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# HD 143454

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 A Combination Nova'' Outburst in Z Andromedae: Nuclear Shell Burning Triggered by a Disk InstabilityWe describe observational evidence for a new kind of interacting binarystar outburst that involves both an accretion instability and anincrease in thermonuclear shell burning on the surface of an accretingwhite dwarf. We refer to this new type of eruption as a combinationnova. In late 2000, the prototypical symbiotic star Z Andromedaebrightened by roughly 2 mag in the optical. We observed the outburst inthe radio with the VLA and MERLIN, in the optical both photometricallyand spectroscopically, in the far-ultraviolet with FUSE, and in theX-rays with both Chandra and XMM-Newton. The 2 year long event had threedistinct stages. During the first stage, the optical rise closelyresembled an earlier, small outburst that was caused by an accretiondisk instability. In the second stage, the hot component ejected anoptically thick shell of material. In the third stage, the shell clearedto reveal a white dwarf whose luminosity remained on the order of104 Lsolar for approximately 1 yr. The eruptionwas thus too energetic to have been powered by accretion alone. Wepropose that the initial burst of accretion was large enough to triggerenhanced nuclear burning on the surface of the white dwarf and theejection of an optically thick shell of material. This outbursttherefore combined elements of both a dwarf nova and a classical nova.Our results have implications for the long-standing problem of producingshell flashes with short recurrence times on low-mass white dwarfs insymbiotic stars. Searching for Flickering Variability in Several Symbiotic Stars and Related Objects: BX Mon, V471 Per, RS Oph, V627 Cas, CI Cam, V886 Her, Z And, T CrB, MWC 560, V407 CygUBVRI photometric observations of 10 symbiotic stars and related objectsobtained in the period 2002-2003 are presented. Analyzing differentiallight curves we found rapid light variations with timescales of tens ofminutes and significant amplitudes in the well-known flickers MWC 560,RS Oph, V407 Cyg and T CrB. MWC 560 and V407 Cyg demonstrate quasiperiodic oscillations (QPO) with similar amplitudes and timescales. Flickering and unusual flare in V627 Cas as well as some indications offlickering presence in BX Mon are detected. The existence of 29 minoscillations in Z And with an amplitude approx 0.02 mag in the U-band isconfirmed. Only one symbiotic star, V471 Per, and both non symbiotic, CICam and V886 Her, seem to be constant on flickering timescales. Nevertheless, small night to night changes in the brightness of V886 Herwere observed as well. BVI photometry and the spectroscopy of Nova Scuti 2005 N.2Our CCD photometry of Nova Scuti 2005 N.2 (=V477 Sct) shows it to be avery fast nova, which is characterized by t2 = 3 andt3 = 6 days, affected by a EB-V ≥ 1.3 magreddening, and which peaked at V ~ 9.8 mag on ~Oct. 12.0 UT. The novawas probably entering a dust condensation episode or brightnessoscillations during the transition phase when it became unobservable forthe seasonal conjunction with the Sun. Absolute spectrophotometry showsit to belong to the He/N class. The emission line width at halfintensity is 2600 km s-1. At least five ripples areidentified in the high resolution emission lines profiles at radialvelocities ranging from -980 to +700 km s-1. The nova eruptedat a large distance from the Sun and at an appreciable height above theGalactic plane, suggesting an association with the Galactic bulge(unusual for a He/N nova). The progenitor was too faint to be recordedon DSS1/2 survey plates, when setting the outburst amplitude to ΔV ≥ 11 mag. The nature of ultraviolet spectra of AG Pegasi and other symbiotic stars: locations, origins, and excitation mechanisms of emission linesA detailed study of ultraviolet spectra of the symbiotic star AG Peg hasbeen undertaken to derive the atomic excitation mechanisms and origin offormation for the lines common in symbiotic systems. More than 600emission lines are observed in spectra from {IUE}, {HST} and {FUSE} ofwhich 585 are identified. Population mechanisms and origin of formationare given for a majority of those lines. Based on the understanding ofthe AG Peg spectra {IUE} data of 19 