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מאמרים קשורים
Rotation Velocities of Red and Blue Field Horizontal-Branch Stars
We present measurements of the projected stellar rotation velocities(vsini) of a sample of 45 candidate field horizontal-branch (HB) starsspanning a wide range of effective temperatures, from red HB stars withTeff~=5000K to blue HB stars with Teff of 17,000K.Among the cooler blue HB stars (Teff=7500-11500 K), weconfirm prior studies showing that, although a majority of stars rotateat vsini<15kms-1, there exists a subset of ``fastrotators'' with vsini as high as 30-35 km s-1. All but one ofthe red HB stars in our sample have vsini<10kms-1, and noanalogous rotation bimodality is evident. We also identify anarrow-lined hot star (Teff~=16,000K) with enhancedphotospheric metal abundances and helium depletion, similar to theabundance patterns found among hot BHB stars in globular clusters, andfour other stars that may also belong in this category. We discussdetails of the spectral line fitting procedure that we use to deducevsini and explore how measurements of field HB star rotation may shedlight on the issue of HB star rotation in globular clusters.
| The determination of Teff for metal-poor A-type stars using V and 2MASS J, H and K magnitudes
Effective temperatures (T_eff) can be determined from (V-J)_0, (V-H)_0and (V-K)_0 colours that are derived from 2MASS magnitudes. This givesanother way to estimate the T_eff of faint blue halo stars (V la 15)whose temperatures are now usually deduced from bo.Transformations (adapted from Carpenter \cite{carp01b}) are used tochange colours derived from the 2MASS data to the Johnson system. T_effis then derived from these colours using an updated Kurucz model. Tablesare given to derive T_eff as a function of (V-J)_0, (V-H)_0 and (V-K)_0for a variety of metallicities and log g suitable for blue horizontalbranch and main sequence stars. The temperatures obtained in this wayare compared with those in the recent literature for various stars with5 <= V <= 15 and T_eff in the range 6500 to 9500 K; systematicdifferences are ~ 100 K. An exception is the sample of BHB starsobserved by Wilhelm et al. (\cite{wbsl99}) whose T_eff are significantlycooler than those we derive by an amount that increases with increasingtemperature. NOAO is operated by the Association of Universities forresearch in Astronomy, Inc., under contract with the National ScienceFoundation. Tables 2 and 3 are only available in electronic form at theCDS 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/391/1039
| Stellar parameters for Pop II A-type stars from IUE spectra and new-ODF ATLAS9 model atmospheres
Stellar parameters for twenty-seven field horizontal branch A-typestars, a post-AGB star (BD +32 2188), and a possible cool sdB star (BD+00 0145) were obtained by fitting the whole IUE energy distributionstaken from the IUE-INES archive to the ultraviolet energy distributionspredicted by new-ODF ATLAS9 model atmospheres, which include theLyman-alpha H-H+ and H-H quasi-molecular absorptions near1400 Å and 1600 Å. The sample of stars was extensivelystudied by Kinman et al. (2000), who derived stellar parameters for themby using visual observations and also an ultraviolet color index. Theeffective temperatures obtained by fitting the IUE spectra to thenew-ODF models agree with T_eff derived by Kinman et al. (2000) for mostof the stars in the sample. The gravities from UV agree with those fromKinman et al. (2000) for stars hotter than about 8700 K, while they arelower, on average, by 0.3 dex for the cooler stars. The same discrepancyis present when log g from the ultraviolet energy distribution iscompared with log g from the visible energy distribution. The differenceis insensitive to reddening, microturbulent velocity, metallicity, ormixing-length parameter for the treatment of the convection. Figures A.1to A.15 are only available in electronic form athttp://www.edpsciences.org
| A spectroscopic study of field BHB star candidates
New spectroscopic observations are presented for a sample of thirty-oneblue horizontal branch (BHB) star candidates that are sufficientlynearby to have reliable proper motions. Comments are given on a furthertwenty-five stars that have previously been suggested as BHB starcandidates but which were not included in our sample. Moderatelyhigh-resolution spectra (lambda /Delta lambda ~ 15 000) of twenty fiveof our program stars were taken with the coudé feed spectrographat Kitt Peak. Twelve of the program stars were also observed with theCAT spectrograph at ESO. Six of these program stars were observed fromboth hemispheres. IUE low-resolution spectra are available for most ofour candidates and were used, in addition to other methods, in thedetermination of their Teff and reddening. A compilation ofthe visual photometry for these stars (including new photometry obtainedat Kitt Peak) is also given. Abundances were obtained from these spectrausing models computed by Castelli with an updated version of the ATLAS9code (Kurucz 1993a). All thirty one candidates are halo stars. Of these,twenty eight are classified as BHB stars because: [(1)]they lie close tothe ZAHB (in a similar position to the BHB stars in globular clusters)in the Teff versus log g plot. For all but one of thesestars, far-UV data were available which were consistent with other data(Strömgren photometry, energy distributions, Hγ profiles) forderiving Teff and log g. [(2)]they have a distribution of kms-1i (<=40 km s-1) that is similar to thatfound for the BHB in globular clusters. Peterson et al. (1995) and Cohen& McCarthy (1997) have shown that the BHB stars in the globularclusters M13 and M92 have a higher km s-1i (<= 40 kms-1) than those in M3 and NGC 288 (<=20 kms-1). The mean deprojected rotational velocity (/line{v}) wascalculated for both the two globular clusters and the nearby BHB starsamples. A comparison of these suggests that both globular cluster kms-1i types are present in our nearby sample. No obvious trendis seen between km s-1i and either (B-V)o or [Fe/H].[(3)]they have -0.99>=[Fe/H]>=-2.95 (mean [Fe/H] -1.67; dispersion0.42 dex), which is similar to that found for field halo RR Lyrae andred HB stars. These local halo field stars appear (on average) to bemore metal-poor than the halo globular clusters. The local sample of redgiant stars given by Chiba & Yoshii (1998) contains a greaterfraction of metal-poor stars than either our halo samples or the haloglobular clusters. The stars in our sample that have a Teffthat exceeds about 8 500 K show the He i (lambda 4471) line with astrength that corresponds to the solar helium abundance. [(4)]they showa similar enhancement of the alpha -elements (< [Mg/Fe]right > =+0.43+/-0.04 and also < [Ti/Fe]right > = +0.44+/-0.02) to thatfound for other halo field stars of similar metallicity. Based onobservations obtained at KPNO, operated by the Association ofUniversities for Research in Astronomy, Inc., under contract with theNational Science Foundation, and the European Southern Observatory,Chile. Tables 4 and 5 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr
| 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.
| Four-color and H-beta photometry for O-A0 type stars in three regions near the galactic equator
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1982A&AS...49..561W&db_key=AST
| 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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| קבוצת-כוכבים: | תאומים |
| התרוממות ימנית: | 06h11m37.25s |
| סירוב: | +26°27'30.1" |
| גודל גלוי: | 9.034 |
| תנועה נכונה: | 55.7 |
| תנועה נכונה: | -57.4 |
| B-T magnitude: | 9.366 |
| V-T magnitude: | 9.062 |
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