ALBERT

Description: During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. Most of the carbon in the Universe is believed to originate from stars with masses up to a few solar masses. Although there is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, the dredge-up itself has never been directly observed. In the last three decades, however, a few stars have been shown to display both carbon- and oxygen-rich material in their circumstellar envelopes. Two models have been proposed to explain this dual chemistry: one postulates that a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch changed the surface chemistry of the star. The other model postulates that oxygen-rich material exists in stable keplerian rotation around the central star. The two models make contradictory, testable, predictions on the location of the oxygen-rich material, either located further from the star than the carbon-rich gas, or very close to the star in a stable disc. Using the Faint Object InfraRed CAmera (FORCAST) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) Telescope, we obtained images of the carbon-rich planetary nebula BD +30° 3639 which trace both carbon-rich polycyclic aromatic hydrocarbons and oxygen-rich silicate dust. With the superior spectral coverage of SOFIA, and using a 3D photoionization and dust radiative transfer model we prove that the O-rich material is distributed in a shell in the outer parts of the nebula, while the C-rich material is located in the inner parts of the nebula. These observations combined with the model, suggest a recent change in stellar surface composition for the double chemistry in this object. This is evidence for dredge-up occurring ~10 3  yr ago.

Description: We use Spitzer observations of the rich population of asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC) to test models describing the internal structure and nucleosynthesis of the most massive of these stars, i.e. those with initial mass above ~4 M . To this aim, we compare Spitzer observations of LMC stars with the theoretical tracks of AGB models, calculated with two of the most popular evolution codes, that are known to differ in particular for the treatment of convection. Although the physical evolution of the two models are significantly different, the properties of dust formed in their winds are surprisingly similar, as is their position in the colour–colour and colour–magnitude diagrams obtained with the Spitzer bands. This model-independent result allows us to select a well-defined region in the ([3.6]–[4.5], [5.8]–[8.0]) plane, populated by AGB stars experiencing hot bottom burning, the progeny of stars with mass M  ~ 5.5 M . This result opens up an important test of the strength hot bottom burning using detailed near-IR ( H and K bands) spectroscopic analysis of the oxygen-rich, high-luminosity candidates found in the well-defined region of the colour–colour plane. This test is possible because the two stellar evolution codes we use predict very different results for the surface chemistry, and the C/O ratio in particular, owing to their treatment of convection in the envelope and of convective boundaries during third dredge-up. The differences in surface chemistry are most apparent when the model stars reach the phase with the largest infrared emission.

Description: Massive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for 13 different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 3756 1D MARCS model atmospheres that spans 3000 ≤ Teff∕K ≤ 8000, − 0.5 ≤log g∕cm s−2 ≤ 5.5, and − 5 ≤ [Fe/H] ≤ 1. We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of 50 126 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between − 0.7 dex and + 0.2 dex for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the [A/Fe] versus [Fe/H] plane by up to 0.1 dex, and it can remove spurious differences between the dwarfs and giants by up to 0.2 dex. The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy. The grids of departure coefficients are publicly available and can be implemented into LTE pipelines to make the most of observational data sets from large spectroscopic surveys.

Description: We present a high-resolution spectroscopic analysis of 62 red giants in the Milky Way globular cluster (GC) NGC 5286. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups, and combined them with photometry of multiple sequences observed along the colour–magnitude diagram. Our principal results are: (i) a broad, bimodal distribution in s -process element abundance ratios, with two main groups, the s -poor and s -rich groups; (ii) substantial star-to-star Fe variations, with the s -rich stars having higher Fe, e.g. $\langle [\mathrm{Fe/H}]\rangle _{s{\hbox{-}\rm rich}} - \langle [\mathrm{Fe/H}]\rangle _{s{\hbox{-}\rm poor}}$  ~ 0.2 dex; and (iii) the presence of O–Na–Al (anti)correlations in both stellar groups. We have defined a new photometric index, c BVI  = ( B – V ) – ( V – I ), to maximize the separation in the colour–magnitude diagram between the two stellar groups with different Fe and s -element content, and this index is not significantly affected by variations in light elements (such as the O–Na anticorrelation). The variations in the overall metallicity present in NGC 5286 add this object to the class of anomalous GCs. Furthermore, the chemical abundance pattern of NGC 5286 resembles that observed in some of the anomalous GCs, e.g. M 22, NGC 1851, M 2, and the more extreme Centauri, that also show internal variations in s -elements, and in light elements within stars with different Fe and s -elements content. In view of the common variations in s -elements, we propose the term s -Fe- anomalous GCs to describe this sub-class of objects. The similarities in chemical abundance ratios between these objects strongly suggest similar formation and evolution histories, possibly associated with an origin in tidally disrupted dwarf satellites.

