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RCoronae Borealis at the 2003 light minimum.
Not Available
| Various aspects of stellar variability in the Magellanic Clouds from the EROS 2 survey
The current status of the search of variable stars in the MagellanicClouds from the EROS 2 survey is briefly presented.
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| The effect of dust obscuration in RR Telescopii on optical and IR long-term photometry and Fe II emission lines
Aims.Infrared and optical photometric and spectroscopic observations ofthe symbiotic nova RR Tel are used to study the effects and propertiesof dust in symbiotic binaries containing a cool Mira component, as wellas showing "obscuration events" of increased absorption, which aretypical for such Miras. Methods: .A set of photometricobservations of the symbiotic nova RR Tel indifferent wavelength bands - visual from 1949 to 2002 and near-infrared({JHKL}) from 1975 to 2002 - are presented. The variability due to thenormal Mira pulsation was removed from the JHKL data, which were thencompared with the American Association of Variable Star Observers'({AAVSO}) visual light curve. The changes of the Fe II emission linefluxes during the 1996-2000 obscuration episode were studied in theoptical spectra taken with the Anglo-Australian telescope. Results: .We discuss the three periods during which the Mira componentwas heavily obscured by dust as observed in the different wavelengthbands. A change in the correlations of J with other infrared magnitudeswas observed with the colour becoming redder after JD 2 446 600.Generally, J-K was comparable, while K-L was larger than typical valuesfor single Miras. A distance estimate of 2.5 kpc, based on the IR data,is given. A larger flux decrease for the permitted than for theforbidden Fe II lines, during the obscuration episode studied, has beenfound. There is no evidence for other correlations with line properties,in particular with wavelength, which suggests obscuration due toseparate optically thick clouds in the outer layers.
| 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.
| Probing circumstellar dust formation through high resolution spectroscopy
The existence of cool regions where dust can condense is explored in RCBstars at minimum.
| 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).
| Molecular outflows in Corona Australis.
Not Available
| Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.
Not Available
| Detection of Near-Infrared CO Absorption Bands in R Coronae Borealis Stars
R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich,pulsating, post-asymptotic giant branch stars that experience massiveirregular declines in brightness caused by circumstellar dust formation.The mechanism of dust formation around RCB stars is not well understood.It has been proposed that CO molecules play an important role in coolingthe circumstellar gas so that dust may form. We report on a survey forCO in a sample of RCB stars. We obtained H- and K-band spectra includingthe first- and second-overtone CO bands for eight RCB stars, theRCB-like star DY Per, and the final-helium-flash star FG Sge. The first-and second-overtone CO bands were detected in the cooler(Teff<6000 K) RCB stars, Z Umi, ES Aql, SV Sge, and DYPer. The bands are not present in the warmer (Teff>6000 K)RCB stars, R CrB, RY Sgr, SU Tau, and XX Cam. In addition,first-overtone bands are seen in FG Sge, a final-helium-flash star thatis in an RCB-like phase at present. Effective temperatures of the eightRCB stars range from 4000 to 7250 K. The observed photospheric COabsorption bands were compared to line-blanketed model spectra of RCBstars. As predicted by the models, the CO bands are strongest in thecoolest RCB stars and not present in the warmest. No correlation wasfound between the presence or strength of the CO bands and dustformation activity in the stars.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| RCoronae Borealis stars at minimum light - UW Cen
Two high-resolution optical spectra of the R Coronae Borealis (R CrB)star UW Cen in decline are discussed. A spectrum from mid-1992 when thestar had faded by 3mag shows just a few differences with the spectrum atmaximum light. The ubiquitous sharp emission lines seen in R CrB at asimilar drop below maximum light are absent. In contrast, a spectrumfrom mid-2002 when the star was 5mag below maximum light shows an arrayof sharp emission lines and a collection of broad emission lines.Comparisons are made with spectra of R CrB obtained during the deep1995-1996 minimum. The many common features are discussed in terms of atorus-jet geometry.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Self-Correlation Analysis of R Coronae Borealis Stars: A Pilot Project
R. Coronae Borealis (RCB) stars are peculiar yellow supergiant starswhich suddenly and unpredictably decrease in brightness by up to severalmagnitudes, then slowly return to normal. Most (perhaps all) RCB starsalso pulsate, and the pulsations may be related to the ejection of thedust clouds which produce the fadings. As a pilot project, we haveapplied self-correlation analysis to two datasets: long-term photometryof R CrB itself by J. D. Fernie, and long-term photometry of severalsouthern RCB stars by P. L. Cottrell, L. Skuljan, and their colleagues.Self-correlation is a simple form of time series analysis which displaysthe cycle-to-cycle behavior of a variable star, averaged over a dataset.It is especially useful for semi-regular variables. Generally, theseasonal pulsation time scales and amplitudes which we derive are inagreement with Fourier analysis of the same datasets. In the case of RCrB, we confirm that there is apparent mode-switching from season toseason.
| Aktivitaten zwischen April und August 2004.
