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The population in the background of open clusters: tracer of the Norma-Cygnus arm
We present colour-magnitude diagrams of open clusters, located in therange 112° < l < 252°, manifesting stellar populations inthe background of clusters. Some of the populations are found to belocated beyond the Perseus arm and may be the part of the Norma-Cygnus(outer) arm. The outer arm seems to be continued from l ~ 120° to~235°. The background populations follow the downward warp of theGalactic plane around l ~ 240°.
| The effect of red noise on planetary transit detection
Since the discovery of short-period exoplanets a decade ago, photometricsurveys have been recognized as a feasible method to detect transitinghot Jupiters. Many transit surveys are now underway, with instrumentsranging from 10-cm cameras to the Hubble Space Telescope. However, theresults of these surveys have been much below the expected capacity,estimated in the dozens of detections per year.One of the reasons is the presence of systematics (`red noise') inphotometric time-series. In general, yield predictions assumeuncorrelated noise (`white noise'). In this paper, we show that theeffect of red noise on the detection threshold and the expected yieldscannot be neglected in typical ground-based surveys. We develop a simplemethod to determine the effect of red noise on photometric planetarytransit detections. This method can be applied to determine detectionthresholds for transit surveys. We show that the detection threshold inthe presence of systematics can be much higher than that with theassumption of white noise, and obeys a different dependence onmagnitude, orbital period and the parameters of the survey. Our methodcan also be used to estimate the significance level of a planetarytransit candidate (to select promising candidates for spectroscopicfollow-up).We apply our method to the OGLE planetary transit search, and show thatit provides a reliable description of the actual detection thresholdwith real correlated noise. We point out in what way the presence of rednoise could be at least partly responsible for the dearth of transitingplanet detections from existing surveys, and examine some possibleadaptations in survey planning and strategy. Finally, we estimate thephotometric stability necessary to the detection of transiting `hotNeptunes'.
| Wide-Field CCD Photometry around Nine Open Clusters
In this paper we study the evolution of the core and corona of nine openclusters using the projected radial density profiles derived fromhomogeneous CCD photometric data obtained with the 105 cm Kiso Schmidttelescope. The age and galactocentric distance of the target clustersvary from 16 to 2000 Myr and 9 to 10.8 kpc, respectively. Barring Be 62,which is a young open cluster, other clusters show a uniform reddeningacross the cluster region. The reddening in Be 62 varies fromE(B-V)min=0.70 mag to E(B-V)max=1.00 mag. Thecoronae of six of the clusters in the present sample are found to beelongated; however, on the basis of the present sample it is notpossible to establish any correlation between the age and shape of thecore. The elongated core in the case of the young cluster Be 62 mayreflect the initial conditions in the parental molecular cloud. Theother results of the present study are as follows: (1) Core radiusrc and corona size rcn/cluster radiusrcl are linearly correlated. (2) The rc,rcn, and rcl are linearly correlated with thenumber of stars in that region. (3) In the age range 10-1000 Myr, thecore and corona shrink with age. (4) We find that in the galactocentricdistance range 9-10 kpc, the core and corona/cluster extent of theclusters increase with the galactocentric distance.
| 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.
| 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.
| A Search for Variable Stars and Planetary Occultations in NGC 2301. II. Variability
We obtained R-band time-series observations of the young, metal-richopen cluster NGC 2301 for 12 nights in 2004 February. B-band images werealso obtained, and color-magnitude diagrams with limits of R=19.5 andB=21.5 were produced. Only a small effort was made to determine clustermembership, as our magnitude limits are far deeper than previouslypublished values. Our photometric precision for the brightest 5 mag ofsources is 1-2 mmag. We determine that for the ~4000 stars that havetime-series data, 56% are variable, and of these, approximately 13% areobserved to exhibit periodic light curves ranging from tens of minutesto days. We present some examples of the light curves obtained, andproduce cuts in variability space based on parameters such as color andamplitude. The percentage of variability is approximately equal acrossall colors, with the majority of variables having amplitudes of 0.15 magor less. In general, redder stars show larger variability amplitudes. Wefind a smooth decline in the number of periodic variables toward longerperiod. This decline is probably due to a transition from intrinsic toextrinsic variability, and also due in part to our limited observingperiod of 12 nights. Essentially all the A and F main-sequence stars inour sample are variable (~2 mmag and larger), and most present complexlight curves containing multiple periods that are suggestive of theirinclusion in the δ Scuti and γ Doradus classes. A variablenon-cluster member giant and two variable white dwarf candidates arediscussed. Our equational description of variability is shown to be anexcellent predictive tool for determining the cumulative fraction ofvariables that will be observed in a photometric survey. Our entire dataset is available electronically.
