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A Method for the Study of Accretion Disk Emission in Cataclysmic Variables. I. The Model
We have developed a spectrum synthesis method for modeling theultraviolet (UV) emission from the accretion disk from cataclysmicvariables (CVs). The disk is separated into concentric rings, with aninternal structure from the Wade & Hubeny disk-atmosphere models.For each ring, a wind atmosphere is calculated in the comoving framewith a vertical velocity structure obtained from a solution of the Eulerequation. Using simple assumptions, regarding rotation and the windstreamlines, these one-dimensional models are combined into a single2.5-dimensional model for which we compute synthetic spectra. We findthat the resulting line and continuum behavior as a function of theorbital inclination is consistent with the observations, and verify thatthe accretion rate affects the wind temperature, leading tocorresponding trends in the intensity of UV lines. In general, we alsofind that the primary mass has a strong effect on the P Cygni absorptionprofiles, the synthetic emission line profiles are strongly sensitive tothe wind temperature structure, and an increase in the mass-loss rateenhances the resonance line intensities. Synthetic spectra were comparedwith UV data for two high orbital inclination nova-like CVs—RW Triand V347 Pup. We needed to include disk regions with arbitrary enhancedmass loss to reproduce reasonably well widths and line profiles. Thisfact and a lack of flux in some high ionization lines may be thesignature of the presence of density-enhanced regions in the wind, oralternatively, may result from inadequacies in some of our simplifyingassumptions.
| The Evolution of Cataclysmic Variables as Revealed by Their Donor Stars
We present an attempt to reconstruct the complete evolutionary pathfollowed by cataclysmic variables (CVs), based on the observedmass-radius relationship of their donor stars. Along the way, we updatethe semi-empirical CV donor sequence presented previously by one of us,present a comprehensive review of the connection between CV evolutionand the secondary stars in these systems, and reexamine most of thecommonly used magnetic braking (MB) recipes, finding that evenconceptually similar ones can differ greatly in both magnitude andfunctional form. The great advantage of using donor radii to infermass-transfer and angular-momentum-loss (AML) rates is that they samplethe longest accessible timescales and are most likely to represent thetrue secular (evolutionary average) rates. We show explicitly that ifCVs exhibit long-term mass-transfer-rate fluctuations, as is oftenassumed, the expected variability timescales are so long that othertracers of the mass-transfer rate—including white dwarf (WD)temperatures—become unreliable. We carefully explore how much ofthe radius difference between CV donors and models of isolatedmain-sequence stars may be due to mechanisms other than mass loss. Thetidal and rotational deformation of Roche-lobe-filling stars produces ~=4.5% radius inflation below the period gap and ~= 7.9% above. Acomparison of stellar models to mass-radius data for non-interactingstars suggests a real offset of ~= 1.5% for fully convective stars(i.e., donors below the gap) and ~= 4.9% for partially radiative ones(donors above the gap). We also show that donor bloating due toirradiation is probably smaller than, and at most comparable to, theseeffects. After calibrating our models to account for these issues, wefit self-consistent evolution sequences to our compilation of donormasses and radii. In the standard model of CV evolution, AMLs below theperiod gap are assumed to be driven solely by gravitational radiation(GR), while AMLs above the gap are usually described by an MB law firstsuggested by Rappaport et al. We adopt simple scaled versions of theseAML recipes and find that these are able to match the data quite well.The optimal scaling factors turn out to be f GR = 2.47± 0.22 below the gap and f MB = 0.66 ± 0.05above (the errors here are purely statistical, and the standard modelcorresponds to f GR = f MB = 1). This revisedmodel describes the mass-radius data significantly better than thestandard model. Some of the most important implications and applicationsof our results are as follows. (1) The revised evolution sequence yieldscorrect locations for the minimum period and the upper edge of theperiod gap; the standard sequence does not. (2) The observed spectraltypes of CV donors are compatible with both standard and revised models.(3) A direct comparison of predicted and observed WD temperaturessuggests an even higher value for f GR, but this comparisonis sensitive to the assumed mean WD mass and the possible existence ofmass-transfer-rate fluctuations. (4) The predicted absolute magnitudesof donor stars in the near-infrared form a lower envelope around theobserved absolute magnitudes for systems with parallax distances. Thisis true for all of our sequences, so any of them can be used to set firmlower limits on (or obtain rough estimates of) the distances toward CVsbased only on P orb and single epoch near-IR measurements.(5) Both standard and revised sequences predict that short-period CVsshould be susceptible to dwarf nova (DN) eruptions, consistent withobservations. However, both sequences also predict that the fraction ofDNe among long-period CVs should decline with P orb above theperiod gap. Observations suggest the opposite behavior, and we discussthe possible explanations for this discrepancy. (6) Approximate orbitalperiod distributions constructed from our evolution sequences suggestthat the ratio of long-period CVs to short-period, pre-bounce CVs isabout 3 × higher for the revised sequence than the standard one.This may resolve a long-standing problem in CV evolution. Tablesdescribing our donor and evolution sequences are provided inelectronically readable form.
