Astronomy & Telescopes

HST SHEL: Enabling Comparative Exoplanetology with HST/STIS

By Keith Cowing
Status Report
June 3, 2024
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HST SHEL: Enabling Comparative Exoplanetology with HST/STIS
The combined STIS G430L and G750L spectrum of WASP-39 b (top left), WASP-121 b (top right), WASP-69 b (bottom left), and WASP-17 b (bottom right). Each color (common between planets) corresponds to a different systematics detrending technique, either using linear or GP methods and either between 1-10 PCs or the 10 initial input vectors. — astro-ph.EP

The Hubble Space Telescope (HST) has been our most prolific tool to study exoplanet atmospheres. As the age of JWST begins, there is a wealth of HST archival data that is useful to strengthen our inferences from JWST.

Notably, HST/STIS and its 0.3-1 μm wavelength coverage extends past JWST’s 0.6 μm wavelength cutoff and holds an abundance of potential information: alkali (Na, K) and molecular (TiO, VO) species opacities, aerosol information, and the presence of stellar contamination. However, time series observations with HST suffer from significant instrumental systematics and can be highly dependent on choices made during the transit fitting process. This makes comparing transmission spectra of planets with different data reduction methodologies challenging, as it is difficult to discern if an observed trend is caused by differences in data reduction or underlying physical processes.

Here, we present the Sculpting Hubble’s Exoplanet Legacy (SHEL) program, which aims to build a consistent data reduction and light curve analysis methodology and associated database of transmission spectra from archival HST observations. In this paper, we present the SHEL analysis framework for HST/STIS and its low-resolution spectroscopy modes, G430L and G750L.

We apply our methodology to four notable hot Jupiters: WASP-39 b, WASP-121 b, WASP-69 b, and WASP-17 b, and use these examples to discuss nuances behind analysis with HST/STIS. Our results for WASP-39 b, WASP-121 b, and WASP-17 b are consistent with past publications, but our analysis of WASP-69 b differs and shows evidence of either a strong scattering slope or stellar contamination. The data reduction pipeline and tutorials are available on Github.

Our final best spectrum for each of our four test planets, shown along with models from Goyal et al. (2019). Note that due to the general lack of features in this wavelength range, there are multiple models equally consistent with the data, but one model is shown as demonstrative of such a family of models. For WASP-39 b, WASP-121 b, and WASP-17 b, due to the similarities in our final spectra, we refer the reader to the planet specific papers and the modeling results therein for an in-depth discussion of the specific properties of the planets’ atmospheres. For WASP-69 b, for which our final spectrum differs significantly from that in past results, we present our best-fit models specifically: a potential haze scattering slope forward model (Goyal et al. 2019) and stellar contamination model. It is impossible to confidently distinguish between these models with the quality of the data. The points in the shaded region are affected by large systematics (see Figure 5) and so are left out of the analysis. — astro-ph.EP

The spectroscopic light curve fits for WASP-69 b, from G430L (two transits, phased together). The left is the linear detrending using 2 PCs as regressors, and the right is the GP detrending using 5 PCs as regressors. The raw data points are shown as more transparent color version of the final detrended points for reference, and each wavelength bin is offset vertically. The beginning and end values of the wavelengths included in the bin are shown above the pre-transit baseline of each spectroscopic light curve (in µm). — astro-ph.EP

Natalie H. Allen, David K. Sing, Néstor Espinoza, Richard O’Steen, Nikolay K. Nikolov, Zafar Rustamkulov, Thomas M. Evans-Soma, Lakeisha M. Ramos Rosado, Munazza K. Alam, Mercedes López-Morales, Kevin B. Stevenson, Hannah R. Wakeford, Erin M. May, Rafael Brahm, Marcelo Tala Pinto

Comments: 36 pages, 25 figures, 4 tables. Accepted for publication in the Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2405.20361 [astro-ph.EP] (or arXiv:2405.20361v1 [astro-ph.EP] for this version)
Submission history
From: Natalie Allen
[v1] Thu, 30 May 2024 16:19:37 UTC (46,312 KB)


Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