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Investigation of the single neutron exposure model for the s-process: the primary nature of the neutron source
The primary nature of the 13C neutron source is verysignificant for the studies of the s-process nucleosynthesis. In thispaper we present an attempt to fit the element abundances observed in 16s-rich stars using parametric model of the single neutron exposure. Thecalculated results indicate that almost all s-elements were made in asingle neutron exposure for nine sample stars. Although a large spreadof neutron exposure is obtained, the maximum value of the neutronexposure will reach about 7.0 mbarn-1, which is close to thetheoretical predictions by the asymptotic giant branch (AGB) model. Thecalculated result is a significant evidence for the primary nature ofthe neutron source. Combining the result obtained in this work and theneutron exposure-initial mass relations, a large spread of neutronexposure can be explained by the different initial stellar mass andtheir time evolution. The possibility that the rotationally inducedmixing process can lead to a spread of the neutron exposure in AGB starsis also existent.
| The origin of the lead-rich stars in the Galactic halo: investigation of model parameters for the s-process
Several stars at the low-metallicity extreme of the Galactic halo showlarge spreads of lead and associated `heavy' s-process elements([Pb/hs]). Theoretically, an s-process pattern should be obtained froman AGB star with a fixed metallicity and initial mass. For the thirddredge-up and the s-process model, several important properties dependprimarily on the core mass of AGB stars. Zijlstra reported that theinitial-to-final mass relation steepens at low metallicity, due to lowmass-loss efficiency. This might affect the model parameters of the AGBstars, e.g. the overlap factor and the neutron irradiation time, inparticular at low metallicity. The calculated results do indeed showthat the overlap factor and the neutron irradiation time aresignificantly small at low metallicities, especially for3.0Msolar AGB stars. The scatter of [Pb/hs] found in lowmetallicities can therefore be explained naturally when varying theinitial mass of the low-mass AGB stars.
| Neutron-Capture Elements in the s- and r-Process-rich Stars: Constraints on Neutron-Capture Nucleosynthesis Processes
The chemical abundances of the very metal-poor double-enhanced stars areexcellent information for setting new constraints on models ofneutron-capture processes at low metallicity. These stars are known as s+ r stars, since they show enhancements of both s-process and r-processelements. The observed abundance ratios for the double-enhanced starscan be explained by those of stars that were polluted by an AGB star andsubsequently accreted very significant amounts of r-process material outof an AIC (accretion-induced collapse) or Type 1.5 supernova. In thispaper we present for the first time an attempt to fit the elementalabundances observed in the s- and r-rich, very metal-poor stars using aparametric model and suggest a new concept of component coefficients todescribe the contributions of the individual neutron-capture processesto double-enhanced stars. We find that the abundance ratios of thesestars are best fitted by enrichments of s- and r-process material. Theoverlap factor in the AGB stars where the observed s-process elementswere produced lies between 0.1 and 0.81. Taking into account thedependence of the initial-final mass relations on metallicity, this widerange of values could possibly be explained by a wide range of core-massvalues of AGB stars at low metallicity. The component coefficient of ther-process is strongly correlated with the component coefficient of thes-process for the double-enhanced stars. This is significant evidencethat the r-process material in double-enhanced stars comes from an AICor Type 1.5 supernova.
| The Calar Alto lunar occultation program: update and new results
We present an update of the lunar occultation program which is routinelycarried out in the near-IR at the Calar Alto Observatory. A total of 350events were recorded since our last report (Fors et al. 2004, A&A,419, 285). In the course of eight runs we have observed, among others,late-type giants, T-Tauri stars, and infrared sources. Noteworthy was apassage of the Moon close to the galactic center, which produced a largenumber of events during just a few hours in July 2004. Results includethe determinations of the angular diameter of RZ Ari,and the projected separations and brightness ratios for one triple and13 binary stars, almost all of which representing first time detections.Projected separations range from 0farcs09 to 0farcs007. We provide aquantitative analysis of the performance achieved in our observations interms of angular resolution and sensitivity, which reach about 0farcs003and K ≈8.5 mag, respectively. We also present a statisticaldiscussion of our sample, and in particular of the frequency ofdetection of binaries among field stars.
| s-Process in low metallicity Pb stars.
