Origin of the mismatch between observed transit Temp signal and simple models for HD 189733 b

Determine whether a more complex stellar-surface geometry (e.g., combining spots and plages) or inconsistencies in stellar atmosphere models should be invoked to explain the discrepancy between the observed disk-averaged temperature variations during transits of HD 189733 b and the predictions from the authors’ toy model based on PHOENIX-derived temperature gradients.

Background

The authors detect a distinct temperature-variation pattern during transits of HD 189733 b using SPIRou (≈1 K increase near ingress/egress and a ≈0.5 K dip at mid-transit). Their toy model, built from PHOENIX atmosphere spectra and accounting for impact parameter and unocculted spots, qualitatively reproduces some features but fails quantitatively, especially for the ingress/egress maxima.

They explicitly state that no simple combination of parameters assuming literature transit geometry fits the observed Temp sequence and identify two candidate explanations requiring investigation: more complex distributions of stellar heterogeneities (spots/plages) and/or inconsistencies in stellar atmosphere models.

References

It remains to be seen if a more complex geometry (e.g., combining spots and plages) or if inconsistencies in stellar atmosphere models should be invoked.

— Measuring Sub-Kelvin Variations in Stellar Temperature with High-Resolution Spectroscopy (2409.07260 - Artigau et al., 11 Sep 2024) in Subsubsection “HD 189733 transit Temp time series” within Section “Transit-induced temperature changes”