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Exploring the History of Stellar Mergers with Chemistry: Examining the Origins of Massive $α$-Enriched Stars using Carbon Isotope Ratios

Zachary G. Maas, Keith Hawkins, Jeffrey M. Gerber, Zoe Hackshaw, Catherine Manea

TL;DR

This work investigates the origins of Massive Alpha-Enhanced Stars (MAES) by measuring metallicities, CNO abundances, and especially the carbon isotopic ratio $^{12}$C/$^{13}$C in 49 red clump stars and four red giants from thin disk, thick disk, and MAES populations. Using high-resolution spectroscopy (R~60,000) around the $^{13}$CN feature at ~8005 Å and synthetic spectra from Turbospectrum, the authors derive atmospheric parameters with BACCHUS, abundances with TSFitPy, and constrain $^{12}$C/$^{13}$C by grid fitting CN lines. They find that 10 MAES red clump stars have $^{12}$C/$^{13}$C consistent with thick-disk values ($^{12}$C/$^{13}$C $\sim$8), while five MAES RC stars exhibit elevated carbon isotopic ratios ($^{12}$C/$^{13}$C $>15$), with some showing AGB mass-transfer signatures and others lacking binary indicators, highlighting multiple MAES formation channels. The carbon isotope ratio thus provides a unique diagnostic to disentangle the evolutionary histories of MAES, complementing kinematics, [C/N], and s-process abundances and informing models of binary interactions and stellar mergers in the Milky Way.

Abstract

Recently discovered massive $α$-enriched (MAE) stars have surface chemistry consistent with members of the older thick disk Milky Way population but high masses ($\sim$ 1.5 - 3 M$_{\odot}$) that suggest these stars are young. The origin of MAE stars is not fully understood although binary interactions are likely an important formation pathway. To better constrain the history of MAE stars, we have measured metallicities, carbon isotope ratios, and CNO abundances in 49 red clump stars and four red giants. Our sample included thin disk, thick disk, and MAE stars to best constrain different formation scenarios. We observed our sample stars using the Tull spectrograph on the McDonald 2.7m telescope and derived abundances using synthetic spectra created by the Turbospectrum radiative transfer code. Overall, we find that 10 of our red clump MAE stars are consistent with the average thick disk carbon isotope ratio of $^{12}$C/$^{13}$C = 8.2 $\pm$ 3.4. We find five MAE stars that have significantly higher carbon isotope ratios ($^{12}$C/$^{13}$C $>$ 15) than stars at similar metallicities. Two of the anomalous stars show abundance patterns consistent with AGB mass transfer while three MAE stars have $^{12}$C/$^{13}$C ratios similar to massive, single RC stars and show no clear signs of binarity from radial velocity monitoring or from the Gaia RUWE measurement. Overall, we find that carbon isotope ratio measurements provide a unique constraint when discerning the possible origins of MAE stars.

Exploring the History of Stellar Mergers with Chemistry: Examining the Origins of Massive $α$-Enriched Stars using Carbon Isotope Ratios

TL;DR

This work investigates the origins of Massive Alpha-Enhanced Stars (MAES) by measuring metallicities, CNO abundances, and especially the carbon isotopic ratio C/C in 49 red clump stars and four red giants from thin disk, thick disk, and MAES populations. Using high-resolution spectroscopy (R~60,000) around the CN feature at ~8005 Å and synthetic spectra from Turbospectrum, the authors derive atmospheric parameters with BACCHUS, abundances with TSFitPy, and constrain C/C by grid fitting CN lines. They find that 10 MAES red clump stars have C/C consistent with thick-disk values (C/C 8), while five MAES RC stars exhibit elevated carbon isotopic ratios (C/C ), with some showing AGB mass-transfer signatures and others lacking binary indicators, highlighting multiple MAES formation channels. The carbon isotope ratio thus provides a unique diagnostic to disentangle the evolutionary histories of MAES, complementing kinematics, [C/N], and s-process abundances and informing models of binary interactions and stellar mergers in the Milky Way.

