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AR Ursae Majoris Discovered to Be a Persistent Radio Polar: Results from a VLA Survey of Magnetic Cataclysmic Variables
We conducted a VLA survey of nine magnetic cataclysmic variable stars(MCVs) at 8.436 GHz in 2003 October, to constrain models of radioemission. A follow-up study was conducted in 2004 September. We obtainedthe first radio detections of AR UMa, the MCV with the highest knownmagnetic field (230 MG), at flux densities of 0.422+/-0.060 mJy (2003October 16) and 0.734+/-0.095 mJy (2004 September 4). We have completedthe sample of radio observations of MCVs out to 100 pc. Surprisingly,the only radio-selected CV, FIRST J1023+0038, was not detected. An upperlimit of 0.20 mJy (4 σ) was obtained. The original radio MCV, AMHer, was detected with a flux density of 0.586+/-0.074 mJy (2003 October17), consistent with previous observations. Typical (4 σ) upperlimits of 0.12 mJy were obtained for other targets. The lack of radioemission from isolated magnetic white dwarfs, as well as fromnonmagnetic CVs (outside of outburst), places constraints on radioemission mechanisms for CVs. We suggest that accretion disks maypreclude radio emission. Also, since we detect radio emission from AMHer in a low state and the pre-CV V471 Tau is a persistent radio source,accretion is not a necessary condition. A secondary with a kilogaussmagnetic field might be necessary. We argue that the emission from ARUMa originates near the secondary. AR UMa joins AM Her and AE Aqr as theonly confirmed persistent radio MCVs, and we suggest avenues of furtherstudy.
| A Dusty Disk around WD 1150-153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks
We report the discovery of excess K-band radiation from a metal-rich DAVwhite dwarf star, WD 1150-153. Our near-infrared spectroscopicobservations show that the excess radiation cannot be explained by a(sub)stellar companion, and is likely to be caused by a debris disksimilar to the other DAZ white dwarfs with circumstellar debris disks.We find that the fraction of DAZ white dwarfs with detectable debrisdisks is at least 14%. We also revisit the problem of explaining themetals in white dwarf photospheres by accretion from the interstellarmedium (ISM). We use the observed interstellar column densities towardstars in close angular proximity and similar distance as DAZ whitedwarfs to constrain the contribution of accretion from the ISM. We findno correlation between the accretion density required to supply metalsobserved in DAZs with the densities observed in their interstellarenvironment, indicating that ISM accretion alone cannot explain thepresence of metals in nearby DAZ white dwarfs. Although ISM accretionwill certainly contribute, our analysis indicates that it is not thedominant source of metals for most DAZ white dwarfs. Instead, thegrowing number of circumstellar debris disks around DAZs suggests thatcircumstellar material may play a more dominant role in polluting thewhite dwarf atmospheres.
| The mass and radius of the M-dwarf in the short-period eclipsing binary RR Caeli
We present new photometry and spectroscopy of the eclipsing whitedwarf-M-dwarf binary star RR Cae. We use timings of the primary eclipsefrom white-light photo-electric photometry to derive a new ephemeris forthe eclipses. We find no evidence for any period change greater thanover a time-scale of 10 yr. We have measured the effective temperatureof the white dwarf, TWD, from an analysis of two highresolution spectra of RR Cae and find TWD = 7540K +/- 175K.We estimate a spectral type of M4 for the companion from the samespectra. We have measured the radial velocity of the white dwarf fromthe Balmer absorption lines and find that the semi-amplitude of thespectroscopic orbit is KWD = 79.3 +/- 3.0kms-1. Wehave combined our radial velocity measurements of the M-dwarf withpublished radial velocities to determine a new spectroscopic orbit forthe M-dwarf with a semi-amplitude of KM = 190.2 +/-3.5kms-1. We have combined this information with an analysisof the primary eclipse to derive relations between the inclination ofthe binary and the radii of the two stars. We use cooling models forhelium white dwarfs with a wide range of hydrogen layer masses todetermine the likely range of the white dwarf radius and, thus, theinclination of the binary and the mass and radius of the M-dwarf. Themass of the M-dwarf is (0.182-0.183) +/- 0.013 Msolar and theradius is (0.203-0.215) +/- 0.013 Rsolar, where the rangesquoted for these values reflect the range of white dwarf models used. Incontrast to previous studies, which lacked a measurement ofKWD, we find that the mass and radius of the M-dwarf arenormal for an M4 dwarf. The mass of the white dwarf is 0.440 +/-0.022Msolar. With these revised masses and radii we find thatRR Cae will become a cataclysmic variable star when the orbital periodis reduced from its current value of 7.3 h to 121min by magnetic brakingin 9-20Gyr. We note that there is night-to-night variability of a fewseconds in the timing of primary eclipse caused by changes to the shapeof the primary eclipse. We speculate as to the possible causes of thisphenomenon.
