Tartalom
Képek
| - (nincs kapcsolódó felvétel) - |
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
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
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Near-infrared observations of candidate extrinsic S stars
Photometric observations in the near infrared for 161 S stars, including18 Tc-rich (intrinsic) stars, 19 Tc-deficient (extrinsic) ones and 124candidates for Tc-deficient S stars, are presented in this paper. Basedon some further investigations into the infrared properties of bothTc-rich and Tc-deficient S stars, 104 candidates are identified as verylikely Tc-deficient S stars. The large number of infrared-selectedTc-deficient S stars provides a convenient way to study the physicalproperties and the evolutionary status of this species of S stars.
| New Hipparcos variables in the Bright Star Catalogue.
Not Available
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude 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.
| On the Variability of K5-M Stars
I investigate the Hipparcos Satellite photometry of K5-M stars to seethe pattern of activity of these stars. A few stars for which furtherstudy is desirable are identified.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| On the Variability of S Stars as Observed by the Hipparcos
The Hipparcos photometry of S type stars shows that they are allvariable. The intrinsic S stars show a larger range of amplitudes thando the extrinsic S stars.
| The nucleosynthesis of heavy elements in low-mass AGB stars
We take 13C(α,n)16O as neutron source, allowfor variation of the mass of the nucleus with thermal pulse number andadopt the unbranched s process path from 56Fe to210Bi in an attempt to explain the overabundance of heavyelements in MS and S stars. We found stars with larger masses havelarger dilution factors because of a stronger convection and that thesestars fall in four different regions in the abundance diagram, thusenabling a rough estimate of the mass to be made. We show that, for a2.5 Msolar star, the third dredge-up stops after a certain number ofpulses because of the formation of a hollow shell structure, so the starwill show no Tc and this can account for the existence of binary systemsconsisting of a ``Tc-no'' AGB star with a main-sequence star companion.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The heavy elements nucleosynthesis in the low mass AGB stars.
Not Available
| Fluorine nucleosynthesis in low-mass AGB stars during the thermal pulse phase.
Not Available
| Infrared emission and dynamics of outlfows in late-type stars
The dynamical structure and infrared emission of winds around late-typestars are studied in a self-consistent model that couples the equationsof motion and radiative transfer. Thanks to its scaling properties, boththe dynamics and IR spectrum of the solution are fully characterized bytauF, the flux-averaged optical depth of the wind. Five typesof dust grains are considered: astronomical silicate, crystallineolivine, graphite, amorphous carbon and SiC, as well as mixtures.Analysis of infrared signatures provides constraints on the grainchemical composition and indications for the simultaneous existence ofsilicate and carbon grains. The abundances of crystalline olivine inSi-dominated grains and of SiC in C-dominated grains are found to belimited to less than or equal to 20%-30%. Furthermore, in carbonaceousgrains carbon is predominantly in amorphous form, rather than graphite.In mixtures, carbonaceous grains tend to dominate the dynamic behaviorwhile silicate and SiC grains dominate the IR signature. The region ofparameter space where radiation pressure can support a given mass-lossrate is identified, replacing the common misconception M nu less than orequal to L*/c, and it shows that radiatively driven windsexplain the highest mass-loss rates observed to date. A new method toderive mass-loss rates from IR data is presented, and its results agreewith other determinations. The theoretical spectra and colors are ingood agreement with observations. IRAS Low Resolution Spectrometerclasses are associated with tauF for various grain materialsand the regions of color-color diagrams expected to be populated bylate-type stars are identified. For a given grain composition, locationin the color-color diagram follows a track with position along the trackdetermined by tauF. We show that cirrus emission can severelyaffect point source measurements to the extent that their listed IRASlong-wavelength fluxes are unreliable. Whenever the listed IRAS flagcirr3 exceeds the listed 60 micrometers flux by more than a factor of 2,the 60 and 100 micrometers fluxes are no longer indicative of theunderlying point source. After accounting of cirrus contamination,essentially all IRAS point sources (95%) located in the relevant regionsof the color-color diagrams can be explained as late-type stars. Thereis no need to invoke time dependent effects, such as detached shells,for example, to explain either the colors or mass-loss rates of thesesources. Although various indications of time varying mass-loss ratesexist in numerous sources, the infrared properties of this class ofstars are well explained as a whole with steady state shows.
| Circumstellar dust envelopes of oxygen-rich AGB stars.
