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Automatic spectral classification of stellar spectra with low signal-to-noise ratio using artificial neural networks
Context. As part of a project aimed at deriving extinction-distances forthirty-five planetary nebulae, spectra of a few thousand stars wereanalyzed to determine their spectral type and luminosity class. Aims: We present here the automatic spectral classification process usedto classify stellar spectra. This system can be used to classify anyother stellar spectra with similar or higher signal-to-noise ratios. Methods: Spectral classification was performed using a system ofartificial neural networks that were trained with a set of line-strengthindices selected among the spectral lines most sensitive to temperatureand the best luminosity tracers. The training and validation processesof the neural networks are discussed and the results of additionalvalidation probes, designed to ensure the accuracy of the spectralclassification, are presented. Results: Our system permits theclassification of stellar spectra of signal-to-noise ratio (S/N)significantly lower than it is generally considered to be needed. ForS/N ? 20, a precision generally better than two spectral subtypes isobtained. At S/N < 20, classification is still possible but has alower precision. Its potential to identify peculiar sources, such asemission-line stars, is also recognized.Based on observations obtained at the 4.2 m WHT telescope of the IsaacNewton Group of Telescopes in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias.
| Comprehensive abundance analysis of red giants in the open clusters NGC 752, 1817, 2360 and 2506
We have analysed high-dispersion echelle spectra (R ? 50 000) ofred giant members for four open clusters to derive abundances for manyelements. The spread in temperatures and gravities being very smallamong the red giants nearly the same stellar lines were employed therebyreducing the random errors. The errors of average abundance for thecluster were generally in the range 0.02-0.07 dex. Our present samplecovers Galactocentric distances of 8.3-10.5 kpc. The [Fe/H] values are-0.02 ± 0.05 for NGC 752, -0.07 ± 0.06 for NGC 2360, -0.11± 0.05 for NGC 1817 and -0.19 ± 0.06 for NGC 2506.Abundances relative to Fe for elements from Na to Eu are equal withinmeasurement uncertainties to published abundances for thin-disc giantsin the field. This supports the view that field stars come fromdisrupted open clusters.
| Chemical abundance analysis of the open clusters Berkeley 32, NGC 752, Hyades, and Praesepe
Context. Open clusters are ideal test particles for studying thechemical evolution of the Galactic disc. However, the number andaccuracy of existing high-resolution abundance determinations, not onlyof [Fe/H], but also of other key elements, remains largely insufficient. Aims: We attempt to increase the number of Galactic open clustersthat have high quality abundance determinations, and to gather all theliterature determinations published so far. Methods: Usinghigh-resolution (R ~ 30 000), high-quality (S/N ? 60 per pixel), weobtained spectra for twelve stars in four open clusters with the fibrespectrograph FOCES, at the 2.2 Calar Alto Telescope in Spain. We employa classical equivalent-width analysis to obtain accurate abundances ofsixteen elements: Al, Ba, Ca, Co, Cr, Fe, La, Mg, Na, Nd, Ni, Sc, Si,Ti, V, and Y. We derived oxygen abundances by means of spectralsynthesis of the 6300 Å forbidden line. Results: We providethe first determination of abundance ratios other than Fe for NGC 752giants, and ratios in agreement with the literature for the Hyades,Praesepe, and Be 32. We use a compilation of literature data to studyGalactic trends of [Fe/H] and [?/Fe] with Galactocentric radius,age, and height above the Galactic plane. We find no significant trends,but some indication for a flattening of [Fe/H] at large Rgc,and for younger ages in the inner disc. We also detect a possibledecrease in [Fe/H] with |z| in the outer disc, and a weak increase in[?/Fe] with Rgc.Based on observations collected with the fiber spectrograph FOCES at the2.2 m Calar Alto Telescope. Also based on data from 2MASS survey and theWEBDA, VALD, NIST, and GEISA online database.Table 12 is available inelectronic form at http://www.aanda.orgFull Table 5 is onlyavailable at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/535/A30
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Coudé-feed stellar spectral library - atmospheric parameters