Description: The stars in the Magellanic Clouds with the largest degree of obscuration are used to probe the highly uncertain physics of stars in the asymptotic giant branch (AGB) phase of evolution. Carbon stars in particular provide key information on the amount of third dredge-up and mass-loss. We use two independent stellar evolution codes to test how a different treatment of the physics affects the evolution on the AGB. The output from the two codes is used to determine the rates of dust formation in the circumstellar envelope, where the method used to determine the dust is the same for each case. The stars with the largest degree of obscuration in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are identified as the progeny of objects of initial mass 2.5–3 M and ~1.5 M , respectively. This difference in mass is motivated by the difference in the star formation histories of the two galaxies, and offers a simple explanation of the redder infrared colours of C-stars in the LMC compared to their counterparts in the SMC. The comparison with the Spitzer colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass fraction X ( C ) ~ 5  x  10 –3 must have been reached by stars of initial mass around 1.5 M . Our results confirm the necessity of adopting low-temperature opacities in stellar evolutionary models of AGB stars. These opacities allow the stars to obtain mass-loss rates high enough (10 –4 M yr –1 ) to produce the amount of dust needed to reproduce the Spitzer colours.

Description: With the exception of Terzan 5, all the Galactic globular clusters that possess significant metallicity spreads, such as Cen and M22, are preferentially the more luminous clusters with extended horizontal branches. Here we present radial velocities and chemical abundances for seven bright giants in the globular cluster M62, a previously little-studied cluster. With M V  = –9.18, M62 is the ninth most luminous Galactic globular cluster and has an extended horizontal branch. Within our sample, we find (i) no evidence for a dispersion in metallicity, [Fe/H], beyond the measurement uncertainties, (ii) star-to-star abundance variations for C, O, Na and Al with the usual correlations between these elements as seen in other globular clusters, and (iii) a global enrichment for the elements Zr, Ba and La at the level [X/Fe] ~= +0.4 dex. For elements heavier than La, the abundance ratios are consistent with the scaled-solar r -process distribution. Below La, the abundances are anomalous when compared to the scaled-solar s -process or r -process distributions. For these elements, the abundance signature in M62 is in agreement with predictions of the s -process from fast-rotating massive stars, although the high [Rb/Y] ratio we measure may be a challenge to this scenario.

Description: We present new theoretical stellar evolutionary models of metal-rich asymptotic giant branch (AGB) stars. Stellar models are evolved with initial masses between 1 and 7 M at Z  = 0.007, and 1 and 8 M at Z  = 0.014 (solar) and at Z  = 0.03. We evolve models with a canonical helium abundance and with helium-enriched compositions ( Y  = 0.30, 0.35, and 0.40) at Z  = 0.014 and 0.03. The efficiency of third dredge-up and the mass range of carbon stars decreases with an increase in metallicity. We predict carbon stars form from initial masses between 1.75 and 7 M at Z  = 0.007 and between 2 and 4.5 M at solar metallicity. At Z  = 0.03, the mass range for C-star production is narrowed to 3.25–4 M . The third dredge-up is reduced when the helium content of the model increases owing to the reduced number of thermal pulses on the AGB. A small increase of Y  = 0.05 is enough to prevent the formation of C stars at Z  = 0.03, depending on the mass-loss rate, whereas at Z  = 0.014, an increase of Y   0.1 is required to prevent the formation of C stars. We speculate that the probability of finding C stars in a stellar population depends as much on the helium abundance as on the metallicity. To explain the paucity of C stars in the inner region of M31, we conclude that the observed stars have Y   0.35 or that the stellar metallicity is higher than [Fe/H]  0.1.

