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Ýlgili Makaleler
Lithium Isotopes in Population II Dwarfs
We address the evolution of lithium in Population II dwarf stars underthe joint effects of microscopic diffusion and tachocline mixing. Thisprocess relies on analytical developments and is also constrained byhelioseismology observations. It was successfully applied to solaranalogs but never investigated in halo stars. It is induced in the upperradiation zone by rotation and a slight differential rotation inlatitude. Consequently we modeled different possible rotation historiesof halo stars, showing that the initial rotation rate had no impact onlithium in the framework of tachocline mixing. We find a negligibleimpact of pre-main-sequence evolution on 7Li independent ofmetallicity provided that [Fe/H]<-1. On the contrary, microscopicdiffusion and tachocline turbulence act on the long term of mainsequence and shape the current 7Li-Teff patternfrom the turnoff down to 5000 K. The tachocline mixing models fit the7Li-Teff relation better than the pure microscopicdiffusion models. We address the issue of warm 7Li-poor starsand conclude that a moderate mass transfer from a companion couldexplain their composition. Finally, we discuss the lithium lighterisotope. The pre-main-sequence and main-sequence 6Lidepletion we compute seems difficult to reconcile with the currentobservations.
| Speckle interferometry of metal-poor stars in the solar neighborhood. II
The results of speckle interferometric observations of 115 metal-poorstars ([m/H] < ‑1) within 250 pc from the Sun and with propermotions µ ≳ 0.2″/yr, made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences,are reported. Close companions with separations ranging from0.034″ to 1″ were observed for 12 objects—G76-21,G59-1, G63-46, G135-16, G168-42, G141-47, G142-44, G190-10, G28-43,G217-8, G130-7, and G89-14—eight of them are astrometricallyresolved for the first time. The newly resolved systems include onetriple star—G190-10. If combined with spectroscopic and visualdata, our results imply a single:binary:triple:quadruple star ratio of147:64:9:1 for a sample of 221 primary components of halo and thick-diskstars.
| Beryllium in Ultra-Lithium-Deficient Halo Stars: The Blue Straggler Connection
There exists a small group of metal-deficient stars that have Liabundances well below the Li plateau that is defined by over 100unevolved stars with temperatures above 5800 K and values of[Fe/H]<-1.0. Abundances of Be have been determined for most of theseultra-Li-deficient stars in order to investigate the cause of the Lideficiencies. These Li-deficient stars have implications on the value ofprimordial Li. High-resolution and high signal-to-noise ratio spectrahave been obtained in the Be II spectral region near 3130 Å forsix ultra-Li-deficient stars with the Keck I telescope and its newUV-sensitive CCD on the upgraded HIRES. The spectrum synthesis techniquehas been used to determine Be abundances. All six stars are found tohave Be deficiencies also. Two have measurable but reduced Be, and fourhave only upper limits on Be. These results are consistent with the ideathat these Li- and Be-deficient stars are analogous to blue stragglers.The stars have undergone mass transfer events (or mergers) that destroyor dilute both Li and Be. The findings cannot be matched by the modelsthat predict that the deficiencies are due to extramixing in a subset ofhalo stars that were initially rapid rotators, with the possibleexception of one star, G139-8. Because the ultra-Li-deficient stars arealso Be-deficient, they appear to be genuine outliers in the populationof halo stars used to determine the Li plateau in that they no longerhave the Li in their atmospheres that was produced in the big bang.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| uvby-β photometry of high-velocity and metal-poor stars. XI. Ages of halo and old disk stars
