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Astron. Astrophys. 318, 485-489 (1997)

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Slowly rotating pulsars and magnetic field decay

Jin Lin Han

Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China (HJL@bao01.bao.ac.cn)

Received 13 June 1995 / Accepted 9 July 1996


Two dozen long period pulsars are separated from the swarm of ordinary pulsars by an obvious gap in the P versus Sd diagram (where [FORMULA]), with a plausible upper boundary for ordinary pulsars. Possible pulsar evolutionary tracks are discussed to explain the diagram in terms of previously suggested scenarios of magnetic field decay. The (P -Sd) diagram is difficult to understand if there is no magnetic field decay during the active life of pulsars. However, if the magnetic fields of neutron stars decay exponentially, almost all slowly rotating pulsars must have been injected with a very long initial spin period of about 2 seconds, which seems impossible.

Based on qualitative analyses, it is concluded that magnetic fields of neutron stars decay as a power-law, with a time scale related to the initial field strengths. The plausible boundary and the gap are suggested to naturally divide pulsars with distinct magnetic "genes", ie. pulsars which were born from strongly magnetized progenitors - such as Bp stars, and pulsars born from normal massive stars. The possibility remains open that a fraction of slowly rotating pulsars were injected with long initial spin periods, while others would have a classical pulsar evolution history. It is suggested that PSR B1849+00 was born in the supernova remnant Kes-79 with an initial period of about 2 seconds.

Key words: pulsars: general – stars: evolution – stars: magnetic fields

Send offprint requests to: J.L. Han

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998