Photospheric Chemical Depletion

• Photospheric chemical depletion is the process where refractory elements (e.g., Ti, Fe, Si) condense onto dust grains, reducing their abundance in stellar photospheres.
• High-resolution spectroscopy and NLTE modeling are used to map depletion profiles and quantify abundance changes as a function of condensation temperature.
• This phenomenon informs our understanding of stellar parameter derivation, disc evolution, and the interplay between gas-dust separation in diverse astrophysical environments.

Photospheric chemical depletion is a phenomenon observed in a range of astrophysical environments wherein the atmospheric abundance of certain chemical elements—most notably refractory species (high condensation temperature elements such as Ti, Fe, Si, and Ca)—is significantly reduced relative to their expected or original values. This effect is typically attributed to selective removal mechanisms, such as condensation onto dust grains and subsequent separation of gas and dust, which alter the chemical composition of the stellar, sub-stellar, or nebular photosphere. Depletion processes impact the interpretation of spectral observations, the derivation of fundamental stellar parameters, and our understanding of underlying mass-transfer, evolutionary, and environmental mechanisms operating in various astrophysical contexts.

1. Mechanisms and Theoretical Foundations

Photospheric chemical depletion most commonly arises from the fractionation of gas and dust in environments that allow or favor condensation of refractory elements onto solid grains, while volatile elements (low condensation temperature species such as S, Zn, and O) remain largely in the gas phase. In classic examples, such as post-AGB and post-RGB binaries with circumbinary discs, mass loss during late stellar evolution leads to the formation of a stable, dusty disc. Refractory elements condense efficiently onto dust, which is then separated from the gas through processes such as radial migration, settling, or disc winds. When the star re-accretes the gas—now depleted in refractories—the photospheric abundances become systematically altered, with marked underabundances in elements with $T_{\rm cond}$ above a threshold indicative of efficient depletion (Mohorian et al., 19 Oct 2025, Mohorian et al., 28 Feb 2025, Menon et al., 18 Mar 2024).

The depletion profile is often parameterized as a function of condensation temperature $T_{\rm cond}$ using a piece-wise model:

• For $T_{\rm cond} < T_{\rm turn-off}$: $[X/\mathrm{H}] = [\mathrm{M/H}]_0$
• For $T_{\rm cond} \ge T_{\rm turn-off}$: $$[X/\mathrm{H}] = [\mathrm{M/H}]_0 + \nabla_{100\,K} \cdot ((T_{\rm cond}-T_{\rm turn-off})/100)$$

where $[X/\mathrm{H}]$ is the logarithmic photospheric abundance of element $X$ relative to hydrogen, $[\mathrm{M/H}]_0$ is the undepleted baseline metallicity, $T_{\rm turn-off}$ is the condensation temperature at which depletion sets in, and $\nabla_{100\,K}$ is the depletion slope (in dex per 100 K in $T_{\rm cond}$) (Mohorian et al., 19 Oct 2025).

In the interstellar medium (ISM) and cold prestellar cores, depletion is governed by freeze-out of molecules such as CO and HCO$^+$ onto icy dust grains, with the rate set by the product of the sticking coefficient $S$, grain number density $n_{\rm grain}$, and geometrical cross-section $\sigma_{\rm grain}$, following the relation $$\tau_{\rm depletion}^{-1} = S n_{\rm grain} \sigma_{\rm grain} v_{\rm th}$$, where $v_{\rm th}$ is the molecular thermal velocity (Maret et al., 2013). Desorption processes, such as cosmic-ray-induced UV photodesorption, partially counterbalance freeze-out, maintaining a low but nonzero gas-phase abundance in otherwise highly depleted regions.

2. Observational Diagnostics and Methodologies

The paper of photospheric chemical depletion employs a combination of high-resolution spectroscopy, spectral synthesis, and detailed modeling. Abundance analyses use either LTE models or, increasingly, NLTE spectral synthesis to correct for nonlocal thermodynamic equilibrium effects, especially in cool, extended, or chemically peculiar atmospheres (Mohorian et al., 19 Oct 2025, Mohorian et al., 28 Feb 2025). Synthesis tools such as E-iSpec, pySME, and the Balder code are applied to derive accurate abundances for dozens of elements, incorporating pre-computed NLTE departure coefficients where needed.

