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Magnetized Massive Stars As Magnetar Progenitors

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Magnetized Massive Stars As Magnetar Progenitors Mon. Not. R. Astron. Soc. 000, ??–?? (2008) Printed 24 October 2018 (MN LATEX style file v2.2) Magnetized massive stars as magnetar progenitors Ren-Yu Hu1⋆ and Yu-Qing Lou1,2,3⋆ 1Physics Department and Tsinghua Centre for Astrophysics (THCA), Tsinghua University, Beijing 100084, China 2Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA 3National Astronomical Observatories, Chinese Academy of Science, A20, Datun Road, Beijing, 100012, China Accepted 2009 February 16. Received 2009 February 06; in original form 2008 December 16 ABSTRACT The origin of ultra-intense magnetic fields on magnetars is a mystery in modern astro- physics. We model the core collapse dynamics of massive progenitor stars with high surface magnetic fields in the theoretical framework of a self-similar general polytropic magnetofluid under the self-gravity with a quasi-spherical symmetry. With the speci- fication of physical parameters such as mass density, temperature, magnetic field and wind mass loss rate on the progenitor stellar surface and the consideration of a re- bound shock breaking through the stellar interior and envelope, we find a remnant compact object (i.e. neutron star) left behind at the centre with a radius of ∼ 106 cm and a mass range of ∼ 1 − 3 M⊙. Moreover, we find that surface magnetic fields of such kind of compact objects can be ∼ 1014 − 1015 G, consistent with those in- ferred for magnetars which include soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs). The magnetic field enhancement factor critically depends on the self-similar scaling index n, which also determines the initial density distribution of the massive progenitor. We propose that magnetized massive stars as magnetar progenitors based on the magnetohydrodynamic evolution of the gravitational core collapse and rebound shock. Our physical mechanism, which does not necessarily re- quire ad hoc dynamo amplification within a fast spinning neutron star, favours the ‘fossil field’ scenario of forming magnetars from the strongly magnetized core collapse inside massive progenitor stars. With a range of surface magnetic field strengths over massive progenitor stars, our scenario allows a continuum of magnetic field strengths from pulsars to magnetars. The intense Lorentz force inside a magnetar may break the crust of a neutron star into pieces to various extents. Coupled with the magne- tar spin, the magnetospheric configuration of a magnetar is most likely variable in the presence of exposed convection, differential rotation, equatorial bulge, bursts of interior magnetic flux ropes as well as rearrangement of broken pieces of the crust. Sporadic and violent releases of accumulated magnetic energies and a broken crust are arXiv:0902.3111v1 [astro-ph.HE] 18 Feb 2009 the underlying causes for various observed high-energy activities of magnetars. Key words: magnetohydrodynamics (MHD) — shock waves — stars: magnetic fields — stars: neutron — supernova remnants — white dwarfs 1 INTRODUCTION recent powerful explosions of SGR J1550-5418 with a short- est spin period of 2.07 s). Most recently, a new Galactic mag- Magnetars are believed to be neutron stars with surface netar is reported with very fast optical flares (Kouveliotou magnetic field strengths considerably stronger than the 2008; Stefanescu et al. 2008; Castro-Tirado et al. 2008), al- quantum critical value of B = 4.4 × 1013 G. There are QED luding a continuum from ordinary dim isolated neutron stars two main types of observational manifestations for magne- to magnetars. The ultra-intense surface magnetic fields on tars: (i) Soft Gamma-ray Repeaters (SGRs) and (ii) Anoma- magnetars are unique in the Universe and they are respon- lous X-ray Pulsars (AXPs). Up to now, six SGRs and ten sible for various high-energy activities, for example the gi- AXPs have been identified observationally (see Mereghetti ant γ-ray flare of SGR 1806-20 (e.g. Hurley et al. 2005; 2008 for a latest list and an extensive review as well as very Palmer et al. 2005). Magnetar-like X-ray emissions are also detected from a rotation-powered pulsar PSR J1846-0258 ∼ × 13 ⋆ E-mail: [email protected] (RYH) and with an inferred intense magnetic field of 4.9 10 G at [email protected], [email protected] (Y-QL) c 2008 RAS 2 R.-Y. Hu and Y.