| The secrets of T Pyxidis. I. UV observations
Aims.We study the UV spectral behavior of the recurrent nova T Pyxduring 16 years of IUE observations. Methods:.We examined both the IUE line-by-line images and theextracted spectra in order to understand the reality and the origin ofthe observed spectral variations. We compare different extractionmethods and their influence on the spectrum of an extendedobject. Results: .The UV continuum of T Pyx hasremained nearly constant in slope and intensity over this time interval,without any indication of long-term trends. The reddening determinedfrom the UV data is EB-V=0.25 ± 0.02. The bestsingle-curve fit to the dereddened UV continuum is a power-lawdistribution ∝λ-2.33. The tail of this curveagrees well with the B, V, and J magnitudes of T Pyx, indicating thatthe contribution of the secondary star is negligible. One peculiaraspect of T Pyx is that most emission lines (the strongest ones beingthose of CIV 1550 and HeII 1640) show substantial changes both inintensity and detectability, in contrast to the near constancy of thecontinuum. Several individual spectra display emission features that aredifficult to identify, suggesting a composite spectroscopic system. Wetentatively ascribe the origin of these transient emission featureseither to loops and jets from the irradiated secondary or to movingknots of the surrounding nebula that are (temporarily) projected infront of the system. The inspection of all IUE line-by-line images hasled to the detection of emission spikes outside the central strip of thespectrum, which in some cases seem associated to known emission featuresin the (main) spectrum. A comparison with other ex-novae reveals asurprising similarity to the spectrum of the very-slow nova HR Del,whose white dwarf primary has a mass that is allegedly about one halfthat of T Pyx.
| The nature of the close magnetic white dwarf + probable brown dwarf binary SDSSJ121209.31+013627.7
Optical time series photometry of the short-period magnetic white dwarf+ probable brown dwarf binary SDSSJ121209.31+013627.7 reveals pulse-likevariability in all bands from i' to u', increasing towards bluerwavelengths and peaking at u'. These modulations are most likely due toa self-eclipsing accretion hot spot on the white dwarf, rotating intoview every 88.43 min. This period is commensurate with the Hαradial velocity period of ~90 min, and consistent with the rotationperiod of the accretor being equal to the binary orbital period. Wecombine our observations with other recently reported results to providean accurate ephemeris. We also detect the system in X-rays with Swift,and estimate the accretion rate at ~10-13Msolaryr-1. We suggest that SDSSJ121209.31+013627.7 is most likelya magnetic cataclysmic variable in an extended state of very lowaccretion, similar to the well-studied polar EF Eri. Alternatively, theputative brown dwarf is not filling its Roche lobe and the system is adetached binary in which the white dwarf is efficiently accreting fromthe wind of the secondary. However, it is unclear whether an L dwarfwind is strong enough to provide the measured accretion rate. We suggestfurther observations to distinguish between the Roche lobe overflow andwind accretion scenarios.Based on observations made with the Faulkes Telescope North, the IsaacNewton Telescope and the Wide Field Camera, the William HerschelTelescope and ULTRACAM high-speed photometer and the Swift spaceobservatory.E-mail: mbu@star.le.ac.uk
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| Mass Determination and Detection of the Onset of Chromospheric Activity for the Substellar Object in EF Eridani
EF Eri is a magnetic cataclysmic variable that has been in a lowaccretion state for the past 9 yr. Low-state optical spectra reveal theunderlying Zeeman-split white dwarf absorption lines. These features areused to determine a value of 13-14 MG as the white dwarf field strength.Recently, 5-7 yr into the low state, Balmer and other emission lineshave appeared in the optical. An analysis of the Hα emission lineyields the first radial velocity solution for EF Eri, leading to aspectroscopic ephemeris for the binary and, using the best availablewhite dwarf mass of 0.6 Msolar, a mass estimate for thesecondary of 0.055 Msolar. For a white dwarf mass of 0.95Msolar, the average for magnetic white dwarfs, the secondarymass increases to 0.087 Msolar. At EF Eri's orbital period of81 minutes, this higher mass secondary could not be a normal star andstill fit within the Roche lobe. The source of the Balmer and otheremission lines is confirmed to be from the substellar secondary, and weargue that it is due to stellar activity. We compare EF Eri'semission-line spectrum and activity behavior to that recently observedin AM Her and VV Pup and attributed to stellar activity. We exploreobservations and models originally developed for V471 Tau, for the RSCVn binaries, and for extrasolar planets. We conclude that irradiationof the secondary in EF Eri and similar systems is unlikely and, inpolars, the magnetic field interaction between the two stars (with apossible tidal component) is a probable mechanism that would concentratechromospheric activity on the secondary near the substellar point of thewhite dwarf.
