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מאמרים קשורים
The Four-Quadrant Phase Mask Coronagraph. IV. First Light at the Very Large Telescope
We present the first high-contrast images obtained at the ESO Very LargeTelescope using a four-quadrant phase mask coronagraph. The two-nightcommissioning was carried out in 2004 January on NACO, the near-IRcamera with adaptive optics at UT4. We evaluated the behavior of thecoronagraph on a variety of astrophysical targets: binary stars,circumstellar disks, and active galactic nuclei. The performance of thecoronagraph is in agreement with our expectations based on numericalsimulations. The phase mask provides a stellar peak attenuation of afactor of about 10 on average (for long exposure), and its performanceis limited only by the phase residuals (mainly low-order aberrations)that are left uncorrected by the adaptive optics system.
| Fe II/Mg II Emission-Line Ratio in High-Redshift Quasars
We present results of the analysis of near-infrared spectroscopicobservations of six high-redshift quasars (z>~4), emphasizing themeasurement of the ultraviolet Fe II/Mg II emission-line strength toestimate the beginning of intense star formation in the early universe.To investigate the evolution of the Fe II/Mg II ratio over a wider rangein cosmic time, we measured this ratio for composite quasar spectra thatcover a redshift range 0<~z<~5 with nearly constant luminosity, aswell as for those that span ~6 orders of magnitude in luminosity. Adetailed comparison of the high-redshift quasar spectra with those oflow-redshift quasars with comparable luminosity shows essentially thesame Fe II/Mg II emission ratios and very similar continuum and linespectral properties, i.e., a lack of evolution of the relativeiron-to-magnesium abundance of the gas in bright quasars since z~=5.Current nucleosynthesis and stellar evolution models predict thatα-elements such as magnesium are produced in massive stars endingin Type II supernovae, while iron is formed predominantly in Type Iasupernovae with intermediate-mass progenitors. This results in an ironenrichment delay of ~0.2-0.6 Gyr. We conclude that intense starformation activity in the host galaxies of z>~4 quasars must havestarted already at an epoch corresponding to zf~=6-9, whenthe age of the universe was ~0.5 Gyr (H0=72 km s-1Mpc-1, ΩM=0.3,ΩΛ=0.7). This epoch corresponds well to thereionization era of the universe.Based on observations collected at the Cerro Tololo Inter-AmericanObservatory, Chile, at the European Southern Observatory, Paranal,Chile, and the W. M. Keck Observatory, Hawaii.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| UVBY CCD photometry of the upper main sequence in Omega Centauri
CCD frames on the uvby system of a field in Omega Centauri are obtainedand analyzed in order to constrain the intrinsic dispersion of heavyelements among its unevolved stars. Results from direct measurements onthe metallicity using m1 and from indirect measures using the colorspread in (b-y) near the turnoff imply an intrinsic dispersion in Fe/Hof about 1 dex, similar to the results of spectroscopic analysis of theevolved stars. The color spread near the turnoff is consistent withlittle spread in age, though the constraint is weak. The variable bluestraggler E39 is recovered showing an additional blue-stragglercandidate. The variation in c1 as a function of magnitude for probableupper-main-sequence members suggests that previous evolutionarycorrections for metal-poor stars were overestimated.
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משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | לויתן |
| התרוממות ימנית: | 00h16m23.46s |
| סירוב: | -22°35'16.9" |
| גודל גלוי: | 7.905 |
| מרחק: | 72.939 פארסק |
| תנועה נכונה: | 23.4 |
| תנועה נכונה: | -39.4 |
| B-T magnitude: | 8.742 |
| V-T magnitude: | 7.975 |
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