Using IRAS observational data, the best-fitting models are searched foroxygen-rich AGB stars in various evolutionary stages from opticallyvisible red giant stars to OH/IR stars with heavy mass loss. The modelinfrared spectral energy distributions of the red giant stars withcircumstellar dust envelopes are calculated with the use of a radiativetransfer code. It is found that the oxygen-rich IRAS AGB starssurrounded by circumstellar dust envelopes are divided into threegroups, which can be clearly distinguished by their mass loss rate andthe grain temperature at the inner boundary of the circumstellar dustenvelopes. One group corresponds to Mira type objects with a moderatemass loss rate and a grain formation temperature of about 500 K, thesecond to OH/IR star type objects with a large mass loss rate and a highgrain formation temperature, and the third to M stars with a very coldgrain temperature at the inner boundary, indicating that the moderatemass loss stopped a considerable time ago in these objects. The highergrain formation temperature of the OH/IR star type objects indicates agrain formation point closer to the stellar surface, suggesting aneffective acceleration of dust grains by radiation pressure. Such aneffective acceleration can play an important role in the mass lossphenomena of OH/IR stars with large mass loss rates. From the modelfitting results, it is suggested that an AGB star undergoes severaldozen phases of moderate mass loss of˙(M)~10^-7^-10^-6^Msun_/yr for a period of10^3^-10^4^years with interruptions of 10^3^-10^4^years during theordinary Mira variable phase before becoming an OH/IR star with a massloss rate exceeding 10^-5^Msun_/yr and a lifetime of about10^3^-10^4^years.
| Companions to bright S and MS stars - Technetium deficiency and binarity
To test the popular hypothesis that technetium-deficient stars ofspectral types S and MS are mass-transfer binaries, we have searched forultraviolet light from the putative hot secondaries in spectra takenwith the SWP spectrograph of IUE. Although most S and MS stars areapparently thermally pulsing AGB (asymptotic giant branch) stars whosesurfaces have been enriched with s-process elements and carbon dredgedup from the interior, those stars whose spectra show enhanced s-processelements but no Tc are widely believed to be cooler analogs of the Ba IIstars, which apparently owe their unusual abundances to prior masstransfer, the Tc from which has decayed away. We report IUE observationsof 15 S and MS stars with the SWP, including the identification of sixhot companions. Assembling all the IUE observations made to date, wefind clear support for the mass-transfer hypothesis, confirming evidencefrom other lines of research. We further discuss the ages of thecompanions and the implications of these discoveries for stellarevolution.
| S stars: infrared colors, technetium, and binarity
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...271..463J&db_key=AST
| Binary S-Stars and Ms-Stars
Not Available
| Asymptotic giant branch stars near the sun
Available red and near-infrared photometry and apparent motions of M, S,and C asymptotic giant branch (AGB) stars in the Bright Star Catalogueare tabulated and discussed. It is shown that the red and near infraredindices normally used for late-type stars are interchangeable except forcarbon stars. The M-type giants are variable with visual amplitudegreater than 0.05 mag. The reddening-free parameter m2 from Genevaphotometry is essentially a temperature parameter for M giants, whilethe reddening-free parameter d is a sensitive detector of blue stellarcompanions. The space density of AGB stars near the sun decreases by afactor of 35 in a temperature range 3800 to 3400 K. Two of the S starsnear the sun were found to have nearly equal space motions and may becomembers of the Arcturus group.
| Fluorine in red giant stars - Evidence for nucleosynthesis
Flourine abundances were determined in several K giants and asymptoticgiant branch (AGB) stars, using the infrared vibration-rotation lines ofHF. The data presented constitute the only available information on thefluorine abundance outside the solar system and provide invaluable cluesto the question of the nucleosynthetic origin of F. The observationsconfirm that F production occurs at a He-burning site and constrain thecurrent models of AGB stars.
| A survey for infrared excesses among high galactic latitude SAO stars
This project involves extending the previous analysis of infraredexcesses among a volume-limited sample of 134 nearby A-K main-sequencestars to a magnitude-limited sample of stars, culled from the SAOCatalog, with excesses determined from the IRAS Point Source Catalogflux density ratios. This new sample includes 5706 B-M type stars, 379of which have infrared excesses. The objective involved use of astatistically complete survey of objects in a standard catalog in orderto assess the frequency with which different physical processes canaffect the infrared output of stars. These processes include, but arenot limited to, orbiting cold particle clouds and the onset of rapidmass loss. It is concluded that cold disks are consistent with theinfrared excesses found among A-G dwarfs and G-K giants in the sample.
| The chemical composition of red giants. III - Further CNO isotopic and s-process abundances in thermally pulsing asymptotic giant branch stars
Near-IR and IR spectra are analyzed to obtain elemental and isotopic C,N, and O abundance, iron peak abundances, and heavy element s-processabundances for a sample of M, MS, and S giants. The compositions of theM giants are similar to those of G and K giants and consistent withpredictions for giants that have deep convective envelopes and so haveexperienced the first dredge-up. The MS and S giants have compositionsthat show the signatures of the third dredge-up that occurs in thermallypulsing AGB stars. The sample shows that C-12, a principal product ofthe He-burning shell, has been added to the envelopes of MS and S stars.The C-12 enrichment is correlated with a more marked enrichment of thes-process elements that are predicted to be synthesized when a neutronsource is ignited in the He-burning shell. The MS and S giants show ahigher N abundance than the M giants, attributed to the expected deepmixing that occurs with the onset of the episodic third dredge-up.