| Dark spot, spiral waves and the SW Sextantis behaviour: it is all about UX Ursae Majoris
We present an analysis of time-resolved, medium-resolution opticalspectroscopic observations of UX Ursae Majoris (UX UMa) in the blue(3920-5250 Å) and red (6100-7200 Å) wavelength ranges thatwere obtained in 1999 April and 2008 March, respectively. The observedcharacteristics of our spectra indicate that UX UMa has been indifferent states during those observations. The blue spectra are verycomplex. They are dominated by strong and broad single-peaked emissionlines of hydrogen. The high-excitation lines of He II ?4686 andthe Bowen blend are quite strong as well. All the lines consist of amixture of absorption and emission components. Using Doppler tomography,we have identified four distinct components of the system: the accretiondisc, the secondary star, the bright spot from the gas stream/discimpact region and the unique compact area of absorption in the accretiondisc seen as a dark spot in the lower left-hand quadrant of thetomograms. In the red wavelength range, both the hydrogen (H?) andneutral helium (He I ?6678 and He I ?7065) lines wereobserved in emission and both exhibited double-peaked profiles. Dopplertomography of these lines reveals spiral structure in the accretiondisc, but in contrast to the blue wavelength range, there is no evidencefor either the dark spot or the gas stream/disc impact region emission,while the emission from the secondary star is weak. During theobservations in 1999, UX UMa showed many of the defining properties ofthe SW Sextantis stars. However, all these features almost completelydisappeared in 2008. We have also estimated the radial velocitysemi-amplitudes K1 and K2 and evaluated the systemparameters of UX UMa. These estimates are inconsistent with previousvalues derived by means of analysis of white dwarf eclipse features inthe light curve in different wavelength ranges.
| Hard X-ray Emission Associated with White Dwarfs. III.
Hard X-ray emission associated with white dwarfs (WDs) can be used todiagnose the presence of late-type binary companions, mass accretionfrom companions, or physical processes with unknown origins. Since ourprevious systematic searches for hard X-ray emission associated withWDs, the Galactic WD catalog has been augmented by >10,000 new WDsfrom the Sloan Digital Sky Survey and new X-ray point-source catalogsfrom XMM-Newton and ROSAT have become available. Therefore, we haveextended the search using the updated catalogs, and found 17 new casesof WDs associated with hard X-ray emission. The 32 WDs associated withhard X-ray emission, from the current and previous searches, can bedivided into five categories: (1) binary WD with a coronal companion,(2) binary WD with mass transfer from a companion, (3) single hot WDwith a hard X-ray component peaking near 1 keV in addition to a softphotospheric component, (4) two PG 1159 stars with very faint X-rayemission in the 0.9-2.0 keV band, and (5) two DA WDs whose photosphericemission component has a hard shoulder extending to 0.5-0.9 keV. Theorigin of the hard X-ray emission in the latter three categories is notyet known. Deeper X-ray observations with higher angular and spectralresolutions are needed to help us understand these WDs' hard X-rayemission.