| The secondary star and distance of the polar V1309 Orionis
Context: . The first phase-resolved JHK light curves of the eclipsingpolar (AM Herculis binary) V1309 Ori are presentedand interpreted. Aims: . We separate the contributions from thesecondary star and from other sources with the aim of determining aphotometric distance. Methods: . Simple model calculations showthat the accretion stream and the cyclotron source on the accretingwhite dwarf are minor contributors to the infrared light, allowing anaccurate determination of spectral type and absolute flux of thesecondary star. Results: . The unilluminated backside of thesecondary star as seen in eclipse has spectral type dM0 to dM0+. Itsdereddened magnitude is K=13.58 at orbital phase φ=0 (eclipse).Using the calibrated surface brightness of M-stars and the publishedmass of the secondary, M_2=0.46 {M}_ȯ, we obtain a distanced=600± 25 pc which scales as M_2^1/2. The radius of theRoche-lobe filling secondary exceeds the main-sequence radius of an M0star by 21+11 -6%. Conclusions: . Thedebated origin of the infrared light of V1309 Ori hasbeen settled in favor of the secondary star as the main contributor andan accurate distance has been derived that will place estimates of theluminosity and synchronization time scale on a more secure basis.
| The new pre-cataclysmic binary PG 2200+085
Aims.We present the results of spectroscopic- and orbit-sampledphotometric observations of the faint UV-excess object PG 2200+085. Methods: .The optical CCD photometry observations of this object wereperformed by the Russian-Turkish 1.5-m telescope RTT150 at the TUBITAKNational Observatory (Turkey). The long-slit optical spectroscopyobservations with 2.6 Å resolution were carried out by 6-mtelescope BTA at the Special Astrophysical Observatory (Russia).Results: .The photometric variations over two nights are almostsinusoidal with an amplitude Δ mV = 0.04m and a periodof P = 0.3186 d. Such a light curve is typical of a detached closebinary with an illumination effect or the ellipsoidal deformation of asecondary star. The observed spectrum clearly displays a featurelessblue continuum of a hot component and a rich absorption-line andmolecular band K-star spectrum. The CaII line profiles with strongemission cores are remarkably similar to those of V471 Tau.Conclusions: .We tentatively classify PG 2200+085 as a pre-cataclysmicbinary of the V471 Tau type.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Spot patterns and differential rotation in the eclipsing pre-cataclysmic variable binary, V471 Tau
We present surface spot maps of the K2V primary star in thepre-cataclysmic variable binary system, V471 Tau. The spot maps show thepresence of large high-latitude spots located at the sub-white dwarflongitude region. By tracking the relative movement of spot groups overthe course of four nights (eight rotation cycles), we measure thesurface differential rotation rate of the system. Our results revealthat the star is rotating rigidly with a surface shear rate, dΩ=1.6 +/- 6mradd-1. The single active star AB Dor has a similarspectral type, rotation period and activity level as the K star in V471Tau, but displays much stronger surface shear (46 < dΩ <58mradd-1). Our results suggest that tidal locking mayinhibit differential rotation; this reduced shear, however, does notaffect the overall magnetic activity levels in active K dwarfs.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| On the interstellar extinction law toward young stars
We have determined the atomic hydrogen column density N HI toward all ofthe young stars from the Taurus-Auriga-Perseus star-forming complex forwhich the corresponding spectra are available in the Hubble SpaceTelescope archive (nine stars) by analyzing the Lyα line profile.We show that the stars studied, except DR Tau, lie not far from the edgeof the gaseous cloud of the star-forming region closest to us or, moreprecisely, inside the outer H I shell of the cloud. This shell with acolumn density of N HI ≃ 6 × 1020 cm-2 surrounds themolecular gas of the cloud composed of a diffuse component (theso-called diffuse screen) in which dense, compact TMC-1 cores areembedded. The properties of the dust grains toward the stars that lie atthe front edge of the cloud most likely differ only slightly from thoseof the interstellar dust outside star-forming regions. This casts doubton the validity of the hypothesis that the extinction curve toward youngstars has an anomalously low amplitude of the 2175 Åbump—such an extinction curve is observed for the field stars HD29647 and HD 283809 toward which the line of sight passes through theTMC-1 core.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Precatalysmic binary systems.
