A Galactic Perspective on the (Unremarkable) Relative Refractory Depletion Observed in the Sun

Abstract

Over the last two decades, the Sun has been observed to be depleted in refractory elements as a function of elemental condensation temperature (\tcond) relative to $\sim80\%$ of its counterparts. We assess the impact of Galactic chemical evolution (GCE) on refractory element--\tcond\ trends for 109,500 unique solar analogs from the GALAH, APOGEE, Gaia RVS, and \cite{bedell18} surveys. We find that a star's \feh\ and \alphafe\ are predictive of its \tcond\ slope (\rsq\ = $15 \pm 5\%$, $23 \pm 10\%$ respectively) while \teff\ and \logg\ contribute more weakly (\rsq\ = $9 \pm 5\%$, $13 \pm 16\%$). The Sun's abundance pattern resembles that of more metal-rich (0.1 dex) and $α$-depleted stars ($-0.02$ dex), suggesting a connection to broader GCE trends. To more accurately model stars' nucleosynthetic signatures, we apply the K-process model from \cite{Griffith24}, which casts each star's abundance pattern as a linear combination of core-collapse and Type Ia supernovae contributions. We find the Sun appears chemically ordinary in this framework, consistent with the intrinsic population scatter expected from stellar nucleosynthesis. We show that refractory element--\tcond\ trends arise because elements with higher \tcond\ have higher contributions from core-collapse supernovae. Refractory element depletion trends primarily reflect nucleosynthetic enrichment patterns shaped by GCE and local ISM inhomogeneities, with these processes accounting for $> 90\%$ of the observed variation within $2σ$. This work highlights how abundance diversity due to local and global chemical enrichment complicates the interpretation of population-scale planet-related chemical signatures in current datasets.

Figures (14)

• Figure 1: Summary of stellar parameters for solar analogs from bedell18 in purple, GALAH DR4 GALAHDR4 in orange, APOGEE DR17 apogeedr17 in blue, and Gaia RVS Rampalli24 in pink. Left: Histograms of distances probed by each survey. bedell18 surveyed stars nearby in the solar neighborhood while our samples from Gaia, APOGEE, and GALAH span further distances. Middle: $T_{\rm eff}$-$\log g$ plane of solar analogs in each sample. Right: $\rm [Fe/H]$-$\rm [\alpha/Fe]$ plane of solar analogs in each sample. GALAH, APOGEE, and Gaia shown as $3\sigma$ contour regions rather than individual scatter point distributions for clarity in middle and right panels.
• Figure 2: $T_{\rm cond}$ linear fits for 10 random stars' refractory abundances as a function of their 50% condensation temperature as reported in Lodders03. Each color represents a different star's elemental abundances, and $\alpha$-elements are noted by blue text labels.
• Figure 3: Distributions of $T_{\rm cond}$ slopes for solar analogs across different surveys. The HARPS sample from bedell18 is shown in solid purple, with resampled slopes using mean abundance uncertainties from GALAH, APOGEE, and Gaia RVS shown in dashed purple. GALAH, APOGEE, and Gaia RVS distributions are shown in orange, blue, and pink, respectively. Injecting larger uncertainties into the HARPS sample broadens the slope distribution and increases the fraction of stars with $T_{\rm cond}$ slopes $< 0$, making the Sun appear less anomalous (though still relatively refractory depleted) and more consistent with the three lower resolution surveys.
• Figure 5: Left: Mean $T_{\rm cond}$ slope ($\frac{\partial[\mathrm{X/Fe}]}{\partial T_{\rm cond}}$) as a function of mean $\rm [\alpha/Fe]$ and colored by mean $\rm [Fe/H]$ with uncertainties on the mean for solar analogs with bedell18 as circles, GALAH as squares, Rampalli24 as diamonds, and APOGEE as stars. There is a positive gradient in $T_{\rm cond}$ slope with $\rm [\alpha/Fe]$ (10$\pm4$E-4 K$^{-1}$) and a similarly strong negative gradient with $\rm [Fe/H]$ as well (-4$\pm1$E-4 K$^{-1}$). Metal-rich, $\alpha$-enriched stars tend to have $\frac{\partial[\mathrm{X/Fe}]}{\partial T_{\rm cond}}$ = 0, consistent with the Sun. The APOGEE stars are systematically offset from the rest of the surveys given their difference in kinematic selection. Right:$\rm [Fe/H]$-$\rm [\alpha/Fe]$ distribution of stars colored by their $T_{\rm cond}$ slope. We confirm what was seen on the left panel: the majority of stars with a $T_{\rm cond}$ slope consistent with zero are metal-rich and depleted in $\rm [\alpha/Fe]$ compared to the Sun. Hexbin cells with mean $T_{\rm cond}$ slopes consistent with zero are centered at $\rm [Fe/H]$ = 0.1 dex and $\rm [\alpha/Fe]$ = $-0.02$ dex.
• Figure 6: Mean $T_{\rm cond}$ slope ($\frac{\partial[\mathrm{X/Fe}]}{\partial T_{\rm cond}}$) as a function of binned absolute vertical distance from the Galaxy's midplane, $|Z|$, colored by mean $\rm [\alpha/Fe]$, shown for each survey: bedell18 as circles, GALAH as squares, Rampalli24 as diamonds, and APOGEE as stars. APOGEE shows systematically higher $T_{\rm cond}$ slopes and occupies larger $|Z|$ on average. While the dominant trend is that higher $|Z|$ corresponds to steeper $T_{\rm cond}$ slopes, APOGEE stars exhibit elevated $T_{\rm cond}$ slopes even compared to other survey bins with similar $\rm [\alpha/Fe]$, suggesting a residual dependence on vertical kinematics or correlated properties such as age. Bins with $<10$ stars are omitted from this figure.