| The age of cataclysmic variables: A kinematical study
Using available astrometric and radial velocity data, the spacevelocities of cataclysmic variables (CVs) with respect to Sun werecomputed and kinematical properties of various sub-groups of CVs wereinvestigated. Although observational errors of systemic velocities(?) are high, propagated errors are usually less than computeddispersions. According to the analysis of propagated uncertainties ofthe computed space velocities, available sample was refined by removingthe systems with the largest propagated uncertainties so that thereliability of the space velocity dispersions was improved. Having adispersion of 51±7kms-1 for the space velocities, CVsin the current refined sample (159 systems) are found to have 5 ±1 Gyr mean kinematical age. After removing magnetic systems from thesample, it is found that non-magnetic CVs (134 systems) have a meankinematical age of 4 ± 1 Gyr. According to 5 ± 1 and 4± 1 Gyr kinematical ages implied by 52 ± 8 and 45 ±7 km s-1 dispersions for non-magnetic systems below andabove the period gap, CVs below the period gap are older than systemsabove the gap, which is a result in agreement with the standardevolution theory of CVs. Age difference between the systems below andabove the gap is smaller than that expected from the standard theory,indicating a similarity of the angular momentum loss time scales insystems with low-mass and high-mass secondary stars. Assuming anisotropic distribution, ? velocity dispersions of non-magnetic CVsbelow and above the period gap are calculated??=30±5kms-1 and??=26±4kms-1. The smalldifference of ? velocity dispersions between the systems below andabove the gap may imply that magnetic braking does not operate in thedetached phase, during which the system evolves from the post-commonenvelope orbit into contact.
| The Anomalous Accretion Disk of the Cataclysmic Variable RW Sextantis
Synthetic spectra covering the wavelength range 900-3000 Å providean accurate fit, established by a ?2 ?analysis, to a combined observed spectrum of RW Sextantis. Twoseparately calibrated distances to the system establish the syntheticspectrum comparison on an absolute flux basis but with two alternativescaling factors, requiring alternative values of \dot{M} for finalmodels. Based on comparisons for a range of \dot{M} values, the observedspectrum does not follow the standard model. Rather than the exponent0.25 in the expression for the radial temperature profile, a value closeto 0.125 produces a synthetic spectrum with an accurate fit to thecombined spectrum. A study of time-series Far Ultraviolet SpectroscopicExplorer spectra shows that a proposed warped or tilted disk is notsupported by the data; an alternative proposal is that an observednon-axisymmetric wind results from an interaction with the mass transferstream debris.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc. underNASA contract NAS5-26555, and the NASA-CNES-CSA Far UltravioletExplorer, which is operated for NASA by the Johns Hopkins Universityunder NASA contract NAS5-32985.
| Far Ultraviolet Spectroscopy of Seven Nova-Like Variables
We present the results of a multicomponent synthetic spectral analysisof the archival far ultraviolet spectra of several key nova-likevariables including members of the SW Sex, RW Tri, UX UMa, and VY Sclsubclasses: KR Aur, RW Tri, V825 Her, V795 Her, BP Lyn, V425 Cas, and HLAqr. Accretion rates as well as the possible flux contribution of theaccreting white dwarf are included in our analysis. Except for RW Tri,which has a reliable trigonometric parallax, we computed the distancesto the nova-like systems using the method of Knigge. Our analysis ofseven archival IUE spectra of RW Tri at its parallax distance of 341 pcconsistently indicates a low mass () white dwarf and an averageaccretion rate, . For KR Aur, we estimate that the white dwarf has , ,and contributes 18% of the far-UV flux, while an accretion disk withaccretion rate at an inclination of 41° contributes the remainder.We find that an accretion disk dominates the far-UV spectrum of V425 Casbut a white dwarf contributes nonnegligibly with approximately 18% ofthe far-UV flux. For the two high state nova-likes, HL Aqr and V825 Her,their accretion disks totally dominate with and , respectively. For BPLyn we find while for V795 Her, we find an accretion rate of . Wediscuss the implications of our results for the evolutionary status ofnova-like variables.
