CRIRES搜寻最低质量恒星周围的行星。I.用氨气池测量高精度近红外径向速度
The CRIRES Search for Planets Around the Lowest-Mass Stars. I. High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell
摘要：
从近红外光谱测量的径向速度是一个潜在的强大工具，以寻找冷恒星和亚恒星物体周围的行星。然而，目前还没有一种技术能产生与可见光中常规获得的径向速度相当的近红外径向速度精度。我们描述了一种从K波段光谱测量这些恒星的高精度相对径向速度的方法。该方法利用充满氨气的玻璃池来校准光谱仪的响应，类似于在可见光中非常成功使用的“碘池”技术。恒星光谱是通过氨细胞和建模的产品的多普勒频移模板光谱的对象和光谱的细胞，卷积与一个可变的仪器配置文件模型。一个复杂的因素是，一个显着的大地吸收线的数量存在于光谱区域包含有用的恒星和氨线。大地线的建模，同时也使用频谱合成与时间分辨模型的大气在天文台。在完整的模型中的自由参数是光谱的波长尺度，仪器配置文件，调整大气模型中的水和甲烷丰度，大地光谱多普勒频移，和恒星多普勒频移。基于对六个月内获得的数百个晚M矮星光谱的分析，该方法的测试表明，在长时间尺度上可以获得约5 m/s的精度，并且在长达一周的时间尺度上可以获得优于3 m/s的精度。所获得的精度是相媲美的预测光子有限的错误，但主要是有限的，在很长的时间尺度上的不完善的建模的大地线。
英文摘要：
Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields near-infrared radial velocity precision comparable to that routinely obtained in the visible. We describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the instrumental profile, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ~5 m/s are obtainable over long timescales, and precisions of better than 3 m/s can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.
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