave space for our assumption and we suggest that new experimental measurements are needed to settle this problem. We also discuss the competitive model based on the super massive star nucleosynthesis.

Reaction rate for carbon burning in massive stars

Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5-4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

A supersonic jet target for the cross section measurement of the 12C(α, γ)16O reaction with the recoil mass separator ERNA

12C(α, γ)16O cross section plays a key-role in the stellar evolution and nucleosynthesis of massive stars. Hence, it must be determined with the precision of about 10% at the relevant Gamow energy of 300keV. The ERNA (European Recoil mass separator for Nuclear Astrophysics) collaboration measured, for the first time, the total cross section of 12C(α, γ)16O by means of the direct detection of the 16O ions produced in the reaction down to an energy of Ecm=1.9MeV. To extend the measurement at lower energy, it is necessary to limit the extension of the He gas target. This can be achieved using a supersonic jet, where the oblique shock waves and expansion fans formed at its boundaries confine the gas, which can be efficiently collected using a catcher. A test version of such a system has been designed, constructed and experimentally characterized as a bench mark for a full numerical simulation using FV (Finite Volume) methods. The results of the commissioning of the jet test version and the design of the new system that will be used in combination with ERNA are presented and discussed.

Uncertainties in s-process nucleosynthesis in massive stars determined by Monte Carlo variations

The $s$-process in massive stars produces the weak component of the $s$-process (nuclei up to $A \sim 90$), in amounts that match solar abundances. For heavier isotopes, such as barium, production through neutron capture is significantly enhanced in very metal-poor stars with fast rotation. However, detailed theoretical predictions for the resulting final $s$-process abundances have important uncertainties caused both by the underlying uncertainties in the nuclear physics (principally neutron capture reaction and $\beta$-decay rates) as well as by the stellar evolution modeling. In this work, we investigated the impact of nuclear-physics uncertainties relevant to the $s$-process in massive stars. Using a Monte-Carlo based approach, we performed extensive nuclear reaction network calculations that include newly evaluated upper and lower limits for the individual temperature dependent reaction rates. We found that most of the uncertainty in the final abundances is caused by uncertainties in the neutron capture rates, while $\beta$-decay rate uncertainties affect only a few nuclei near $s$-process branchings. The $s$-process in rotating metal-poor stars shows quantitatively different uncertainties and key reactions, although the qualitative characteristics are similar. We confirmed that our results do not significantly change at different metallicities for fast rotating massive stars in the very low metallicity regime. We highlight which of the identified key reactions are realistic candidates for improved measurement by future experiments.

New insights from cosmic gamma rays

The measurement of gamma rays from cosmic sources at ~MeV energies is one of the key tools for nuclear astrophysics, in its study of nuclear reactions and their impacts on objects and phenomena throughout the universe. Gamma rays trace nuclear processes most directly, as they originate from nuclear transitions following radioactive decays or high-energy collisions with excitation of nuclei. Additionally, the unique gamma-ray signature from the annihilation of positrons falls into this astronomical window and is discussed here: Cosmic positrons are often produced from β-decays, thus also of nuclear physics origins. The nuclear reactions leading to radioactive isotopes occur inside stars and stellar explosions, which therefore constitute the main objects of such studies. In recent years, both thermonuclear and core-collapse supernova radioactivities have been measured though 56Ni, 56Co, and 44Ti lines, and a beginning has thus been made to complement conventional supernova observations with such measurements of the prime energy sources of supernova light created in their deep interiors. The diffuse radioactive afterglow of massive-star nucleosynthesis in gamma rays is now being exploited towards astrophysical studies on how massive stars feed back their energy and ejecta into interstellar gas, as part of the cosmic cycle of matter through generations of stars enriching the interstellar gas and stars with metals. Large interstellar cavities and superbubbles have been recognised to be the dominating structures where new massive-star ejecta are injected, from 26Al gamma-ray spectroscopy. Also, constraints on the complex interiors of stars derive from the ratio of 60Fe/26Al gamma rays. Finally, the puzzling bulge-dominated intensity distribution of positron annihilation gamma rays is measured in greater detail, but still not understood; a recent microquasar flare provided evidence that such objects may be prime sources for positrons in interstellar space, rather than distributed nucleosynthesis. We also briefly discuss the status and prospects for astronomy with telescopes for the nuclear-radiation energy window.