additional symbiotic stars areinvestigated and differences and similarities of their spectra arediscussed. Fe II fluorescence lines pumped by strong emission linesbetween 1000 and 2000 Å are observed in 13 of these systems. Someof the symbiotic systems belonging to the subclass symbiotic novae havemore than 100 Fe II fluorescence lines in the ultraviolet wavelengthregion. Forbidden lines are detected for 13 of the stars, mostly fromhighly-ionized spectra such as Ar V, Ne V and Mg V. Further, [Mg VI] and[Mg VII] lines are observed in a symbiotic star (AG Dra) for the firsttime. Five of the symbiotic stars have broad white-dwarf wind profiles({FWHM} > 400 km s-1) for a few lines in their spectra.The stars with no such broad lines can be divided into two similarlysized groups, one where all lines have FWHM less than 70 kms-1 and the other where one, a few or all of the broad({FWHM} > 400 km s-1) lines of AG Peg have an enhancedbroad wing (110-140 km s-1). The OGLE-II event sc5_2859: a classical nova outburst?Context: .The OGLE-II event sc5_2859 was previously identified as thethird longest microlensing event ever observed. Aims: .Additionalphotometric observations from the EROS (Expérience de Recherched'Objets Sombres) survey and spectroscopic observations of the candidatestar are used to test the microlensing hypothesis. Methods: .Thecombined OGLE and EROS data provide a high quality coverage of the lightcurve. The colour of the sc5_2859 event is seen to change with time. Aspectrum taken in 2003 exhibits a strong Hα emission line.Results: .The additionnal data show that the OGLE-II sc5_2859 event isactually a classical nova outburst. Rapid Hα Variability in T Coronae BorealisWe report on a search conducted for variability in the Hα emissionline of the recurrent nova T CrB with a time resolution of 10-15minutes. This is comparable to the timescale of the photometricflickering observed in this object. This is the first time thatobservations of the short-timescale emission-line variation have beenmade for this system. On two nights (1999 January 6 and 7), we detectedstatistically significant variability (at the 99% confidence level) inthe Hα line profile. This variability is confined to the centralpart of the emission line (+/-100 km s-1), althoughFWZI(Hα)~800 km s-1. The variability in the lineprofile is accompanied by variability of the total equivalent width,EW(Hα): +/-8% for 1999 January 6 and +/-6% for 1999 January 7(calculated from the mean EW value). Assuming Keplerian motion, thevariability is generated at a distance of ~20-30 Rsolar fromthe white dwarf, which is approximately the radius of the ring that thestream of gas forms as it flows away from the L1 Lagrangian point. Forthree other nights we are only able to put upper limits on thevariability, ΔEW(Hα): +/-2% for 1998 April 15, +/-4% for1998 August 2, and +/-3% for 1998 August 3.Based on observations obtained at the National Astronomical ObservatoryRozhen, Bulgaria. Emission line variability of RS Ophiuchi*We report that the Hα emission line of RS Oph was stronglyvariable during our 2004 observations on a time-scale of one month. Theline consisted of both a double-peaked central narrow component [fullwidth at half-maximum (FWHM) ~ 220 kms-1] and a stronglyvariable broad one (FWHM > 2000 kms-1). The base of theHα line was very broad, with full width at zero intensity ~4600kms-1 on all spectra from 1986 to 2004. The variability ofthe broad component extends from -2000 to +2000 kms-1. Mostprobably this is due either to blobs ejected from the white dwarf (witha typical blob mass estimated to be ~10-10Msolar)or to a variable accretion disc wind. We also detected variability ofthe HeIIλ4686 line on a time-scale shorter than 1 d. The possibleorigin is discussed. Bipolar jet growth and decline in Hen 3-1341: a direct link to fast wind and outburst evolution*We report on and investigate the evolution and disappearance in thesymbiotic star Hen 3-1341 of collimated bipolar jets, which take theform of symmetrically displaced components of emission lines. Frommodelling of the emission-line spectrum it turns out that the accretingwhite dwarf (WD) in quiescence has TWD~ 1.2 ×105 K and RWD~ 0.14 Rsolar, for aluminosity of 3.8 × 103 Lsolar, and it isstably burning hydrogen on the surface at a rate of , feeding ionizingphotons to a radiation