Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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.
| Simultaneous X-ray and Ultraviolet Observations of the SW Sextantis Star DW Ursae Majoris
We present the first pointed X-ray observation of DW Ursae Majoris, anovalike cataclysmic variable (CV) and one of the archetype members ofthe SW Sextantis class, obtained with the XMM-Newton satellite. Thesedata provide the first detailed look at an SW Sex star in the X-rayregime (with previous X-ray knowledge of the SW Sex stars limitedprimarily to weak or non-detections in the ROSAT All Sky Survey). It isalso one of only a few XMM-Newton observations (to date) of any highmass transfer rate novalike CV, and the only one in the evolutionarilyimportant 3-4 hr orbital period range. The observed X-ray spectrum of DWUMa is very soft, with ~95% of the detected X-ray photons at energies<2 keV. The spectrum can be fit equally well by a one-componentcooling flow model, with a temperature range of 0.2-3.5 keV, or atwo-component, two-temperature thermal plasma model, containing hard(~5-6 keV) and soft (~0.8 keV) components. The X-ray light curve of DWUMa shows a likely partial eclipse, implying X-ray reprocessing in avertically extended region, and an orbital modulation, implying astructural asymmetry in the X-ray reprocessing site (e.g., it cannot bea uniform corona). We also obtained a simultaneous near-ultravioletlight curve of DW UMa using the Optical Monitor on XMM-Newton. Thislight curve is similar in appearance to published optical-UV lightcurves of DW UMa and shows a prominent deep eclipse. Regardless of theexact nature of the X-ray reprocessing site in DW UMa, the lack of aprominent hard X-ray total eclipse and very low fraction of high energyX-rays point to the presence of an optically and geometrically thickaccretion disk that obscures the boundary layer and modifies the X-rayspectrum emitted near the white dwarf.
| Wavelet and R/S analysis of the X-ray flickering of cataclysmic variables
Context. Recently, wavelets and R/S analysis have been used asstatistical tools to characterize the optical flickering of cataclysmicvariables. Aims: Here we present the first comprehensive study ofthe statistical properties of X-ray flickering of cataclysmic variablesin order to link them with physical parameters. Methods: Weanalyzed a sample of 97 X-ray light curves of 75 objects of all classesobserved with the XMM-Newton space telescope. By using the waveletsanalysis, each light curve has been characterized by two parameters,? and ?, that describe the energy distribution of flickeringon different timescales and the strength at a given timescale,respectively. We also used the R/S analysis to determine the Hurstexponent of each light curve and define their degree of stochasticmemory in time. Results: The X-ray flickering is typicallycomposed of long time scale events (1.5 ? ? ? 3), withvery similar strengths in all the subtypes of cataclysmic variables (-3? ? ? -1.5). The X-ray data are distributed in a muchsmaller area of the ? - ? parameter space with respect tothose obtained with optical light curves. The tendency of the opticalflickering in magnetic systems to show higher ? values than thenon-magnetic systems is not encountered in the X-rays. The Hurstexponents estimated for all light curves of the sample are larger thanthose found in the visible, with a peak at 0.82. In particular, we donot obtain values lower than 0.5. The X-ray flickering presents apersistent memory in time, which seems to be stronger in objectscontaining magnetic white dwarf primaries. Conclusions: Thesimilarity of the X-ray flickering in objects of different classestogether with the predominance of a persistent stochastic behavior canbe explained in terms of magnetically-driven accretion processes actingin a considerable fraction of the analyzed objects.Table 1 is only available in electronic form at http://www.aanda.org
| 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.
| 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.
| Hot stars observed by XMM-Newton. I. The catalog and the properties of OB stars
Aims: Following the advent of increasingly sensitive X-rayobservatories, deep observations of early-type stars became possible.However, the results for only a few objects or clusters have until nowbeen reported and there has been no large survey comparable to thatbased upon the ROSAT All-Sky Survey (RASS). Methods: A limitedsurvey of X-ray sources, consisting of all public XMM-Newtonobservations (2XMMi) and slew survey data (XMMSL1), is now available.The X-ray counterparts to hot, massive stars have been searched for inthese catalogs. Results: About 300 OB stars were detected withXMM-Newton. Half of them were bright enough for a spectral analysis tobe possible, and we make available the detailed spectral properties thatwere derived. The X-ray spectra of O stars are represented well by low(<1 keV) temperature components and seem to indicate that anabsorption column is present in addition to the interstellarcontribution. The X-ray fluxes are well correlated with the bolometricfluxes, with a scatter comparable to that of the RASS studies and thuslarger than found previously with XMM-Newton for some individualclusters. These results contrast with those of B stars that exhibit alarge scatter in the LX - LBOL relation, noadditional absorption being found, and the fits indicate a plasma athigher temperatures. Variability (either within one exposure or betweenmultiple exposures) was also investigated whenever possible: short-termvariations are far more rare than long-term ones (the former affects afew percent of the sample, while the latter concerns between one thirdand two thirds of the sources). Conclusions: This paper presentsthe results of the first high-sensitivity investigation of the overallhigh-energy properties of a sizable sample of hot stars.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA), and accessed via the 2XMMi and XMM slew surveycatalogs. Tables 1 and 5 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/1055 PostdoctoralResearcher FNRS. Visiting astronomer, UNAM-Morelos (Mexico).
