Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Mapping the Local Galactic Halo. I. Optical Photometry of Cool Subdwarf Candidates
Optical (BVRI) photometric measurements of a sample of 564 candidatecool subdwarfs in the nearby halo are presented. The stars generallyspan the color range 0.4<(B-V)<1.8 hence, the sample is composedof early F- through early M-type stars on the subdwarf sequence. Thesample is selected from the revised NLTT catalog of Gould and Salim andSalim and Gould via a reduced proper motion diagram. The photometry isprecise and accurate; in particular, for stars with 9
| 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.
| A Survey of Proper-Motion Stars. XVII. A Deficiency of Binary Stars on Retrograde Galactic Orbits and the Possibility that ω Centauri is Related to the Effect
We compare the frequency of field binary stars as a function of Galacticvelocity vectors and find a deficiency of such stars on stronglyretrograde orbits. Metal-poor stars moving on prograde Galactic orbitshave a binary frequency of 28%+/-3%, whereas the retrograde stars'binary frequency is only 10%+/-2% for V<=-300 km s-1. Nosuch binary deficiencies are seen for the U or W velocities, nor for[Fe/H]. Some mechanism exists that either disrupts binary systems orpreferentially adds single stars moving primarily on retrograde orbits.Theoretical analyses and critical evaluations of our observational dataappear to rule out preferential disruption of preexisting binary starsdue to such causes as tidal interactions with massive gravitationalperturbers, including giant molecular clouds, black holes, or theGalactic center.Dynamically evolved stellar ensembles, such as globular clusters,provide a possible source of single stars. Three lines of evidence ruleout this explanation. First, there is no mechanism to significantlyenhance dissolution of clusters moving on retrograde orbits. Second, astudy of globular clusters moving on prograde and retrograde orbits andwith perigalacticon distances such that they are unlikely to be affectedstrongly by central tidal effects shows that clusters moving on progradeGalactic orbits may be more evolved dynamically than clusters moving onretrograde orbits. Finally, we have undertaken a comprehensive searchfor star streams that might be discernible. Monte Carlo modelingsuggests that our sample may include one moving group, but it containsonly five stars. Although the Galactic orbit of this group passes nearthe Galactic center, it is not moving on a retrograde Galactic orbit andfalls short by a factor of at least 20 in supplying the necessary numberof single stars.There is one intriguing possibility to explain our results. A dissolveddwarf galaxy may have too large a velocity spread to be easily detectedin our sample using our technique. However, dwarf galaxies appear tooften show element-to-iron versus [Fe/H] abundance patterns that are notsimilar to the bulk of the stellar field and cluster halo stars. Weexplore the s-process elements Y and Ba. Eight stars in our sample havesuch elemental abundances already measured and also lie in the criticaldomain with -1.6<=[Fe/H]<=-1.0 and V<=-300 km s-1.The admittedly small samples appears to show a bimodal distribution in[Y/Fe], [Ba/Fe], and [α/Fe], where ``α'' represents anaverage abundance of Mg, Si, Ca, and Ti. This behavior is reminiscent ofthe difference in the abundances found between the globular clusterω Centauri and other globular clusters. It is also intriguing thatthe stars most similar to ω Cen in their chemical abundances showa relatively coherent set of kinematic properties, with a modestvelocity dispersion. The stars less like ω Cen define adynamically hot population. The binary frequency of the stars in ωCen does not appear to be enhanced, but detailed modeling of the radialvelocity data remains to be done.
| 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.
| A photometric study of the Seyfert 1 galaxy Markarian 509
Not Available
| The general catalogue of trigonometric [stellar] paralaxes
Not Available
| Subdwarf studies. I - UBVRI photometry of NLTT stars
UBVRI photometry is presented for a sample of 1656 southern stars,including 1211 that were previously unmeasured, drown from the NLTTproper-motion catalog. The catalog is shown to be a rich source ofsubdwarfs. The normalized ultraviolet excess delta (U - B)0.6,photometric parallax, and interstellar reddening are calculated for eachstar when possible. Photometric parallaxes are compared withtrigonometric parallaxes from the literature. It is found that theformer do not have systematic errors greater than about 25 percent. Inagreement with other studies, the bluest subdwarfs are found at B - V =0.35. The selection of the program stars on the basis of large reducedproper motions restricted subgiant contamination of the sample to about5 percent and increased the discovery fraction of halo stars relative todisk stars. The claim is made here that the sample can be used toinvestigate the abundance distribution of the halo. The sample includesstars with ultraviolet excesses characteristic of disk abundances butwith velocities up to 150 km/s. These are believed to be stars that,quite expectedly, reside in the high-velocity tail of the disk velocitydistribution.
| Carbon and nitrogen abundances in metal-poor dwarfs of the solar neighborhood
Intermediate-resolution C, N, and Fe abundance spectra for 83 subdwarfsin the lowest metallicity range (-1.5 to -3.2), obtained with the LickObservatory 3.1-m Shane telescope, are analyzed. The effects ofsubgiants and binaries on the sample are examined. The relation between(B-V) and (V-K) and effective temperature is studied. C/Fe, N/Fe, andFe/H abundances were determined using the model-atmosphere technique ofspectrum synthesis. The derived abundance values are compared with datafrom previous investigations. It is observed that there is goodcorrelation between the Fe/H abundance values derived from theintermediate-resolution spectra and those based on high-resolutionspectra; C/Fe abundance values are constant; and there is an upturn inthe C/Fe abundance values at the very lowest metallicities.
| G. P. Kuiper's spectral classifications of proper-motion stars
Spectral classifications are listed for over 3200 stars, mainly of largeproper motion, observed and classified by Kuiper during the years1937-1944 at the Yerkes and McDonald Observatories. While Kuiper himselfpublished many of his types, and while improved classifications are nowavailable for many of these stars, much of value remains. For many ofthe objects, no other spectral data exist.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Kova |
| Sað Açýklýk: | 20h56m39.95s |
| Yükselim: | -09°39'19.2" |
| Görünürdeki Parlaklýk: | 10.202 |
| özdevim Sað Açýklýk: | -62.4 |
| özdevim Yükselim: | -356.7 |
| B-T magnitude: | 10.859 |
| V-T magnitude: | 10.257 |
Kataloglar ve belirtme:
|
