Insights into the planetary dynamics of HD 206893 with ALMA

Radial substructure in the form of rings and gaps has been shown to be ubiquitous among protoplanetary discs. This could be the case in exoKuiper belts as well, and evidence for this is emerging. In this paper we present ALMA observations of the debris/planetesimal disc surrounding HD 206893, a system that also hosts two massive companions at 2 and 11 au. Our observations reveal a disc extending from 30 to 180 au, split by a 27 au wide gap centred at 74 au, and no dust surrounding the reddened brown dwarf (BD) at 11 au. The gap width suggests the presence of a 0.9 MJup planet at 74 au, which would be the third companion in this system. Using previous astrometry of the BD, combined with our derived disc orientation as a prior, we were able to better constrain its orbit finding it is likely eccentric (0:14+0:05 􀀀0:04). For the innermost companion, we used RV, proper motion anomaly and stability considerations to show its mass and semi-major axis are likely in the range 10–100 MJup and 1.4–2.7 au. These three companions will interact on secular timescales and perturb the orbits of planetesimals, stirring the disc and potentially truncating it to its current extent via secular resonances. Finally, the presence of a gap in this system adds to the growing evidence that gaps could be common in wide exoKuiper belts. Out of 6 wide debris discs observed with ALMA with enough resolution, 4–5 show radial substructure in the form of gaps.

以环与间隙为形式的径向子结构（radial substructure）已被证实广泛存在于原行星盘（protoplanetary discs）之中。系外柯伊伯带（exoKuiper belts）大概率也存在此类结构，相关实证正不断涌现。本文呈现了围绕HD 206893的碎屑/星子盘（debris/planetesimal disc）的阿塔卡马大型毫米/亚毫米波阵列（ALMA）观测结果，该系统还拥有两颗位于2 au与11 au处的大质量伴天体。我们的观测显示，该盘的延展范围为30 au至180 au，被一处以74 au为中心、宽度达27 au的间隙分割，且在11 au处的红化褐矮星（brown dwarf, BD）周围未探测到尘埃。该间隙的宽度暗示，74 au处存在一颗质量为0.9倍木星质量（MJup）的行星，这将成为该系统的第三颗伴天体。结合此前对该褐矮星的天体测量数据，以及我们推导得到的盘取向作为先验信息，我们得以更严格地约束其轨道，发现其轨道偏心率约为0.14^{+0.05}_{-0.04}，大概率呈偏心状态。对于最内侧的伴天体，我们通过视向速度（Radial Velocity, RV）、自行异常（proper motion anomaly）以及稳定性考量，证明其质量与半长轴（semi-major axis）大概率处于10–100 MJup与1.4–2.7 au的范围内。这三颗伴天体将在长期时标（secular timescales）上发生相互作用，并扰动星子的轨道，搅动该盘，并可能通过长期共振（secular resonances）将其截断至当前的延展范围。最后，该系统中间隙的存在，进一步佐证了宽域系外柯伊伯带中间隙可能较为普遍的结论。在已通过阿塔卡马大型毫米/亚毫米波阵列（ALMA）观测、且分辨率足够的6颗宽域碎屑盘中，有4至5颗呈现出以间隙为形式的径向子结构。