M. Livio

Accretion disc viscosity: how big is alpha?

We consider observational and theoretical estimates of the accretion disc viscosity parameter a. We find that in thin, fully ionized discs, the best observational evidence suggests a typical range a ∼ 0.1-0.4, whereas the relevant numerical simulations tend to derive estimates for a which are an order of magnitude smaller. We discuss possible reasons for this apparent discrepancy.

The Rest-Frame Ultraviolet Luminosity Density of Star-forming Galaxies at Redshifts z > 3.5*

We have measured the rest-frame λ ~ 1500 A comoving specific luminosity density of star-forming galaxies at redshift 3.5 < z < 6.5 (Lyman break galaxies [LBGs]) selected from deep, multiband images taken with the Hubble Space Telescope and the Advanced Camera for Surveys, obtained as part of the Great Observatories Origins Deep Survey (GOODS). The samples cover ~0.09 deg2 and are also relatively deep, reaching between 0.2L and 0.5L, depending on the redshift, where L is the characteristic UV luminosity of LBGs at z ~ 3. The specific luminosity density appears to be nearly constant with redshift over the range 3 < z < 6, although the measure at z ~ 6 remains relatively uncertain, because it depends on the accurate estimate of the faint counts of the z ~ 6 sample. If LBGs are fair tracers of the cosmic star formation activity, our results suggest that at z ~ 6, namely, at less than ~7% of the current cosmic age, the universe was already producing stars as vigorously as it did near its maximum several gigayears later, at 1 z 3.

Extracting Energy from Black Holes: The Relative Importance of the Blandford-Znajek Mechanism

We critically assess the role of the Blandford-Znajek mechanism in the powering of outflows from accretion disk-fed black holes. We argue that there is no reason to suppose that the magnetic field threading the central spinning black hole differs significantly in strength from that threading the central regions of the disk. In this case, we show that the electromagnetic output from the inner disk regions is expected in general to dominate over that from the hole. Thus the spin (or not) of the hole is probably irrelevant to the expected electromagnetic power output from the system. We also point out that the strength of the poloidal field in the center of a standard accretion disk has been generally overestimated, and discuss scenarios which might give rise to more significant central poloidal fields.

The Evolution of Disk Galaxies in the GOODS-South Field: Number Densities and Size Distribution*

We examine the evolution of the sizes and number densities of disk galaxies using the high-resolution images obtained by the Great Observatories Origins Deep Survey (GOODS) with the Advanced Camera for Surveys on the Hubble Space Telescope. The multiwavelength images are used to classify galaxies based on their rest-frame B-band morphologies out to redshift . In order to minimize the effect of selection biases, we confine our z ∼ 1.25 analysis to galaxies that occupy the region of the magnitude-size plane where the survey is ∼90% complete at all redshifts. The observed size distribution is consistent with a lognormal distribution as seen for the disk galaxies in the local universe and does not show any significant evolution over the redshift range . We 0.25 ≤ z ≤ 1.25 find that the number densities of disk galaxies remains fairly constant over this redshift range, although a modest evolution by a factor of 4 may be possible within the 2 j uncertainties. Subject headings: galaxies: evolution — galaxies: formation — galaxies: fundamental parameters — galaxies: structure

Variability in black hole accretion discs

Observations of accreting systems often show significant variability (10–20 per cent of accretion luminosity) on time-scales much longer than expected for the disc regions releasing most of the luminosity. We propose an explicit physical model for disc variability, consistent with Lyubarskiis general scheme for solving this problem. We suggest that local dynamo processes can affect the evolution of an accretion disc by driving angular momentum loss in the form of an outflow (a wind or jet). We model the dynamo as a small-scale stochastic phenomenon, operating on roughly the local dynamical time-scale. We argue that large-scale outflow can only occur when the small-scale random processes in neighbouring disc annuli give rise by chance to a coherent large-scale magnetic field. This occurs on much longer time-scales, and causes a bright large-amplitude flare as a wide range of disc radii evolve in a coherent fashion. Most of the time, dynamo action instead produces small-amplitude flickering. We reproduce power spectra similar to those observed, including a 1/ƒ power spectrum below a break frequency given by the magnetic alignment time-scale at the inner disc edge. However, the relation between the black hole mass and the value of the break frequency is less straightforward than often assumed in the literature. The effect of an outer disc edge is to flatten the spectrum below the magnetic alignment frequency there. We also find a correlation between the variability amplitude and luminosity, similar to that found in some active galactic nuclei.

The formation of binary and single nuclei of planetary nebulae

We model the evolution of binary systems and explore the possibilities of formation of single and binary planetary nebula nuclei (PNNs). Under the assumption that all stars are born in binary systems, we obtain a Galactic rate of PNN formation of 0.87 yr -1 . The model suggests that about 20% of all PNNs are single, while 66% are CO white dwarfs accompanied by main-sequence stars. Our results are consistent with the observations of close binary PNNs. However, a direct comparison is difficult due to poor statistics and selection effects which bias the observed sample. We used the obtained results to place constraints on parameters of common envelope evolution

The Nature of the Supersoft X-Ray Source RX J0513-69

We present spectroscopy and photometry of the LMC supersoft binary system RX~J0513.9-6951. We derive a refined spectroscopic period of P

ON THE RATE OF NOVAE IN GALAXIES OF DIFFERENT TYPES

=0.761pm0.004

Is the Bardeen-Petterson effect responsible for the warping and precession in NGC 4258?

~d, which is consistent with the value obtained from long term photometric monitoring (P

How special is the Solar system

=0.76278pm0.00005