Alpha Enhancement and the Formation of Planetary Systems · Ryan Schmitt, 2010
In 2010, as a senior astrophysics student, I spent a semester measuring the elemental chemistry of three exoplanet host stars using the ELODIE archive and a manual spectral analysis toolkit. The question was simple: does a star’s alpha-element enhancement tell us something about how its planets formed? Sixteen years later, I’m answering it properly.
The analysis used the ELODIE archive at Observatoire de Haute-Provence — a 1.93m telescope with a fiber echelle spectrograph at R~42,000. Spectra were retrieved via SPLAT-VO, a virtual observatory tool with a manual GUI for spectrum inspection and normalization.
Stellar atmosphere modeling used SPECTRUM (R. O. Gray) with ATLAS9/Kurucz model grids, and equivalent widths were measured using the BLACKWEL and ABUNDANCE programs — a C++/GUI toolchain developed for classroom use at the time.
The solar reference was Lodders (2003), and no NLTE corrections were applied.
The targets
HD 89307 — a G0V solar analog selected as the cleanest exoplanet host star available in ELODIE. The goal was to establish a baseline comparison star as close to solar as possible, then examine how the chemistry deviated in the other targets.
Gliese 581 — an M3 red dwarf with two candidates in the habitable zone at the time (Gliese 581c and 581d). The low effective temperature (~3500 K) made ATLAS9 modeling unreliable for an M dwarf, and the analysis showed this clearly.
The Sun served as the primary calibration anchor, as it always must.
Key 2010 results
What the 2010 data said
Despite the limitations of the toolchain, several results held up and a few anomalies appeared that have gone unresolved ever since.
VALIDFe abundances for HD 89307 confirmed solar-like. [Fe/H] = +0.03 ± 0.09 dex, consistent with its classification as a solar analog and with subsequent literature values.
VALIDAlpha-element enhancement detected in HD 89307. Modest [Mg/Fe] and [Si/Fe] elevation above solar, consistent with thin-disk kinematics and age estimates of ~6 Gyr. The alpha-enhancement hypothesis partially supported.
ANOMALYAnomalous Ni, Co, and Si abundances in Gliese 581. The values diverged significantly from what would be expected for an M dwarf of its metallicity — but ATLAS9 was never designed for stars below ~4000 K. Whether this is astrophysical or an artifact of the model mismatch remains open.
LIMITATIONOnly ~10 Fe I lines used for Blackwell diagram convergence. Modern EW analysis uses 50–100+ Fe I/II lines to anchor the stellar parameters. The small line set inflated uncertainties and prevented reliable microturbulence determination.
VALIDIonization equilibrium method shown to be viable on the available data. Fe I / Fe II balance was achieved for HD 89307 within the uncertainty, establishing that the approach is correct in principle — it just needed better data.
Methodology evolution
EXOPLANET CODEX — METHODOLOGY
From thesis to frontier: 15 years of pipeline evolution
In 2010 Ryan measured stellar abundances using SPECTRUM and the ELODIE archive for his astrophysics thesis. The Exoplanet Codex rebuilds that work on a modern foundation — better data, better physics, open science.
2010 Senior thesis
2014 iSpec + HARPS era
2021 Asplund 3D NLTE solar scale
2024 JWST rocky planets
2026 Codex v1 ← now
Future 3D NLTE + M dwarfs
3×
higher spectral resolution HARPS vs ELODIE
27
elements measured vs a limited subset
0.05
dex target precision vs ~0.1–0.2 dex typical
WHAT CHANGED AND WHY IT MATTERS
2010 THESIS
EXOPLANET CODEX (2026)
Spectrograph
ELODIE R~42,000 Ceres/Vesta solar reference
Spectrograph
HARPS R~115,000 Direct solar feed (Dumusque 2021)
eliminates asteroid systematics
Radiative transfer
SPECTRUM (Gray & Corbally 1994) No molecular lines
Type A + Type B solar zero-point Full quadrature budget per element
publishable error bars
Reproducibility
Manual, thesis only
Reproducibility
Git-versioned, automated pipeline Open science at exoplanetcodex.org
fully reproducible
Elements measured
13
Elements measured
27 (canonical list: Fe I/II, C, O, N, Mg, Si, S, Ca, Ti, Co, Ni, Na, Al, K, P, Ba, Y, Eu, Mn, Cr, V, Sc, Cu, Zr, Li, Sr)
"The same wall I hit in 2010 — optical HARPS can’t recover carbon well — is still there. The difference now is that we know exactly why, and we have Turbospectrum, UV composites, and NLTE corrections to work around it."
WHY THIS MATTERS FOR ROCKY PLANET SCIENCE
The C/O ratio of a host star sets the mineralogy of its planets. A star with C/O > 0.8 builds carbon-rich worlds (graphite, SiC); below that threshold you get silicate-dominated planets like Earth. SPECTRUM-era pipelines had ~0.1–0.15 dex systematic errors on Mg and Si — large enough to shift a planet from "rocky like Earth" to "different composition" in the models.
Turbospectrum + NLTE corrections + astrophysical log gf calibration targets <0.05 dex precision on the key elements. That’s the threshold where stellar chemistry becomes a reliable input to planetary interior models.
▸The core physics. Stellar atmosphere theory, LTE line formation, and the Kurucz model grid are still the backbone of precision abundance work in 2026.
▸The ionization equilibrium method. Balancing Fe I and Fe II to derive log g is still the gold standard. We were doing it right in 2010, just with too few lines.
▸The scientific question. Alpha enhancement as a window into planet formation is still an active research area. Papers continue to be published on [Mg/Fe], [Si/Fe], and the Mg/Si ratio as proxies for rocky planet interior structure.
▸HD 89307’s Fe abundance. The literature value for HD 89307 ([Fe/H] ≈ +0.02) aligns well with the 2010 result. The solar-analog conclusion was correct.
What we’d do differently
▸Never analyze M dwarfs with ATLAS9. Gliese 581 required a PHOENIX or MARCS model grid designed for cool stars. The anomalous abundances are almost certainly model artifacts, not astrophysics.
▸Use at least 40 Fe lines. Ten lines is not enough to reliably establish excitation equilibrium. The stellar parameter uncertainties were dominated by line-selection noise.
▸Document the uncertainty budget formally. The 2010 thesis reports scatter but not stellar parameter sensitivity (ΔTeff, Δlog g, Δvturb contributions). This is now standard practice.
▸Use a version-controlled, automated pipeline from day one. Manual GUI workflows make results difficult to reproduce and impossible to re-run when a new line list or solar reference supersedes the old one.