Using high-precision spectroscopy to scrutinize solar model atmospheres

我们最近在德国塔筒望远镜（VTT）安装了一个激光频率梳，与VTT的高分辨率摄谱仪光纤耦合。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。这对应于0.5ms-1的多普勒频移。有了这台目前独一无二的仪器，我们打算以前所未有的精确度进行基本观测，作为对我们对太阳大气层了解的一次严峻考验。这使我们能够准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进行比较。太阳颗粒化的温度和速度模式的耦合产生了特征线不对称性（称为C形）和线位移（称为对流蓝移）。通过我们的观测，我们将仔细检查基于复杂数值模拟的理论预测的有效性。由于谱线不对称性和对流蓝移都取决于日心角，即，在太阳大气的视角上，我们能够消除温度和速度分布的模糊性。在太阳上的每一个位置，我们都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，振幅为几百ms-1，其值远低于10 ms-1。由于使用频率梳进行的可再现绝对校准，我们可以可靠地将不同时间进行的联合收割机测量组合起来，因为系统效应被消除了。太阳模型大气的实验观测对天体物理学具有重要意义。这些模型被用来从观测和计算的线宽的比较中得出太阳中重元素的丰度。太阳的金属丰度又被用来校准恒星的丰度。同时，太阳模型大气是研究太阳磁场的基础，是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;这是宇宙磁场物理学的基础，作为一个最有价值的附带成果，我们将使用相同的仪器，加上适当的实验室灯，来确定一系列具有天体物理学意义的光谱线的波长。我们将实现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.