We consider a sample of very metal-poor, C-rich, s-rich and lead-richstars observed at high-resolution spectroscopy, and some recentspectroscopic data of C+s-rich stars obtained at moderate resolution.The spectroscopic data of these stars are interpreted with AGBtheoretical models of different 13C-pocket efficiencies,initial mass and initial r-enrichment. When lead is not measured we giveour theoretical prediction. The observed stars are not on the AGB phase,but are main sequence or giant stars. They acquired the C and senrichments by mass transfer in a close binary system from the moremassive companion while on the AGB (now a white dwarf). A considerablefraction of the stars show both high s and r enrichments. To explain thes+r enriched stars we assume a parental cloud already enriched inr-elements. The measurement of Nb is an indicator of an extrinsic AGB ina binary system. The intrinsic indicator [hs/ls] constrains the initialmass, while [Pb/hs] and [Pb/ls] are a measure of the s-processefficiency. The apparent discrepancies of C and N abundances may bereconciled by assuming a strong cool bottom process occurring during theAGB. An important primary production of light elements, from Ne to Si,increasing with the star mass, is predicted for AGB models at very lowmetallicity, induced by n capture on primary 22Ne and itsprogenies.
| The Binary Frequency Among Carbon-enhanced, s-Process-rich, Metal-poor Stars
We discuss radial velocities for a sample of carbon-enhanced,s-process-rich, very metal-poor (CEMP-s) stars, analyzed withhigh-resolution spectroscopy obtained over multiple epochs. We find that~68% of the stars in the sample show evidence of radial velocityvariations. The expected detection fraction for these stars, adoptingthe measured binary fraction in the field (~60%) and assuming that theyshare the same period and eccentricity distribution, is ~22%. Even ifone assumes that the true binary fraction of these stars is 100%, theexpected detection percentage is ~36%. These values indicate that thebinary fraction among CEMP-s stars is higher than the field binaryfraction, suggesting that all of these objects are in double (ormultiple) systems. The fact that the observed frequency of velocityvariation exceeds the expected detection fraction in the case of anassumed binary fraction of 100% is likely due to a more restricteddistribution of orbital periods for these objects, as compared to normalfield binaries. Our results indicate that CEMP-s stars are themetal-poor analogs of classical CH stars.Based in part on observations collected at the European SouthernObservatory, Paranal, Chile (ESO Programme 167.D-0173).INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5,I-35122 Padova, Italy.
| The first stars: what we know and do not know.
Not Available
| First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity
We present an abundance analysis of the very metal-poor, carbon-enhancedstar CS 29497-030. Our results indicate that this unusually hot turnoffstar (Teff = 6650 K, log g = 3.5) has a metallicity [Fe/H] =-2.8, and exhibits large overabundances of carbon ([C/Fe] = +2.38),nitrogen ([N/Fe] = +1.88), and oxygen ([O/Fe] = +1.67). This star alsoexhibits a large enhancement in its neutron-capture elements; thepattern follows that expected to arise from the s-process. Inparticular, the Pb abundance is found to be very high with respect toiron ([Pb/Fe] = +3.5), and also with respect to the second peaks-process elements (e.g., Ba, La, Ce, Nd), which fits into the newlyintroduced classification of lead (Pb) stars. The known spectroscopicbinary status of this star, along with the observed s-process abundancepattern, suggest that it has accreted matter from a companion, whichformerly was an Asymptotic Giant-Branch (AGB) star. In a preliminaryanalysis, we have also identified broad absorption lines of metallicspecies that suggest a large axial rotational velocity for this star,which may be the result of spin-up associated with the accretion ofmaterial from its previous AGB companion. In addition, this star isclearly depleted in the light element Li. When considered along with itsrather high inferred temperature, these observations are consistent withthe expected properties of a very low metallicity halo blue straggler.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A)).Table \ref{tab6} is only available in electronic form athttp://www.edpsciences.org
| Lead and mathbf s-process elements in stars of various metallicities: AGB predictions compared with observation
We present AGB predictions for all heavy elements within a large rangeof 13C-pocket efficiencies for stars of differentmetallicities, and compare them in detail with a number of spectroscopicobservations of s-rich and lead-rich in the Galaxy. The current conceptof the s-process efficiency, specified by the [hs/ls] index, is shown tobe inappropriate for the metal poor AGB stars and a second independentindex, [Pb/hs] or [Pb/ls], needs to be introduced. The state-of-the-artconcerning the interpretation of lead stars allows a very large spreadof [Pb/hs] in metal poor stars, as typically observed. We discussagreements and discrepancies for a large range of elements.