Context. Empirical libraries of stellar spectra play an important rolein different fields. For example, they are used as reference for theautomatic determination of atmospheric parameters, or for buildingsynthetic stellar populations to study galaxies. The CFLIB(Coudé-feed library, Indo-US) database is at present one of themost complete libraries, in terms of its coverage of the atmosphericparameters space (T{eff}, log g and [Fe/H]) and wavelengthcoverage 3460-9464 Å at a resolution of 1 Å FWHM. Althoughthe atmospheric parameters of most of the stars were determined fromdetailed analyses of high-resolution spectra, for nearly 300 of the 1273stars of the library at least one of the three parameters is missing.For the others, the measurements, compiled from the literature, areinhomogeneous. Aims: In this paper, we re-determine theatmospheric parameters, directly using the CFLIB spectra, and comparethem to the previous studies. Methods: We use the ULySS programto derive the atmospheric parameters, using the ELODIE library as areference. Results: Based on comparisons with several previousstudies we conclude that our determinations are unbiased. For the 958 F,G, and K type stars the precision on T{eff}, log g, and[Fe/H] is respectively 43 K, 0.13 dex and 0.05 dex. For the 53 M starsthey are 82 K, 0.22 dex and 0.28 dex. And for the 260 OBA type stars therelative precision on T{eff} is 5.1%, and on log g, and[Fe/H] the precision is respectively 0.19 dex and 0.16 dex. Theseparameters will be used to re-calibrate the CFLIB fluxes and to producesynthetic spectra of stellar populations.Tables 2 and 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/525/A71
| Red giants in open clusters. XIV. Mean radial velocities for 1309 stars and 166 open clusters
Context: Radial velocities have proved to be an efficient method formembership determination if there are at least 2 or 3 red giants in acluster. They are necessary for galactic studies, but are still missingfor many open clusters. Aims: We present the final catalogues of along-term observing programme performed with the two coravelspectrovelocimeters for red giants in open clusters. The main aims wereto detect spectroscopic binaries and determine their orbital parameters,determine the membership, and compute mean velocities for the stars andopen clusters. Methods: We computed weighted mean radial velocities for1309 stars from 10 517 individual observations, including the systemicradial velocities from spectroscopic orbits and for cepheids. Results:The final results are contained in three catalogues collecting 10 517individual radial velocities, mean radial velocities for 1309 redgiants, and mean radial velocities for 166 open clusters among whichthere are 57 new determinations. We identified 891 members and 418non-members. We discovered a total of 288 spectroscopic binaries, amongwhich 57 are classified as non-members. In addition 27 stars were judgedto be variable in radial velocities and they are all red supergiants. Conclusions: The present material, combined with recent absolute propermotions, will permit various investigation of the galactic distributionand space motions of a large sample of open clusters. However, thedistance estimates still remain the weakest part of the necessary data.This paper is the last one in this series devoted to the study of redgiants in open clusters based on radial velocities obtained with thecoravel instruments.Based on observations collected at the Haute-Provence Observatory(France) and on observations collected with the Danish 1.54-m telescopesat the European Southern Observatory, La Silla, Chile. Full Tables [seefull textsee full textsee full textsee full textsee full text] to [seefull textsee full textsee full textsee full textsee full text] are onlyavailable and Tables [see full textsee full textsee full textsee fulltextsee full text] and [see full textsee full textsee full textsee fulltextsee full text] are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/485/303
| The Benchmark Cluster Reddening Project. III. A Comparison of Reddening Values Derived from F and K Stars in NGC 752
A protocol for reddening analysis developed in previous papers in thisseries is reviewed here and applied to NGC 752. The protocol includes(1) detailed scrutiny of the histories of reddening determinations forprogram clusters, (2) adherence to statistical norms, (3) qualitycontrol of all reddening techniques, (4) a primary focus on techniqueswith limited metallicity sensitivity, (5) zero-point control of sourcedata, (6) use of the solar neighborhood to establish a zero point forreddening values, (7) use of a standard permitting meaningful quantitiesto be as small as a few millimagnitudes, and (8) rejection of ad hocdata assessments. After certain published results are set aside, it isfound that a reddening value derived for F stars by using a techniquethat is insensitive to blanketing disagrees by about 0.04 mag with avalue derived for K giants by using DDO photometry. This disagreementdoes not appear to be an artifact of the use of questionable data, andit persists if additional techniques are considered. Partly because twocomparable reddening results for K giants are afterward found todisagree with each other, the problem is attributed to a blanketingeffect (with an unknown source) on the reddening values derived for theK giants. Pending a final resolution of that problem, the adopted valueof E(B-V) for the cluster (44+/-3.4 mmag) is based solely on the initialF-star analysis. A discussion of the potential usefulness of smallstandard errors like the one derived here for E(B-V) is given. In aparallel investigation, it is found that no definitive value of [Fe/H]can be assigned to NGC 752 at present because a statisticallysignificant disagreement between two published high-precisionmetallicities is detected.
| Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
We present a new set of model predictions for 16 Lick absorption lineindices from Hδ through Fe5335 and UBV colors for single stellarpopulations with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from-1.3 to +0.3, and variable abundance ratios. The models are based onaccurate stellar parameters for the Jones library stars and a new set offitting functions describing the behavior of line indices as a functionof effective temperature, surface gravity, and iron abundance. Theabundances of several key elements in the library stars have beenobtained from the literature in order to characterize the abundancepattern of the stellar library, thus allowing us to produce modelpredictions for any set of abundance ratios desired. We develop a methodto estimate mean ages and abundances of iron, carbon, nitrogen,magnesium, and calcium that explores the sensitivity of the variousindices modeled to those parameters. The models are compared to high-S/Ndata for Galactic clusters spanning the range of ages, metallicities,and abundance patterns of interest. Essentially all line indices arematched when the known cluster parameters are adopted as input.Comparing the models to high-quality data for galaxies in the nearbyuniverse, we reproduce previous results regarding the enhancement oflight elements and the spread in the mean luminosity-weighted ages ofearly-type galaxies. When the results from the analysis of blue and redindices are contrasted, we find good consistency in the [Fe/H] that isinferred from different Fe indices. Applying our method to estimate meanages and abundances from stacked SDSS spectra of early-type galaxiesbrighter than L*, we find mean luminosity-weighed ages of theorder of ~8 Gyr and iron abundances slightly below solar. Abundanceratios, [X/Fe], tend to be higher than solar and are positivelycorrelated with galaxy luminosity. Of all elements, nitrogen is the morestrongly correlated with galaxy luminosity, which seems to indicatesecondary nitrogen enrichment. If that interpretation is correct, thisresult may impose a lower limit of 50-200 Myr to the timescale of starformation in early-type galaxies. Unlike clusters, galaxies show asystematic effect whereby higher order, bluer, Balmer lines yieldyounger ages than Hβ. This age discrepancy is stronger for lowerluminosity galaxies. We examine four possible scenarios to explain thistrend. Contamination of the bluer indices by a metal-poor stellarpopulation with a blue horizontal branch cannot account for the data.Blue stragglers and abundance-ratio effects cannot be ruled out, as theycan potentially satisfy the data, even though this can only be achievedby resorting to extreme conditions, such as extremely high [O/Fe] orspecific blue-straggler frequencies. The most likely explanation is thepresence of small amounts of a young/intermediate-age stellar populationcomponent. We simulate this effect by producing two-component models andshow that they provide a reasonably good match to the data when the massfraction of the young component is typically a few percent. Ifconfirmed, this result implies star formation has been extended inearly-type galaxies, and more so in less massive galaxies, which seemsto lend support to the ``downsizing'' scenario. Moreover, it impliesthat stellar population synthesis models are capable of constraining notonly the mean ages of stellar populations in galaxies, but also theirage spread.
| The stellar population of the Rosat North Ecliptic Pole survey
Context: .X-ray surveys are a very efficient mean of detecting youngstars and therefore allow us to study the young stellar population inthe solar neighborhood and the local star formation history in the lastbillion of years. Aims: .We want to study the young stellarpopulation in the solar neighborhood, to constrain its spatial densityand scale height as well as the recent local star formation history. Methods: .We analyze the stellar content of the ROSAT North EclipticPole survey, and compare the observations with the predictions derivedfrom stellar galactic model. Since the ROSAT NEP survey is sensitive atintermediate fluxes is able to sample both the youngest stars and theintermediate age stars (younger than 109 years), linking theshallow and deep flux surveys already published in the literature. Results: .We confirm the existence of an excess of yellow stars inour neighborhood previously seen in shallow survey, which is likely dueto a young star population not accounted for in the model. However theexcellent agreement between observations and predictions of dM starscasts some doubt on the real nature of this active population.
