Using high-precision spectroscopy to scrutinize solar model atmospheres

• Applicant: Professor Dr. Wolfgang Schmidt
• Subject Area: Astrophysics and Astronomy
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 275523552

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.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Rolf Schlichenmaier