Abundance Analysis of Chemically Depleted Post-AGB/Post-RGB Binaries with Faint Discs

TL;DR

This study probes the link between circumbinary disc evolution and photospheric depletion in eight post-AGB/post-RGB binaries with faint discs by performing a homogeneous NLTE abundance analysis using high-resolution optical spectra. A two-piece linear model, characterized by $[M/H]_{0}$, $T_{ m turn-off}$, and $ abla_{ m 100\,K}$, captures the depletion profiles, which are found to be saturated and exhibit bimodality in both $T_{ m turn-off}$ (roughly $900$ K and $1300$ K) and depletion gradient. When combined with full and transition disc samples, faint discs split into full-like and transition-like subgroups, suggesting distinct disc-formation pathways and evolutionary stages; a correlation between $L_{ m IR}/L_ullet$ and depletion slope is evident in transition discs. The results imply that faint discs are the late stages of disc dissipation, likely shaped by gas–dust fractionation, disc geometry, and possible planet formation, and motivate future interferometric studies and expanded homogeneous abundance analyses across Galactic and Magellanic Cloud systems.

Abstract

Post-AGB and post-RGB binaries with stable circumbinary discs provide key insights into late stellar and disc evolution, revealing how binary interactions shape disc structure and stellar surface composition. A defining trait of such systems is the observed underabundance of refractory elements in the stellar photosphere relative to volatile elements -- photospheric chemical depletion -- resulting from the star accreting volatile-rich circumstellar gas. In this study, we investigated the link between photospheric depletion and disc evolution by focusing on post-AGB/post-RGB binaries with low infrared excess (hereafter ``faint disc'' targets). We analysed high-resolution optical spectra from HERMES/Mercator and UVES/VLT for 6 Galactic and 2 LMC targets. Using \texttt{E-iSpec}, we homogeneously derived atmospheric parameters and chemical abundances of 29 elements from carbon to europium, and included NLTE corrections for 15 elements from carbon to barium that we calculated using pySME and pre-computed grids of departure coefficients. All targets exhibit `saturated' depletion patterns, which we characterised using two-piece linear fits defined by three parameters: initial metallicity ([M/H]$_0$), turn-off temperature ($T_{\rm turn-off}$), and depletion scale ($\nabla_{\rm 100 K}$). Among several findings, we highlight the bimodal distribution of $T_{\rm turn-off}$ in faint disc targets, which allows classification into two subgroups analogous to full discs with continuous, optically thick dust ($T_{\rm turn-off}$ > 1 100 K), and transition discs with inner clearing ($T_{\rm turn-off}$ < 1 100 K). Our results imply that faint disc targets likely represent the final stages of disc dissipation, highlighting the diversity of depletion profiles, the complexity of disc-binary interactions, and the need to understand the rarity and evolution of faint disc systems.

Figures (12)

• Figure 1: Updated IR colour-colour plot of post-AGB/post-RGB binary stars in the Galaxy and in the Magellanic Clouds. NIR magnitudes ($H$ and $K$) are adopted from 2MASS 6X, while MIR magnitudes ($W_1$ and $W_3$) are adopted from AllWISE (for more details, see Section \ref{['ssec:sdopht']}). Grey markers represent the extended sample of post-AGB/post-RGB binaries in the Galaxy kluska2022GalacticBinaries, in the Small Magellanic Cloud kamath2014SMC, and in the Large Magellanic Cloud kamath2015LMC. Coloured markers represent the post-AGB/post-RGB binaries homogeneously studied with E-iSpec+pySME (for more details, see Section \ref{['ssec:dscall']}): red circles mark the full disc targets mohorian2024EiSpec, yellow stars mark the transition disc targets mohorian2025TransitionDiscs, and blue squares mark the faint disc targets (this study). Adopted contours represent the demarcation between different categories of disc targets kluska2022GalacticBinaries.
• Figure 2: Spectra of all faint disc targets across the region featuring lines from the volatile S and Zn. Each spectrum is RV corrected, normalised, and offset in flux for clarity. Red dashed vertical lines mark the positions of spectral line peaks (for more details, see Section \ref{['ssec:anaspc']}).
• Figure 3: Spectral energy distribution plots of faint disc targets in the Galaxy and the LMC. The red solid curve corresponds to the reddened Kurucz model atmosphere, while the black solid curve represents Kurucz model atmosphere after de-reddening and scaling to the object. A legend within the plot clarifies the meaning of the symbols and colours used.
• Figure 4: Elemental abundances of a subsample of post-AGB/post-RGB binaries with faint discs as functions of condensation temperature lodders2003CondensationTemperatureswood2019CondensationTemperatures. For ID explanation, see Table \ref{['tab:alllum']}. The legend for the symbols and colours used is included within the plot. "NLTE insensitive" abundances are derived from spectral lines of V ii and Cr ii; for more details, see Section \ref{['ssec:anaspc']}).
• Figure 5: Elemental abundances of a subsample of post-AGB/post-RGB binaries with faint discs as functions of condensation temperature lodders2003CondensationTemperatureswood2019CondensationTemperatures. For ID explanation, see Table \ref{['tab:alllum']}. The legend for the symbols and colours used is included within the plot. "NLTE insensitive" abundances are derived from spectral lines of V ii and Cr ii; for more details, see Section \ref{['ssec:anaspc']}).