Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The long-term phenomenon in U Mon.
The nature and origin of the long-term variability in U Mon has beeninvestigated using multicolour photometry and high-resolution echellespectroscopy. The photometry shows long-term dimming, damping andreddening of the system, as well as the extremely variable nature ofthis modulation. The new spectroscopic observations show enhanced Halphaemission during long-term light minimum. Our radial velocities werecombined with those from the literature to determine orbital parametersfor the U Mon system (P2600 d, e=0.43). Obscuration by dust atcertain orbital phases explains the light decrease and reddening, whilethe enhanced H-alpha emission and damping of the light and colour curvesmay indicate that mass loss or interaction is occurring at (or close to)periastron passage in the U Mon system.
| R Scuti nach der Jahrtausendwende.
Not Available
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
| The light curve of the semiregular variable L2 Puppis - II. Evidence for solar-like excitation of the oscillations
We analyse visual observations of the pulsations of the red giantvariable L2 Puppis (L2 Pup). The data cover 77 yrbetween 1927 and 2005, thus providing an extensive empirical base forcharacterizing properties of the oscillations. The power spectrum of thelight curve shows a single mode resolved into multiple peaks under anarrow envelope. We argue that this results from stochastic excitation,as seen in solar oscillations, with a mode lifetime of about 5 yr. Therandom fluctuations in phase also support this idea. A comparison with XCam, a true Mira star with the same pulsation period, and W Cyg, a truesemiregular star, illustrates the basic differences in phase behaviours.The Mira shows very stable phase, consistent with excitation by theκ-mechanism, whereas W Cyg shows large phase fluctuations thatimply stochastic excitation. We find L2 Pup to beintermediate, implying that both mechanisms play a role in itspulsation. Finally, we also checked the presence of low-dimensionalchaos and could safely exclude it.
| Infall and SiO emission in V838 Mon
We present moderate- and high-resolution infrared spectroscopy of thepeculiar eruptive variable V838 Mon, which underwent a series ofremarkable outbursts in early 2002. During the period covered by ourobservations, 2002 December-2003 December, the near-infrared spectrumcontinued to show many of the characteristics of a very cool supergiant.However, throughout this period the spectrum also revealed strong andvariable SiO first overtone emission, and Paβ emission. The 2003December spectrum contained a series of TiI lines with inverse P Cygniprofiles. This is clear evidence that some material is falling inwardtowards the star.
| The semi-regular variable RS Cancri: one period, two, or many?
Not Available
| Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.
Not Available
| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
Not Available
| Strong dust processing in circumstellar discs around 6 RV Tauri stars. Are dusty RV Tauri stars all binaries?
We present extended Spectral Energy Distributions (SEDs) of sevenclassical RV Tauri stars, using newly obtained submillimetre continuummeasurements and Geneva optical photometry supplemented with literaturedata. The broad-band SEDs show a large IR excess with a black-body slopeat long wavelengths in six of the seven stars, R Sct being thenoticeable exception. This long wavelength slope is best explainedassuming the presence of a dust component of large grains in thecircumstellar material. We show that the most likely distribution of thecircumstellar dust around the six systems is that the dust resides in adisc. Moreover, very small outflow velocities are needed to explain thepresence of dust near the sublimation temperature and we speculate thatthe discs are Keplerian. The structure and evolution of these compactdiscs are as yet not understood but a likely prerequisite for theirformation is that the dusty RV Tauri stars are binaries.
| Time-Series Analysis of Variable Star Data
Time-series analysis is a rich field of mathematical and statisticalanalysis, in which physical understanding of a time-varying system canbe gained through the analysis of time-series measurements. There areseveral different techniques of time-series analysis that can beusefully applied to variable star data sets. Some of these techniquesare particularly useful for data found in the AAVSO InternationalDatabase. In this paper, I give a broad overview of time-series analysistechniques useful for variable star data, along with some practicalsuggestions for the application of different techniques to differenttypes of variables. Included are elementary discussions of traditionalFourier methods, along with wavelet and autocorrelation analysis.
| Discrete Fourier Analysis of the Light Curve of S Persei
A discrete Fourier analysis was performed on the validated S Perseivisual light curve data as obtained from the American Association ofVariable Star Observers (AAVSO). These observations span just over acentury, from February of 1903 to July of 2003. This analysis was anattempt to find the fundamental periods of the variability of the redsupergiant S Persei. Inspection of the S Per light curve indicates alikely complex combination of sinusoids of differing periods. UsingFourier analysis, four periods of various relative strengths wereextracted from these data: 745, 797, 952, and 2857 days. Although someof these periods are similar to earlier results, they seem to indicate amore complex result than has previously been determined.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
Not Available
| Evidence for Low-dimensional Chaos in Semiregular Variable Stars
We present an analysis of the photometric observations of the lightcurves of the five large-amplitude, irregularly pulsating stars R UMi,RS Cyg, V CVn, UX Dra, and SX Her. First, multiperiodicity is eliminatedfor these pulsations; i.e., they are not caused by the excitation of asmall number of pulsation modes with constant amplitudes. Next, on thebasis of energetics we also eliminate stochasticity as a cause, leavinglow-dimensional chaos as the only alternative. We then use a global flowreconstruction technique in an attempt to extract quantitativeinformation from the light curves and to uncover common physicalfeatures in this class of irregular variable stars that straddle the RVTau to the Mira variables. Evidence is presented that the pulsationalbehavior of R UMi, RS Cyg, V CVn, and UX Dra takes place in afour-dimensional dynamical phase space, suggesting that two vibrationalmodes are involved in the pulsation. A linear stability analysis of thefixed points of the maps further indicates the existence of a two-moderesonance, similar to the one we had uncovered earlier in R Sct. Theirregular pulsations are the result of a continual energy exchangebetween two strongly nonadiabatic modes, a lower frequency pulsationmode and an overtone that are in a close 2:1 resonance. The evidence isparticularly convincing for R UMi, RS Cyg, and V CVn, but much weakerfor UX Dra. In contrast, the pulsations of SX Her appear to be morecomplex and may require a six-dimensional space.
| A weakly non-adiabatic one-zone model of stellar pulsations: application to Mira stars
There is growing observational evidence that the irregular changes inthe light curves of certain variable stars might be due to deterministicchaos. Supporting these conclusions, several simple models of non-linearoscillators have been shown to be capable of reproducing the observedcomplex behaviour. In this paper, we introduce a non-linear,non-adiabatic one-zone model intended to reveal the factors leading toirregular luminosity variations in some pulsating stars. We have studiedand characterized the dynamical behaviour of the oscillator as the inputparameters are varied. The parametric study implied values correspondingto stellar models in the family of long period variables and inparticular of Mira-type stars. We draw attention to certain solutionsthat reproduce with reasonable accuracy the observed behaviour of somepeculiar Mira variables.
| Studies of the RV Tauri phenomenon.
Not Available
| Annual report of the director for fiscal year 2001-2002.
Not Available
| Beobachtungsegebnisse Bundesdeutsche Arbeitsgemainschaft fur Veranderliche Sterne e.V.
Not Available
| Observability of Scattered-Light Echoes around Variable Stars and Cataclysmic Events
Scattered-light echoes from variable and cataclysmic stars offer one ofthe most effective means to probe the structure and composition ofcircumstellar and interstellar media. I build a simple model oflight-echo surface brightness by considering the source spectrum and thedust density, geometry, and scattering efficiency. I use this model toinvestigate whether echoes should be observable around short andlong-period giants, cataclysmic variables, and supernovae. Onlysupernovae are expected to illuminate material on both circumstellar andinterstellar scales. Giant and post-AGB stars (e.g., Cepheids and Miras)with high mass-loss rates can produce observable echoes within theircircumstellar envelopes. Echoes from novae and dwarf novae are probablydetectable only from nearby material, and only in unusually dense gas. Ipresent characteristic exposure times to image such echoes over a widerange of wavelengths for ground-based and Hubble Space Telescopeobservations. I apply these results to analyze the dust properties ofthe recently reported echoes around SN 1993J, finding the dust in M81 tohave a grain-size distribution and chemical composition consistent withGalactic dust. Optimal observing strategies for echo detection are alsodiscussed.