| Seven-Color Vilnius Photometry of the Open Cluster NGC 752
New photoelectric seven-color observations in the Vilnius system arepresented for 65 stars in the region of the open cluster NGC 752. Basedon individual stars with accurate photometric classifications, wedetermine the apparent distance modulus (m-M)V = 8.38±0.14 and the mean reddening to the cluster EY-V=0.027± 0.010, or EB-V= 0.034± 0.013 (the errorsgiven are the standard deviations for one star). The mean photometricmetallicity for the main-sequence stars, [Fe/H] = -0.14± 0.03, isfound to be slightly lower than that derived for the red clump giants,[Fe/H] = -0.08± 0.09. This difference suggests that red giants inlater evolutionary phases may not have zero-age surface values of[Fe/H]. We made use of the least-squares minimization techniques to fitthe Padova theoretical isochrones to the CMD, when the binary starpopulation is taken into account. By varying the distance modulus,metallicity and age, the best match has been found between the sevenmagnitudes and colors of the observed stars and those of model binaries,which gives the distance modulus by 0.2 mag smaller than that derivedfrom individual stars, i.e., (m-M)V = 8.18, a closely similarmetallicity ([Fe/H] = -0.12), and age of 1.6 Gyr. With these results,the fraction of photometric binaries among the main-sequence stars is≥40%.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Extended Strömgren Photoelectric Photometry in NGC 752
Photoelectric photometry on the extended Strömgren system (uvbyCa)is presented for 7 giants and 21 main-sequence stars in the old opencluster NGC 752. Analysis of the hk data for the turnoff stars yields anew determination of the cluster mean metallicity. From 10 single-starmembers, [Fe/H]=-0.06+/-0.03, where the error quoted is the standarderror of the mean and the Hyades abundance is set at [Fe/H]=+0.12. Thisresult is unchanged if all 20 stars within the limits of the hkmetallicity calibration are included. The derived [Fe/H] is in excellentagreement with past estimates, using properly zeroed m1 data,transformed moderate-dispersion spectroscopy, and recent high-dispersionspectroscopy.
| A New Definition for the Ca4227 Feature: Is Calcium Really Underabundant in Early-Type Galaxies?
We have investigated the abundance of calcium in early-type galaxies bymeasuring the strength of the Ca I λ4227 absorption line in theirintegrated spectra. The database used is the large sample of early-typegalaxy integrated spectra in Caldwell and coworkers. We have measured Caabundances from the Ca I λ4227 feature both by using the LickCa4227 index and by defining a new index, Ca4227r, thatavoids the CN4216 molecular band in the continuum on the blueward sideof the line. With the new index definition we measure Ca abundances thatare systematically ~0.3 dex higher than with the Lick Ca4227 index. Theresult is that with the new index definition we obtain higher [Ca/Fe]abundances in early-type galaxies, which are more consistent with theirwell-known [Mg/Fe] overabundances. Hence, we suggest that Ca might beslightly enhanced, relative to Fe, in early-type galaxies.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The formation of galaxy bulges: Spectrophotometric constraints
We have measured Mg2, Fe 5270 and Fe 5335 spectrophotometricindices (LICK system) in the bulge of 89 galaxies, mostly spirals fromthe Héraudeau (\cite{her96}) sample. The indices are reduced to anull velocity dispersion and normalized to an aperture of 0.2h-1 kpc. The mean errors are 0.009 mag on Mg2, and0.3 Å on the iron indices. These measurements almost double theamount of similar data already available on spiral galaxies. Our dataconfirm the existence of the relation between Mg2, andsigma0, the central stellar velocity dispersion; we find aneven tighter relation between Mg2, andVmrot, the maximum rotational velocity of thegalaxy, deduced from HI observations. For the most massive bulges, thesecorrelations may be interpreted as a mass-metallicity relation. However,the presence of young stellar populations, traced by the detection of[OIII] lambda 5007 Å, emission, provides clear evidence that ageeffects do play a role. Since the contribution of the young populationis anti-correlated to the mass of the galaxy, it continues theMg2, vs. sigma0 , relation toward thelow-sigma0, region and globally increases its slope. We alsopresent evidence for a new positive correlation between Fe indices andsigma0, and for a significant correlation between theline-strength indices and the total or disk luminosity. We propose tomodel the whole sequence of bulges within the folowing framework: bulgesare composed of a primary population formed prior to the disk, duringthe initial collapse, and of a secondary population formed during itsevolution. The whole family of bulges can be classified into threeclasses: (A) the bulges dominated by young populations are generallysmall, have ionized gas, low velocity dispersion and low line strengths;(B) the bulges dominated by the primary population lie along themass-metallicity sequence defined for elliptical galaxies; and (C) thebulges where the secondary population is significant are lessMg-over-abundant than (B)-type bulges and deviate from theMg2, vs. sigma0, relation of elliptical galaxies.Based on observations collected at the Observatoire de Haute-Provence.Table 3 is presented in electronic form only at the CDS. Tables 1 and 2are also available form at the CDS, Strasbourg, via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/366/68
