High-speed photometry of the dwarf nova SS Cyg

P. K. Shternberg State Astronomical Institute. Translated from Astrofizika, Vol. 29, No. 3, pp. 433–439, November–December, 1988.     

On the accretion disc properties in eclipsing dwarf nova EM Cyg

In this paper we analyze the behavior of the unusual dwarf nova EM Cyg using the data obtained in April–October, 2007, at the Vyhorlat observatory (Slovak Republic) and in September, 2006, at the Crimean Astrophysical Observatory (Ukraine). During our observations, EM Cyg has shown outbursts in every 15–40 days. Because on the light curves of EM Cyg a partial eclipse of the accretion disc is observed, we applied the eclipse mapping technique to a reconstruction of the temperature distribution in the eclipsed parts of the disc. Calculations of the accretion rate for the system were made for the quiescent and the outburst states of activity for various distances.     

The effect of an optically thin region on stability of an accretion disk around a supermassive black hole

We analyse conditions of the innermost portion of an accretion disk and establish a set of equations for this region. A stable innermost region may exist, which can probably explain the observed UV and X-ray spectra, avoiding the unstable emission. We then discuss the detailed radial structure of a disk around a black hole for typical AGN parameters and obtain different kinds of- relationships for different regions of a disk. On the basis of this, we discuss the stability. A new type of cycle is present, which we call a double S shaped cycle. In this cycle, the extent of accretion rate variability is much larger than that in dwarf nova cycles. This probably solves the problem of violent variability of AGN. In the meantime, the very high accretion rate at the hottest state in limit cycles in the unstable region may provide continuous injection of matter to the jet and power the relativistic motion of the jet.     

On the asphericity of nova remnants caused by rotating white dwarf envelopes

The effect of rotating white dwarf envelopes in determining the structure of nova shells is examined. This is achieved by numerical hydrodynamic simulations of the flows around a binary star system. In previous studies of remnant formation, this rotation has not been included.
It is found that the structures formed in the flow are more consistent with observations of nova shells than the previous theoretical studies. The shells produced by the nova become more prolate with increasing white dwarf envelope rotation. Hence the rotation of white dwarf envelopes must be included in any future discussion of remnant formation.
A possible method of identifying the dominant process by which mixing of accreted and white dwarf matter takes place is suggested.     

Estimation of plasma parameters in an accretion column near the surface of accreting white dwarfs from their flux variability

We consider the behavior of matter in the accretion column that emerges under accretion in binary systems near the surface of a white dwarf. The plasma heated in a standing shock wave near the white dwarf surface efficiently radiates in the X-ray energy band. We suggest a method for estimating post-shock plasma parameters, such as the density, temperature, and height of the hot zone, from the power spectrum of its X-ray luminosity variability. The method is based on the fact that the flux variability amplitude for the hot region at various Fourier frequencies depends significantly on its cooling time, which is determined by the parameters of the hot zone in the accretion column. This allows the density and temperature of the hot matter to be estimated. We show that the characteristic cooling time can be efficiently determined from the break frequency in the power spectrum of the X-ray flux variability for accreting white dwarfs. The currently available X-ray instruments do not allow such measurements to be made because of an insufficient collecting area, but this will most likely become possible with new-generation large-area X-ray spectrometers.     

Importance of the accretion process in asteroid thermal evolution: 6 Hebe as an example

Abstract— Widespread evidence exists for heating that caused melting, thermal metamorphism, and aqueous alteration in meteorite parent bodies. Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of ²⁶Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high‐grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories.