Contents
Images
Upload your image
DSS Images Other Images
Related articles
MACHO 82.8405.15: the First Extragalactic Depleted RV Tauri Star
RV Tauri stars are a heterogeneous subclass of the Population IICepheids, probably in the post-AGB stage of evolution. Many RV Tauristars show some degree of depletion in their photospheres, which iscaused by a gas-dust separation in the circumstellar environment and asubsequent gas accretion, making the photosphere devoid of refractoryelements. The most favorable circumstance for this process to occur iswhen the circumstellar dust is trapped in a disk. The presence of a diskin evolved objects is likely related to binarity. In this contribution,we present the results of our abundance study of the first depleted RVTauri star in the Large Magellanic Cloud, MACHO 82.8405.15, based onhigh-resolution VLT-UVES spectra. While our analysis of other LMC RVTauri stars is still in progress, it is clear that, also in the LMC,depletion seems to be quite common amongst RV Tauri objects.
| On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae
We report on multiepoch optical and near-infrared spectroscopy aroundthe first-overtone rovibrational band of CO in the pulsating yellowhypergiant ρ Cas, one of the most massive stars in the Galaxy and acandidate SN II progenitor. We argue that the double cores of the COabsorption lines, which have previously been attributed to separatecircumstellar shells expelled during its recurrent outbursts, result infact from a superposition of a wide absorption line and a narrow centralemission line. The CO line doubling returns over subsequent pulsationcycles, where the superposed line emission assumes its largest intensitynear phases of maximum light. We find that the morphology and behaviorof the CO band closely resemble the remarkable ``line-splittingphenomenon'' also observed in optical low-excitation atomic lines. Basedon radiative transport calculations, we present a simplified model ofthe near-infrared CO emission emerging from cooler atmospheric layers inthe immediate vicinity of the photosphere. We speculate that the kinetictemperature minimum in our model results from a periodicpulsation-driven shock wave. We further discuss a number of alternativeexplanations for the origin of the ubiquitous emission-line spectrum,possibly due to a quasi-chromosphere or a steady shock wave at theinterface of a fast expanding wind and the interstellar medium. Wepresent a number of interesting spectroscopic similarities between ρCas and other types of cool variable supergiants, such as the RV Tau andR CrB stars. We further propose a possibly common mechanism for theenigmatic outburst behavior of these luminous pulsating cool stars.
| Post-AGB stars as testbeds of nucleosynthesis in AGB stars
We construct a data base of 125 post-AGB objects (including R CrB andextreme helium stars) with published photospheric parameters (effectivetemperature and gravity) and chemical composition. We estimate themasses of the post-AGB stars by comparing their position in the (logT{eff}, log g) plane with theoretical evolutionary tracks ofdifferent masses. We construct various diagrams, with the aim of findingclues to AGB nucleosynthesis. This is the first time that a large sampleof post-AGB stars has been used in a systematic way for such a purposeand we argue that, in several respects, post-AGB stars should be morepowerful than planetary nebulae to test AGB nucleosynthesis. Our mainfindings are that: the vast majority of objects which do not showevidence of N production from primary C have a low stellar mass(Mstar < 0.56 Mȯ); there is no evidencethat objects which did not experience 3rd dredge-up have a differentstellar mass distribution than objects that did; there is clear evidencethat 3rd dredge-up is more efficient at low metallicity. The sample ofknown post-AGB stars is likely to increase significantly in the nearfuture thanks to the ASTRO-F and follow-up observations, making theseobjects even more promising as testbeds for AGB nucleosynthesis.
| Keplerian discs around post-AGB stars: a common phenomenon?
Aims.We aim at showing that the broad-band SED characteristics of oursample of post-AGB stars are best interpreted, assuming thecircumstellar dust is stored in Keplerian rotating passivediscs.Methods.We present a homogeneous and systematic study of theSpectral Energy Distributions (SEDs) of a sample of 51 post-AGB objects.The selection criteria to define the whole sample were tuned to coverthe broad-band characteristics of known binary post-AGB stars. The wholesample includes 20 dusty RV Tauri stars from the General Catalogue ofVariable Stars (GCVS). We supplemented our own Geneva optical photometrywith literature data to cover a broad range of fluxes from the UV to thefar-IR.Results.All the SEDs display very similar characteristics: alarge IR excess with a dust excess starting near the sublimationtemperature, irrespective of the effective temperature of the centralstar. Moreover, when available, the long wavelength fluxes show ablack-body slope indicative of the presence of a component of large mmsized grains.Conclusions.We argue that in all systems, gravitationallybound dusty discs are present. The discs must be puffed-up to cover alarge opening angle for the central star and we argue that the discshave some similarity with the passive discs detected around youngstellar objects. We interpret the presence of a disc to be a signaturefor binarity of the central object, but this will need confirmation bylong-term monitoring of the radial velocities. We argue that dusty RVTauri stars are those binaries which happen to be in the Population IIinstability strip.
| A strongly s-process enriched RV Tauri star in the LMC.
A detailed abundance analysis is presented of an intriguing object inthe Large Magellanic Cloud that links the class of RV Tauri stars to thepost-AGB phase of evolution: MACHO 47.2496.8. The spectrum, taken withVLT-UVES, is dominated by molecular lines of carbon bearing molecules,together with strong transitions of s-process species. Detailed spectralsyntheses were made using a state-of-the-art carbon rich MARCS model. Asurprisingly low metallicity ([Fe/H] = -1.4), together with strongcarbon (C/O>2) and s-process overabundances were found, reachingvalues of [ls/Fe] = +1.2 for the light s-process elements, and even[hs/Fe] = +2.1 for the heavy ones. The strong s-process enhancements arein agreement with the theoretical expectations at that metallicity. Onlythe combination of a low lead content and a high [hs/ls] is not easilyexplained by the current nucleosynthetic models. It is not clear whetherthis star is intrisically or extrinsically enriched, but severalarguments favour an intrisic enrichment, implying the object to be agenuine post carbon (N-type) AGB star. With the low metallicity and aluminosity at the very low end of the carbon star luminosity function,MACHO 47.2496.8 represents the final evolutionary state of a star of lowinitial mass.Based on observations collected at ESO, Chile (programme 074.D-0619(A))
| Binary post-AGB stars and their Keplerian discs .
In this contribution we give a progress report on our systematic studyof a large sample of post-AGB stars. The sample stars were selected onthe basis of their infrared colours and the selection criteria weretuned to discover objects with hot dust in the system. We started a veryextensive, multi-wavelength programme which includes the analysis of ourradial velocity monitoring; our optical high-resolution spectra; ourgroundbased N-band spectral data as well as the Spitzer full spectralscans; the broad-band SED and the high spatial-resolutioninterferometric experiments with the VLTI. In this contribution wehighlight the main results obtained so far and argue that all systems inour sample are indeed binaries, which are surrounded by dusty Kepleriancircumbinary discs. The discs play a lead role in the evolution of thesystems.
| 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.
| DZ UMa - RV oder SR?
Not Available
| R Scuti nach der Jahrtausendwende.
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.
| High-resolution spectroscopy of the high galactic latitude RV Tauri star CE Virginis
Analysis of the surface composition of the suspected cool RV Tauri starCE Virginis shows no systematic trend in depletions of elements withrespect to condensation temperature. However, there is a significantdepletion of the elements with respect to the first ionization potentialof the element. The derived Li abundance of logɛ (Li) = 1.5 +/-0.2 indicates production of Li in the star. Near-infrared coloursindicate sporadic dust formation close to the photosphere.
| Aenderungen der Programme Standardsterne, 2000 und Langperiodische.
Not Available
| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
Not Available
| 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.
| The structure of radiative shock waves. V. Hydrogen emission lines
We considered the structure of steady-state plane-parallel radiativeshock waves propagating through the partially ionized hydrogen gas oftemperature T1 = 3000 K and density 10-12 gcm-3leρ1≤10-9 gcm-3. The upstream Mach numbers range within6≤M1≤14. In frequency intervals of hydrogen lines theradiation field was treated using the transfer equation in the frame ofthe observer for the moving medium, whereas the continuum radiation wascalculated for the static medium. Doppler shifts in Balmer emissionlines of the radiation flux emerging from the upstream boundary of theshock wave model were found to be roughly one-third of the shock wavevelocity: -δ V≈ (1/3 U1. The gas emitting the Balmerline radiation is located at the rear of the shock wave in the hydrogenrecombination zone where the gas flow velocity in theframe of the observer is approximately one-half of the shock wavevelocity: -V*≈(1)/(2) U1. The ratio of theDoppler shift to the gas flow velocity of δ V/V* ≈0.7 results both from the small optical thickness of the shock wave inline frequencies andthe anisotropy of the radiation field typical for the slab geometry. Inthe ambient gas with density of ρ1 10-11g cm-3 the flux in the Hα frequency interval revealsthe double structure of the profile. A weaker Hβ profile doublingwas found for ρ1≳ 10-10 g cm-3and U1 50 km s-1. The unshifted redwardcomponent of the double profile is due to photodeexcitation accompanyingthe rapid growth of collisional ionization in the narrow layer in frontof the discontinuous jump.
