arXiv subjects

M. M. Miller Bertolami

Publications and source records attributed to M. M. Miller Bertolami.

At least 19 recordsLinked to original sources

arXiv2026

Stars on the ascending helium giant branch I. From white dwarf merger to helium giant: the evolutionary state of the rapidly rotating hot subdwarf HE 1518-0948

Hot subdwarf stars with masses above $0.8 M_\odot$ ascend the helium giant branch after the end of core helium burning, before entering the white dwarf cooling track or exploding as type Ib/c supernovae. Such massive helium stars are typically expected to form through the stripping of an intermediate mass star by a binary companion after which some hydrogen is still expected to be retained. However, the subclass of extreme helium rich hot subdwarfs (He-sdOs) shows no or very weak hydrogen traces, and their low binary fraction suggests that they are either created through single-star evolution triggered by a late hot flash in a low-mass red giant or the merger of two helium white dwarfs. Most He-sdOs are located close to the helium zero-age main sequence, while a small number exhibit much lower surface gravities, indicating inflated radii. Whether these objects are evolutionarily connected to the main He-sdO population remains unclear. In this work, we analyse the luminous, extreme helium-rich, low-surface-gravity sdO HE 1518-0948 (HE 1518) through a detailed spectroscopic study of optical and ultraviolet data. A comparison with evolutionary models indicates that HE 1518 is the product of a massive double helium white dwarf merger and is currently undergoing helium shell burning while ascending the helium giant branch. This makes HE 1518 one of only a few known objects located in this sparsely populated region of the Hertzsprung-Russell diagram. Such stars provide valuable laboratories for studying the evolution of massive hot subdwarfs beyond core helium burning, and their high luminosities allow them to be detected at large distances.

astro-ph.SR↗

arXiv2026

Formation and Evolution of [Wolf-Rayet] Planetary Nebulae through a Late Thermal Pulse

We present the first radiation-hydrodynamical simulations of the formation of a born-again planetary nebula (PN) triggered by a late thermal pulse (LTP). The 2D radiation-hydrodynamic simulations, performed with the {\sc pluto} code, have been consistently coupled to stellar evolution calculations using the Modules for Experiments in Stellar Astrophysics ({\sc mesa}) code. Very particularly the stellar evolution model uses (i) updated opacity tables for H-deficient, C-rich mixtures during the LTP, and (ii) a mass-loss prescription tailored for H-deficient [Wolf-Rayet]([WR])-type winds during the post-LTP phase. Our stellar model reproduces the nearly complete depletion of H expected after an LTP event, while matching the observed abundances and spectral types of iconic [WR]-type central stars of PNe. The simulations show for the first time that the H-deficient LTP ejecta forms a transient double-shell structure which, after $\sim$1000 yr, becomes fully mixed with the H-rich PN. The ejecta mass ($\sim3.4\times10^{-4}$~M$_\odot$) is too small to leave a lasting imprint on the nebular abundances, predicting H-rich PNe around [WR] central stars. The injection of LTP material into the hot bubble drives turbulence, clump formation, and enhanced mixing, providing an explanation to the larger expansion velocities and larger turbulent nebular structures of PNe with [WR] central stars compared to those with H-rich central stars. These results provide robust support for the born-again scenario as the origin of H-deficient [WR] central stars within H-rich PNe.

astro-ph.SR↗

arXiv2025

Local mixing length theory with compositional effects:\ First application to asymptotic giant branch evolution