Not Available
| Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs
We report the detection, from the Far Ultraviolet Spectroscopic Explorer(FUSE) data, of phosphorus in the atmospheres of GD71 and two similar DAwhite dwarfs. This is the first detection of a trace metal in thephotosphere of the spectrophotometric standard star GD71. Collectively,these objects represent the coolest DA white dwarfs in whichphotospheric phosphorus has been observed. We use a grid of homogeneousnon-local thermodynamic equilibrium synthetic spectra to measureabundances of [P/H]=-8.57+0.09-0.13,-8.70+0.23-0.37 and-8.36+0.14-0.19 in GD71, RE J1918+595 and REJ0605-482 respectively. At the observed level we find that phosphorushas no significant impact on the overall energy distribution of GD71. Weexplore possible mechanisms responsible for the presence of this elementin these stars, concluding that the most likely is an interplay betweenradiative levitation and gravitational settling, possibly modified byweak mass loss.
| An XMM-Newton Study of the Coronae of σ2 Coronae Borealis
We present results of XMM-Newton Guaranteed Time observations of the RSCVn binary σ2 Coronae Borealis. The spectra obtainedwith the Reflection Grating Spectrometers and the European PhotonImaging Camera MOS2 were simultaneously fitted with collisionalionization equilibrium plasma models to determine coronal abundances ofvarious elements. Contrary to the solar first ionization potential (FIP)effect, in which elements with a low FIP are overabundant in the coronacompared to the solar photosphere, and contrary to the ``inverse'' FIPeffect observed in several active RS CVn binaries, coronal abundanceratios in σ2 CrB show a complex pattern, as supportedby similar findings in the Chandra HETGS analysis of σ2CrB with a different methodology by Osten and coworkers in 2003. Low-FIPelements (<10 eV) have abundance ratios relative to Fe that areconsistent with the solar photospheric ratios, whereas high-FIP elementshave abundance ratios that increase with increasing FIP. We find thatthe coronal Fe abundance is consistent with the stellar photosphericvalue, indicating that there is no metal depletion inσ2 CrB. However, we obtain a higher Fe absoluteabundance than Osten and coworkers did. Except for Ar and S, ourabsolute abundances are about 1.5 times larger than those reported byOsten and coworkers. However, a comparison of their model with ourXMM-Newton data (and vice versa) shows that both models work adequatelyin general. We find, therefore, no preference for one methodology overthe other for deriving coronal abundances. Despite the systematicdiscrepancy in absolute abundances, our abundance ratios are very closeto those obtained by Osten and coworkers. Finally, we confirm themeasurement of a low density in O VII (<4×1010cm-3) but could not confirm the higher densities measured inspectral lines formed at higher temperatures that were derived by otherstudies of σ2 CrB due to the lower spectral resolutionof the XMM-Newton grating spectrometers.
