HAT-P-70b through the Eyes of MAROON-X: Constraining Elemental Abundances of Metals and Insights on Atmosphere Dynamics

Abstract: Ultra-hot Jupiters (UHJs) are exceptional laboratories for studying planetary atmospheres under extreme irradiation conditions. With close-in tidally locked orbits, these planets can have daysides hot enough for metals to be significantly ionized while still maintaining nightsides cold enough for refractory species to potentially condense. We present an analysis of the ultra-hot Jupiter HAT-P-70b taken with the MAROON-X high-resolution spectrograph. Using cross-correlations, we detect 14 neutral and singly ionized species, including Fe I, Fe II, Ti I, Ca I, Ca II, Cr I, Na I, V I, Mn I, Ni I, Mg I, Ba II, O I, and Sr I, with tentative evidence for H I, Co I, and K I. The absorption signals exhibit blueshifts on the order of a few $\mathrm{km\,s^{-1}}$, consistent with day-to-night winds. We further constrain relative abundances with atmospheric retrievals and demonstrate that some inferred elemental abundance ratios depend strongly on modeling assumptions. In particular, we show that a well-mixed retrieval approach neglecting ionization can strongly bias highly ionizable elements such as Ca and Ti. Accounting for the effects of equilibrium chemistry and thermal ionization generally results in inferred elemental abundance ratios that are closer to expectations for a solar-like composition, although not in all cases. Interestingly, we find a distinct nickel enrichment on HAT-P-70b, adding to the growing number of UHJ studies where the Ni abundance is seemingly enhanced. Our results underline the importance of considering physical and chemical atmospheric processes such as ionization when interpreting high-resolution transmission spectra of UHJs.

• The paper employs high-resolution spectroscopy with MAROON-X to constrain metal abundances and detect clear Doppler blueshifts indicating day-to-night winds on HAT-P-70b.
• It contrasts free and equilibrium retrieval methods, revealing systematic biases and significant deviations in Ti, Ca, Ni, and Sr abundance ratios.
• The findings support incomplete nightside cold-trapping of Ti and persistent supersolar Ni and Sr levels, challenging current atmospheric and formation models.

Atmospheric Composition and Dynamics of HAT-P-70b Probed with High-Resolution Optical Spectroscopy

Introduction

Ultra-hot Jupiters (UHJs) are an extreme class of close-in gas giants with equilibrium temperatures in excess of 2000 K, placing them in a unique regime where refractory elements remain in the gas phase and their atmospheres support vigorous thermal inversions, strong winds, and disequilibrium chemistry. HAT-P-70b, with a $T_{\rm eq} = 2562$ K, is of particular interest due to its placement in the transition region for nightside cold-trapping of refractory species such as Ti. Previous studies delivered conflicting abundance measurements and hinted at rich atmospheric dynamics, motivating further spectroscopic characterization. This work employs red-optical high-resolution transmission spectroscopy with the MAROON-X instrument to provide a comprehensive chemical inventory of HAT-P-70b’s atmosphere and execute a robust retrieval analysis that differentiates between various abundance inference frameworks.

Observations and Methods

Two MAROON-X transit datasets of HAT-P-70b were obtained, with the instrument resolving power of $R \sim 85,000$ and coverage of 500–920 nm. Data reduction combined advanced detrending procedures, including principal component analysis and empirical alignment to the stellar rest-frame, to suppress telluric and stellar residuals without compromising the planetary signal. The analysis adopted both cross-correlation and atmospheric retrieval paradigms:

• Model Generation: Transmission spectra for cross-correlation were produced using the SCARLET forward model, including up-to-date neutral and singly ionized line lists and equilibrium chemistry.
• Cross-Correlation Analysis: Employed to detect individual species via the Doppler-shifted signal of the planet, providing clear discrimination between metals and their ionic variants.
• Retrieval Analysis: Both free retrievals (species abundance altitudinally invariant and unconstrained by equilibrium relations) and equilibrium retrievals (enforced ionization/dissociation balance with independently-varying elemental ratios) were explored.

Chemical Inventory and Atmospheric Winds

The cross-correlation analysis resulted in unambiguous detections of Fe I, Fe II, Ti I, Ca I, Ca II, Cr I, Na I, V I, Mn I, Ni I, Mg I, Ba II, O I, and Sr I. Marginal signals for H I, Co I, and K I were also reported. Notably, all robust atomic and ionic detections exhibit consistent blueshifts ($\sim$ few km/s), an unequivocal signature of dayside-to-nightside winds expected for tidally locked UHJs with substantial day-night temperature gradients. Kinematic traces are temporally invariant through transit, implying symmetric wind geometry between the morning and evening limbs. This wind signature supports 3D atmospheric circulation models predicting global scale flows and vigorous mixing.