bounded circumstellar nebula extending for ~17au. The WD underwent a multimaxima outburst lasting from 1998 to 2004during which its H-burning envelope reacted to a probable small increasein the mass accretion by expanding and cooling to Teff~ 1× 104 K and R~ 20 Rsolar, mimicking anA-type giant that radiated a total of ~6 × 1044 erg, atan average rate of ~1 × 103 Lsolar. Bipolarjets developed at the time of outburst maximum and their strengthdeclined in parallel with the demise of the fast wind from the inflatedWD, finally disappearing when the wind stopped halfway to quiescence,marking a 1:1 correspondence between jets presence and feeding action ofthe fast wind. The total mass in the jets was Mjet~ 2.5× 10-7 Msolar for a kinetic energy ofEkinjet~ 1.7 × 1042(sini)-1 erg, corresponding to ~0.3(sini)-1 percent of the energy radiated during the whole outburst. We suggest thatthe spectroscopic search for jets in symbiotic stars could pay higherdividends if focused on the outburst phases characterized by maximumwind intensity. Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.Not Available SWIFT/BAT Detections of Hard X-ray Sources IITable of BAT survey source identifications described in ATEL #668. IR Photometry of the Symbiotic Star BF Cyg: Detection of the Red Giant's Ellipsoidal Brightness VariabilityWe present the results of our IR photometric observations of theclassical symbiotic star BF Cyg acquired in 1978 2003. The variabilityrange in the J and K bands was 0.2m. A periodic component in thecool star’s brightness variations is clearly visible, its periodbeing half the orbital one and its J amplitude being 0.15m. Thiscomponent is associated with the ellipsoidal shape of the red giant,which model calculations show fills its Roche lobe. This is required inorder to reproduce ellipsoidal brightness variability with such a largeamplitude: the calculated amplitude for a red giant filling 90% of itsRoche lobe is half the observed value. At the same time, it was notpossible to confidently chose the optimum component-mass ratio, q = Mgiant /Mhot, and orbital inclination, i, from possible values in theranges q = 2 4, i = 70° 90°. Including the contribution from thehot radiation sources (the hot component and ionized envelope), whichvary with a period equal to the orbital period, has a considerableinfluence on the estimated parameters associated with the redgiant’s ellipsoidal brightness variations, and this contributioncannot be neglected. The deviations of the observed from the calculatedlight curve are irregular, with the rms deviation being σ(O-C)≈ 0.04m. Chandra Observations of the Recurrent Nova IM NormaeThe recurrent nova IM Nor was observed twice in X-rays with the ChandraACIS-S, 1 and 6 months after the optical outburst. It was not detectedin the first observation, with an upper limit on the X-ray luminosity inthe 0.2-10 keV rangeLX<4.8×1030(dkpc-1)2ergs s-1 (where d is the distance to the nova). Five monthslater, a hard X-ray source withLX=(1.4-2.5)×1032(dkpc)2 ergss-1 was detected. The X-ray spectrum appears to be thermal,but we cannot rule out additional components due to unresolved emissionlines. A blackbody component is likely to contribute to the observedspectrum, but it has bolometric luminosityLbol=2.5×1033(dkpc-1)2ergs s-1 therefore, it is not sufficiently luminous to becaused by a central white dwarf that is still burning hydrogen on thesurface. An optical spectrum, taken 5 months postoutburst, indicates nointrinsic reddening of the ejecta. Therefore, we conclude that the shellhad already become optically thin to supersoft X-rays, but nuclearburning had turned off or was in the process of turning off at thistime. We discuss why this implies that recurrent novae, even the rareones with long optical decays like IM Nor, indicating a large envelopemass, are not statistically significant as Type Ia supernova candidates. Colliding Winds in Symbiotic Binary Systems. I. Analytic and Numerical SolutionsWe present new formulations of binary colliding wind models appropriateto symbiotic star systems. The derived models differ from previousformulations in assuming mixing of the shocked material from bothincoming streams, rather than postulating a self-sustaining contactdiscontinuity. The CWb model (colliding winds, binary) extends the workof Girard and Willson by the derivation