During the evolution of stars on the asymptotic giant branch (AGB), thermal pulses lead to the formation of strongly stratified layers in the outer regions of the CO core, which might lead to inversions in the chemical gradient. Such inversions would produce instabilities beyond the ones predicted by the Schwarzschild criterion and the standard use of mixing length theory (MLT). We used a set of MLT equations that consider the impact of the background chemical gradients. This extension of MLT is referred to in this work as MLT$\sharp$, to make a distinction between both prescriptions. We applied MLT$\sharp$ in tandem with the more general Ledoux instability criterion. We computed the evolution in the AGB phase and compared the chemical profiles resulting from MLT, MLT$\sharp$ and the double diffusive GNA theory. We continued the evolution through a post-AGB thermal pulse and performed a pulsational analysis of the resultant GW Vir models to asses $g$-mode pulsation periods. Finally, we tested our results with pulsation properties of known GW Vir stars derived from recent observations. We find that the much simpler MLT$\sharp$ set of equations closely reproduces the results from the GNA theory. As such, MLT$\sharp$ offers a simple way to include chemically driven convection in stellar evolution computations. Stellar evolution simulations show that Rayleigh-Taylor and thermohaline instabilities can play an important role during the TP-AGB. We obtained significantly different chemical profiles using a standard MLT approach compared to those resulting from our MLT$\sharp$ and GNA computations. Our adiabatic pulsational analysis shows that these differences in the chemical stratification leave clear mode-trapping signatures in the pulsation spectrum of the GW Vir models.

astro-ph.SR↗

arXiv2025

Catalogue of central stars of extragalactic planetary nebulae

Context. Central stars of planetary nebulae (CSPNe) are essential for understanding the final evolutionary stages of low- and intermediate-mass stars. However, their study in extragalactic environments remains challenging due to their intrinsic faintness and the limited availability of high-quality data. Aims. We aim to provide a comprehensive and up-to-date catalogue of extragalactic CSPNe in order to enable a more complete view of their physical properties across different galactic environments and metallicities. Methods. The catalogue was assembled using data from the most recent and reliable literature sources. Priority was given to collecting effective temperatures and luminosities that have either been directly reported or consistently derived. When available, spectral types or specific spectral features -- such as P-Cygni profiles or broad H$α$ emission lines -- were also included. This approach allowed for broader characterisation of the sample, even when accurate classifications are not available. Results. We present a new compilation of extragalactic CSPNe -- the largest to date -- comprising over 800 objects located in the Magellanic Clouds, NGC 300, NGC 5128, and fifteen other nearby galaxies. This catalogue enables, for the first time, a global comparison of CSPNe physical parameters beyond the Milky Way. Updated Hertzsprung-Russell diagrams are provided featuring CSPNe from seven different galaxies, revealing trends and outliers that merit further investigation. The catalogue represents a valuable resource for future spectroscopic follow-up and for improving our understanding of post-AGB evolution in diverse galactic contexts.

astro-ph.GA↗

arXiv2025

The $i$-processes nucleosynthesis during the formation of He-rich hot-subdwarf stars

It has been shown that proton ingestion episodes can happen in the formation of hot-subdwarf stars, and that neutron-capture processes are possible in those cases. Moreover, some helium-rich hot subdwarfs display extraordinarily high abundances of heavy elements such as Zr, Yr and Pb on their surfaces. We explore under which conditions neutron-capture processes can occur in late helium core flashes, i.e. those occurring in the cores of stripped red-giant stars. We compute evolutionary models through the helium core flash and the subsequent hydrogen ingestion episode in stripped red giant stars. Stellar structure models are then used in post-processing to compute the detailed evolution of neutron-capture elements. We find that for metallicities of $10^{-3}$ and below, neutron densities can be as high as $10^{15}\,$cm$^{-3}$ and intermediate neutron capture processes occur in some of our models. The results depend very strongly on the H-envelope mass that survives after the stripping. Interestingly, we find that computed abundances in some of our models closely match the element abundances up to tin observed for EC 22536-5304, the only well-studied star for which the hot-flasher scenario assumed in our models is the most likely evolutionary path. Intermediate neutron capture processes can occur in the He-core flash experienced by the cores of some stripped red giants, and might be connected to the abundances of heavy elements observed in some helium-rich hot-subdwarf stars. The agreement between the observed abundances in EC 22536-5304 and those of our models offers support to our nucleosynthesis calculations. Moreover, if confirmed, the idea that heavy element abundances retain signatures of the different evolutionary channels opens the possibility that heavy element abundances in iHe-sdOB stars can be used to infer their evolutionary origin.