| S stars without technetium - The binary star connection
An exploratory survey of non-Mira MS and S star radial velocities andthe He I 10830 A triplet are used to test the assertion that S starswithout Tc are spectroscopic binaries, probably with white-dwarfcompanions. It is found that the He I 10830 A triplet is a prominentfeature of the spectra of S stars without Tc, but the He I line isundetectable in the spectra of most S stars without Tc. Also, whenradial-velocity variations attributable to orbital motion are detectedfor S stars without Tc, the variations have a higher frequency that thatof S stars with Tc. The results suggest that the S stars without Tc arespectroscopic binaries and are probably related to the G and K giantBarium stars.
| Detection of 51 new 86 GHz SiO, V = 1, J = 2-1 masers associated with IRAS point sources
This paper reports the results of a search for southern stellar SiO, v =1, J = 2-1 masers in a survey of IRAS point sources. Out of the 116observed southern sources, 51 identified as new SiO, v = 1, J = 2-1masers, on the basis of their position in IRAS color-color plot. Ofthese, 42 masers have stellar identifications in IPSC, two arepreviously known non-IRAS IR objects, and six have no identification inIPSC.
| S-process-enriched cool stars with and without technetium - Clues to asymptotic giant branch and binary star evolution
A search for Tc in a sample of 40 MS and S stars is reported. Of thetotal sample, it is found that 38 percent of these s-process-enrichedstars do not show Tc. It is argued that the space density of theseTc-poor MS and S stars, relative to M giants, is comparable to the spacedensity, relative to normal G and K giants, of the barium stars, whichalso lack Tc and whose s-process enrichment is probably the result ofbinary mass transfer in the past. It is suggested that the MS and Sstars are composed of two major groups: Tc-containing stars which arecurrently thermally pulsing AGB stars undergoing third dredge-up andTc-poor stars which represent the coolest members of the barium starclass.
| On the absence of young white dwarf companions to five technetium stars
A search for hot companions to five stars of type MS and S has beencarried out using the IUE satellite. No hot companions were detected forthe MS stars HR 85, 4647, 6702, and 8062, and the S star HR 8714. Limitson the luminosities of possible white dwarf companions provide lowerlimits of 2-5x10 to the 8th yr to the ages of any degenerate companions.All five stars exhibit strong Tc I lines, and the presence oftechnetium, with a half-life of 2.1x10 to the 5th yr, signifies recentnucleosynthesis. The limits on the ages of possible white dwarfcompanions that are equal to or greater than 1000 half-lives of Tcexclude the possibility that the s-process elemental enhancement seen inthese MS and S stars resulted from mass transfer from a more highlyevolved companion (as is probably the mechanism by which barium starsare created). These MS and S stars represent a sample of true thermallypulsing asymptotic giant-branch stars.
| IRAS catalogues and atlases - Atlas of low-resolution spectra
Plots of all 5425 spectra in the IRAS catalogue of low-resolutionspectra are presented. The catalogue contains the average spectra ofmost IRAS poiont sources with 12 micron flux densities above 10 Jy.
| Radial velocities of southern stars obtained with the photoelectric scanner CORAVEL. III - 790 late-type bright stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&AS...59...15A&db_key=AST
| The radial velocities of 116 southern red stars
Using a photoelectric speedometer based on the radial velocityspectrometer of Griffin (1967), radial velocities of 116 southern redstars, many of them semi-regular variables, were measured. Theinstrument was placed at the Newtonian focus of the 130-inch (4.2-m)camera of the Coude spectrograph of the Mount Stromlo 74-inch (1.88-m)reflector. The stellar spectrum was focused on a mask 50 mm in length,designed to match the spectrum of the M-giant Beta Pegasi. The inversedispersion was 2.5 angstroms per millimeter in the second order. Thewavelength range used (5338-5449 angstroms) was chosen because it isbetween two TiO bands and so suffers less blanketing in M stars. Resultsare tabulated and compared with standard values.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Északi Vízikígyó |
| Rektaszcenzió: | 12h13m13.00s |
| Deklináció: | -34°07'32.0" |
| Vizuális fényesség: | 6.5 |
| Távolság: | 279.33 parszek |
| RA sajátmozgás: | -34.5 |
| Dec sajátmozgás: | 7.8 |
| B-T magnitude: | 8.332 |
| V-T magnitude: | 6.619 |
Katalógusok és elnevezések:
|