New uvby-β data are provided for 442 high-velocity and metal-poorstars; 90 of these stars have been observed previously by us, and 352are new. When combined with our previous two photometric catalogues, thedata base is now made up of 1533 high-velocity and metal-poor stars, allwith uvby-β photometry and complete kinematic data, such as propermotions and radial velocities taken from the literature. Hipparcos, plusa new photometric calibration for Mv also based on theHipparcos parallaxes, provide distances for nearly all of these stars;our previous photometric calibrations give values for E(b-y) and [Fe/H].The [Fe/H], V(rot) diagram allows us to separate these stars intodifferent Galactic stellar population groups, such as old-thin-disk,thick-disk, and halo. The X histogram, where X is our stellar-populationdiscriminator combining V(rot) and [Fe/H], and contour plots for the[Fe/H], V(rot) diagram both indicate two probable components to thethick disk. These population groups and Galactic components are studiedin the (b-y)0, Mv diagram, compared to theisochrones of Bergbusch & VandenBerg (2001, ApJ, 556, 322), toderive stellar ages. The two thick-disk groups have the meancharacteristics: ([Fe/H], V(rot), Age, σW') ≈ (-0.7dex, 120 km s-1, 12.5 Gyr, 62.0 km s-1), and≈(-0.4, 160, 10.0, 45.8). The seven most metal-poor halo groups,-2.31 ≤ [Fe/H] ≤ -1.31, show a mean age of 13.0 ± 0.2(mean error) Gyr, giving a mean difference from the WMAP results for theage of the Universe of 0.7 ± 0.3 Gyr. These results for the agesand components of the thick disk and for the age of the Galactic halofield stars are discussed in terms of various models and ideas for theformation of galaxies and their stellar populations.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| Metal-poor Field Blue Stragglers: More Evidence for Mass Transfer
We report radial velocity studies of five candidate metal-poor fieldblue stragglers, all known to be deficient in lithium. Four of the fivestars are single-lined spectroscopic binaries, with periods ranging from302 to 840 days, and low orbital eccentricities, in agreement withsimilar behavior found for other blue straggler candidates by Preston& Sneden and Carney et al. The limited data available for lithiumabundances indicate that all blue straggler binaries have depletedlithium, but that constant velocity stars generally have normal lithiumabundances. This suggests that the ``lithium gap'' for hot metal-poormain-sequence stars may not exist or lies at higher temperatures thanfound in the Hyades. Our results and those of Preston & Sneden showhigher values of vrotsini for the binary stars than those ofcomparable temperature constant velocity stars. The orbital periods aretoo long for tidal effects to be important, implying that spin-up duringmass transfer when the orbital separations and periods were smaller isthe cause of the enhanced rotation. The mass function distribution issteeper for the blue straggler binary stars than that of lower masssingle-lined spectroscopic binaries, indicating a narrower range insecondary masses. We argue that if all secondaries are white dwarfs withthe same mass, it is probably around 0.55 Msolar. The modelsof Rappaport et al., applied to white dwarf secondaries, suggest thatthe orbital elements of all metal-poor binary blue stragglers areconsistent with stable mass transfer, with the possible exception ofG202-65.Some of the results presented here used observations made with theMultiple Mirror Telescope, a joint facility of the SmithsonianInstitution and the University of Arizona.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/167
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Rapid Rotation of Ultra-Li-depleted Halo Stars and Their Association with Blue Stragglers
Observations of 18 halo main-sequence turnoff stars, four of which areextremely deficient in Li, show that three of the Li-poor ones havesubstantial line broadening. We attribute this to stellar rotation.Despite the great ages of halo stars, for G202-65, BD +51°1817, andWolf 550 we infer vsini=8.3+/-0.4, 7.6+/-0.3, and 5.5+/-0.6 kms-1, respectively. The stated errors are 3 σ. For CD-31°19466 we derive a 3 σ upper limit vsini<2.2 kms-1. The three rotating stars are known spectroscopicbinaries. We explain the high rotation velocities in terms of mass andangular momentum transfer onto the surface of the turnoff star from aninitially more massive donor. Estimates of the specific angular momentumof accreted material indicate that quite small transfer masses couldhave been involved, although the unknown subsequent spin-down of theaccretor prevents us from assigning definitive values for each star. Theaccretor is now seen as an ultra-Li-deficient star whose origin makes ita low-mass counterpart of field blue stragglers. The Li could have beendestroyed before or during the mass transfer episode. Such objects mustbe avoided in studies of the primordial Li abundance and ininvestigations into the way normal single stars process their initialLi. Based on observations obtained with the University College Londonechelle spectrograph (UCLES) on the Anglo-Australian Telescope (AAT) andwith the Utrecht echelle spectrograph (UES) on the William HerschelTelescope (WHT).