astro-ph.SR↗

arXiv2024

Explorando el impacto de los gradientes químicos en los procesos de mezcla del interior estelar

During the various steps of stellar evolution are formed convectives zones that alter the chemical stratification in stars. Usually, in astrophysics is used the Mixing Length Theory (MLT) for modeling the convective movement and, in general, it is used with the Schwarzschild instability criterion, which neglects the impact of chemical composition gradients in the development of convection. However, towards the end of central helium burning and during the thermal pulses in the Asymptotic Giant Branch (AGB) are produced stratification processes with inversions in the chemical gradient that would produce instabilities beyond the ones predicted by the Schwarzschild criterion. These instabilities would alter the chemical profile in the white dwarfs, with respect to the one predicted by MLT, having observable consequences in the pulsational modes of such objects. In the present work we will explore an extension of MLT in which we will consider the chemical instabilities as generators of convectives and non-convectives instabilities. This theory will be applied in stellar evolution models in comparison with standard MLT and a double diffusive mixing theory, discussing the benefits and shortcomings of each one.

astro-ph.SR↗

arXiv2023

A self-synthesized origin of heavy metals in hot subdwarf stars?

Some He-rich hot subdwarf stars (He-sdOBs) present high abundances of trans-iron elements, such as Sr, Y, Zr and Pb. Diffusion processes are important in hot subdwarf stars, and it is thought that the high abundances of heavy elements in these stars are due to the action of radiative levitation. However, during the formation of He-sdOBs, hydrogen can be ingested into the convective zone driven by the He-core flash. It is known that episodes in which protons are being ingested into He-burning convective zones can lead to neutron-capture processes and the formation of heavy elements. In this work we aim to explore for the first time if neutron-capture processes can occur in late He-core flashes happening in the cores of the progenitors of He-sdOBs. We compute a detailed evolutionary model of a stripped red-giant star with a stellar evolution code with a nuclear network comprising 32 isotopes. Then we post-process the stellar models in the phase of He and H burning with a post-processing nucleosynthesis code with a nuclear network of 1190 species that allows us to follow the neutron-capture processes in detail. We find the occurrence of neutron-capture processes in our model, with neutron densities reaching a value of $\sim5\times10^{12}\,{\rm cm}^{-3}$. We find that the trans-iron elements are enhanced in the surface by 1 to 2 dex as compared to initial compositions. Moreover, the relative abundance pattern $[{\rm X}_i/\rm{Fe}]$ produced by neutron-capture processes closely resembles those observed in some He-sdOBs, hinting at a possible self-synthesized origin of the heavy elements in these stars. We conclude that intermediate neutron-capture processes can occur during a proton ingestion event in the He-core flash of stripped red-giant stars. This mechanism offers a natural channel to produce the heavy elements observed in some of the He-sdOBs.

astro-ph.SR↗

arXiv2023

Uncertainties in the 12C+12C reaction rate and their impact on the composition of ultra-massive WDs

Stars with initial masses 7 Msun . MZAMS . 9 Msun reach temperatures high enough to ignite C under degenerate conditions after the end of He-core burning (Garcia-Berro & Iben 1994). These isolated stars are expected to evolve into the so-called super AGB (SAGB) phase and may end their lives as ultra-massive ONe WDs (see Siess 2006,2007, 2010; Camisassa et al. 2019, and references therein). The exact proportions of O and Ne found in the core at the end of the SAGB phase will determine the cooling times and pulsational properties of these WDs. Uncertainties affecting the rates of nuclear reactions occurring during the C burning phase should have a measurable impact on the distribution of 16O, 20Ne, 23Na and 24Mg and, consequently, on the evolution of the WD. Here we present a study of the impact of uncertainties in the 12C(12C, α)20Ne and 12C(12C, p)23Na nuclear reaction rates (and their branching ratios) on the chemical structure of intermediate- to high-mass progenitors at the end of the C-burning phase. Using the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA) we computed evolutionary sequences for stars with initial masses 7.25<= MZAMS /Msun <=8.25, from the ZAMS to the SAGB phase, adopting different prescriptions for the 12C+12C burning rates. We found that adopting lower reaction rates for the 12C+12C burning delays C-ignition by at most 2700 yrs, and the ignition takes place in a position further from the center. Our results shows that differences in the 20Ne central abundances remain modest, below 14%.

