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| Paper: |
Advancing Our Understanding of the Interstellar Medium with the Habitable Worlds Observatory |
| Monograph: |
11, HWO25 Proceedings Part II: Mission Framework, Technology, and Broader Contributions |
| Page: |
87 |
| Authors: |
Adam M. Ritchey |
| DOI: |
10.26624/JCTR9733 |
| Abstract: |
Significant advances in our understanding of the physics and chemistry of the interstellar medium require a next-generation large-aperture UV space telescope, with characteristics like those proposed for the Habitable Worlds Observatory. Here, I describe some of the confounding uncertainties and limitations imposed by the current generation of UV instrumentation (primarily the STIS and COS instruments onboard the Hubble Space Telescope). Examples include the still-very-uncertain gas-phase abundances of the key dust-forming elements carbon and silicon. These elements are difficult to study in the interstellar medium because their UV transitions are either extremely strong (and saturated) or are so weak that very high signal-to-noise ratio spectra are required to detect the absorption lines. To date, only a handful of high-quality carbon and silicon abundance measurements exist for sight lines probing the local Galactic ISM. This severely limits our understanding of the composition and abundances of interstellar dust grains and how those properties change with changing interstellar conditions. Similarly, the interstellar abundances of neutron-capture elements are poorly understood because the associated absorption lines are weak and difficult to detect. Finally, the gas densities and kinetic temperatures in neutral diffuse clouds can be studied effectively using observations of the fine-structure excitation of neutral oxygen. However, this technique is currently limited by the weakness of the absorption features arising from the excited levels. A large-aperture UV space telescope would yield breakthroughs in each of these areas by providing an efficient method of obtaining very high signal-to-noise ratio spectra for a large sample of Galactic sight lines. |
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