ABSTRACT
The assembly history of the Milky Way (MW) is a rapidly evolving subject, with numerous small accretion events and at least one major merger proposed in the MW’s history. Accreted alongside ...these dwarf galaxies are globular clusters (GCs), which act as spatially coherent remnants of these past events. Using high precision differential abundance measurements from our recently published study, we investigate the likelihood that the MW clusters NGC 362 and NGC 288 are galactic siblings, accreted as part of the Gaia-Sausage-Enceladus (GSE) merger. To do this, we compare the two GCs at the 0.01 dex level for 20 + elements for the first time. Strong similarities are found, with the two showing chemical similarity on the same order as those seen between the three LMC GCs, NGC 1786, NGC 2210-, and NGC 2257. However, when comparing GC abundances directly to GSE stars, marked differences are observed. NGC 362 shows good agreement with GSE stars in the ratio of Eu to Mg and Si, as well as a clear dominance in the r- compared to the s-process, while NGC 288 exhibits only a slight r-process dominance. When fitting the two GC abundances with a GSE-like galactic chemical evolution model, NGC 362 shows agreement with both the model predictions and GSE abundance ratios (considering Si, Ni, Ba, and Eu) at the same metallicity. This is not the case for NGC 288. We propose that the two are either not galactic siblings, or GSE was chemically inhomogeneous enough to birth two similar, but not identical clusters with distinct chemistry relative to constituent stars.
Abstract
Of all the light elements, the evolution of lithium (Li) in the Milky Way is perhaps the most difficult to explain. Li is difficult to synthesize and is easily destroyed, making most stellar ...sites unsuitable for producing Li in sufficient quantities to account for the protosolar abundance. For decades, novae have been proposed as a potential explanation for this “Galactic Li problem,” and the recent detection of
7
Be in the ejecta of multiple nova eruptions has breathed new life into this theory. In this work, we assess the viability of novae as dominant producers of Li in the Milky Way. We present the most comprehensive treatment of novae in a galactic chemical evolution code to date, testing theoretically and observationally derived nova Li yields by integrating metallicity-dependent nova ejecta profiles computed using the binary population synthesis code
binary
_
c
with the galactic chemical evolution code
OMEGA+
. We find that our galactic chemical evolution models which use observationally derived Li yields account for the protosolar Li abundance very well, while models relying on theoretical nova yields cannot reproduce the protosolar observation. A brief exploration of physical uncertainties including single-stellar yields, the metallicity resolution of our nova treatment, common-envelope physics, and nova accretion efficiencies indicates that this result is robust to physical assumptions. Scatter within the observationally derived Li yields in novae is identified as the primary source of uncertainty, motivating further observations of
7
Be in nova ejecta.
Abstract
Analysis of inclusions in primitive meteorites reveals that several short-lived radionuclides (SLRs) with half-lives of 0.1–100 Myr existed in the early solar system (ESS). We investigate ...the ESS origin of
107
Pd,
135
Cs, and
182
Hf, which are produced by
slow
neutron captures (the
s
-process) in asymptotic giant branch (AGB) stars. We modeled the Galactic abundances of these SLRs using the
OMEGA+
galactic chemical evolution (GCE) code and two sets of mass- and metallicity-dependent AGB nucleosynthesis yields (Monash and FRUITY). Depending on the ratio of the mean-life
τ
of the SLR to the average length of time between the formations of AGB progenitors
γ
, we calculate timescales relevant for the birth of the Sun. If
τ
/
γ
≳ 2, we predict self-consistent isolation times between 9 and 26 Myr by decaying the GCE predicted
107
Pd/
108
Pd,
135
Cs/
133
Cs, and
182
Hf/
180
Hf ratios to their respective ESS ratios. The predicted
107
Pd/
182
Hf ratio indicates that our GCE models are missing 9%–73% of
107
Pd and
108
Pd in the ESS. This missing component may have come from AGB stars of higher metallicity than those that contributed to the ESS in our GCE code. If
τ
/
γ
≲ 0.3, we calculate instead the time (
T
LE
) from the last nucleosynthesis event that added the SLRs into the presolar matter to the formation of the oldest solids in the ESS. For the 2
M
⊙
,
Z
= 0.01 Monash model we find a self-consistent solution of
T
LE
= 25.5 Myr.