of an adiabatic temperature, theconsideration of radiative cooling, the inclusion of thermal pressuresin the incoming winds, and the treatment of interaction shells of finitethickness and density. The finite thickness of the interaction shellallows for calculation of its radiative intensity distribution. The CWcmodel (colliding winds, concentric) is a similar extension of the modelof Kwok, Purton, and Fitzgerald. It is derived in a manner parallel tothat of the CWb model, thereby facilitating a unification of the twomodels. A unified model is desired since wind collisions in symbioticsystems should include aspects of both CWb and CWc interactions. Twoexamples of model applications are presented: a comparison of the fluxdensities arising from colliding winds (CWb model) with those arisingfrom the ionization of the surrounding medium (STB model) in thegalactic population of symbiotic stars, and model imaging of thesymbiotic nova HM Sge. Time resolved spectroscopy of CI Aql.Not Available Disentangling the composite continuum of symbiotic binaries. I. S-type systemsWe describe a method of disentangling the composite, 0.12-5 μmcontinuum of symbiotic binaries. The observed SED is determined by theIUE/HST archival spectra and flux-points corresponding to the opticalUBVRI and infrared JHKLM photometric measurements. The modeled SED isgiven by superposition of fluxes from the cool giant, hot stellar sourceand nebula including the effect of the Rayleigh scattering process andconsidering influence of the iron curtain absorptions. We applied thismethod to 21 S-type symbiotic stars during quiescence, activity andeclipses. We isolated four main components of radiation and determinedtheir properties. (i) Stellar radiation from the giant corresponds to aunique luminosity class - normal giants. Characteristic luminosities are1600 ± 200 and 290 ± 30 Lȯ for red andyellow giants, respectively in our sample of objects. (ii) Hot objectradiation during quiescence consists of the nebular and stellarcomponent. The former radiates at a mean electron temperature of 19 000K and its amount of emission suggests a mass-loss rate from giants viathe wind at dot MW = a few × 10-7Mȯ yr-1. Radiation of the latter conformswell with that of a black-body photosphere at a characteristictemperature of 105 000 K. The corresponding effective radii are a factorof 10 larger than those of white dwarfs, which thus precludes observingthe accretor's surface. Extreme cases of AX Per and V443 Her, for whichthe hot star temperature from the fit is not capable of producing thenebular emission, signal a disk-like structure of the hot stellar sourceeven during quiescence. (iii) Hot object radiation during activityconsists of three components - the stellar and the low- andhigh-temperature nebular radiation. The stellar radiation satisfies thatof a black-body photosphere at a low characteristic temperature of 22000 K (we call it the 1st type of outbursts) or at a very highcharacteristic temperature of ≈165 000 K (2nd type of outbursts). Allthe active objects with a high orbital inclination show features of the1st-type of outbursts (here Z And, AE Ara, CD-43circ14304, TXCVn, BF Cyg, CH Cyg, CI Cyg, AR Pav, AX Per), while AG Dra representsthe 2nd-type. The presence of a two-temperature type of UV spectrum andan enlargement of effective radii of the stellar source by a factor of 10 with respect to the quiescent values during the 1st-type of outburstsuggest an expansion of an optically thick medium at the orbital planein the form of a disk. The low-temperature nebula radiates at a meanelectron temperature of 14 000 K and is subject to eclipses, while thehigh-temperature nebula, which is seen during eclipses as the onlycomponent, is characterized by Te > 30 000 K. Radiativeand geometric properties of the main sources of radiation allowed us toreconstruct a basic structure of the hot object during the 1st-type ofoutburst. There is an edge-on disk around the accretor. Its outer flaredrim represents a warm pseudophotosphere of the hot stellar source, whoseradiation is Rayleigh attenuated and affected by the iron curtainabsorptions in the neutral gas concentrated at the orbital plane. Thelow-temperature nebula is placed just above/below the disk with aconcentration at its edge as to be subject to eclipses and to“see” well the central ionizing source. High above/below theorbital plane, there is a hot nebular emitting region. Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclustersThe availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165} Fe II fluorescence in symbiotic starsFe 0 fluorescence by PAR has been investigated in eight symbiotic starshaving a wide range in temperature of the hot component and orbitalperiod. The data used are spectra obtained from the IUE archive. Allpumping lines investigated in this work are in the short wavelengthregion of IUE (1200-2000 Å), except for He 0 λ 1084.942 andO 0 λ 1032.041. The resulting Fe 0 fluorescence lines are mainlyin the long wavelength region (2000-3300 Å), but a few fall in thesame region as the pumping lines. The aim is to understand the optimalconditions for formation of Fe 0 fluorescence lines caused by PAR. Threeof the selected systems, RR Tel, AG Peg and V1016 Cyg, have 10-30 activeFe 0 channels. Two conditions connect those systems to each other: Thehot component is a white dwarf of extreme temperature (80 .103-150 . 103 K) and all three systems are socalled symbiotic novae and have had outbursts during the last 150 years.Three systems, AG Dra, RW Hya and R Aqr, have only 2-3 active Fe 0channels. In the two remaining systems, CI Cyg and T CrB, Fe 0fluorescence lines were totally absent. These two systems have twofeatures in common: The emission strength of highly ionized elements isless than in most symbiotic systems, and the hot component is suspectedto be an accreting main sequence star rather than a white dwarf. The New View on the Long-Therm Flickering in T CrBThe big scatter of data on the light curve of T CRB is identified withthe flickering activity of the system. The data performed during April1996 have a falling trend and they can be a part of a downward branch ofa long-term and energetically powerful flare. Estimated energy 21035 J and the duration of this event were compared with thetheoretical assumptions based on three typical physical scenarios, whichcan be the source of flickering. The dissipation of magnetic loops andthe existence of the turbulent eddies are energetically deficient, butthese scenarios are real in the case of less powerfull flares. The realexplanation could be the instability of the secondary and variable masstransfer rate through the Lagrangian point, therefore through wholedisc. Spectroscopic Diagnostics of Symbiotic Stars. II. Statistical Analyses of Highly Resolved Emission-Line ProfilesWe obtained highly resolved spectroscopic data of 34 symbiotic stars atHα, He II λ 4686, and [O III ] λ 5007. We analyzed the line profilesstatistically to obtain clear common characteristics of the emissionnebulae in symbiotic stars. We first carried out coarse analyses, whichsuggested differences in the line profiles between the quiescent andoutburst phases. Second, we de-convolved the line profiles with multipleGaussian components, and statistically dealt with each component inorder to extract certain characteristics as a group. In the case ofalmost edge-on binary orbits, the relation between the characteristicvelocities of the main, the broad wing, and the absorption components ofHα versus the orbital phase can be explained by the existence andinteraction of stellar winds from cool and hot stars. The difference inthe width of the main component of He II λ4686 between the quiescent and the outburst phases and the changes intheir radial velocities with the orbital phase support the idea thatHe++ gas would distribute around a hot star. Double-peakedprofiles of [O III ] λ 5007 lines and theirrelation to the orbital phase can be explained by bi-conical flows. Flickering variability of T Coronae Borealis We present electro-photometric UBV and high-speed U-band flickeringobservations of the recurrent nova T CrB during a period when its Ubrightness varies by more than 2 mag. The V band is dominated by theellipsoidal variability of the red giant; however, the variability ofthe hot component also causes ~0.15-mag variations in V. We define a setof parameters that characterize the flickering. The Fourier spectra ofall 27 nights are similar to each other. The power spectral density ofthe variations has a power-law component (~f-1.46 onaverage). We do not detect a dependence of the Fourier spectra andautocorrelation function on the brightness