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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.
| Spectroscopy of Hot Stars in the Galactic Halo. II. The Identification and Classification of Horizontal-Branch and Other A-Type Stars
We discuss a spectroscopic and photometric technique that enables theidentification and classification of field horizontal-branch (FHB) andother A-type stars, even from relatively low signal-to-noise ratiomedium-resolution spectra. This technique makes use of broadband UBVcolors predicted from model atmosphere calculations and Balmer lineprofiles and Ca II K equivalent widths determined from synthetic spectrato estimate the physical parameters T_eff, log g, and [Fe/H] for starsin the effective temperature range 6000-10,000 K. A comparison of ourmethod with high signal-to-noise ratio spectra of standard starsindicates a scatter in the derived parameters of sigma(T_eff)=+/-250 K,sigma(log g)=+/-0.14 dex, and sigma([Fe/H])=+/-0.12 dex. This precisionallows for a separation of low surface gravity FHB and other, generallyhigher surface gravity, A-type (and somewhat later) stars. We alsodevelop a synthetic-template comparison technique, which is veryeffective in the identification of metallic-line and peculiar A-typestars. A detailed investigation of the influence of noise in the spectraon the determination of physical parameters shows that, for spectra withsignal-to-noise ratios in the range 10
| Lithium in high velocity A and F stars: Constraints on the blue straggler phenomenon
We have determined equivalent widths for a number of atomic lines,including the lambda 6707 resonance doublet of Li, from spectra of asample of candidate early-type high-velocity stars (including a numberof blue straggler candidates) and low-velocity comparison stars. We findthat the high-velocity candidates can be divided into three distinctgroups: (1) stars with low abundances both of Li and of metals ingeneral; (2) stars with low abundances of Li but only slightdeficiencies of the other metals; and (3) stars with nearly normalamounts of both Li and other metals. We tentatively identify thesegroups as (1) halo and thick-disk blue stragglers; (2) old, thinnishdisk blue stragglers; and (3) nearly normal Population l stars whichprobably lie in the tail of the usual Pop. l velocity distribution. Thekinematic properties of the three groups are consistent with thisinterpretation. Li is a fragile element which is easily destroyed in thehot interiors of main-sequence stars, and has previously been shown tobe depleted in the atmospheres of blue stragglers in the open clusterM67. Our results on objects that we have identified in this paper asold, halo or thick disk, field blue stragglers suggest that lithiumdepletion is a general property of blue stragglers; this argues thatlarge scale mixing must be a part of any mechanism that produces bluestragglers. Our observations of low lithium abundances in a number ofmetal-normal, high-velocity, A stars, suggest that the high-latitude Astars observed by Rodgers an d collaborators are probably not producedin the collison of a Magellanic Cloud-like object with the Galactic diskin the past few Gyr. A more likely hypothesis (that is consistent withour lithium abundances) is that the high latitude, normal gravity,metal-rich, A star population consists primarily of blue stragglers.These data also have allowed a redetermination of the relationshipbetween the Stromgren metallicity index delta mo and (M/H)for low-metallicity objects.
| Early type high-velocity stars in the solar neighborhood. IV - Four-color and H-beta photometry
Results are presented from photometric obaservations in the Stromgrenuvby four-color and H-beta systems of early-type high-velocity stars inthe solar neighborhood. Several types of photometrically peculiar starsare selected on the basis of their Stromgren indices and areprovisionally identified as peculiar A stars, field horizontal-branchstars, metal-poor stars near the Population II and old-disk turnoffs,metal-poor blue stragglers, or metallic-line A stars. Numerousphotometrically normal stars were also found.
| Early-type high-velocity stars in the solar neighborhood. I - List of candidates
Suspected nearby high-velocity stars of spectral types B and A areidentified by their proper motions, which are generally too small forthe stars to have been included in previous catalogs of high-velocitystars. The same selection process is then extended to spectral type Fstars, in order to both provide a companion sample and investigatewhether the hypothetical high-velocity, metal-rich population continuesto later spectral types. Of the presented list of 371 stars withconfirmed proper motions, 168 are of A and B spectral types. Photometricand spectroscopic observations of these stars are required to eliminatethose with misclassified spectra and convert proper motions into spacevelocities. The identification of type of stars is important forunderstanding apparent exceptions to the correlation between stellarages, metal abundances, and kinematic properties.
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Observation and Astrometry data
| Constellation: | Pisces |
| Right ascension: | 01h24m42.30s |
| Declination: | +07°00'05.2" |
| Apparent magnitude: | 9.575 |
| Proper motion RA: | 7.4 |
| Proper motion Dec: | -149.8 |
| B-T magnitude: | 9.925 |
| V-T magnitude: | 9.604 |
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