Feature Catalogue — 183 Spectroscopic Features

Canonical mode

All features listed here are extracted in spectro_only=True mode. Total: 183 features across 9 families.

Feature count clarification

Earlier documentation cited 174 features (FeatureEngineer output only). The canonical count of 183 includes ~9 match_* columns added by ProcessingPipeline during the line-matching step. See spectro_only=True Mode.

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Feature families overview

Family	Count	Physical domain	Example features
Balmer lines	~25	Temperature	Hα EW, Hβ prominence, Hγ FWHM
Ca II H&K	~20	Metallicity / activity	Ca II K EW, Ca II H prominence
Na / Mg / α-elements	~15	Chemistry / gravity	Na D EW, Mg b EW, Mg b prominence
Molecular bands	~20	Cool stars (T_eff < 4000 K)	TiO Δm, CaH index, MgH band
Fe & metals	~25	Metallicity	Fe I λ5270 EW, Fe I λ4668
Continuum indices	~30	Temperature / gravity	B−V synthetic, fblue/fred, colour slopes
Line ratios	~20	Temperature / metallicity	Ca II K / Hβ, Fe / Mg
Line-profile moments	~15	Pressure / rotation	FWHM, asymmetry, kurtosis
match_* columns	~13	Line-matching (ProcessingPipeline)	match_CaIIH_prom, match_Hβ_EW

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Key features by SHAP importance

These features rank highest in the SHAP analysis on the canonical spectro_only=True run (see SHAP Interpretability):

SHAP rank	Feature name	Family	Physical interpretation
1	num_feature_CaIIK_prom	Ca II	Ca II K line prominence — metallicity proxy
2	num_feature_CaIIK_ew	Ca II	Ca II K equivalent width
3	num_feature_CaIIK_fwhm	Ca II	Ca II K line width (pressure broadening)
4	num_feature_CaIIH_prom	Ca II	Ca II H prominence
5	num_feature_CaIIH_ew	Ca II	Ca II H equivalent width
6	num_feature_Ha_ew	Balmer	H-alpha equivalent width
7	num_feature_Ha_prom	Balmer	H-alpha prominence
8	num_feature_Mgb_ew	Metals	Mg b equivalent width
9	num_feature_Mgb_prom	Metals	Mg b prominence
10	num_feature_Balmer_temp_index	Balmer	Combined Balmer temperature index

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Key features by PCA loading (PC1 — thermal axis)

These features contribute most to PC1 (ρ(PC1, T_eff) = +0.831):

PC1 rank	Feature	Loading w	Interpretation
1	W(Hα)	+0.173	Hα equivalent width — T_eff indicator
2	Continuum asymmetry	+0.172	Blue/red slope
3	W(Mg b)	−0.171	Mg b EW
4	Synthetic B−V	−0.163	Colour temperature proxy
5	f_blue / f_red	+0.161	Flux ratio

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Feature families in detail

Balmer series (Hα to Hθ)

Hydrogen Balmer absorption lines are the primary temperature indicators in stellar spectra. For each line, three measurements are extracted:

• EW (equivalent width): integrated line strength in Å
• Prominence: peak depth relative to local pseudo-continuum
• FWHM: full width at half maximum in Å

Features: Ha_ew, Ha_prom, Ha_fwhm, Hb_ew, Hb_prom, Hb_fwhm, ...

Hα FWHM vs Gaia DR3 T_eff — strong correlation confirming physical grounding

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Ca II H&K doublet

Ca II H (3968 Å) and K (3933 Å) are the strongest lines in stellar optical spectra. They serve simultaneously as:

• Metallicity proxies (line strength ∝ Ca abundance)
• Chromospheric activity indicators (emission cores in active stars)

These are the top SHAP-ranked features in the canonical model.

Ca II K prominence vs Gaia DR3 [Fe/H] — physical correlation between top SHAP feature and metallicity

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Molecular bands (cool stars)

For T_eff < ~4,000 K, molecular absorption bands become the dominant spectral features:

Band	Molecule	Wavelength range	Physical use
TiO	Titanium oxide	Multiple (5000–9000 Å)	Cool K-M stars
CaH	Calcium hydride	6382–6389 Å	M subdwarfs vs giants
MgH	Magnesium hydride	4780–4850 Å	Cool star gravity indicator
VO	Vanadium oxide	7850–7860 Å	Very cool M stars

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SNR quality check

SNR distribution across the 43,019-spectrum dataset — quality filtering at SNR_g > threshold

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Colour indices

Synthetic colour indices (B−V, V−I, etc.) derived from the spectroscopic continuum

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Column naming convention

num_feature_{line}_{measurement}

Examples:
  num_feature_CaIIK_ew       → Ca II K equivalent width
  num_feature_Ha_fwhm        → Hα full width at half maximum
  num_feature_Mgb_prom       → Mg b prominence
  num_feature_Balmer_temp_index → combined Balmer temperature index
  match_CaIIH_prom           → Ca II H match from ProcessingPipeline

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See also

• spectro_only=True Mode — which features are included/excluded
• Physical Purity & Leakage — why non-physical features are excluded
• Feature Engineering — how features are computed
• SHAP Interpretability — feature importance analysis
• PCA — Physical Interpretation — PCA loadings by feature family