Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Electron density diagnostic potential of the SiX ion and its application in Procyon
Theoretical electron density sensitive line ratiosR1-R6 of SiX soft X-ray emission lines arepresented. We found that these line ratios are sensitive to electrondensity ne, and the ratio R1 is insensitive toelectron temperature Te. For reliable determination of theelectron density of laboratory and astrophysical plasmas, atomic data,such as electron impact excitation rates, are very important. Ourresults reveal that the discrepancy of the line ratios from differentatomic data calculated with the distorted wave (DW) approximation andthe R-matrix method is up to 19 per cent at ne= 2 ×108cm-3. We applied the theoretical intensityratio R1 to the Low Energy Transmission Grating Spectrometer(LETGS) spectrum of the solar-like star Procyon. By comparing theobserved value (1.29) with the theoretical calculation, the derivedelectron density ne is 2.6 ×108cm-3, which is consistent with that derivedfrom (CV <8.3 × 108cm-3). When thetemperature structure of the Procyon corona is taken into account, thederived electron density increases from ne= 2.6 ×108 to 2.8 × 108cm-3.
| New Times of Minima of Eclipsing Binary Systems and of Maximum of SXPHE Type Stars
We present 64 photoelectric minima observations of 31 eclipsingbinaries. We also report three new times of maxima of three SXPHE typepulsating stars.
| Strichspuraufnahmen zur Veraenderlichenbeobachtung.
Not Available
| Beobachtung heller Veraenderlicher mit CCD.
Not Available
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| The Effect of a Third Body on the Orbital Parameters of the Secondary Component in the System of Algol
The perturbing effects of the third component on the orbital parametersof the secondary component in the system of Algol are numericallyestimated using the method of the theory of perturbation. The equationsof secular perturbation are derived using the method of integration ofthe average value to solve the Lagrangian equations. The results showthat the perturbing effect of the third body on the orbital parametersof the secondary component is not negligible.
| X-ray luminosity function of faint point sources in the Milky Way
We assessed the contribution to the X-ray (above 2 keV) luminosity ofthe Milky Way by different classes of low-mass binary systems and singlestars. We began by using the RXTE Slew Survey of the sky at|b|>10° to construct an X-ray luminosity function (XLF) of nearbyX-ray sources in the range 1030 ergs-1
| Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
| Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| PHASES differential astrometry and the mutual inclination of the V819 Herculis triple star system
V819 Herculis is a well-studied triple star system consisting of a"wide" pair with 5.5 year period, one component of which is a 2.2-dayperiod eclipsing single-line spectroscopic binary. Differentialastrometry measurements from the Palomar High-precision AstrometricSearch for Exoplanet Systems (PHASES) are presented and used todetermine a relative inclination between the short- and long-periodorbits of 23.6 ± 4.9 degrees. This represents only the sixthunambiguous determination of the mutual inclination of orbits in ahierarchical triple system. This result is combined with those for theother five systems for analysis of the observed distribution of mutualinclinations in nearby triple systems. It is found that thisdistribution is different than that which one would expect from randomorientations with statistical significance at the 94% level;implications for studying the spatial distribution of angular momentumin star forming regions is discussed.
| Automatic classification of eclipsing binaries light curves using neural networks
In this work we present a system for the automatic classification of thelight curves of eclipsing binaries. This system is based on aclassification scheme that aims to separate eclipsing binary systemsaccording to their geometrical configuration in a modified version ofthe traditional classification scheme. The classification is performedby a Bayesian ensemble of neural networks trained with Hipparcos data ofseven different categories including eccentric binary systems and twotypes of pulsating light curve morphologies.
| Gravity-darkening exponents in semi-detached binary systems from their photometric observations. II.
This second part of our study concerning gravity-darkening presents theresults for 8 semi-detached close binary systems. From the light-curveanalysis of these systems the exponent of the gravity-darkening (GDE)for the Roche lobe filling components has been empirically derived. Themethod used for the light-curve analysis is based on Roche geometry, andenables simultaneous estimation of the systems' parameters and thegravity-darkening exponents. Our analysis is restricted to theblack-body approximation which can influence in some degree theparameter estimation. The results of our analysis are: 1) For four ofthe systems, namely: TX UMa, β Per, AW Cam and TW Cas, there is avery good agreement between empirically estimated and theoreticallypredicted values for purely convective envelopes. 2) For the AI Drasystem, the estimated value of gravity-darkening exponent is greater,and for UX Her, TW And and XZ Pup lesser than corresponding theoreticalpredictions, but for all mentioned systems the obtained values of thegravity-darkening exponent are quite close to the theoretically expectedvalues. 3) Our analysis has proved generally that with the correction ofthe previously estimated mass ratios of the components within some ofthe analysed systems, the theoretical predictions of thegravity-darkening exponents for stars with convective envelopes arehighly reliable. The anomalous values of the GDE found in some earlierstudies of these systems can be considered as the consequence of theinappropriate method used to estimate the GDE. 4) The empiricalestimations of GDE given in Paper I and in the present study indicatethat in the light-curve analysis one can apply the recent theoreticalpredictions of GDE with high confidence for stars with both convectiveand radiative envelopes.
