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Comparing Extinction Models with a Sample of Elliptical Galaxies, Star Clusters, and the Extinction at the Galactic Center
We performed comparisons of our interstellar extinction models, whichare based on the hypothesis that gas and dust are homogeneously mixedand make use of the dust-to-gas ratio to make extinction predictions,with samples of objects for which other methods of estimating extinctionare available in the literature: giant elliptical (gE) galaxies, starclusters, and the extinction toward the Galactic center. We used thesame sample of gE galaxies that Burstein selected in 2003 for acomparison he made between the extinction predicted by Burstein &Heiles and by Schlegel and coworkers, to extend the comparison to ourmodels. Due to the quality of the sample of gE galaxies, we slightlytuned the parameters of our model A, getting what we call the improvedmodel A2. We found 3 σ-limited rms differences in E(B-V) around0.017 mag in the comparison of the predictions of this model with thetwo methods mentioned above. Our comparisons take into account the zeropoints recommended by Burstein. The correlation coefficient obtained inthe comparisons is around 0.86. These results bring to light that ourmodel can be safely used for extragalactic work in order to estimate theextinction in our Galaxy, as an alternative method to the predictions ofBurstein & Heiles and Schlegel and coworkers. In the comparison ofour results with the globular cluster data we found rms differences inE(B-V) equal to 0.32 and 0.30 for our models A and S, respectively. Forthe open clusters we made comparisons using different samples; the rmsdifferences were typically 0.25. In the Galactic center, comparing ourmodel A with the extinction map in the K band provided by Dutra andcoworkers, we found an rms difference in AK equal to 0.16,which is relatively small, considering the high extinction values inthat region.
| Element abundances in the metal-rich open cluster NGC 6253
Context: We have carried out a big FLAMES survey of 10 Galactic openclusters aiming at different goals. One of them is the determination ofchemical abundances, to put constraints on the radial metallicitygradient in the disk and its evolution. One of the sample clusters isthe very metal-rich NGC 6253. Aims: We have obtained UVES highresolution spectra of seven candidate cluster members (from the turn offup to the red clump) with the goal of determining the chemicalcomposition of NGC 6253 and of investigating its origin and role in theinterpretation of the radial metallicity gradient in the disk. Methods:Equivalent width analysis and spectral synthesis were performed usingMOOG and Kurucz model atmospheres. Results: We derived abundances ofFe, α-, and Fe-peak elements, the light element Na, and thes-process element Ba. Excluding two likely non-members and the clumpgiant, whose metallicity from equivalent widths is overestimated, wefind an average [Fe/H] = +0.36±0.07 (rms) for the cluster. Formost of the other elements we derive solar abundance ratios.Based on observations collected at ESO telescopes under program072.D-0550(A). Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/185
| K-band magnitude of the red clump as a distance indicator
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
| Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity
Aims.We present new CORAVEL radial-velocity observations andphotoelectric photometry in the UBV and DDO systems for a sample ofpotential members of the red-giant branches of NGC 6192, NGC 6208 andNGC 6268, three open clusters projected close to the Galactic centerdirection. We also examine the properties of a sample of 42 inner diskopen clusters projected towards almost the same direction as the threeclusters. Methods: .Cluster members and red field giants werediscriminated by using the CORAVEL radial-velocity data and by applyingtwo photometric criteria. Interstellar reddening and metal content ofthe clusters were derived from combined BV and DDO data. Results:.Cluster membership for five red giants in NGC 6192, three in NGC 6208and three in NGC 6268 has been confirmed by the analysis of thephotometric and kinematic data. Photometric membership probabilitiesshow very good agreement with those obtained from CORAVEL radialvelocities. Three new spectroscopic binaries were discovered among thered giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B-V)colour excesses were derived. Conclusions: .The overallmetallicities were found to be nearly solar for NGC 6208 and above solarfor NGC 6192 and NGC 6268. Most of the clusters located closer than 2kpc from the Sun in the considered direction are slightly more reddenedthan the absorption resulting from the Baade's window absorption law.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| The Bologna Open Cluster Chemical Evolution Project: Midterm Results from the Photometric Sample
