jrasc june 1998 final - The Royal Astronomical Society of Canada

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jrasc june 1998 final - The Royal Astronomical Society of Canada

Second LightThe Baseball Model of Neutron Star Spinsby Leslie J. SageWe know that almost all neutron stars are born whenmassive stars explode as supernovae, and that pulsarsare rapidly rotating neutron stars. Pulsars, however,move very differently from the precursor massive stars. Theyrotate much more rapidly than the pre-supernova star and theyzoom through space with average speeds of about 400 km s –1 ,while the average speed of a massive star in our Galaxy is lessthan 30 km s –1 . Moreover, pulsars seem to have very strongmagnetic fields. Why they should have such properties hasbeen very unclear to astronomers.Pulsars emit beams of radiation that sweep over the Earth,like a searchlight passing over a ship atsea; they are normally detected by radiotelescopes, but occasionally the pulsesare seen in X-rays or visible light. “Normal”pulsars can have periods as short as 0.03seconds, and as long as a few seconds.That is a direct measure of the speed withwhich the neutron star rotates — the ideaof an object with the mass of the Sunspinning on its axis thirty times a secondis quite something! (Millisecond pulsarsrotate even faster, but it is generally believedthat they are not born that way.)Henk Spruit (Max-Planck-Institutfür Astrophysik) and Sterl Phinney (Caltech)think they have an explanation for theobserved properties of pulsars, which tiesthem all together (see 14 May issue ofNature).A common explanation of themagnetic field strength and spin rate hasbeen that if you compressed the Sun downto the size of a neutron star, you wouldget values for both that are comparableto what are observed. (The Sun is usedhere as an analogue for the core of the massive progenitor star.)For such an explanation to work, the core of the supernovaprogenitor has to be able to spin freely; it has to be disconnectedfrom the slowly rotating envelope. That has not been thoughtto be a problem, but Spruit and Phinney show that the magneticfield will in fact keep the core rotating at the same slow speedas the envelope! So, being able to explain the magnetic fieldstrength essentially kills the explanation for the spin rate —the slowly rotating core has 1000 times less angular momentumthan is needed to explain the pulsar observations.Spruit and Phinney have looked at an alternative explanationImagine hittinga baseball witha bat — if youdon’t hit the ballsquarely, then itspins (often as a“pop fly” or a“foul”).for pulsars’ spins. Even though the exact mechanism for kickingthe newly born neutron stars has not been identified, theexistence of a kick is not in any doubt. Moreover, it seems thatthe most probable time to kick the star is during the supernovaexplosion, when there is lots of energy to spare. They proposethat the kick itself is responsible for most of the spin of thestar. Imagine hitting a baseball with a bat — if you don’t hitthe ball squarely, then it spins (often as a “pop fly” or a “foul”).Much of the energy of your hit has gone into the spin of theball, rather than the desired forward bulk kinetic energy thatyou need to get the ball to the outfield. They propose that thesame thing happens with neutron starsas they are born — the more squarelythe birth kick hits the neutron star, thefaster it will move through space but themore slowly it will spin. As the kickbecomes increasingly off-centre, moreof the energy will go into the spin, andless into the space velocity. Spruit andPhinney use their model to predict acorrelation between the spin rates andspace velocities of pulsars, and comparethe model with observations.The correspondence between modelprediction and observations is reasonable,but not striking. Given that the very originand duration of the kicks is completelyunknown, a discrepancy between theobservations and predictions is not reallysurprising, because the model predictionsare very sensitive to the duration of thekick. (Different types of kicks will havedifferent durations.) Of necessity, Spruitand Phinney had to pick a particular typeof kick in order to produce their model;they predict more quickly rotating, highvelocityneutron stars than are observed. The discrepancy mayin fact indicate that the kick lasts a second or more (muchlonger than the model assumed) — the longer-lasting kicksmay put more energy into rotation than the shorter ones. Wheredoes it leave the model? It seems to be a very clever idea andgood way to connect pulsars to the progenitor stars, but eitherthe type of kick will need to be identified or some otherobservational support for the model will need to be found.One remaining prediction of the model is that manyneutron stars will be born rotating relatively slowly, probablytoo slowly to be seen as radio pulsars. We ought to be able to124JRASC June/juin 1998

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