| More lead stars
The standard model for the operation of the s-process in asymptoticgiant branch (AGB) stars predicts that low-metallicity ([Fe/H] <~ -1)AGB stars should exhibit large overabundances of Pb and Bi as comparedto other s-elements. The discovery of the first three such ``leadstars'' (defined as stars enriched in s-elements with [Pb/hs] >~ 1,hs being any of Ba, La or Ce) among CH stars has been reported in aprevious paper (Van Eck et al. \cite{VanEck-01}). Five more CH stars(with [Fe/H] ranging from -1.5 to -2.5) are studied in the presentpaper, and two of them appear to be enriched in lead (with [Pb/Ce] =~0.7). The Pb I line at lambda4057 .812 Å is detected and clearlyresolved thanks to high-resolution spectra (R = lambda /Delta lambda =135ts000 ). The abundances for these two stars (HD 198269 and HD 201626)are consistent with the predictions for the s-process operating inlow-metallicity AGB stars as a consequence of the ``partial mixing'' ofprotons below the convective hydrogen envelope. Another two stars (HD189711 and V Ari) add to a growing number of low-metallicity stars (alsoincluding LP 625-44 and LP 706-7, as reported by Aoki et al.\cite{Aoki2001}) which do not conform to these predictions. Variationson the canonical proton-mixing scenario for the operation of thes-process in low-metallicity stars, that could account for thesediscrepant stars, are briefly discussed.Based on observations carried out at the European Southern Observatory(La Silla, Chile; Program 65.L-0354) and at the Observatoire de HauteProvence (operated by CNRS, France).
| 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
| 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
| Chemical Composition of Carbon-Rich, Very Metal-Poor Subgiant LP 625-44 Observed with the Subaru/HDS
We have obtained high-resolution (R ~ 90000) spectra of the carbon- ands-process-element-rich, very metal-poor ([Fe/H] = -2.7) subgiant LP625-44, as well as those of HD 140283 (a metal-poor subgiant with normalabundance ratio) for a comparison, with the High Dispersion Spectrograph(HDS) for the Subaru Telescope for detailed abundance study. The excessof oxygen in LP 625-44 seems to be remarkable (perhaps by nearly afactor 10) compared with that of HD 140283 derived from the O I tripletaround 7770Å, though the oxygen abundance derived from these linesis uncertain. The Na enhancement in LP 625-44 is by about a factor 50,suggesting hydrogen burning in the 22Ne-rich layer in anasymptotic giant branch star which produced the abundance pattern ofthis object. In our new spectrum of LP 625-44, the Pb Iλ3683Å line has been detected, confirming the Pb abundance[log ∈(Pb) ~ 1.9] derived from the Pb I λ4057Å line byprevious work. The abundance ratio of s-process elements at the secondpeak (e.g., La, Ce, and Nd) to that at the third peak (Pb) in LP 625-44is significantly higher (by a factor 5) than that in other threes-process element-rich objects recently studied by van Eck et al. Recenttheoretical studies have modeled s-process nucleosynthesis in theradiative layer of asymptotic giant branch stars in the inter-pulsephase. The above results mean that these processes produced a largescatter in the abundance ratios, or different processes (e.g., s-processnucleosynthesis during thermal pulses) contributed to heavy elements inthe early Galaxy.