Study of the 26Al(n,p)26Mg and 26Al(n,α)23Na reactions using the 27Al(p,p')27Al inelastic scattering reaction

26Al was the first cosmic radioactivity ever detected in the galaxy as well as one of the first extinct radioactivity observed in refractory phases of meteorites. Its nucleosynthesis in massive stars is still uncertain mainly due to the lack of nuclear information concerning the 26Al(n,p)26Mg and 26 Al(n,α)23Na reactions. We report on a single and coincidence measurement of the 27Al(p,p')27Al(p)26Mg and 27Al(p,p')27Al(α)23Na reactions performed at the Orsay TANDEM facility aiming at the spectroscopy study of 27Al above the neutron threshold. Fourteen states are observed for the first time within 350 keV above the 26Al+n threshold.

S-process production in rotating massive stars at solar and low metallicities

Rotation was shown to have a strong impact on the structure and light element nucleosynthesis in massive stars. In particular, models including rotation can reproduce the primary nitrogen observed in halo extremely metal-poor (EMP) stars. Additional exploratory models showed that rotation may enhance $s$-process production at low metallicity. Here we present a large grid of massive star models including rotation and a full $s$-process network to study the impact of rotation on the weak $s$-process. We explore the possibility of producing significant amounts of elements beyond the strontium peak, which is where the weak $s$-process usually stops. We used the Geneva stellar evolution code coupled to an enlarged reaction network with 737 nuclear species up to bismuth to calculate $15-40\,\text{M}_\odot$ models at four metallicities ($Z = 0.014,10^{-3}$, $10^{-5}$, and $10^{-7}$) from the main sequence up to the end of oxygen burning. We confirm that rotation-induced mixing between the convective H-shell and He-core enables an important production of primary $^{14}$N and $^{22}$Ne and $s$-process at low metallicity. At low metallicity, even though the production is still limited by the initial number of iron seeds, rotation enhances the $s$-process production, even for isotopes heavier than strontium, by increasing the neutron to seed ratio. The increase in this ratio is a direct consequence of the primary production of $^{22}$Ne. Despite nuclear uncertainties affecting the $s$-process production and stellar uncertainties affecting the rotation-induced mixing, our results show a robust production of $s$ process at low metallicity when rotation is taken into account. Considering models with a distribution of initial rotation rates enables to reproduce the observed large range of the [Sr/Ba] ratios in (carbon-enhanced and normal) EMP stars.

Effects of axions on nucleosynthesis in massive stars

We investigate the effect of axion cooling on nucleosynthesis in a massive star with $16{M}_{\ensuremath{\bigodot}}$ by a standard stellar evolution calculation. We find that axion cooling suppresses nuclear reactions in carbon, oxygen, and silicon burning phases because of the extraction of the energy. As a result, larger amounts of the already synthesized neon and magnesium remain without being consumed to produce further, heavier elements. Even in the case with axion-photon coupling constant ${g}_{a\ensuremath{\gamma}}={10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$, which is six times smaller than the current upper limit, the amount of neon and magnesium that remain just before the core-collapse supernova explosion is considerably larger than the standard value. This implies that we could give a more stringent constraint on ${g}_{a\ensuremath{\gamma}}$ from the nucleosynthesis of heavy elements in massive stars.

Code dependencies of pre-supernova evolution and nucleosynthesis in massive stars: evolution to the end of core helium burning

Massive stars are key sources of radiative, kinetic, and chemical feedback in the universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes---GENEC, KEPLER and MESA---to compute non-rotating stellar models of $15~\mathrm{M}_\odot$, $20~\mathrm{M}_\odot$, and $25~\mathrm{M}_\odot$ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The GENEC and KEPLER models hold physics assumptions used in large grids of published models. The MESA code was set up to use convective core overshooting such that the CO core masses are consistent with those obtained by GENEC. For all models, full nucleosynthesis is computed using the NuGrid post-processing tool MPPNP. We find that the surface abundances predicted by the models are in reasonable agreement. In the helium core, the standard deviation of the elemental overproduction factors for Fe to Mo is less than $30\,\%$---smaller than the impact of the present nuclear physics uncertainties. For our three initial masses, the three stellar evolution codes yield consistent results. Differences in key properties of the models, e.g., helium and CO core masses and the time spent as a red supergiant, are traced back to the treatment of convection and, to a lesser extent, mass loss. The mixing processes in stars remain the key uncertainty in stellar modelling. Better constrained prescriptions are thus necessary to improve the predictive power of stellar evolution models.