Not Available
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
Not Available
| Optical Properties of the Circumstellar Dust around Stars with Aperiodic Fadings
Multicolor observations of 21 rapid, irregular variable stars withAlgol-like fadings and of R Coronae Borealis are used to derive theoptical extinction coefficients of the circumstellar dust associatedwith these objects, θ(λ). We used more than 3600 uniformmulti-color brightness measurements obtained in a rigorous U BV R systemat the Terskol High-altitude Observing Station. The mean extinctioncoefficient θ for the circumstellar dust for this sample of starscoincides almost exactly with the interstellar extinction coefficient,θis, but is somewhat higher in the ultraviolet. We suggest anexplanation for this difference.
| The Dust Envelope of R CrB in June 2001: Detection of a Compact IR Source
We present the results of a joint analysis of JHK interferometric andUBVJHKLM photometric observations of RCrB acquired in June 2001. Thebaseline for the IOTA interferometer was 21.18 m. During theobservations, the star was in its bright state in the V band and nearits maximum brightness in the L band. Our analysis reveals an IR sourcethat is considerably smaller than the extended dust envelope discoveredearlier. We identify this compact IR source with the emission from agroup of dust clouds. The linear scale (diameter) of the IR source was din,c ≈ 13.5D * (its angular diameter is θin,c≈6.4 mas).About 7% of the star’s radiation was obscured by this group ofclouds, which contributed 14% of the total IR excess of R CrB and22% of the K-band flux. The color temperature of the compact sourcewas only 300 K higher than the color temperature of the extendeddust envelope. The inner boundary of the extended dust envelope had adiameter of d in,e ≈ 90D * (θin,e≈43 mas).
| Kinematics of the envelope of the post-AGB star V510 PupNucleus of a future planetary nebula
We have carried out a detailed identification of lines in the opticalspectrum of the post-AGB star V510 Pup associated with the infraredsource IRAS 08005-2356 based on observations with high spectralresolution. Absorption lines of the ions FeII, TiII, CrII, and YII arepresent at wavelengths from 4549 to 8546 Å. The absorption by YIIand other s-process elements is anomalously strong, and the absorptionis also strong in the circumstellar C2 Swan bands. The profiles of mostof the lines (of hydrogen and metals) display P Cygniabsorption-emission profiles. All the absorption lines are shiftedtoward the blue, suggesting an outflow of stellar material. Theexpansion velocity of the envelope derived from the Swan bands arisingthere is V exp =42 km/s. The highest wind velocity determined from theabsorption wings of the FeII(42) P Cygni profiles reaches 240 km/s.Based on the star’s kinematic characteristics and the amount ofinterstellar absorption, it is at a distance of d≈3-4 kpc, whichcorresponds to an absolute magnitude of M v≈-6m.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Abundances of Neutron-Capture Elements in the Hot Extreme Helium Stars V1920 Cygni and HD 124448
Analysis of Hubble Space Telescope Space Telescope Imaging Spectrographultraviolet spectra of two hot extreme helium (EHe) stars, V1920 Cyg andHD 124448, provides the first measurements of abundances ofneutron-capture elements for EHe stars. Although the two stars havesimilar abundances for elements up through the iron group, they differstrikingly in their abundances of heavier elements: V1920 Cyg isenriched by a factor of 30 in light neutron-capture elements (Y/Fe andZr/Fe) relative to HD 124448. These differences in abundances ofneutron-capture elements among EHe stars are exhibited by the R CrBstars and are evidence supporting the view that there is an evolutionaryconnection between these two groups of hydrogen-deficient stars. Also,the abundances of Y and Zr in V1920 Cyg provide evidence that at leastone EHe star went through an s-process synthesis episode in its earlierevolution.Based on observations obtained with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| IOTA Observation of the Circum-stellar Envelope of R CrB
Not Available
| JHK'-Band IOTA Interferometry of the Mira Star T Cep and the Circumstellar Environment of R CrB
Not Available
| A High-Resolution Spectrum of the R Coronae Borealis Star V2552 Ophiuchi
Photometry and low-resolution spectroscopy have added V2552 Oph to therare class of R Coronae Borealis variables. We confirm thisclassification of V2552 Oph through a comparison of our high-resolutionoptical spectrum of this star and that of R CrB and other F-type membersof the class. We show that V2552 Oph most closely resembles Y Mus and FHSct, stars in which Sr, Y, and Zr are enhanced.
| The Newly Active R Coronae Borealis Star, V2552 Ophiuchi
In 2001, V2552 Oph (CD -22°12017, Had V98) quickly faded by severalmagnitudes in a manner typical of the R Coronae Borealis (RCB) stars.Photometry of V2552 Oph obtained over 70 years previous to 2001 shows noindication of variability. Optical spectra of this star subsequentlyconfirmed that V2552 Oph is a member of the hydrogen-deficient,carbon-rich RCB class of variables. It resembles the warm(Teff~7000 K) RCB stars such as R Coronae Borealis itself.Other RCB stars, such as XX Cam and Y Mus, have experienced similarperiods of inactivity, going decades without significant dust formation.Further observations of V2552 Oph will be of great interest since thereis an opportunity to monitor an RCB star that may be moving fromprolonged inactivity into an active phase of dust production.
| Jesse Leonard Greenstein (1909-2002)
Not Available
| 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.
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| Csillagkép: | Északi Korona |
| Rektaszcenzió: | 15h48m34.40s |
| Deklináció: | +28°09'24.0" |
| Vizuális fényesség: | 5.85 |
| Távolság: | 1851.852 parszek |
| RA sajátmozgás: | -2.2 |
| Dec sajátmozgás: | -11.9 |
| B-T magnitude: | 6.614 |
| V-T magnitude: | 5.959 |
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