Using high-precision spectroscopy to scrutinize solar model atmospheres

在德国真空塔望远镜（VTT）上，我们最近安装了一个激光频率梳，与VTT的高分辨率光谱仪光纤耦合。这种设置使我们能够以10-9的精度确定500 nm和700 nm之间的光谱线的绝对波长。这相当于0.5 ms-1的多普勒频移。有了这个目前独一无二的仪器，我们打算以前所未有的精度进行基本观测，作为对我们对太阳大气理解的严峻考验。与绝对波长一起，我们将测量光谱仪的仪器展宽。这使我们能够精确地确定所选光谱线的等效宽度、位置和不对称性，并将这些量与太阳模型大气的预测结果进行比较。太阳颗粒的温度和速度模式的耦合产生了一种特征的线不对称（称为c形）和线移（称为对流蓝移）。通过我们的观察，我们将仔细检查基于复杂数值模拟的理论预测的有效性。由于线不对称和对流蓝移都取决于日心角，即太阳大气中的视角，因此我们能够解开温度和速度分布中的模糊性。在太阳上的每个位置，我们需要几个小时的观测时间来减少由近地表对流和太阳振荡引起的多普勒频移，它们的时间尺度为几分钟，振幅为100毫秒-1，远低于10毫秒-1。由于消除了系统效应，使用频率梳进行可重复的绝对校准，我们可以可靠地组合在不同时间进行的测量。太阳模型大气的实验检测对天体物理学具有重要的基础意义。这些模型通过比较观测到的线宽和计算出的线宽来推算出太阳中重元素的丰度。太阳的金属丰度反过来被用来校准恒星的丰度。同时，太阳大气模型对于研究太阳磁场是必不可少的。它们是我们理解太阳活动区辐射驱动磁对流的基础。在小尺度上对磁力和流体动力相互作用的研究只能在太阳上进行；它是宇宙磁场物理学的基础。作为最有价值的副产品，我们将使用相同的仪器，加上合适的实验室灯，来确定一系列天体物理学感兴趣的光谱线的波长。我们将达到0.005 pm （2 ms-1）的精度，至少比以前的测量好一个数量级。

At the German Vacuum-Tower-Telescope (VTT) we recently installed a Laser Frequency Comb, fiber-coupled with the high-resolution spectrograph of the VTT. This setup allows us to determine the absolute wavelengths of the spectral lines between 500 nm and 700 nm with a precision of 10-9.. This corresponds to a Doppler shift of 0.5 ms-1. With this presently unique instrument we intend to perform fundamental observations with unprecedented accuracy as an acid test of our understanding of the solar atmosphere.Together with the absolute wavelength we will measure the instrumental broadening of the spectrograph. This allows us to exactly determine the equivalent widths, the positions and asymmetries of selected spectral lines and to compare these quantities with predictions of solar model atmospheres. The coupling of temperature and velocity pattern of the solar granulation produces a characteristic line asymmetry (known as C-shape) and line shift (known as convective blue shift). With our observations we will scrutinize the validity of theoretical predictions based on complex numerical simulations.Since both line asymmetry and convective blue shift depend on the heliocentric angle, i.e., on the view angle in the solar atmosphere, we are able to disentangle ambiguities in the temperature and velocity distributions. At each location on the Sun we need several hours of observing time to reduce the Doppler shifts caused by near-surface convection and solar oscillations, with their time scales of several minutes and their amplitudes of several 100 ms-1, to values well below 10 ms-1. Thanks to the reproducible absolute calibration with the frequency comb we can reliably combine measurements taken at different times, since systematic effects are eliminated. The experimental inspection of solar model atmospheres is of fundamental importance for astrophysics. These models are used to derive the abundance of heavy elements in the Sun from a comparison of observed and computed line widths. The solar metallicity in turn is used to calibrate stellar abundances. At the same time, solar model atmospheres are essential for the investigation of the solar magnetism; they are the basis for our understanding of radiatively driven magneto-convection in solar active regions. The investigation of the interaction of magnetic and hydrodynamic forces on small scales can only be done on the Sun; it is the basis for the physics of cosmic magnetic fields.As a most valuable spin-off, we will use the identical instrument, together with suitable laboratory lamps, to determine the wavelengths of a series of spectral lines that are of astrophysical interest. We will achieve an accuracy of 0.005 pm (2 ms-1), at least one order of magnitude better than previous measurements.