An analysis of close binaries (CB) based on photometric measurements (VI)

The paper is devoted to the actual problematics in the determination of orbital and physical parameters of active CB on the basis of the interpretation of photometric observations. One solves the problem in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding CB model (Djuraevi, 1992a) are given a priori (direct problem) and by determining the parameters of the given model for which the best fit between the synthetic light curve and the observations is achieved (inverse problem) (Djuraevi, 1992b). In this paper, the above procedure is applied to a particular case of an interpretation of CB light curves WZ Cep and FT Lupi.     

An analysis of close binaries (CB) based on photometric measurements (VII)

The paper is devoted to the actual problematics in the determination of orbital and physical parameters of active CB on the basis of the interpretation of photometric observations. One solves the problem in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding CB model (Djuraevi, 1992a) are given a priori (direct problem) and by determining the parameters of the given model for which the best fit between the synthetic light curve and the observations is achieved (inverse problem) (Djuraevi, 1992b). In this paper, the above procedure is applied to a particular case of an interpretation of CB light curves AU Ser and RV Corvi.     

An analysis of active close binaries (CB) based on photometric measurements

This is the first in a series of papers devoted to the actual problematics in the determination of orbital and physical parameters of active CB on the basis of the interpretation of photometric observations. One solves the problem in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding CB model are givena priori (direct problem) and by determining the parameters of the given model for which the best fit between the synthetic light curve and the observations is achieved (inverse problem (see Djura?evi?, 1991). In the first article of the series one presents the basis of the model developed for the synthesis of asymmetric, deformed, light curves of active CB with spots on their components. The modelling of the CB systems is based on the principles orginated in the Wilson and Devinney (1971; hereafter referred to as WD) model for the synthesis of a light curve generalised to include also the case of a nonsynchronous rotation of the components. The shapes of the components correspond to the equipotentials in the Roche model so that the critical Roche limits can be filled up to an arbitrary degree. In a spherical-coordinate system the surfaces of the components are divided into a large number of elementary cells whose intensity and angular radiation distribution are determined by the star temperature. limb darkening, gravitational darkening, and by the effect of reflection in the system. The active regions are approximated with circular spots. The presence of spots (dark or hot) enables to explain the asymmetry and depressions on the light curves of active CB. The model enables to be also interpreted the light curves of classic CB (without spots).     

An analysis of active close binaries (CB) based on photometric measurements

In order to enable a successful application of the realised CB models (presented in the two former articles of the series) in the analysis of the observed light curves, an efficient method unifying the best properties of the gradient method and of the differential-corrections one into a single algorithm (Djuraevi, 1991) is proposed. This method is realised by modifying the Marquardt (1963) algorithm. The inverse problem is solved in an iterative cycle of corrections to the model elements based on a nonlinear least-square method.The interpretation of photometric observations is based on the choice of optimal model parameters yielding the best agreement between an observed light curve and the corresponding synthetic one. Some of these parameters can be determineda priori in an independent way, while the others are found by solving the inverse problem.The programmes for analysing light curves find the optimal system parameters rapidly, reliably and correctly. The corresponding programme support enables an evident graphic presentation of the results.     

An Analysis of the Light Curves of the Early-Type Binary BF Aurigae

The UBV observations of the massive binary BF Aur were made at the Ankara University Observatory during 1988, 1989 and 1996. Asymmetry of the light curves, arising from unequal height of successive maxima, indicates that the system is active. By analysing these observations in the framework of the Roche model (including the presence of bright regions on the components) one obtains a semidetached configuration of the system, with the cooler secondary component filling its Roche lobe. The analysis of the light curves yields consistent solutions for mass ratio q = m₂/m₁ somewhat less than one. The influence of the mass transfer on the change of the system-orbital-period is relatively small. The upward parabolic character of the O–C diagram (Zhang et al., 1993) indicates a mass transfer from the less massive secondary to the more massive primary. This inturn requires the less massive secondary to fill its Roche lobe. This is consistent with our solution. Based on these facts we introduced the following working hypothesis. At the place where the gas stream from the secondary falls on the primary, relatively small in size but a high temperature contrast active hot-spot (hs) region is formed. As a result of the heating effect caused by the irradiation of the hot-spot region, on the secondary's side facing the hot spot a bright-spot (bs) region is formed. The bright-spot region is larger in size but with significantly lower temperature than the hot spot. This region can be treated as a ‘reflection cap’. By analysing the light curves in the framework of this working hypothesis the basic parameters of the system and the active regions are estimated. The problem is solved in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding Close Binary (CB) Roche model (Djurašević, 1992a) are given a priori (the direct problem) and by determining the parameters of the given model for which the best fit between the synthetic light curve and the observations is achieved (the inverse problem) (Djurašević, 1992b). This revised version was published online in July 2006 with corrections to the Cover Date.     

