发表机构
Johns Hopkins University; Space Telescope Science Institute; Carnegie Institution for Science; Leiden University（约翰斯·霍普金斯大学; 空间望远镜科学研究所; 卡内基科学研究所; 莱顿大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
利用HST/STIS近紫外透射光谱，在WASP-76b高层大气中探测到Mg II双线逃逸吸收，估算质量损失率约10^12.14克/秒，表明无需极端镁富集即可发生流体动力学逃逸。
AI 中文摘要
近紫外（NUV）观测可通过金属的强共振线独特地探测超热木星（UHJs）中的低密度、高海拔气体。WASP-76b是一颗研究充分的超热木星，先前已探测到难熔金属和复杂的昼夜化学过程，使其成为NUV波段大气逃逸诊断的主要目标。我们展示了WASP-76b的HST/STIS E230M（2274-3119埃）透射光谱。虽然宽带（约200埃）和窄带（4埃）透射光谱未显示光谱特征，但速度分辨的单线分析揭示了Mg II双线（2796埃和2803埃，k线和h线）两个分量的明显过量吸收。Mg II信号局限于蓝翼（多普勒速度[-50, -10] km/s），并通过更窄的0.5埃分箱恢复；推断的吸收超过光学连续谱半径和洛希瓣尺度，表明存在高度延伸的高层大气。我们根据ATES估算质量损失率约为10^12.14克/秒。目前的NUV数据未改善对低层大气性质的约束，与高层大气正演模型的比较不利于极端镁富集的情景。结合从光学+红外光谱推断的近太阳金属丰度，这表明Mg II吸收可产生于逃逸的高层大气，无需极端镁富集，与流体动力学逃逸定性一致。正在进行的HST项目扩展了聚焦于NUV观测的超热木星样本，将能够对逃逸大气中的成分和运动学进行比较测试。
英文摘要
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 $\mathring{\mathrm{A}}$) transmission spectroscopy of WASP-76b. While broad- (~ 200 $\mathring{\mathrm{A}}$) and narrow-band (4 $\mathring{\mathrm{A}}$) 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 $\mathring{\mathrm{A}}$ and 2803 $\mathring{\mathrm{A}}$, $k$ 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 $\mathring{\mathrm{A}}$ 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 $\dot{M}\sim10^{12.14}\,\mathrm{g\,s^{-1}}$ 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.
Comments27 pages; 14 figures and 9 tables. Accepted for publication in AJ
