Exoplanets and Stellar Physics

Abstract:

Abstract Silicon is a key refractory element in giant planet atmospheres, which governs the formation of magnesium-silicate clouds, and reflects the quantity of silicates accreted during formation. While observations of directly imaged giant exoplanets have focused on the measurement of volatile species (e.g. CO, H2O), high-resolution spectroscopy with CRIRES+ M-band provides access to gas phase silicon chemistry in sub-stellar atmospheres, through the ro-vibrational band head of SiO at 4 μm. Here, we present the first results of the CRIMSON survey of silicon chemistry in directly imaged companions with CRIRES+ M-band. We report the strong detection of gaseous SiO (S/N = 7.5) in the directly imaged companion TWA 5 B, with an atmospheric abundance of log(SiO) = $-3.56^{+0.42}_{-0.32}$ VMR, providing access to the refractory content of the atmosphere. The high retrieved SiO abundance implies the absence of significant magnesium-silicate cloud condensation, and thus the atmospheric silicon abundance is contained almost entirely within the observed gas phase SiO. Using the detection of refractory silicon, together with strong detections of the volatile species CO (S/N = 9.1) and H2O (S/N = 18.8), we measure a stellar C/O and a marginally sub-stellar O/Si and C/Si, but a super-stellar Si/H ([Si/H]⋆ = $1.41^{+0.42}_{-0.32}$). Collectively, these volatile-to-refractory ratios are consistent with formation through core-accretion beyond the CO snowline, or gravitational instability followed by substantial solid enrichment. Finally, we discuss how gas phase SiO provides a unique diagnostic of the cloud properties in hot gas-giants, and can be used to probe the dominant cloud species forming across the directly imaged planet and isolated brown dwarf populations.

Abstract:

Ultrahot 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 UHJ 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 kilometers per second, 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.

Abstract:

Magnetic fields are a key factor in the evolution of planets and their atmospheres, but they are still poorly constrained for exoplanets owing to limited direct observations. Ultra-hot Jupiters provide a new avenue to probe magnetic effects, as the circulation of their highly ionized atmospheres could be directly sensitive to the atmospheric magnetic field. However, it remains unclear whether the impact of these magnetic effects can be observed directly and used to constrain the magnetic field strength. With high spectral resolution observations targeting the planetary iron lines, we measure the Doppler shift and thus the wind speed of seven transiting ultra-hot Jupiters. Here we find a clear decrease of wind speed with increasing planetary temperature, which is a trend inconsistent with purely hydrodynamic mechanisms but naturally reproduced by magnetic drag. From this relationship, we estimate the possible strength of magnetic fields of hot giant planets to at most a few gauss, which is comparable with the Jovian equatorial field. Our results support the idea that magnetic fields affect the atmospheric circulation of ultra-hot Jupiters and could provide a crucial benchmark for scaling laws used to predict magnetic fields in exoplanets, from hot Jupiters to rocky Earths, with additional implications for future direct observations.