An analysis of close binaries (CB) based on photometric measurements (VIII)

The author considers current problems in the determination of the orbital and physical parameters for active close binaries (CB) of W Ser type based on the interpretation of photometric observations (Djuraevi, 1991). In this particular case, one analyses the light curves of CB SX Cas in the framework of the accretion-disc model explained in the second paper of this series (Djuraevi, 1992a). The light curve is analysed by applying the inverse-problem method described in the third paper of the series (Djuraevi, 1992b). On the basis of the observational material in theB andV filters (Shao, 1967), the light curves, the fundamental orbital and physical parameters of the system are estimated. In the paper a graphical illustration of the obtained solutions is also given. The obtained results confirm the general picture about the active processes in CB of W Ser type.     

An analysis of close binaries (CB) based on photometric measurements (V)

The author considers the current problematics in the determination of the orbital and physical parameters for active close binaries (CB) of W Ser type based on the interpretation of photometric observations. In the particular case one analyses the light curves of CB RX Cas in the framework of the accretion-disc model explained in the second paper of this series (Djuraevi, 1992a). The change of the light curves with that of the system's physical-activity phase is analysed and the orbital and physical parameters of the system are determined for the maximum, minimum and the transition regime of the physical activity by applying the inverse-problem method described in the third paper of the series (Djuraevi, 1992b). In the paper a graphical illustration of the solutions obtained is also given.     

Real-time abnormal light curve detection based on a Gated Recurrent Unit network

Targeting the problem of high real-time requirements in astronomical data processing, this paper proposes a real-time early warning model for light curves based on a Gated Recurrent Unit(GRU) network.Using the memory function of the GRU network, a prediction model of the light curve is established, and the model is trained using the collected light curve data, so that the model can predict a star magnitude value for the next moment based on historical star magnitude data. In this paper,we calculate the difference between the model prediction value and the actual observation value and set a threshold. If the difference exceeds the set threshold, the observation value at the next moment is considered to be an abnormal value,and a warning is given. Astronomers can carry out further certification based on the early warning and in combination with other means of observation. The method proposed in this paper can be applied to real-time observations in time domain astronomy.     

An analysis of close binaries (CB) based on photometric measurements

The subject of the paper is the problem of stellar differntial rotation in close binaries (CB) ofRS CV n type. The differential-rotation parameters we find on the basis of the migration of the depression in the light curves caused by the spot effect over the orbital phase. For that purpose, a simple model (Bussoet al., 1985) and inverse-problem procedure, based on the Marquardt (1963) algorithm, are used. To verify the obtained solutions, the SIMPLEX algorithm (Torczon, 1991) is applied, suitable for the nonlinear parameter optimisation. This algorithm enables a correct solution of the nonlinear equation system describing the differential rotation. The procedure is applied in the determination of the parameters of differential rotation forCV Cam, VV Mon andSS Boo binaries.     

An Analysis of Gsc0008.324, Newly Discovered Binary Star

A separate solution of the R *light curves was carried out using the 1998version of the Wilson's synthetic light curve code. In this paper we considered observations of Robb et al. (1999) for analysis. The photometry showed that star exhibits variable and asymmetric light curve. The asymmetric in the light curve is fitted with a spot on the surface of the hotter component. The fundamental orbital and physical parameters of this system have determined. The mass ratio is defined to be always q ≤ 1. The results indicate that the system is a late-type detached binary so that the components seem to be a very close pair of dwarfs. This revised version was published online in July 2006 with corrections to the Cover Date.     