We describe a long-term project aimed at deriving information on thechemical evolution of the Galactic disk from a large sample of openclusters. The main property of this project is that all clusters areanalyzed in a homogeneous way to guarantee the robustness of the rankingin age, distance, and metallicity. Special emphasis is devoted to theevolution of the earliest phases of the Galactic disk evolution, forwhich clusters have superior reliability with respect to other types ofevolution indicators. The project is twofold: on one hand we derive theage, distance, and reddening (and indicative metallicity) byinterpreting deep and accurate photometric data with stellar evolutionmodels, and on the other hand, we derive the chemical abundances fromhigh-resolution spectroscopy. Here we describe our overall goals andapproaches and report on the midterm project status of the photometricpart, with 16 clusters already studied, covering an age interval from0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. Theimportance of quantifying the theoretical uncertainties by deriving thecluster parameters with various sets of stellar models is emphasized.Stellar evolution models assuming overshooting from convective regionsappear to better reproduce the photometric properties of the clusterstars. The examined clusters show a clear metallicity dependence on thegalactocentric distance and no dependence on age. The tight relationbetween cluster age and magnitude difference between the main-sequenceturnoff and the red clump is confirmed.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| New fundamental parameters for the inner disk open cluster Lyngå 11
We have obtained CCD BVIKC photometry down to V ˜ 21.5for the open cluster Lyngå 11 and its surrounding field. Thispreviously unstudied object appears to be rich and highly absorbed. Acluster angular radius of 4.5′ ± 0.5′, equivalent to(3.0 ± 0.3) pc, is estimated from star counts in 100-pixel a sideboxes distributed throughout the whole observed field. Our analysissuggests that Lyngå 11 is moderately young and probably of solarmetallicity. Adopting the theoretical metal content Z = 0.02, whichprovides the best global fit, we derive an age of (630 ± 70) Myr.Simultaneously, colour excesses E(B ‑ V) = 0.70 and E(V ‑ I)= 0.85 and an apparent distance modulus V ‑ MV = 14.0are obtained. These results place Lyngå 11 at a distance of (2.3± 0.5) kpc from the Sun and ˜6.5 kpc from the Galacticcentre. The properties of a sample of clusters aligned along theline-of-sight of Lyngå 11 are examined as well.
| The Sloan Digital Sky Survey monitor telescope pipeline
The photometric calibration of the Sloan Digital Sky Survey (SDSS) is amulti-step process which involves data from three different telescopes:the 1.0-m telescope at the US Naval Observatory (USNO), FlagstaffStation, Arizona (which was used to establish the SDSS standard starnetwork); the SDSS 0.5-m Photometric Telescope (PT) at the Apache PointObservatory (APO), New Mexico (which calculates nightly extinctions andcalibrates secondary patch transfer fields); and the SDSS 2.5-mtelescope at APO (which obtains the imaging data for the SDSS proper).In this paper, we describe the Monitor Telescope Pipeline, MTPIPE, thesoftware pipeline used in processing the data from the single-CCDtelescopes used in the photometric calibration of the SDSS (i.e., theUSNO 1.0-m and the PT). We also describe transformation equations thatconvert photometry on the USNO-1.0m u'g'r'i'z' system to photometry theSDSS 2.5m ugriz system and the results of various validation tests ofthe MTPIPE software. Further, we discuss the semi-automated PT factory,which runs MTPIPE in the day-to-day standard SDSS operations atFermilab. Finally, we discuss the use of MTPIPE in current SDSS-relatedprojects, including the Southern u'g'r'i'z' Standard Star project, theu'g'r'i'z' Open Star Clusters project, and the SDSS extension (SDSS-II).
| High-resolution spectroscopy of the old open cluster Collinder 261: abundances of iron and other elements
We present the analysis of high resolution spectra of six red giantstars in the old open cluster Collinder 261. Reddening values forindividual stars, derived from the relation between colours andtemperatures (deduced from our fully spectroscopic analysis) areconsistent with previous determinations based on photometry. For thiscluster we derive an iron abundance of [Fe/H] = - 0.03 ± 0.03. Wealso obtain the abundances of light metals (O, Na and Al),α-elements (Mg, Si, Ca, Ti), elements of the Fe-group (Sc, Cr, Mn,Co, Ni) and the neutron-capture element Ba. No intrinsic star-to-starscatter is present in any of these elements within our sample. Wecompare our findings with previous investigations on this cluster,discussing in detail differences in analysis methods and results.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
| Deepsky delights.