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| V3885 Sagittarius: A Comparison with a Range of Standard Model Accretion Disks
A \widetilde{?}^2 analysis of standard model accretion disksynthetic spectrum fits to combined Far Ultraviolet SpectroscopicExplorer and Space Telescope Imaging Spectrograph spectra of V3885Sagittarius, on an absolute flux basis, selects a model that accuratelyrepresents the observed spectral energy distribution. Calculation of thesynthetic spectrum requires the following system parameters. Thecataclysmic variable secondary star period-mass relation calibrated byKnigge in 2006 and 2007 sets the secondary component mass. A mean whitedwarf (WD) mass from the same study, which is consistent with anobservationally determined mass ratio, sets the adopted WD mass of 0.7 Msun, and the WD radius follows from standard theoreticalmodels. The adopted inclination, i = 65°, is a literature consensus,and is subsequently supported by \widetilde{?}^2 analysis. The masstransfer rate is the remaining parameter to set the accretion disk Teff profile, and the Hipparcos parallax constrains thatparameter to \dot{M}=(5.0± 2.0)× 10^{-9} M_{&sun;} yr^{-1}by a comparison with observed spectra. The fit to the observed spectraadopts the contribution of a 57, 000 ± 5000 K WD. The model thusprovides realistic constraints on \dot{M} and T eff for alarge \dot{M} system above the period gap.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc. underNASA contract NAS5-26555, and the NASA-CNES-CSA Far UltravioletExplorer, which is operated for NASA by the Johns Hopkins Universityunder NASA contract NAS5-32985.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Simulations of the Boundary Layer Between a White Dwarf and Its Accretion Disk
Using a 2.5D time-dependent numerical code we recently developed, wesolve the full compressible Navier-Stokes equations to determine thestructure of the boundary layer (BL) between the white dwarf (WD) andthe accretion disk in nonmagnetic cataclysmic variable systems. In thispreliminary work, our numerical approach does not include radiation. Inthe energy equation, we either take the dissipation function (?)into account or we assume that the energy dissipated by viscousprocesses is instantly radiated away (? = 0). For a slowly rotatingnonmagnetized accreting WD, the accretion disk extends all the way tothe stellar surface. There, the matter impacts and spreads toward thepoles as new matter continuously piles up behind it. We carry outnumerical simulations for different values of the alpha-viscosityparameter (?), corresponding to different mass accretion rates. Inthe high viscosity cases (? = 0.1), the spreading BL sets off agravity wave in the surface matter. The accretion flow movessupersonically over the cusp making it susceptible to the rapiddevelopment of gravity wave and/or Kelvin-Helmholtz shearinginstabilities. This BL is optically thick and extends more than 30°to either side of the disk plane after only 3/4 of a Keplerian rotationperiod (tK = 19 s). In the low viscosity cases (? =0.001), the spreading BL does not set off gravity waves and it isoptically thin.
| On the Accretion Rates of SW Sextantis Nova-like Variables
We present accretion rates for selected samples of nova-like variableshaving IUE archival spectra and distances uniformly determined using aninfrared method by Knigge. A comparison with accretion rates derivedindependently with a multiparametric optimization modeling approach byPuebla et al. is carried out. The accretion rates of SW Sextantisnova-like systems are compared with the accretion rates of non-SWSextantis systems in the Puebla et al. sample and in our sample, whichwas selected in the orbital period range of three to four and a halfhours, with all systems having distances using the method of Knigge.Based upon the two independent modeling approaches, we find nosignificant difference between the accretion rates of SW Sextantissystems and non-SW Sextantis nova-like systems insofar as opticallythick disk models are appropriate. We find little evidence to suggestthat the SW Sex stars have higher accretion rates than other nova-likecataclysmic variables (CVs) above the period gap within the same rangeof orbital periods.
| A Time-Series Analysis of the H? Emission Line in V3885 Sagitarii
Flickering is a phenomenon related to mass accretion observed among manyclasses of astrophysical objects. In this paper we present a study offlickering emission lines and the continuum of the cataclysmic variableV3885 Sgr. The flickering behavior was first analyzed throughstatistical analysis and the power spectra of lightcurves.Autocorrelation techniques were then employed to estimate the flickeringtimescale of flares. A cross-correlation study between the line and itsunderlying continuum variability is presented. The cross-correlationbetween the photometric and spectroscopic data is also discussed.Periodograms, calculated using emission-line data, show a behavior thatis similar to those obtained from photometric datasets found in theliterature, with a plateau at lower frequencies and a power-law athigher frequencies. The power-law index is consistent with stochasticevents. The cross-correlation study indicates the presence of acorrelation between the variability on H? and its underlyingcontinuum. Flickering timescales derived from the photometric data wereestimated to be 25 min for two lightcurves and 10min for one of them.The average timescales of the line flickering is 40 min, while for itsunderlying continuum it drops to 20 min.