| 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.
| A Search for Variable Stars and Planetary Occultations in NGC 2301. I. Techniques
We observed the young open cluster NGC 2301 for 14 nights in 2004February using the orthogonal transfer CCD camera OPTIC. We usedpoint-spread function shaping techniques (``square stars'') during theobservations, allowing us to obtain a larger dynamic range (4.5 mag) ofhigh photometric precision results (<=2 mmag). These results arebetter than similar observing campaigns using standard CCD imagers. Thispaper discusses our observational techniques and presents initialresults for the variability statistics found in NGC 2301. Details of thevariability statistics as functions of color, variability type, stellartype, and cluster location will appear in Paper II.
| Stellar Variability in the Young, Metal Rich Open Cluster NGC 2301
Using an Orthogonal Transfer CCD camera (OPTIC), we observed the young,metal rich open cluster NCG 2301 over a 14 night period in Feb. 2004.Six adjacent fields centered on the cluster were observed in sequenceeach night with a field repeat cadence of about 12 minutes. Ourphotometric precision was near 1-2 mmag for the top 5 magnitudes ofdynamic range of the detector, a vast improvement over normal CCDobservations. Using a reduced chi-squared of greater than three as ourdetermining variability statistic, we find that 56% of all our pointsources are variable. Of these variables, 64% are periodic (at a 99%confidence level). Details on this variability in terms of stellar type,cluster location, etc. will be presented and discussed. Partial supportof this research was provided by the NSF.
| 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.
| Search for variable stars in the open cluster NGC 2539
We present UBVI absolute and V-band time-series CCD photometric resultsfor the intermediate-age open cluster NGC 2539. From the empiricalzero-age main sequence and theoretical isochrone fitting in color-colorand color-magnitude diagrams, we estimated the physical parameters ofthis cluster as follows: the color excess of E(B-V)=0.06 +/- 0.03, thedistance modulus of (V-MV)0=10.2 +/- 0.1 and theage of log t = 8.8 ( ~ 630 Myr). By carefully examining the time-seriesimages, we discovered seven new variable stars in the observed clusterfield. Considering the light curves, periods and positions on thecolor-magnitude diagram, we classified them as five eclipsing binarystars, one delta Scuti star and one gamma Doradus candidate. The gammaDoradus candidate might not be a cluster member because it is locatedredder by about Delta (B-V) = 0.2 than the cool edge of gamma Doradusinstability strip.
| 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.
| 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.
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
| The CFHT Open Star Cluster Survey. I. Cluster Selection and Data Reduction
We present this paper in conjunction with a companion paper as the firstresults in the Canada-France-Hawaii Telescope Open Star Cluster Survey.This survey is a large BVR imaging data set of 19 open star clusters inour Galaxy. This data set was taken with the CFH12K mosaic CCD(42'×28'), and the majority of the clusters were imaged underexcellent photometric, subarcsecond seeing, conditions. The combinationof multiple exposures extending to deep (V~25) magnitudes with short(<=10 s) frames allows for studies ranging from faint white dwarfstars to bright turnoff, variable, and red giant stars. The primary aimof this survey is to catalog the white dwarf stars in these clusters andestablish observational constraints on the initial-final massrelationship for these stars and the upper mass limit to white dwarfproduction. Additionally, we hope to better determine the properties ofthe clusters, such as age and distance, and also test evolution anddynamical theories by analyzing luminosity and mass functions. In orderto more easily incorporate these data in further studies, we haveproduced a catalog of positions, magnitudes, colors, and stellarityconfidence for all stars in each cluster of the survey. This reduction,along with the computed calibration parameters for all three nights ofthe observing run will encourage others to use these data in differentastrophysical studies outside of our goals. Additionally, the data setis reduced using the new TERAPIX photometric reduction package, PSFex,which is found to compare well with other packages. This paper isintended both as a source for the astronomical community to obtaininformation on the clusters in the survey and as a detailed reference ofreduction procedures for further publications of individual clusters. Wediscuss the methods employed to reduce the data and compute thephotometric catalog. We reserve both the scientific results for eachindividual cluster and global results from the study of the entiresurvey for future publications. The first of these further publicationsis devoted to the old rich open star cluster, NGC 6819, and appears as acompanion paper in the same issue of the Journal.
