An independent determination of Fomalhaut b's orbit and the dynamical effects on the outer dust belt
Hervé Beust, Jean-Charles Augereau, Amy Bonsor, J. Graham, Paul Kalas, J. Lebreton, Anne-Marie Lagrange, Steve Ertel, Virginie Faramaz, Philippe Thebault
备注：
17 pages, 15 figures, accepted for publication in Astronomy \& Astrophysics
The nearby star Fomalhaut harbours a cold, moderately eccentric dust belt with a sharp inner edge near 133 au. A low-mass, common proper motion companion (Fom b), was discovered near the inner edge and was identified as a planet candidate that could account for the belt morphology. However, the most recent orbit determination based on four epochs of astrometry over eight years reveals a highly eccentric orbit that appears to cross the belt in the sky plane projection. We perform here a full orbital determination based on the available astrometric data to independently validate the orbit estimates previously presented. Adopting our values for the orbital elements and their associated uncertainties, we then study the dynamical interaction between the planet and the dust ring, to check whether the proposed disk sculpting scenario by Fom b is plausible. We used a dedicated MCMC code to derive the statistical distributions of the orbital elements of Fom b. Then we used symplectic N-body integration to investigate the dynamics of the dust belt, as perturbed by a single planet. Different attempts were made assuming different masses for Fom b. We also performed a semi-analytical study to explain our results. Our results are in good agreement with others regarding the orbit of Fom b. We find that the orbit is highly eccentric, is close to apsidally aligned with the belt, and has a moderate mutual inclination relative to the belt plane of. If coplanar, this orbit crosses the disk. Our dynamical study then reveals that the observed planet could sculpt a transient belt configuration with a similar eccentricity to what is observed, but it would not be simultaneously apsidally aligned with the planet. This transient configuration only occurs a short time after the planet is placed on such an orbit (assuming an initially circular disk), a time that is inversely proportional to the planet's mass, and that is in any case much less than the 440 Myr age of the star. We constrain how long the observed dust belt could have survived with Fom b on its current orbit, as a function of its possible mass. This analysis leads us to conclude that Fom b is likely to have low mass, that it is unlikely to be responsible for the sculpting of the belt, and that it supports the hypothesis of a more massive, less eccentric planet companion Fom c.
Resolving the outer ring of HD 38206 using ALMA and constraining limits on planets in the system
HD 38206是天鸽座星协中的一颗A0V恒星，拥有一个由IRAS首次发现的碎片盘。斯皮策和赫歇尔的进一步观测表明，这个圆盘有两个组成部分，可能类似于太阳系的小行星和柯伊伯带。这颗恒星的年轻年龄使其成为直接成像行星搜索的主要目标。系统中可能的行星可以使用碎片盘来限制。在这里，我们提出了第一ALMA观测系统的柯伊伯带，并适合他们使用的前向建模MCMC方法。我们探测到一个扩展的尘埃盘，峰值在180 au左右，宽度为140 au。椎间盘接近边缘，显示出不对称的初步迹象，最佳偏心率为$0.25^{+0.10}_{-0.09}$。我们使用拟合参数来确定冷带内部行星质量的限制。我们确定至少需要四颗行星，每颗行星的最小质量为0.64 M $_J$，以清除小行星和柯伊伯带之间的差距。如果我们假设最外层的行星负责圆盘的搅动，其内缘的位置和圆盘的偏心率，那么我们可以更紧密地预测其偏心率，质量和半长轴为$e_{\rm{p}}=0.34^{+0.20}_{-0.13}$，$m_{\rm{p}}=0.7^{+0.5}_{-0.3}\,\rm{M}_{\rm{J}}$和$a_{\rm{p}}=76^{+12}_{-13}\,\rm{au}$。
