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Merging in the common envelope and the origin of early R-type stars
Context. Binary systems experiencing one or two common envelope episodesduring the red giant branch or the Hertzsprung gap phases can produce asingle star, evolving along the Hayashi track, as a final outcome. Evenif these objects are expected to be very common in nature, a properdescription of their evolution and physical properties is still missing.Moreover, this scenario (red giant merging scenario) has been invoked asthe progenitor systems of early-R stars, by assuming that the physicalconditions developed as a consequence of the cores merging could producethe mixing into the convective envelope of fresh carbon that wassynthesized during the He-flash. Aims: We analyze in detail thered giant merging scenario to verify if the resulting star develops thephysical conditions suitable for a dredge-up of C-enriched material fromthe core to the envelope. Methods: We performed 3D simulations ofthe merging stars, to check whether He is burnt efficiently during theformation of a self-sustained disk. We therefore did 1D computations ofthe accretion phase occurring after the merging and of the followingevolution up to the settling of quiescent He-burning in the center. Weadopted different assumptions on the amount of angular momentumtransferred from the disk to the core and on the angular momentumtransport. Results: Efficient He-burning does not occur duringthe merging, because a very high temperature (T > 108 K)at the disk/He-core interface develops only for a few minutes. Ourcomputations show that the accretion process is the leading parameter indetermining the final properties of the merged object. In particular,the thermal energy delivered by the accreted matter determines theheating of the whole newborn core, thus preventing the developing ofhighly degenerate physical conditions. This occurrence determines theonset of the He-burning with an He-flash milder and closer to thecenter, as compared to standard RGB stars. Rotation and differentangular momentum transport efficiency plays a secondary role bydetermining the exact location of the first He-flash. In none of thecomputed models is material formed in the He-core mixed into theconvective envelope, because the H-burning shell, which always activeduring the He-flashes and later on, acts as a barrier. Conclusions: In the red giant merging scenario, the physical conditionssuitable for both a peculiar He-flash and the dredging-up of C-enrichedmaterial never occur. Our results speak against the possibility thatsuch an evolutionary scenario could represent the progenitor system ofearly R-stars.
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) are only available in electronic form atthe 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/514/A2
| The chemical composition of carbon stars. The R-type stars
Aims. The aim of this work is to shed some light on the problem of theformation of carbon stars of R-type from a detailed study of theirchemical composition. Methods: We use high-resolution and highsignal-to-noise optical spectra of 23 R-type stars (both early- andlate-types) selected from the Hipparcos catalogue. The chemical analysisis made using spectral synthesis in LTE and state-of-the-art carbon-richspherical model atmospheres. We derive their CNO content (including the12C/13C ratio), average metallicity, lithium, andlight (Sr, Y, Zr) and heavy (Ba, La, Nd, Sm) s-element abundances. Theobserved properties of the stars (galactic distribution, kinematics,binarity, photometry and luminosity) are also discussed. Results:Our analysis shows that late-R stars are carbon stars with identicalchemical and observational characteristics as the normal (N-type) AGBcarbon stars. The s-element abundance pattern derived can be reproducedby low-mass AGB nucleosynthesis models where the 13C(?,n)16O reaction is the main neutron donor. We confirm theresults of the sole previous abundance analysis of early-R stars, namelythat they are carbon stars with near solar metallicity showing enhancednitrogen, low 12C/13C ratios and no s-elementenhancements. In addition, we have found that early-R stars have Liabundances larger than expected for post RGB tip giants. We also findthat a significant number (~40%) of the early-R stars in our sample arewrongly classified, probably being classical CH stars and normal Kgiants. Conclusions: On the basis of the chemical analysis, weconfirm the previous suggestion that late-R stars are just misclassifiedN-type carbon stars in the AGB phase of evolution. Their photometric,kinematic, variability and luminosity properties are also compatiblewith this. In consequence, we suggest that the number of true R stars isconsiderably lower than previously believed. This alleviates the problemof considering R stars as a frequent stage in the evolution of low-massstars. We briefly discuss the different scenarios proposed for theformation of early-R stars. The mixing of carbon during an anomalousHe-flash is favoured, although no physical mechanism able to triggerthat mixing has been found yet. The origin of these stars still remainsa mystery.