astro-ph.SR↗

arXiv2022

Chemistry and physical properties of the born-again planetary nebula HuBi 1

The central star of the planetary nebula (PN) HuBi\,1 has been recently proposed to have experienced a very late thermal pulse (VLTP), but the dilution of the emission of the recent ejecta by that of the surrounding H-rich old outer shell has so far hindered confirming its suspected H-poor nature. We present here an analysis of the optical properties of the ejecta in the innermost regions of HuBi\,1 using MEGARA high-dispersion integral field and OSIRIS intermediate-dispersion long-slit spectroscopic observations obtained with the 10.4m Gran Telescopio de Canarias. The unprecedented tomographic capability of MEGARA to resolve structures in velocity space allowed us to disentangle for the first time the H$α$ and H$β$ emission of the recent ejecta from that of the outer shell. The recent ejecta is found to have much higher extinction than the outer shell, implying the presence of large amounts of dust. The spatial distribution of the emission from the ejecta and the locus of key line ratios in diagnostic diagrams probe the shock excitation of the inner ejecta in HuBi\,1, in stark contrast with the photoionization nature of the H-rich outer shell. The abundances of the recent ejecta have been computed using the {\sc mappings v} code under a shock scenario. They are found to be consistent with a born-again ejection scenario experienced by the progenitor star, which is thus firmly confirmed as a new "born-again" star.

astro-ph.SR↗

arXiv2021

EuCAPT White Paper: Opportunities and Challenges for Theoretical Astroparticle Physics in the Next Decade

Astroparticle physics is undergoing a profound transformation, due to a series of extraordinary new results, such as the discovery of high-energy cosmic neutrinos with IceCube, the direct detection of gravitational waves with LIGO and Virgo, and many others. This white paper is the result of a collaborative effort that involved hundreds of theoretical astroparticle physicists and cosmologists, under the coordination of the European Consortium for Astroparticle Theory (EuCAPT). Addressed to the whole astroparticle physics community, it explores upcoming theoretical opportunities and challenges for our field of research, with particular emphasis on the possible synergies among different subfields, and the prospects for solving the most fundamental open questions with multi-messenger observations.

astro-ph.HE↗

arXiv2021

The importance of thermal torques on the migration of planets growing by pebble accretion

A key process in planet formation is the exchange of angular momentum between a growing planet and the protoplanetary disc, which makes the planet migrate through the disc. Several works show that in general low-mass and intermediate-mass planets migrate towards the central star, unless corotation torques become dominant. Recently, a new kind of torque, called the thermal torque, was proposed as a new source that can generate outward migration of low-mass planets. While the Lindblad and corotation torques depend mostly on the properties of the protoplanetary disc and on the planet mass, the thermal torque depends also on the luminosity of the planet, arising mainly from the accretion of solids. Thus, the accretion of solids plays an important role not only in the formation of the planet but also in its migration process. In a previous work, we evaluated the thermal torque effects on planetary growth and migration mainly in the planetesimal accretion paradigm. In this new work, we study the role of the thermal torque within the pebble accretion paradigm. Computations are carried out consistently in the framework of a global model of planet formation that includes disc evolution, dust growth and evolution, and pebble formation. We also incorporate updated prescriptions of the thermal torque derived from high resolution hydrodynamical simulations. Our simulations show that the thermal torque generates extended regions of outward migration in low viscosity discs. This has a significant impact in the formation of the planets.

astro-ph.EP↗

arXiv2020

Giant planet formation at the pressure maxima of protoplanetary disks II. A hybrid accretion scenario