of the object. Havingsubtracted the contribution of the red giant, we show that theflickering amplitude correlates with the average flux of the accretingcomponent. A comparison with CH Cyg and MWC 560 indicates that theflickering of T CrB is more stable (at least during the time of ourobservations) than that in these jet-ejecting symbiotic stars. The dataare available in electronic form from the authors. Rapid Variations in RS Oph Observed by OMC/INTEGRALNot Available HeII lambda4686 observations of T Coronae BorealisWe present observations of HeII_4686 of the recurrent nova T CrB. Thespectra were obtained during the period 1997-2002. The HeII4686 emissionline is detected on a few occasions. The presence of the line isconnected with periods of brightening in U band and higher massaccretion rate. Sometimes, the line seems to be blue shifted relative tothe calculated velocity of the hot component. Fluctuation of the Mass Transfer Rate in T CRBA large scatter in the data on the light curve of T CrB is explained bythe flickering activity of the system. The data of April 1996 have afalling trend, and this can be a part of downward branch of a long-termand energetically powerful flare. The estimated energy (2×1035 J) and the duration of this event were compared withtheoretical assumptions based on the three physical scenarios, which canbe flickering sources. The dissipation of magnetic loops and theexistence of the turbulent eddies are energetically deficient, but thesescenarios are real in the case of less powerful flares. The realexplanation could be instability of the secondary component and thevariable mass transfer rate through the Lagrangian point. An Active Quarter of the Century in Research of Cataclysmic VariablesA brief history of our efforts to understand the internal structure ofcataclysmic variables during the last quarter of the 20th century ispresented. We describe the methods used for their investigation and themain results. Classification of Spectra from the Infrared Space Observatory PHT-S DatabaseWe have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA). Surface Hydrogen-burning Modeling of Supersoft X-Ray Binaries: Are They Type Ia Supernova Progenitors?Nova explosions occur on the white dwarf (WD) component of a cataclysmicvariable stellar system that is accreting matter lost by a companion. AType Ia supernova (SN Ia) explosion is thought to result when a WD, in asimilar binary configuration, grows in mass to the Chandrasekhar limit.Here, we present calculations of accretion of solar matter, at a varietyof mass accretion rates, onto hot (2.3×105 K), luminous(30 Lsolar), massive (1.25, 1.35 Msolar)carbon-oxygen WDs. In contrast to our nova simulations, where the WD hasa low initial luminosity and a thermonuclear runaway (TNR) occurs andejects material, these simulations do not eject material (or only asmall fraction of the accreted material), and the WD grows in mass. Ahydrogen TNR does not occur because hydrogen fuses to helium in thesurface layers, and we call this process surface hydrogen burning (SHB).As the helium layer grows in mass, it gradually fuses either to carbonand oxygen or to more massive nuclei, depending on the WD mass and massaccretion rate. If such a WD were to explode in a SN Ia event,therefore, it would show neither hydrogen nor helium in its spectrum asis observed. Moreover, the luminosities and effective temperatures ofour simulations agree with the observations of some of the supersoftX-ray binary sources, and therefore, our results strengthen previousspeculation that some of them (CAL 83 and CAL 87, for example) areprobably progenitors of SN Ia explosions. Finally, we have achieved SHBfor values of the mass accretion rate that almost span the observedvalues of the cataclysmic variables. Spectral and Luminosity Classification of Symbiotic Star Cool Components with Near-Infrared PhotometryWe have used the absolutely calibrated Wing eight-color near-infraredphotometric system to quantitatively derive spectral types andluminosity classes for the cool components of 12 symbiotic stars. Weexamine the advantages and limitations of the system as applied tosymbiotic systems. We find that three systems, CI Cyg, T CrB, and S149,have CN strengths corresponding to luminosity class II. For severalsystems there is a correlation between photometric