| Cluster AgeS Experiment (CASE): deficiency of observed dwarf novae in globular clusters
We present the results of a search for dwarf novae (DNe) in globularclusters (GCs). It is based on the largest available homogeneous sampleof observations, in terms of the time-span, number of observations andnumber of clusters. It includes 16 Galactic GCs and yielded two newcertain DNe: M55-CV1 and M22-CV2. All previously known systems locatedin our fields were recovered, too. We surveyed M4, M5, M10, M12, M22,M30, M55, NGC 288, NGC 362, NGC 2808, NGC 3201, NGC 4372, NGC 6362, NGC6752, ? Centauri (NGC 5139) and 47 Tucanae (NGC 104). Thediscovery of two DNe, namely M55-CV1 and M22-CV2, was already reportedby Kaluzny et al. and Pietrukowicz et al., respectively. In theremaining 14 GCs, we found no certain new DNe. Our result raises thetotal number of known DNe in the Galactic GCs to 12 DNe, distributedamong seven clusters. Our survey recovered all three already knownerupting cataclysmic variables (CVs) located in our fields, namelyM5-V101, M22-CV1, and V4 in the foreground of M30. To assess theefficiency of the survey, we analysed images with inserted artificialstars mimicking outbursts of the prototype DNe SS Cygni and U Geminorum.Depending on the conditions, we recovered between 16-100 per cent ofthese artificial stars. The efficiency seems to be predominantlyaffected by duty cycle/time-sampling and much less bydistance/magnitude. Except for saturated tiny collapsed cores of M30,NGC 362 and NGC 6752 (and also the dense core of NGC 2808), crowdingeffects in the V band were avoided by our image subtraction techniqueaugmented with auxiliary unsaturated B-band images. Our results clearlydemonstrate that in GCs common types of DNe are very rare indeed.However, great care must be taken before these conclusions can beextended to the CV population in GCs.
| Tomographic Simulations of Accretion Disks in Cataclysmic Variables -- Flickering and Wind
Both continuum and emission line flickering are phenomena directlyassociated with the mass-accretion process. In this work we simulatedaccretion-disk Doppler maps, including the effects of winds andflickering flares. Synthetic flickering Doppler maps were calculatedand the effect of the flickering parameters on the maps was explored.Jets and winds occur in many astrophysical objects where accretion disksare present. Jets are generally absent among the cataclysmic variables(CVs), but there is evidence of mass loss by wind in many objects. CVsare ideal objects to study accretion disks, and consequently to studythe wind associated with these disks. We also present simulations ofaccretion disks, including the presence of a wind with orbital phaseresolution. Synthetic H? line profiles in the optical region wereobtained and their corresponding Doppler maps were calculated. Theeffect of the wind simulation parameters on the wind line profiles wasalso explored. From this study we verified that optically thick linesand/or emission by diffuse material into the primary Roche lobe arenecessary to generate single peaked line profiles, often seen in CVs.The future accounting of these effects is suggested for interpretingDoppler tomography reconstructions.
| Spatial distribution and galactic model parameters of cataclysmic variables
The spatial distribution, galactic model parameters and luminosityfunction of cataclysmic variables (CVs) in the solar neighbourhood havebeen determined from a carefully established sample of 459 CVs. Thesample contains all of the CVs with distances computed from theperiod-luminosity-colours (PLCs) relation of CVs which has been recentlyderived and calibrated with 2MASS photometric data. It has been foundthat an exponential function fits best to the observationalz-distributions of all of the CVs in the sample, non-magnetic CVs anddwarf novae, while the sech2 function is more appropriate fornova-like stars and polars. The vertical scaleheight of CVs is 158± 14 pc for the 2MASS J-band limiting apparent magnitude of 15.8.On the other hand, the vertical scaleheights are 128 ± 20 and 160± 5 pc for dwarf novae and nova-like stars, respectively. Thelocal space density of CVs is found to be ˜3 ×10-5 pc-3 which is in agreement withthe lower limit of the theoretical predictions. The luminosity functionof CVs shows an increasing trend toward higher space densities at lowluminosities, implying that the number of short-period systems should behigh. The discrepancies between the theoretical and observationalpopulation studies of CVs will almost disappear if for the z-dependenceof the space density the sech2 density function is used.
