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1. Carbon Isotope Ratios in Planetary Nebulae: The Unexpected Enhancement of 13C

   Ziurys, L.M.; Schmidt, D.R.; Woolf, N.J. (July 2020, The astrophysical journal)

   The 12C/13C ratio has been measured toward a sample of planetary nebulae (PNe) using millimeter observations of CO, HCN, HNC, CN, and other species, conducted with the 12 m antenna and the Submillimeter Telescope of the Arizona Radio Observatory. The observed nebulae spanned the entire lifetime of PNe, from ∼900 to 12,000 yr, and include well-known objects such as NGC 7293 (Helix), NGC 6720 (Ring), and NGC 2440, as well as relatively unexplored nebulae (M3–28, M2–48, and M3–55). In most cases, multiple molecules and transitions were used in the ratio determination, resulting in the most accurate values available to date, with 10%–40% uncertainties. The ratios found were unexpectedly low, lying in the range 12C/13C ∼1.0 ± 0.7–13.2 ± 4.9, with an average value of 3.7—drastically less than found in the envelopes of C-rich AGB stars, and, in some cases, lower than the minimum value achieved in equilibrium CNO burning. Such low values are expected for the two O-rich nebulae studied (M2–9 and M2–48), because of insufficient third dredge-up events. However, most of the PNe observed were clearly carbon-rich, as deduced from the large number of C-bearing molecules present in them. Because nucleosynthesis ceases in the PN stage, both the C/O and the 12C/13C ratios must reflect abundances at the end of the AGB. These consistently low 12C/13C ratios, combined with the bipolar/multipolar morphologies of all planetary nebulae observed, suggest an explosive process involving proton-capture occurred at the AGB–PN transition. 

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2. Iron-peak Element Abundances in Warm Very Metal-poor Stars

   https://doi.org/10.3847/1538-4357/accd62  

   Sneden, Christopher; Boesgaard, Ann_Merchant; Cowan, John_J; Roederer, Ian_U; Den_Hartog, Elizabeth_A; Lawler, James_E (July 2023, The Astrophysical Journal)

   Abstract We have derived new detailed abundances of Mg, Ca, and the Fe-group elements Sc through Zn (Z= 21−30) for 37 main-sequence turnoff very metal-poor stars ([Fe/H] ≲−2.1). We analyzed Keck HIRES optical and near-UV high signal-to-noise spectra originally gathered for a Be abundance survey. Using typically ∼400 Fe-group lines with accurate laboratory transition probabilities for each star, we have determined accurate LTE metallicities and abundance ratios for neutral and ionized species of the 10 Fe-group elements as well asαelements Mg and Ca. We find good neutral/ion abundance agreement for the six elements that have detectable transitions of both species in our stars in the 3100–5800 Å range. Earlier reports of correlated Sc−Ti−V relative overabundances are confirmed, and appear to slowly increase with decreasing metallicity. To this element trio we add Zn; it also appears to be increasingly overabundant in the lowest-metallicity regimes. Co appears to mimic the behavior of Zn, but issues surrounding its abundance reliability cloud its interpretation. 

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3. Additional fluorine abundance determinations in evolved stars

   https://doi.org/10.1051/0004-6361/201935286  

   Abia, C.; Cristallo, S.; Cunha, K.; de Laverny, P.; Smith, V. V. (May 2019, Astronomy & Astrophysics)

   We present new fluorine abundance measurements for a sample of carbon-rich asymptotic giant branch (AGB) stars and two other metal-poor evolved stars of Ba/CH types. The abundances are derived from IR, K -band, high-resolution spectra obtained using GEMINI-S/Phoenix and TNG/Giano-b. Our sample includes an extragalactic AGB carbon star belonging to the Sagittarius dSph galaxy. The metallicity of our stars ranges from [Fe/H] = 0.0 down to − 1.4 dex. The new measurements, together with those previously derived in similar stars, show that normal (N-type) and SC-type AGB carbon stars of near solar metallicity present similar F enhancements, discarding previous hints that suggested that SC-type stars have larger enhancements. These mild F enhancements are compatible with current chemical-evolution models pointing out that AGB stars, although relevant, are not the main sources of this element in the solar neighbourhood. Larger [F/Fe] ratios are found for lower-metallicity stars. This is confirmed by theory. We highlight a tight relation between the [F/⟨s⟩] ratio and the average s-element enhancement [⟨s⟩/Fe] for stars with [Fe/H] > −0.5, which can be explained by the current state-of-the-art low-mass AGB models assuming an extended 13 C pocket. For stars with [Fe/H] < −0.5, discrepancies between observations and model predictions still exist. We conclude that the mechanism of F production in AGB stars needs further scrutiny and that simultaneous F and s-element measurements in a larger number of metal-poor AGB stars are needed to better constrain the models. 

