Using Binary Population Synthesis to Calculate the Yields of Low- and Intermediate-Mass Binary Populations at Low Metallicity

TL;DR

This study applies binary population synthesis with binary_c to model low- and intermediate-mass ($0.7-7\,M_{\odot}$) stellar populations at $Z=0.0001$ and quantify how binary evolution shifts AGB yields. Key findings include a $\sim$37% reduction in TP-AGB stars, $\sim$38% less C, and a $\sim$35-40% drop in s-process yields when binaries are included, with hot-bottom burning stars providing substantial but uncertain N yields due to wind mass-loss prescriptions. The work also reveals a pronounced, time-dependent N overproduction between $300-700$ Myr after formation driven by mass transfer and mergers in lower-mass binaries, and highlights large uncertainties arising from binary evolution and TP-AGB mass-loss treatments. Overall, binary evolution significantly alters the chemical output and its temporal evolution, underscoring the need to integrate binary physics into chemical evolution models and to improve constraints on CE and mass-loss processes. Data products (yields and delay-time distributions) will be made available online to enable broader use in Galactic chemical evolution studies.

Abstract

Asymptotic giant branch (AGB) stars are important to chemical evolution at metallicity $Z \sim 0.0001$ ($\text{[Fe/H]} \approx -2.2$) as they contribute significantly to the production of nitrogen, lead, and dust in the early Universe. The contribution of AGB stars to the chemical evolution of the Universe is often quantified using the chemical yields from single AGB stars. Binary evolution challenges our understanding of chemical evolution as binary phenomena such as mergers and mass transfer episodes can significantly alter the stellar evolution pathways and yields. In this work, we use binary population synthesis code binary_c to model populations of low and intermediate-mass ($\sim 0.7-7 \, M_{\rm \odot}$) stars at metallicity $Z = 0.0001$. Our binary star populations predict $\sim 37\%$ fewer thermally-pulsing AGB stars than our single star populations, leading to a $\sim 40\%$ decrease in the amount of ejected C and a $\sim 35-40\%$ reduction in elements synthesised through the slow neutron capture process. The uncertainty introduced by the mass-loss from stellar winds on the AGB makes the impact of binary evolution on the total amount of ejected N uncertain. The total N yield ejected by our binary star populations ranges from a $17\%$ to a $36\%$ decrease compared to our single star populations. However, our binary populations overproduce N by over an order of magnitude during the period $300-700\,$Myr after formation.

Figures (5)

• Figure 1: We compare the fit for $f_{\rm Tdrop}$ described by Equation \ref{['eq:HBBfit_old']} (Standard) to our new fit described by Equation \ref{['eq:HBBfit']} (New Fit), and the models presented in Karakas2010. We show $f_{\rm Tdrop}$ as a function of $M_{\rm env}/M_{\rm env, 1TP}$.
• Figure 2: Total stellar yield of C (top) and N (bottom) as a function of initial stellar mass. Here, we compare the results from detailed stellar evolution codes to those from our single stars models from our model sets as described in Table \ref{['tab:5pops']}. All results from detailed stellar evolution codes are calculated with $Z=0.0001$, except for Ventura2002 which uses $Z=0.0002$. Model set $B10$ describes our models where mass-loss on the TP-AGB is calculated using Bloecker1995 with $\eta = 0.1$.
• Figure 3: Here we compare the population N yields of our single and binary star populations, calculated from model set $B02$. For our binary population, we show the contribution of the binary primary, secondary, and post-merger stars to the total population N yield. We show these results as a function of the initial single or binary-star mass. We bin the yield contribution of our secondary and post-merger stars by the initial mass of their binary primary stars. We stack the contributions from each component of the binary population, with their summation equalling the total population yield.
• Figure 4: Here we show the average of the percentage change in the total elemental yields of our binary star populations from our single star populations from our five model sets. For the data labelled 'Binary Population', we are comparing our populations with a binary fraction of 1 to populations with a binary fraction of 0. The error bars indicate one standard deviation of the average, highlighting the variation introduced by our choice of mass-loss on the TP-AGB. For the data labelled 'Binary Population (P+S Isolated Evolution)', we are showing the average and one standard deviation of our results where we evolve the stellar components of our binary-star population as if they are single.
• Figure 5: Here, we show the average net C and N yield of our stellar populations as a function of time. We are comparing our populations where the binary fraction is 0 (single star population) and 1 (binary star population) following a single burst of star formation. We show our results up to $1\,$Gyr after formation, and we bin with a $100\,$Myr time-step. The histograms are transparent and overlapping. The error bars indicate one standard deviation in the average population yield, calculated from our five model sets.