A model of light-curve synthesizing for dwarf novae and the analysis of the OY Car observations by application of the inverse-problem method

The paper contains a model synthesizing the light curves of novae and novae-like stars, as well as of active close binaries (CB) in the phase of an intensive matter exchange between the components with accretion onto a white dwarf. The model considers the radial and azimuthal temperature distributions in the disk enabling a successful interpretation of asymmetrically deformed light curves characteristic for these systems. The analysis of the observed light curves is performed by using the inverse-problem method (Djuraevi, 1992b) adapted to this model. In the particular case the parameters for the dwarf-novaOY Car are estimated on the basis of the U and B observations (Woodet al., 1989).The synthetic light curves obtained through the inverse-problem solving, as a whole, fit the observations well which indicates that it is possible to estimate the system parameters on the basis of the model proposed here.The obtained results indicate a complex hot-spot structure approximated in the model with two components—a central part and a surrounding spot larger in size. The central hot-spot part (temperature about 10000 K is surrounded asymmetrically by the larger spot lower in temperature (about 7000 K). The radiation of the central hot spot is beamed forward by about 20°. The angular size of the hot-spot central part is about 5°, the centre longitude is 322°, whereas for the surrounding spot the size is about 33° and the longitude of the centre about 300°.For the mass ratio of the componentsq=0.102 one finds for the orbit inclination about 83°.8. The analysis shows that the disk radius is about 51% of the corresponding Roche lobe radius.Based on the U and B light curves the quiescent disk-edge temperature is estimated to about 5500 K (U), i.e. 4400 K (B). The disk-radial-temperature profile is much flatter than in the steady-state-approximation case. Beginning from the edge towards the disk centre the temperature slowly increases attaining about 7200 K (U), i.e. 5700 K (B) near the white dwarf. The differences in the solutions for the U and B light curves can be due to deviations in the disk radiation from the black-body approximation assumed in the present model. Expressed in the units of the distance between the component centres [D=1] the disk size is estimated to about 0.304 [D=1], its thickness to 0.014 [D=1], and the white-dwarf radius to about 0.02 [D=1]. The white-dwarf temperature is about 15000 K.The obtained results are in a relatively good agreement with the system parameters estimated earlier (Woodet al., 1989). This indicates that the proposed model of the system and the corresponding inverse-problem method briefly presented here are fully applicable to the analysis of active CB light curves in this evolutionary phase. Though the model given here includes a number of approximations, it enables an independent procedure in the observational-material analysis based on the light-curve synthesis and on the application of the inverse-problem method. Results obtained by applying such an independent method can also serve as a reasonable way in testing the solutions obtained by utilising the earlier approaches.     

Simulations and interpretation of the 6-cm MERLIN images of the classical nova V723 Cas

We compare the predictions of simple models for the radio emission from classical novae with the MERLIN radio observations of nova V723 Cas (Nova Cas 1995). Spherically symmetric and ellipsoidal radiative transfer models are implemented in order to generate synthetic emission maps. These are then convolved with an accurate representation of the u v coverage of MERLIN. The parameters and geometry of the shell model are based on those returned by fitting models to the observed light curve. This allows direct comparison of the model images with the nine 6-cm MERLIN images of V723 Cas.
It is found that the seemingly complex structure (clumping, apparent rotation) evident in the observations can actually be reproduced with a simple spherical emission model. The simulations show that a 24-h track greatly reduces the instrumental effects and the synthetic radio map is a closer representation of the true (model) sky brightness distribution. It is clear that interferometric arrays with sparse u v coverage (e.g. MERLIN, VLBA) will be more prone to these instrumental effects especially when imaging ring-like objects with time-dependent structure variations. A modelling approach such as that adopted here is essential when interpreting observations.     

Analysis of light curves produced by surface inhomogeneities on magnetic CP stars

The problem of the determination of surface brightness distribution parameters from the observed CP2-star variability, usually explained with the “oblique rotator model”, is discussed. A simple geometrical model of the surface brightness distribution is derived from the common properties of the observed light curves of these stars. This “spot model” which is supported from the known facts concerning the magnetic field structure and the surface distributions of chemical elements serves as a basis of the special inverse problem: the determination of the number of large scale inhomogeneities, their locations and extents and further parameters, from all the observed light curves of a given star. A suitable technique for solving the special inverse problem is explained. The problem of ambiguity which even arises for the proposed simple model and, in connection with that, the remaining possibilities to win the relevant information on the inhomogeneities of surface brightness are discussed. For the purpose of illustration, the result of the light curve analysis of the CP2 star HD 8441 is given.     

