On the production of He, C, and N by low- and intermediate-mass stars: a comparison of observed and model-predicted planetary nebula abundances

Abstract

The primary goal of this paper is to make a direct comparison between the measured and modelpredicted 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 ≤ 4 M ⊙ ) were inferred using two published sets of post-AGB model tracks and L and T ef f 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 M ⊙ , i.e., lower than ∼ 5 M ⊙ 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.

Impact of convective boundary mixing on the TP-AGB

“…This result indicates that studies that aim to link AGB or PNe abundances to the initial stellar progenitor masses, or ages of the host population, should rely on stellar models that include CBM at all pre-AGB phases. This is in agreement with recent hints of HBB taking place in stars with initial masses as low as M ZAMS ∼ 3M (Henry et al 2018;Fang et al 2018;Davis et al 2019).…”

“…Overall the effect of including additional CBM during core burning phases can play a significant role in changing the outcome of TP-AGB calculations, and given the evidence in favour of the existence of CBM during the core burning stages this effect needs to be addressed. This evidence is also reinforced by the recent hints of the existence of HBB in stars with initial masses as low as M ZAMS ∼ 3M (Henry et al 2018;Fang et al 2018;Davis et al 2019).…”

Self-consistent Predictions for LIER-like Emission Lines from Post-AGB Stars

“…There is evidence that nucleosynthetic processing by low-and intermediate-mass stars does not alter the alpha abundances, but does enhance He, N and C abundances compared to typical H ii regions (Karakas 2010;Maciel et al 2017). We enhance C and N relative to O following Henry et al (2018): log 10 (N/O) = −0.5 and log 10 (C/O) = 0 for the α-enhanced 12 + log O/H = 8.89.…”

Neutron-capture Elements in Planetary Nebulae: First Detections of Near-infrared [Te iii] and [Br v] Emission Lines* †

“…The enrichments for NGC 7027 are consistent with a progenitor initial mass of around 3±0.5M e as estimated by Zhang et al (2005) and other studies (see Sterling et al 2016, for references). The Kr enrichment of IC 418 suggests a lower mass for its progenitor star, which has been estimated as M= 1.75 M e (Morisset & Georgiev 2009) and as M = 1.4± 0.5 M e (Henry et al 2018).…”