| 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.
| Modeling UX Ursae Majoris: An Abundance of Challenges
We present a system model for optical and far-UV spectra of thenova-like variable UX UMa involving a white dwarf, secondary star, gasstream, hot spot, and accretion disk using our code BINSYN and based onan initially adopted system distance. Calculated SED intensity datasuccessfully fit successive tomographically extracted annuli longward ofthe Balmer limit but require a postulated ``iron curtain'' shortward ofthe Balmer limit that is applied to the annulus section closest to thesecondary star, while postulated recombination emission fills in themodel SED shortward of the Balmer limit and is applied to the annulussection more remote from the secondary star. The same model fits UBV1954 light curves by Walker and Herbig. Fits to HST FOS spectra areapproximate but require assumed time-variable changes in the SED.Comparable effects, possibly involving variable absorption, afflict FUSEspectra. Fits to IUE spectra by the model show time-dependent residualsthat indicate changes in the accretion disk temperature profile,possibly indicative of a slightly variable ? from the secondarystar. Using model-based component light contributions and theimprovement on the Bailey relation by Knigge we determine the systemdistance and mass transfer rate.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.
| 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.
| The Structure of the Local Interstellar Medium. V. Electron Densities
We present a comprehensive survey of C II* absorption detections towardstars within 100 pc in order to measure the distribution of electrondensities present in the local interstellar medium (LISM). Using highspectral resolution observations obtained by GHRS and STIS on board HST,we searched for all detections of LISM C II* absorption. We identify 13sight lines with 23 individual C II* absorption components, whichprovide electron density measurements. We employ several strategies todetermine more accurate C II column densities from the saturated C IIresonance line, including constraints of the line width from theoptically thin C II* line, constraints from independent temperaturemeasurements of the LISM gas based on line widths of other ions, andthird, using measured S II column densities as a proxy for C II columndensities. The distribution of electron densities based on using S II asa proxy for C II is similar to the distribution based on carbon alone,while significantly tighter, and proves to be a promising technique toavoid grossly overestimating the C II column density based on thesaturated line profile. The sample of electron densities appearsconsistent with a lognormal distribution and an unweighted mean value ofne(C IISII)=0.11+0.10-0.05cm-3. Seven individual sight lines probe the LocalInterstellar Cloud (LIC), and all present a similar value for theelectron density, with a weighted mean of ne(LIC)=0.12+/-0.04cm-3. Given some simple assumptions, the range of observedelectron densities translates into a range of thermal pressures,P/k=3300+5500-1900 K cm-3. This workgreatly expands the number of electron density measurements and providesimportant constraints on the ionization, abundance, and evolutionarymodels of the LISM.
| 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.
| Synthetic Spectrum Constraints on a Model of the Cataclysmic Variable QU Carinae
Neither standard model SEDs nor truncated standard model SEDs fitobserved spectra of QU Carinae with acceptable accuracy over the range900-3000 Å. Nonstandard model SEDs fit the observation setaccurately. The nonstandard accretion disk models have a hot regionextending from the white dwarf to R=1.36RWD, a narrowintermediate-temperature annulus, and an isothermal remainder to thetidal cutoff boundary. The models include a range of ? valuesbetween 1.0×10-7 and 1.0×10-6Msolar yr-1 and limiting values of MWDbetween 0.6 and 1.2 Msolar. A solution withMWD=1.2 Msolar is consistent with an empiricalmass-period relation. The set of models agree on a limited range ofpossible isothermal region Teff values between 14,000 and18,000 K. The model-to-model residuals are so similar that it is notpossible to choose a best model. The Hipparcos distance, 610 pc, isrepresentative of the model results. The orbital inclination is between40° and 60°.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.
| 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.
| Investigation of Cataclysmic variables and related objects with the INTEGRAL satellite
Examples of the results of investigation of cataclysmic variables (CVs)and related objects with the ESA INTEGRAL satellite mostly within theCore Programme are presented and discussed. It is shown that theINTEGRAL satellite serves as an efficient tool to study these objectsand that not only the onboard high-energy telescopes, but also the smalloptical camera OMC provide valuable scientific information.
