Is PSR 1257+12 a Spun-up Pulsar?

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Session 64 -- Pulsars
Display presentation, Thursday, 9:20-4:00, Heller Lounge Room

[64.06] Is PSR 1257+12 a Spun-up Pulsar?

Kaiyou Chen (T-6, Los Alamos National Lab)

Observed properties of many short period millisecond pulsars ($P < 10$ ms) near the galactic disk support the standard spin-up scenario for their origins. Radio pulse properties of these ms pulsars (PSR 1937+21, PSR 1957+20, PSR 1855+09 and PSR 1953+29) are consistent with those expected from substantially spun-up neutron stars. The presence of low mass companions in extremely circular orbits in three out four cases indicates rather long mass transfer histories. \bigskip \noindent However, the radio pulse profile of PSR 1257+12 is more similar to the pulse profiles of most canonical pulsars than these ms pulsars mentioned above. The existence of two companion planets around this pulsar is unique among the ms pulsars in the galactic disk. Here we suggest that this ms pulsar may have a different origin: core coalescence of a close binary system. We consider the evolution of a close binary consisting with two main sequence stars of masses 7 $M_\odot$ and 5 $M_\odot$ respectively. When the more massive one evolves into a giant, it is assumed that mass can be transfered to the companion through Roche-lobe overflow and the primary evolves into a compact core with mass about 1 $M_\odot$. When the secondary (which now has mass $11M_\odot$) evolves into a giant, the mass transfer may bring the evolved primary into the envelope of the secondary, leading to the merger of the two cores. If the coalesced core collapses through a SN II, a neutron star with a rotation period of a few ms is expected. A neutron star formed by this scenario may also have a relatively weak magnetic field. Because the merged star inherits large angular momentum of the binary ($J=M_1M_2\sqrt{Ga/(M_1+M_2)}$, where $a\sim 10^{12}$ cm is the initial separation of the binary), the typical angular momentum of the material in the envelope of the merged star is rather high ($\sim \sqrt{Ga(M_1+M_2)}$). The fallback material with mass of few percent solar mass may form a gas disk at radius $10^{10-13}$ cm. The evolution of the gas disk may lead to the formation of planets. \bigskip \noindent This work was performed under auspices of the US DOE.

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