Contents
Images
Upload your image
DSS Images Other Images
Related articles
The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars.
| 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.
| 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).
| Erster Nachweis von Staubwolken um einen R CrB-Stern.
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.
| The cool Galactic R Coronae Borealis variable DY Persei
Results of first CCD photometry during the recent deep light decline,and high-resolution spectroscopy, are presented for DY Persei. Thespectra show variable blueshifted features in the sodium D lines. The Ci lines are strong whereas neutron-capture elements are not enhanced.The isotopic 13CN(2, 0) lines relative to 12CN areof similar strength with those for the carbon star U Hya. All theseconfirm the RCB nature of DY Per and the existence of ejected clouds. Atleast two clouds are revealed at -197.3 and -143.0 km s-1. Astar was detected about 0.4 arcsec to the west and 2.5 arcsec to thenorth from DY Per. This anonymous companion, with observed colourindices (B-V) = 0.68 and (V-R) ≃ 1.1, may be a foreground star.
| 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 R Coronae Borealis stars: carbon abundances from forbidden carbon lines
Spectra of several R Coronae Borealis (RCB) stars at maximum light havebeen examined for the [CI] 9850- and 8727-Åabsorption lines. The9850-Åline is variously blended with an FeII and CN lines, butpositive identifications of the [CI] line are made for RCrB and SUTau.The 8727-Åline is detected in the spectrum of the five starsobserved in this wavelength region. Carbon abundances are derived fromthe [CI] lines using the model atmospheres and atmospheric parametersused by Asplund et al.Although the observed strength of a CI line is constant from cool to hotRCB stars, the strength is lower than predicted by an amount equivalentto a factor of 4 reduction of the gf-value of a line. Asplund et al.dubbed this `the carbon problem' and discussed possible solutions.The [CI] 9850-Åline seen clearly in RCrB and SUTau confirms themagnitude of the carbon problem revealed by the CI lines. The [CI]8727-Åline measured in five stars shows an enhanced carbonproblem. The gf-value required to fit the observed [CI] 8727-Ålineis a factor of 15 less than the well-determined theoretical gf-value. Wesuggest that the carbon problem for all lines may be alleviated to someextent by a chromospheric-like temperature rise in these stars. The risefar exceeds that predicted by our non-local thermodynamic equilibriumcalculations, and requires a substantial deposition of mechanicalenergy.
| Aus der Sektion Kataklysmische Sterne: Aktivitaten zwischen August - No vember 2004.
Not Available
| 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).
| First detection of dust clouds around R CrB variable stars
From VLT/NACO diffraction-limited images of RY Sgr, we report the firstdirect detection of heterogeneities in the circumstellar envelope of a RCoronae Borealis variable star. Several bright and very large dustclouds are seen in various directions at several hundred stellar radiifrom RY Sgr, revealing high activity for the ejection of stellarmaterial by R CrB variables. These observations do support the currentinterpretation that optically thick dust clouds are formed around thesurface of this type of variable stars and, when passing between thestar and the observer, produce the huge and sudden declinescharacterizing these objects in visible light. This is the first directconfirmation of a scenario proposed about 70 years ago.Based on observations collected with the VLT/UT4 Yepun telescope(Paranal Observatory, ESO, Chile) using the NACO instrument (program ID71.D-0543A).
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Photometry of R Coronae Borealis Stars during the Recovery Phase of their Declines
The photometric observations (UBVRI) of nine cool R Coronae Borealis(RCB) stars have been collected at Mt John University Observatory, NewZealand, over a period of twelve years. The analysis of themagnitude-colour and colour-colour diagrams for the recovery phasedemonstrates that all declines exhibit a similar asymptotic approach totheir normal brightness. Declines return to maximum brightness along aline with essentially the same slope that does not depend on the star orthe depth of the decline. Assuming a uniform obscuration of thephotosphere by the dust cloud during the recovery phase, the extinctionproperties of the material were determined. The ratio of total toselective extinction (RV) for the RCB stars in our sample isin the range 2.5 to 4.6, indicating that the obscuring dust hasextinction properties similar to that of the interstellar dust.Observations have been compared with the theoretical extinction curvesfor different sorts of grains.
| The case for asymmetric dust around a C-rich asymptotic giant branch star
JHKL observations of the mass-losing carbon Mira variable IRAS15194-5115 (II Lup) extending over about 18 yr are presented anddiscussed. The pulsation period is 575 d and has remained essentiallyconstant over this time span. The star has undergone an extensiveobscuration minimum during this time. This is complex and, like suchminima in similar objects (e.g. R For), does not fit the modelpredictions of a simple long-term periodicity. Together with thehigh-resolution observations of Lopez et al., the results suggest thatthe obscuration changes are caused by the formation of dust clouds oflimited extent in the line of sight. This is an R Coronae Borealis-type(RCB-type) model. The effective reddening law at J and H is similar tothat found for R For.
| Annual report of the director for fiscal year 2001-2002.
Not Available
| New R CrB-Type Star HadV98
HadV98 = CoD-22 12017 = GSC 6825.253 is a variable star discovered byKatsumi Haseda. This object has been recently confirmed to be a new RCrB star (Hasselbach et al. 2002), as had been originally proposed by usat the time of the discovery alert in VSNET. We studied the light curvebased on Haseda's photographic material and ASAS-3 V-band data. Therewas only major fading episode in the past decade. The fading started in2001 May was composed of at least two minima, and followed by a slowrecovery.
| Wer beobachtet mit? SU Tauri.