| 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
| Red supergiants in the LMC - III: luminous F and G stars
New BVRI observations for 40 and spectrophotometric measurements for 23F to G LMC supergiant candidates (and 3 galactic F to G supergiants) arepresented. The errors of the BVRI data are 0.01 to 0.03 mag in mostcases. The wavelength range of the spectra is 3400 to 6400 Angstroms,their resolution 10 Angstroms. The mean error of the fluxes is 0.03 mag.Spectral indices measuring the strengths of the Hβ , Hγ ,Hdelta , NaD and CaII H+K lines, the CHα_ {0} and CNbeta_ {0}bands, of the Balmer jump and the slope of the continuum redwards arediscussed as measures of effective temperature and luminosity on thebasis of galactic stars with accurate MK types and parallaxes. TheHγ line and the continuum gradient are very good temperaturecriteria, the CHα_ {0} band and especially the Balmer jump forluminosity. The luminosity classification given for F to G supergiantcandidates in the LMC in the literature is often doubtful. 5 of the 23stars observed spectrophotometrically turn out to be probably galacticforeground dwarfs on the basis both of the Balmer jump and thecomparison of their flux distributions with synthetic ones based on theKurucz model atmospheres. Surface gravities derived purely on the basisof flux distributions and such ones given by models of stellar evolutionagree with each other for dwarfs and giants only. For supergiants theformer are about 1.0 dex higher than the latter. As a consequenceeffective temperatures and metallicities given by these two methodsdeviate from each other for such stars, too. The intrinsic colours andtemperatures of galactic and LMC supergiants do not differ. Withabsolute magnitudes up to -9.6 mag the upper luminosity limit in the LMCdoes not exceed that in the Galaxy, where Ia-0 supergiants haveMV of up to -9.5 mag. The metallicities of the supergiantsshow a rather large scatter. Nevertheless the mean metallicities of 0.02+/- 0.09 dex for the Galaxy and -0.26 +/- 0.10 dex for the LMC agreewell with other observations.
| Red giants in open clusters. VIII. NGC 752
The results of an 18-year radial-velocity survey of 30 red giants in thefield of the open cluster NGC 752 are presented. The membership of 15stars is confirmed. Four spectroscopic binaries have been discoveredamong the members and three orbits have been determined for H75, 110 and208, with periods of 3321, 127 and 5276 days respectively. The binaryfrequency (27%) is normal. A search for additional members in a widesurrounding area (2degr ) yielded two possible new members: both areclump red giant candidates. The red giant distribution in thecolour-magnitude diagram is somewhat unusual, with a clump containing 8stars and a second, fainter feature extending to the blue, defined by 3or 4 stars, which is not accounted for by theoretical isochrones. Basedon observations collected at the Haute-Provence Observatory (France)
| Quantitative spectral classification of galactic disc K-M stars from spectrophotometric measurements
New spectral observations for 47 southern galactic red supergiantsobtained with the new RUBIKON spectrophotometer (developed at theAstronomisches Institut der Ruhr-Universitat Bochum) at the Bochum 61-cmtelescope on La Silla are presented. The spectra range from 4800 to 7700A and their resolution is 10 A. The mean error of absolute fluxes is0.028 mag and that of relative fluxes 0.021 mag. The spectra will beavailable at the Strasbourg Stellar Database (CDS). Together with datataken from recently published spectral catalogues, the new observationshave been used to define spectral indices as measures of the strengthsof the following features: Fe i+TiOalpha_1, Mgb+TiOalpha_0,NaD+TiOgamma'_1, TiOgamma'_0 and TiOgamma_1 systems. The indices havebeen checked against errors introduced by reductions, interstellarreddening and different resolutions of different spectral catalogues,and have been found to be very insensitive to all these effects.Therefore, different catalogues may be combined without any loss ofaccuracy and homogeneity. The mean error of a single index has beenfound to be 0.011 mag. For stars from K4 to M7, a strong temperaturedependence is found for all indices. For the Fe i+TiO and especially theMgb+TiO features, a strong dependence on luminosity has also beenobserved. These indices therefore have been combined to form aluminosity index, while the others together form a spectral index. Thecombined indices have been calibrated in terms of MK data using thestepwise linear regression technique, and may be used for quantitativetwo- dimensional spectral classification of late K- and M-type stars.The mean error of the classification is 0.6 of spectral subtype and 0.8of luminosity class, which is much higher than would be expected fromthe uncertainty of the indices alone (which, e.g., for an M4 giantcorrespond to an uncertainty of 0.1 of spectral subtype and 0.3 ofluminosity class). This may be explained by the uncertainty of theoriginal MK classifications and the variability of some programme stars.
