Trans-Magnetosonic Accretion in a Black Hole Magnetosphere

We present the critical conditions for hot trans-fast magnetohydrodynamical (MHD) flows in a stationary and axisymmetric black-hole magnetosphere. To accrete onto the black hole, the MHD flow injected from a plasma source with low velocity must pass through the fast magnetosonic point after passing through the ``inner'' or ``outer'' Alfven point. We find that a trans-fast MHD accretion solution related to the inner Alfven point is invalid when the hydrodynamical effects on the MHD flow dominate at the magnetosonic point, while the other accretion solution related to the outer Alfven point is invalid when the total angular momentum of the MHD flow is seriously large. When both regimes of the accretion solutions are valid in the black hole magnetosphere, we can expect the transition between the two regimes. The variety of these solutions would be important in many highly energetic astrophysical situations.Comment: 27 pages, 12 figures, accepted to Ap

Efficiency of Magnetized Thin Accretion Disks in the Kerr Metric

The efficiency of thin disk accretion onto black holes depends on the inner boundary condition, specifically the torque applied to the disk at the last stable orbit. This is usually assumed to vanish. I estimate the torque on a magnetized disk using a steady magnetohydrodynamic inflow model originally developed by Takahashi et al., 1990. I find that the efficiency epsilon can depart significantly from the classical thin disk value. In some cases epsilon > 1, i.e. energy is extracted from the black hole.Comment: 11 pages, 3 figures, aastex, submitted to ApJ Letter

Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264

We present some results from a UVI photometric study of a field in the young open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main sequence stars is linked to rotation. We confirm that U-V excess is well correlated with H-alpha equivalent width for the stars in our sample. We show that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun) there is also a significant association between U-V excess and rotation, in the sense that slow rotators are more likely to show excess U-band emission and variability. This constitutes significant new evidence in support of the disk-locking paradigm.Comment: Accepted by ApJ Letter

Magnetar Spindown, Hyper-Energetic Supernovae, and Gamma Ray Bursts

The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birth of a neutron star in a successful core-collapse supernova, is accompanied by a neutrino-driven wind. For magnetar-strength ($\sim10^{15}$ G) large scale surface magnetic fields, this outflow is magnetically-dominated during the entire cooling epoch.Because the strong magnetic field forces the wind to co-rotate with the protoneutron star,this outflow can significantly effect the neutron star's early angular momentum evolution, as in analogous models of stellar winds (e.g. Weber & Davis 1967). If the rotational energy is large in comparison with the supernova energy and the spindown timescale is short with respect to the time required for the supernova shockwave to traverse the stellar progenitor, the energy extracted may modify the supernova shock dynamics significantly. This effect is capable of producing hyper-energetic supernovae and, in some cases, provides conditions favorable for gamma ray bursts. We estimate spindown timescales for magnetized, rotating protoneutron stars and construct steady-state models of neutrino-magnetocentrifugally driven winds. We find that if magnetars are born rapidly rotating, with initial spin periods ($P$) of $\sim1$ millisecond, that of order $10^{51}-10^{52}$ erg of rotational energy can be extracted in $\sim10$ seconds. If magnetars are born slowly rotating ($P\gtrsim10$ ms) they can spin down to periods of $\sim1$ second on the Kelvin-Helmholtz timescale.Comment: 16 pages, 5 figures, emulateap

The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres

Recent simulations of force-free, degenerate (ffde) black hole magnetospheres indicate that the fast mode radiated from (or near) the event horizon can modify the global potential difference in the poloidal direction orthogonal to the magnetic field, V, in a black hole magnetosphere. There is a fundamental contradiction in a wave that alters V coming from near the horizon. The background fields in ffde satisfy the ``ingoing wave condition'' near the horizon (that arises from the requirement that all matter is ingoing at the event horizon), yet outgoing waves are radiated from this region in the simulation. Studying the properties of the waves in the simulations are useful tools to this end. It is shown that regularity of the stress-energy tensor in a freely falling frame requires that the outgoing (as viewed globally) waves near the event horizon are redshifted away and are ineffectual at changing V. It is also concluded that waves in massless MHD (ffde) are extremely inaccurate depictions of waves in a tenuous MHD plasma, near the event horizon, as a consequence black hole gravity. Any analysis based on ffde near the event horizon is seriously flawed.Comment: 9 pages to appear in ApJ Letter

