MRI Development: EXPRES - the Extreme Precision Spectrograph for Exoplanet Studies

在过去的二十年里，使用地面和天基天文台进行的广泛调查已经编目了我们太阳系以外的数千颗行星。不幸的是，除了围绕母星的轨道周期和行星的近似表面温度之外，大多数发现技术只能提供很少的更多信息。最令人感兴趣的是行星的质量，因为这一点，加上对其大小的估计，导致对其组成(岩石或气体)的粗略评估。行星质量是通过它们对母星施加的引力来获得的，而像地球这样相对较轻的行星对恒星运动的影响很小。事实上，要发现岩石的类地系外行星，需要开发技术来测量恒星的周期性运动，其速度远低于每秒1米，或者说大约相当于一个人在人行道上散步的速度。D·费舍尔博士(耶鲁大学)致力于实现这一目标，他建造了一台专门用于系外行星研究的仪器，并将其用于亚利桑那州弗拉格斯塔夫外高山上的一台新望远镜上。利用多普勒效应准确测量恒星径向速度的关键是稳定性：光谱仪有稳定的温度和压力环境，望远镜对仪器的照明稳定，波长校准源稳定。费舍尔博士的计划包括建造一台专门为系外行星研究设计的仪器，并将其用于望远镜上，每年有相当数量的专门夜晚使用。通过将整个光谱仪封闭在一个温度控制的真空室中，对输入图像进行光纤加扰，并使用法布里-珀罗标准具光源进行校准，就可以满足稳定性标准。法布里-珀罗标准具光源的通带波长最终与稳定性为10^11的原子钟绑定在一起。费舍尔博士希望使用该仪器发现并描述多达100个围绕附近恒星运行的类似地球的恒星。并首次获得太阳系外可能存在生命的频率的可靠估计。美国国家科学基金会天文科学部通过参与重大研究仪器计划和数学和物理科学局多学科活动办公室为研究系外行星的精密仪器的开发提供资金。

Extensive surveys over the past two decades using ground- and space-based observatories have cataloged thousands of planets beyond our solar system. Unfortunately, most discovery techniques are able to provide little more information than the orbital period around the parent star and the approximate surface temperature of the planet. Of vital interest is the planet's mass, since this, coupled with an estimate of its size, leads to a rough assessment of its composition (rocky or gaseous). Planetary masses are obtained through the gravitational attraction they exert on the parent star, with comparatively lightweight planets like the Earth providing only feeble influences on stellar motions. Indeed, the discovery of rocky, Earthlike exoplanets requires the development of techniques that can measure periodic motions of a star well below 1 meter per second, or approximately the speed of a person strolling down a sidewalk. Dr. D. Fischer (Yale University) seeks to accomplish exactly goal this by constructing an instrument especially for exoplanet study and using it on a new telescope high in the mountains outside Flagstaff, Arizona.The key to accurate radial velocity measurements of a star using the Doppler effect is stability: a stable temperature and pressure environment for the spectrograph, stable illumination of the instrument by the telescope, and a stable wavelength calibration source. Dr. Fischer's plan includes the construction of an instrument specifically designed for exoplanet study and its use on a telescope for a sizable, dedicated number of nights each year. The stability criteria will be met by enclosing the entire spectrograph in a temperature-controlled vacuum chamber, fiber-scrambling the input image, and calibration using a Fabry-Perot etalon source whose passband wavelengths are ultimately tied to an atomic clock with a stability of one part in 10^11. Using the instrument, Dr. Fischer expects to find and characterize as many as 100 Earth-like analogs orbiting nearby stars, and for the first time obtain reliable estimates of the frequency of possible life hosts beyond our solar system.Funding for the development of a precision instrument for the study of exoplanets is being provided by NSF's Division of Astronomical Sciences through its participation in the Major Research Instrumentation program and by the Office of Multidisciplinary Activities of the Directorate for Mathematical and Physical Sciences.