| New findings based on long-term photometric observations of the eclipsing binary V471 Tauri
The post-common envelope and pre-cataclysmic binary V471 Tau has beenobserved by the authors since 1973. At least a complete light curve in Band V bands and more than two eclipse timings were obtained in eachyear. All the available data published so far (including the authors')have been collected and analysed for the brightness and orbital periodchanges. The system brightened about 0.22 mag in both B and V bands moreor less regularly up to 1997 and started to decrease afterwards. Asearch for periodicity of this variation yields a period longer than 85yr. In addition to this long-period variation, a small amplitude ofabout 0.08 mag and short time-interval fluctuations on the meanbrightness have been detected. The variations of the mean brightnesshave been discussed and plausible causes suggested. The changes of theapparent period have been attributed to a third body. Analysis of allthe `observed-calculated' (O-C) data yields a period of 32.4 yr, with asemi-amplitude of 151 s and an eccentricity of 0.30 for the third-bodyorbit. For orbital inclinations greater than 34° the mass of thethird body would possibly match to a brown dwarf. One of the mostinteresting features in the light curve of V471 Tau is the decrement ofthe eclipse depth with time. The depth of the eclipse in the B band hasbeen decreased from 0.082 to 0.057 mag over 34 yr. Subtracting thevariation of the depth due to the brightening of the red dwarf star, theactual variation in depth, originated from from the white dwarf, wasfound to be about 0.012 mag. This change in the brightness of thecompact object has been attributed to the mass accretion from itsprimary component via thermally driven wind and/or flare-like events.
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Results from the Whole Earth Telescope - Indian Contributions
The Indian team at ISRO has been part of the Whole Earth Telescope (WET)team since 1988 when we first participated in the campaign on V471 Tau.We have been part of many other observing campaigns over the lastdecade. This presentation traces the circumstances leading to ourjoining the WET team and how useful the coverage from the Indianlongitude has been. The results of several pulsators from the WET runsduring which we participated are also described. These includePG1159-035 the prototype of the GWVir type of stars, RE J 0751 +14 acataclysmic variable, PG 1336-018 a binary with an sdB pulsator andfinally HR 1217 a roAp star. The paper concludes with what thelimitations are in our obser-vations and how we can overcome them in thefuture.
| The Loopy Ultraviolet Line Profiles of RU Lupi: Accretion, Outflows, and Fluorescence
We present far-ultraviolet (FUV) spectra of the classical T Tauri starRU Lup covering the 912-1710 Å spectral range, as observed by theHubble Space Telescope STIS and the Far Ultraviolet SpectroscopicExplorer satellite. We use these spectra, which are rich in emission andabsorption lines, to probe both the accreting and outflowing gas.Absorption in the Lyα profile constrains the extinction toAV~0.07 mag, which we confirm with other diagnostics. Weestimate a mass accretion rate of (5+/-2)×10-8Msolar yr-1 using the optical-NUV accretioncontinuum. The accreting gas is also detected in bright, broad lines ofC IV, Si IV, and N V, which all show complex structures across the lineprofile. Many other emission lines, including those of H2 andFe II, are pumped by Lyα. RU Lup's spectrum varies significantlyin the FUV; our STIS observations occurred when RU Lup was brighter thanseveral other observations in the FUV, possibly because of a high massaccretion rate.
| Six detached white-dwarf close binaries
We determine the orbits of four double-degenerate systems (DDs),composed of two white dwarfs and two white-dwarf-M-dwarf binaries. Thefour DDs, WD1022+050, WD1428+373, WD1824+040 and WD2032+188, showorbital periods of 1.157155(5), 1.15674(2), 6.26602(6) and 5.0846(3) d,respectively. These periods combined with estimates for the masses ofthe brighter component, based on their effective temperatures, allow usto constrain the masses of the unseen companions. We estimate that theupper limit for the contribution of the unseen companions to the totalluminosity in the four DDs ranges between 10 and 20 per cent. In thecase of the two white-dwarf-M-dwarf binaries, WD1042-690 and WD2009+622,we calculate the orbital parameters by fitting simultaneously theabsorption line from the white dwarf and the emission core from the Mdwarf. Their orbital periods are 0.337083(1) and 0.741226(2) d,respectively. We find signatures of irradiation on the inner face of thecompanion to WD2009+622. We calculate the masses of both components fromthe gravitational redshift and the mass-radius relationship for whitedwarfs and find masses of 0.75-0.78 and 0.61-0.64Msolar forWD1042-690 and WD2009+622, respectively. This indicates that the starsprobably reached the asymptotic giant branch in their evolution beforeentering a common envelope phase. These two white-dwarf-M-dwarf binarieswill become cataclysmic variables, although not within a Hubble time,with orbital periods below the period gap.
| Where Are the Magnetic White Dwarfs with Detached, Nondegenerate Companions?