Not Available
| Metal Abundances in Extremely Distant Galactic Old Open Clusters. I. Berkeley 29 and Saurer 1
We report on high-resolution spectroscopy of four giant stars in theGalactic old open clusters Berkeley 29 and Saurer 1 obtained with HIRESat the Keck Telescope. These two clusters possess the largestGalactocentric distances yet known for open star clusters and thereforeare crucial objects to probe the chemical pattern and evolution of theoutskirts of the Galactic disk. We find that [Fe/H]=-0.38+/-0.14 and-0.44+/-0.18 for Saurer 1 and Berkeley 29, respectively. On the basis ofthese data, we first revise the fundamental parameters of the clustersand then discuss them in the context of the Galactic disk radialabundance gradients. Both clusters seem to significantly deviate fromthe general trend, suggesting that the outer part of the Galactic diskunderwent a completely different evolution compared with the inner disk.In particular, Berkeley 29 is clearly associated with the Monocerosstream, whereas Saurer 1 exhibits very different properties. Theabundance ratios suggest that the chemical evolution of the outer diskwas dominated by the Galactic halo.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| Iron abundances from high-resolution spectroscopy of the open clusters NGC 2506, NGC 6134, and IC 4651
This is the first of a series of papers devoted to deriving themetallicity of old open clusters to study the time evolution of thechemical abundance gradient in the Galactic disk. We present detailediron abundances from high resolution (R⪆ 40 000) spectra of severalred clump and bright giant stars in the open clusters IC4651, NGC 2506 and NGC6134. We observed 4 stars of NGC 2506, 3 stars of NGC 6134 and5 stars of IC 4651 with the FEROS spectrograph with the ESO 1.5 mtelescope; moreover, 3 other stars of NGC 6134 were observed with theUVES spectrograph on Kueyen (VLT UT2). After excluding the cool giantsnear the red giant branch tip (one in IC 4651 and one in NGC 2506), wefound overall [Fe/H] values of -0.20±0.01, rms = 0.02 dex (2stars) for NGC 2506, +0.15±0.03, rms = 0.07 dex (6 stars) for NGC6134 and +0.11±0.01, rms = 0.01 dex (4 stars) for IC 4651. Themetal abundances derived from a line analysis for each star wereextensively checked using spectrum synthesis of about 30 to 40 Fe Ilines and 6 Fe II lines. Our spectroscopic temperatures providereddening values in good agreement with literature data for theseclusters, strengthening the reliability of the adopted temperature andmetallicity scale. Also, gravities from the Fe equilibrium of ionizationagree quite well with expectations based on cluster distance moduli andevolutionary masses.Based on observations collected at ESO telescopes under programme65.N-0286, and in part 169.D-0473.
| On the recent star formation history of the Milky Way disk
We have derived the star formation history of the Milky Way disk overthe last 2 Gyr from the age distribution diagram of a large sample ofopen clusters comprising more than 580 objects. By interpreting the agedistribution diagram using numerical results from an extensive libraryof N-body calculations carried out during the last ten years, wereconstruct the recent star formation history of the Milky Way disk.Under the assumption that the disk has never been polluted by anyextragalactic stellar populations, our analysis suggests thatsuperimposed on a relatively small level of constant star formationactivity mainly in small-N star clusters, the star formation rate hasexperienced at least five episodes of enhanced star formation lastingabout 0.2 Gyr with production of larger clusters. This cyclic behaviourshows a period of 0.4+/-0.1 Gyr and could be the result of density wavesand/or interactions with satellite galaxies. On the other hand, the starformation rate history from a volume-limited sample of open clusters inthe solar neighbourhood appears to be consistent with the overall starformation history obtained from the entire sample. Pure continuous starformation both in the solar neighbourhood and the entire Galactic diskis strongly ruled out. Our results also indicate that, in the Milky Waydisk, about 90% of open clusters are born with N<=150 and the slopein the power-law frequency distribution of their masses is about -2.7when quiescent star formation takes place. If the above results arere-interpreted taking into consideration accretion events onto the MilkyWay, it is found that a fraction of the unusually high number of openclusters with ages older than 0.6 Gyr may have been formed in disruptedsatellites. Problems arising from the selection effects and the ageerrors in the open cluster sample used are discussed in detail.
