Earth's Upper Atmosphere Glows in the Ultraviolet
When ultraviolet light from the Sun strikes the Earth, hydrogen and oxygen atoms in the upper layers of the Earth’s atmosphere scatter this light, generating what is known as geocoronal emission, or airglow. This ultraviolet airglow is captured by UV sensitive instruments within the Earth's exosphere, such as the FUV spectrographs on board HST. The Space Telescope Imaging Spectrograph (STIS) has high enough spatial resolution to create a separate airglow spectrum and can remove this from the science spectrum. This is not the case for the Cosmic Origins Spectrograph (COS), which instead prioritizes the spectral resolution of faint point sources. A combination of greater airglow contamination and vignetting in the spatial axis prevents airglow from being subtracted from COS science spectra. For cool, main sequence stars, this obscures stellar Lyman-alpha (Lyα) emission at 1216 Å and oxygen emission at 1302, 1305, and 1306 Å. These stellar emission features drive photochemistry in exoplanet atmospheres, and additionally serve as backlights for exoplanet transits in the FUV, making it important to know what the underlying emission features look like.
Recovering Stellar Emission from HST-COS Spectra
Using archival airglow observations with the COS/G130M grating, I developed airglow templates which are applicable to a wide variety of observation settings. Building upon early airglow subtraction techniques, I led the development of a subtraction method which can recover and reconstruct the underlying stellar Lyα and oxygen emission. I developed a GUI which performs this airglow subtraction and stellar recovery of the previously obscured emission features. In addition to airglow, Lyα also suffers from absorption from the interstellar medium (ISM) as the starlight makes its way to Earth. The GUI is equipped to reconstruct the obscured Lyα flux based on previous techniques developed for STIS spectra. Of 171 archival dwarf star spectra, the GUI was able to successfully recover ~100 Lyα and oxygen profiles. The recovered emission was then used to develop ways to predict the Lyα and oxygen emission of other F-, G-, K-, and M-type dwarf stars.
For more details, see my paper, and to access the airglow subtraction GUI, click here!
For more details, see my paper, and to access the airglow subtraction GUI, click here!
Reconstructing Lyα Emission of Classical T Tauri Stars
Building upon the success of the airglow subtraction and stellar recovery GUI developed for main sequence stars, the models were adapted so that they can be applied to protostellar objects known as Classical T Tauri Stars (CTTSs). A CTTS is a young F-, G-, K-, or M-type star in the making, accreting material from the protoplanetary disk (PPD) surrounding it. Much like main sequence stars, the FUV spectra of CTTSs are dominated by Lyα emission. As a consequence, protostellar Lyα emission is an important driver of disk photochemistry and evolution, however directly observing this emission proves to be impossible for the same reasons as observing the Lyα profiles of main sequence stars (see above). I am currently leading the development of two new CTTS models which simultaneously fit the emission of the CTTS, absorption by an atomic hydrogen outflow from the Lyα, additional absorption by the intervening ISM, and airglow contamination. By better understanding the Lyα profiles of CTTSs, we can better understand the environments in which exoplanets are actively forming. Stay tuned for an upcoming publication on this work, followed by an update to the GUI!
© February 2024