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Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Deepsky delights.
Not Available
| Caroline Herschel as observer
Not Available
| CCD-Δ a and BVR photometry of NGC 7296
The first CCD photometric investigation of the open cluster NGC 7296 upto now was performed within the narrow band Δ a photometricsystem, which enables us to detect peculiar objects. A deeperinvestigation of that cluster followed, using the standard BVR-Besselfilter set. The age and E(B-V) was determined independently to log t =8.0 ± 0.1 and 0.15 ± 0.02, respectively by using Δ aand broadband photometry. In total five Be/Ae objects and two metal-weakstars showing significant negative Δ a-values as well as oneclassical chemically peculiar star could be identified within thatintermediate age open cluster.Based on observations at the Hvar Observatory, University of Zagreb andthe Leopold-Figl Observatory for Astrophysics, University of Vienna.
| 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.
| Stellar Photometry Using Old Photographic Plates
The precision of various methods of stellar photometry on oldphotographic plates is investigated using the original plates or theirdigitized copies from the plate collection of the Maria MitchellObservatory (MMO), It is shown, in particular, that the simple and fastmethod of eye photometry is comparable in precision to the traditional"objective" methods using a microphotometer to measure the plates ofimage analysis software to measure digital copies of small parts of theplates obtained with a CCD camera. All these methods provide photometricaccuracy of +/-0.1-0.2 magnitude on the MMO plates. It is demonstratedthat the high-performance commercially available scanner AgfaScan T5000used for plate digitization at the MMO produces images that can bemeasured to a considerably higher precision of +/-0.05 magnitude, whichis sufficient for most purposes of photographic stellar photometry. Theresults of this investigation may be of interest to those who use oldplates for stellar photometry, as well as to those who look for anadequate, fast, and relatively inexpensive scanner to digitize theirplate archive.
| Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas
We present a catalogue (CSOCA ) of stars residing in 520 Galactic opencluster sky areas which is the result of the kinematic (proper motion)and photometric member selection of stars listed in the homogeneousAll-sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). We describethe structure and contents of the catalogue, the selection procedureapplied, and the proper motion and photometric membership constraintsadopted. In every cluster area the CSOCA contains the complete list ofthe ASCC-2.5 stars regardless of their membership probability. Forevery star the CSOCA includes accurate J2000 equatorial coordinates,proper motions in the Hipparcos system, BV photometric data in theJohnson system, proper motion and photometric membership probabilities,as well as angular distances from the cluster centers for about 166 000ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types,multiplicity and variability flags from the ASCC-2.5, and radialvelocities with their errors from the Catalogue of Radial Velocities ofGalactic Stars with high precision Astrometric Data (CRVAD) are alsogiven.
| Distances to Cepheid open clusters via optical and K-band imaging
We investigate the reddening and main-sequence-fitted distances to 11young, Galactic open clusters that contain Cepheids. Each clustercontains or is associated with at least one Cepheid variable star.Reddening to the clusters is estimated using the U-B:B-V colours of theOB stars and the distance modulus to the cluster is estimated via B-V:Vand V-K:V colour-magnitude diagrams. Our main-sequence fitting assumesthat the solar-metallicity zero-age main sequence of Allen appliesuniversally to all the open clusters, although this point iscontroversial at present. In this way we proceed to calibrate theCepheid period-luminosity (PL) relation and find MV=-2.87× logP- 1.243 +/- 0.09, MK=-3.44 × logP- 2.21 +/-0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band givingan overall distance modulus to the LMC of μ0= 18.51 +/-0.10. This is in good agreement with the previous Cepheid PL-K result ofLaney & Stobie at μ0= 18.51 +/- 0.09 and with theHipparcos parallax-calibrated Cepheid PL-K estimate of Feast &Catchpole at μ0= 18.66 +/- 0.10 when no account is takenof the LMC metallicity.We also find that the two-colour U-B:B-V diagrams of two importantclusters are not well fitted by the standard main-sequence line. In onecase, NGC 7790, we find that the F stars show a UV excess and in thesecond case, NGC 6664, they are too red in U-B. Previous spectroscopicestimates of the metallicity of the Cepheids in these clusters appear tosuggest that the effects are not due to metallicity variations. Otherpossible explanations for these anomalies are positional variations inthe dust reddening law and contamination by foreground or backgroundstars.