Nucleosynthesis ofAl26in massive stars: NewAl27states aboveαand neutron emission thresholds

The ${}^{26}\mathrm{Al}$ radioisotope is of great importance for understanding the chemical and dynamical evolution of our galaxy. Among the possible stellar sources, massive stars are believed to be the main producer of this radioisotope. Understanding ${}^{26}\mathrm{Al}$ nucleosynthesis in massive stars requires estimates of the thermonuclear reaction rates of the $^{26}\mathrm{Al}$${(n,p)}^{26}$Mg, $^{26}\mathrm{Al}$${(n,\ensuremath{\alpha})}^{23}$Na, and $^{23}\mathrm{Na}$${(\ensuremath{\alpha},p)}^{26}$Mg reactions. These reaction rates depend on the spectroscopic properties of ${}^{27}\mathrm{Al}$ states above the neutron and alpha thresholds. In this context, the $^{27}\mathrm{Al}$${(p,{p}^{\ensuremath{'}})}^{27}$Al* reaction was studied at 18 MeV using a high-resolution Enge Split-Pole spectrometer. States from the ground state up to excitation energies of $\ensuremath{\approx}$14 MeV were populated. While up to the ${}^{23}\mathrm{Na}$ + $\ensuremath{\alpha}$ threshold no additional states are observed, we report for the first time 30 new levels above the ${}^{23}\mathrm{Na}$ + $\ensuremath{\alpha}$ threshold and more than 30 new states above the ${}^{26}\mathrm{Al}$ + n threshold for which excitation energies are determined with an uncertainty of 4--5 keV.

Experimental neutron capture data of58Ni from the CERN n_TOF facility

The $^{58}$Ni $(n,\gamma)$ cross section has been measured at the neutron time of flight facility n_TOF at CERN, in the energy range from 27 meV up to 400 keV. In total, 51 resonances have been analyzed up to 122 keV. Maxwellian averaged cross sections (MACS) have been calculated for stellar temperatures of kT$=$5-100 keV with uncertainties of less than 6%, showing fair agreement with recent experimental and evaluated data up to kT = 50 keV. The MACS extracted in the present work at 30 keV is 34.2$\pm$0.6$_\mathrm{stat}\pm$1.8$_\mathrm{sys}$ mb, in agreement with latest results and evaluations, but 12% lower relative to the recent KADoNIS compilation of astrophysical cross sections. When included in models of the s-process nucleosynthesis in massive stars, this change results in a 60% increase of the abundance of $^{58}$Ni, with a negligible propagation on heavier isotopes. The reason is that, using both the old or the new MACS, 58Ni is efficiently depleted by neutron captures.

Detection of a Noble Gas Molecular Ion, 36 ArH + , in the Crab Nebula

Noble gas molecules have not hitherto been detected in space. From spectra obtained with the Herschel Space Observatory, we report the detection of emission in the 617.5- and 1234.6-gigahertz J = 1-0 and 2-1 rotational lines of (36)ArH(+) at several positions in the Crab Nebula, a supernova remnant known to contain both molecular hydrogen and regions of enhanced ionized argon emission. Argon-36 is believed to have originated from explosive nucleosynthesis in massive stars during core-collapse supernova events. Its detection in the Crab Nebula, the product of such a supernova event, confirms this expectation. The likely excitation mechanism for the observed (36)ArH(+) emission lines is electron collisions in partially ionized regions with electron densities of a few hundred per centimeter cubed.