A key empirical diagnostic is the pattern of [X/Fe] or [X/Zn] versus $T_{\rm cond}$. Refractory elements exhibit strong underabundances relative to volatiles, typically in a "saturated" regime where the abundance falls off sharply above a critical $T_{\rm turn-off}$. Another major diagnostic is the high [S/Ti] ratio: since S is volatile and Ti is refractory, large [S/Ti] values directly trace depletion efficiency (Mohorian et al., 28 Feb 2025). NLTE corrections are essential for minimizing line-to-line scatter and securing robust patterns.

In some environments (e.g., polluted white dwarfs), abundance analyses must account for contamination by circumstellar material. In these cases, a combined disk+photosphere spectral model—incorporating disk opacity, temperature gradients, and additional absorption features—is necessary to derive true photospheric abundances. Failure to model the circumstellar contribution can result in systematic errors of up to 1 dex in the inferred metal content (Bourdais et al., 14 Oct 2024).

3. Astrophysical Contexts and Diversity of Manifestations

Photospheric chemical depletion is a pervasive phenomenon but manifests differently depending on astrophysical context:

• Evolved Binaries and Circumbinary Discs: In post-AGB/post-RGB binaries with circumbinary discs, nearly all studied objects exhibit strong, saturated depletion of refractories, often classified by the disc structure: "full" discs (continuous, optically thick) exhibit higher $T_{\rm turn-off}$ values; "transition" discs (with inner dust clearing) have lower turn-off temperatures (Mohorian et al., 19 Oct 2025). Faint disc systems likely represent advanced disc dissipation, exhibiting diverse depletion profiles.
• Prestellar Cores and ISM: In the ISM and especially in cold prestellar cores, depletion (here largely as molecular freeze-out) removes molecules such as CO from the gas, reducing their abundances by 2–3 orders of magnitude in dense regions. Cosmic-ray photodesorption and grain growth modify the depletion strength, with grain growth slowing down freeze-out due to reduced total grain surface area (Maret et al., 2013).
• Protoplanetary and Planet-Forming Discs: In disks around very low-mass stars, strong photospheric oxygen depletion in the innermost regions (C/O $\sim$ 100) suppresses oxygen-bearing species (e.g. H$_2$O, CO$_2$) and produces hydrocarbon-rich spectra. This is typically caused by gaps that prevent inward migration of O-rich icy grains, fundamentally altering inner disk chemistry (Kanwar et al., 15 Aug 2025).
• Field Stars and Dwarfs: In cool stars, such as M dwarfs, depletion of metals onto dust grains occurs in the photosphere due to condensation at low $T_{\rm eff}$. This leads to weakened metal lines and enhanced continuum opacity. BT-Settl model atmospheres account for these processes by self-consistent treatment of dust formation and depletion (Rajpurohit et al., 2017).
• Young Star Clusters: In G/K-dwarf members of young open clusters (e.g., M35), rotation and magnetic activity modulate photospheric lithium depletion, with rapid rotators showing higher lithium abundance due to reduced mixing to the Li-burning region—an effect analogous in diagnostic power but not strictly equivalent in mechanism to classic refractories/volatiles depletion (Jeffries et al., 2020).
• High-Redshift Galaxies: NIRSpec observations reveal a pattern of gas-phase silicon depletion (low [Si/O]) and selective iron enhancement ([Fe/O] $>$ 0) in chemically young, star-forming galaxies at $z=4$–7, paralleling depletion processes seen in stars and the ISM, and pointing to rapid dust condensation after core-collapse supernova enrichment (Isobe et al., 22 Sep 2025).

