The Lya line of atomic hydrogen is often a luminous component of the radiation emitted by distant galaxies. Except for those galaxies which have a substantial central source of non-stellar ionizing radiation, most of the Lya radiation emitted by galaxies is generated within regions of the interstellar medium which are photoionized by starlight. Conversely, much of the energy radiated by photoio...

We review various prescriptions which have been proposed for the turbulent transport of matter and angular momentum in differentially rotating stellar radiation zones. A new prescription is presented for the horizontal transport associated with the anisotropic shear turbulence which is produced by the differential rotation in latitude; this ‘β-viscosity’ is drawn from torque measurements in the...

I overview the current understanding of the evolution of massive black hole (MBH) binaries in the center of the host stellar system. One of the main questions is whether the stellar dynamical effect can make the MBH binary hard enough that they can merge through gravitational wave radiation. So far, theories and numerical simulations suggested otherwize, since the hardening timescale becomes ve...

Do " isolated " planetary mass objects orbit invisible stellar mass companions? Abstract We propose that the " isolated " planetary mass objects observed by Zapa-tero Osorio et al in the σ Orionis cluster might actually be in orbit around invisible stellar mass companions such as mirror stars. The existence of such objects can be motivated by the dark matter problem and the observation of MACHO...

The central parsec of the Galaxy contains dozens of massive stars with a cumulative mass loss rate ∼ 10 M⊙ yr . Shocks among these stellar winds produce the hot plasma that pervades the central part of the galaxy. We argue that these stellar wind shocks also efficiently accelerate electrons and protons to relativistic energies. The relativistic electrons inverse Compton scatter the ambient ultr...

Context: An impulsive increase in luminosity by one half or more of the Eddington value will lead to ejection of all optically thin plasma from Keplerian orbits around the radiating star, if gravity is Newtonian and the PoyntingRobertson drag is neglected. Radiation drag may bring some particles down to the stellar surface. On the other hand, general relativistic calculations show that gravity ...

The capture and gradual inspiral of stellar mass objects by a massive black hole at the centre of a galaxy has been proposed as one of the most promising source of gravitational radiation to be detected by LISA. Unfortunately rate estimates for this process suffer from many uncertainties. Here we report on the use of our newly developed Monte Carlo stellar dynamics code to tackle this problem. ...

We present an interpretation of the phenomenological relations between the spectral peak, isotropic luminosity and duration of long gamma ray bursts that have been discovered by Amati et al., Ghirlanda et al., Firmani et al., and Liang & Zhang. In our proposed model, a jet undergoes internal dissipation which prevents its bulk Lorentz factor from exceeding 1/θ (θ being the jet opening angle) un...

We present new 0.05 00 (22 AU) resolution narrow-band Hubble Space Telescope Planetary Camera images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present 0.02 00 (9 AU) resolution UV images of 7 externally illuminated proto-stellar environments and the rst UV spectra that cover the spectral range between 1400 and 3000 A. We discuss 43 objects for which t...

Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt ac...