| DDO Metal Abundances of High-Luminosity Late-Type Stars in Galactic Open Clusters
Results from UBV and DDO photometry are presented for 54 high-luminositylate-type stars in the fields of 23 open clusters. The probability ofcluster membership for each observed star is evaluated using twoindependent photometric criteria. It is found that 32 stars are verylikely cluster members, the remaining ones being almost certainly fieldobjects. The recently improved calibrations of the DDO system have beenused to derive MK spectral types, effective temperatures, andmetallicities, while E(B-V) color excesses have been determined throughknown photometric and spectroscopic procedures. The DDO metallicitiesrange between values typical of moderately metal-poor ([Fe/H]=~ -0.3) tomoderately metal-rich ([Fe/H] =~ 0.2) clusters. The masses of thecluster giants range between 1 and 4 solar masses, with the scatterwithin a cluster being less than 1 solar mass. (SECTION: StellarClusters and Associations)
| The photosphere and chromosphere of the RS Canum Venaticorum star, II Pegasi I. Spots and chromospheric emission in 1991.
We report on high-resolution spectroscopy of the RSCVn binary IIPeg,along with contemporaneous UBV(RI)_KC_ photometry, including partlysimultaneous U-band flare monitoring, during the third quarter of 1991.The spectroscopy was taken at two epochs separated by more than 6(1)/(2)weeks or seven rotations of IIPeg. At the first epoch we have observedthe three chromospheric lines, Hα, Hβ and HeID_3_, over asingle rotation and at the second epoch we observed almost the entirevisible region, for a substantial fraction of one rotation, includingHα-Hɛ and CaIIH. Our photometric results show that therewas an unusually complicated spot distribution at this epoch, with atleast three centres of activity discernable. We demonstrate, for thefirst time on a spotted late-type star, that these centres of spotactivity have significantly different mean temperatures from oneanother. Furthermore, our derived spot temperatures are markedly lowerthan those previously derived from such multicolour photometry and moresimilar to those derived spectroscopically. Using multiple repeatexposures on each spectral line within each night, we are confident thatwe have derived a mean, non-flaring spectrum at each observationalphase. Furthermore, the line flux variations at the two epochs repeatvery well, suggesting that the mean global chromospheric structures arestable on a 1(1)/(2) months time scale. We discuss the implications ofthe mean spectra for the non-flaring atmosphere of IIPeg and concludethat, in the region of formation of the Hα line, velocity fieldsare required to account for the line profile. These results suggestupward motions of the deep chromosphere with a less dramatic downflow inthe high chromosphere. We examine the variation of the chromosphericline fluxes as a function of spot visibility and conclude that there isonly a very loose spatial correlation of chromsopheric heating withspots. The motions suggested from the Hα profile variations appearto be spot-associated.