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| Astrophysics in 2002
This has been the Year of the Baryon. Some low temperature ones wereseen at high redshift, some high temperature ones were seen at lowredshift, and some cooling ones were (probably) reheated. Astronomerssaw the back of the Sun (which is also made of baryons), a possiblesolution to the problem of ejection of material by Type II supernovae(in which neutrinos push out baryons), the production of R CoronaeBorealis stars (previously-owned baryons), and perhaps found the missingsatellite galaxies (whose failing is that they have no baryons). A fewquestions were left unanswered for next year, and an attempt is made todiscuss these as well.
| Self-Correlation Analysis of RV Tauri Stars and Related Objects
We have used self-correlation-a simple form of variogram analysis-tostudy 33 RV Tauri and related stars in the LMC, using MACHO data. Weconfirm the periods and classifications of Alcock et al. and discuss afew stars of special interest. We find that self-correlation is a usefuladjunct to Fourier analysis, especially for stars whose classificationis based on their cycle-to-cycle behavior. In particular, it canidentify stars whose behavior is more complicated than the standard``alternating deep and shallow minima'' and begin to investigate thequestion of whether the Population II Cepheids, the RV Tauri variables,and the SRd variables form a continuous sequence from periodicity toirregularity. Our results also emphasize that the RV Tauri phenomenonhas two dimensions: the relative depths of adjacent minima and thenumber of cycles over which the alternating minima persist.
| Beobachtungsegebnisse Bundesdeutsche Arbeitsgemainschaft fur Veranderliche Sterne e.V.
Not Available
| Fading of light maximum and linear polarization variation in the carbon Mira R Leporis
Polarimetry of R Lep obtained over the years 1991-2002 is presented.During this period the star underwent an episode of fading of thebrightness at light maximum, after an interval of about 35 yr. Ananalysis of the data shows that the percentage linear polarizationincreased as the fading progressed, attained a maximum of slightly over3 per cent in the V band close to the epoch of minimum, and remainedmore or less at the same level during and well after the recovery tonormal brightness. The polarization, apparently, originated from thecircumstellar envelope above the region where the dust that caused thefading in the star condensed. The physical mechanism that causes therather large polarization during fadings is perhaps selective extinctionby aligned foreground grains produced by the passage of shocks throughthe circumstellar envelope. The small-amplitude, short-term fluctuationsin polarization observed in R Lep, which appear to be superposed on thefading-related large-amplitude variation, are probably pulsation-relatedand arise from the inner zones of the circumstellar envelope.
| Detection of Zinc in the Very Metal-Poor Post-AGB Star HR 4049
We report on the detection of two Zn I lines at4722.15Å and 4810.53Å in the high-quality spectrum of thevery metal-poor post-AGB star HR4049, which was obtained with the HighDispersion Spectrograph attached to the Subaru Telescope. The strengthsof these lines indicate an appreciable underabundance of Zn by ~ -1.3dex relative to the Sun. The fact that this volatile element, similarlyto others belonging to the same group (e.g., C, N, O, S), does notconform to the extreme depletion ( > 4 dex) of refractory metals(e.g., Fe), strongly suggests that grain formation has something to dowith the origin of the chemical peculiarity. This (not extremely butsignificantly) subsolar value of [Zn/H] is quantitatively discussed inconnection with those of other volatile species, especially with respectto S. We also detected a new Fe II line at5159.03Å along with the already known Fe II4923.93Å line; based on these two lines the Fe abundance of HR4049is determined to be ~ 2.8 ([Fe/H] ~ -4.7).
| System Description and First Light Curves of the Hungarian Automated Telescope, an Autonomous Observatory for Variability Search
Having been operational at Kitt Peak for more than a year, the prototype(HAT-1) of the Hungarian Automated Telescope (HAT) has been used for anall-sky variability search of the northern hemisphere. This smallautonomous observatory is recording the brightness of stars in the rangeof IC~6-13 mag with a telephoto lens and its9deg×9deg field of view, yielding a datarate of ~106 photometric measurements per night. We givebrief hardware and software descriptions of the system, controlled by asingle PC running RealTime Linux OS. We provide an overview ofsite-specific details and quantify the astrometric and photometriccapabilities of HAT. As a demonstration of system performance, we give asample of 60 short-period variables in a single selected field, allbright, with I<13 mag, where only 14 were known before. Depending onthe observing strategy, a search for extrasolar planet transits is alsoa feasible observing program. We conclude with a short discussion onfuture directions. Further information can be found at the HAT Webpage.3
| Coordinates and Identifications of Harvard Variables
Coordinates and identifications are presented for 726 Harvard Variablestars and suspected variables, discovered or studied by D. Hoffleit andannounced in Harvard Bulletins 874, 884, 887, 901, and 902; plus 141others, previously known, lying in the same fields.
