Magnetic massive stars as magnetar progenitors

1. Magnetic massive stars as magnetar progenitors Ren-Yu HU and Yu-Qing LOU Physics Department and Tsinghua Centre for Astrophysics (THCA), Tsinghua University (Based on Hu & Lou 2009, MNRAS, in press, astro-ph/0902.3111)

2. This research was supported in part by Tsinghua Centre for Astrophysics (THCA), by the National Natural Science Foundation of China (NSFC) grants 10373009 and 10533020 and by the National Basic Science Talent Training Foundation (NSFC J0630317) at Tsinghua University, and by the SRFDP 20050003088 and 200800030071 and the Yangtze Endowment from the Ministry of Education and Tsinghua University. Acknowledgments Compact Stars in the QCD phase diagram II Beijing, China

3. Two propositions on the origin of ultra-intense magnetic fields of magnetars (SGRs, AXPs) : Ultra-Intense Magnetic Fields • Neutron Star Dynamo • “Fossil-fields” (Duncan & Thompson 1992; Thompson & Duncan 1993) Require fast-spining neutron stars at the beginning (Period~1-2 ms) Based on magnetic flux conservation so far Supported by population synthesis study (Ferrario & Wickramasinghe 2006, 2008) Compact Stars in the QCD phase diagram II Beijing, China

4. Examine quantitatively the “fossil field” scenario Study the conditions for magnetar formation. Model magnetized massive stars with a quasi-spherical general polytropic magnetofluid under self-gravity (Wang & Lou 2008; Lou & Hu 2009) Our purpose Our approach Compact Stars in the QCD phase diagram II Beijing, China

5. A magnetofluid under self-gravity in quasi-spherical symmetry General Polytropic Magnetofluid Mass Conservation Momentum Equation Magnetic Induction Equation General Polytropic EoS Self-Similar Transformation Compact Stars in the QCD phase diagram II Beijing, China

6. Several integrals Nonlinear ODEs General Polytropic Magnetofluid nx-v=0 -> m=0 : a central cavity h: magnetic parameter q: self-similar parameter q=2(γ+n-2)/(3n-2) : singular surface Compact Stars in the QCD phase diagram II Beijing, China

7. We determine the self-similar parameters from density, temperature, magnetic field and mass loss rate on the surface of progenitor. We proceed the integration inwards. We should insert MHD shocks in the solutions to cross the singular surface. Progenitor Surface Finite velocity solutions at large x, Back to dimensional variables: x-> ∞ means either t->0+ or r-> ∞ Compact Stars in the QCD phase diagram II Beijing, China

8. Quasi-static solutions can cross the singular surface by MHD shocks. • We consider different sound speed (temperature) across a shock front. • Entropy increases from upstream to downstream sides. MHD Shock Jump Conditions Compact Stars in the QCD phase diagram II Beijing, China

9. There exists a kind of asymptotic solutions v=LxK, α=A0x-2/n+NxK-1-2/n at small x. When x->0, t->∞, v->0 (Quasi-Static). The magnetic Lorentz force and the thermal pressure force are in the same order of magnitude at small x. Quasi-Static Solutions Compact Stars in the QCD phase diagram II Beijing, China

10. n= 0.673 q=0 γ= 1.327 h= 1.52×10-4 Rs= 109 cm at 1 s One numerical example Compact Stars in the QCD phase diagram II Beijing, China

11. On the surface of the progenitor (1012 cm): On the surface of the neutron star (106 cm): The numerical example Density = 2.5×10-5 g cm-3, Temperature = 3×104 K, Mass loss rate = 10-6 solar mass per year, Mass = 5.59 solar mass, Magnetic field = 103 G ( may arise from dynamo or “fossil fields” from ISM). Mass = 2.15 solar mass, Magnetic field = 4.70×1014 G. Compact Stars in the QCD phase diagram II Beijing, China

12. The ratio between the neutron star mass and the progenitor mass: fM = Mi,ult/Mo,ini = (A0/A)λ2/n (ri/ro)3-2n The ratio between the magnetic field on the neutron star surface and that on the progenitor surface: fB = <B2i,ult>1/2/<B2o,ini>1/2 = (A0/A)λ2/n(ri/ro)1-2n Mass Ratio and Magnetic Ratio Compact Stars in the QCD phase diagram II Beijing, China

13. Since we have fB/ fM = (ro/ri)2, Considering the TOV limit of neutron star masses Magnetar B-M Relation Compact Stars in the QCD phase diagram II Beijing, China

14. Magnetic enhancement across a shock <B2t>1/21/<B2t>1/22 = ρ1/ρ2 = 2/[(γ+1)Mach12]+(γ+1)/(γ-1) The final magnetic amplification factor depends on n, q, and shock properties. The Amplification Factor Compact Stars in the QCD phase diagram II Beijing, China

15. fB < 1012 =(ro/ri)2 Larger q, smaller fB Median shock speed corresponds to smallest fB Here we may resolve the difficulty posed by Petit et al. (2008) The Amplification Factor Compact Stars in the QCD phase diagram II Beijing, China

16. If the progenitor is a magnetic massive star with a surface magnetic field strength of ~103G, it would have a good chance to produce a magnetar at the center of its supernova remnant. The magnetar magnetic field is proportional to the magnetar mass and the progenitor magnetic field. The TOV limit actually limits the magnetar magnetic field. If the progenitor is extremely magnetized, the new born magnetar may power the short GRB. We could naturally expect a continuum from dim isolated neutron stars to magnetars. In this scenario, magnetars should be slowly rotating neutron stars. Conclusions Compact Stars in the QCD phase diagram II Beijing, China