Jim Fuller edited sectionDiscussion_an.tex  about 9 years ago

Commit id: b5916b7ea7c0a71fe44c83300f72c6b597d02b4c

deletions | additions      

       

Stochastic influxes of IGW also lead to minimum core rotation rates, which may be realized given very efficient prior core spin-down via IGW/magnetic torques. Such a spin-down is not unreasonable, especially given that the cores of low mass red giant stars rotate slower than can be accounted for using hydrodynamic mechanisms or magnetic torques via the Tayler-Spruit dynamo \citep{cantiello:14}. It is thus quite plausible that massive star cores are efficiently spun down via waves/magnetic torques, after which they are stochasticly spun up via waves launched during O/Si burning. If this mechanism determines the core spin rate before death, it entails a Maxwellian distribution in spin frequency, with typical spin periods of $300 \, {\rm s} \lesssim P \lesssim 10^4 \, {\rm s}$. We thus find it extremely unlikely that magnetic torques can enforce very large pre-collapse spin periods as claimed by \cite{spruit:98}.   If AM is conserved during the supernova, stochastic IGW spin-up entails NS birth periods of $20 \, {\rm ms} \lesssim P \lesssim 400 \, {\rm ms}$, albeit with significant uncertainty. These estimates are comparable to spin periods of typical young, slowly rotating NSs (\citealt{lai:96,gotthelf:13}), and for the broad inferred birth spin period distribution of $P \lesssim 500 \, {\rm ms}$ for ordinary pulsars (\citealt{faucher:06,popov:10,gullon:14}). Therefore, stochastic wave spin-up could be the dominant mechanism in determining the rotation periods of pre-collapse SN cores and newborn NSs. In this scenario, there is little or no correlation between the spin of the progenitor and the spin of the NS it spawns. We note that this scenario is very similar to the stochastic spin-up scenario proposed by \cite{spruit:98}, except that the AM depostion occurs before core-collapse, and the source of the momentum AM  (convectively excited IGW) is somewhat better understood. % We note that the AM deposited by IGW also entails a characteristic momentum deposition of $P_{\rm ex} \sim I_c \Omega_{\rm ex}/r_c$, which corresponds to a core velocity of $v_{\rm ex} \sim P_{\rm ex}/M_c$. Our calculations entail typical ``kick" velocities of $ 10^2 \, {\rm km} \, {\rm s}^{-1} \lesssim v_{\rm ex} \lesssim 7 \times 10^2 \, {\rm km} {\rm s}^{-1} $, very similar to typical NS kick velocities ***REF***. It may therefore be possible that stochastic momentum/AM deposition accounting for NS kicks/spins occurs in a similar fashion outlined in \cite{spruit:98}, but with the momentum deposition occuring {\it before} core collapse via IGW excited by vigorous shell burning.   There is ample evidence that {\it some} CC events occur with rapidly rotating cores. In particular, long GRBs almost certainly require a rapidly rotating central engine \citep{Woosley_1993,Yoon_2006,Woosley_2006,Metzger_2011}, and the picture advanced above must break down in certain (although somewhat rare) circumstances. It is not immediately clear what factors contribute to the high spin rate in GRB progenitors, as our analysis was restricted to ``typical" NS progenitors with $10 M_\odot \lesssim M \lesssim 20 M_\odot$, which explode to produce type-IIp supernovae during a red supergiant phase \citep[See e.g.][]{Smartt_2009}. We speculate that GRB progenitors (if occurring in effectively single star systems) have {\it never} undergone a red supergiant phase, as torques via magnetic fields and/or waves are likely to spin-down spin down  the helium core by coupling it with the huge AM reservoir contained in the slowly rotating convective envelope. Alternatively, it may be possible that stars with very massive He cores, which exhibit more vigorous pre-SN burning phases, can generate stochastic wave spin-up strong enough to produce a GRB. A third possibility is that spin-up via mass transfer/tidal torques in binary systems is required by for  GRB production. The population of massive stars approaching death is complex, and factors such as initial mass, rotation, metallicity, binarity, magnetic fields, overshoot, mixing, winds, etc., will all contribute to the anatomy of aging massive stars. We have argued that AM transport via convectively driven waves IGW  is likely to be an important factor in most massive stars. But it is not immediately obvious how this picture will change in different scenarios, e.g., electron capture supernovae, very massive $(M_i \gtrsim 40 M_\odot)$ stars, interacting binaries, etc. We hope to explore these issues in subsequent works.