The heating of ions downstream of the x‐line during magnetic reconnection is explored using full‐particle simulations, test particle simulations, and analytic analysis. Large‐scale particle ...simulations reveal that the ion temperature increases sharply across the boundary layer that separates the upstream plasma from the Alfvénic outflow. This boundary layer, however, does not take the form of a classical switch‐off shock as discussed in the Petschek reconnection model, so the particle heating cannot be calculated from the magnetohydrodynamic, slow‐shock prediction. Test particle trajectories in the fields from the simulations reveal that ions crossing the narrow boundary into the exhaust instead behave like pickup particles: they gain both a directed outflow and an effective thermal speed given by the flow speed v0 of the exhaust. The detailed dynamics of these particles are explored by taking 1‐D cuts of the simulation data across the exhaust, transforming to the deHoffman‐Teller frame, and calculating explicitly the increment in the temperature, miv02/3, with mi, the ion mass. We compare the model predictions with the temperature increment in solar wind exhausts measured by the ACE and Wind spacecraft, confirming that the temperature increment is proportional to the ion mass. The Wind data from 22 high‐shear exhaust encounters confirm the scaling of the proton temperature increment with the square of the exhaust velocity. However, the temperature increments are consistently lower than the model prediction. Implications for understanding the production of high‐energy ions in flares and the broader universe are discussed.
In collisionless and weakly collisional plasmas, such as hot accretion flows onto compact objects, the magnetorotational instability (MRI) can differ significantly from the standard (collisional) ...MRI. In particular, pressure anisotropy with respect to the local magnetic-field direction can both change the linear MRI dispersion relation and cause nonlinear modifications to the mode structure and growth rate, even when the field and flow perturbations are very small. This work studies these pressure-anisotropy-induced nonlinearities in the weakly nonlinear, high-ion-beta regime, before the MRI saturates into strong turbulence. Our goal is to better understand how the saturation of the MRI in a low-collisionality plasma might differ from that in the collisional regime. We focus on two key effects: (i) the direct impact of self-induced pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii) the influence of pressure anisotropy on the ‘parasitic instabilities’ that are suspected to cause the mode to break up into turbulence. Our main conclusions are: (i) The mirror instability regulates the pressure anisotropy in such a way that the linear MRI in a collisionless plasma is an approximate nonlinear solution once the mode amplitude becomes larger than the background field (just as in magnetohyrodynamics). This implies that differences between the collisionless and collisional MRI become unimportant at large amplitudes. (ii) The break up of large-amplitude MRI modes into turbulence via parasitic instabilities is similar in collisionless and collisional plasmas. Together, these conclusions suggest that the route to magnetorotational turbulence in a collisionless plasma may well be similar to that in a collisional plasma, as suggested by recent kinetic simulations. As a supplement to these findings, we offer guidance for the design of future kinetic simulations of magnetorotational turbulence.
Accretion discs with masses ∼10−3–0.1 M⊙ are believed to form during the merger of a neutron star (NS) with another NS and the merger of a NS with a black hole (BH). Soon after their formation, such ...hyperaccreting discs cool efficiently by neutrino emission and their composition is driven neutron-rich by pair captures under degenerate conditions. However, as the disc viscously spreads and its temperature drops, neutrino cooling is no longer able to offset viscous heating and the disc becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the disc's remaining mass. We calculate the thermal evolution and nuclear composition of viscously spreading accretion discs formed from compact object mergers using one-dimensional height-integrated simulations. We show that freeze-out from weak equilibrium necessarily accompanies the disc's late-time transition to an advective state. As a result, hyperaccreting discs generically freeze-out neutron-rich (with electron fraction Ye∼ 0.2–0.4), and their late-time outflows robustly synthesize rare neutron-rich isotopes. Using the measured abundances of these isotopes in our Solar system, we constrain the compact object merger rate in the Milky Way to be ≲10−5 (Md,0/0.1 M⊙)−1 yr−1, where Md,0 is the average initial mass of the accretion disc. Thus, either the NS–NS merger rate is at the low end of current estimates or the average disc mass produced during a typical merger is ≪0.1 M⊙. Based on the results of current general relativistic merger simulations, the latter constraint suggests that prompt collapse to a BH is a more common outcome of NS–NS mergers than the formation of a transient hypermassive NS. We also show that if most short-duration gamma-ray bursts (GRBs) are produced by compact object mergers, their beaming fraction must exceed fb≈ 0.13(Md,0/0.1 M⊙), corresponding to a jet half-opening angle ≳30° (Md,0/0.1 M⊙)1/2. This is consistent with other evidence that short-duration GRB outflows are less collimated than those produced in long-duration GRBs.
