Multiple time scales in cataclysmic binaries. The low-field magnetic dwarf nova DO Draconis

We study the variability of the cataclysmic variable DO Dra, on time-scales of between minutes and decades. The characteristic decay time dt/dm=0.902(3) days/mag was estimated from our 3 nights of CCD R observations. The quiescent data show a photometric wave with a cycle about 303(15)d. We analyzed the profile of the composite (or mean) outburst. We discovered however, that a variety of different outburst heights and durations had occurred, contrary to theoretical predictions. With increasing maximum brightness, we find that the decay time also increases; this is in contrast to the model predictions, which indicate that outbursts should have a constant shape. This is interpreted as representing the presence of outburst-to-outburst variability of the magnetospheric radius. A presence of a number of missed weak narrow outbursts is predicted from this statistical relationship. A new type of variability is detected, during 3 subsequent nights in 2007: periodic (during one nightly run) oscillations with rapidly-decreasing frequency from 86 to 47 cycles/day and a semi-amplitude increasing from 0.06 to 0. 10, during a monotonic brightness increase from 14. 27 to 14. 13. This phenomenon was observed only during an unusually prolonged event of about 1 mag brightening in 2007 (lasting till autumn), during which no (expected) outburst was detected. We refer to this behaviour as to the transient periodic oscillations (TPO). To study this new and interesting phenomenon, new regular photometric and spectral (in a target of opportunity mode) observations are required.Comment: 12pages, 8figures, accepted in Astronomy and Astrophysic

Orbital stability of solutions to the problem on capillary-gravity waves in two-fluid layer

Going back to N. E. Kochin [1] problem about capillary-gravity waves on interface of two-fluid flow is considered as bifurcation problem with spectral parameters

Secondary electron emission yield in the limit of low electron energy

Secondary electron emission (SEE) from solids plays an important role in many areas of science and technology.1 In recent years, there has been renewed interest in the experimental and theoretical studies of SEE. A recent study proposed that the reflectivity of very low energy electrons from solid surface approaches unity in the limit of zero electron energy2,3,4, If this was indeed the case, this effect would have profound implications on the formation of electron clouds in particle accelerators,2-4 plasma measurements with electrostatic Langmuir probes, and operation of Hall plasma thrusters for spacecraft propulsion5,6. It appears that, the proposed high electron reflectivity at low electron energies contradicts to numerous previous experimental studies of the secondary electron emission7. The goal of this note is to discuss possible causes of these contradictions.Comment: 3 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba, Ital

Magnetic field and unstable accretion during AM Herculis low states

A study of AM Her low states in September 1990 and 1991 and June-July 1997 is reported from a coordinated campaign with observations obtained at the Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory. Spectra obtained at different dates when the source was in low states at a comparable V magnitude, show the presence of strong Zeeman absorption features and marked changes in emission lines with a day-to-day reappearance of the HeII (4686\AA) emission lines in 1991. Despite this variability, the magnetic field inferred from the fitting of the absorption spectrum with Zeeman hydrogen splitting, is remarkably constant with a best value of (12.5$\pm$0.5)MG. Detailed analysis of the UBVRI light curves shows the presence of repetitive moderate amplitude ($\sim$ 0.3-0.5 mag) flares predominantly red in colour. These flares are attributed to small accretion events and are compared to the large ($\sim$ 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We suggest that the general flaring activity observed during the low states is generated by accretion events. The different characteristics of the flares (colour and polarization) are the results of different shock geometries depending on the net mass accretion flux.Comment: accepted in Astronomy & Astrophysics (Main Journal), 10 pages, 6 Figures, Late

Complete Parallax and Proper Motion Solutions For Halo Binary-Lens Microlensing Events

A major problem in the interpretation of microlensing events is that the only measured quantity, the Einstein time scale t_E, is a degenerate combination of the three quantities one would like to know, the mass, distance, and speed of the lens. This degeneracy can be partly broken by measuring either a "parallax" or a "proper motion" and completely broken by measuring both. Proper motions can easily be measured for caustic-crossing binary-lens events. Here we examine the possibility (first discussed by Hardy & Walker) that one could also measure a parallax for some of these events by comparing the light curves of the caustic crossing as seen from two observatories on Earth. We derive analytic expressions for the signal-to-noise ratio of the parallax measurement in terms of the characteristics of the source and the geometry of the event. For Galactic halo binary lenses seen toward the LMC, the light curve is delayed from one continent to another by a seemingly minuscule 15 seconds (compared to t_E ~ 40 days). However, this is sufficient to cause a difference in magnification of order 10%. To actually extract complete parallax information (as opposed to merely detecting the effect) requires observations from three non-collinear observatories. Parallaxes cannot be measured for binary lenses in the LMC but they can be measured for Galactic halo binary lenses seen toward M31. Robust measurements are possible for disk binary lenses seen toward the Galactic bulge, but are difficult for bulge binary lenses.Comment: Revised to take account of important work by Hardy & Walker (1995

Nova-Like Cataclysmic Variable TT Ari: QPO Behaviour Coming Back From Positive Superhumps

We study the variability of the nova-like cataclysmic variable TT Ari, on time-scales of between minutes and months. The observations in the filter R were obtained at the 40-cm telescope of the Chungbuk National University (Korea). TT Ari was in the "negative superhump" state after its return from the "positive superhump" state, which lasted 8 years. The ephemeris for 12 best pronounced minima is $T_{min}=BJD 2453747.0700(47)+0.132322(53)E.$ where numbers in digits are errors in units of the last digit. The phases of minima may reach 0.2, indicating non-eclipse nature of these minima. The quasi-periodic oscillations (QPO) are present with a mean "period" of 21.6 min and mean semi-amplitude of 36 mmag. This value is consistent with the range 15-25 minutes reported for previous "negative superhump" states and does not support the hypothesis of secular decrease of the QPO period. Either the period, or the semi-amplitude show significant night-to-night variations. According to the position at the two-parameter diagrams, the interval of observations was splitted into 5 parts, showing different characteristics which are discussed in this paper. The system is an excellent laboratory to study processes resulting in variations at time-scales from seconds to decades and needs further monitoring at various states of activity.Comment: 14pages,10figures, accepted in Astronomy and Astrophysic

Stellar models of evolved secondaries in CVs

In this paper we study the impact of chemically evolved secondaries on CV evolution. We find that when evolved secondaries are included a spread in the secondary mass-orbital period plane comparable to that seen in the data is produced for either the saturated prescription for magnetic braking or the unsaturated model commonly used for CVs. We argue that in order to explain this spread a considerable fraction of all CVs should have evolved stars as the secondaries. The evolved stars become fully convective at lower orbital periods. Therefore, even if there was an abrupt decrease in magnetic braking for fully convective stars (contrary to open cluster data) it would not be expected to produce a sharp break in the period distribution for CVs. We also explore recent proposed revisions to the angular momentum loss rate for single stars, and find that only modest increases over the saturated prescription are consistent with the overall observed spindown pattern. We compare predictions of our models with diagnostics of the mass accretion rate in WDs and find results intermediate between the saturated and the older braking prescription. Taken together these suggest that the angular momentum loss rate may be higher in CV secondaries than in single stars of the same rotation period, but is still significantly lower than in the traditional model. Alternative explanations for the CV period gap are discussed.Comment: 24 pages, 9 figures. Submitted to Ap