| A spectroscopic survey for lambda Bootis stars. III. Final results
In the third paper of a series dedicated to the spectroscopic survey fornew lambda Bootis stars, we present all new and confirmed members of thegroup as well as a detailed analysis of the observed sample. The natureof this small group of chemically peculiar stars of the upper mainsequence still challenges our understanding of processes like diffusion,mass-loss and accretion. The typical abundances pattern (nearly solarvalues for C, N, O and S whereas the Fe-peak elements are moderate tostrong underabundant) can still not be explained by any proposed theory.Hence, the significant increase of new members gives the opportunity toinvestigate the group properties in more detail. We report the discoveryof 26 new members of the group and the confirmation of 18 candidatesfrom the literature. This almost triples the number of known lambdaBootis stars. The existence of one member in the young open cluster NGC2264 and four members in the Orion OB1 association proves that thelambda Bootis phenomenon already works at very early stages of stellarevolution. Recent results from the Hipparcos mission have shown that thewell established lambda Bootis stars of the Galactic field comprise thewhole area from the Zero Age Main Sequence to the Terminal Age MainSequence (~ 109 yr for an A-type star). There is a continuoustransition between very young and rather evolved evolutionary stages. Wefind that the overall percentage of lambda Bootis type among all normaltype stars in the spectral range from B8 to F4 is 2% in the Galacticfield as well as in open clusters. Furthermore, 44 metal-weak objectsare listed which might be connected with the lambda Bootis phenomenon.Our biased sample (chosen by photometric boxes) is not distinguishedfrom all A-type stars in the corresponding spectral region by therotational velocity distribution. Only for the luminosity classes IV andIII (especially for the cooler program stars) the determined mean v sini values are very high compared to those of the literature. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| Hipparcos Trigonometric Parallaxes and the Distance Scale for Open Star Clusters
Hipparcos trigonometric parallaxes are used to estimate the distances tothe maximum possible number of open star clusters (OSC); distance moduliare estimated for 45 clusters with maximum heliocentric distances ofabout 1000 pc. The latter value can serve as an estimate of the limit towhich it still makes sense to use Hipparcos trigonometric parallaxes todetermine the distances to small groups composed of 6-10 sufficientlybright stars. A systematic correction to the distance moduli of clustersfrom the homogeneous catalog of OSC parameters (Loktin et al. 1997,2000) is estimated, which turns out to be independent of the clusterage.
| Search for short-period variable stars in the open cluster NGC 2301
Time-series CCD photometric observations of an intermediate age opencluster NGC 2301 were performed to search for variable stars,particularly gamma Dor stars and delta Sct stars. We also carried outUBVI photometry to study physical properties of variable stars in thecluster. We derived observational parameters of the cluster from UBVIphotometric results. The best fit of the empirical ZAMS in a color-colordiagram gives a reddening value of E(B-V) = 0.05 +/- 0.03. We alsoestimated a distance modulus of (V-MV)0 = 9.6 +/-0.1 and an age of log t = 8.4 +/- 0.1, from the ZAMS and isochronefitting in a color-magnitude diagram. Using V time-series data, wecarefully examined light variations of 923 stars fainter than 10\fm0,and discovered nine short-period variable stars; two gamma Dorcandidates, five eclipsing binaries, one Slowly Pulsating B star and onemagnetic Chemically Peculiar variable candidate. Two gamma Dorcandidates with relatively short periods of around 0.3 days are locatedin a redder region (about 0\fm1 in B-V color) than the red edge of deltaSct instability strip in the color-magnitude diagram.
| Search for Variable Stars in Two Old Open Clusters: NGC2506 and NGC2420
We present results of photometric observations of two old open clusters:NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCDphotometric observations were carried out to search for variable starsand to investigate their physical properties. From the UBVI photometryof NGC 2506, we obtain interstellar reddening of E(B-V)=0.04+/-0.03 mag,distance modulus (V-M_V)_0=12.5+/-0.1 mag, and age log(t/yr)=9.25. Frommonitoring of 590 stars with 304 CCD frames taken for ten nights, threedelta Sct stars and one eclipsing binary star are discovered in NGC2506. For the other cluster, NGC 2420, we have examined light variationsof 505 stars using 347 time-series data obtained for five nights,finding no variable star. It should be noted that we have found no gammaDor star among main-sequence stars near F0 in these two old openclusters, which is consistent with the suggestion that gamma Dor-typephenomenon occurs in stars younger than log(t/yr)=8.4. On the otherhand, it has been known that color-magnitude diagrams of these twoclusters show well-established binary sequences, implying high incidenceof binary systems: >=20% for NGC 2506 and approx 50% for NGC 2420.However, only one eclipsing binary star was found in these two clusters.