Metal Abundances, Ionization, and Retrieval Systematics

Relative abundances were inferred using both well-mixed and chemical equilibrium retrievals, focusing on Ti, Ca, Cr, Ni, Mg, V, Mn, Co, and Sr relative to Fe. The critical findings are as follows:

• Free Retrievals: Constant-with-altitude abundances yield subsolar ratios for elements with low ionization thresholds (Ti, Ca, Sr) due to the inability to capture altitude-dependent ionization.
• Equilibrium Retrievals: Inclusion of thermal ionization produces marked changes. Ionically susceptible species (Ca, Sr, Ti) show abundance ratios substantially increased relative to the free method, converging toward solar, with the exception of V, Ni, and Sr, which diverge further from solar values, particularly with Ni/Fe and Sr/Fe enhancement factors as high as $\sim 20$–$40\times$ solar.
• Degeneracies and Limitations: The spectrum’s line-depth sensitivity to relative abundance, scale height, and continuum normalization creates parameter degeneracies that challenge unique interpretation, especially for planetary mass inference from transmission spectra.

Titanium Cold-Trapping and Condensation Chemistry

HAT-P-70b’s Ti/Fe derived from equilibrium chemistry is marginally subsolar. This, alongside the detection of both Ti I and Ti II, rules out total nightside cold-trapping. The Ti/V ratio is strongly subsolar even in equilibrium retrievals, a trend persisting across UHJs in this transition regime. This reflects partial but incomplete condensation of Ti on the cooler nightside, with significant atmospheric mixing returning it to the photosphere, supporting theoretical cold-trap and advection models.

Supersolar Nickel and Strontium: Model or Chemistry?

Both Ni and Sr emerge as supersolar (relative to Fe) by more than an order of magnitude in equilibrium retrievals, confirming similar results from MAROON-X emission and other UHJs. The persistent nature of this enrichment, unmitigated by masking of strong individual lines or use of different line lists, points to unresolved model inadequacies (e.g., incomplete line data, unknown opacities, non-equilibrium chemistry, or unaccounted photoionization particularly for Fe) as well as possible atmospheric or formation history anomalies. These results cannot yet be uniquely attributed to physical processes without further spectroscopic and theoretical investigation, but highlight the limitations of even state-of-the-art ionization-inclusive equilibrium retrievals.

Constraining Planetary Mass from Transmission Data

The equilibrium retrieval’s planetary mass posterior is bimodal (peaks at 1.4 and 1.8 $M_{\rm Jup}$), a direct result of degeneracies between $M_p$, temperature structure, and chemical/opacity parameters via their mutual effect on scale height and line contrast. The marginalized mass is consistent with current spectroscopic and RV limits, but highlights the poor constraining power of transmission data alone when independent constraints are not available.

Practical and Theoretical Implications

The results reinforce several critical considerations for the study of UHJ atmospheres:

• High-Resolution Spectroscopy: Demonstrates the power of high-S/N, R $>$ 50,000 optical spectroscopy for identifying multiple atomic/ionic species and constraining winds in exoplanet atmospheres inaccessible to current space-based instruments.
• Retrieval Framework Fidelity: Inference of elemental ratios for highly ionizable species demands retrievals that explicitly include altitude-dependent ionization; otherwise, significant systematic biases result, especially for Ti, Ca, and Sr.
• Model Limitations: Systematic Ni and Sr overabundances suggest either missing physics (e.g., unmodeled photochemistry, photoionization, or vertical mixing) or technical issues (e.g., incomplete line lists, unrecognized blends), motivating further lab and theoretical investigations.
• Cold-Trap Regime Mapping: HAT-P-70b populates the transition regime between strong and weak nightside cold-trapping, making it a testbed for condensation and mixing models; continued comparative spectroscopic observations across a range of $T_{\rm eq}$ and metallicity regimes will further constrain these processes.
• Planetary Formation and Migration: Persistent Ni overabundance (if confirmed physical) could point toward non-solar accretion, planetesimal delivery, or other exotic formation histories.

Conclusion

The analysis of HAT-P-70b with MAROON-X sets new reference standards for high-resolution atmospheric inventories of UHJs, delivering robust simultaneous detections of a broad suite of metal species, characterizing the dynamic atmospheric circulation, and illuminating the systematic challenges and physical implications of metal abundance retrievals under extreme irradiation. The evidence for partial yet incomplete nightside cold-trapping of Ti, large-scale day-to-night winds, and persistent unexplained enhancements of Ni/Sr exemplify the complexity of interpreting UHJ atmospheres and establish the need for next-generation instruments and improved theoretical models to resolve the composition, structure, and evolution of exoplanetary atmospheres in the ultra-hot regime (2605.07873).