| Discovery of lead stars with the ESO 3.6-m telescope and CES
Not Available
| Discovery of three lead-rich stars
About half of the stable nuclei heavier than iron are believed to besynthesized during the late stages of evolution of stars with masses inthe range 0.8-8 solar masses. These elements are then expelled into theinterstellar medium through stellar winds after being `dredged up'towards the surface of the stars. These processes occur when the star isin the `asymptotic giant branch' (AGB) phase of its life. Nuclei (mainlyiron) deep inside the star slowly capture neutrons and progressivelybuild up heavier elements (the `s-process'). For AGB stars that formedearly in the history of the Galaxy, and that therefore have very lowabundances of elements heavier than helium (`metals'), models predictthat the s-process will accumulate synthesized material with atomicweights in the Pb-Bi region. Such stars will therefore have largeoverabundances of lead relative to other heavy elements. Here we reportthe discovery of large amounts of lead in three metal-poor stars(HD187861, HD196944 and HD224959). Our analysis shows that these starsare more enriched in lead than in any other element heavier than iron.The excellent agreement between the observed and predicted abundancesreinforces our current understanding of the detailed operation of thes-process deep in the interiors of AGB stars.
| Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observed s-Process Abundances
We present the results of s-process nucleosynthesis calculations forasymptotic giant branch (AGB) stars of different metallicities anddifferent initial stellar masses (1.5 and 3 Msolar), and wepresent comparisons of them with observational constraints fromhigh-resolution spectroscopy of evolved stars over a wide metallicityrange. The computations were based on previously published stellarevolutionary models that account for the third dredge-up phenomenonoccurring late on the AGB. Neutron production is driven by the13C(α,n)16O reaction during the interpulseperiods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13C is manufacturedlocally by proton captures on the abundant 12C; a few protonsare assumed to penetrate from the convective envelope into the radiativelayer at any third dredge-up episode, when a chemical discontinuity isestablished between the convective envelope and the He- and C-richzones. A weaker neutron release is also guaranteed by the marginalactivation of the reaction 22Ne(α,n)25Mgduring the convective thermal pulses. Owing to the lack of a consistentmodel for 13C formation, the abundance of 13Cburnt per cycle is allowed to vary as a free parameter over a wideinterval (a factor of 50). The s-enriched material is subsequently mixedwith the envelope by the third dredge-up, and the envelope compositionis computed after each thermal pulse. We follow the changes in thephotospheric abundance of the Ba-peak elements (heavy s [hs]) and thatof the Zr-peak ones (light s [ls]), whose logarithmic ratio [hs/ls] hasoften been adopted as an indicator of the s-process efficiency (e.g., ofthe neutron exposure). Our model predictions for this parameter show acomplex trend versus metallicity. Especially noteworthy is theprediction that the flow along the s-path at low metallicities drainsthe Zr and Ba peaks and builds an excess at the doubly magic208Pb, which is at the termination of the s-path. We thendiscuss the effects on the models of variations in the crucialparameters of the 13C pocket, finding that they are notcritical for interpreting the results. The theoretical predictions arecompared with published abundances of s-elements for AGB giants ofclasses MS, S, SC, post-AGB supergiants, and for various classes ofbinary stars, which supposedly derive their composition by mass transferfrom an AGB companion. This is done for objects belonging both to theGalactic disk and to the halo. The observations in general confirm thecomplex dependence of neutron captures on metallicity. They suggest thata moderate spread exists in the abundance of 13C that isburnt in different stars. Although additional observations are needed,it seems that a good understanding has been achieved of s-processoperation in AGB stars. Finally, the detailed abundance distributionincluding the light elements (CNO) of a few s-enriched stars atdifferent metallicities are examined and satisfactorily reproduced bymodel envelope compositions.
