Abstract
We present a comprehensive study of the abundance evolution of the elements from H to U in the Milky Way halo and local disc. We use a consistent chemical evolution model, ...metallicity-dependent isotopic yields from low and intermediate mass stars and yields from massive stars which include, for the first time, the combined effect of metallicity, mass loss, and rotation for a large grid of stellar masses and for all stages of stellar evolution. The yields of massive stars are weighted by a metallicity-dependent function of the rotational velocities, constrained by observations as to obtain a primary-like 14N behaviour at low metallicity and to avoid overproduction of s-elements at intermediate metallicities. We show that the Solar system isotopic composition can be reproduced to better than a factor of 2 for isotopes up to the Fe-peak, and at the 10 per cent level for most pure s-isotopes, both light ones (resulting from the weak s-process in rotating massive stars) and the heavy ones (resulting from the main s-process in low and intermediate mass stars). We conclude that the light element primary process (LEPP), invoked to explain the apparent abundance deficiency of the s-elements with A < 100, is not necessary. We also reproduce the evolution of the heavy to light s-elements abundance ratio (hs/ls) – recently observed in unevolved thin disc stars – as a result of the contribution of rotating massive stars at sub-solar metallicities. We find that those stars produce primary F and dominate its solar abundance and we confirm their role in the observed primary behaviour of N. In contrast, we show that their action is insufficient to explain the small observed values of $\rm ^{12}C/^{13}C$ in halo red giants, which is rather due to internal processes in those stars.
Abstract
The decomposition of the Solar system abundances of heavy isotopes into their s- and r- components plays a key role in our understanding of the corresponding nuclear processes and the ...physics and evolution of their astrophysical sites. We present a new method for determining the s- and r- components of the Solar system abundances, fully consistent with our current understanding of stellar nucleosynthesis and galactic chemical evolution. The method is based on a study of the evolution of the solar neighborhood with a state-of-the-art 1-zone model, using recent yields of low and intermediate mass stars as well as of massive rotating stars. We compare our results with previous studies and we provide tables with the isotopic and elemental contributions of the s- and r-processes to the Solar system composition.
We present a new set of zero metallicity models in the range 13-80 M sub(middot in circle) together to the associated explosive nucleosynthesis. These models are fully homogeneous with the solar ...metallicity set we published in Limongi & Chieffi and will be freely available at the Online Repository for the FRANEC Evolutionary Output Web site. A comparison between these yields and an average star that represents the average behavior of most of the very metal-poor stars in the range -5.0 < Fe/H < -2.5 confirms previous findings that only a fraction of the elemental X/Fe may be fitted by the ejecta of standard core collapse supernovae.
ABSTRACT
Most neutron capture elements have a double production by r- and s-processes, but the question of production sites is complex and still open. Recent studies show that including stellar ...rotation can have a deep impact on nucleosynthesis. We studied the evolution of Sr and Ba in the Milky Way. A chemical evolution model was employed to reproduce the Galactic enrichment. We tested two different nucleosynthesis prescriptions for s-process in massive stars, adopted from the Geneva group and the Rome group. Rotation was taken into account, studying the effects of stars without rotation or rotating with different velocities. We also tested different production sites for the r-process: magneto rotational driven supernovae and neutron star mergers. The evolution of the abundances of Sr and Ba is well reproduced. The comparison with the most recent observations shows that stellar rotation is a good assumption, but excessive velocities result in overproduction of these elements. In particular, the predicted evolution of the Sr/Ba ratio at low metallicity does not explain the data at best if rotation is not included. Adopting different rotational velocities for different stellar mass and metallicity better explains the observed trends. Despite the differences between the two sets of adopted stellar models, both show a better agreement with the data assuming an increase of rotational velocity towards low metallicity. Assuming different r-process sources does not alter this conclusion.
Dust grains are classically thought to form in the winds of asymptotic giant branch (AGB) stars. However, there is increasing evidence today for dust formation in supernovae (SNe). To establish the ...relative importance of these two classes of stellar sources of dust, it is important to know the fraction of freshly formed dust in SN ejecta that is able to survive the passage of the reverse shock and be injected in the interstellar medium. With this aim, we have developed a new code, GRASH_Rev, that allows following the dynamics of dust grains in the shocked SN ejecta and computing the time evolution of the mass, composition, and size distribution of the grains. We considered four well-studied SNe in the Milky Way and Large Magellanic Cloud: SN 1987A, CasA, the Crab nebula, and N49. These sources have been observed with both Spitzer and Herschel, and the multiwavelength data allow a better assessment the mass of warm and cold dust associated with the ejecta. For each SN, we first identified the best explosion model, using the mass and metallicity of the progenitor star, the mass of 56Ni, the explosion energy, and the circumstellar medium density inferred from the data. We then ran a recently developed dust formation model to compute the properties of freshly formed dust. Starting from these input models, GRASH_Rev self-consistently follows the dynamics of the grains, considering the effects of the forward and reverse shock, and allows predicting the time evolution of the dust mass, composition, and size distribution in the shocked and unshocked regions of the ejecta. All the simulated models aagree well with observations. Our study suggests that SN 1987A is too young for the reverse shock to have affected the dust mass. Hence the observed dust mass of 0.7−0.9 M⊙ in this source can be safely considered as indicative of the mass of freshly formed dust in SN ejecta. Conversely, in the other three SNe, the reverse shock has already destroyed between 10−40% of the initial dust mass. However, the largest dust mass destruction is predicted to occur between 103 and 105 yr after the explosions. Since the oldest SN in the sample has an estimated age of 4800 yr, current observations can only provide an upper limit to the mass of SN dust that will enrich the interstellar medium, the so-called effective dust yields. We find that only between 1−8% of the currently observed mass will survive, resulting in an average SN effective dust yield of (1.55 ± 1.48) × 10-2M⊙. This agrees well with the values adopted in chemical evolution models that consider the effect of the SN reverse shock. We discuss the astrophysical implications of our results for dust enrichment in local galaxies and at high redshift.