| All-Sky Relative Opacity Mapping Using Nighttime Panoramic Images
An all-sky cloud-monitoring system that generates relative opacity mapsover many of the world's premier astronomical observatories isdescribed. Photometric measurements of numerous background stars arecombined with simultaneous sky brightness measurements to differentiatethin clouds from sky glow sources such as airglow and zodiacal light.The system takes a continuous pipeline of all-sky images and comparesthem to canonical images taken on other nights at the same siderealtime. Data interpolation then yields transmission maps covering almostthe entire sky. An implementation of this system is currently operatingthrough the Night Sky Live network of CONCAM3s located at CerroPachón (Chile), Mauna Kea (Hawaii), Haleakala (Hawaii), SALT(South Africa), and the Canary Islands (Northwestern Africa).
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| Geometrical constraints upon the unipolar model of V407 Vul and RXJ0806.3+1527
V407 Vul and RX J0806.3+1527 are X-ray emitting stars with X-ray lightcurves that are 100 per cent modulated on periods of 569 and 321 s,respectively. These periods and no others are also seen at optical andinfrared wavelengths. These properties have led to the suggestion thatthe periods are the orbital periods of ultracompact pairs of whitedwarfs. There are two main double white dwarf models: the unipolarinductor model analogous to the Jupiter-Io system and the direct impactmodel analogous to Algol. In this paper, we consider geometricalconstraints on the unipolar inductor model, in particular what parametervalues (component masses, orbital inclination and magnetic colatitude)can describe the X-ray and optical light curves. We find that for adipole field on the primary star, the unipolar inductor model fails tomatch the data on V407 Vul for any combination of parameters and canonly match RX J0806.3+1527 if the sparser set of observations of thisstar have been unluckily timed.
| Vibrato, ma non troppo.
Not Available
| Caroline Herschel as observer
Not Available
| The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue
Not Available
| Aus den IBVS.
Not Available
| Wer beobachtet mit? Beta Persei (Algol).
Not Available
| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
Not Available
| Oscillations of Optical Emission from Flare Stars and Coronal Loop Diagnostics
Based on an analogy between stellar and solar flares, we investigate theten-second oscillations detected in the U and B bands on the star EVLac. The emission pulsations are associated with fast magnetoacousticoscillations in coronal loops. We have estimated the magnetic field, B≈ 320 G; the temperature, T ≈ 3.7 × 107 K; and the plasmadensity, n ≈ 1.6 × 1011 cm-3, in the region of energy release.We provide evidence suggesting that the optical emission source islocalized at the loop footpoints.
| Two- and Three-Dimensional Hydrodynamical Simulations of Mass Transfer in Semidetached Binaries with Explicit Radiative Cooling and Self-Absorption in Their Gaseous Envelopes
We have modeled the mass transfer in three Algol-type binaries usingtwo-and three-dimensional numerical hydrodynamical techniques. Radiativecooling and absorption in the envelopes surrounding the components ofthe close binaries are explicitly taken into account. The resultingenvelope temperatures are consistent with those observed. We derivedthese temperatures by considering the energy balance in the moving gas,including heating due to the numerical viscosity that is inherent to thesimulations and radiative cooling. The radiative cooling was calculatedassuming an optically thin plasma in a state of ionization balance.Taking into account both of these processes yielded temperatures of 400030 000 K in the disk around the accreting primary and in the commonenvelope surrounding the close binary. Our computations also indicatethat the envelope morphology is not sensitive to the radiative-coolingmodel used.
| Bright X-Ray Flares in Orion Young Stars from COUP: Evidence for Star-Disk Magnetic Fields?
