2026/03/25 by Prajwal Niraula, Benjamin V. Rackham, Julien de Wit +4 · 1 voice
#astro-ph.SR #astro-ph.EP
HAT-P-11 is a well-studied, active K dwarf hosting an eccentric, misaligned transiting sub-Neptune. As part of the HST Stellar Treasure Trove program (HST-AR-17551), we analyze absolutely calibrated out-of-transit \HST spectra from STIS and WFC3 across the G430L, G750L, G102, and G141 bandpasses to constrain the surface heterogeneity of HAT-P-11 and its impact on transmission spectroscopy. Grid-based spectral retrievals using NewEra PHOENIX models robustly favor two-component photospheres in the WFC3 G102 and G141 data, with a ∼4950 K photospheric component and a cooler (∼3400 K) component covering 26--33% of the stellar disk. By contrast, retrievals on the STIS optical spectra do not yield a satisfactory fit, reflecting current limitations of stellar atmosphere models in the optical regime compared to the \HST observational precision. We contextualize these results using long-term photometric monitoring and chromospheric activity indices. The inferred high spot covering fractions are broadly consistent with the elevated photometric variability observed during the Kepler era (f\rm spot∼10--20%) but are in tension with the much lower rotational amplitudes observed from TESS in the mid 2020s (f\rm spot∼1--10%). This secular decline in variability is mirrored by a ∼20% decrease in the Ca ii H&K index. These results imply that HAT-P-11 undergoes comparatively quiescent phases that offer more favorable windows for atmospheric characterization, which serendipitously coincided with some of the recent JWST observations. More generally, our study demonstrates that multi-epoch, space-based stellar spectra provides a physically grounded pathway for mitigating stellar contamination in high-precision transmission spectra in the JWST era.