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| CCD UVBY Hβ Photometry in Clusters. I. The Open Cluster Standard, IC 4651
CCD photometry of the intermediate-age open cluster IC 4651 on theuvbyHβ system is presented and analyzed. By using a combination ofthe information from the color-magnitude diagram (CMD) and thecolor-color diagrams, a sample of 98 highly probable main-sequencecluster members with high photometric accuracy is isolated. From thissample, adopting the intrinsic color relation of Olsen,E(b-y)=0.062+/-0.003 and [Fe/H]=+0.077+/-0.012, where the errors quotedare the standard errors of the mean and refer to the internal errorsalone. Use of the Nissen intrinsic color relation produces E(b-y)=0.071and [Fe/H]=+0.115. Adopting the lower reddening, a direct main-sequencefit to the Hyades with (m-M)=3.33 leads to (m-M)=10.15, while isochroneswith convective overshoot and zeroed to the Hyades produce an age of1.7+/-0.1 Gyr, with an excellent match to the morphology of the turnoff.The higher reddening produces (m-M)=10.3 and an age lower by 0.1 Gyr.Comparison with the CMD of NGC 3680 shows that the two clusters havevirtually identical morphology, which in combination with their similarcompositions produces identical ages. Coincidentally, the shifts in theCMD necessary to superpose the two clusters require that the apparentmoduli of IC 4651 and NGC 3680 be the same, whileE(b-y)4651=E(b-y)3680+0.04.
| Search for Low-Instability Strip Variables in the Young Open Cluster NGC 2516
In this paper we revise and complete the photometric survey of theinstability strip of the southern open cluster NGC 2516 published byAntonello & Mantegazza. No variable stars with amplitudes largerthan 0.02 mag were found. However, by means of an accurate analysisbased on a new statistical method, two groups of small-amplitudevariables have been disentangled: one with periods of less than 0.25days (probably delta Scuti stars) and one with periods of greater than0.025 days. The position in the H-R diagram and the apparent timescalemay suggest that the stars of the second group belong to a recentlydiscovered new class of variables, named gamma Dor variables. Theycertainly deserve further study. We also present a comparison betweenthe results of the photometric survey and the available pointed ROSATobservations of this cluster.
| Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2556T&db_key=AST
| A spectroscopic survey for lambda Bootis stars. I. Strategy, techniques and first results
In recent years, the chemically peculiar (CP) stars of the upper mainsequence have become a fruitful field for the testing of astrophysicaltheories. Processes such as diffusion, convection and mass loss havebeen developed theoretically and introduced into models. The group oflambda Bootis stars, however, is remarkable among the chemicallypeculiar stars as they are nonmagnetic, Population I, A to F-type dwarfswhich show significant underabundances of metals (except for C, N, O andS). Unfortunately, the small number of confirmed members of the lambdaBootis class makes a sound statistical analysis of their propertiesimpossible. Thus, it is still difficult to decide between the twotheories - mass loss with diffusion and the accretion theory - whichhave been proposed to explain the origin of these stars. We thereforehave started a spectroscopic survey to find new lambda Bootis stars inthe field as well as in open clusters and associations. The presence oflambda Bootis stars in open clusters and associations would permit adetermination of the ages of these stars, and thus would yield animportant test for distinguishing between the two theories. In thispaper we describe the selection of candidates using photometriccriteria, the basic requirements, spectroscopic follow-up observationsand results from the first three observing runs. Special care was takento avoid misclassification of our programme stars (e.g. lambda Bootisstars are often confused with intermediate Population II, He-weak orhigh v sin i stars), using a refined MK system. The discovery of atleast six new lambda Bootis stars (including three in the Orion OB1association) shows the efficacy of our selection criteria. Based onobservations obtained at the Observatoire de Haute-Provence and the DarkSky Observatory.
| Parameters of open star clusters from uvby-beta photometry.
Not Available
| Cluster membership determinations from proper motion surveys
Not Available
| Chemical Evolution of the Galactic Disk: Evidence for a Gradient Perpendicular to the Galactic Plane
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2813P&db_key=AST
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