Astrophysical axion bounds diminished by screening effects

‘‘Invisible axions’’ could be produced in stellar interiors through Compton- and Primakoff-type photoproduction and through bremsstrahlung processes. We point out that in a plasma screening effects lead to important reductions of these emission rates. Limits on the axion mass and interaction strength are thereby relaxed to values less restrictive than limits previously thought to be firm. For the case of the Sun the Primakoff rate is reduced by two orders of magnitude. This process is the dominant emission mechanism for Kim-Shifman-Vainshtein-Zakharov- (KSVZ) type axions which do not couple directly to electrons. The mass limit is then relaxed by an order of magnitude to m_a<20 eV. Dine-Fischler-Srednicki (DFS) axions couple directly to electrons and the dominant solar emission process becomes bremsstrahlung from electron-nucleus and electron-electron collisions while the previously thought dominant Primakoff rate is now suppressed. The mass limit accidentally remains at m_a<3 eV. Model-dependent bounds based on axion emission from red giants are m_a<1 eV (KSVZ) and m_a<0.06 eV (DFS). The mass limits for the DFS axions are understood for the special case where the free parameter 2 cos²β of the model equals unity. Our results can be easily translated to other hypothetical pseudoscalar particles if they are light compared with typical stellar temperatures.