| CO 1st overtone spectra of cool evolved stars: Diagnostics for hydrodynamic atmosphere models
We present spectra covering the wavelength range 2.28 to 2.36 mu m at aresolution of Delta lambda = 0.0007 mu m (or R = 3500) for a sample of24 cool evolved stars. The sample comprises 8 M supergiants, 5 M giants,3 S stars, 6 carbon stars, and 2 RV Tauri variables. The wavelengthscovered include the main parts of the 12C16O v =2-0 and 3-1 overtone bands, as well as the v = 4-2 and 13CO v= 2-0 bandhead regions. CO lines dominate the spectrum for all the starsobserved, and at this resolution most of the observed features can beidentified with individual CO R- or P-branch lines or blends. Theobserved transitions arise from a wide range of energy levels extendingfrom the ground state to E/k > 20 000 K. We looked for correlationsbetween the intensities of various CO absorption line features and otherstellar properties, including IR colors and mass loss rates. Two usefulCO line features are the v = 2-0 R14 line, and the CO v = 2-0 bandhead.The intensity of the 2-0 bandhead shows a trend with K-[12] color suchthat the reddest stars (K-[12] > 3 mag) exhibit a wide range in 2-0bandhead depth, while the least reddened have the deepest 2-0 bandheads,with a small range of variation from star to star. Gas mass loss ratesfor both the AGB stars and the red supergiants in our sample correlatewith the K-[12] color, consistent with other studies. The data implythat stars with dot M_gas < 5x 10-7 Msuny-1 exhibit a much narrower range in the relative strengthsof CO 2-0 band features than stars with higher mass loss rates. Therange in observed spectral properties implies that there are significantdifferences in atmospheric structure among the stars in this sample.Figures 4-9, 11-14, 16, 17, 19-21, 23, 24 are only avalaible inelectronic form at http://www.edpsciences.org
| Interstellar extinction in the California Nebula region.
Vilnius seven-color photometry has been obtained for 238 stars down to ~13 mag in the area of the California Nebula inPerseus. For nearly all of the stars, photometric spectral classes,luminosity classes, absolute magnitudes, interstellar reddenings,extinctions and distances are determined. The ``extinction versusdistance'' diagrams give evidence for the presence of one dust layer at~ 160 pc distance in the direction of the CaliforniaNebula and its nearest surroundings and of two dust layers atdistances of 160 pc and 300 pc north-west of the nebula, in thedirection of the dark clouds L 1449 and L1456. The front layer contributes extinction between 0.3 and1.3 mag, and the second layer gives about 1 mag of additionalextinction. It appears likely that the front dust layer is the extensionof the Taurus dark clouds. The second dust layer probably belongs to thecomplex of dark clouds found in other areas of the southern part ofPerseus (in the directions of the open cluster IC348, the reflection nebula NGC 1333, etc.).Both cloud complexes run more or less parallel to the Galactic plane.
| R Centauri: An Unusual Mira Variable in a He-Shell Flash
We present an analysis of AAVSO visual observations of the Mira variableR Cen from 1918 to 2000. The period of the dominant mode has beensteadily decreasing from 550 days at JD 2,434,000 (1951) to its presentvalue of 505-510 days. In the same interval, the pulsational amplitudehas decreased by 3 mag, from 5.5-11.8 V to 6.3-9.1 V. We suggest thatboth are caused by a He-shell flash, as the period decrease is similarto that of other He-shell flash stars such as R Hya, R Aql, and T UMi.The period change is consistent with the luminosity drop expectedimmediately after the flash, as predicted by He-shell flash models forstars of 2-3 Msolar or less. The light curve shows thefamiliar pattern of alternating deep and shallow minima, giving theappearance of double maxima. While the amplitude of the main mode hasdecreased 3 mag in the last 50 years, the amplitude of the secondarymode near 274 days has remained almost constant, so that the doublemaxima have nearly vanished from the light curve in recent years. Thepower spectrum between 1930 and 1966 shows harmonics up to 8 times themain frequency at 1/548 cycle day-1. The most likelyexplanation for the double-peaked light curve is a resonance between twomodes.
| Aus der Sektion "Halb-und Unregelmassige".
Not Available
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| Photometric Investigation of the California Nebula Region: Catalogs
Vilnius seven-color photometry has been obtained for 238 stars down to13th mag in the area of the California Nebula in Perseus. For nearly allof the stars photometric spectral classes, luminosity classes, absolutemagnitudes, interstellar reddenings, extinctions and distances aredetermined. The present paper contains the results of photometry and theparameters of the stars.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Ταύρος |
| Right ascension: | 04h47m06.73s |
| Declination: | +26°10'45.6" |
| Apparent magnitude: | 9.512 |
| Proper motion RA: | 6.2 |
| Proper motion Dec: | -4 |
| B-T magnitude: | 11.48 |
| V-T magnitude: | 9.675 |
Catalogs and designations:
|