| New variable stars in open clusters. I. Methods and results for 20 open clusters
We present high precision CCD photometry of 1791 objects in 20 openclusters with an age of 10 Myr to 1 Gyr. These observations wereperformed within the Δ a photometric system which is primarilyused to detect chemically peculiar stars of the upper main sequence.Time bases range from 30 min up to 60 days with data from severalnights. We describe the time series analysis reaching a detection limitof down to 0.006 mag for apparent variability. In total, we havedetected 35 variable objects of which four are not members of theircorresponding clusters. The variables cover the entireHertzsprung-Russell diagram, hence they are interesting targets forfollow-up observations.Based on observations obtained at Complejo Astronómico elLeoncito (CASLEO), operated under the agreement between the ConsejoNacional de Investigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba y San Juan; ESO-La Silla and UTSO-Las Campanas.
| Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627
Charge-coupled device (CCD) photometry in the Johnson V, Kron-Cousins Iand Washington CMT1 systems is presented in the field of thepoorly known open cluster NGC 2627. Four independent Washingtonabundance indices yield a mean cluster metallicity of [Fe/H]=-0.12 +/-0.08, which is compatible with the existence of a radial gradient in theGalactic disc. The resultant colour-magnitude diagrams indicate that thecluster is an intermediate-age object of 1.4 Gyr. Based on the best fitsof the Geneva group's isochrones to the (V, V-I) and (T1,C-T1) diagrams, we estimate E(V-I) = 0.25 +/- 0.05 andV-MV= 11.80 +/- 0.25 for logt= 9.15, and E(C-T1) =0.23 +/- 0.07 and T1-MT1= 11.85 +/-0.25 for logt= 9.10, respectively, assuming solar metal content. Thederived reddening value E(C-T1) implies E(B-V) = 0.12 +/-0.07 and a distance from the Sun of 2.0 +/- 0.4 kpc. Using the WEBDAdata base and the available literature, we re-examined the overallproperties of all the open clusters with ages between 0.6 and 2.5 Gyr.We identified peaks of cluster formation at 0.7-0.8, 1.0-1.1, 1.6-1.7and 2.0-2.1 Gyr, separated by relative quiescent epochs of ~0.2-0.3 Gyr.We also estimated a radial abundance gradient of -0.08 +/- 0.02, whichis consistent with the most recent determinations for the Galactic disc,but no clear evidence for a gradient perpendicular to the Galactic planeis found.
| Theoretical isochrones for the Delta a photometric system
We have calculated theoretical isochrones for the photometric Delta asystem to derive astrophysical parameters such as the age, reddening anddistance modulus for open clusters. The Delta a system samples the fluxdepression at 520 nm which is highly efficient to detect chemicallypeculiar (CP) objects of the upper main sequence. The evolutionarystatus of CP stars is still a matter of debate and very important totest, for example, the dynamo and diffusion theories. In fact, thedynamo or fossil origin of the magnetic fields present in this kind ofstar is still not clear. Using the stellar evolutionary models by Claret(\cite{Cla95}), a grid of isochrones with different initial chemicalcompositions for the Delta a system was generated. The published dataof 23 open clusters were used to fit these isochrones with astrophysicalparameters (age, reddening and distance modulus) from the literature. Asan additional test, isochrones with the same parameters for Johnson UBVdata of these open clusters were also considered. The fits show a goodagreement between the observations and the theoretical grid. We findthat the accuracy of fitting isochrones to Delta a data without theknowledge of the cluster parameters is between 5 and 15%.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| BVR Photometry of Old Open Clusters
Not Available
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| δ Scuti stars and their related objects
δ Scuti stars are a group of stars located on or a little abovethe main sequence of H-R diagram with spectral type from A3 to F5. Theyare low amplitude single or multi period pulsators with period shorterthan 0.3 d. Within the same area there are several groups of variablesor special stars correlated with them, e.g., Dwarf Cepheids, γ Dorvariables, Blue Stragglers, Am stars, Ap stars, ROAp variables, λBoo variables and δ Del variables. In this paper a general reviewin this field, including the number of new variables discovered after1995, is presented. The most reliable period variation rates for all thehigh amplitude variables and several low amplitude variables are listed.Statistic shows the higher the rotation rate v sin i is, the lower thelight variation amplitude is. Thus within young open clusters highamplitude variables cannot be found. The amplitudes-periods distributionhave 3 peaks with the highest of 1.0 mag in V at 0.17 d in period. Forδ Scuti variables in stellar systems the shorter the averageperiod is, the lower the metallicity and the older the age of thestellar system are.