| 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.
| 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.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| Astrometry of the Open Cluster M25
The young open star cluster M25 (IC 4725) is located in the direction ofthe galactic center, near much irregular absorption features onSagittarius arm. This system is found to be at a distance of 600 pc,with a median age of 9.45 × 107 years and a distancefrom the galactic plane of -52.82 pc The mass data available in theliterature has been gathered and many statistical methods have beenapplied for this cluster. Depending only on these methods, the stellardensity, the distribution of dark matter, the luminosity and the massfunctions have been estimated. The center of the cluster has beendefined, it is shifted by 45 arc sec in the northeast direction. Theradius of the cluster is found to be 4 pc. More than 220 stars with meanreddening of 0.50 mag and absorption of 1.62 mag are found inside thisaria. The total mass of the cluster has been estimated with the mass ofthe interstellar matter (gas and dust). It is found to be 1937Msolar, whereas about 24% of the material mass of the clusterhas remained as interstellar matter after the processes of formation.
| The Distance Scale for Classical Cepheid Variables
New radii, derived from a modified version of the Baade-Wesselink (BW)method that is tied to published KHG narrowband spectrophotometry, arepresented for 13 bright Cepheids. The data yield a best-fittingperiod-radius relation given bylog=1.071(+/-0.025)+0.747(+/-0.028)logP0. In combination with other high-quality radiusestimates recently published by Laney & Stobie, the new data yield aperiod-radius relation described bylog=1.064(+/-0.0006)+0.750(+/-0.006)logP0, which simplifies to ~P3/4.The relationship is used to test the scale of Cepheid luminositiesinferred from cluster zero-age main-sequence (ZAMS) fitting, for whichwe present an updated list of calibrating Cepheids located in stellargroups. The cluster ZAMS-fitting distance scale tied to a Pleiadesdistance modulus of 5.56 is found to agree closely with the distancescale defined by Hipparcos parallaxes of cluster Cepheids and alsoyields Cepheid luminosities that are a good match to those inferred fromthe period-radius relation. The mean difference between absolute visualmagnitudes based on cluster ZAMS fitting,C, and those inferred for 23 clusterCepheids from radius and effective temperature estimates,BW, in the sense of C-BW is+0.019+/-0.029 s.e. There is no evidence to indicate the need for amajor revision to the Cepheid cluster distance scale. The absolutemagnitude differences are examined using available [Fe/H] data for thecluster Cepheid sample to test the metallicity dependence of theperiod-luminosity relation. Large scatter and a small range ofmetallicities hinder a reliable estimate of the exact relationship,although the data are fairly consistent with predictions from stellarevolutionary models. The derived dependence isΔMV(C-BW)=+0.06(+/-0.03)-0.43(+/-0.54)[ Fe/H].
| 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
| Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations. I. Association Cas OB2 and the small-amplitude Cepheid SU Cassiopeae
The small-amplitude Cepheid SU Cas and four membersof the association Cas OB2 (HD 16893, HD17327a and b, HD 17443) were investigated,using high-resolution CCD spectra. The following results were obtained:1) All these objects have the same metallicity values, close to that ofthe Sun; 2) Elemental abundance indicates that SU Cas is a post firstdredge-up star with an age from 1 108 to 1.45 108yr, and it is not crossing the Cepheid instability strip for the firsttime. The mean value of log g = 2.35 corresponds to pulsations in thefundamental tone, although errors in gravity estimations provideovertone pulsations. The questions about its pulsational mode andmembership in Cas OB2 remained open; 3) HD17327a is a slowly rotating HgMn-star with the highest heliumcontent among such objects, while HD 16893 also has a manganeseoverabundance and might be classified as an Am-star; 4) HD17327b and HD 17443 are rapidly rotating main-sequence stars,while HD 17443 has a helium content comparable with that of the Sun.
