Ground-based astrometric planet detection and confirmations

我们的目标是确认两个径向速度（RV）的行星候选人在双星系统HD 19994和γ Ceph通过测量的天体测量位移（'摆动'）的行星主机恒星引起的。对于RV行星，由于未知的轨道倾角i，只有行星质量的下限m sin i是已知的。主星星的天体测量摆动产生了行星的轨道倾角，从而产生了它的质量。星星GJ 876是第一个被哈勃太空望远镜探测到这种摆动的例子（Benedict et al. 2002）。正如以前其他人所指出的（见Pravdo & Shaklan 1996）和我们在这里所证明的那样，使用VLT和Keck AO可以实现从地面到亚毫角秒范围的天体测量精度。我们正在观测可见双星，其中一颗恒星有一颗RV行星候选者，因此天体测量摆动被视为双星分离的变化。HD 19994是一颗约2.3“”的星星，其中一颗有RV行星候选者，摆动为0.16 mas（由于m · sin i质量极限中的未知i，摆动甚至更大），而仙王座γ的摆动≥ 0.32 mas。第一次成功的好天气运行是在2004年12月与纳科在ESO VLT完成的。这里给出的结果表明，我们可以达到~ 100 μas/μ hr的相对天体测量精度（hr是以小时为单位的观测时间），这是单孔径相对天体测量在地面上所达到的最高精度。这些观测也可作为一个可行性方案，以研究8至10米天文望远镜的天体测量能力，特别是关于探测和发现系外行星的能力。如果成功的话，我们可以对RV方法不起作用的恒星周围的系外行星进行更大规模的调查。许多仪器和设施可用于此类计划，如NIRC-2/Keck，CIAO/Subaru或NIRI/Gemini-North，以及几年后的Linc-Nirwana/LBT。

We aim to confirm two radial velocity (RV) planet candidates in the binary stellar systems HD 19994 and γ Ceph by measuring the astrometric displacement (’wobble’) of planet host stars induced by the planet. For RV planets, only a lower mass limit m sin i of the planetary mass is known due to the unknown orbit inclination i. The astrometric wobble of the host star yields the planet orbital inclination and, hence, its mass. The star GJ 876 was the first case, where this wobble was detected with the Hubble Space Telescope (Benedict et al. 2002). As shown by others before (see Pravdo & Shaklan 1996) and demonstrated by us here, one can achieve an astrometric precision in the sub-milli-arc-second range from the ground with VLT and Keck AO. We are observing visual binary stars, where one of the stars has a RV planet candidate, so that the astrometric wobble is seen as change in binary separation. In the case of HD 19994, a ~ 2.3′′ binary star, one with a RV planet candidate, the wobble is 0.16 mas (or even more due to the unknown i in the m · sin i mass limit), and for γ Ceph, its ≥ 0.32 mas. The first successfull good weather run was done at ESO VLT with NACO in Dec 2004. The results presented here demonstrate that we can reach a relative astrometric precision of ~ 100 μas/√hr (hr is observing time in hours), the best precision ever achieved in single-aperture relative astrometry from the ground. These observations also serve as a feasibility program to investigate the astrometric capabilities of 8- to 10-m telescopes with AO, in particular regarding the detection and discovery of exo-planets. If successfull, we can embark on a larger survey for exo-planets around stars where the RV method does not work well. Numerous instruments and facilities are available for such programs, like NIRC-2/Keck, CIAO/Subaru or NIRI/Gemini-North, and in a few years Linc-Nirwana/LBT.