We have calculated the stellar β-decay rate of the important s-process branching point 134Cs based on the state-of-the-art shell model calculations. At typical s-process temperatures (T ∼ 0.2–0.3 ...GK), our new rate is one order of magnitude lower than the widely used rate from Takahashi and Yokoi (hereafter TY87). The impact on the nucleosynthesis in AGB stars is investigated with various masses and metallicities. Our new decay rate leads to an overall decrease in the 134Ba/136Ba ratio, and well explains the measured ratio in meteorites without introducing the i-process. We also derive the elapsed time from the last AGB nucleosynthetic event that polluted the early solar system to be >28 Myr based on the 135Cs/133Cs ratio, which is consistent with the elapsed times derived from 107Pd and 182Hf. The s-process abundance sum of 135Ba and 135Cs is found to increase, resulting in a smaller r-process contribution of 135Ba in the solar system.
High-dispersion observations of the Na I D lambdalambda5890, 5896 and K I lambdalambda7665, 7699 interstellar lines, and the diffuse interstellar band at 5780 A in the spectra of 32 Type Ia ...supernovae are used as an independent means of probing dust extinction. We show that the dust extinction of the objects where the diffuse interstellar band at 5780 A is detected is consistent with the visual extinction derived from the supernova colors. This strongly suggests that the dust producing the extinction is predominantly located in the interstellar medium of the host galaxies and not in circumstellar material associated with the progenitor system. One quarter of the supernovae display anomalously large Nai column densities in comparison to the amount of dust extinction derived from their colors. Remarkably, all of the cases of unusually strong Na I D absorption correspond to "Blueshifted" profiles in the classification scheme of Sternberg et al. This coincidence suggests that outflowing circumstellar gas is responsible for at least some of the cases of anomalously large Na i column densities. Two supernovae with unusually strong Nai D absorption showed essentially normal K I column densities for the dust extinction implied by their colors, but this does not appear to be a universal characteristic. Overall, we find the most accurate predictor of individual supernova extinction to be the equivalent width of the diffuse interstellar band at 5780 A, and provide an empirical relation for its use. Finally, we identify ways of producing significant enhancements of the Na abundance of circumstellar material in both the single-degenerate and double-degenerate scenarios for the progenitor system.
ABSTRACT
We conduct a line-by-line differential analysis of a sample of 125 co-moving pairs of stars (dwarfs and subgiants near solar metallicity). We obtain high-precision stellar parameters with ...average uncertainties in effective temperature, surface gravity, and metallicity of 16.5 K, 0.033 and 0.014 dex, respectively. We classify the co-moving pairs of stars into two groups, chemically homogeneous (conatal; |ΔFe/H| ≤ 0.04 dex) and inhomogeneous (non-conatal), and examine the fraction of chemically homogeneous pairs as a function of separation and effective temperature. The four main conclusions from this study are (1) A spatial separation of Δs = 106 au is an approximate boundary between homogeneous and inhomogeneous pairs of stars, and we restrict our conclusions to only consider the 91 pairs with Δs ≤ 106 au; (2) There is no trend between velocity separation and the fraction of chemically homogeneous pairs in the range Δv ≤ 4 km s−1; (3) We confirm that the fraction of chemically inhomogeneous pairs increases with increasing Teff and the trend matches a toy model of that expected from planet ingestion; (4) Atomic diffusion is not the main cause of the chemical inhomogeneity. A major outcome from this study is a sample of 56 bright co-moving pairs of stars with chemical abundance differences ≤ 0.02 dex (5 per cent) which is a level of chemical homogeneity comparable to that of the Hyades open cluster. These important objects can be used, in conjunction with star clusters and the Gaia ‘benchmark’ stars, to calibrate stellar abundances from large-scale spectroscopic surveys.