| 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
| Comparative abundance analysis of the hot main sequence stars and their progeny in open cluster M 25
Remarkable inconsistencies between elemental abundances in the mainsequence stars and their progeny F-G supergiants are discussed.Comparative abundance analysis of the hot main sequence stars, thecepheid U Sgr and two cool supergiants belonging to young open cluster M25 is performed. The detected disaccord in the abundances of carbon,oxygen and other elements between these stars having a common origin butoccupying at present different evolutionary stages may be due to thefact that the chemical anomalies observed in B stars are caused by themechanism of the radiative diffusion in the upper atmosphere layers. Thechemical composition of B stars determined spectroscopically may notreflect correctly their true chemical composition, nor the chemicalcomposition of the interstellar medium. On the other hand such abundanceanomalies are not expected for F-G supergiants which have suffered thelarge scale mixing in the red giant phase. The observed abundances forthese objects are much more reliable as a reference point in the studyof galactic chemical evolution. Three new Be stars are discovered in M25. Our study has doubled the number of Be stars known in this cluster.
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| The First 50 Years at Palomar: 1949-1999 The Early Years of Stellar Evolution, Cosmology, and High-Energy Astrophysics
An account is given of the history of two observational programs set forthe Palomar 200-inch telescope, one by Walter Baade and the other byEdwin Hubble near the start of the scheduled operation of the telescope50 years ago. The review is partly an assessment of whether, and howwell, these programs have been carried to completion, and partly anaccount of the response of Palomar to new discoveries and developmentsnot foreseen in 1950. Stellar evolution, the discovery of variations inthe metallicity of stars of different populations, the chemicalevolution of the Galaxy, the Cepheid P-L relation, the redshift-distancerelation of the expanding universe, and the extragalactic distance scaleare discussed as they relate to the predictions for progress on theprograms set out by Baade and Hubble. Not foreseen was the invention anddevelopment of radio astronomy and high energy astrophysics, leading tothe discovery of radio galaxies, quasars, and the gradual realization ofviolent events, both in stars and in galaxies. The review is highlyrestricted to these subjects, covering only three areas among thetotality of the work in observational astrophysics studied during thefirst 50 years at Palomar.
| Astrometric radial velocities. I. Non-spectroscopic methods for measuring stellar radial velocity
High-accuracy astrometry permits the determination of not only stellartangential motion, but also the component along the line-of-sight. Suchnon-spectroscopic (i.e. astrometric) radial velocities are independentof stellar atmospheric dynamics, spectral complexity and variability, aswell as of gravitational redshift. Three methods are analysed: (1)changing annual parallax, (2) changing proper motion and (3) changingangular extent of a moving group of stars. All three have significantpotential in planned astrometric projects. Current accuracies are stillinadequate for the first method, while the second is marginally feasibleand is here applied to 16 stars. The third method reaches high accuracy(<1 km s(-1) ) already with present data, although for some clustersan accuracy limit is set by uncertainties in the cluster expansion rate.Based (in part) on observations by the ESA Hipparcos satellite
| HIPPARCOS data for two open clusters containing cepheids
Using proper motion data from the Hipparcos satellite it is shown thatthe cepheids S Nor and U Sgr are members of the open clusters NGC 6087and M 25, respectively. The precision of the Hipparcos data is needed toconclusively carry out a membership test. Parallax data, also obtainedfrom Hipparcos, do not contradict the proper motion results.
| Corridors in the regions of young open stellar clusters.