| A photometric and radial-velocity analysis of the intermediate-age open cluster NGC 752
Using all available proper-motion and radial-velocity data, includingnew radial-velocity observations obtained for this investigation,probable members of the open cluster NGC 752 have been identified.Photoelectric data on six systems have been transformed and collated toform an internally consistent sample on the BV system. Binaries havebeen identified using photometric and radial-velocity data, including aphotographic survey for variability and the radial-velocity observationsof this study. Analysis of the data leads to the following clusterparameters and their probable errors: E(B - V) = 0.035 plus or minus0.005 mag, Fe/H = -0.15 plus or minus 0.05 dex, and (m - M) = 8.25 plusor minus 0.10 mag. The spread in color among stars in thecolor-magnitude diagram (cmd) along the main sequence from the turnoffto the unevolved main sequence is the consequence of a rich populationof binaries. Due to its age and the comprehensive data available for thecluster, NGC 752 provides an ideal test of a variety of evolutionaryphenomena. Comparison with theoretical isochrones normalized in aninternally consistent manner leads to the conclusion that the morphologyand distribution of stars in the cmd can best be matched using modelsthat include convective overshoot, particularly those of Schaller et al.(1992, A&AS, 96, 269). Despite their differences, the traditional andthe overshoot isochrones both imply very similar ages, 1.9 plus or minus0.2 Gyr and 1.7 plus or minus 0.1 Gyr, respectively, for the cluster.The Li abundances for the giants confirm that the giant branch isdominated by clump stars and first-ascent giants below the luminosity ofthe clump. The position and size of the Li dip among the main-sequencestars, compared to the Hyades, is readily explained by stellar evolutionwith convective overshoot. It is predicted that among turnoff stars inthe intermediate-age range Li will cease to be a unique function of ageat a given color. Chromospheric flux is shown to be a well-definedfunction of color for single, unevolved stars, identical to that foundfor the Hyades, and the relation for NGC 752 falls within theVaughan-Preston gap. However, the slope of the relation requires thatincreasing color implies increasing age for the bluer portion of theweak-emission boundary. The combined effect of small samples, randomerrors, emission limits, a possible selection bias in favor of turnoffstars, and metallicity corrections is to render highly questionable anyinterpretation of time-variable star formation within the Galaxy basedupon chromospheric ages.
| Evolved GK stars near the Sun. 2: The young disk population
From a sample of nearly 2000 GK giants a group of young disk stars withwell determined space motions has been selected. The zero point of theluminosity calibrations, both from the ultraviolet flux (modifiedStroemgren system) and that in the region of 4200 to 4900 A (DDOsystem), show a discontinuity of about a half magnitude at the border ofthe young disk and old disk domains. The population separation is basedon the space velocity components, which are also an age discriminant,with the population interface near 2 x 109 yr, based onmodels with convective overshoot at the core. This age corresponds togiant masses near 1.7 solar mass, near the critical mass separating theyoung stars that do not burn helium in degenerate cores from older starsthat do. Ten percent of both populations show CN anomalies in that thederived value of P(Fe/H) from CN (Cm) and fromFe(M1) differ by more than 0.1 dex and the weak and strong CNstars occur equally in the old disk but the weak CN stars predominate inthe young disk. Peculiar stars, where flux distortions affect theluminosity calibrations, are of the CH+(Ba II) and CH-(weak G band)variety and represent less than 1% of the stars in both populations. Theyoung disk giants are restricted to ages greater than about109 yr, because younger stars are bright giants orsupergiants (luminosity class 2 or 1), and younger than about 2 x109 yr, because the old disk-young disk boundary occurs near1.7 solar mass. The distribution of heavy element abundances, P(Fe/H),for young disk giants is both more limited in range (+/- 0.4 dex) and isskewed toward higher abundances, compared with the nearly normaldistribution for old disk giants. The distribution of (U,V) velocityvectors gives (U,V,W) and their dispersions = (+17.6 +/- 18.4, -14.8 +/-8.4, -6.9 +/- 13.0) and (+3.6 +/- 38.4, -20.7 +/- 27.5, -6.7 +/-17.3)km/s for young and old disk giants, respectively.
| Multicolour photoelectric photometry of the open cluster NGC 752
In the open cluster NGC 752, 89 stars are observed in the Vilniusseven-color photometric system, including probable members fainter thanobserved hitherto. Spectral classification of observed stars are madeand individual reddening values are determined. The mean reddening ofthe cluster stars is equivalent E(B - V) = 0.025 m. By fitting on colorindex-magnitude diagrams the cluster Main Sequence with the Hyades onethe distance modulus (m - M)0 = 8.15 +/- 0.15 m is found. For 11 redgiant members of the cluster absolute magnitudes are derived andcompared with ones of the standard calibration. It is obtained that thecluster giants are on average 0.6 m fainter than giants of thecorresponding spectral class in the solar vicinity.