phase and measuredspectral type. Near-Infrared Spectroscopy of V1493 Aquilae and V4642 Sagittarii: Two Novae with Unusual Spectral FeaturesWe present 0.8-2.5 μm spectroscopy of two novae with uncommonspectral features at different stages in their decline after outburst.Nova Aql 1999 No. 1 (V1493 Aql) was a very fast nova that initiallyexhibited a rapid decline in brightness that flattened by day 20 andactually reversed to show an unusual secondary peak. The near-infraredspectrum was observed during this secondary peak at 46 days afterinitial peak brightness. Blended low-excitation lines such as H IBrackett and Paschen and O I largely populated the spectrum, which alsoshowed a strong continuum declining monotonically toward the red(1.4-2.5 μm). He II lines were just beginning to emerge. The overallspectral appearance was much more representative of the period a fewdays after outburst. A possible explanation for this, and the secondarypeak in the visible light curve, is that the nova experienced a secondperiod of mass loss, but of a more continuous rather than explosivenature. Nova Sgr 2000 (V4642 Sgr) was a moderately fast nova, observed160 days after peak brightness. High-excitation lines, including coronallines, were present in the spectrum. However, their profiles weredistinctly different from those of the lower excitation lines. Alsopresent were four emission lines that have only recently been detectedin the spectra of novae and which remain unidentified. The Optical Gravitational Lensing Experiment. Ellipsoidal Variability of Red Giants in the Large Magellanic CloudWe used the OGLE-II and OGLE-III photometry of red giants in the LargeMagellanic Cloud to select and study objects revealing ellipsoidalvariability. We detected 1546 candidates for long period ellipsoidalvariables and 121 eclipsing binary systems with clear ellipsoidalmodulation. The ellipsoidal red giants follow a period--luminosity (PL)relationship (sequence E), and the scatter of the relation is correlatedwith the amplitude of variability: the larger the amplitude, the smallerthe scatter.We note that some of the ellipsoidal candidates exhibit simultaneouslyOGLE Small Amplitude Red Giants pulsations. Thus, in some cases the LongSecondary Period (LSP) phenomenon can be explained by the ellipsoidalmodulation.We also select about 1600 red giants with distinct LSP, which are notellipsoidal variables. We discover that besides the sequence D in the PLdiagram known before, the LSP giants form additional less numeroussequence for longer periods. We notice that the PL sequence of theellipsoidal candidates is a direct continuation of the LSP sequencetoward fainter stars, what might suggest that the LSP phenomenon isrelated to binarity but there are strong arguments against such apossibility.About 10% of the presented light curves reveal clear deformation by theeccentricity of the system orbits. The largest estimated eccentricity inour sample is about 0.4.All presented data, including individual BVI observations and findingcharts are available from the OGLE Internet archive. Hα variability of the recurrent nova T Coronae BorealisWe analyze Hα observations of the recurrent nova TCrB obtained during the last decade. For the first time theHα emission profile is analyzed after subtraction of the red giantcontribution. Based on our new radial velocity measurements of theHα emission line we estimate the component masses of TCrB. It is found that the hot component is most likely amassive white dwarf. We estimate the inclination and the componentmasses to be i≃67o,MWD≃1.37±0.13 Mȯ andMRG≃1.12±0.23 Mȯ, respectively. The radial velocity of the central dip in the Hα profile changesnearly in phase with that of the red giant's absorption lines. Thissuggests that the dip is most likely produced by absorption in thegiant's wind.Our observations cover an interval when the Hα and the U-band fluxvary by a factor of 6, while the variability in B and V is muchsmaller. Based on our observations, and archival ultraviolet and opticaldata we show that the optical, ultraviolet and Hα fluxes stronglycorrelate. We argue that the presence of an accretion disc can accountfor most of the observed properties of T CrB.Based on observations obtained at Rozhen National AstronomicalObservatory, Bulgaria.
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