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4. Chemical evolution of fluorine in the Milky Way

   https://doi.org/10.1093/mnras/stac3180  

   Womack, Kate A.; Vincenzo, Fiorenzo; Gibson, Brad K.; Côté, Benoit; Pignatari, Marco; Brinkman, Hannah E.; Ventura, Paolo; Karakas, Amanda (November 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Fluorine has many different potential sites and channels of production, making narrowing down a dominant site of fluorine production particularly challenging. In this work, we investigate which sources are the dominant contributors to the galactic fluorine by comparing chemical evolution models to observations of fluorine abundances in Milky Way stars covering a metallicity range of −2 < [Fe/H] < 0.4 and upper limits in the range of −3.4 < [Fe/H] < −2.3. In our models, we use a variety of stellar yield sets in order to explore the impact of varying both asymptotic giant branch (AGB) and massive star yields on the chemical evolution of fluorine. In particular, we investigate different prescriptions for initial rotational velocity in massive stars as well as a metallicity-dependent mix of rotational velocities. We find that the observed [F/O] and [F/Fe] abundance ratios at low metallicity and the increasing trend of [F/Ba] at [Fe/H] ≳ −1 can only be reproduced by chemical evolution models assuming, at all metallicities, a contribution from rapidly rotating massive stars with initial rotational velocities as high as 300 km s−1. A mix of rotational velocities may provide a more physical solution than the sole use of massive stars with vrot  =  300 km s−1, which are predicted to overestimate the fluorine and average s-process elemental abundances at [Fe/H] ≳ −1. The contribution from AGB stars is predicted to start at [Fe/H] ≈ −1 and becomes increasingly important at high metallicity, being strictly coupled to the evolution of the nitrogen abundance. Finally, by using modern yield sets, we investigate the fluorine abundances of Wolf–Rayet winds, ruling them out as dominant contributors to the galactic fluorine. 

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5. Chemical Abundances of Eight Highly-extincted Milky Way Planetary Nebulae*

   https://doi.org/10.3847/1538-3881/ac8a45  

   Manea, Catherine; Dinerstein, Harriet L.; Sterling, N. C.; Zeimann, Greg (October 2022, The Astronomical Journal)

   Abstract Low- and intermediate-mass (0.8M_⊙<M< 8M_⊙) stars that evolve into planetary nebulae (PNe) play an important role in tracing and driving Galactic chemical evolution. Spectroscopy of PNe enables access to both the initial composition of their progenitor stars and products of their internal nucleosynthesis, but determining accurate ionic and elemental abundances of PNe requires high-quality optical spectra. We obtained new optical spectra of eight highly-extincted PNe with limited optical data in the literature using the Low Resolution Spectrograph 2 on the Hobby–Eberly Telescope. Extinction coefficients, electron temperatures and densities, and ionic and elemental abundances of up to 11 elements (He, N, O, Ne, S, Cl, Ar, K, Fe, Kr, and Xe) are determined for each object in our sample. Where available, astrometric data from Gaia eDR3 is used to kinematically characterize the probability that each object belongs to the Milky Way's thin disk, thick disk, or halo. Four of the PNe show kinematic and chemical signs of thin disk membership, while two may be members of the thick disk. The remaining two targets lack Gaia data, but their solar O, Ar, and Cl abundances suggest thin disk membership. Additionally, we report the detection of broad emission features from the central star of M 3–35. Our results significantly improve the available information on the nebular parameters and chemical compositions of these objects, which can inform future analyses. 

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