| A comprehensive analysis of the cool RCB star DY Persei
Context: Theoretically, the number of cool Galactic R Coronae Borealisstars (RCBs) should be greater than the warm RCBs, however to date, onlya few candidates have been detected. Aims: Observations of theextremely cool RCB candidate, DY Per, and the anonymous nearby star (the“Taipei star”) are presented to specify its fundamentalparameters and evolutionary status. Methods: CCD BVRI photometryand low-resolution spectroscopy at deep light decline was carried out in2004. A high-resolution spectrum was gathered near the maximum light in2002, and a qualitative analysis was made relative to the typical carbonstars of various types. Results: Near the light maximums, thenearby anonymous star with a separation of ~2.5 arcsec reported byZa?s et al. (2005a, A&A, 438, L13) was outshined by DY Per.However, in the B and V band the components are resolved at deep lightdecline and the nearby star is somewhat brighter in B than DY Peritself. BV(R) individual magnitudes of the components were estimatedusing PSF photometry for the first time. The light decline in 2004 wasthe deepest ever observed for DY Per, B = 18.16 mag. The individualcolour indices of DY Per at light decline, (B-V) = 2.35 and (V-R) ?1.9, are not typical for carbon stars. In the spectrum, absorptionfeatures of C2 and CN molecules prevail, however, prominentC2 absorption bands are significantly veiled at deep-lightdecline and a broad (FWHM ~ 600 km s-1) emission feature ofNa i D12 is visible. Emissions are suspected in the C2 (1,0)bandhead and in Ca i line at 4227 Å. Analysis of DY Per relativeto the sample of carbon stars, verifies the significant hydrogendeficiency, high carbon abundance, relatively high12C/13C ratio and solar metallicity. Thus DY Percould be a prototype of cool extreme Galactic RCBs. The nearby star doesnot vary appreciably in the B and V bands, B = 17.8 mag. With theobserved colour indices (B-V) = 1.00 and (V-R) ? 0.6, it may be aG0 dwarf not physically related to DY Per, although the distances toboth stars are similar, d ~ 1.5 kpc.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| A detailed spectroscopy of the carbon-rich star BD +57°2161
An LTE abundance analysis based on high-resolution spectra is presentedfor the carbon-rich star BD +57°2161, whose evolutionary status isunknown. With [C/Fe] =+0.4 dex and a mean s-process overabundance of[s/Fe] ≃ +1.5 dex the peculiar atmospheric composition of BD+57°2161 is confirmed. The 12C/13C abundanceratio was found to be about 10. The mild iron deficiency, [Fe/H] = -0.2,supports the idea that BD +57°2161 could be an old-disk-populationobject. Radial-velocity measurements confirm the binary nature of thestar. Therefore the peculiar chemical composition could be due to themass transfer from the secondary - AGB star in the past. Orbitalparameters are estimated for another star of this group, BD +75°348.
| 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
| Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars
The Hipparcos Intermediate Astrometric Data for carbon stars have beenreprocessed using an algorithm which provides an objective criterion forrejecting anomalous data points and constrains the parallax to bepositive. New parallax solutions have been derived for 317 cool carbonstars, mostly of types R and N. In this paper we discuss the results forthe R stars. The most important result is that the early R stars (i.e.,R0 - R3) have absolute magnitudes and V-K colors locating them among redclump giants in the Hertzsprung-Russell diagram. The average absolutemagnitude MK for early R-type stars (with V - K < 4) hasbeen derived from a Monte-Carlo simulation implicitly incorporating allpossible biases. It appears that the simulated magnitude distributionfor a population with a true Gaussian distribution of mean MK= -2.0 and intrinsic standard deviation 1.0 mag provides a satisfactorymatch to the observed distribution. These values are consistent with theaverage absolute magnitude MK = -1.6 for clump red giants inthe solar neighborhood (Alves 2000). Further, early R-type stars arenon-variable, and their infrared photometric properties show that theyare not undergoing mass loss, properties similar to those of the redclump giants. Stars with subtypes R4 - R9 tend to be cooler and havesimilar luminosity to the N-type carbon stars, as confirmed by theirposition in the (J-H, H-K) color-color diagram. The sample of earlyR-type stars selected from the Hipparcos Catalogue appears to beapproximately complete to magnitude K0 ~ 7, translating intoa completeness distance of 600 pc if all R stars had MK= -2(400 pc if MK= -1). With about 30 early R-type stars in thatvolume, they comprise about 0.04% (0.14% for MK= -1) of thered clump stars in the solar neighborhood. Identification with the redclump locates these stars at the helium core burning stage of stellarevolution, while the N stars are on the asymptotic giant branch, wherehelium shell burning occurs. The present analysis suggests that for asmall fraction of the helium core burning stars (far lower than thefraction of helium shell-burning stars), carbon produced in the interioris mixed to the atmosphere in sufficient quantities to form a carbonstar. Based on observations from the Hipparcos astrometric satelliteoperated by the European Space Agency (ESA 1997).
| 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.
| Infrared observations of peculiar carbon stars.
We present a uniform and high quality set of infrared photometric (JHK)observations of the 6 peculiar carbon giant stars V Ari, UV Cam, BD+34911, TU Gem, BD+57 2161 and BD+34 4134. All of these belong to the smallgroup of known cool CH giants populating the Galactic halo. Comparisonof the J-H and H-K colours to "normal" C stars show our stars to be"bluer" (i.e., having lower values of J-H and H-K) than the bulk of theGalactic C stars. Comparison with synthetic JHK colours reveal 5 of our6 stars as having considerably lower metallicities and/or highertemperatures than the bulk. Using standard assumptions we deriveestimates of their effective temperatures, gravities, luminosities anddistances. Their derived luminosities place them close to (or below) thetheoretical first He shell flash luminosity, although other observationsindicate their carbon excess to be intrinsic.
| 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.
| Catalogue of CH and metal-deficient barium stars
Not Available
| Spectral classification of carbon stars by means of photoelectric photometry of line strengths.
Abstract image available at:http://adsabs.harvard.edu/abs/1977PASJ...29..731Y
| CH-like stars
The properties of CH-like stars are discussed on the basis ofYamashita's (1972) classification of some 290 carbon stars. The CH-likestars are shown to be early-type red carbon stars. Their spectra arefound to be very similar to those of CH stars (typical high-velocityPopulation II carbon stars), but their proper motions and radialvelocities reveal no evidence of high velocity. The spectra are alsofound to be similar to those of Ba II stars with enhanced carbonfeatures. Abundance anomalies in the spectra of various types of oldpeculiar stars are briefly discussed. It is noted that 16 CH-like starshave been discovered thus far.
| Radial Velocities of Nineteen Carbon Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974PASJ...26..159Y&db_key=AST
| A general catalogue of cool carbon stars
Not Available
| The magnitudes, colors and motions of stars of spectral class R.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958AJ.....63..477V&db_key=AST
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