Itzhak Goldman

Interpretation of the Spatial Power Spectra of Neutral Hydrogen in the Galaxy and in the Small Magellanic Cloud

Recent 21 cm radio observations of H I regions in the Small Magellanic Cloud have revealed spatial power spectra of the intensity that are quite similar in shape to those previously deduced for the Galaxy. The similarity, in spite of the differences in the physical parameters between the Galaxy and the SMC, suggests that the shape of the power spectra reflects some underlying mechanism that is not too sensitive to the environmental specifics. In this paper we present an interpretation for the observational power spectra in terms of a large-scale turbulence in the interstellar medium in which the emitting H I regions are embedded. The turbulence gives rise to density fluctuations that lead to the observed intensity fluctuations in the H I regions. The observational power spectra are used to deduce the turbulence spectral function. In the SMC, the turbulences largest eddies are comparable in scale to the SMC itself. This implies that turbulent mixing should have smoothed out any large-scale abundance gradients. Indeed, this seems to be the case, observationally. The turbulence is also expected to amplify and shape up the large-scale magnetic field. Indeed, the observational data indicate the existence of a large-scale disordered field of the strength expected from energy equilibrium with the turbulent velocity field. The large-scale turbulence is most probably generated by instabilities in the large-scale flows induced by the tidal close encounter with the LMC ~2 × 108 yr ago. The lifetime of the largest eddies is ~4 × 108 yr, so the turbulence has not yet had enough time to decay and persists even though the energy source is no longer there.

Turbulently generated magnetic fields in clusters of galaxies

The typical scale and velocity of the dominant, turbulent eddies excited by the motion of galaxies in clusters are determined from basic considerations which are valid if a steady state is attained. Hydrodynamic turbulence in the intracluster plasma presumably generates magnetic fields; our estimates of the typical scale and mean strength of these fields in cluster cores are about 10 kpc and at most a few 10 −7 G. Implications of fields with these properties on the detectability of diffuse hard X-ray emission from clusters, and on transport properties in the intracluster space, are discussed

Turbulent convection in thin accretion disks

A self-consistent solution for a thin accretion disk with turbulent convection is presented. The disk viscosity and the convective flux are derived from a physical model for turbulence, and expressed in terms of the local physical conditions of the disk which, in turn, are controlled by the former two. In the gas pressure region, two distinct solutions are obtained. In one, the convective flux is much larger than the radiative flux and the blackbody region extends over the entire gas pressure region and could also extend down to the inner boundary of the disk. In this solution the temperature profile is close to adiabatic. In the other solution, the convective flux is about a third of the total flux, and there exist the gas pressure blackbody and electron scattering regions as well as the radiation pressure region. In the radiation pressure region, the temperature profile is very close to adiabatic, and the disk is geometrically thin and optically thick even for super Eddington accretion rates. The fraction of the convective flux, out of the total flux, increases with the accretion rate, and for accretion rates comparable to the Eddington limit is close to 1. This variation stabilizes the, radiation pressure region, so that the disk regions are secularily stable.

SIMILARITIES BETWEEN THE INNER SOLAR SYSTEM AND THE PLANETARY SYSTEM OF PSR B1257+12

We call attention to the surprising similarity between the newly discovered planetary system around PSr B1257+12 and the inner solar system. The similarity is in the ratios of the orbital radii and the masses of the three planets.

Decrease of gravitational mass due to neutrino emission and shock revival in supernovae

Neutrino emission from the newly formed neutron star reduces the stars gravitational mass and thus reduces the depth of the gravitational potential well, out of which the mantle and envelope layers have to be pushed. We estimate that this effect can contribute ∼10% of the energy required for a Type II supernova explosion. It can thus facilitate the shock revival in the delayed-explosion scenario

PSR 0655 + 64 : an astrophysical laboratory for testing relativistic gravity theories

The binary radio pulsar PSR 0655+64 is shown to be an effective astrophysical laboratory for testing gravity theories that violate the strong equivalence principle (SEP). Its observed spin-down rate has been used by us recently to test one aspect of SEP violation: the variability of the gravitational constant with cosmic time. Here it is suggested that PSR 0655+64 has the characteristics required for testing another aspect of SEP violation: the emission of dipole gravitational radiation. The companion of PSR 0655+64 is a white dwarf with a specific binding energy much smaller than that of the neutron star

The effective tidal viscosity in close solar-type binaries

A major problem confronting the understanding of tidal evolution of close solar-type binaries is the inefficiency of the turbulent convection. The value of the effective viscosity estimated, in the framework of the mixing length theory (MLT), implies circularization timescales which are almost two orders of magnitude longer than observed. Moreover, the reduction of the effective viscosity due to the fast time-variation of the tidal shear in short period binaries, increases the discrepancy to about three orders of magnitude. This state of affairs has motivated suggestions that tidal orbital evolution, notably circularization occurs mainly during the pre-main-sequence phase. However, observational data accumulated over the recent decades imply that circularization does occur during the the main-sequence phase (Mazeh 2008). In this work, we examine the possibility that the apparent inefficiency of turbulent convection is merely a shortcoming of MLT approach. Indeed, a recent 3D numerical simulation (Penev et al. 2007), suggests that the true convective viscosity is probably larger than the MLT value and that the reduction due to the time-variation of the shear is not drastic. We employ a model for stellar turbulent convection (Canuto, Goldman & Mazzitelli 1996) to evaluate the effective viscosity both for a steady for and time dependent tidal shear. The model is physically based, self-consistent, and accounts for the full spectrum of the turbulent eddies. It has been found advantageous, compared to the MLT, in many applications. We use an analytic approximation to the turbulent spectrum to obtain the reduction of the efficiency due to the time-variation of the tide. The results are: (i) an enhanced effective viscosity (by a factor of ∼ 4.5), and more importantly (ii) only a mild reduction due to the time-variation of the tidal shear. Overall, for binaries with orbital period of 15 days the discrepancy is “only” a factor of ∼30 down from a factor of ∼1000. These encouraging results should motivate an investigation of rigorous non-analytic solutions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)