Nature vs. Nurture: Distinguishing Effects from Stellar Processing and Chemical Evolution on Carbon and Nitrogen in Red Giant Stars

TL;DR

This work demonstrates that the initial [C/N] composition of stars, which varies with metallicity and alpha-enhancement, meaningfully influences surface [C/N] after the first dredge-up. By leveraging subgiants as proxies for birth abundances and combining APOGEE DR17 with APOKASC3 asteroseismic masses, the authors map how [C/N] evolves through RGB and RC phases, finding that [C/N] serves as a mass diagnostic only in certain regimes (notably below ~1.5 M⊙ for LRGB and extending to higher masses for RC). They also find clear evidence of extra mixing in high-alpha stars at low metallicities, while low-alpha giants show little compelling mixing in [C/N], and RGB mass loss signals are weak. The results challenge solar-scaled C and N in standard stellar models and have implications for stellar physics and population modeling, with future surveys expected to broaden the calibration across wider parameter space.

Abstract

The surface [C/N] ratios of evolved giants are strongly affected by the first dredge-up (FDU) of nuclear-processed material from stellar cores. C and N also have distinct nucleosynthetic origins and serve as diagnostics of mixing and mass loss. We use subgiants to find strong trends in the birth [C/N] with [Fe/H], which differ between the low-$α$ and high-$α$ populations. We demonstrate that these birth trends have a strong impact on the surface abundances after the FDU. This effect is neglected in current stellar models, which use solar-scaled C and N. We map out the FDU as a function of evolutionary state, mass, and composition using a large and precisely measured asteroseismic dataset in first-ascent red giant branch (RGB) and core He-burning, or red clump (RC), stars. We describe the domains where [C/N] is a useful mass diagnostic and find that the RC complements the RGB and extends the range of validity to higher mass. We find evidence for extra mixing on the RGB below [Fe/H]= -0.4, matching literature results, for high-$α$ giants, but there is no clear evidence of mixing in the low-$α$ giants. The predicted signal of mass loss is weak and difficult to detect in our sample. We discuss implications for stellar physics and stellar population applications.

Figures (19)

• Figure 1: The samples in this paper with evolution tracks and the full DR17 samples for context. The lines are MIST stellar evolution tracks of 1 $M_\odot$ (dotted) and 1.6 $M_\odot$ (dashed) at solar metallicity. These tracks have markers denoting the onset, midpoint, and completion of the FDU as well as the beginning of re-contraction at the RGB bump. Due to the FDU beginning and ending slowly, the onset and completion are marked at the points where 10% and 90% of the total [C/N] change has occurred, respectively. Unlike the subgiants, not all giants from APOGEE are used because they are restricted to the APOKASC3 sample.
• Figure 2: [C/N] versus log g for the APOGEE sample, with the same tracks and colouring as Figure \ref{['fig:HR']}.
• Figure 3: Density in the [Mg/Fe]-[Fe/H] plane for the APOGEE DR17 subgiant sample and the APOKASC3 giant sample. The solid line indicates the boundary between high- and low-$\alpha$ populations used in this paper, and the dashed lines show boundary shifted by 0.02 dex which reflects the actual cuts used to define the samples.
• Figure 4: [C/Fe], [N/Fe], and [C/N] versus [Fe/H] for both low-$\alpha$ and high-$\alpha$ subgiant samples. The points represent bins of 200 stars with error bars representing the standard error of the values in the bin. The solid lines are the second-order polynomial fit of the data, and the shaded region indicates the error of the fit. The dashed and dash-dotted lines show the birth values employed by YREC and MIST models respectively. The single points in the corner of the plot show the median measurement errors of the respective parameters shown in the plot. The regression fit coefficients are given in Table \ref{['tab:subgiantfe']}
• Figure 5: [C/Mg], [N/Mg], and [C/N] versus [Mg/H] for both the low-$\alpha$ and high-$\alpha$ subgiant samples. It has the same plotting conventions as Figure \ref{['fig:cfebin']}. The regression fit coefficients are given in Table \ref{['tab:subgiantfe']}