Cosmic Gamma-Ray Spectroscopy

Penetrating gamma- rays require complex instrumentation for astronomical spectroscopy measurements of gamma-rays from cosmic sources. A combination of multiple-interaction detectors in space and post-processing of detector events on ground have lead to a spectroscopy performance which is now capable to provide new astrophysical insights. Spectral signatures in the MeV regime originate from transitions in atomic nuclei, stimulated by either radioactive decays or high-energy nuclear collisions such as with cosmic rays. Lines have been detected from radioactive isotopes produced in stellar and supernova nuclear burning, and from energetic-particle interactions in solar flares. Radioactive-decay gamma-rays from 56Ni directly reflect the power source of supernova light. 44Ti is produced in core-collapse supernova interiors and the largely unknown and dynamical conditions herein. From 26Al and 60Fe which are distributed in interstellar space from massive-star nucleosynthesis over millions of years. Additionally, nuclear de-excitation lines have been measured in solar-flare events, and convey information a b out energetic particle production in these outbursts, and t heir interaction in the solar atmosphere. Annihilating posit rons add another very special astrophysical source, which has been puzzling so far, with its characteristic gamma-rays at 511 keV; it has been measured both in such solar flares, and throughout the interstellar medium of our Milky Way galaxy. - We discuss instrumentation and data processing for cosmic gamma-ray spectroscopy, and the astrophysical issues and insights from these measurements.

NUCLEOSYNTHETIC CONSTRAINTS ON THE MASS OF THE HEAVIEST SUPERNOVAE

Assuming a Salpeter initial mass function and taking the solar abundances as a representative sample, we explore the sensitivity of nucleosynthesis in massive stars to the truncation of supernova explosions above a certain mass. It is assumed that stars of all masses contribute to nucleosynthesis by their pre-explosive winds, but above a certain limiting main sequence mass, the presupernova star becomes a black hole and ejects nothing more. The solar abundances from oxygen to atomic mass 90 are fit quite well assuming no cut-off at all, i.e., by assuming all stars up to 120 solar masses make successful supernovae. Little degradation in the fit occurs if the upper limit is reduced to 25 solar masses. The limit can be further reduced, but the required event rate of supernovae in the remaining range rises rapidly to compensate for the lost nucleosynthesis of the more massive stars. The nucleosynthesis of the s-process declines precipitously and the production of species made in the winds, e.g., carbon, becomes unacceptably large compared with elements made in the explosion, e.g., silicon and oxygen. However, by varying uncertain physics, especially the mass loss rate for massive stars and the rate for the neon-22 to magnesium-25 reaction rate, acceptable nucleosynthesis might still be achieved with a cutoff as low as 18 solar masses. This would require a supernova frequency three times greater than the fiducial value obtained when all stars explode in order to produce the required oxygen-16. The nucleosynthesis of iron-60 and aluminum-26 is also examined.

Ba and Eu abundances in M 15 giant stars

To investigate the Ba and Eu abundances for a sample of 63 giant stars in the globular cluster M15. This is the largest sample of M15 giants stars for which Ba abundances have been determined and, due to the target selection of the original research programme, the Ba abundances are complete along the red giant branch. Stellar parameters were taken from the previous key study and a microturbulence-surface gravity relation was determined for precise measurement of the Ba line at 6496.898 Angstroms, which has a high sensitivity to microturbulence. Element abundances for Ba, La, Eu, Ca, Ni and Fe were calculated using spectrum synthesis and equivalent widths techniques. A bimodal distribution in Ba, Eu and La abundances was found within the sample. The low Ba,Eu,La mode had mean abundances of <[Ba/H]>=-2.41+/-0.16, <[Eu/H]>=-1.80+/-0.08 and <[La/H]>=-2.19+/-0.13 while the high Ba,Eu,La mode had mean abundances of <[Ba/H]>=-2.00+/-0.16, <[Eu/H]>=-1.65+/-0.13 and <[La/H]>=-1.95+/-0.11. Both modes are indicative of a pollution scenario dominated by the r-process, hence contributions from explosive nucleosynthesis of massive stars. There may be evidence of further enhancement by another heavy element process and of potential anticorrelations in Na-O for both modes indicating a complex formation and evolution history for M15.