We have analyzed a number of intense X-ray flares observed in theChandra Orion Ultradeep Project (COUP), a 13 day observation of theOrion Nebula Cluster (ONC), concentrating on the events with the higheststatistics (in terms of photon flux and event duration). Analysis of theflare decay allows to determine the physical parameters of the flaringstructure, particularly its size and (using the peak temperature andemission measure of the event) the peak density, pressure, and minimumconfining magnetic field. A total of 32 events, representing the mostpowerful ~=1% of COUP flares, have sufficient statistics and aresufficiently well resolved to grant a detailed analysis. A broad rangeof decay times are present in the sample of flares, withτlc (the 1/e decay time) ranging from 10 to 400 ks. Peakflare temperatures are often very high, with half of the flares in thesample showing temperatures in excess of 100 MK. Significant sustainedheating is present in the majority of the flares. The magneticstructures that are found, from the analysis of the flare's decay, toconfine the plasma are in a number of cases very long, with semilengthsup to ~=1012 cm, implying the presence of magnetic fields ofhundreds of G (necessary to confine the hot flaring plasma) extending tocomparable distance from the stellar photosphere. These very large sizesfor the flaring structures (length L>>R*) are not foundin more evolved stars, where, almost invariably, the same type ofanalysis results in structures with L<=R*. As the majorityof young stars in the ONC are surrounded by disks, we speculate that thelarge magnetic structures that confine the flaring plasma are actuallythe same type of structures that channel the plasma in themagnetospheric accretion paradigm, connecting the star's photospherewith the accretion disk.
| Evolution of Close Binary Systems
We collect data on the masses, radii, etc., of three classes of closebinary stars: low-temperature contact binaries (LTCBs), near-contactbinaries (NCBs), and detached close binaries (DCBs). We restrictourselves to systems in which (1) both components are, at leastarguably, near the main sequence, (2) the periods are less than a day,and (3) there is both spectroscopic and photometric analysis leading toreasonably reliable data. We discuss the possible evolutionaryconnections between these three classes, emphasizing the roles played bymass loss and angular momentum loss in rapidly rotating cool stars. Wedescribe a new mechanism, differential rotation as observed in the Sun,which can explain the remarkable efficiency of heat transport in theouter envelopes of contact binaries.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| 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.
| Evidence for Superhumps in the Radio Light Curve of Algol and a New Model for Magnetic Activity in Algol Systems
Extensive radio data of two Algol systems and two RS CVn binaries werereanalyzed. We found evidence for a new periodicity that we interpret asa superhump in β Per (Algol), for which it may have been expectedaccording to its semidetached nature and low binary mass ratio. Theconcluded presence of an accretion disk (or an annulus) is consistentwith previous studies of optical data and numerical simulations ofβ Per. In our model, the 50 day period, previously found in theradio data of β Per, is explained as the apsidal precession of theelliptical accretion disk or annulus. If our interpretation is correct,this is the first detection of the superhump phenomenon in the radio andthe first observation of superhumps in Algol systems. According to ourresult, the accretion disk or annulus in β Per precesses in spiteof its non-Keplerian nature, and therefore, this phenomenon is notrestricted to the classical Keplerian accretion disks in compactbinaries. We propose that in Algol systems with short orbital periods,which have accretion disks or annuli, the disk is magnetically active inaddition to the cool secondary star. The magnetic field in the diskoriginates from amplification of the seed field in the magnetizedmaterial transferred from the secondary. The disk and stellar fieldsinteract with each other, with reconnection of the field lines causingflares and particle acceleration. Relativistic particles are trapped inthe field and directed toward the polar regions of the secondary starbecause of the dipole structure of its magnetic field. These systemsare, therefore, somewhat analogous to the RS CVn systems, which have twomagnetically interacting stars. Our proposed model for the magneticactivity in Algol systems provides a simple explanation for the observedproperties of β Per in the radio wavelengths, and in particular,for the presence of quiescent gyrosynchrotron emission near the polarregion of the secondary star, where electrons are difficult to confineif the field lines are open as in normal, single, magnetic stars. Itfurther explains the recent discovery that the Doppler shifts of theemission lines in the X-ray data of β Per are somewhat lower thanwhat is expected from the orbital motion of the secondary star. Wepropose that the superhump variation in the radio is generated byenhanced reconnection when the elongated side of the elliptic accretiondisk is the closest to the cool star. This leads to flares andenhancement in particle acceleration and is manifested as strongergyrosynchrotron radiation. The observed superhump period, at3.037+/-0.013 days (~6% longer than the orbital period), was used todeduce a binary mass ratio of ~0.23 in β Per, which is consistentwith previous studies. Our finding opens the possibility to extend thesuperhump phenomenon to Algol systems and to test the theories ofprecessing accretion disks in various types of interacting binaries. Itprovides a new method to estimate the mass ratios in these binaries. Italso offers new insights and improves our understanding of thecomplicated magnetic interaction and feedback between mass transferdynamics, time-dependent disk accretion, and induced magnetic activityin the Algol and related systems.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
Каталог и designations:
|
