Observations and Analysis of the Photospheric Magnetic Fields on Dwarf g, K and M Stars.
Abstract
I have developed an improved technique for the analysis of magnetic broadening in stellar absorption line profiles. Unlike previous methods, the new technique is based on a model that includes radiative transfer effects and the full Zeeman patterns. The effects of weak blends on the profiles can be reduced by comparing identical lines in two stars of the same spectral type, one of which is chromospheric inactive. After adjusting for differences in line strength and doppler broadening, the difference profile can be modeled to determine both the fraction of the stellar surface covered by magnetic regions (f) and the mean field strength in these regions (B). Accuracies of about 20% in B and f are possible. I find that previous Zeeman analysis methods systematically overestimate f, especially for cooler stars, due to their neglect of line saturation and blends. I have applied the new technique to two sets of high resolution, low noise spectra. The first set consists of 29 stars, spanning spectral types from GO to M4.5 and a broad range of rotational rates and ages. I have made the first ever detection of photospheric magnetic fields on a BY Draconis variable (EQ Vir) and a dMe flare star (AD Leo). Several major trends are apparent from the results. The stellar magnetic field strength is in pressure equilibrium with the photospheric gas pressure and is thus a function of spectral type. The filling factor, f is a linear function of angular velocity. The total magnetic flux on a star is thus a function of the star's rotation rate, in agreement with simple dynamo theories. A constant B and an f value decreasing with time explains the observed decline in stellar rotational velocities and activity levels with age. There is no correlation between f and B, or between B and rotation. Contrary to some theoretical models, I find dM stars have little magnetic flux. The highest filling factors are found on flare stars. I have also used the new technique to study the spatial and temporal behavior of f and B on the active star Epsilon Eri. Eleven observations were made over two months with contemporaneous measurements of the chromospheric Ca II emission flux. I find no correlation between B and phase or Ca II flux. The filling factor, however, shows a weak phase dependence and is linearly proportional to the Ca II emission, consistent with many interpretations of the solar data.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1987
- Bibcode:
- 1987PhDT.........3S
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Field Strength;
- G Stars;
- K Stars;
- Radiative Transfer;
- Stellar Magnetic Fields;
- Stellar Rotation;
- Zeeman Effect;
- Absorption Spectra;
- Angular Velocity;
- Dwarf Stars;
- M Stars;
- Astronomy