Recent observations of protoplanetary disks have revealed ring-like structures that can be associated to pressure maxima. Pressure maxima are known to be dust collectors and planet migration traps. Most of planet formation works are based either on the pebble accretion model or on the planetesimal accretion model. However, recent studies proposed the possible formation of Jupiter by the hybrid accretion of pebbles and planetesimals. We aim to study the full process of planet formation consisting of dust evolution, planetesimal formation and planet growth at a pressure maximum in a protoplanetary disk. We compute, through numerical simulations, the gas and dust evolution, including dust growth, fragmentation, radial drift and particle accumulation at a pressure bump. We also consider the formation of planetesimals by streaming instability and the formation of a moon-size embryo that grows into a giant planet by the hybrid accretion of pebbles and planetesimals. We find that pressure maxima in protoplanetary disks are efficient collectors of dust drifting inwards. The condition of planetesimal formation by streaming instability is fulfilled due to the large amount of dust accumulated at the pressure bump. Then, a massive core is quickly formed (in $\sim 10^4$ yr) by the accretion of pebbles. After the pebble isolation mass is reached, the growth of the core slowly continues by the accretion of planetesimals. The energy released by planetesimal accretion delays the onset of runaway gas accretion, allowing a gas giant to form after $\sim$1 Myr of disk evolution. The pressure maximum also acts as a migration trap. Pressure maxima in protoplanetary disks are preferential locations for dust traps, planetesimal formation by streaming instability and planet migration traps. All these conditions allow the fast formation of a giant planet by the hybrid accretion of pebbles and planetesimals.

astro-ph.EP↗

arXiv2020

An in-depth reanalysis of the alleged type Ia supernova progenitor Henize 2-428

The nucleus of the planetary nebula Hen2-428 is a short orbital-period (4.2h), double-lined spectroscopic binary, whose status as a potential supernova type Ia progenitor has raised some controversy in the literature. With the aim of resolving this debate, we carried out an in-depth reanalysis of the system. Our approach combines a refined wavelength calibration, thorough line-identifications, improved radial-velocity measurements, non-LTE spectral modeling, as well as multi-band light-curve fitting. Our results are then discussed in view of state-of-the-art stellar evolutionary models. Besides systematic zero-point shifts in the wavelength calibration of the OSIRIS spectra which were also used in the previous analysis of the system, we found that the spectra are contaminated with diffuse interstellar bands. Our Voigt-profile radial velocity fitting method, which considers the additional absorption of these diffuse interstellar bands, reveals significantly lower masses ($M_1=0.66\pm0.11M_\odot$ and $M_2=0.42\pm0.07M_\odot$) than previously reported and a mass ratio that is clearly below unity. Our spectral and light curve analyses lead to consistent results, however, we find higher effective temperatures and smaller radii than previously reported. Moreover, we find that the red-excess that was reported before to prove to be a mere artifact of an outdated reddening law that was applied. Our work shows that blends of HeII 5412A with diffuse interstellar bands have led to an overestimation of the previously reported dynamical masses of Hen2-428. The merging event of Hen2-428 will not be recognised as a supernova type Ia, but most likely leads to the formation of a H-deficient star. We suggest that the system was formed via a first stable mass transfer episode, followed by common envelope evolution, and it is now composed of a post-early AGB star and a reheated He-core white dwarf.

astro-ph.SR↗

arXiv2020

Catalogue of central stars of planetary nebulae: Expanded edition

Planetary nebulae represent a potential late stage of stellar evolution, however the central stars (CSPNe) are relatively faint and therefore pertinent information is merely available for <20% of the Galactic sample. Consequently, the literature was surveyed to construct a new catalogue of 620 CSPNe featuring important spectral classifications and information. The catalogue supersedes the existing iteration by 25%, and includes physical parameters such as luminosity, surface gravity, temperature, magnitude estimates, and references for published spectra. The marked statistical improvement enabled the following pertinent conclusions to be determined: the H-rich/H-poor ratio is 2:1, there is a deficiency of CSPNe with types [WC 5-6], and nearly 80% of binary central stars belong to the H-rich group. The last finding suggests that evolutionary scenarios leading to the formation of binary central stars interfere with the conditions required for the formation of H-poor CSPN. Approximately 50% of the sample with derived values of log L, log Teff, and log g, exhibit masses and ages consistent with single stellar evolutionary models. The implication is that single stars are indeed able to form planetary nebulae. Moreover, it is shown that H-poor CSPNe are formed by higher mass progenitors. The catalogue is available through the Vizier database.

