Unlocking HST's Stellar Treasure Trove: Stellar Activity Minima for HAT-P-11 Offer Prime Windows for Transmission Spectroscopy

Abstract

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 \texttt{STIS} and \texttt{WFC3} across the \textsc{G430L}, \textsc{G750L}, \textsc{G102}, and \textsc{G141} bandpasses to constrain the surface heterogeneity of HAT-P-11 and its impact on transmission spectroscopy. Grid-based spectral retrievals using NewEra \texttt{PHOENIX} models robustly favor two-component photospheres in the \texttt{WFC3} G102 and G141 data, with a ${\sim}4950$\,K photospheric component and a cooler ($\sim$3400\,K) component covering 26{--}33\% of the stellar disk. By contrast, retrievals on the \texttt{STIS} optical spectra do not yield a satisfactory fit, reflecting current limitations of stellar atmosphere models in the optical regime compared to the \HST{} observational precision. We contextualize these results using long-term photometric monitoring and chromospheric activity indices. The inferred high spot covering fractions are broadly consistent with the elevated photometric variability observed during the \textit{Kepler} era ($f_{\rm spot}$$\sim$10--20\%) but are in tension with the much lower rotational amplitudes observed from TESS in the mid 2020s ($f_{\rm spot}$$\sim$1--10\%). This secular decline in variability is mirrored by a $\sim$20\% decrease in the Ca\,\textsc{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.

Figures (11)

• Figure 1: Summary of selected space-based observations of HAT-P-11. Top: Four years of Kepler short-cadence photometry showing pronounced rotational modulation and numerous spot-crossing events. Middle: Observation timeline for five facilities---Kepler, TESS, HST, Spitzer, and JWST---spanning optical to infrared wavelengths. Bottom: The TESS light curve, displayed on the same normalized-flux scale as the Kepler data for direct comparison. The rotational modulation amplitude in TESS is smaller than in Kepler. The high-cadence data are binned to highlight the modulation signal. The rotational periods inferred from both datasets are consistent with the previously reported value of 29.2$\pm$0.5 days.
• Figure 2: Optical and near-infrared spectra of HAT-P-11. The HST spectra of HAT-P-11 were obtained with STIS (G430L, two visits; G750L, one visit) and WFC3 (G102, five visits; G141, one visit), spanning 0.3--1.7 $\micron$. We show these data alongside an IRTF/SpeX spectrum ($R{\sim}2000$ of HAT-P-11, which better resolves the Ca ii infrared triplet. The Ca ii region is highlighted in the main panel and shown in the inset to illustrate the effect of spectral resolution in G750L, G102, and SpeX data. All spectra are vertically scaled to match the STIS/G750L continuum level. After accounting for differences in resolution and wavelength sampling, the spectra from the various HST modes are consistent at the 1--2% level.
• Figure 3: Stellar spectral modeling of HAT-P-11 from blue to red wavelengths (STIS/G430L, STIS/G750L, WFC3/G102, and WFC3/G141). Fits are performed on the averaged spectra for each mode, with uncertainties estimated from the data scatter. In each pair of flux and residual panels, the top panel shows the observed spectrum (gray) and the best-fit combined model (red), with residuals displayed below. The first pair of rows shows the single-component model, the second pair the two-component model, and the third pair the three-component model. Contributions from the individual components are indicated by colored curves (blue, orange, and green), and their sum is shown in red. The Bayesian Information Criterion (BIC) for each fit is shown in the upper panels; the preferred model in each wavelength range is highlighted with a red box.
• Figure 4: Time series of stellar temperature components and spot filling factor from two-component spectral fits. Top: Retrieved values of the photospheric temperature ($T_1$) and spot temperature ($T_2$) for each HST epoch. For the STIS datasets (gray), the inferred photosphere and spot temperatures strongly overlap, indicating that the two-component model does not meaningfully separate distinct temperature components; these results are not considered reliable due to significant residuals (see \ref{['sec:modelFidelity']}) and are shown for completeness only. Bottom: Corresponding inferences of the spot filling factor ($f_2$) as a function of epoch.
• Figure 5: Comparison of five WFC3/G102 spectra obtained at different epochs. Top: Extracted stellar flux for each epoch shown without scaling, demonstrating consistency in flux levels and spectral shape. Middle: The same spectra, vertically offset for clarity. Bottom: The ratio of the brightest epoch to the dimmest. The brightest spectrum is elevated by 0.35% relative to the faintest, with no wavelength-dependent structure. This uniform offset indicates instrumental stability and suggests that the stellar inhomogeneities remain relatively stable across five visits.