| A New, Bright, Short-Period, Emission Line Binary in Ophiuchus
The 11th magnitude star LS IV-08°3 has been classified previously asan OB star in the Luminous Stars survey, or alternatively as a hotsubdwarf. It is actually a binary star. We present spectroscopy,spectroscopic orbital elements, and time-series photometry fromobservations made at the Kitt Peak National Observatory 2.1 m, StewardObservatory 2.3 m, MDM Observatory 1.3 m and 2.4 m, Hobby-Eberly 9.2 m,and Michigan State University 0.6 m telescopes. The star exhibitsemission of varying strength in the cores of H and He I absorptionlines. Emission is also present at 4686 Å (He II) and near4640/4650 Å (N III/C III). Time-series spectroscopy collected from2005 July to 2007 June shows coherent, periodic radial velocityvariations of the H? line, which we interpret as orbital motionwith a period of 0.1952894(10) days. High-resolution spectra show thatthere are two emission components, one broad and one narrow, moving inantiphase, as might arise from an accretion disk and the irradiated faceof the mass donor star. Less coherent, low-amplitude photometricvariability is also present on a timescale similar to the orbitalperiod. Diffuse interstellar bands indicate considerable reddening,which however is consistent with a distance of ~100-200 pc. The star isthe likely counterpart of a weak ROSAT X-ray source, whose propertiesare consistent with accretion in a cataclysmic variable (CV) binarysystem. We classify LS IV-08°3 as a new member of the UX UMasubclass of CV stars.Based in part on observations obtained with the Hobby-Eberly Telescope,which is a joint project of the University of Texas at Austin, thePennsylvania State University, Stanford University,Ludwig-Maximillians-Universität München, andGeorg-August-Universität Göttingen.
| A Statistical Study of Accretion Disk Model Spectra for Cataclysmic Variables
We have performed a statistical test of the currently used accretiondisk models for cataclysmic variables (CVs) using a set of 33 CVs withsteady disks (10 old novae and 23 nova-like systems). The mass transferrate () for each system was also calculated. Ultraviolet (UV) data werefitted by model spectra using a multiparametric optimization method,aiming to constrain the values. It was verified that these accretiondisk models fail to fit both color and flux simultaneously, aspreviously noted when composite stellar atmosphere models were fitted tothe UV spectra of CVs by Wade. By applying such models to a sample ofnovae and nova-like CVs, we confirm that the limb-darkening effect mustbe taken into account when estimating mass transfer rates, especiallyfor high-inclination systems. Important fitting degeneracies of thebasic disk parameters are analyzed. Our simulations suggest that toreproduce the observations a revision of the temperature profile, atleast in the innermost parts of the disk, seems to be required, andpossibly the vertical distribution of the viscosity should be revised.In addition, an optically thin layer or an extended disk componentshould be considered. This component may be physically represented by adisk wind and/or a chromosphere. A physical description of theemission-line profiles may help to break the degeneracies that appearwhen only the continuum is analyzed. The average value of found fornova-like systems is ~9.3 × 10-9 Modotyr-1, while ~1.3 × 10-8 Modotyr-1 is found for old classical novae. No clear evidence isfound for either the presence or absence of a correlation between andthe orbital period. Such correlation analysis was performed for highaccretion rate systems (15 nova-like systems and 10 old novae), but wewere not able to find a well-defined correlation as found by Patterson.By measuring the equivalent width of the emission lines (C IV?1550 and He II ?1640) we found a lack of systems with lowand strong UV emission lines. A correlation between the equivalent widthof such lines and the orbital inclination (i) was also confirmed.
| New Ultraviolet Observations of AM CVn
We have obtained observations of the ultraviolet spectrum of AM CVn, anultrashort-period helium cataclysmic variable, using the Space TelescopeImaging Spectrograph aboard the Hubble Space Telescope (HST). Weobtained data in time-tag mode during two consecutive orbits of HST,covering 1600-3150 and 1140-1710 Å, respectively. The meanspectrum is approximately flat in f?. The absorptionprofiles of the strong lines of N V, Si IV, C IV, He II, and N IV areblueshifted and in some cases asymmetric, evidencing a wind that ispartly occulted by the accretion disk. There is weak redshifted emissionfrom N V and He II. The profiles of these lines vary mildly with time.The light curve shows a decline of ~20% over the span of theobservations. There is also flickering and a 27 s (or 54 s) "dwarf novaoscillation," revealed in a power-spectrum analysis. The amplitude ofthis oscillation is larger at shorter wavelengths. We assemble andillustrate the spectral energy distribution of AM CVn from theultraviolet to the near-infrared. Modeling the accretion phenomenon inthis binary system can in principle lead to a robust estimate of themass accretion rate on to the central white dwarf, which is of greatinterest in characterizing the evolutionary history of the binarysystem. Inferences about the mass accretion rate depend strongly on thelocal radiative properties of the disk, as we illustrate. Uncertainty inthe distance of AM CVn and other parameters of the binary systemcurrently limit the ability to confidently infer the mass accretionrate.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with program8159.