ABSTRACT We present a new theoretical estimate for the birthrate of R Coronae Borealis (RCB) stars that is in agreement with recent observational data. We find the current Galactic birthrate of RCB ...stars to be 25% of the Galactic rate of Type Ia supernovae, assuming that RCB stars are formed through the merger of carbon-oxygen and helium-rich white dwarfs. Our new RCB birthrate (1.8 × 10−3 yr−1) is a factor of 10 lower than previous theoretical estimates. This results in roughly 180-540 RCB stars in the Galaxy, depending on the RCB lifetime. From the theoretical and observational estimates, we calculate the total dust production from RCB stars and compare this rate to dust production from novae and born-again asymptotic giant branch (AGB) stars. We find that the amount of dust produced by RCB stars is comparable to the amounts produced by novae or born-again post-AGB stars, indicating that these merger objects are a viable source of carbonaceous pre-solar grains in the Galaxy. There are graphite grains with carbon and oxygen isotopic ratios consistent with the observed composition of RCB stars, adding weight to the suggestion that these rare objects are a source of stardust grains.
ABSTRACT
Novae are some of the most commonly detected optical transients and have the potential to provide valuable information about binary evolution. Binary population synthesis codes have emerged ...as the most effective tool for modelling populations of binary systems, but such codes have traditionally employed greatly simplified nova physics, precluding detailed study. In this work, we implement a model treating H and He novae as individual events into the binary population synthesis code binary_c. This treatment of novae represents a significant improvement on the ‘averaging’ treatment currently employed in modern population synthesis codes. We discuss the evolutionary pathways leading to these phenomena and present nova event rates and distributions of several important physical parameters. Most novae are produced on massive white dwarfs, with approximately 70 and 55 per cent of nova events occurring on O/Ne white dwarfs for H and He novae, respectively. Only 15 per cent of H-nova systems undergo a common-envelope phase, but these systems are responsible for the majority of H nova events. All He-accreting He-nova systems are considered post-common-envelope systems, and almost all will merge with their donor star in a gravitational-wave-driven inspiral. We estimate the current annual rate of novae in M31 (Andromeda) to be approximately 41 ± 4 for H novae, underpredicting the current observational estimate of $65^{+15}_{-16}$, and 0.14 ± 0.015 for He novae. When varying common-envelope parameters, the H nova rate varies between 20 and 80 events per year.
We investigate the effect of helium enrichment on the evolution and nucleosynthesis of low-mass asymptotic giant branch (AGB) stars of 1.7M sub(middot in circle) and 2.36M sub(middot in circle) with ...a metallicity of Z = 0.0006 (Fe/H approximately -1.4). We calculate evolutionary sequences with the primordial helium abundance (Y = 0.24) and with helium-enriched compositions (Y = 0.30, 0.35, 0.40). For comparison, we calculate models of the same mass but at a lower metallicity Z = 0.0003 (Fe/H approximately -1.8) with Y = 0.24. Post-processing nucleosynthesis calculations are performed on each of the evolutionary sequences to determine the production of elements from hydrogen to bismuth. Elemental surface abundance predictions and stellar yields are presented for each model. The models with enriched helium have shorter main sequence and AGB lifetimes, and they enter the AGB with a more massive hydrogen-exhausted core than the primordial helium model. The main consequences are as follows: (1) low-mass AGB models with enhanced helium will evolve more than twice as fast, giving them the chance to contribute sooner to the chemical evolution of the forming globular clusters, and (2) the stellar yields will be strongly reduced relative to their primordial helium counterparts. An increase of Delta Y = 0.10 at a given mass decreases the yields of carbon by up to approximately 60% and of fluorine by up to 80%; it also decreases the yields of the s-process elements barium and lanthanum by approximately 45%. While the yields of first s-process peak elements strontium, yttrium, and zirconium decrease by up to 50%, the yields of rubidium either do not change or increase.
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