The Visual Light Curve Of C/1995 O1 (Hale-Bopp) From Discovery To Late 1997

We present a light curve of C/1995 O1 (Hale-Bopp) compiledfrom more than 3000 visual observations of the comet made by members of the The Astronomer Group world-wide. These observations cover the period from discovery through to the end of 1997. The light curve shows that the rate of brightening of the comet varied widely at different times, with rapid rates of brightening at high heliocentric distance pre-perhelion and a comparably rapid post-perihelion fade. There is no evidence that the comet was suffering a large photometric outburst when first discovered, although a small outburst can be identified at perihelion. At least five difficult brightening regimes can be identified in the light curve between discovery and perihelion. From 2.5 AU to perihelion the rate of brightening with decreasing heliocentric distance was typical for “fairly” new comets(n ∼ 3.5, where “n” is the power law exponent of the heliocentric distance), although this was preceded by a period of very slow brightening with n ∼ 1 from r ∼ 4.0 AU to r ∼ 2.8 AU and followed by an initially more rapid brightening which appears to be related to the on-set of rapid water sublimation activity. We derive the light curve parameters at different stages of the comet's apparition. This revised version was published online in July 2006 with corrections to the Cover Date.     

An analysis of active close binaries (CB) based on photometric measurements

For the purpose of analysing light curves of active CB with an accretion disc being at the evolutional phase of an intensive matter exchange between the components a model for light-curve synthesis has been realized where the attention is given to systems like W Ser not sufficiently examined yet with regard that in them the accretion disc is formed around an ordinary star. In the paper one uses the elements presented in the first paper of this series. The model can successfully describe the essential characteristics of the observed light curves due to existence of an accretion disc and a hot spot, as well as those originated in the temperature distribution along the disc radius. The system components are considered in the framework of the nonsynchronous Roche model and the accretion disc of a constant thickness lies in the orbital plane around the star capturing the matter of the neighbouring component.The primary surrounded by the disc is situated relatively well within the Roche oval and its rotation can be significantly nonsynchronous. Near the Lagrange equilibrium pointL ₁ flows from the secondary (which fills the Roche limit) the gas stream nourishing the disc. In the zone where the stream touches the lateral side of the disc a hot spot is formed.The proposed model enables estimating of the basic orbital and physical parameters of active type W Ser CB (Djuraevi, 1991) on the basis of photometric measurements by applying the inverse-problem method.     

Stellar disruption by a supermassive black hole: is the light curve really proportional to t−5/3?

In this paper, we revisit the arguments for the basis of the time evolution of the flares expected to arise when a star is disrupted by a supermassive black hole. We present a simple analytic model relating the light curve to the internal density structure of the star. We thus show that the standard light curve proportional to   t ^−5/3  only holds at late times. Close to the peak luminosity the light curve is shallower, deviating more strongly from   t ^−5/3  for more centrally concentrated (e.g. solar type) stars. We test our model numerically by simulating the tidal disruption of several stellar models, described by simple polytropic spheres with index γ. The simulations agree with the analytical model given two considerations. First, the stars are somewhat inflated on reaching pericentre because of the effective reduction of gravity in the tidal field of the black hole. This is well described by a homologous expansion by a factor which becomes smaller as the polytropic index becomes larger. Secondly, for large polytropic indices wings appear in the tails of the energy distribution, indicating that some material is pushed further away from parabolic orbits by shocks in the tidal tails. In all our simulations, the   t ^−5/3  light curve is achieved only at late stages. In particular, we predict that for solar-type stars, this happens only after the luminosity has dropped by at least 2 mag from the peak. We discuss our results in the light of recent observations of flares in otherwise quiescent galaxies and note the dependence of these results on further parameters, such as the star/hole mass ratio and the stellar orbit.     

The Cosmological Problem as Initial Value Problem on the Observer's Past Light Cone: Observations

In a previous paper it was shown how cosmological models can be characterized by few initial data on the observer's past light cone. Part of the inner geometry of the past cone as well as some matter variables taken on the cone (in the simplest case the velocity and density of a cosmological fluid) determine a world model uniquely within the past light cone. Since the initial data to some degree enter the relations between observable quantities such as redshift, luminosity, angular diameter, number counts, distortion parameters and background radiation intensity, it might be possible to determine the required initial data from observations. This problem is discussed in the present article for some observable relatios involving the quantities just mentioned.     

