A High-Resolution NUV Transmission Spectrum of KELT-9b: Mg II and Fe II Escaping from the Hottest Known Giant Planet
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| Format: | Preprint |
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2025
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| _version_ | 1866918259585974272 |
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| author | Baldwin, Austin Lothringer, Joshua D. Santos, Leonardo A. Dos Sing, David K. Rustamkulov, Zafar Nikolov, Nikolay K. Valenti, Jeff Wakeford, Hannah R. |
| author_facet | Baldwin, Austin Lothringer, Joshua D. Santos, Leonardo A. Dos Sing, David K. Rustamkulov, Zafar Nikolov, Nikolay K. Valenti, Jeff Wakeford, Hannah R. |
| contents | We present high-resolution NUV observations from Hubble Space Telescope's (HST) Space Telescope Imaging Spectrograph (STIS) data for the hottest known gas planet, KELT-9b. Observations were collected with STIS/E230M (2300-3000 $Å$, R$\sim$ 30,000) and we de-correlate systematic effects from the telescope using jitter detrending. We show the clear presence of the Mg II doublet at 2800 $Å$ and Fe II at 2600 $Å$ in KELT-9b. The Mg II is measured above the planet's Roche transit radius, indicating it is escaping. We fit 1D NLTE atmospheric escape models to these features, demonstrating a significant loss of mass in KELT-9b's atmosphere ($\dot{M} \approx 10^{12} $ g/s); we also find a remarkably high line-broadening corresponding to a velocity of about $50-75$ km/s, and a net blueshift of the Mg II doublet greater than 30 km/s. Future 3D MHD modeling of the spectrum and gas kinematics is likely needed to explain these observations. We interpret these results in the context of the Mg II ``Cosmic Shoreline" and show that the detection of escaping Mg II in KELT-9b and the non-detection in WASP-178b are consistent with the hypothesis that stars hotter than $T_{\mathrm{eff}} \sim$ 8250~K have relatively low levels of XUV radiation due to the lack of a chromosphere. Therefore planets around such early-type stars experience a different degree of atmospheric escape. This result highlights the importance of XUV irradiation in driving atmospheric escape inside and outside the Solar System. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2512_19662 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | A High-Resolution NUV Transmission Spectrum of KELT-9b: Mg II and Fe II Escaping from the Hottest Known Giant Planet Baldwin, Austin Lothringer, Joshua D. Santos, Leonardo A. Dos Sing, David K. Rustamkulov, Zafar Nikolov, Nikolay K. Valenti, Jeff Wakeford, Hannah R. Earth and Planetary Astrophysics We present high-resolution NUV observations from Hubble Space Telescope's (HST) Space Telescope Imaging Spectrograph (STIS) data for the hottest known gas planet, KELT-9b. Observations were collected with STIS/E230M (2300-3000 $Å$, R$\sim$ 30,000) and we de-correlate systematic effects from the telescope using jitter detrending. We show the clear presence of the Mg II doublet at 2800 $Å$ and Fe II at 2600 $Å$ in KELT-9b. The Mg II is measured above the planet's Roche transit radius, indicating it is escaping. We fit 1D NLTE atmospheric escape models to these features, demonstrating a significant loss of mass in KELT-9b's atmosphere ($\dot{M} \approx 10^{12} $ g/s); we also find a remarkably high line-broadening corresponding to a velocity of about $50-75$ km/s, and a net blueshift of the Mg II doublet greater than 30 km/s. Future 3D MHD modeling of the spectrum and gas kinematics is likely needed to explain these observations. We interpret these results in the context of the Mg II ``Cosmic Shoreline" and show that the detection of escaping Mg II in KELT-9b and the non-detection in WASP-178b are consistent with the hypothesis that stars hotter than $T_{\mathrm{eff}} \sim$ 8250~K have relatively low levels of XUV radiation due to the lack of a chromosphere. Therefore planets around such early-type stars experience a different degree of atmospheric escape. This result highlights the importance of XUV irradiation in driving atmospheric escape inside and outside the Solar System. |
| title | A High-Resolution NUV Transmission Spectrum of KELT-9b: Mg II and Fe II Escaping from the Hottest Known Giant Planet |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2512.19662 |