| A new absolute magnitude calibration with 2MASS for cataclysmic variables
Using reliable trigonometric measurements, we find that the absolutemagnitude of cataclysmic variables depends on the orbital period andde-reddened (J ‑ H)0 and (H ‑Ks)0 colours of 2MASS (Two Micron All Sky Survey)photometric system. The calibration equation covers the ranges0.032d < Porb ⩽ 0.454d,‑0.08 < (J ‑ H)0 ⩽ 1.54, ‑0.03 <(H ‑ Ks)0 ⩽ 0.56 and 2.0
| Albus 1: A Very Bright White Dwarf Candidate
We have serendipitously discovered a previously unknown, bright source(BT=11.75+/-0.07 mag) with a very blueVT-Ks color, which we have named Albus 1. Aphotometric and astrometric study using Virtual Observatory tools hasshown that it possesses an appreciable proper motion and magnitudes andcolors very similar to those of the well-known white dwarf G191-B2B. Weconsider Albus 1 as a DA-type white dwarf located at about 40 pc. If itsnature is confirmed, Albus 1 would be the sixth brightest isolated whitedwarf in the sky, which would make it an excellent spectrophotometricstandard.
| A Synthetic Spectrum and Light-Curve Analysis of the Cataclysmic Variable IX Velorum
Spectrum synthesis analysis of FUSE and STIS spectra for the cataclysmicvariable IX Vel shows that it is possible to achieve a close syntheticspectrum fit with a mass transfer rate of M?=5×10-9Msolar yr-1 and a modified standard modeltemperature profile. The innermost four annuli of the accretion diskmodel, extending to r/rWD~4, are isothermal; beyond thatlimit the temperatures follow the standard model. A light synthesis fitto K-band photometry requires shallow eclipses of the accretion disk rimand secondary star limb. The geometry constrains the orbital inclinationto i=57deg+/-2deg. The synthetic light curverequires a vertically extended accretion disk rim, beyond that predictedby gravitational equilibrium, to shadow the secondary star. The enhancedvertical extension is consistent with recent MHD predictions for CVaccretion disks. Matching differing observed heights of alternate K-bandlight-curve maxima requires a warm rim region downstream of theintersection point of the mass transfer stream with the accretion diskrim. The temperature of the warm region is inconsistent with expectationfor a bright spot associated with a rim shock.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555, and the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Emission-Line Flickering from the Secondary Star in Cataclysmic Variables? A Study of V3885 Sagittarii
Spectrophotometric observations of the H? and He I ?6678emission lines of the nova-like cataclysmic variable V3885 Sgr arepresented and analyzed. The binary orbital period was determined asP=0.20716071(22) days. Doppler tomography was performed with bothH? and He I lines. Disk radial emissivity profiles were alsocomputed. The tomography mapping of flickering sources was performedusing the H? line, from which we concluded that the flickering isnot uniformly distributed on the disk. The observed tomogram of theflickering was compared with simulations, suggesting that the mostintense flickering source in the H? is not located in theaccretion disk. It is proposed that the main line-flickering sourcemight be associated with the illuminated secondary star.