| A Survey of Proper-Motion Stars. XIV. Spectroscopic Binaries among Metal-poor Field Blue Stragglers
We summarize the results from a program of monitoring the radialvelocities of 10 metal-poor, high-velocity field stars whose colors are0.01 to 0.13 mag bluer than main-sequence turnoffs ofcomparable-metallicity globular clusters. Two of the candidate halo bluestragglers (BD +72 94 and BD +40 1166) show no signs of velocityvariability, one (HD 84937) shows only weak signs of variability, one(BD +25 1981) appears to be a very long-period binary, and six (BD -122669, HD 97916, HD 106516, BD +51 1817, G66-30, and G202-65) aresingle-lined spectroscopic binaries, with periods ranging from 167 to844 days. Velocity coverage for the four candidates without orbitalsolutions ranges from 15.9 to 19.0 years. The orbital eccentricities areall low, e<0.30 and =0.11. Five of the six binary orbitshave very low eccentricities, with =0.07. We have reanalyzedthe velocity data from Preston & Sneden and have derived orbitalsolutions similar to theirs for 10 of the spectroscopic binaries amongtheir ``blue metal-poor'' stars with [Fe/H]<=-0.6. We confirm theirconclusion that the binary frequency is high; we find 47+/-10% if weinclude only the definite binaries with [Fe/H]<=-0.6. Our orbitalsolutions for the seven binaries with periods longer than 20 days allhave low eccentricities, with e<=0.26 and =0.11. Theseorbital characteristics are very similar to the Ba II, CH, subgiant CH,and dwarf carbon stars, suggesting that mass transfer has been involvedin their formation. Of the five binary stars in our program withpublished abundances of lithium, all have been found to be deficient(and one in beryllium as well). In contrast, two of the three apparentlysingle stars have published lithium abundances and show no deficiency.The mass functions for the six binaries in our program and seven similarsystems studied by Preston & Sneden are consistent with their unseencompanions all being white dwarfs with M~0.55 Msolar andrandom orbital inclinations. Taking all of our observations and those ofothers together, we argue that the results are consistent with all fieldblue stragglers being binary systems with long periods and loweccentricities, the primary stars being deficient in lithium and thesecondary stars being normal-mass white dwarfs. All these properties aresuggestive of a blue-straggler formation model that involves masstransfer. For six of the 13 stars in the two programs for whichs-process elemental abundances are available, no signs of enhancementare discernible, suggesting that the donor star was a first-ascent redgiant. For the star with the longest orbital period (1307 days), CS22956-028, s-process abundance enhancements have been reported. Thisstar may be a precursor to the subgiant CH class, as suggested by Luck& Bond. Some of the results presented here used observations madewith the Multiple Mirror Telescope, a joint facility of the SmithsonianInstitution and the University of Arizona.
| Abundances and Evolution of Lithium in the Galactic Halo and Disk
We have measured the Li abundance of 18 stars with -2<~[Fe/H]<~-1and 6000<~Teff<~6400 K, a parameter range that waspoorly represented in previous studies. We examine the Galactic chemicalevolution (GCE) of this element, combining these data with previoussamples of turnoff stars over the full range of halo metallicities. Wefind that A(Li) increases from a level of ~2.10 at [Fe/H]=-3.5 to ~2.40at [Fe/H]=-1.0, where A(Li)=log10(n(Li)/n(H))+12.00. We comparethe observations with several GCE calculations, including existingone-zone models and a new model developed in the framework ofinhomogeneous evolution of the Galactic halo. We show that Li evolved ata constant rate relative to iron throughout the halo and old disk epochsbut that during the formation of young disk stars, the production of Lirelative to iron increased significantly. These observations can beunderstood in the context of models in which postprimordial Li evolutionduring the halo and old disk epochs is dominated by Galactic cosmic-rayfusion and spallation reactions, with some contribution from theν-process in supernovae. The onset of more efficient Li production(relative to iron) in the young disk coincides with the appearance of Lifrom novae and asymptotic giant branch (AGB) stars. The major challengefacing the models is to reconcile the mild evolution of Li during thehalo and old disk phases with the more efficient production (relative toiron) at [Fe/H]>-0.5. We speculate that cool-bottom processing(production) of Li in low-mass stars may provide an importantlate-appearing source of Li, without attendant Fe production, that mightexplain the Li production in the young disk. Based on observationsobtained with the University College London échelle spectrograph(UCLES) on the Anglo-Australian Telescope (AAT) and the Utrechtéchelle spectrograph (UES) on the William Herschel Telescope(WHT).
| Ultra-Lithium-deficient Halo Stars and Blue Stragglers: A Common Origin?
