The growth of helium burning cores

Helium burning in the convective cores of horizontal branch and `red clump' stars appears to involve a process of `ingestion' of unburnt helium into the core, the physics of which has not been identified yet. I show here that a limiting factor controlling the growth is the buoyancy of helium entering the denser C+O core. It yields a growth rate which scales directly with the convective luminosity of the core, and agrees with constraints on core size from current asteroseismology.Comment: Accepted for publication in A&

Jets from compact objects

Some topics in the theory of jets are reviewed. These include jet precession, unconfined jets, the origin of knots, the internal shock model as a unifying theme from protostellar jets to Gamma-ray bursts, relations between the Blandford-Znajek and MHD disk-wind models, and jet collimation in magnetic acceleration models.Comment: To appear in Highly Energetic Physical Processes .... (IAU Symp 195) P. C. H. Martens and S. Tsuruta, ed

Semiconvection: numerical simulations

A grid of numerical simulations of double-diffusive convection is presented for the astrophysical case where viscosity (Prandtl number Pr) and solute diffusivity (Lewis number Le) are much smaller than the thermal diffusivity. As in laboratory and geophysical cases convection takes place in a layered form. The proper translation between subsonic flows in a stellar interior and an incompressible (Boussinesq) fluid is given, and the validity of the Boussinesq approximation for the semiconvection problem is checked by comparison with fully compressible simulations. The predictions of a simplified theory of mixing in semiconvection given in a companion paper are tested against the numerical results, and used to extrapolate these to astrophysical conditions. The predicted effective He-diffusion coefficient is nearly independent of the double-diffusive layering thickness $d$. For a fiducial main sequence model (15 $M_\odot$) the inferred mixing time scale is of the order $10^{10}$ yr. An estimate for the secular increase of $d$ during the semiconvective phase is given. It can potentially reach a significant fraction of a pressure scale height.Comment: arXiv admin note: substantial text overlap with arXiv:1012.585

Why pulsars rotate and move: kicks at birth

RADIO pulsars are thought to born with spin periods of 0.02-0.5 s and space velocities of 100-1000 km/s, and they are inferred to have initial dipole magnetic fields of 10^{11}-10^{13}. The average space velocity of a normal star in the Milky Way is only 30 km/s, which means that pulsars must receive a substantial 'kick' at birth. Here we propose that the birth characteristics of pulsars have a simple physical connection with each other. Magnetic fields maintained by differential rotation between the core and envelope of the progenitor would keep the whole star in a state of approximately uniform rotation until 10 years before the explosion. Such a slowly rotating core has 1000 times less angular momentum than required to explain the rotation of pulsars. Although the specific physical process that 'kicks' the neutron star at birth has not been identified, unless its force is exerted exactly head-on, it will also cause the neutron star to rotate. We identify this process as the origin of the spin of pulsars. Such kicks will cause a correlation between the velocity and spin vectors of pulsars. We predict that many neutron stars are born with periods longer than 2 s, and never become radio pulsars.Comment: To appear in Nature. Press embargo till publishe

Magnetically powered prompt radiation and flow acceleration in GRB

The physics of GRB powered by a magnetic energy flux is reviewed. Magnetic fields are natural for transmitting the energy from the central compact object to the small amount of baryons required for a GRB. When dissipation of the flux of magnetic energy by reconnection inside the flow is taken into account, the magnetic model assumes several more convincing properties. For baryon loading typical of observed GRB, most of the dissipation takes place just outside photosphere, so that prompt emission is produced efficiently, and the magnetic field strength in this region is high, resulting in efficient synchrotron emission. Remarkably, the dissipation also causes very efficient acceleration of the bulk flow. This effect is illustrated with a classical hydrodynamic equivalent. In this context, the distinction between the flux of magnetic energy $cB^2/8\pi$ and the Poynting flux $cB^2/4\pi$ is important, and an interpretation of the Poynting flux as a `magnetic enthalpy flux' illuminating. Numerical and analytical results for flow acceleration and the relative contribution of photospheric (thermal) and nonthermal emission as functions of the asymptotic bulk Lorentz factor are given. The transition between X-ray flashes and true GRB is predicted at $\Gamma\approx 100$.Comment: To appear (in shortened form) in Proceedings "Gamma Ray Bursts in the Afterglow Era, Third Workshop" (Rome, Sept 2002