The effect of constant horizontial magnetic fields on model atmospheres of AP stars

Abstract

view Abstract Citations (21) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Effect of Constant Horizontal Magnetic Fields on Model Atmospheres of AP Stars LeBlanc, F. ; Michaud, G. ; Babel, J. Abstract Model atmospheres have been constructed in LTE for A-type magnetic stars. The Lorentz force caused by the interaction between the ambipolar diffusion of hydrogen and the magnetic field, has been taken into account in the hydrostatic equation. Calculations were carried out using a modified version of the ATLAS line blanketed program with metal solar abundances, but no helium. Models with Teff = 8500, 10,000 and 12,000 K, log g = 4.0 and constant horizontal magnetic fields up to 10 kG in the 12,000 K model and up to 8 kG in the cooler ones are presented. The Lorentz force increases effective gravity in the hydrogen ionization region and thus modifies the structureof magnetic atmospheres. In a Teff = 10,000 K star, effective gravity is up to 7 times greater than gravity in the presence of a 5 kG horizontal field in the hydrogen ionization zone. The pressure gradient is proportionately increased in the ionization region of these magnetic Ap stars. The effect is present in stars with Teff up to at least 12,000 K but the effect decreases as Teff further increased and hydrogen is completely ionized in the atmosphere. Balmer lines are broadened in these magnetic atmospheres leading to a variation of 0.01 mag in the beta index in the Teff - 10,000 and 12,000 K models. This is smaller than the variation of 0.02 mag with rotational period typically observed in Ap stars. This still might play a role in explaining the beta variations of stars like 56 Ari and CU Vir. Observed metallicity gradients and magnetic field configurations could also play a role. The effect on Balmer lines in the Teff = 8500 K model is considerably reduced, since the ionization zone occurs at too large an optical depth for the pressure gradient increase to affect observations. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174492 Bibcode: 1994ApJ...431..388L Keywords: Abundance; Ambipolar Diffusion; Magnetic Stars; Stellar Atmospheres; Stellar Magnetic Fields; Stellar Models; Atmospheric Ionization; Balmer Series; Computational Astrophysics; Lorentz Force; Pressure Gradients; Astrophysics; DIFFUSION; STARS: ATMOSPHERES; STARS: MAGNETIC FIELDS; STARS: PECULIAR full text sources ADS | data products SIMBAD (2)