The Sloan Digital Sky Survey has already more than doubled the sample ofwhite dwarfs with spectral classifications, the subset with detached Mdwarf companions, and the subset of magnetic white dwarfs. In the courseof assessing these new discoveries, we have noticed a curious,unexpected property of the total lists of magnetic white dwarfs and ofwhite dwarf plus main-sequence binaries: there appears to be virtuallyzero overlap between the two samples! No confirmed magnetic white dwarfhas yet been found in such a pairing with a main-sequence star. The samestatement can be made for the samples of white dwarf-M dwarf pairs inwide, common proper motion systems. This contrasts with the situationfor interacting binaries, in which an estimated 25% of the accretingsystems have a magnetic white dwarf primary. Alternative explanationsare discussed for the observed absence of magnetic whitedwarf-main-sequence pairs, but the recent discoveries of very lowaccretion rate magnetic binaries pose difficulties for each. A plausibleexplanation may be that the presence of the companion and the likelylarge mass and small radius of the magnetic white dwarf (relative tononmagnetic degenerate dwarfs) may provide a selection effect againstthe discovery of the latter in such binary systems. More carefulanalysis of the existing samples may yet uncover members of this classof binary, and the sample sizes will continue to grow. The question ofwhether the mass and field distributions of the magnetic primaries ininteracting binaries are similar to those of the isolated magnetic whitedwarfs (including those in wider binaries) must also be answered.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin-up
The zero-age main-sequence star AB Dor and the K dwarf component of theV471 Tau close binary have essentially identical rotation rates andspectral types. An analysis of their high-resolution Chandra X-rayspectra reveals remarkably similar coronal characteristics in terms ofboth temperature structure and element abundances. Both stars showdepletions of low first ionization potential (FIP) elements by factorsof ~3, with higher FIP elements showing more mild depletions. Noevidence for enhancements of very low FIP (<7 eV) elements, such asNa, Al, and Ca, as compared to other low-FIP elements, was found. Theabundance anomaly pattern for AB Dor and V471 Tau is similar to,although less extreme than, the abundance anomalies exhibited by activeRS CVn-type binaries. While we find statistically significant structurein the underlying differential emission measure distributions of thesestars over narrow temperature intervals, this structure is stronglydependent on the lines used in the analysis and is probably spurious. Onthe basis of their X-ray similarities, we conclude that the exactevolutionary state of a star has little effect on coronalcharacteristics and that the parameters that dominate coronal structureand composition are simply the rotation rate and spectral type.
| Inferring Coronal Structure from X-Ray Light Curves and Doppler Shifts: A Chandra Study of AB Doradus
The Chandra X-Ray Observatory continuously monitored the single coolstar AB Dor for a period lasting 88 ks (1.98Prot) in 2002December with the Low-Energy Transmission Grating HRC-S. The X-ray lightcurve shows rotational modulation with three peaks that repeat in twoconsecutive rotation cycles. These peaks may indicate the presence ofcompact emitting regions in the quiescent corona. Centroid shifts as afunction of phase in the strongest line profile, O VIII λ18.97,indicate Doppler rotational velocities with a semiamplitude of 30+/-10km s-1. By taking these diagnostics into account along withconstraints on the rotational broadening of line profiles (provided byarchival Chandra High-Energy Transmission Grating Fe XVII and FarUltraviolet Spectroscopic Explorer Fe XVIII profiles), we can constructa simple model of the X-ray corona that requires two components. One ofthese components is responsible for 80% of the X-ray emission and arisesfrom the pole and/or a homogeneously distributed corona. The secondcomponent consists of two or three compact active regions that causemodulation in the light curve and contribute to the O VIII centroidshifts. These compact regions account for 16% of the emission and arelocated near the stellar surface with heights of less than0.3R*. At least one of the compact active regions is locatedin the partially obscured hemisphere of the inclined star, while anotherof the active regions may be located at 40°. High-quality X-ray datasuch as these can test the models of the coronal magnetic fieldconfiguration as inferred from magnetic Zeeman Doppler imaging.
