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uvby-β photometry of high-velocity and metal-poor stars. X. Stars of very low metal abundance: Observations, reddenings, metallicities, classifications, distances, and relative ages
uvby(-β) photometry has been obtained for an additional 411 verymetal-poor stars selected from the HK survey, and used to derive basicparameters such as interstellar reddenings, metallicities, photometricclassifications, distances, and relative ages. Interstellar reddeningsadopted from the Schlegel et al. (\cite{schlegel}) maps agree well withthose from the intrinsic-color calibration of Schuster & Nissen(\cite{schuster89}). [Fe/H] values are obtained from the CaII K lineindex of the HK survey combined with the uvby and UBV photometry. Thec0,(b-y)0 diagram is seen to be very useful forclassifying these very metal-poor field stars into categories similar tothose derived from globular cluster color-magnitude diagrams; the HKsurvey has detected metal-poor candidates extending from the red-giantto the blue-horizontal branch, and from the horizontal branch tosubluminous stars. Distances derived from UBV photometry agreereasonably well with those from uvby, considering the paucity of goodcalibrating stars and the extrapolations required for the mostmetal-poor stars. These very metal-poor stars are compared to M 92 inthe c0,(b-y)0 diagram, and evidence is seen forfield stars 1-3 Gyrs younger than this globular cluster; uncertaintiesin the [Fe/H] scale for M 92 would only tend to increase this agedifference. Significant reddening uncertainties for M 92 are unlikelybut might decrease this difference. The significance of these youngervery metal-poor stars is discussed in the context of Galactic evolution,mentioning such possibilities as hierarchical star-formation/mass-infallof very metal-poor material and/or accretion events whereby thismaterial has been acquired from other (dwarf) galaxies with differentformation and chemical-enrichment histories.Based on observations collected at the H. L. Johnson 1.5 m telescope atthe Observatorio Astronómico Nacional at San Pedro Mártir,Baja California, México, and at the Danish 1.5 m telescope, LaSilla, Chile.Tables 1-9 are 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/422/527
| 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
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Observation and Astrometry data
| Constellation: | Αιγόκερως |
| Right ascension: | 20h31m20.29s |
| Declination: | -17°08'19.1" |
| Apparent magnitude: | 7.722 |
| Distance: | 389.105 parsecs |
| Proper motion RA: | -3.4 |
| Proper motion Dec: | -1.4 |
| B-T magnitude: | 9.905 |
| V-T magnitude: | 7.903 |
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