The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster

We have determined new rotation periods for 404 stars in the Orion Nebula Cluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on La Silla, Chile. Mass estimates are available for 335 of these and most have M < 0.3 M_sun. We confirm the existence of a bimodal period distribution for the higher mass stars in our sample and show that the median rotation rate decreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. While the spread in angular momentum (J) at any given mass is more than a factor of 10, the majority of lower mass stars in the ONC rotate at rates approaching 30% of their critical break-up velocity, as opposed to 5-10% for solar-like stars. This is a consequence of both a small increase in observed specific angular momentum (j=J/M) and a larger decrease in the critical value of j with decreasing mass. Perhaps the most striking fact, however, is that j varies by so little - less than a factor of two - over the interval 0.1-1.0 M_sun. The distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in nature to what is found in the Pleiades (age ~ 100 My). These observations provide a significant new guide and test for models of stellar angular momentum evolution during the proto-stellar and pre-main sequence phases.Comment: 11 pages, 3 figure

Reliability of astrophysical jet simulations in 2D. On inter-code reliability and numerical convergence

This document is the Accepted Manuscript version of the following article: M. Krause and M. Camezind, 'Reliability of astrophysical jet simulations in 2D', Astronomy & Astrophysics, Vol. 380 (III): 789-804, December 2001, the version of record is available online at doi: DOI: 10.1051/0004-6361:20011452. © ESO 2001In the present paper, we examine the convergence behavior and inter-code reliability of astrophysical jet simulations in axial symmetry. We consider both pure hydrodynamic jets and jets with a dynamically significant magnetic field. The setups were chosen to match the setups of two other publications, and recomputed with the MHD code NIRVANA. We show that NIRVANA and the two other codes give comparable, but not identical results. We explain the differences by the different application of artificial viscosity in the three codes and numerical details, which can be summarized in a resolution effect, in the case without magnetic field: NIRVANA turns out to be a fair code of medium efficiency. It needs approximately twice the resolution as the code by Lind (Lind et al. 1989) and half the resolution as the code by Kössl (Kössl & Müller 1988). We find that some global properties of a hydrodynamical jet simulation, like e.g. the bow shock velocity, converge at 100 points per beam radius (ppb) with NIRVANA. The situation is quite different after switching on the toroidal magnetic field: in this case, global properties converge even at 10 ppb. In both cases, details of the inner jet structure and especially the terminal shock region are still insufficiently resolved, even at our highest resolution of 70 ppb in the magnetized case and 400 ppb for the pure hydrodynamic jet. The magnetized jet even suffers from a fatal retreat of the Mach disk towards the inflow boundary, which indicates that this simulation does not converge, in the end. This is also in definite disagreement with earlier simulations, and challenges further studies of the problem with other codes. In the case of our highest resolution simulation, we can report two new features: first, small scale Kelvin-Helmholtz instabilities are excited at the contact discontinuity next to the jet head. This slows down the development of the long wavelength Kelvin-Helmholtz instability and its turbulent cascade to smaller wavelengths. Second, the jet head develops Rayleigh-Taylor instabilities which manage to entrain an increasing amount of mass from the ambient medium with resolution. This region extends in our highest resolution simulation over 2 jet radii in the axial direction.Peer reviewe

Are jets in symbiotic stars driven by magnetic fields?

We compare two scenarios to launch jets -- formation by MHD processes or formation by thermal pressure in the boundary layer (BL) -- with respect to their compatibility with observational data of jets in symbiotic stars, especially in the well studied jet source MWC 560. Finally, we discuss points of further research to be done.Comment: 4 pages, accepted for publication in A & A Letter