Not Available
| Relation Between Light Weakening and Equivalent Widths of Circumstellar Na I D-Lines in Stars with R CRB-Type Variability
High-resolution spectra in the Na I D-line region for target stars withR Coronae Borealis type variability are analyzed. Two types of relationsbetween light weakening and equivalent widths of circumstellar lines arefound, one of them being considerably different from a similar relationfor the interstellar medium.
| Winds in R Coronae Borealis Stars
We present new spectroscopic observations of the He I λ10830 linein R Coronae Borealis (RCB) stars that provide the first strong evidencethat most, if not all, RCB stars have winds. It has long been suggestedthat when dust forms around an RCB star, radiation pressure acceleratesthe dust away from the star, dragging the gas along with it. The newspectra show that nine of the 10 stars observed have P Cygni orasymmetric blueshifted profiles in the He I λ10830 line. In allcases, the He I line indicates a mass outflow with a range of intensityand velocity. Around the RCB stars, it is likely that this state ispopulated by collisional excitation rather thanphotoionization/recombination. The line profiles have been modeled withan SEI code to derive the optical depth and the velocity field of thehelium gas. The results show that the typical RCB wind has a steepacceleration with a terminal velocity of V&infy;=200-350 kms-1 and a column density of N~1012 cm-2in the He I λ10830 line. There is a possible relationship betweenthe light curve of an RCB star and its He I λ10830 profile. Starsthat have gone hundreds of days with no dust formation episodes tend tohave weaker He I features. The unusual RCB star V854 Cen does not followthis trend, showing little or no He I absorption despite high mass-lossactivity. The He I λ10830 line in R CrB itself, which has beenobserved at four epochs between 1972 and 2001, seems to show a P Cygnior asymmetric blueshifted profile at all times, whether it is in declineor at maximum light.
| 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
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| Recent declines of RS Telescopii, UW Centauri, and V Coronae Australis
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.
| The 1998 decline of V854 Centauri
Abstarct Photometry andlow- to high-resolution spectroscopy have been obtained throughout the1998 decline of V854 Centauri. This is the most complete coverage of adecline of this star. Various emission lines which are typical of an RCBdecline have been observed. Their evolution is consistent with theclassification into three groups according to theE1/E2/BL line-region model. However, some of theline characteristics indicate a different origin from that suggested bythe model. The low-excitation sharp emission lines (E2group), visible throughout the whole decline, show a blueshift of about3 km s-1 and a complex profile indicating a shell with itsouter region expanding faster than the inner parts. The estimatedposition of this region is at about 3.5 R*, assuming the dustcloud condensation occurs at about 2 R*. The high-excitationlines of CI, OI and MgI, classified as E1, exhibit a redshiftof a few km s-1 relative to the stellar velocity, indicatinga possible photospheric origin. The broad lines (BL group) show aconstant absolute flux throughout the decline phase and therefore theregion of their origin does not seem to be affected or obscured by thedust cloud causing the decline. The high-velocity NaI D absorptioncomponents, at a velocity of about -400 km s-1, show aconstantly increasing blueshift interpreted as an acceleration of thegas associated with the dust cloud formation.
| Photometric Observations of R Coronae Borealis in the Optical and Infrared
The results of long-term photometric observations of R CrB in the UBVJHKLM bands are presented. The temporal and color characteristics of theemission of the star itself and of its extended dust envelope areanalyzed in detail. No stable harmonic has been found in the semiregularvariations of the optical brightness of R CrB. Two harmonics withperiods P≈3.3 and 11.3 yrs have been detected in the brightnessvariations of the dust envelope; the minima of these variationscoincided in 1999, resulting in a record decrease in the LM brightnessof the envelope. This by chance coincided in time with a deep minimum ofthe visual brightness of the star, resulting in a unique decrease in thetotal brightness of the star and dust envelope. This enabled estimationof the bolometric flux of the hot dust clouds, which made up only a fewper cent of the bolometric flux of the dust envelope. The brightnessvariations of the dust envelope are not accompanied by appreciable colorchanges and are associated with variations of its optical depth τ(V)in the range 0.2 0.4. The dust envelope forms at a large and fairlyconstant distance from the star , from material in its stellar wind,whose intensity obeys a Reimers law. No variations synchronous withthose of the optical depth of the dust envelope, in particular, with theperiod P≈3.3 yrs, have been found in the optical emission of R CrB,suggesting that the stellar wind is not spherically symmetric. The dustenvelope consists of small grains (a gr≤0.01 µm), while theclouds screening the star from the observer are made up of large grains(a gr≈0.1 µm). The activity of R CrB, whose nature is unclear,is reflected in variations of the stellar-wind intensity and theappearance of dust clouds in the line of sight: these variations arerepeated by corresponding changes in the optical depth of the dustenvelope with a delay of 4 years (the time for a particle moving atV env≈45 km/s to move from the star to the boundary of the dustenvelope).
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/967
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | いて座 |
| Right ascension: | 19h16m32.80s |
| Declination: | -33°31'18.0" |
| Apparent magnitude: | 6.25 |
| Distance: | 2777.778 parsecs |
| Proper motion RA: | 8 |
| Proper motion Dec: | -0.3 |
| B-T magnitude: | 7.72 |
| V-T magnitude: | 6.902 |
Catalogs and designations:
|
INHABITED SKY
