Axion Mass Limits from Cooling Neutron Stars
  The thermal evolution of a neutron star is studied by including the energy loss due to axion emission. Two axion models and three types of neutron-star matter equation of state are used with the effects of nucleon superfluidity properly taken into account. In comparison with the observational data of PSR0656+14 from ROSAT, the upper limits on the axion mass are found to be $m_a < 0.06 - 0.3$ eV and 0.08$ -$ 0.8 eV for the KSVZ and DFSZ axion models, respectively, with the soft equation of state giving the most stringent limits. 
Hideyuki Umeda(1), Naoki Iwamoto(2), Sachiko Tsuruta(3), Letao Qin(3),   and Ken'ichi Nomoto(1) ((1) Research Center for the...