Description: The dimensions of Fanaroff–Riley class I jets and the stellar densities at galactic centres imply that there will be numerous interactions between the jet and stellar winds. These may give rise to the observed diffuse and ‘knotty’ structure of the jets in the X-ray, and can also mass load the jets. We performed modelling of internal entrainment from stars intercepted by Centaurus A's jet, using stellar evolution- and wind codes. From photometry and a code-synthesized population of 12 Gyr ( Z  = 0.004), 3 Gyr ( Z  = 0.008) and 0–60 Myr ( Z  = 0.02) stars, appropriate for the parent elliptical NGC 5128, the total number of stars in the jet is ~8 10 8 . Our model is energetically capable of producing the observed X-ray emission, even without young stars. We also reproduce the radio through X-ray spectrum of the jet, albeit in a downstream region with distinctly fewer young stars, and recover the mean X-ray spectral index. We derive an internal entrainment rate of ~2.3 10 –3 M  yr –1 which implies substantial jet deceleration. Our absolute nucleosynthetic yields for the Asymptotic Giant Branch stellar population in the jet show the highest amounts for 4 He, 16 O, 12 C, 14 N and 20 Ne. If some of the events at ≥55 EeV detected by the Pierre Auger Observatory originate from internal entrainment in Centaurus A, we predict that their composition will be largely intermediate-mass nuclei with 16 O, 12 C and 14 N the key isotopes.

Description: The dimensions of Fanaroff–Riley class I jets and the stellar densities at galactic centres imply that there will be numerous interactions between the jet and stellar winds. These may give rise to the observed diffuse and ‘knotty’ structure of the jets in the X-ray, and can also mass load the jets. We performed modelling of internal entrainment from stars intercepted by Centaurus A's jet, using stellar evolution- and wind codes. From photometry and a code-synthesized population of 12 Gyr ( Z  = 0.004), 3 Gyr ( Z  = 0.008) and 0–60 Myr ( Z  = 0.02) stars, appropriate for the parent elliptical NGC 5128, the total number of stars in the jet is ~8 10 8 . Our model is energetically capable of producing the observed X-ray emission, even without young stars. We also reproduce the radio through X-ray spectrum of the jet, albeit in a downstream region with distinctly fewer young stars, and recover the mean X-ray spectral index. We derive an internal entrainment rate of ~2.3 10 –3 M  yr –1 which implies substantial jet deceleration. Our absolute nucleosynthetic yields for the Asymptotic Giant Branch stellar population in the jet show the highest amounts for 4 He, 16 O, 12 C, 14 N and 20 Ne. If some of the events at ≥55 EeV detected by the Pierre Auger Observatory originate from internal entrainment in Centaurus A, we predict that their composition will be largely intermediate-mass nuclei with 16 O, 12 C and 14 N the key isotopes.

Description: Motivated by unexplained observations of low sulphur abundances in planetary nebulae (PNe) and the PG1159 class of post-asymptotic giant branch (AGB) stars, we investigate the possibility that sulphur may be destroyed by nucleosynthetic processes in low-to-intermediate mass stars during stellar evolution. We use a 3 M , Z  = 0.01 evolutionary sequence to examine the consequences of high and low reaction rate estimates of neutron captures on to sulphur and neighbouring elements. In addition, we have also tested high and low rates for the neutron producing reactions 13 C(α,n) 16 O and 22 Ne(α,n) 25 Mg. We vary the mass width of a partially mixed zone (PMZ), which is responsible for the formation of a 13 C pocket and is the site of the 13 C(α,n) 16 O neutron source. We test PMZ masses from zero up to an extreme upper limit of the entire He-intershell mass at 10 –2 M . We find that the alternative reaction rates and variations to the PMZ have almost no effect on surface sulphur abundances and do not reproduce the anomaly. To understand the effect of initial mass on our conclusions, 1.8 and 6 M evolutionary sequences are also tested with similar results for sulphur abundances. We are able to set a constraint on the size of the PMZ, as PMZ sizes that are greater than half of the He-intershell mass (in the 3 M model) are excluded by comparison with neon abundances in PNe. We compare the 1.8 M model's intershell abundances with observations of PG1159–035, whose surface abundances are thought to reflect the intershell composition of a progenitor AGB star. We find general agreement between the patterns of F, Ne, Si, P and Fe abundances and a very large discrepancy for sulphur where our model predicts abundances that are 30–40 times higher than those observed in the star.