| Notices to investigation of symbiotic binaries. III. Approximation of the Roche lobe parameters for asynchronously rotating star in a binary system
We derive approximative analytical formulas for the basic parameters ofthe Roche lobe, its radius and the position of the L1-point,for asynchronously rotating component in a binary system. Our solutionis valid in the range of the mass ratio 0.1 < q < 10 and theparameter 1 ≤ p ≤ 20 (p = Porb/Prot).Deviations between numerical solution and that given by our analyticalapproximation are less than 7%.
| Reconstructing the evolution of white dwarf binaries: further evidence for an alternative algorithm for the outcome of the common-envelope phase in close binaries
We determine the possible masses and radii of the progenitors of whitedwarfs in binaries from fits to detailed stellar evolution models anduse these to reconstruct the mass-transfer phase in which the whitedwarf was formed. We confirm the earlier finding that in the first phaseof mass transfer in the binary evolution leading to a close pair ofwhite dwarfs, the standard common-envelope formalism (theα-formalism) equating the energy balance in the system (implicitlyassuming angular momentum conservation) does not work. An algorithmequating the angular momentum balance (implicitly assuming energyconservation) can explain the observations. This conclusion is now basedon 10 observed systems rather than three. With the latter algorithm (theγ-algorithm) the separation does not change much for approximatelyequal-mass binaries. Assuming constant efficiency in the standardα-formalism and a constant value of γ, we investigate theeffect of both methods on the change in separation in general andconclude that when there is observational evidence for strong shrinkageof the orbit, the γ-algorithm also leads to this. We then extendour analysis to all close binaries with at least one white dwarfcomponent and reconstruct the mass-transfer phases that lead to thesebinaries. In this way we find all possible values of the efficiency ofthe standard α-formalism and of γ that can explain theobserved binaries for different progenitor and companion masses. We findthat all observations can be explained with a single value of γ,making the γ-algorithm a useful tool to predict the outcome ofcommon-envelope evolution. We discuss the consequences of our findingsfor different binary populations in the Galaxy, including massivebinaries, for which the reconstruction method cannot be used.
| The structure of our stellar system.
Not Available
| RXJ2130.6+4710 - an eclipsing white dwarf-M-dwarf binary star
We report the detection of eclipses in the close white-dwarf-M-dwarfbinary star RXJ2130.6+4710. We present light curves in the B, V and Ibands and fast photometry obtained with the three-channel CCD photometerUltracam of the eclipse in the u', g' and r' bands. The depth of theeclipse varies from 3.0 mag in the u' band to less than 0.1 mag in the Iband. The times of mid-eclipse are given by the ephemerisBJD(mid-eclipse) = 2452785.681876(2) + 0.521035625(3) E, where figuresin parentheses denote uncertainties in the final digit. We presentmedium-resolution spectroscopy from which we have measured thespectroscopic orbits of the M dwarf and white dwarf. We estimate thatthe spectral type of the M dwarf is M3.5Ve or M4Ve, although the data onwhich this is based are not ideal for spectral classification. We havecompared the spectra of the white dwarf with synthetic spectra from purehydrogen model atmospheres to estimate that the effective temperature ofthe white dwarf is Teff= 18000 +/- 1000 K. We have used thewidth of the primary eclipse and duration of totality measured preciselyfrom the Ultracam u' data combined with the amplitude of the ellipsoidaleffect in the I band and the semi-amplitudes of the spectroscopic orbitsto derive masses and radii for the M dwarf and white dwarf. The M dwarfhas a mass of 0.555 +/- 0.023 Msolar and a radius of 0.534+/- 0.053 Rsolar, which is a typical radius for stars of thismass. The mass of the white dwarf is 0.554 +/- 0.017 Msolarand its radius is 0.0137 +/- 0.0014 Rsolar, which is theradius expected for a carbon-oxygen white dwarf of this mass andeffective temperature. The light curves are affected by frequent flaresfrom the M dwarf and the associated dark spots on its surface can bedetected from the distortions to the light curves and radial velocities.RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star thatwill start mass transfer at a period above the period gap forcataclysmic variables.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | שור |
| התרוממות ימנית: | 03h50m24.97s |
| סירוב: | +17°14'47.4" |
| גודל גלוי: | 9.464 |
| מרחק: | 46.795 פארסק |
| תנועה נכונה: | 127.5 |
| תנועה נכונה: | -22.9 |
| B-T magnitude: | 10.512 |
| V-T magnitude: | 9.551 |
קטלוגים וכינוים:
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