astro-ph.GA↗

arXiv2019

Spectral analysis of the hybrid PG 1159-type central stars of the planetary nebulae Abell 43 and NGC 7094

Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the He-rich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of $\mathrm{H}\approx\,20\,$% by mass (AFTP), $\approx 1\,$% (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of $T_\mathrm{eff} = 115\,000\pm 5\,000\,$K and determine surface gravities of $\log (g\,/\,\mathrm{cm/s^2}) = 5.6\pm 0.1$ for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of $0.57^{+0.07}_{-0.04}\,M_\odot$ and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of $0.25 \pm 0.03$ and $0.15 \pm 0.03$, respectively, and a Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution timescales expected from the AFTP scenario and alternatives should be explored.

astro-ph.SR↗

arXiv2019

Hot UV-bright stars of galactic globular clusters

We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce. We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of -2.3<[Fe/H]<-1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, two are evolving away from the HB, one is a foreground white dwarf, and one is a white dwarf merger. So spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. Adding published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low.

astro-ph.SR↗

arXiv2019

Thermal torque effects on the migration of growing low-mass planets

As planets grow the exchange of angular momentum with the gaseous component of the protoplanetary disc produces a net torque resulting in a variation of the semi-major axis of the planet. For low-mass planets not able to open a gap in the gaseous disc this regime is known as type I migration. Pioneer works studied this mechanism in isothermal discs finding fast inward type I migration rates that were unable to reproduce the observed properties of extrasolar planets. In the last years, several improvements have been made in order to extend the study of type I migration rates to non-isothermal discs. Moreover, it was recently shown that if the planet's luminosity due to solid accretion is taken into account, inward migration could be slowed down and even reversed. In this work, we study the planet formation process incorporating, and comparing, updated type I migration rates for non-isothermal discs and the role of planet's luminosity over such rates. We find that the latter can have important effects on planetary evolution, producing a significant outward migration for the growing planets.

astro-ph.EP↗

arXiv2017

On the Production of He, C and N by Low and Intermediate Mass Stars: A Comparison of Observed and Model-Predicted Planetary Nebula Abundances

The primary goal of this paper is to make a direct comparison between the measured and model-predicted abundances of He, C and N in a sample of 35 well-observed Galactic planetary nebulae (PN). All observations, data reductions, and abundance determinations were performed in house to ensure maximum homogeneity. Progenitor star masses (M < 4M_sun) were inferred using two published sets of post-AGB model tracks and L and T_eff values. We conclude the following: 1) the mean values of N/O across the progenitor mass range exceeds the solar value, indicating significant N enrichment in the majority of our objects; 2) the onset of hot bottom burning appears to begin around 2 solar masses, i.e., lower than ~5 M_sun implied by theory; 3) most of our objects show a clear He enrichment, as expected from dredge-up episodes; 4) the average sample C/O value is 1.23, consistent with the effects of third dredge-up; and 5) model grids used to compare to observations successfully span the distribution over metallicity space of all C/O and many He/H data points but mostly fail to do so in the case of N/O. The evident enrichment of N in PN and the general discrepancy between the observed and model-predicted N/O abundance ratios signal the need for extra-mixing as an effect of rotation and/or thermohaline mixing in the models. The unexpectedly high N enrichment that is implied here for low mass stars, if confirmed, will likely impact our conclusions about the source of N in the Universe.

astro-ph.SR↗