| Polarimetry of evolved stars. III. RV Tau and R CrB stars
We present broadband optical polarimetry, and broadband optical andinfrared photometry, of eight RV Tau-type and five R CrB-type stars;much of the photometry and polarimetry was obtained simultaneously. Fornine of the objects polarimetric data is reported for the first time. Wehave estimated and subtracted the interstellar component ofpolarization, allowing us to determine the level of intrinsicpolarization. In some cases this is =~ 1%-2% even when the star is in abright photometric state. We consider this to be evidence for thepresence of permanent clumpy non-spherical dust shells around the RV Tauand R CrB-type stars we observed. Our polarimetric and photometric datalead us to conclude that, for most of our programme stars, neutralextinction must be significant in their circumstellar envelopes. Apartfrom the brightness variations due to pulsations and changes in theeffective temperature of stars, there is clear evidence ofwavelength-independent flux variations - with amplitude from 0fm 5 to1fm 0 - implying the presence of large (a>~ 0.15 mu m) dustparticles. Rapid ( ~ 2 hours) evolution of the infrared fluxdistribution at the level of ~ 0fm 6 in the JHKL bands was detected inthe RV Tau star R Sct.Table 2 is only available in electronic form at the CDS via anomymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/405Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| The Cepheids of Population II and Related Stars
The Type II Cepheids include most intrinsic variables with periodsbetween 1 and about 50 days, except for the classical Cepheids and theshortest semiregular variables of type M. The Type II Cepheids may bedivided in groups by period, such that the stars with periods beween 1and 5 days (BL Her class), 10-20 days (W Vir class), and greater than 20days (RV Tau class) have differing evolutionary histories. The chemicalcomposition of Type II Cepheids reflects the material they were madefrom as modified by their internal nuclear evolution and mixing.Finally, RV Tau stars are affected by mass loss by dust and speciesattached to the dust. The populations to which the various classes ofType II Cepheids are assigned constitute important clues to the originand evolution of the halo of our Galaxy and the dwarf spheroidal systemsfrom which at least part of the halo seems to have been accreted.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| Period-doubling events in the light curve of R Cygni: Evidence for chaotic behaviour
A detailed analysis of the century long visual light curve of thelong-period Mira star R Cygni is presented and discussed. The data werecollected from the publicly available databases of the AFOEV, the BAAVSSand the VSOLJ. The full light curve consists of 26655 individual pointsobtained between 1901 and 2001. The light curve and its periodicity wereanalysed with help of the O-C diagram, Fourier analysis andtime-frequency analysis. The results demonstrate the limitations ofthese linear methods. The next step was to investigate the possiblepresence of low-dimensional chaos in the light curve. For this, asmoothed and noise-filtered signal was created from the averaged dataand with the help of time delay embedding, we have tried to reconstructthe attractor of the system. The main result is that R Cygni shows suchperiod-doubling events that can be interpreted as being caused by arepetitive bifurcation of the chaotic attractor between a period 2Torbit and chaos. The switch between these two states occurs in a certaincompact region of the phase space, where the light curve ischaracterized by ~ 1500-day long transients. The Lyapunov spectrum wascomputed for various embedding parameters confirming the chaoticattractor, although the exponents suffer from quite high uncertaintybecause of the applied approximation. Finally, the light curve iscompared with a simple one zone model generated by a third-orderdifferential equation which exhibits well-expressed period-doublingbifurcation. The strong resemblance is another argument for chaoticbehaviour. Further studies should address the problem of global flowreconstruction, including the determination of the accurate Lyapunovexponents and dimension.
| The extended atmosphere and evolution of the RV Tau star, R Scuti
We analyze ISO/SWS spectra of the RV Tau star R Scuti. The infraredspectra are dominated by H2O emission bands. The near- andmid-infrared excess is attributed to H2O; the dustcontribution is less important. We also identify CO, SiO andCO2 bands. The various molecular emission bands originatefrom an extended atmosphere, an atmosphere above the photosphere. Theextended atmosphere of R Sct is formed from matter which gradually havelifted up from the photosphere through the pulsations of the star. Incontrast to the abundant molecules around the star, the silicate dustfeature is weak and the dust mass-loss rate is only dotMd=10-11 Msun yr-1. Thisimplies that there might be a process to inhibit dust formation frommolecules. RV Tau stars are commonly considered as post-AGB stars. Whilea detached dust envelope around R Sct is consistent with such aninterpretation, we show that its period evolution is slower thanexpected. We argue that R Sct may be a thermal-pulsing AGB star,observed in a helium-burning phase. Based on observations with ISO, anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, The Netherlands and the UK) with theparticipation of ISAS and NASA. The SWS is a joint project of SRON andMPE.