| The effective temperatures of carbon-rich stars
We evaluate effective temperatures of 390 carbon-rich stars. Theinterstellar extinction on their lines of sights was determined andcircumstellar contributions derived. The intrinsic (dereddened) spectralenergy distributions (SEDs) are classified into 14 photometric groups(HCi, CVj and SCV with i=0,5 and j=1,7). The newscale of effective temperatures proposed here is calibrated on the 54angular diameters (measured on 52 stars) available at present from lunaroccultations and interferometry. The brightness distribution on stellardiscs and its influence on diameter evaluations are discussed. Theeffective temperatures directly deduced from those diameters correlatewith the classification into photometric groups, despite the large errorbars on diameters. The main parameter of our photometric classificationis thus effective temperature. Our photometric < k right >1/2 coefficients are shown to be angular diameters on arelative scale for a given photometric group, (more precisely for agiven effective temperature). The angular diameters are consistent withthe photometric data previously shown to be consistent with the trueparallaxes from HIPPARCOS observations (Knapik, et al. \cite{knapik98},Sect. 6). Provisional effective temperatures, as constrained by asuccessful comparison of dereddened SEDs from observations to modelatmosphere predictions, are in good agreement with the values directlycalculated from the observed angular diameters and with those deducedfrom five selected intrinsic color indices. These three approaches wereused to calibrate a reference angular diameter Phi 0 and theassociated coefficient CT_eff. The effective temperatureproposed for each star is the arithmetic mean of two estimates, one(``bolometric'') from a reference integrated flux F0, theother (``spectral'') from calibrated color indices which arerepresentative of SED shapes. Effective temperatures for about 390carbon stars are provided on this new homogeneous scale, together withvalues for some stars classified with oxygen-type SEDs with a total of438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given.Objects with strong infrared excesses and optically thick circumstellardust shells are discussed separately. The new effective temperaturescale is shown to be compatible and (statistically) consistent with thesample of direct values from the observed angular diameters. Theeffective temperatures are confirmed to be higher than the mean colortemperatures (from 140 to 440 K). They are in good agreement with thepublished estimates from the infrared flux method forTeff>= 3170 K, while an increasing discrepancy is observedtoward lower temperatures. As an illustration of the efficiency of thephotometric classification and effective temperature scale, the C/Oratios and the Merrill-Sanford (M-S) band intensities are investigated.It is shown that the maximum value, mean value and dispersion of C/Oincrease along the photometric CV-sequence, i.e. with decreasingeffective temperature. The M-S bands of SiC2 are shown tohave a transition from ``none'' to ``strong'' at Teff =~(2800+/- 150right ) K. Simultaneously, with decreasing effectivetemperature, the mean C/O ratio increases from 1.04 to 1.36, thetransition in SiC2 strength occurring while 1.07<= C/O<= 1.18. This research has made use of the Simbad database operatedat CDS, Strasbourg, France. Table 10 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)}or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/178
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
| Re-processing the Hipparcos Transit Data and Intermediate Astrometric Data of spectroscopic binaries. I. Ba, CH and Tc-poor S stars
Only 235 entries were processed as astrometric binaries with orbits inthe Hipparcos and Tycho Catalogue (\cite{Hipparcos}). However, theIntermediate Astrometric Data (IAD) and Transit Data (TD) made availableby ESA make it possible to re-process the stars that turned out to bespectroscopic binaries after the completion of the Catalogue. This paperillustrates how TD and IAD may be used in conjunction with the orbitalparameters of spectroscopic binaries to derive astrometric parameters.The five astrometric and four orbital parameters (not already known fromthe spectroscopic orbit) are derived by minimizing an objective function(chi 2) with an algorithm of global optimization. This codehas been applied to 81 systems for which spectroscopic orbits becameavailable recently and that belong to various families ofchemically-peculiar red giants (namely, dwarf barium stars, strong andmild barium stars, CH stars, and Tc-poor S stars). Among these 81systems, 23 yield reliable astrometric orbits. These 23 systems make itpossible to evaluate on real data the so-called ``cosmic error''described by Wielen et al. (1997), namely the fact that an unrecognizedorbital motion introduces a systematic error on the proper motion.Comparison of the proper motion from the Hipparcos catalogue with thatre-derived in the present work indicates that the former are indeed faroff the present value for binaries with periods in the range 3 to ~ 8years. Hipparcos parallaxes of unrecognized spectroscopic binaries turnout to be reliable, except for systems with periods close to 1 year, asexpected. Finally, we show that, even when a complete orbital revolutionwas observed by Hipparcos, the inclination is unfortunately seldomprecise. Based on observations from the Hipparcos astrometric satelliteoperated by the European Space Agency (ESA 1997).