Not Available
| The overlapping open clusters NGC 1750 and NGC 1758. III. Cluster-field segregation and clusters physical parameters
Cluster-field segregation and determination of physical parameters ofthe open clusters NGC 1750 and NGC 1758 were performed from CCD andphotographic photometric and astrometric surveys previously published bythe authors. Membership probabilities were computed to a brightnesslimit of R=15 mag through a non-standard method in a fullynon-parametric approach, using proper motions, positions and photometry.This study allowed several parameters for NGC 1750 and NGC 1758 to bedetermined, including position, size, density profile, absorption,distance, age, luminosity function and mass. The common colour excess ofboth clusters is E(B-V)=0.34 mag. NGC 1750 is slightly younger (log t =8.3), closer (V-M_v=9.0 mag) and less concentrated than NGC 1758 (log t= 8.60 and V-M_v=9.4 mag). We also conclude that NGC 1746 is anerroneous assignation (not corresponding to any real open cluster), andthat NGC 1750 and NGC 1758 are overlapping clusters, but they do notconstitute a gravitationally bounded system.
| Parameters of open star clusters from uvby-beta photometry.
Not Available
| Cluster membership determinations from proper motion surveys
Not Available
| Performance of the Extreme Ultraviolet Explorer Imaging Telescopes
The Extreme Ultraviolet Explorer (EUVE) observatory, successfullylaunched on 1992 June 7, provided astronomers with the first observatorydedicated to observation in the extreme ultraviolet. The scientificobjectives of EUVE include an all-sky survey in four spectral bands, adeep survey in two spectral bands, and a spectroscopic survey carriedout through a NASA guest observer program. We describe here the hardwarecomponents of the four telescopes used to carry out both the all-skysurvey and the deep survey, and we present the imaging and photometricperformance characteristics of the instruments as determined from groundcalibration and in-orbit data.
| Chemical Abundances of Galactic Cepheid Variables that Calibrate the P-L Relation
We have completed spectroscopic studies of 23 Galactic cepheids todetermine possible metallicity effects on measuring the zero point ofthe P-L relation. We find a spread of 0.4 dex in [Fe/H], which is notthe result of either observational scatter or of a metallicity gradientwithin the Galactic disk. We find a smaller, less significant spread of0.2 dex in [alpha /Fe]. Our results are robust, determined by constancyof [Fe/H] derived at differing temperatures throughout the pulsationalcycle, and abundances of two dwarfs compared to U Sgr in the cluster M25. We discuss briefly the effect of metallicity on the P-L relation. Wealso argue that EV Sct and QZ Nor are overtone pulsators, and that thelatter is not a member of the cluster NGC 6067.
| Be stars in open clusters.
Not Available
| An improved calibration of Cepheid visual and infrared surface brightness relations from accurate angular diameter measurements of cool giants and supergiants.
We have calibrated optical and near-infrared surface brightness - colourrelations for cool giant and supergiant stars using high-precisionangular diameters of these stars determined from Michelsoninterferometry. We find that the giant and supergiant relations areundistinguishable over a wide range of intrinsic colours. Weindependently determine the slopes of these relations obeyed by Cepheidvariables and find that in all the diagrams considered, these agree verywell with the slopes derived from the stable giants and supergiants.Forcing the slopes to the values derived from the Cepheids, we determinea very precise value of the zero point of the surface brightness -colour relations valid for Cepheid variables, which is 3.947+/-0.003.This value is in agreement with the one derived from the Cepheideffective temperature scale of Pel (1978), and from the lunaroccultation angular diameter of the Cepheid ζ Gem (Ridgway etal.1982). We apply our newly calibrated surface brightness - colourrelations to the cluster Cepheid U Sgr to find its radius and distancefrom the optical V, V-R_J_ and the infrared V, V-K and K, J-KBarnes-Evans technique. While the numerical values derived from thethree different versions of the technique do agree within 1-σ, thenear-infrared distance and radius values are 5-10 times more accuratethan the optical one; in particular, the distance and radius of the starderived from the V, V - K solution are 592 +/-4 pc and 48.7 +/-0.3 solarradii, respectively, with errors less than 1 percent. We briefly discussthe potential of the near-infrared versions of the Barnes-Evanstechnique to set a very accurate (0.02mag) zero point to Cepheidperiod-luminosity relations and thus make a very important contributionto the absolute calibration of the extragalactic distance scale.
| BVRIJHK period-luminosity relations for Galactic classical Cepheids.
Not Available
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