3D Hydrodynamical Simulations of Colliding Wind Binaries: TheoryConfronts Observations

The link between gas dynamical models and observations is crucial. The general progress in numerical simulations must be accompanied by predictions for observable quantities, which not only allow to test the models or parts of them against observations but which also improve the understanding of observational data. In this paper we focus on predictions for observations, for three examples of 3Dhydrodynamical simulations of binary star systems, and the techniques required for their derivation. The examples include synthetic, optically thin Doppler broadened line profiles for colliding wind symbiotic binaries, the UV light curve of an accretion model for the symbiotic binary RW Hya, and the X-ray light curve of the WR+O binaryγ Velorum. The main purpose is to emphasize the importance of such studies and to illustrate the capabilities of the emploiedtools. The tools are all contained in the A-MAZE code package we have developed and are publicly available. Seminar for Applied Mathematics This revised version was published online in July 2006 with corrections to the Cover Date.     

Cristobalite- and tridymite-bearing clasts in Parnallee (LL3) and Farmington (L5)

Abstract— A set of cristobalite- and tridymite-rich igneous clasts (CB1 to CB8) have been found in Parnallee (LL3.6). They consist of clinoenstatite, minor feldspathic mesostasis and cristobalite veined by endiopsideaugite. The largest clast, CB8, is 1.6 cm in diameter and contains veined tridymite and cristobalite, clinoenstatite (zoned to ferroaugite and pyroxferroite Fs_75.6Wo_20.0) and plagioclase. Compared to bulk ordinary chondrites (OC), the bulk clasts are depleted in Al (0.02–0.8× OC), Na and K and enriched in Si (1.6–2.0× OC) and Ca (1.3–4.5× OC). Bulk CB8 has LREE > HREE (La/Lu = 1.6) with a positive Eu anomaly (Eu/Eu^* = 2.4). Textural observations suggest that the clasts cooled rapidly (24–420 °C/h) above 1200 °C. Clasts CB1—CB8 contain the isotopically heaviest O yet found in ordinary chondrites (up to δ¹⁷O = +8.7%_o, δ¹⁸O = +11.6%_o). Enrichment in the heavy isotopes of O is dependent on the proportion of cristobalite (or tridymite) in the clasts. A regression line CRIL (Cristobalite Line), with slope 0.77, is defined by the isotopic compositions of CB1—CB8, the Farmington clast and ordinary chondrite chondrules. An ¹⁶O-poor gas reservoir, whose composition must lie at some point along the extension of CRIL, has undergone varying degrees of isotopic exchange with most ordinary chondrite material. Silica polymorphs have undergone the greatest degree of exchange because of their open, framework structures. Silicon in CB1—CB8 has normal isotopic ratios. A model is proposed that involves differentiation of H-group material through extraction of volatile elements in a vapour phase, loss of an Fe-Ni-S melt and metastable crystallisation (60–70%) of olivine. The calculated residual liquid is silica-oversaturated and its subsequent predicted crystallisation sequence resembles that preserved in CB1—CB8. This model may require two stages of heating, the second one prior to cristobalite crystallisation (if the silica polymorph crystallises within its predicted stability field of > 1500 °C). Isotopic exchange took place either when CB1—CB8 were ejected from their parent body due to impact or near the surface of the parent body, perhaps in an ejecta blanket setting. The latter option is preferred because it is more consistent with our igneous model.     

V723 Cas (Nova Cassiopeiae 1995): MERLIN observations from 1996 to 2001

MERLIN observations of the unusually slow nova V723 Cas are presented. Nine epochs of 6-cm data between 1996 and 2001 are mapped, showing the initial expansion and brightening of the radio remnant, the development of structure and the final decline. A radio light curve is presented and fitted by the standard Hubble flow model for radio emission from novae in order to determine the values of various physical parameters for the shell. The model is consistent with the overall development of the radio emission. Assuming a distance of 2.39 (±0.38) kpc and a shell temperature of 17 000 K, the model yields values for expansion velocity of  414 ± 0.1 km s^-1  and shell mass of  1.13 ± 0.04 × 10⁻⁴ M_⊙  . These values are consistent with those derived from other observations although the ejected masses are rather higher than theoretical predictions. The structure of the shell is resolved by MERLIN and shows that the assumption of spherical symmetry in the standard model is unlikely to be correct.