Abstract

Recently discovered massive -enriched (MAE) stars have surface chemistry consistent with members of the older thick disk Milky Way population but high masses ( 1.5 - 3 M) that suggest these stars are young. The origin of MAE stars is not fully understood although binary interactions are likely an important formation pathway. To better constrain the history of MAE stars, we have measured metallicities, carbon isotope ratios, and CNO abundances in 49 red clump stars and four red giants. Our sample included thin disk, thick disk, and MAE stars to best constrain different formation scenarios. We observed our sample stars using the Tull spectrograph on the McDonald 2.7m telescope and derived abundances using synthetic spectra created by the Turbospectrum radiative transfer code. Overall, we find that 10 of our red clump MAE stars are consistent with the average thick disk carbon isotope ratio of C/C = 8.2 3.4. We find five MAE stars that have significantly higher carbon isotope ratios (C/C 15) than stars at similar metallicities. Two of the anomalous stars show abundance patterns consistent with AGB mass transfer while three MAE stars have C/C ratios similar to massive, single RC stars and show no clear signs of binarity from radial velocity monitoring or from the Gaia RUWE measurement. Overall, we find that carbon isotope ratio measurements provide a unique constraint when discerning the possible origins of MAE stars.

Paper Structure

This paper contains 19 sections, 5 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Sample Selection and Observations
  3. Selecting Stars in Different Stellar Populations
  4. Observation Summary and Data Reduction
  5. Abundance Measurement Methodology
  6. Atmospheric Parameter Measurements with BACCHUS
  7. $^{13}$C, N, and O Abundances with TSFitPy
  8. Abundance Uncertainty and Literature Comparisons
  9. Comparison to Afşar et al. 2012
  10. Comparison to LAMOST Parameters
  11. Other Literature Comparisons
  12. Final Uncertainties
  13. Discussion
  14. Star Kinematics and Li Abundance Estimates
  15. Carbon Isotope Ratios for Red Clump Stars
  16. ...and 4 more sections

Figures (5)

  • Figure 1: Left: The stars selected from huang20 and analyzed in this work are represented as green stars for MAE stars, blue circles for thick disk stars, and orange diamonds for thin disk stars. Gray points are the rest of the LAMOST RC catalog that was unobserved. The figure shows ages compared to [$\alpha$/Fe] enrichment with parameters from huang20. The mean age and [$\alpha$/Fe] uncertainties for our sample stars is shown as a black x. Right: We show a color-magnitude diagram for our targets selected from huang20. Contours represent 16$\%$, 50$\%$, 84$\%$ density values for the total sample. The extinction correction methodology is described in subsection \ref{['subsec:sample']}.
  • Figure 2: C, N, O, and Mg abundances are plotted for the RC stars in our sample. Representative error bars for [N/Fe] measurements are shown in the gold cross. Most symbols are the same as Fig. \ref{['fig:sample_abun_phot']} with the addition of Kepler MAES targets added as black triangles. The top two panels include [C/Fe] ratios and [N/Fe] ratios of probable RC stars from afsar12 as pink squares, lagarde24 as brown hexagons, and tautvaisien13 as cyan triangles. The bottom two panels include O and Mg abundances from bensby14, represented as gray circles. Red giant abundances are discussed in section \ref{['subsec:red_giant']}.
  • Figure 3: Atmospheric parameter comparison from Table \ref{['tab:atmoparams']} with LAMOST parameters from huang20. The symbols are the same as Fig. \ref{['fig:sample_abun_phot']} and the dashed line represents no difference between the LAMOST value and our measurement.
  • Figure 4: The carbon isotope ratios from our sample stars are plotted with reference to metallicity (left) and the stellar mass (right panel). Masses were adopted from sun20. The symbols are the same as Fig. \ref{['fig:cnomg_abund']}.
  • Figure 5: The carbon isotope ratios from our sample stars are plotted with reference to metallicity (left) and the surface gravity (right panel). The symbols are the same as Fig. \ref{['fig:cnomg_abund']}. The solid points are stars within the RC photometric sample (shown in Fig. \ref{['fig:sample_abun_phot']}).