| Rotation of stars in NGC 6134. A comparison of delta Scuti stars and non-variable stars
We present results of spectroscopic observations of selected stars inthe southern open cluster NGC 6134. We have determined the rotationalvelocities of the six known delta Scuti stars in NGC 6134 as well asseveral other non-variable stars with similar colour temperature inorder to investigate if v sin i and variability is somehow connected: wefind no such correlation. We also compare the distribution of v sin i ofdelta Scuti stars and non-variable stars with four other well-studiedopen clusters to look for any systematic behaviour, but we find noconclusive evidence for v sin i and variability to be connected. We havealso used the spectra to carry out an abundance analysis of the deltaScuti stars in NGC 6134 to confirm the high metal content of thecluster. We find [Fe/H] = +0.38+/-0.05 which is in agreement with theresult obtained from Strömgren photometry. We also present Delta aphotometry of the cluster, but we find no chemical peculiar stars basedon this index. Based in part on observations obtained at the EuropeanSouthern Observatory at La Silla and UTSO-Las Campanas, Chile.
| Two highly reddened young open clusters located beyond the Sagittarius arm
We present the results of CCD BVI Johnson-Cousins photometry down to V ~19 mag in the regions of the unstudied stellar groups Pismis 23 and BH222, both projected close to the direction towards the Galactic centre.We measured V magnitude and B-V and V-I colours for a total of 928 starsin fields of about 4arcmin x4arcmin . Pismis 23 is conclusively aphysical system, since a clear main sequence and other meaningfulfeatures can be seen in the colour-magnitude diagrams. The reality ofthis cluster is also supported by star counts carried out within andoutside the cluster field. For Pismis 23 we derive colour excessesE(B-V) = 2.0 +/- 0.1 and E(V-I) = 2.6 +/- 0.1, a distance from the Sunof 2.6 +/- 0.6 kpc (Z = -19 pc) and an age of 300 +/- 100 Myr (assumingsolar metal content). BH 222 appears to be a young open cluster formedby a vertical main sequence and by a conspicuous group of luminous,typically red supergiant stars. We derived for this cluster a colourexcess of E(V-I) = 2.4 +/- 0.2, a distance from the Sun of 6.0 +/- 2.7kpc (Z = -46 pc) and an age of 60 +/- 30 Myr. The resulting reddeningand distance estimates place these two young objects among the mostreddened and distant open clusters known in the direction towards theGalactic centre. They are located beyond the Sagittarius arm, close tothe direction where this arm probably bifurcates into two arms. Based onobservations made at the University of Toronto (David DunlapObservatory) 24-inch telescope, Las Campanas, Chile. Tables \ref{t2} and3 are only available 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/388/179
| CCD photometric search for peculiar stars in open clusters. III. NGC 2439, NGC 3960, NGC 6134, NGC 6192 and NGC 6451
In total, more than 550 objects within the fields of five open clusterswere investigated in order to find classical chemically peculiar stars.One bona-fide CP candidate was detected in each of the open clusters NGC3960, NGC 6192 and NGC 6451. All three objects seem to be members of thecorresponding cluster, taking the results from our photometry as well asfrom the literature. For NGC 6192 and NGC 6451 we found widely differentinterstellar reddening values and therefore distances and ages in theliterature. From an analysis of published Johnson UBVRI photometry, wewere able to rule out a rather old age for NGC 6451. This is alsosupported by the finding of one peculiar object in this cluster withspectral type of about A2. Five variable objects within the field of NGC6134 were detected. The variability exceeds 10 times (or typically 0.03mag) the standard deviation of the weighted individual measurements. Butsince our observations were obtained mostly within one hour (typicalintegration times of five minutes per filter) on several nights within45 days, no light curves could be derived. For this cluster, some Amcandidates were reported for which we were not able to detect asignificant positive Delta a-value. Based on observations at ESO-LaSilla and\ UTSO-Las Campanas.
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
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