4. Quantitative Profiles and Classification

The chemical depletion profile in photospheric abundances is quantitatively characterized by a two-piece linear fit to $[\mathrm{X}/\mathrm{H}]$ vs. $T_{\rm cond}$, controlled by three key parameters:

• [M/H]$_0$: Baseline metallicity, set by volatiles.
• $T_{\rm turn-off}$: Critical condensation temperature marking the onset of significant depletion; observed to be bimodal in faint disc systems, separating full-disc analogues ($T_{\rm turn-off} \gtrsim 1100\,\mathrm{K}$) from transition discs ($T_{\rm turn-off} \lesssim 1100\,\mathrm{K}$) (Mohorian et al., 19 Oct 2025).
• $\nabla_{100\,\mathrm{K}}$: Depletion slope (dex per 100 K in $T_{\rm cond}$); steeper values imply more efficient depletion of refractory elements.

These quantified patterns are crucial for classifying depletion regimes, constraining disc properties, and linking present-day abundance signatures to evolutionary histories.

5. Connections to Disc, Binary, and Stellar Evolution

Photospheric chemical depletion is tightly coupled to disc and binary evolution. In post-AGB/post-RGB binaries, the presence and structure of a circumbinary disc directly impact the efficiency and profile of depletion, as evidenced by the link between disc IR excess, disc type (full vs. transition), and the observed depletion gradient and turn-off temperature (Mohorian et al., 19 Oct 2025, Mohorian et al., 28 Feb 2025). Faint disc systems with saturated depletion patterns likely represent late-stage disc dissipation, providing a final evolutionary window on the process.

Disc chemistry (O-rich vs. C-rich) further modulates depletion efficiency. For instance, post-AGB binaries with C-rich discs often retain the nucleosynthetic signature of the third dredge-up (carbon and s-process enhancements) rather than display strong depletion, highlighting the influence of condensation sequence and chemistry (Menon et al., 18 Mar 2024). The differential efficiency of depletion for various elements is directly traceable to condensation curves and refractoriness.

Magnetic activity, starspots, and rotation modulate depletion of lithium in young stars through their influence on internal mixing, energy transport, and core temperature, reflecting the complexity of depletion signatures beyond simple condensation-driven gas/dust fractionation (Jeffries et al., 2020).

6. Comparative Depletion Across Astrophysical Systems

While the fundamental mechanism—separation of refractory-rich dust from volatile-rich gas—operates in diverse environments (from evolved binaries to galaxies, protoplanetary discs, and the ISM), the manifestation of depletion patterns varies with environmental conditions and evolutionary history. In the ISM as well as in post-AGB binaries and young star systems with transition discs (λ Boo stars), the volatile/refractory abundance ratio ([S/Ti], [Zn/Ti], [X/Zn]) is a powerful diagnostic. However, depletion in post-AGB/post-RGB binaries with transition discs is markedly stronger (several dex in [S/Ti]) than in young transition-disc systems, resembling (but typically exceeding) the depletion signatures found in the ISM (Mohorian et al., 28 Feb 2025).

In extragalactic settings, the analogy is supported by observations of strong silicon depletion onto dust in the ISM of $z=4$–7 galaxies, paralleling the patterns seen in stellar photospheres and confirming rapid dust condensation following massive star enrichment (Isobe et al., 22 Sep 2025).

7. Implications and Future Directions

Discrete and continuous monitoring of chemical depletion profiles using NLTE-corrected, high-resolution spectroscopy and advanced atmospheric and disc models is imperative for disentangling the effects of gas-dust separation, accretion history, and intrinsic stellar nucleosynthetic signatures. Systematic comparison across environments—combining stellar, circumstellar, and extragalactic observations—clarifies the timescales and evolutionary endpoints of depletion phenomena and provides constraints on the dynamics of disc evolution, binary interaction, and the chemical preconditions for planet formation.

Key open questions include the detailed efficiency of depletion as a function of disc structure, the influence of secondary parameters such as disc chemistry and photophysics, and the full evolutionary sequence connecting initial dust condensation and final surface abundance patterns. Improved multi-epoch spectroscopic surveys, modeling of non-LTE effects, and integration with hydrodynamical and chemical evolution codes will further refine the interpretation and astrophysical impact of photospheric chemical depletion.