Equilibrium H2+/H- Abundance Ratios in MARCS Atmosphere Models
Abstract
We investigate the equilibrium abundance ratio of the molecular hydrogen ion H2+ and the negative hydrogen ion H- in stellar atmospheres using compact Saha-type equilibrium expressions and layer-by-layer partial pressures from MARCS model atmospheres. The analysis distinguishes the temperature-dependent coefficient ratio KH2+(T)/KH-(T) from the full abundance ratio, which also depends on the local ionisation factor NH+/Ne. The internal partition function ZH2+(T) is included in the H2+ equilibrium coefficient. We processed 51,994 MARCS models, corresponding to 2,911,664 atmospheric layers, and computed N(H2+)/N(H-) directly from the tabulated equilibrium partial pressures, constructing the first systematic map of this ratio across the MARCS grid. H2+ exceeds H- in 827,350 layers, corresponding to F=0.284, or 28.4% of all analysed layers. Although H- remains dominant in the global logarithmic mean, with ⟨ log10[N(H2+)/N(H-)] ⟩=−4.229, H2+ becomes comparable to or more abundant than H- in a substantial subset of layers. Its relative significance increases toward higher Teff, lower [Fe/H], and deeper layers. The balance is therefore controlled by the combined local thermodynamic and ionisation structure, not by temperature alone. These ratios are equilibrium abundance diagnostics and do not imply opacity dominance; assessing opacity requires wavelength-dependent cross-sections and radiative-transfer calculations.
Related
- High-resolution X-ray spectroscopy with XRISM/Resolve reveals super-solar abundance ratios in Virgo/M87
- When the core ate the atmosphere
- Atmosphere mitigation in cosmic microwave background observations using multifrequency time-domain component separation
- Thermodynamic and structural properties of the thin atmosphere of a white dwarf
- The GAPS Programme at TNG. LXXXI. The puzzle of the HeI absorption signal in the atmosphere of V1298Taub with GIARPS@TNG
BibTeX
@misc{sarsembayeva2026equilibrium,
title = {{Equilibrium \(\mathrmH_2^+/\mathrmH^-\) Abundance Ratios in MARCS Atmosphere Models}},
author = {Aiganym Sarsembayeva and Hernando Quevedo},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.30529},
eprint = {2608.30529},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.30529},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Equilibrium H_2^+/H^- Abundance Ratios in MARCS Atmosphere Models
AU - Aiganym Sarsembayeva
AU - Hernando Quevedo
PY - 2026
DA - 2026/08/31
PB - arXiv
DO - 10.48550/arxiv.2608.30529
UR - https://arxiv.org/abs/2608.30529
AB - We investigate the equilibrium abundance ratio of the molecular hydrogen ion H_2^+ and the negative hydrogen ion H^- in stellar atmospheres using compact Saha-type equilibrium expressions and layer-by-layer partial pressures from MARCS model atmospheres. The analysis distinguishes the temperature-dependent coefficient ratio K_H_2^+(T)/K_H^-(T) from the full abundance ratio, which also depends on the local ionisation factor N_H^+/N_e. The internal partition function Z_H_2^+(T) is included in the H_2^+ equilibrium coefficient. We processed 51,994 MARCS models, corresponding to 2,911,664 atmospheric layers, and computed N(H_2^+)/N(H^-) directly from the tabulated equilibrium partial pressures, constructing the first systematic map of this ratio across the MARCS grid. H_2^+ exceeds H^- in 827,350 layers, corresponding to F=0.284, or 28.4% of all analysed layers. Although H^- remains dominant in the global logarithmic mean, with ⟨ log_10[N(H_2^+)/N(H^-)] ⟩=−4.229, H_2^+ becomes comparable to or more abundant than H^- in a substantial subset of layers. Its relative significance increases toward higher T_eff, lower [Fe/H], and deeper layers. The balance is therefore controlled by the combined local thermodynamic and ionisation structure, not by temperature alone. These ratios are equilibrium abundance diagnostics and do not imply opacity dominance; assessing opacity requires wavelength-dependent cross-sections and radiative-transfer calculations.
ER -