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Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| 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.
| Spectroscopy of SN 1987A at 0.9-2.4μm: days 1348-3158
We present near-infrared spectroscopic observations of SN 1987A coveringthe period 1358 to 3158 d post explosion. This is the first time that IRspectra of a supernova have been obtained to such late epochs. Thespectra comprise emission from both the ejecta and the bright,ring-shaped circumstellar medium (CSM). The most prominent CSM emissionlines are recombination lines of H I He I, and forbidden lines of [SIII] and [Fe II]. The ejecta spectra include allowed lines of H I, He Iand Na I and forbidden lines of [Si I], [Fe I], [Fe II] and possibly [SI]. The intensity ratios and widths of the H I ejecta lines areconsistent with a low-temperature Case B recombination spectrum arisingfrom non-thermal ionization/excitation in an extended,adiabatically-cooled H envelope, as predicted by several authors. Theslow decline of the ejecta forbidden lines, especially those of [Si I],indicates that pure non-thermal excitation was taking place, drivenincreasingly by the decay of 44Ti. The ejecta iron exhibitsparticularly high velocities (4000-4500 km s-1), supportingscenarios where fast radioactive nickel is created and ejected justafter the core bounce. In addition, the ejecta lines continue to exhibitblueshifts with values ~-200 to -800 km s-1 to at least day2000. These blueshifts, which first appeared around day 600, probablyindicate that very dense concentrations of dust persist in the ejecta,although an alternative explanation of asymmetry in the excitationconditions is not ruled out.
| High-Redshift Quasars and Star Formation in the Early Universe
In order to derive information on the star formation history in theearly universe, we observed six high-redshift (z~=3.4) quasars in thenear-infrared to measure the relative iron and Mg II emission strengths.A detailed comparison of the resulting spectra with those oflow-redshift quasars show essentially the same Fe II /Mg II emissionratios and very similar continuum and line spectral properties,indicating a lack of evolution of the relative iron to magnesiumabundance of the gas since z~=3.4 in bright quasars. On the basis ofcurrent chemical evolution scenarios of galaxies, where magnesium isproduced in massive stars ending in Type II Supernovae (SNe II), whileiron is formed predominantly in SNe Ia with a delay of ~1 Gyr andassuming as cosmological parameters H0=72 km s-1Mpc-1, ΩM=0.3, andΩΛ=0.7, we conclude that major star formationactivity in the host galaxies of our z~=3.4 quasars must have startedalready at an epoch corresponding to zf~=10, when the age ofthe universe was less than 0.5 Gyr. Based on observations collected atthe European Southern Observatory, La Silla, Chile.
| Imaging the host galaxies of high-redshift radio-quiet QSOs
We present new deep K-band and optical images of four radio-quiet QSOsat z approximately = 1 and six radio-quiet QSOs at z approximately =2.5, as well as optical images only of six more at z approximately =2.5. We have examined the images carefully for evidence of extended'fuzz' from any putative QSO host galaxy. None of the z approximately =2.5 QSOs shows any extended emission, and only two of the zapproximately = 1 QSOs show marginal evidence for extended emission. Our3 sigma detection limits in the K images, mK approximately =21 for an isolated source, would correspond approximately to anunevolved Lstar elliptical galaxy at z = 2.5 or 2-3 magfainter than an Lstar elliptical at z = 1, although ourlimits on host galaxy light are weaker than this due to the difficultyof separating galaxy light from QSO light. We simulate simple models ofdisk and elliptical host galaxies, and find that the marginal emissionaround the two z approximately = 1 QSOs can be explained by disks orbulges that are approximately 1-2 mag brighter than an unevolvedLstar galaxy in one case and approximately 1.5-2.5 magbrighter than Lstar in the other. For two other zapproximately = 1 QSOs, we have only upper limits (L approximately =Lstar). The hosts of the high-redshift sample must be nobrighter than about 3 mag above an unevolved Lstar galaxy,and are at least 1 magnitude fainter than the hosts of radio-loud QSOsat the same redshift. If the easily detected K-band light surrounding aprevious sample of otherwise similar but radio-loud QSOs is starlight,then it must evolve on timescales of greater than or approximately equalto 108 yr (e.g., Chambers & Charlot 1990); therefore ournon-detection of host galaxy fuzz around radio-quiet QSOs supports theview that high-redshift radio-quiet and radio-loud QSOs inhabitdifferent host objects, rather than being single types of objects thatturn their radio emission on and off over short timescales. This isconsistent with the general trend at low redshifts that radio-loud QSOsare found in giant elliptical galaxies while radio-quiet QSOs are foundin less luminous disk galaxies. It also suggests that the processesresponsible for the spectacular properties of radio-loud AGNs at highredshifts might not be generally relevent to the (far more numerous)radio-quiet population.
| JHKLM standard stars in the ESO system
A list of 199 standard stars suitable for the ESO standard photometricsystem at JHKLM is given. Faint stars (although brighter than K = 7.7)to be used on larger telescopes are included. This list is based on ananalysis of all infrared photometric observations carried out at LaSilla from 1979 until 1989 inclusive. The accuracy of the data (about0.02 mag. at J, H, K, L, and M) is similar to the one achieved at SAAOand CTIO. Comparisons with these systems, as well as with the AAO andMSSO systems, are made: it is shown that the ESO system is very close tothe other ones, with the exception of CTIOs.
| Southern JHKL standards
The basis for the current SAAO standard photometric system at JHKL isgiven. This depends on an extensive investigation involving 230 starsdistributed around the sky. The accuracy is estimated at + or - 0.02 magfor J, H and K and + or - 0.05 mag for L.
| Hyades and Sirius supercluster members brighter than magnitude(V) 7.1. I - The first six hours of right ascension
This is the first of four papers listing the probable members of theHyades and Sirius superclusters among stars brighter than V = 7.1 mag.The star sample is contained in the Bright Star Catalogue and itsSupplement, augmented with a further supplement of 550 stars foundduring various observing programs over the past 40 years. Accurate,four-color and H-beta, or (RI), photometry is available for most of thesupercluster members. The criteria for membership are the comparisons ofthe proper motion, radial velocity, and luminosity obtained from thesupercluster parameters with the observed motions and the luminosityderived from the photometric parameters. New proper motions, based onall available catalogs, have been derived for the additional 550 starsdiscussed here, as well as for all supercluster members.
| Infrared standard stars
The results of an observational program aimed at setting up a network offaint near-infrared standards of sufficient accuracy are reported. Thenetwork covers both northern and southern hemispheres and includesstandards red enough to provide at least a limited check on colortransformations. The standards are set up at J (1.2 micron), H (1.6micron), K (2.2 microns), and L (3.5 microns), and their H2O and COmolecular absorption indices are determined. The problem of colortransformations between observatories is discussed briefly. Allmagnitudes presented are transformed to the natural system defined bythe CIT observations.
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| Takýmyýldýz: | Ocak |
| Sað Açýklýk: | 03h10m42.81s |
| Yükselim: | -39°03'05.6" |
| Görünürdeki Parlaklýk: | 6.995 |
| Uzaklýk: | 123.762 parsek |
| özdevim Sað Açýklýk: | -19.7 |
| özdevim Yükselim: | 8.7 |
| B-T magnitude: | 7.191 |
| V-T magnitude: | 7.012 |
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