Spectroscopic Follow-up of Young High-$α$ Dwarf Star Candidates: Still Likely Genuinely Young

TL;DR

We address the existence and origin of genuinely young high-$\alpha$ stars, a challenge to standard Galactic chemical evolution due to their alpha-enhancement at intermediate ages. The approach combines high-resolution PEPSI spectroscopy of candidate stars with Li as a mass-transfer diagnostic, gyrochronology-based ages from rotation periods, multi-epoch RV screening to exclude close binaries, and differential abundance comparisons with doppelgänger stars to test youth robustly. The study identifies three genuinely young high-$\alpha$ dwarfs (ages around $5$ Gyr, $[Fe/H] \sim -0.5$) with Li $\sim 0.5$ dex and Al $\sim 0.1$ dex elevations, plus one possible merger product, suggesting a mixed-origin population. These results support a scenario in which genuinely young high-$\alpha$ stars arise from local, self-enriched environments (potentially in the outer disk) while acknowledging a substantial contribution from binary interactions, and they motivate larger samples and PLATO-driven asteroseismology for refined ages.

Abstract

The question of whether genuinely young high-$α$ stars exist has been discussed for over a decade since their discovery from asteroseismology of giant stars as it is challenging to break the degeneracy between the binary interaction and the genuinely young scenarios. Young high-$α$ stars are hard to explain with traditional chemical evolution model as the high-$α$ disk is typically associated with the early epoch of star formation in the Milky Way. Combined with recent advances of gyrochronology, and that $^7$Li can serve as an unambiguous indicator for identifying merger products in dwarfs thanks to its low burning temperature, we identified young high-$α$ dwarf candidate stars through their fast rotation in a previous study. In this paper, we performed high-resolution spectroscopic follow-up of these candidates using Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI), and confirm 3 additional stars that are most likely genuinely young. Together with the star from the earlier paper, we find three out of four of them center around [Fe/H]=-0.5 dex, are ~5 Gyr old, and have a similar amount of elevated Li (~0.5 dex) and Al (~0.1 dex) compared to stars with matching $\log g$, $T_{\rm eff}$, Mg, and Fe within observational uncertainties, hinting at their common formation pathway.

Figures (7)

• Figure 1: Fitting results for PEPSI using BACCHUS compared to results from APOGEE DR17 APOGEEDR17 in red and GALAH DR3 Galahdr3 in black. The bias ($\Delta$) and variance ($\sigma$) are shown in each panel. Abundances measured from a PEPSI solar spectrum are also shown for reference to track potential abundance offsets.
• Figure 2: The left column shows the [Mg/Fe]-[Fe/H] plane for the APOGEE sample (top left) and GALAH sample (bottom left). The background gray points show the Kepler dwarf stars in the APOGEE field or the K2 dwarfs in the GALAH field for reference. The dark gray points show the high-$\alpha$ stars in each survey for reference. The points show the [Mg/Fe] and [Fe/H] values from APOGEE DR17 and GALAH DR3. The red dashed lines --- [Mg/Fe] = $-$0.1[Fe/H]+0.1 for APOGEE, and [Mg/Fe] = $-$0.03[Fe/H]+0.15 for GALAH --- show our separation for the high- and low-$\alpha$ disk, using the background reference. The right column shows the Kiel diagram for the APOGEE sample (top right) and GALAH sample (bottom right). Again, the background gray points show the reference stars, the dark gray points show the high-$\alpha$ stars with $-0.6$ dex $<$ [Fe/H] $<-0.4$ dex, and the larger points show the values from the public survey data releases for the PEPSI stars, colored by their metallicity values. Since we fixed $\log g$ and $T_{\rm eff}$ while fitting the PEPSI spectra, we do not have a different set of values from our new observations. The points outlined in black show the abundance measurements for the truly young high-$\alpha$ stars that will be described in Section \ref{['subsec:truly young']}.
• Figure 3: Detail analysis for 03413908+2359163. Top: K2 light curve with the best-fit model (left), the Lomb-Scargle Periodogram where the red line shows the period detected and the red dotted lines show the half and double periods (middle), and the folded light curve onto the detected period (right). Middle left: Korg best-fit model (red lines) around the Li line. The vertical black dashed lines show the window for the Li line. The black points show the PEPSI spectra for this targeted star, and the blue line shows the synthetic spectrum of a star with the same stellar parameters but with [Li/H] = 0 dex. Middle right: BACCHUS best-fit model (red) to the data for the Mg 6318.7 line and the Ca 6493.8 line. The vertical dashed line shows the centers of the lines. The blue lines show the synthetic spectra of a low-$\alpha$ star with the same stellar parameters generated from BACCHUS for comparison. Bottom left: Abundance comparisons with doppelgänger stars (detail see Section \ref{['subsec:abund_doppel']}). The orange points show refractory elements used to vet planet engulfment events. Bottom right: RV measurements and binary exclusion tests for this star. In the binary exclusion test plot showing mass ratio vs orbital period, the blue vertical line shows the synchronized binary limit at 15 days, binary stars with an orbital period less than 15 days should be synchronized. The lines show 95% confidence level for binary exclusion assuming a typical age of a high-$\alpha$ star of 10 Gyr, analyzed with PEPSI RVs (black) and APOGEE RVs (red) separately. The dashed lines show those for an intermediate age high-$\alpha$ star of 4 Gyr.
• Figure 4: Similar as Figure \ref{['fig:3']} but for 03435417+1719250. The differences here is for the comparison with doppelgänger stars in the bottom left panel, we compared both with doppelgängers selecting on agreeing [Mg/Fe] (circle points) or agreeing $[\mathrm{\alpha}/\mathrm{Fe}]$ (square points) as this star likely exhibit an abnormally high [Mg/Fe] values. This is a GALAH-K2 candidate star.
• Figure 5: Similar as Figure \ref{['fig:3']} but for 01084954-0030464. Here we use the Ca 6439.1 line. This is a GALAH-K2 candidate star.