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Near-infra-red spectroscopic ages of massive star clusters in M 82
Context: Like other starburst galaxies, M 82 hosts compact, massive(>5 × 105 M?) young star clustersthat are interesting both in their own right and as benchmarks forpopulation synthesis models. Aims: In addition to assessing orreassessing the properties of some of the brightest near-IR sources in M82, this paper addresses the following questions. Can populationsynthesis models at ?/? ? ? 750 adequatelyreproduce the near-IR spectral features and the energy distribution ofthese clusters between 0.8 and 2.4 ?m? How do the derived clusterproperties compare with previous results from optical studies? Methods: We analyse the spectra of 5 massive clusters in M 82, usingdata acquired with the spectrograph SpeX on the InfraRed TelescopeFacility (NASA/IRTF) and a new population synthesis tool with a highlyimproved near-IR extension, based on a recent collection of empiricaland theoretical spectra of red supergiant stars. Results: Weobtain excellent fits across the near-IR with models at quasi-solarmetallicity and a solar neighbourhood extinction law. Spectroscopybreaks a strong degeneracy between age and extinction in the near-IRcolours in the red supergiant-dominated phase of evolution. Theestimated near-IR ages cluster between 9 and 30 Myr, i.e. the ages atwhich the molecular bands due to luminous red supergiants are strongestin the current models. They do not always agree with opticalspectroscopic ages. Adding optical data sometimes leads to the rejectionof the solar neighbourhood extinction law. This is not surprisingconsidering small-scale structure around the clusters, but it has nosignificant effect on the near-IR based spectroscopic ages. Conclusions: The observed IR-bright clusters are part of the mostrecent episode of extended star formation in M 82. The near-IR study ofclusters that are too faint for optical observation adds importantelements to the age distribution of massive clusters in dustystarbursts. Further joint optical and near-IR spectroscopic studies willprovide strong constraints on the uncertain physics of massive stars onwhich population synthesis models rest.
| CCD photometry and astrometry of visual double and multiple stars of the HIPPARCOS Catalogue. II. CCD photometry and differential astrometry of 288 southern ``Intermediate'' systems
We present photometric and astrometric data of about 280 visual doublestars of the ``intermediate'' class, i.e. with angular separationsmainly in the range 2arcsec < rho < 12arcsec . The observationshave been obtained in 1991-92 with a CCD camera attached to the 91 cmDutch telescope at ESO La Silla, Chile. Differential magnitudes of thedouble star components as well as magnitudes and colour indices of theindividual components have been determined in the Cousins V and Ipassbands with an internal error of about 0.005 mag and an externalaccuracy of less than 0.03 mag. In addition, angular separations havebeen secured to an internal accuracy of 0.004arcsec and position anglesto about 0.05degr. Tables 1 to 4 are only available in electronic format the CDS via ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html. Based on observationscollected at the European Southern Observatory, La Silla, Chile.
| CCD measurements of visual binaries
CCD measurements of visual double stars were obtained with the ESO 1.5 mdanish reflector. All binaries observed are candidates for the HIPPARCOSInput Catalogue. More than 400 observations have been made in four clearnights. The accuracy obtained is comparable to the accuracy of thephotographic technique, but the observing and reduction times are oneorder of magnitude smaller.
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