| The donor stars of cataclysmic variables
We carefully consider observational and theoretical constraints on theglobal properties of secondary stars in cataclysmic variable stars(CVs). We then use these constraints to construct and test a complete,semi-empirical donor sequence for CVs with orbital periodsPorb <= 6 h. All key physical and photometric parametersof CV secondaries (along with their spectral types) are given as afunction of Porb along this sequence. This provides abenchmark for observational and theoretical studies of CV donors andevolution.The main observational basis for our donor sequence is an empiricalmass-radius relationship for CV secondaries. Patterson and co-workershave recently shown that this can be derived from superhumping and/oreclipsing CVs. We independently revisit all of the key steps in thisderivation, including the calibration of the period excess-mass ratiorelation for superhumpers and the use of a single representative primarymass for most CVs. We also present an optimal technique for estimatingthe parameters of the mass-radius relation that simultaneously ensuresconsistency with the observed locations of the period gap and the periodminimum. We present new determinations of these periods, findingPgap,+ = 3.18 +/- 0.04 h (upper edge), Pgap,- =2.15 +/- 0.03 h (lower edge) and Pmin = 76.2 +/- 1.0 min(period minimum).We test the donor sequence by comparing observed and predicted spectraltypes (SpTs) as a function of orbital period. To this end, we update theSpT compilation of Beuermann and co-workers and show explicitly that CVdonors have later SpTs than main-sequence (MS) stars at all orbitalperiods. This extends the conclusion of the earlier study to theshort-period regime (Porb < 3 h). We then compare ourdonor sequence to the CV data, and find that it does an excellent job ofmatching the observed SpTs. Thus the empirical mass-radius relationyields just the right amount of radius expansion to account for thelater-than-MS spectral types of CV donors. There is remarkably littleintrinsic scatter in both the mass-radius and SpT-Porbrelations, which confirms that most CVs follow a unique evolution track.The donor sequence exhibits a fairly sharp drop in temperature,luminosity and optical/infrared flux well before the minimum period.This may help to explain why the detection of brown dwarf secondaries inCVs has proven to be extremely difficult.We finally apply the donor sequence to the problem of distanceestimation. Based on a sample of 22 CVs with trigonometric parallaxesand reliable 2MASS data, we show that the donor sequence correctlytraces the upper envelope of the observedMJHK-Porb distribution. Thus robust lower limitson distances can be obtained from single-epoch infrared observations.However, for our sample, these limits are typically about a factor of 2below the true distances.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Spiral waves and the secondary star in the nova-like variable V3885 Sgr
We present seven nights' blue (4300-5000Å) spectroscopy of thenova-like variable star V3885 Sgr. The line spectrum shows a typicalcombination of broad absorption and emission in Hγ, Hβ andHeI, which is associated with the accretion disc. We also observeantiphased narrow emission, which we attribute to irradiation of thesecondary star. The HeIIλ4686 and NIII-CIII-CIV emission linesare devoid of structure and are most likely formed in an outflow. Wemeasure radial velocity shifts in the absorption and emission lines,from which we fit an orbital period of 4.97126 +/- 0.00036h. From thevelocity semi-amplitudes of the disc and companion star, we are able toconstrain the binary mass ratio to q > 0.7.The phase-folded spectra provide dense coverage of the entire orbitalcycle. Doppler tomograms of the hydrogen and HeI lines reveal spiralstructure in the accretion disc and the irradiated donor star. Webelieve that this is the first unambiguous detection of spiral waves ina nova-like variable.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| FUSE observations of V592 Cas: the continuing enigma of orbitally modulated spectral signatures of accretion disc winds
By exploiting Far Ultraviolet Spectroscopic Explorer (FUSE) satellitetime-series data, we have probed the relation between accretion discgeometry and bipolar outflow in the low-inclination, nova-likecataclysmic variable (CV) V592 Cas. Our results show that the outflow isstrictly modulated on the orbital period and not on the (negative orpositive) superhump periods of this system. This implies that theprecession of either the disc eccentricity or its tilt is not directlyaffecting the structure and global dynamics of the outflow. Theprincipal variability in the ultraviolet resonance lines ischaracterized by Doppler shifts of the entire blueward absorptiontroughs, in concert for a range of low- and high-ionization cases. Therepetitive behaviour is asymmetric and highly non-sinusoidal over theorbital cycle of the system. The disc wind characteristics derived hereallow V592 Cas to be added to a growing number of non-magnetic CVs whichexhibit orbitally modulated wind variability. The origin of thedeparture in axisymmetry, which has to be fixed in the binary starframe, remains to be established.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
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Membro dei seguenti gruppi:
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Osservazione e dati astrometrici
| Costellazione: | Sagittario |
| Ascensione retta: | 19h47m40.53s |
| Declinazione: | -42°00'26.4" |
| Magnitudine apparente: | 10.333 |
| Moto proprio RA: | 30 |
| Moto proprio Dec: | -40.3 |
| B-T magnitude: | 10.307 |
| V-T magnitude: | 10.331 |
Cataloghi e designazioni:
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