Robert T. Emmering

Magnetic acceleration of broad emission-line clouds in active galactic nuclei

It is proposed that broad emission lines in active galactic nuclei are formed by dense clouds in a molecular, hydromagnetic wind accelerated radiatively and centrifugally away from an accretion disk orbiting a massive black hole. These clouds are supposed to be photoionized by the UV continuum produced in the innermost radii of the accretion disk where the radiation flux is sufficient to evaporate the dust grains, that is, at a radius ∼0.2L 1/2 ion,46 pc

Hydrodynamic instabilities in supernova remnants : self-similar driven waves

The initial interaction of a supernova with its surroundings involves the uniformly expanding, roughly power-law density profile of the outer parts. If the supernova gas has a steep power-law density profile and the surrounding density can be described by ρ ∞ r -s , with s = 0 for a uniform interstellar medium and s = 2 for a stellar wind, the interaction region is given by a self-similar solution. The profiles of the physical quantities in the shocked region show that the flow is subject to convective or Rayleigh-Taylor instability; the convective growth rate is largest at the contact discontinuity between the shocked supernova gas and the shocked surroundings

Shocked relativistic magnetohydrodynamic flows with application to pulsar winds

The time-dependent behavior of a shocked spherically symmetric relativistic fluid with tangential magnetic field is investigated, considering the case where the boundaries of the shocked fluid move at constant velocity so that self-similar solutions exist. The behavior of the fluid in the ultrarelativistic regime is compared to that in the nonrelativistic regime; there is a smooth transition between these limits. If a magnetic field is present, the magnetic pressure becomes increasingly important with distance from the shock wave; the gas pressure vanishes at the contact discontinuity that bounds the flow. Analytic expressions are given which describe the flow. The solutions can be applied to the evolution of shocked relativistic pulsar winds, which are probably observed as Crab-like supernova remnants. A model for the Crab Nebula, based on the steady-state model of Kennel and Coroniti (1984), indicates that sigma = 0.0016, where sigma is twice the ratio of magnetic to particle energy in the wind as measured in the fluid frame. This is about half the value suggested by Kennel and Coroniti and is much smaller than the value that might be expected for a pulsar wind. 17 references.

The intrinsic luminosity and initial period of pulsars

The observed properties of pulsars are modeled on the assumption that they evolve by emitting magnetic dipole radiation and have exponentially decaying magnetic fields. The models are constructed by using Monte Carlo techniques to generate a Galactic sample of pulsars having particular distributions of initial periods, magnetic dipole moments, and positions. By choosing a relationship between the radio luminosity L(r), the period (P), and the P derivative, it is determined which pulsars of the generated sample could be detected by one of the major pulsar surveys. Detailed account is taken of known selection effects in these surveys. Models in which the size of the pulsar emission cone is dependent on the period are considered. The properties of the detectable pulsars are compared with the properties of the observed pulsars, and the initial distributions and luminosity relation of the model are adjusted until a satisfactory agreement is obtained. A crucial point is that the observed relation between L(r) and P, P derivative is affected by selection effects and does not apply to the actual pulsar population. It is found that the best fits are obtained with models in which the pulsars are injected with relatively long periods, about 0.5 sec.

Illuminating a red supergiant wind around SN 1987A

A recently discovered light echo at a radius of about 9 arcsec from SN 1987A is interpreted as the result of a circumstellar shell at a radius of 4.5 pc from the SN. The shell may have its origin in the deceleration of the pre-SN red supergiant wind by the pressure of the surrounding medium. The required pressure is p/k = about 1000 per cu cm K. The echo is predicted to expand and contract over the next 30 yr with a declining surface brightness and a broadening of its width. Colors of the echo compared to those of the larger echoes may show differences in the wavelength dependence of scattering of the circumstellar grains compared to interstellar grains. 30 refs.

Infrared supernova light curves and asymmetric stellar mass loss

Infrared dust emission echos from Type II supernovae are a natural consequence of the heating of circumstellar dust by the supernova light. Red supergiants, the likely progenitors of most Type II supernovae, are known in some cases to have asymmetric circumstellar envelopes. It is noted that an asymmetric dust distribution can have a substantial effect on the evolution of an infrared echo and results are presented for an ellipsoidal dust distribution. The angle between the symmetry axis and the line of sight is unknown in any particular case so that detailed observations of a number of supernovae will be necessary to test for asymmetries. Asymmetries may also be observable in the radio structure of supernovae and in a possible scattered-light echo. 16 references.

Circumstellar infrared and scattered light echoes from SN 1987A

The observed light curve and temperature evolution and the theoretically predicted initial UV burst of SN 1987A are used to construct models for the IR echo and scattered-light echo arising from circumstellar dust formed in the wind from the supernova progenitors red supergiant phase. Observations of narrow UV emission lines suggest that some of this wind has been swept into a shell with a radius of order 10 to the 18th cm by the progenitors blue supergiant wind. The UV observations also indicate a high abundance of oxygen with respect to carbon, which implies that the circumstellar dust should be composed primarily of silicate grains. Echo models are constructed by including the detailed emission and scattering properties of silicate grains and a spectrum of grain sizes. It is concluded that an IR echo has probably not been observed as of August, 1987. 46 refs.

The interstellar light echoes around SN 1987A

The observed light echoes around SN 1987A are modeled by dust in planar clouds roughly 120 and 330 pc in front of the supernova. The expected echo profiles depend on the observed light curve and on the cloud thicknesses. The surface brightness and colors of the echoes are modeled using a mix of silicate and graphite grains appropriate to the Large Magellanic Cloud (LMC). The model colors approximately agree with the observed ones. The observed surface brightness suggests that the dust column density in the echo clouds is comparable to the total dust column density in the LMC toward SN 1987A. The model predicts the presence of a narrow ultraviolet echo outside the visual echo; it should be detectable with future ultraviolet observatories and will give information on the supernova near the phase of shock breakout. 21 references.