| RU Cen and SX Cen: Two strongly depleted RV Tauri stars in binary systems. The RV Tauri photometric b phenomenon and binarity
We present a chemical abundance analysis on the basis of highsignal-to-noise and high-resolution (lambda /Delta lambda ~ 48 000)optical spectra of two RV Tauri stars RU Cen and SX Cen. With an [Fe/H]= -1.9 and a [Zn/Fe] = +0.9 for RU Cen and a [Fe/H] = -1.1 and a [Zn/Fe]= +0.6 for SX Cen, both stars of spectroscopic class B display strongdepletion of refractory elements in their photospheres. Our CORALIEradial velocity measurements prove the stars to be members of binarysystems and a detailed construction of the spectral energy distributionindicate the presence of a large amount of hot circumstellar dust.Moreover, the orbital period of SX Cen of around 600 days is similar tothe published period of mean magnitude variation in the light curve (RVTauri phomometric class b phenomenon). All these observations indicatethe presence of a stable circumbinary disk in the objects and strengthenthe model that this is a necessary condition for the depletion processto take place. Based on observations collected at the European SouthernObservatory in Chile (64.L-0117(A), 67.D-0054(A)), on the 1.2 m SwissEuler telescope on La Silla and on the 70 cm Swiss photometric telescopealso on La Silla.
| Aufsuchkarten fur intrinsic Variable Stars in Brno.
Not Available
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Polarimetry of 167 Cool Variable Stars: Data
Multicolor photoelectric polarimetry is presented for 167 stars, most ofwhich are variable stars. The observations constitute a data set thatfor some stars covers a time span of 35 yr. Complex variations are foundover time and wavelength and in both the amount of polarization and itsposition angle, providing constraints for understanding the polarizingenvironments in and around these cool stars.
| Mass, linear momentum and kinetic energy of bipolar flows in protoplanetary nebulae
We have studied the CO emission from protoplanetary nebulae (PPNe). Oursample is composed of 37 objects and includes, we think, all wellidentified PPNe detected in CO, together with the two yellow hypergiantsemitting in CO and one young PN. We present a summary of the existing COdata, including accurate new observations of the 12CO and13CO J=1-0 and J=2-1 lines in 16 objects. We identify in thenebulae a slowly expanding shell (represented in the spectra by acentral core) and a fast outflow (corresponding to the line wings), thatin the well studied PPNe is known to be bipolar. Excluding poor data, weend up with a sample of 32 sources (including the 16 observed by us);fast flows are detected in 28 of these nebulae, being absent in only 4.We present a method to estimate from these data the mass, ``scalar''momentum and kinetic energy of the different components of the molecularoutflows. We argue that the uncertainties of our method can hardly leadto significant overestimates of these parameters, althoughunderestimates may be present in not well studied objects. The totalnebular mass is often as high as ~1 Msun, and the mass-lossrate, that (presumably during the last stages of the AGB phase)originated the nebula, had typical values ~10-4Msun yr-1. The momentum corresponding to this massejection process in most studied nebulae is accurately coincident withthe maximum momentum that radiation pressure, acting through absorptionby dust grains, is able to supply (under expected conditions). Weestimate that this high-efficiency process lasts about 1000-10 000 yr,after which the star has ejected a good fraction of its mass and the AGBphase ends. On the other hand, the fast molecular outflows, that haveprobably been accelerated by shock interaction with axial post-AGB jets,carry a significant fraction of the nebular mass, with a very highmomentum (in most cases between 1037 and 1040 g cms-1) and very high kinetic energy (usually between1044 and 1047 erg). In general, yellow hypergiantsand post-AGB objects with low initial mass show nebular masses andmomenta that are, respectively, higher and lower than these values. Wecompare the momenta of the fast outflows with those that can be suppliedby radiation pressure, taking into account the expected shortacceleration times and some effects that can increase the momentumtransfer. We find that in about 80% of PPNe, the fast molecular flowshave too high momenta to be powered by radiation pressure. In some casesthe momentum of the outflow is ~1000 larger than that carried byradiation pressure; such high factors are difficult to explain evenunder exceptional conditions. Wind interaction is the basic phenomenonin the PN shaping from the former AGB envelopes; we conclude that thisinteraction systematically takes place along a dominant direction andthat this process is not powered by radiation pressure. Due to the lackof theoretical studies, the possible momentum source remains a matter ofspeculation.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Scudo |
| Ascensione retta: | 18h47m29.00s |
| Declinazione: | -05°42'18.0" |
| Magnitudine apparente: | 5.2 |
| Distanza: | 431.034 parsec |
| Moto proprio RA: | -45.2 |
| Moto proprio Dec: | -30 |
| B-T magnitude: | 7.188 |
| V-T magnitude: | 5.581 |
Cataloghi e designazioni:
|
INHABITED SKY
