We investigate the choice of stellar population for use as the astrometric grid for the Space Interferometry Mission (SIM). SIM depends on the astrometric stability of about 2000 stars, the so-called grid, against which the science measures are referenced. Low-metallicity and thus relatively high luminosity K giants are shown to be the population of choice, when available. The alternative, nearby G dwarfs, is shown to be susceptible to unmodeled motions induced by gas-giant planetary companions, should there be a significant population of such companions. Radial velocity filtering is quite efficient in selecting grid members from the K giants with yields exceeding 50% if filtering at 30 m s~(-1) (1 σ) is available. However, if the binary fraction of the G dwarfs approaches 100% as some studies suggest, the yield of stable systems would be in the range of 15% at best (with 10 m s~(-1) filtering). Use of the initial SIM measurement as a final filter is shown not to be critical in either case, although it could improve the yield of stable grid members. For a grid composed of weak-lined K giants, the residual contamination by large unmodeled motions will amount to about 3% (and rises to about 10% if a 60 m s~(-1) radial velocity criterion is used). The selective introduction of quadratic terms in the proper motion solutions during the postmission phase of data reduction can reduce contamination to a remarkable 1% or better in either case. Analytic estimates based on circular orbits are developed that show how these results come about.
展开▼
机译:我们调查了用作恒星干涉网格(SIM)的天体网格的恒星种群的选择。 SIM取决于约2000颗恒星(所谓的网格)的天文稳定性,并参考科学方法。低金属含量,因此具有相对较高的发光度K巨人(如果有)是首选。如果附近有大量G矮星伴星,则表明它们容易受到气巨行星伴星引起的非模型运动的影响。如果可以在30 m s〜(-1)(1σ)下进行过滤,则径向速度过滤在从K个巨人中选择网格成员方面非常有效,其产量超过50%。但是,如果G矮星的二元分数接近100%(如某些研究表明的那样),则稳定系统的产量最多将在15%的范围内(使用10 m s〜(-1)过滤)。尽管将初始SIM卡测量用作最终过滤器可以提高稳定网格成员的产量,但这两种方法都不是至关重要的。对于由弱衬里的K巨人组成的网格,大的未建模运动造成的残留污染将达到大约3%(如果使用60 m s〜(-1)径向速度准则,则将增加到大约10%)。在数据缩减的后期阶段,在适当的运动解决方案中有选择地引入二次项可以在两种情况下将污染降低到显着的1%或更高。提出了基于圆形轨道的解析估计,以显示这些结果是如何产生的。
展开▼