| Robust Models for Phase Shifts in Accreting Binary Stars
Radial velocity studies of accreting binary stars commonly use accretiondisk emission lines to determine the radial velocity of the primary starand therefore the mass ratio. These emission-line radial velocity curvesare often shifted in phase from the expected motion of the primary.These phase shifts cast doubt on the use of disk emission lines in thedetermination of mass ratios. We present a systematic study of phaseshifts, using data from the literature to distinguish between possibleexplanations of the phase shift. We find that one widely adopted classof models is contradicted by observations. We present a generalized formof another class of models, which we call ``measurement-offset models.''We show that these models are quantitatively consistent with existingdata. We consider the implications of adopting measurement-offsetmodels, for both disk structure and determination of binary parameters.Specifically, we describe how measurement-offset models may be usedimprove determinations of the mass ratio based on disk emission lines.This could be a valuable new tool in determining masses of importantastrophysical objects such as accreting neutron stars and black holes.
| 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.
| The variability plane of accreting compact objects
Recently, it has been shown that soft-state black hole X-ray binariesand active galactic nuclei populate a plane in the space defined by theblack hole mass, accretion rate and characteristic frequency. We showthat this plane can be extended to hard-state objects if one allows aconstant offset for the frequencies in the soft and the hard state.During a state transition, the frequencies rapidly move from one scalingto the other depending on an additional parameter, possibly the discfraction. The relationship among frequency, mass and accretion rate canbe further extended by including weakly accreting neutron stars (NSs).We explore if the lower kHz quasi-periodic oscillations of NSs and thedwarf nova oscillations of white dwarfs can be included as well anddiscuss the physical implications of the found correlation.
| 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
| 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.
| Theoretical Orbital Period Distributions of Cataclysmic Variables above the Period Gap: Effects of Circumbinary Disks
Population synthesis tools are used to investigate the population ofnonmagnetic cataclysmic variables with unevolved main-sequence-likedonors at orbital periods greater than 2.75 hr. In addition to theangular momentum losses associated with gravitational radiation,magnetic braking, and mass loss from the system, we also include theeffects of circumbinary disks on the evolution. For a fractional massinput rate into the disk, corresponding to 3×10-4 ofthe mass transfer rate, the model systems exhibit a bounce at orbitalperiods greater than 2.75 hr. The simulations reveal that (1) somesystems can exist as dwarf novae throughout their lifetime, (2) dwarfnovae can evolve into novalike systems, and (3) novalike systems canevolve back into dwarf novae during their postbounce evolution to longerorbital periods. Among these subclasses, novalike cataclysmic variableswould be the best candidates to search for circumbinary disks atwavelengths >~10 μm. The theoretical orbital period distributionis in reasonable accord with the combined population of dwarf novae andnovalike systems above the period gap, suggesting the possibility thatsystems with unevolved donors need not detach and evolve below theperiod gap as in the disrupted magnetic braking model. The resultingpopulation furthermore reveals the possible presence of systems withsmall mass ratios and a preference of O/Ne/Mg white dwarfs in dwarf novasystems in comparison to novalike systems. The novalike populationfurthermore shows a lack of systems with high-mass white dwarfs. Theimportance of observational bias in accounting for the differingpopulations is examined, and it is shown that an understanding of theseeffects is necessary in order to confront the theoretical distributionswith the observed ones in a meaningful manner.
| 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.
| A search for evidence of irradiation in Centaurus X-4 during quiescence
Aims.We present a study of the neutron star X-Ray Transient Cen X-4. Ouraim is to look for any evidence of irradiation of the companion with adetailed analysis of its radial velocity curve, relative contribution ofthe donor star and Doppler tomography of the main emission lines.Methods: .To improve our study all our data are compared with a set ofsimulations that consider different physical parameters of the system,like the disc aperture angle and the mass ratio. Results: .Weconclude that neither the radial velocity curve nor the orbitalvariation of the relative donor's contribution to the total flux areaffected by irradiation. On the other hand, we do see emission from thedonor star at H? and HeI ?5876 which we tentativelyattribute to irradiation effects. In particular, the H? emissionfrom the companion is clearly asymmetric and we suggest is produced byirradiation from the hot-spot. Finally, from the velocity of the HeI?5876 spot we constrain the disc opening angle to?=7°-14°.
| 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.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Yelken |
| Sað Açýklýk: | 08h15m18.97s |
| Yükselim: | -49°13'20.7" |
| Görünürdeki Parlaklýk: | 9.441 |
| özdevim Sað Açýklýk: | -26.7 |
| özdevim Yükselim: | -96.1 |
| B-T magnitude: | 9.412 |
| V-T magnitude: | 9.439 |
Kataloglar ve belirtme:
|