| Dust extinction and intrinsic SEDs of carbon-rich stars. II. The hot carbon stars
The present work is an extension of a recent study by Knapik &Bergeat (\cite{knapik}, henceforth called Paper I) of the spectralenergy distributions (SEDs) of about 300 cool carbon-rich variables andof the interstellar extinction observed on their line of sights. Themethods were originally developed for Semi-Regular (SR) and Irregular(L)-variables. Shortly, this is a kind of a pair method making usesimultaneously of the whole SED from UV to IR. Our approach is appliedhere to the galactic carbon-rich giants with bluer SEDs, namely the hotcarbon (HC) stars, including many ``constant'' stars and a minority ofvariables: AC Her a RV Tau star, the R Coronae Borealis (RCB) stars andothers. Some HdC (i.e. carbon-rich hydrogen deficient stars) and Ba IIstars are also considered. The total number of studied HC stars amountsto about 140. With few exceptions, the colour excesses for interstellarextinction are found in good agreement with the field values from mapspublished in the literature, taking into account the approximatedistances to our stars from HIPPARCOS data (\cite{esa}, henceforthcalled ESA) or binarity. We propose a classification scheme with sixphotometric groups (or boxes: HC0 to HC5) from the bluest to the reddestSEDs. Oxygen-rich SEDs earlier than HC0, are attributed to the hotteststars (AC Her, most RCB-variables and a few others). Previous findingsare confirmed of a junction between oxygen-rich and carbon-rich SEDs atspectral type G. The latest (HC5) group is immediately close to theearliest one in Paper I, namely CV1. The sequence of groups then goesregularly from HC0 to CV6. Substantial infrared excesses with respect toour solutions are found in HD 100764 a HC1 carbon star, AC Her a G0g RVTau star, and the RCB stars classified in either HC or oxygen-groups.The colour excesses at maximum light can usually be attributed tointerstellar reddening, with neutral circumstellar (CS) reddening (largegrains) or no CS extinction at all on the line of sight (non sphericalgeometry) as possible explanations. The latter model (disc or patchydistribution through successive puffs) is favoured. Two RCB variablesfor which we exploit SEDs on a rising branch (V CrA) or minimum light(RS Tel), show CS laws, respectively a selective extinction compatiblewith small grains and an extinction partly neutral indicative of largegrains on the line of sight. This research has made use of the Simbaddatabase operated at CDS, Strasbourg, France.}\fnmsep\thanks{Partiallybased on data from the ESA HIPPARCOS astrometrysatellite}\fnmsep\thanks{Tables~3 and 4 are only available in electronicform at the CDS via anonymous ftp 130.79.128.5
| Insights into the formation of barium and Tc-poor S stars from an extended sample of orbital elements
The set of orbital elements available for chemically-peculiar red giant(PRG) stars has been considerably enlarged thanks to a decade-longCORAVEL radial-velocity monitoring of about 70 barium stars and 50 Sstars. When account is made for the detection biases, the observedbinary frequency among strong barium stars, mild barium stars andTc-poor S stars (respectively 35/37, 34/40 and 24/28) is compatible withthe hypothesis that they are all members of binary systems. Thesimilarity between the orbital-period, eccentricity and mass-functiondistributions of Tc-poor S stars and barium stars confirms that Tc-poorS stars are the cooler analogs of barium stars. A comparative analysisof the orbital elements of the various families of PRG stars, and of asample