We observed the occultation of the star Gaia DR2 4056440205544338944 by (28978) Ixion. The event was observed from two Lowell Observatory sites, using the 4.3 m Lowell Discovery Telescope (LDT), near Happy Jack, AZ, USA, and a 0.32 m telescope co-mounted with the Titan Monitoring telescope on Lowell’s Mars Hill campus in Flagstaff, AZ. The LDT chord, at 44.86 s, was roughly 30% longer than the longest predicted possible chord. Under the assumption of a spherical body, Ixion’s fitted diameter D = 709.6 0.2 km. The LDT light-curve profile was used to place an upper limit on the surface pressure P < 2 μbar on any possible atmosphere of Ixion. At the distance of Ixion, the occulted star had a fitted projected diameter of 19.25 0.3 km assuming uniform disk illumination, giving a stellar angular diameter of 0.675 0.010 mas. Using the Gaia EDR3 parallax of 0.565 mas, the stellar radius is . The measured size is consistent with prior spectral classification of this star as a reddened mid-M giant. This is one of only a modest number of M5 III stars to have a directly measured diameter, and is more distant than most.

我们观察到 (28978) Ixion 掩星盖亚 DR2 4056440205544338944。该事件是从洛厄尔天文台的两个站点观测到的，使用位于美国亚利桑那州哈比杰克附近的 4.3 m 洛厄尔发现望远镜 (LDT) 以及与位于弗拉格斯塔夫洛厄尔火星山校区的泰坦监测望远镜共同安装的 0.32 m 望远镜， AZ。LDT 和弦为 44.86 秒，比最长的预测可能和弦长约 30%。在球体假设下，Ixion 的拟合直径D = 709.6 0.2 km。LDT 光曲线轮廓用于设置表面压力P < 2 μ 的上限禁止任何可能的 Ixion 气氛。在 Ixion 的距离，假设均匀的圆盘照明，这颗被掩星的恒星的拟合投影直径为 19.25 0.3 公里，恒星角直径为 0.675 0.010 mas。使用 0.565 mas 的 Gaia EDR3 视差，恒星半径为. 测得的大小与先前将这颗恒星光谱归类为变红的中 M 巨星一致。这是仅有的少数具有直接测量直径的 M5 III 恒星之一，并且比大多数恒星更远。