Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars
Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the "low-T/|W| instability"), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T/|W| instability, we find that they do so only in a small range of field strengths. Below 4×10^(13) G, poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5×10^(14) G, magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally, we report that the detailed development of the low-T/|W| instability, including its growth rate, depends strongly on the particular numerical methods used. The high-order methods we employ suggest that growth might be considerably slower than found in some previous simulations.
Additional Information© 2014 American Physical Society. Received 12 May 2014; published 17 November 2014. We extend our thanks to D. Lai for inspiring this investigation, to M. Boyle for advice on several occasions, and to F. Hébert for catching errors in the text. We also extend our gratitude to S. Bernuzzi, R. De Pietri, B. Giacomazzo, and L. Rezzolla, whose correspondence after the first version of this paper helped to clarify the comparison between their simulations and ours. The authors at Cornell gratefully acknowledge support from National Science Foundation (NSF) Grants No. PHY- 1306125 and No. AST-1333129, while the authors at Caltech acknowledge support from NSF Grants No. PHY-1440083 and No. AST-1333520 and NSF CAREER Award No. PHY-1151197. Authors at both Caltech and Cornell also thank the Sherman Fairchild Foundation for their support. F. Foucart gratefully acknowledges support from the Vincent and Beatrice Tremaine Postdoctoral Fellowship, from the NSERC of Canada, from the Canada Research Chairs Program, and from the Canadian Institute for Theoretical Astrophysics. Finally, the authors at WSU acknowledge support through NASA Grant No. NNX11AC37G and NSF Grant No. PHY-1402916. Some computations were performed on the GPC supercomputer at the SciNet HPC Consortium , funded by the Canada Foundation for Innovation under the auspices of Compute Canada, the Government of Ontario, Ontario Research Fund–Research Excellence, and the University of Toronto. This work also used the Extreme Science and Engineering Discovery Environment (XSEDE) through allocations No. TG-PHY100033 and No. PHY990002, supported by NSF Grant No. OCI-1053575. Additionally, this research was performed in part using the Zwicky computer system operated by the Caltech Center for Advanced Computing Research and funded by NSF MRI No. PHY-0960291 and the Sherman Fairchild Foundation.
Published - PhysRevD.90.104014.pdf
Submitted - 1405.2144v1.pdf