of chemically-normal, binary giants in open clusters, revealsseveral interesting features. The eccentricity - period diagram of PRGstars clearly bears the signature of dissipative processes associatedwith mass transfer, since the maximum eccentricity observed at a givenorbital period is much smaller than in the comparison sample of normalgiants. be held The mass function distribution is compatible with theunseen companion being a white dwarf (WD). This lends support to thescenario of formation of the PRG star by accretion of heavy-element-richmatter transferred from the former asymptotic giant branch progenitor ofthe current WD. Assuming that the WD companion has a mass in the range0.60+/-0.04 Msb ȯ, the masses of mild and strong barium starsamount to 1.9+/-0.2 and 1.5+/-0.2 Msb ȯ, respectively. Mild bariumstars are not restricted to long-period systems, contrarily to what isexpected if the smaller accretion efficiency in wider systems were thedominant factor controlling the pollution level of the PRG star. Theseresults suggest that the difference between mild and strong barium starsis mainly one of galactic population rather than of orbital separation,in agreement with their respective kinematical properties. There areindications that metallicity may be the parameter blurring the period -Ba-anomaly correlation: at a given orbital period, increasing levels ofheavy-element overabundances are found in mild barium stars, strongbarium stars, and Pop.II CH stars, corresponding to a sequence ofincreasingly older, i.e., more metal-deficient, populations. PRG starsthus seem to be produced more efficiently in low-metallicitypopulations. Conversely, normal giants in barium-like binary systems mayexist in more metal-rich populations. HD 160538 (DR Dra) may be such anexample, and its very existence indicates at least that binarity is nota sufficient condition to produce a PRG star. This paper is dedicated tothe memory of Antoine Duquennoy, who contributed many among theobservations used in this study
| Carbon isotope ratio in carbon stars of the galactic halo.
We analysed CN red system (~8000Å) and C_2_ Swan system(~4700Å) to know carbon isotope ratios (^12^C/^13^C) for carbonstars in the Galactic halo, named CH stars. The isotope ratios areobtained for 6 CH stars by the curve-of-growth analysis of the isolated^12^CN and ^13^CN lines. In this analysis, we compared directly ^12^CNand ^13^CN lines of similar intensities (iso-intensity method), and theresulting ^12^C/^13^C ratios are almost independent of the modelatmosphere and its parameters. The ^13^CN lines appear to be too weak insome CH stars, for which we applied the spectral synthesis method to thestronger C_2_ Swan band, obtained ^12^C/^13^C ratios for two stars andestimated the lower limits of ^12^C/^13^C ratios for two stars. In thiscase, however, the results depend on model atmosphere and itsparameters. Results from our present and previous works show that mostof them (12 stars) distribute around ^12^C/^13^C~10 and two stars havevery high values (^12^C/^13^C>=500). The distribution of ^12^C/^13^Cratios in CH stars is different from that of the population I carbonstars as well as population II oxygen-rich giants (G~K types). The CHstars of very high ^12^C/^13^C ratios can be explained by dredge-up of^12^C due to 3α-process as in population I carbon stars (N-type).On the other hand the formation of the CH stars with low ^12^C/^13^Cratios requires the large supply of ^12^C followed by a process ofdecreasing ^12^C/^13^C ratio.
| Chemical composition of metal-poor carbon stars in the halo.