We present data for four ultra-Li-deficient, warm, halo stars. The Lideficiency of two of these is a new discovery. Three of the four starshave effective temperatures Teff~6300 K, in contrast topreviously known Li-deficient halo stars, which spanned the temperaturerange of the Spite plateau. In this paper we propose that these andpreviously known ultra-Li-deficient halo stars may have had theirsurface lithium abundances reduced by the same mechanism as produceshalo field blue stragglers. Even though these stars have yet to revealthemselves as blue stragglers, they might be regarded as``blue-stragglers-to-be.'' In our proposed scenario, the surfaceabundance of Li in these stars could be destroyed (1) during the normalpre-main-sequence single-star evolution of their low-mass precursors,(2) during the post-main-sequence evolution of an evolved mass donor,and/or (3) via mixing during a mass-transfer event or stellar merger.The warmest Li-deficient stars at the turnoff would be regarded asemerging ``canonical'' blue stragglers, whereas cooler ones representsub-turnoff-mass blue-stragglers-to-be. The latter are presently hiddenon the main sequence, Li depletion being possibly the clearest signatureof their past history and future significance. Eventually, themain-sequence turnoff will reach down to their mass, exposing thoseLi-depleted stars as canonical blue stragglers when normal stars of thatmass evolve away. Arguing against this unified view is the observationthat the three Li-depleted stars at Teff~=6300 K are allbinaries, whereas very few of the cooler systems show evidence forbinarity; it is thus possible that two separate mechanisms areresponsible for the production of Li-deficient main-sequence halo stars.Based on observations obtained with the University College Londonéchelle spectrograph (UCLES) on the Anglo-Australian Telescope(AAT) and the Utrecht échelle spectrograph (UES) on the WilliamHerschel Telescope (WHT).
| Extremely Metal-Poor Stars. III. The Li-depleted Main-Sequence Turnoff Dwarfs
We present abundances of 14 elements for the metal-poor,near--main-sequence turnoff stars G66-30, G139-8, and G186-26, which arewell known to possess less than 1/5--1/10 the value of Li/H observed infield halo Spite Plateau dwarfs and are thought by many to have beenproduced in the currently accepted standard big bang cosmology. Thestars have [Fe/H] = -1.53, -2.24, and -2.68, respectively, and for thesevalues their collective abundance patterns are not abnormal. That is tosay, there is no common abundance abnormality that one might associatewith their Li deficiencies. In G186-26, we find an overabundance of theheavy neutron-capture elements that increases with atomic mass. Wemeasure [Ba/Fe] = +0.35, whereas most stars of this metallicity have[Ba/Fe] ~ -0.5, together with smaller enhancements for Sr and possiblyY. Such supersolar, neutron-capture, element abundances, however, areexhibited by ~25% of stars having [Fe/H] ~ -2.7. No such enhancement isfound in G66-30 or G139-8, and the simplest explanation for the data ofthe three stars is that there is no connection between Li depletion andthe abundance patterns of the heavy neutron-capture elements. G66-30 andG186-26 appear to be multiple systems, and we discuss the possible roleof binarity in producing the observed Li depletion. We concur with J. A.Thorburn that this group of objects is not the progeny of bluestragglers, and we discuss the alternative that some of the Li-depletedstars may be the Population II counterparts of the disk populationsubgiant CH, barium dwarf, and F str lambda 4077 stars. As suggested byothers, the most likely explanation for the abundance anomalies in theseobjects, which includes universal Li deficiency, involves mass transferacross a binary during the asymptotic giant branch evolutionary phase ofthe erstwhile primary of the system. We note that while such masstransfer might be expected to produce Li depletion, variations in C, N,and the heavy neutron-capture elements are possible but not allnecessary. Neither G66-30 nor G139-8 shows enhancement of C or theneutron-capture elements, and no information is available for N. Furtherwork is necessary to settle the issue. If mass transfer is responsiblefor Li depletions, such stars justifiably could be excluded frominvestigations of the primordial Li abundance. In the absence ofevidence for such mass transfer, however, the possibility remains thatsome of these objects, if not all, are extreme examples of a processthat has affected all Plateau stars.