-Q. Lou the centre of supernova remnant Kes75 (e.g. Gavriil et al. couple of early B stars, for example the B0.5V star HD 37061 2008; Archibald et al. 2008). (∼ 650 G, e.g. Hubrig et al. 2006). Petit et al. (2008a,b) Recent observations have also provided clues connecting carried out systematic spectropolarimetric observations to magnetars with very massive progenitor stars, for example search for magnetic fields on all massive OB stars in the an infrared elliptical ring or shell was discovered surround- Orion Nebula Cluster star-forming region. Strong magnetic ing the magnetar SGR 1900+14 (e.g. Wachter et al. 2008). fields of the order of kG were inferred on 3 stars out of a However, the formation of magnetars, especially the origin of sample of 8. The existence of strong magnetic fields on OB the ultra-intense magnetic field, remains an important open stars even appears somewhat overwhelming in contrast to issue. There are two major contending physical scenarios, very few magnetars that have been discovered so far. viz. the dynamo scenario versus the fossil-field scenario. With the assumption that neutron stars form dur- Duncan & Thompson (1992) and Thompson & Duncan ing the collapse of massive progenitors in the Galac- < < (1993) explored the turbulent dynamo amplification, occur- tic disc with 8 ∼ M/M⊙ ∼ 45 (stellar masses in ring primarily in the convection zone of the progenitor, as the main-sequence phase), and ∼ 8 percent of massive well as in a differentially rotating nascent neutron star, and stars have surface magnetic fields higher than ∼ 1000 concluded that very strong magnetic field, in principle up G, Ferrario & Wickramasinghe (2006) estimated that these to ∼ 3 × 1017 G, may be created. The dynamo mechanism high-field massive progenitors gave birth to 24 neutron stars > 14 requires an extremely rapid rotation of a nascent neutron with magnetic field ∼ 10 G, consisting a major part of star with a spin period of a few milliseconds. However, the magnetars. While the fossil-field scenario appears plausible current population of magnetars appears to be slow rota- from the perspective of statistics, it is highly instructive to tors, having spin periods in the range of ∼ 2 − 12 s (e.g. have a more direct magnetohydrodynamic (MHD) model Mereghetti 2008). Therefore, neutron star dynamo scenario description for the core collapse of high-field massive pro- for magnetars faces a considerable challenge to account for genitor stars and to check whether compact remnants left the fact of slowly rotating magnetars as observed so far. behind MHD rebound shocks do possess ultra-intense mag- The fossil-field scenario for the mag- netic fields. netism of compact objects was first pro- In this paper, we attempt to model magnetized massive posed to explain magnetic white dwarfs (e.g. progenitor stars with a quasi-spherical general polytropic Braithwaite & Spruit 2004; Wickramasinghe & Ferrario magnetofluid under the self-gravity (Wang & Lou 2008; Lou 2005; Ferrario & Wickramasinghe 2005; Lou & Wang 2007). & Hu 2009). We examine semi-analytic and numerical so- It is conceivable that the magnetic field of white dwarfs may lutions to explore the self-similar MHD evolution emerging be of fossil origin from the main-sequence phase of their from dynamic processes of core collapse and rebound shock progenitors, and the attempt to link magnetic white dwarfs travelling in the stellar envelope with a wind mass loss. More with their main-sequence progenitors naturally makes the specifically, we adopt a general polytropic equation of state chemically peculiar Ap and Bp stars as plausible candi- (EoS) p = κ(r, t)ργ with p, ρ, γ, and κ respectively be- dates. Observations of Auri`ere et al. (2003) have shown ing the gas pressure, mass density, polytropic index and a that chemically peculiar Ap and Bp stars are generally proportional coefficient dependent on radius r and time t. magnetic indeed, with a surface magnetic field of ∼ 100 G Here, κ is closely related to the ‘specific entropy’ and is not by Zeeman splittings. In general, magnetic field strengths necessarily a global constant. By ‘specific entropy’ conser- fall in the range of ∼ 3 × 102 − 3 × 104 G (e.g. Braithwaite & vation along streamlines, another key parameter q arises in Spruit 2004 and references therein). Magnetic white dwarfs self-similar dynamics (see Wang & Lou 2008). For κ being may be created as a result of rebound shock explosion a global constant, or equivalently q = 0, the general poly- (Lou & Wang 2007) and may further give rise to novel tropic EoS reduces to a conventional polytropic EoS. By magnetic modes of global stellar oscillations (Lou 1995). By further setting γ = 1, a conventional polytropic gas reduces the magnetic flux conservation during the stellar evolution, to an isothermal gas (e.g. Lou & Shen 2004). We also require Ferrario & Wickramasinghe (2005) argued that stellar γ > 1 to ensure a positive specific enthalpy p/(γ − 1). magnetic fields (∼ 100 G) in their main-sequence phase Chiueh & Chou (1994) studied the isothermal MHD by can be enhanced up to the range of ∼ 106 − 109 G on the including the magnetic pressure gradient force in the radial surface of magnetic white dwarfs. This fossil-field scenario momentum equation. Yu & Lou (2005), Yu, Lou, Bian & Wu is supported by the statistics for the mass and magnetic (2006), Wang & Lou (2007) and Wang & Lou (2008) gener- field distributions of magnetic white dwarfs. alized the self-similar hydrodynamic framework by including Based on the same scenario and a similar physical argu- a completely random transverse magnetic field with the ap- ment, Ferrario & Wickramasinghe (2006) further suggested proximation of a ‘quasi-spherical’ symmetry (e.g.
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