In an attempt to increase the sample of metal-deficient late-type (i.e.cool) halo carbon stars analysed (only 3 such stars have previously beenanalysed spectroscopically), we obtained high-resolution visual spectraof 5 more candidates (and analysed in addition existing archive IRspectra for one of the stars) from the recent catalogue of Sleivyt e& Bartkevicius (1990), HD 25408 (C5,3J), HD 42272 (C5,4 CH), HD59643 (C6,2CH), HD 189711 (C4,3 CH) and HD 197604 (C4,2 CH). From thespectra we have derived C/O ratios, N/C ratios, and metal abundances. Ifthe oxygen abundance was fixed at logA(O)=7.4/8.3 (assuming that itfollows the trend of oxygen overabundance relative to iron found in halostars in general) we can furthermore derive [C/Fe] and [N/Fe]. New modelatmospheres of metal-poor carbon stars were calculated with continuumopacity sources and molecular lines of CO, CN, C_2_, HCN, C_2_H_2_ andC_3_. Two of the stars, HD 25408 and HD 42272, turned out not to be CHstars. The other three stars, although late-type, showed the C/O and[Fe/H] ratios common in early-type CH stars. From the total sample ofthe six confirmed cool halo-CH stars now analysed, we find evidence thatnot all metal-poor low mass halo carbon stars can have formed due tomass transfer in binary systems, as is usually assumed. At least 3 ofthe stars, and possible more, are likely to have formed as intrinsiccarbon stars, with some similarities to the carbon star population inthe dwarf galaxies.
| A New Version of the Catalog of CH and Related Stars (CH95 Catalog)
A new version of the catalog of CH and related stars contains 244 fieldstars and 17 globular cluster stars. Here a list of these stars withtheir coordinates, their positions in the HR diagram and somestatistical diagrams is presented. The catalog will soon be available inthe printed and computerized versions.
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The B jRI Photometric System
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..281G&db_key=AST
| CH stars in the Large Magellanic Cloud and in our Galaxy
Infrared JHK photometry is given for all the proposed LMC CH stars inthe list of Hartwick and Cowley (43 stars). Their luminosities range upto M(bol) of -6 or brighter, making them the most massive carbon starsin the LMC. However, the kinematic discussion of Hartwick and Cowleysuggests an age of a few Gyr for them. They may thus be the products ofbinary mergers, i.e. evolutionary products of blue stragglers of anintermediate-age population. Some analogous stars in the halo of theGalaxy may have evolved from globular cluster type blue stragglers.
| The CH Stars. III. Heavy Element Abundances
Abstract image available at:http://adsabs.harvard.edu/abs/1992AJ....104.1997V
| The CH stars. II - Carbon, nitrogen, and oxygen abundances
Complete CNO abundances are obtained for a sample of red-giant CH stars.Analysis of the abundances shows that there are large excesses of carbonand nitrogen in the atmosphere while all indications are that the oxygenabundances follow the normal pattern observed among field G and Kgiants. A correlation between the C/N and C-13/C-12 seems to exist inthe red giant CH stars. Taken together, the excess of C and N nuclei,the large C/O ratios, and the C-12/C-13 and C/N ratios favor a scenariofor the creation of CH stars in which Roche lobe overflow followed bysome mechanism which mixes the accreted material into regions of thestar, where it participates in CN burning to some degree. The s-processelement abundance patterns of the CH stars are investigated, and it isfound that the overall level of enhancement is greater in the CH starsthan in barium stars. The distribution of the element favors the heavys-process peak more than in barium stars. The abundance pattern are bestmodeled by exposure of s-process seed nuclei to a single irradiation.These abundance patterns are shown to arise naturally from the operationof the C-13 neutron source in low-mass low-metallicity AGB stars.
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לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | דגים |
| התרוממות ימנית: | 00h02m08.02s |
| סירוב: | -02°49'12.3" |
| גודל גלוי: | 9.511 |
| תנועה נכונה: | -39.4 |
| תנועה נכונה: | -94.2 |
| B-T magnitude: | 10.925 |
| V-T magnitude: | 9.628 |
קטלוגים וכינוים:
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