| A Survey of Proper Motion Stars. XIII. The Halo Population
Based on our expanded sample of metallicities and kinematics for a largesample of stars selected from the Lowell Proper Motion Catalog, we studyseveral questions relating to the halo stellar population(s) in ourGalaxy. For [m/H]≤-1.4, there does not seem to be any variation with[m/H] in the mean values of the V velocity (i.e., angular momentumrelated to that in the disk) or the Galactic orbital eccentricities. Further, in spite of the strong kinematical biases in our sample, starswith very low metallicities are found that have small V velocities (highorbital angular momenta) and low orbital eccentricities. These resultscontradict the model that the metal-poor stars are a single populationthat is only the relic of the earliest stages of the Galaxy's collapse. There are signs that some of the metal-poor stars in the solarneighborhood are due to accretion events and, perhaps, also to theearliest stages of the formation of the Galactic disk. Regardingaccretion, we confirm Majewski's [ApJS, 78, 87 (1992)] finding of aretrograde rotation among stars that reach S kpc or more from the plane. These stars do not show any radial metallicity gradient, and may beyounger on average than dynamically hot, metal-poor stars closer to theplane. These latter stars show net prograde rotation and a radialmetallicity gradient, suggestive of a dissipative process in theearliest stages of disk formation. The correlation between metallicityand perigalacticon found by Ryan & Norris [AJ, 101, 1835 (1991a)]disappears when care is taken to exclude the stars that may have beenaccreted by our Galaxy. The field star results complement those forglobular clusters found by other workers, notably Zinn (1993), whoargued for two populations of metal-poor clusters, one apparently inretrograde rotation with no radial metallicity gradient and slightlyyounger ages, and the other with prograde rotation, a weak radialmetallicity gradient, and slightly older ages. The field stars andglobular clusters do differ slightly, however. Their metallicitydistributions differ, with the field stars showing a larger fraction ofthe most metal-poor stars. This could be caused by accretion of Dracodwarf galaxy-like objects, with very low metallicities and no globularclusters. We see in our data, particularly in the V vs>Rapo< plane, possible signs of large-scale kinematicsubstructure suggestive of specific accretion events. We also see signsfor the Preston et al. [AJ, 108, 538 (1994)] low-metallicity,intermediate kinematics, and younger age stellar population. However,the strength of the signal in our data suggests that a fairly largefraction of its stars may be old. On the other hand, the "away" versus"toward" mystery of Croswell et al. [Al, 93, 1445 (1987)] hasdisappeared: the numbers of stars approaching and receding from theplane agree with expectations. Finally, we point out that the model ofNorris [ApJ, 431, 645 (1994)] for a proto-disk population that is hotterdynamically than the accreted halo components does not agree with ourexpanded data sample. We suggest that the proto-disk component wasdynamically cooler when the mean metallicity was very low.
| A survey of proper motion stars. 12: an expanded sample
We report new photometry and radial velocities for almost 500 stars fromthe Lowell Proper Motion Catalog. We combine these results with ourprior sample and rederive stellar temperatures based on the photometry,reddening, metallicities (using chi squared matching of our 22,500 lowSignal to Noise (S/N) high resolution echelle spectra with a grid ofsynthetic spectra), distances, space motions, and Galactic orbitalparameters for 1269 (kinematics) and 1261 (metallicity) of the 1464stars in the complete survey. The frequency of spectroscopic binariesfor the metal-poor ((m/H) less than or equal to -1.2) stars with periodsshorter than 3000 days is at least 15%. The spectroscopic binaryfrequency for metal-rich stars ((m/H) greater than -0.5) appears to belower, about 9%, but this may be a selection effect. We also discussspecial classes of stars, including treatment of the double-linedspectroscopic binaries, and identification of subgiants. Four possiblenew members of the class of field blue stragglers are noted. We pointout the detection of three possible new white dwarfs, six broad-lined(binary) systems, and discuss briefly the three already knownnitrogen-rich halo dwarfs. The primary result of this paper will beavailable on CD-ROM, in the form of a much larger table.
| Two new lithium-deficient population II dwarfs
Observations are reported which show that G122-69 and G139-8 are normal,extremely metal-poor subdwarfs with no detectable lithium. This resultchallenges the contention that the most metal-deficient halo stars showno dispersion in surface lithium abundance beyond the observationalscatter; the ratio of extreme subdwarfs with unmeasurably low lithiumabundances may be close to 20 percent. Attempts to identify stellarN(Li) with the lithium fraction generated by big bang nucleosynthesismay therefore have been premature. Larger samples and a more completeunderstanding of the dispersion must play a key role in further studiesof this kind.