| Metallicities and radial velocities of old open clusters
Medium resolution spectra of giant stars in 24 open clusters with agesgreater than about 1 Gyr have been obtained and analyzed to provideradial velocities accurate to 10-15 km/s and (Fe/H) metallicitiesaccurate to 0.15 dex. The radial velocities of the older cluster systemsare consistent with those expected from the reflex of solar motion andsimple Galactic rotation. The cluster kinematics suggest strongly thatthey are drawn from the old thin disk stellar population. An abundancegradient in the Galactic disk of -0.09 +/- 0.02 dex/kpc is found. Thisgradient is in good agreement with that found for samples of youngeropen clusters, suggesting that there has been little change in theGalactic radial abundance gradient over time. The cluster sample showsno correlation between age and metallicity, regardless of Galactocentricradius. The clusters show an unexpected correlation between the Mg linestrength and age of the cluster at a fixed metallicity. Possible reasonsfor this are discussed.
| A new library of stellar optical spectra
Attention is given to a new digital optical stellar library consistingof spectra covering 3510-8930 R at 11-A resolution for 72 differentstellar types. These types extend over the spectral classes O-M andluminosity classes I-V. Most spectra are of solar metallicity stars butsome metal-rich and metal-poor spectra are included. This new library isquantitatively compared to two previously published libraries. It offersseveral advantages over them: it is photometrically well-calibratedindividually and consistently from star to star. Good temperature andluminosity coverage has been achieved. The incorporation of stars withwell-determined temperature, metallicity, and surface gravity parametersincreases the accuracy of the spectral type assigned to each compositelibrary star.
| A critical appraisal of published values of (Fe/H) for K II-IV stars
'Primary' (Fe/H) averages are presented for 373 evolved K stars ofluminosity classes II-IV and (Fe/H) values beween -0.9 and +0.21 dex.The data define a 'consensus' zero point with a precision of + or -0.018 dex and have rms errors per datum which are typically 0.08-0.16dex. The primary data base makes recalibration possible for the large(Fe/H) catalogs of Hansen and Kjaergaard (1971) and Brown et al. (1989).A set of (Fe/H) standard stars and a new DDO calibration are given whichhave rms of 0.07 dex or less for the standard star data. For normal Kgiants, CN-based values of (Fe/H) turn out to be more precise than manyhigh-dispersion results. Some zero-point errors in the latter are alsofound and new examples of continuum-placement problems appear. Thushigh-dispersion results are not invariably superior to photometricmetallicities. A review of high-dispersion and related work onsupermetallicity in K III-IV star is also given.
| The studies of proper motions in the regions of open clusters. II - NGC 752
Relative proper motions of 1777 stars in the region of the open clusterNGC 752 have been determined from the PDS measurements of eleven Pulkovonormal astrograph plates. In a 110-arcmin-diameter circle around thecluster, 175 stars have membership probabilities P greater than 0percent. The total number of cluster stars, obtained by weighting themwith membership probabilities, reaches 130. The catalog of propermotions is complete down to m(pg) = 150 mag, the limiting value is 154mag. The average standard error of the catalog proper motions variesfrom + or - 000075 arcsec/yr to + or - 00011 arcsec/yr for the rest. Theluminosity function for cluster stars shows a deficiency of faint stars.
| The catalogue of equivalent line widths in the spectra of A- and F-stars.
Not Available
| Radial velocities of more old open clusters
Moderate-resolution CCD spectra in the Mg b region of red giants in 13old or metal-poor open clusters were obtained with the kitt Peak 2.1-mtelescope. Radial velocities for these giants, measured bycross-correlation techniques, are accurate to 10 km/s. For the sixclusters in the sample with previous measurements, the velocities are ingood agreement with published values. Results are presented for sevenclusters (NGC 1193, NGC 1817, NGC 6819, NGC 7142, Be 21, King 8, IC 166)with no previous radial-velocity measurements. The majority of clustersin the sample, supplemented with ten more clusters older than 1 Gyrwhich have published velocities, show radial velocities which areconsistent with those expected from the reflex of solar motion andsimple galactic rotation. A few of the clusters, including the newlymeasured NGC 1193 and NGC 1817, show surprisingly large radialvelocities, indicating significant deviation from the general rotationof the remaining old cluster system.
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