重新审视TWA 27B的氦和氢吸积指标：近自由落体速度下的弱质量流
Revisiting the Helium and Hydrogen Accretion Indicators at TWA 27B: Weak Mass Flow at Near-Freefall Velocity
AI总结：
重新分析TWA 27B的氦氢发射线，测得吸积率极低（~5e-9 MJ/yr），表明其形成已近结束，并展示JWST探测低质量吸积的能力。
摘要：
TWA 27B（2M1207b）是首个直接成像的行星质量（MP ~ 5 MJ）伴星（Chauvin等人，2004年），并已通过JWST/NIRSpec在0.9–5.3微米波段进行了观测（Luhman等人，2023年）。为了理解TWA 27B的吸积特性，我们搜索了连续谱扣除后的近红外氦和氢发射线，并测量了它们的宽度和光度。我们以4.3σ的显著性探测到He I三重线，以及NIRSpec覆盖的所有帕申系谱线（Pa α、Pa β、Pa γ、Pa δ），显著性为4–5σ。三条最亮的布拉开系谱线（Br α、Br β、Br γ）以及Pf γ和Pf δ为2–3σ的暂定探测。我们提供了其他氢线的上限，包括通过哈勃太空望远镜档案数据得到的H α上限。有三条谱线可以被可靠地反卷积，揭示出固有宽度Δv = 67±9 km/s，这是表面自由落体速度的60%。谱线光度似乎显著过高，无法归因于色球活动。将谱线光度转换为吸积率，使用行星质量的标度关系得到Mdot ~ 5e-9 MJ/yr，而使用外推的恒星标度则得到Mdot ~ 0.1e-9 MJ/yr。其中几条谱线是在如此低质量吸积体上的首次探测。微弱的吸积率表明形成过程可能已经结束。这项分析表明，JWST可用于测量行星质量天体上的低谱线发射质量吸积率，促使更深入地搜索为TWA 27B提供物质的质量储库，并暗示其他年轻直接成像天体可能——迄今未知——也在吸积。
英文摘要：
TWA 27B (2M1207b) is the first directly-imaged planetary-mass (MP ~ 5 MJ) companion (Chauvin et al. 2004) and was observed at 0.9--5.3 micron with JWST/NIRSpec (Luhman et al. 2023). To understand the accretion properties of TWA 27B, we search for continuum-subtracted near-infrared helium and hydrogen emission lines and measure their widths and luminosities. We detect the He I triplet at 4.3 sigma and all Paschen-series lines covered by NIRSpec (Pa alpha, Pa beta, Pa gamma, Pa delta) at 4--5 sigma. The three brightest Brackett-series lines (Br alpha, Br beta, Br gamma) as well as Pf gamma and Pf delta are tentative detections at 2--3 sigma. We provide upper limits on the other hydrogen lines, including on H alpha through Hubble Space Telescope archival data. Three lines can be reliably deconvolved to reveal an intrinsic width Delta v = 67+-9 km/s, which is 60% of the surface freefall velocity. The line luminosities seem significantly too high to be due to chromospheric activity. Converting line luminosities to an accretion rate yields Mdot ~ 5e-9 MJ/yr when using scaling relationships for planetary masses, and Mdot ~ 0.1e-9 MJ/yr with extrapolated stellar scalings. Several of these lines represent first detections at an accretor of such low mass. The weak accretion rate implies that formation is likely over. This analysis shows that JWST can be used to measure low line-emitting mass accretion rates onto planetary-mass objects, motivates deeper searches for the mass reservoir feeding TWA 27B, and hints that other young directly-imaged objects might -- hitherto unbeknownst -- also be accreting.
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