Minor tweaks to text 968436868 Assigned ID: 4330 ----- Attempting Submission 1 (Wed May 24 13:53:11 GMT 2023) ----- HST Phase I Proposal 4330 successfully submitted. Receipt: # 4330-1 ----- Attempting Submission 2 (Wed May 24 18:33:00 GMT 2023) ----- Opening windows into the atmospheres of other worlds is undoubtedly among the most pivotal contributions to HST's scientific legacy. Exoplanetary transmission spectra have yielded insights into the their composition, temperatures, and pressures, providing a foundation for the new era of atmospheric characterization with JWST. However, it has recently become clear that host-star photospheric heterogeneities may limit the exoplanet legacy of HST - and even JWST - by giving rise to spectral features that mimic or mask those of planetary atmospheres, leading to erroneous inferences. Fortunately, the very same HST datasets provide exquisite out-of-transit stellar spectra that can be leveraged to mitigate this effect. While a few recent studies have shown the utility of studying out-of-transit HST stellar spectra, most remain untouched. Here we propose to thoroughly tap this stellar treasure trove via a large, uniform analysis of all 25,000 archival HST transit exposures to reveal the properties of hosts' photospheres. Through this analysis, we will establish an HST Legacy Library of time-dependent spectra of exoplanet host stars, constraints on their photospheric spectral components and covering fractions, and estimates of their spectral contributions to exoplanet transmission spectra. As Legacy Data Products, we will also release the tools we develop for the data reduction, stellar heterogeneity modeling, and transmission spectra corrections as open-source Python libraries. The insights and tools developed through this work will cement HST's legacy of revealing the properties of both exoplanets and their host stars and pave the way for future studies with both HST and JWST. PI Dr. Rackham is an expert in observations of exoplanetary transmission spectroscopy from space and the ground, as well as modeling and correcting for the impact of stellar photospheric heterogeneity on transmission spectra. He is the PI of HST program GO 16446 in Cycle 28 with WFC3 and recently co-lead NASA ExoPAG Study Analysis Group 21, focused on understanding the impact of stellar photospheric heterogeneity on space-based transmission spectra. Co-PI Prof. Apai is an expert in exoplanetary science and high-precision time-resolved HST spectroscopy. He led the Cloud Atlas HST Large Treasury program and the Extrasolar Storms Spitzer Exploration Science program. He is the PI of HST GOs 12314, 12551, 13176, 13418, 15947, 14241, AR 14278, and SNAP 12550. Co-PI Prof. de Wit is an expert in exoplanetary science with extensive experience with HST observations with a particular focus on atmospheric characterization. He is the PI of HST GOs 14500, 14873, 14900, 15304, and a significantly contributing Co-I of HST GOs 14493 and 16875. Co-I de Beurs is an NSF Graduate Research Fellow with expertise in disentangling stellar activity and planetary signals in observational data. Co-I Garcia is an expert in exoplanet detection and exoplanetary atmospheric characterization with HST, and the lead developer of the open-source and modular software prose. Co-I Dr. Giampapa is an expert in solar and stellar physics, including stellar dynamos, stellar cycles and magnetic activity, exoplanet system characterization, and asteroseismology. Co-I Dr. Hedges is an expert in data analysis and systematics removal with space-based telescope data including Kepler/K2, TESS, and HST. She has extensive experience building transmission spectra of exoplanet atmospheres from HST data. Co-I Hedges is PI of HST GO 15428. Co-I Dr. Mansfield is an expert in exoplanet atmospheric characterization with HST. She is the PI of HST GO 16168 in Cycle 28 with WFC3. Co-I Prof. Wakeford is an expert in exoplanetary atmospheric characterization with HST and cloud modeling in exoplanetary atmospheres. She is the PI of HST GOs 14169 and 14918. Stellar Physics and Stellar Types Exoplanets and Exoplanet Formation 0 Legacy false false false false false false SUPPORTED