Neutron Stars and Pulsars

The strongest fields encountered in astrophysical environments are found for neutron stars, the end product of stellar core collapse at the start of the supernova event for massive stars. These objects, having about 1 Mq but radii of order 10 km, have been observed to have fields upwards of 10 G. Two indirect means have been used to infer the strengths of these fields, which defy direct detection 

The source for stellar magnetism, at least in the proto-stellar phase, must lie in the environment Measurements of dust-induced polarization in nearby molecular clouds, especially Taurus-Auriga and p Ophiuchus, show that the magnetic field is complex and pervasive throughout the parent cloud in which star formation is occurring. 

Additional evidence for large-scale magnetism in the interstellar medium comes from the radio observation of supernova remnants, strong synchrotron sources that show the effects of the expanding blast wave in the redirection and shock-amplification of the ambient galactic field. 

There are two methods for measuring the magnetic field of the interstellar medium. The most direct method is the detection of the Zeeman splitting of the 21-cm line of neutral hydrogen, or the splitting of the ground state transitions of the OH molecule. Both of these are present in low-density clouds that pervade the interstellar medium and that have low enough internal temperatures that the lines are not too broad for the measurement. 

Individual HI clouds having temperatures of order 100 K display internal fields of order 10 /xG, larger than that inferred for the low-density gas and consistent with flux-freezing in the clouds. OH transitions, which are col-lisionally excited and arise from masers that are pumped in the presence of strong infrared sources, provide similar field measurements. 

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