| Observed upper limits on lithium abundances in blue stragglers
High-dispersion echelle spectra which include the Li I 6708 A line arereported for four blue stragglers. Two are field Population II starswith Fe/H near -1.4, while the other two are members of the open clusterM 67. The Li I line is not detected in the spectra of any of these fourblue stragglers; consequent upper limits on the surface lithiumabundances are further deduced from suitable model atmospheres. Theupper limits are significantly lower than the initial lithium abundancesinferred for the respective stellar populations associated with theseblue stragglers. The negative results for the two metal-deficient starsmay reveal the existence of a narrow 'lithium gap' along the PopulationII main sequence. Alternatively, some mechanism associated with theformation and evolution of halo blue stragglers may account for theirreduced atmospheric lithium abundance. The negative results in M 67argue that blue stragglers do not result from delayed star formation,and that blue stragglers do not remain near the main sequence forextended lifetimes as a result of interior mixing, unless the surfacesof such stars are also mixed.
| Subdwarf Studies. III. The Halo Metallicity Distribution
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991AJ....101.1865R&db_key=AST
| A survey of proper motion stars. IX - The galactic halo's metallicity gradient
Using data already presented for a survey of proper motion stars and theBahcall, Schmidt, and Soneira (1983) model of the Galaxy, Galacticorbital parameters are computed, including planar and three-dimensionaleccentricities, apo- and perigalacticon distances, and maximum distancesreached above/below the plane, based on extreme values for R and theabsolute value of Z over 15 azimuthal periods. The orbital data are usedto bin the survey's stars by apogalacticon and maximum Z distances. Inan attempt to isolate a halo population sample, analyses are restrictedto those stars that lag behind the local standard of the rest's circularorbital velocity by 50, 100, 150, and 200 km/s. The mean metallicitiesof the stars in a variety of Rapo and Zmax bins are compared .
| A survey of proper-motion stars. III - Reddenings, distances, and metallicities
Further data on the Lowell proper-motion stars surveyed by Carney andLatham (1987) are presented. Both new and published photometry aresummarized for 286 of these stars. Included are R-I data for 64 stars,uvby (or by) data for 221 stars (of which are included new results for152 stars), and JHK (or K) data for 238 stars (of which are included newresults for 180 stars). The procedures used to estimate the reddeningand photometric parallax of each star are discussed. The metallicitiesfor 818 stars, based on 5795 spectra, determined using a new method,described in an earlier paper, which compares synthetic spectra to thelow-signal-to-noise spectra obtained for radial velocities are alsopresented. The reddening, distance, and metallicity are interdependentand have been determined in a self-consistent manner.
| A survey of proper-motion stars. I - UBV photometry and radial velocities
The background, motivation, and goals of a photometric and spectroscopicsurvey of over 900 stars selected from the Lowell Proper Motion Surveywithout any metallicity bias are discussed, and 1225 new UBV measures of867 stars with V = 7-16 mag, and a new mean radial velocities for 914stars based on 5815 high-resolution spectra are presented. Theradial-velocity data indicate the binary fraction of th high-velocitystars probably exceeds 25 percent.
| High-tangential-velocity stars
Two lists of high tangential velocity (greater than 100 km/s) starscontained in the Lowell Northern Hemisphere proper motion surveycatalogue have been compiled, the stellar distances being inferred fromeither trigonometric or spectroscopic parallaxes. The information givenincludes equatorial coordinates, corrected photographic magnitudes,proper motions, trigonometric or spectroscopic parallaxes, and spectraltypes.
| Spectral classification of high-proper-motion stars
Spectral types have been found for about 900 stars of high proper motioncontained in the Lowell Observatory Northern Hemisphere proper-motionstar survey using all blue-region objective prism plates. The spectralclassification criteria are given. About eighty stars of largetangential velocity have been classified using slit spectrograms takenwith a 36-in. reflector. A new calibration of Luyten's absolutemagnitude vs reduced proper motion relation is made, and its dependenceon spectral type is investigated.
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| Takýmyýldýz: | Herkul |
| Sað Açýklýk: | 16h35m58.58s |
| Yükselim: | +45°51'59.3" |
| Görünürdeki Parlaklýk: | 11.068 |
| özdevim Sað Açýklýk: | -158.6 |
| özdevim Yükselim: | 154.5 |
| B-T magnitude: | 11.694 |
| V-T magnitude: | 11.12 |
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