Context.
Luminous blue variable stars (LBVs) are massive evolved stars that suffer sporadic and violent mass-loss events. They have been proposed as the progenitors of some core-collapse supernovae ...(SNe), but this idea is still debated because of a lack of strong evidence. As supernova remnants (SNRs) can carry in their morphology the fingerprints of the progenitor stars as well as of the inhomogeneous circumstellar medium (CSM) sculpted by the progenitors, the study of SNRs from LBVs could help to place core-collapse SNe in context with the evolution of massive stars.
Aims.
We investigate the physical, chemical, and morphological properties of the remnants of SNe originating from LBVs in order to search for signatures in the ejecta distribution and morphology of the remnants that could reveal the nature of the progenitors.
Methods.
As a template of LBVs, we considered the LBV candidate Gal 026.47+0.02. We selected a grid of models that describe the evolution of a massive star with properties consistent with those of Gal 026.47+0.02 and its final fate as a core-collapse SN. We developed a three-dimensional hydrodynamic model that follows the post-explosion evolution of the ejecta from the breakout of the shock wave at the stellar surface to the interaction of the SNR with a CSM characterized by two dense nested toroidal shells, parametrized in agreement with multi-wavelength observations of Gal 026.47+0.02.
Results.
Our models show a strong interaction of the blast wave with the CSM which determines an important slowdown of the expansion of the ejecta in the equatorial plane where the two shells lay, determining a high degree of asymmetry in the remnant. After ≈10 000 yr of evolution, the ejecta show an elongated shape forming a broad jet-like structure caused by the interaction with the shells and oriented along the axis of the toroidal shells. Models with high explosion energy show Fe-rich internal ejecta distributions surrounded by an elongated Si-rich structure with a more diffuse O-rich ejecta all around. Models with low explosion energy instead show a more homogeneous distribution of chemical elements with a very low presence of Fe-group elements.
Conclusions.
The geometry and density distribution of the CSM where a LBV star goes SN are fundamental in determining the properties of the resulting SNR. For all the LBV-like progenitors explored here, we found that the remnants show a common morphology, namely elongated ejecta with an internal jet-like structure, which reflects the inhomogeneous and dense pre-SN CSM surrounding the star.
TOPoS Bonifacio, P; Caffau, E; Spite, M ...
Astronomy and astrophysics (Berlin),
7/2015, Volume:
579
Journal Article
Peer reviewed
In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very ...common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. We analyze both X-shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbo-spectrum line formation code. We propose an interpretation of this bi-modality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements.
ABSTRACT
Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the ...question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr, and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.
Context. The spatial distribution of elemental abundances in the disc of our Galaxy gives insights both on its assembly process and subsequent evolution, and on the stellar nucleogenesis of the ...different elements. Gradients can be traced using several types of objects as, for instance, (young and old) stars, open clusters, HII regions, planetary nebulae. Aims. We aim to trace the radial distributions of abundances of elements produced through different nucleosynthetic channels – the α-elements O, Mg, Si, Ca and Ti, and the iron-peak elements Fe, Cr, Ni and Sc – by use of the Gaia-ESO IDR4 results for open clusters and young-field stars. Methods. From the UVES spectra of member stars, we have determined the average composition of clusters with ages > 0.1 Gyr. We derived statistical ages and distances of field stars. We traced the abundance gradients using the cluster and field populations and compared them with a chemo-dynamical Galactic evolutionary model. Results. The adopted chemo-dynamical model, with the new generation of metallicity-dependent stellar yields for massive stars, is able to reproduce the observed spatial distributions of abundance ratios, in particular the abundance ratios of O/Fe and Mg/Fe in the inner disc (5 kpc <RGC< 7 kpc), with their differences, that were usually poorly explained by chemical evolution models. Conclusions. Oxygen and magnesium are often considered to be equivalent in tracing α-element abundances and in deducing, for example, the formation timescales of different Galactic stellar populations. In addition, often α/Fe is computed combining several α-elements. Our results indicate, as expected, a complex and diverse nucleosynthesis of the various α-elements, in particular in the high metallicity regimes, pointing towards a different origin of these elements and highlighting the risk of considering them as a single class with common features.
Recent improvements in stellar models for intermediate-mass stars and massive stars (MSs) are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetimes τ 25 Myr, ...in order to re-examine the origins of now extinct radioactivities that were alive in the solar nebula. The Galactic inheritance broadly explains most of them, especially if r-process nuclei are produced by neutron star merging, according to recent models. Instead, 26Al, 41Ca, 135Cs, and possibly 60Fe require nucleosynthetic events close to the solar formation. We outline the persisting difficulties to account for these nuclei by intermediate-mass stars (2 M/M 7-8). Models of their final stages now predict the ubiquitous formation of a 13C reservoir as a neutron capture source; hence, even in the presence of 26Al production from deep mixing or hot bottom burning, the ratio 26Al/107Pd remains incompatible with measured data, with a large excess in 107Pd. This is shown for two recent approaches to deep mixing. Even a late contamination by an MS encounters problems. In fact, the inhomogeneous addition of supernova debris predicts nonmeasured excesses on stable isotopes. Revisions invoking specific low-mass supernovae and/or the sequential contamination of the presolar molecular cloud might be affected by similar problems, although our conclusions here are weakened by our schematic approach to the addition of SN ejecta. The limited parameter space that remains to be explored for solving this puzzle is discussed.
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