The accretion-induced collapse (AIC) of a white dwarf to form a neutron star can leave behind a rotationally supported disc with mass of up to ∼ 0.1 M⊙. The disc is initially composed of free ...nucleons but as it accretes and spreads to larger radii, the free nucleons recombine to form helium, releasing sufficient energy to unbind the remaining disc. Most of the ejected mass fuses to form 56Ni and other iron group elements. We present spherically symmetric radiative transfer calculations of the transient powered by the radioactive heating of this ejecta. We estimate the ejecta composition using nucleosynthesis calculations in the literature and explore the sensitivity of our results to uncertainties in the ejecta kinematics. For an ejecta mass of 10−2 M⊙ (3 × 10−3 M⊙), the light curve peaks after ≲ 1 d with a peak bolometric luminosity ≃ 2 × 1041 erg s−1 (≃ 5 × 1040 erg s−1); the decay time is ≃ 4 (2) d. Overall, the spectra redden with time reaching U−V≃ 4 after ≃ 1 d; the optical colours (B−V) are, however, somewhat blue. Near the peak in the light curve, the spectra are dominated by Doppler-broadened Nickel features, with no distinct spectral lines present. At d, strong calcium lines are present in the infrared, although the calcium mass fraction is only ∼ 10−4.5. If rotationally supported discs are a common byproduct of AIC, current and upcoming transient surveys such as the Palomar Transient Factory should detect a few AIC per year for an AIC rate of ∼ 10−2 of the Type Ia rate. We discuss ways of distinguishing AIC from other rapid, faint transients, including. Ia's and the ejecta from binary neutron star mergers.
We extend previous theories of stochastic ion heating to account for the motion of ions along the magnetic field B. We derive an analytic expression for the temperature ratio T sub(perpendiculari)/T ...sub(perpendicularp) in the solar wind assuming that stochastic heating is the dominant ion heating mechanism, where T sub(perpendiculari) is the perpendicular temperature of species i and T sub(perpendicularp) is the perpendicular proton temperature. This expression describes how T sub(perpendiculari)/T sub(perpendicularp) depends upon U sub(i) and beta sub(||p), where U sub(i) is the average velocity along B of species i in the proton frame and beta sub(||p) is the ratio of the parallel proton pressure to the magnetic pressure, which we take to be <, ~ 1. We compare our model with previously published measurements of alpha particles and protons from the Wind spacecraft. We find that stochastic heating offers a promising explanation for the dependence of T sub(perpendicularalpha)/T sub(perpendicularp) on U sub(alpha) and beta sub(||p) when the fractional cross helicity and Alfven ratio at the proton-gyroradius scale have values that are broadly consistent with solar-wind measurements. We also predict how the temperatures of other ion species depend on their drift speeds.
Several recent observations suggest that gas-poor (dissipationless) mergers of elliptical galaxies contribute significantly to the build-up of the massive end of the red sequence at z≲ 1. We perform ...a series of major merger simulations to investigate the spatial and velocity structure of the remnants of such mergers. Regardless of orbital energy or angular momentum, we find that the stellar remnants lie on the fundamental plane defined by their progenitors, a result of virial equilibrium with a small tilt due to an increasing central dark matter fraction. However, the locations of merger remnants in the projections of the fundamental plane – the Faber–Jackson and Re–M* relations – depend strongly on the merger orbit, and the relations steepen significantly from the canonical scalings (L∝σ4e and Re∝M0.6*) for mergers on radial orbits. This steepening arises because stellar bulges on orbits with lower angular momentum lose less energy via dynamical friction on the dark matter haloes than do bulges on orbits with substantial angular momentum. This results in a less tightly bound remnant bulge with a smaller velocity dispersion and a larger effective radius. Our results imply that the projections of the fundamental plane – but not necessarily the plane itself – provide a powerful way of investigating the assembly history of massive elliptical galaxies, including the brightest cluster galaxies at or near the centres of galaxy clusters. We argue that most massive ellipticals are formed by anisotropic merging and that their fundamental plane projections should thus differ noticeably from those of lower mass ellipticals even though they should lie on the same fundamental plane. Current observations are consistent with this conclusion. The steepening in the L–σe relation for luminous ellipticals may also be reflected in a corresponding steepening in the MBH–σe relation for massive black holes.
Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their ...explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44Ti.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Measurements of small-scale turbulent fluctuations in the solar wind find a non-zero right-handed magnetic helicity. This has been interpreted as evidence for ion cyclotron damping. However, ...theoretical and empirical evidence suggests that the majority of the energy in solar wind turbulence resides in low-frequency anisotropic kinetic Alfven wave fluctuations that are not subject to ion cyclotron damping. We demonstrate that a dissipation range comprised of kinetic Alfven waves also produces a net right-handed fluctuating magnetic helicity signature consistent with observations. Thus, the observed magnetic helicity signature does not necessarily imply that ion cyclotron damping is energetically important in the solar wind.
We present time-dependent axisymmetric magnetohydrodynamic simulations of the interaction of a relativistic magnetized wind produced by a proto-magnetar with a surrounding stellar envelope, in the ...first ∼10 s after core collapse. We inject a super-magnetosonic wind with into a cavity created by an outgoing supernova shock. A strong toroidal magnetic field builds up in the bubble of plasma and magnetic field that is at first inertially confined by the progenitor star. This drives a jet out along the polar axis of the star, even though the star and the magnetar wind are each spherically symmetric. The jet has the properties needed to produce a long-duration gamma-ray burst (GRB). At ∼5 s after core bounce, the jet has escaped the host star and the Lorentz factor of the material in the jet at large radii ∼1011 cm is similar to that in the magnetar wind near the source. Most of the spindown power of the central magnetar escapes via the relativistic jet. There are fluctuations in the Lorentz factor and energy flux in the jet on a ∼ 0.01–0.1 s time-scale. These may contribute to variability in GRB emission (e.g. via internal shocks).
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