Detection of escaping magnesium in the upper atmosphere of WASP-76b
Abstract
Near-ultraviolet (NUV) observations can uniquely probe low-density, high-altitude gas in ultra-hot Jupiters (UHJs) via strong resonance lines of metals. WASP-76b is a well-studied UHJ with prior detections of refractory metals and complex day-night chemistry, making it a prime target for atmospheric escape diagnostics in the NUV. We present HST/STIS E230M (2274 - 3119 mathringA) transmission spectroscopy of WASP-76b. While broad- (~ 200 mathringA) and narrow-band (4 mathringA) transmission spectra do not show spectral features, a velocity-resolved single line analysis reveals clear excess absorption in both components of the Mg II doublet (2796 mathringA and 2803 mathringA, k and h lines). The Mg II signal is localized to the blue wing (Doppler velocities [-50, -10] km s−1) and is recovered with narrower 0.5 mathringA bins; the inferred absorption exceeds both the optical continuum radius and the Roche-lobe scale, suggesting a highly extended upper atmosphere. We estimate a mass-loss rate of Ṁ∼1012.14 g s−1 from ATES. The present NUV data do not improve constraints on lower-atmosphere properties, and a comparison with upper-atmosphere forward models disfavor scenarios with extreme Mg enrichment. Together with the near-solar metallicity inferred from optical+IR spectra, this suggests that the Mg II absorption can arise in an escaping upper atmosphere without requiring extreme Mg enrichment, qualitatively consistent with hydrodynamic escape. Ongoing HST programs expanding the sample of UHJs focused on NUV observations will enable comparative tests of composition and kinematics in escaping atmospheres.
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BibTeX
@misc{rosado2026detection,
title = {{Detection of escaping magnesium in the upper atmosphere of WASP-76b}},
author = {Lakeisha M. Ramos Rosado and Leonardo A. Dos Santos and Joshua D. Lothringer and David K. Sing and Johanna Teske and Aline A. Vidotto and David Ehrenreich and Julia V. Seidel and H. Jens Hoeijmakers and Munazza K. Alam and Monika Lendl},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.09128},
eprint = {2610.09128},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.09128},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Detection of escaping magnesium in the upper atmosphere of WASP-76b
AU - Lakeisha M. Ramos Rosado
AU - Leonardo A. Dos Santos
AU - Joshua D. Lothringer
AU - David K. Sing
AU - Johanna Teske
AU - Aline A. Vidotto
AU - David Ehrenreich
AU - Julia V. Seidel
AU - H. Jens Hoeijmakers
AU - Munazza K. Alam
AU - Monika Lendl
PY - 2026
DA - 2026/10/06
PB - arXiv
DO - 10.48550/arxiv.2610.09128
UR - https://arxiv.org/abs/2610.09128
AB - Near-ultraviolet (NUV) observations can uniquely probe low-density, high-altitude gas in ultra-hot Jupiters (UHJs) via strong resonance lines of metals. WASP-76b is a well-studied UHJ with prior detections of refractory metals and complex day-night chemistry, making it a prime target for atmospheric escape diagnostics in the NUV. We present HST/STIS E230M (2274 - 3119 mathringA) transmission spectroscopy of WASP-76b. While broad- (~ 200 mathringA) and narrow-band (4 mathringA) transmission spectra do not show spectral features, a velocity-resolved single line analysis reveals clear excess absorption in both components of the Mg II doublet (2796 mathringA and 2803 mathringA, k and h lines). The Mg II signal is localized to the blue wing (Doppler velocities [-50, -10] km s^−1) and is recovered with narrower 0.5 mathringA bins; the inferred absorption exceeds both the optical continuum radius and the Roche-lobe scale, suggesting a highly extended upper atmosphere. We estimate a mass-loss rate of Ṁ∼10^12.14 g s^−1 from ATES. The present NUV data do not improve constraints on lower-atmosphere properties, and a comparison with upper-atmosphere forward models disfavor scenarios with extreme Mg enrichment. Together with the near-solar metallicity inferred from optical+IR spectra, this suggests that the Mg II absorption can arise in an escaping upper atmosphere without requiring extreme Mg enrichment, qualitatively consistent with hydrodynamic escape. Ongoing HST programs expanding the sample of UHJs focused on NUV observations will enable comparative tests of composition and kinematics in escaping atmospheres.
ER -