The HST Stellar Treasure Trove

HST Legacy Archival Program Constraining Stellar Contamination in Transmission Spectroscopy

The HST Stellar Treasure Trove is a large effort to uniformly reprocess archival Hubble Space Telescope time-series spectroscopy of transiting exoplanets and quantify stellar and planetary signals in transmission spectra.

Grant: HST AR 17551   â€˘   Last updated: —

Unlocking the Stellar Treasure Trove

Spatially heterogeneous stellar photospheres (spots, faculae, and other active regions) imprint wavelength-dependent signals on measured transit depths. These stellar spectral imprints can bias inferred planetary radii, slopes, and molecular abundances—especially for planets orbiting active K and M dwarfs.

By leveraging the breadth of archival HST transit observations, the Stellar Treasure Trove provides uniform reductions and stellar-contamination constraints, enabling robust comparisons across targets and instruments and supporting interpretation of JWST spectra and future atmospheric surveys.

Stellar contamination as an astrophysical systematic

Quantify the impact of unocculted and occulted heterogeneity on transmission spectra, and provide contamination-aware products designed for downstream atmospheric inference.

Uniform, comparable archival products

Apply consistent reduction and modeling assumptions across a broad sample of HST time-series observations, producing a coherent library for population-level analysis.

Bridge HST to JWST and future missions

Provide constraints and priors that inform JWST interpretation and guide observing strategies for characterizing small planets around active stars.

Targets

The HST Stellar Treasure Trove comprises 13,271 exposures of 16 transiting-planet host stars spanning effective temperatures from 2,566 to 6,776 K. The archive includes time-series spectroscopy from six STIS and WFC3 observing modes, providing broad optical-to-near-infrared coverage across M, K, G, and F host stars.

Target Spectral Type STIS/CCD (orbits) WFC3/UVIS (orbits) WFC3/IR (orbits)
WASP-121 F3V G430L (11) · G750L (5) — G102 (10) · G141 (62)
WASP-17 F5V G430L (11) · G750L (5) G280 (5) G102 (10) · G141 (10)
WASP-12 F8V G430L (15) · G750L (5) — G102 (16) · G141 (58)
WASP-19 G8V G430L (8) · G750L (4) — G141 (25)
WASP-39 G9V G430L (10) · G750L (5) G280 (20) G102 (5) · G141 (10)
55 Cancri K0V G430L (22) · G750L (21) — G141 (24)
HAT-P-26 K2V G430L (11) · G750L (5) G280 (16) G102 (5) · G141 (10)
HD 189733 K2V G430L (35) · G750M (28) — G141 (30)
HAT-P-11 K3V G430L (10) · G750L (5) — G102 (24) · G141 (9)
HAT-P-12 K4V G430L (9) · G750L (4) G280 (15) G141 (11)
K2-3 K9V — — G141 (42)
AU Mic M1V G430L (6) · G750L (6) — G102 (8) · G141 (8)
K2-18 M2.5V G430L (1) — G141 (45)
GJ 436 M2.5V G430L (29) · G750L (8) — G141 (25)
GJ 1214 M5V G430L (2) · G750L (4) — G141 (77)
TRAPPIST-1 M8V G430L (1) — G141 (43)

Papers

Axisymmetric High Spot Coverage on Exoplanet Host HD 189733 A

Isaac S. Narrett, Benjamin V. Rackham, and Julien de Wit. 2024. The Astronomical Journal 167, 107

In the first paper from the HST Stellar Treasure Trove program, we show that absolutely calibrated HST/WFC3/G141 stellar spectra can reliably recover the temperatures and full-disk filling factors of distinct photospheric components. For HD 189733 A, the spectra imply that cool spots cover roughly 40% of the visible disk—far more spottedness than the modest rotational modulation measured by TESS might suggest. The two measurements are reconciled if much of the spot coverage is distributed approximately axisymmetrically, such as within densely populated latitude bands or near polar regions. This establishes HST stellar spectra as a direct probe of disk-integrated photospheric heterogeneity while showing why rotational variability alone can substantially understate total spot coverage.

Three epochs of absolutely calibrated HST/WFC3/G141 spectra of HD 189733 A, with rows comparing one-, two-, and three-component photospheric models and their residuals
Across all three HST/WFC3/G141 epochs, a two-component photosphere describes the stellar spectra substantially better than a single-component model and returns consistent physical properties. The agreement shows that these spectra can constrain both photospheric component temperatures and disk-integrated filling factors.
TESS light curves of HD 189733 A showing modest rotational flux variations compared with example spot-modulation models
TESS records only modest rotational variability from HD 189733 A, despite the roughly 40% spot filling factor inferred from the HST spectra. This apparent tension demonstrates that rotational modulation measures asymmetry in the surface pattern, not the star’s total spot coverage.
Illustrations of HD 189733 A with extensive dark spots arranged in latitude bands or polar regions, compared with a more sparsely spotted non-axisymmetric surface
Large spot filling factors can coexist with modest rotational variability when much of the coverage is approximately axisymmetric—for example, in heavily populated latitude bands or polar regions. Only the smaller non-axisymmetric component strongly modulates the observed brightness as the star rotates.

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

Prajwal Niraula, Benjamin V. Rackham, Julien de Wit, Daniel Apai, Mark S. Giampapa, David Berardo, and Chia-Lung Lin. 2026. The Astronomical Journal 172, 112

HD 189733 A established that HST stellar spectra can recover full-disk photospheric heterogeneity even when rotational variability reveals only part of the activity pattern. HAT-P-11 extends that approach across an unusually rich temporal baseline. Combining Kepler and TESS light curves with HST-derived spot filling factors and ground-based Ca II H&K measurements reveals strong long-term evolution: HAT-P-11 was highly active during the Kepler era, then became substantially quieter by the mid-2020s. That relatively quiet state coincided with JWST observations in 2024 and 2025, providing particularly favorable windows for atmospheric characterization of HAT-P-11 b.

Timeline of HAT-P-11 observations from 2009 through 2025, with strongly modulated Kepler and more weakly variable TESS light curves alongside the epochs of HST, Spitzer, and JWST observations
More than a decade of observations provides the temporal context for HAT-P-11. Kepler recorded strong rotational modulation, whereas later TESS data show much weaker variability; the timeline also locates the HST, Spitzer, and 2024–2025 JWST observations within that long evolution.
Long-term activity history of HAT-P-11 comparing HST spot filling factors, Kepler and TESS variability estimates, Ca II H and K measurements, and the 2024 and 2025 JWST observing epochs
HST-derived spot filling factors, Kepler and TESS variability, and independent Ca II H&K measurements together trace HAT-P-11 from a highly active state toward a much quieter one. The marked JWST epochs in 2024 and 2025 fall within this recent activity minimum.
Comparison of HAT-P-11 light curves and transit profiles during the Kepler, TESS 2022, and TESS 2024 epochs, showing diminishing rotational modulation and spot-crossing structure
The change is visible directly in the light curves: the Kepler era shows strong rotational modulation and prominent spot-crossing structure, while the later TESS observations—especially in 2024—show much weaker activity signatures.

Contact

  • Email

    brackham@mit.edu

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