Kinematical and orbital properties for selected southern high — Velocity stars

Using the model of the Galaxy presented by Eggen, Lynden-Bell and Sandage (1962), plane galactic orbits have been calculated for several southern high-velocity stars which possess parallax, proper motion, and radial velocity data. Extensive lists of both raw and computed data for these stars are included. Published values ofU-B andB-V for some of these stars were used in plots of each of the orbital parameters versusU-B, B-V, or the ultraviolet excess (U-B). Also, a comparison is made between the H-R diagrams for the southern high-velocity star group and that of M3, a globular cluster, and again for M67, an old open cluster. The high-velocity star group is found to resemble an old open cluster more than a globular cluster.     

Evolution of Population II stars

The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998     

Photographicuvby photometry of the open cluster M52

Photographicuvby photometry of 15 stars in the open cluster M52 (NGC 7654) has been obtained. The distance modulusV _o–M _v=11.3±0.1 and mean cluster reddeningE(B–V)=0.57 are determined. Some basic physical characteristics of 30 stars with observeduvby values are obtained as well as a numerical estimation of the open cluster age 9.6×10⁷ yr is made.     

Population i pulsating stars

On the basis of our age estimations of Population I pulsating stars in our Galaxy (Tsvetkov, 1986a), the mean ages of 6 open star clusters containing 21 Delta Scuti-variables and of 8 star clusters and associations containing 13 classical cepheids, have been evaluated. These mean cluster age estimations weighted according to the probabilities for different evolutionary phases of the pulsating stars, are obtained in the evolutionary track systems of Iben (1967) and Paczyski (1970); the cluster ages are larger in the former system. Our results are compared with those obtained from various methods by other authors. Clusters with classical cepheids and with Delta Scuti-stars have ages, respectively, in the ranges 10⁷–10⁸ years and 10⁶–10⁹ years. It is shown that the use of simple period-age(-colour) relations for Population I pulsating stars gives sufficiently accurate cluster age estimations. By use of our period-age relations for classical cepheids (Tsvetkov, 1986a), the mean ages of 56 other star clusters and associations in our Galaxy, the Magellanic Clouds, and M 31 galaxy have been estimated in both systems of tracks. The results are generally in agreement with those obtained from various methods by other authors. The use of Population I pulsating stars in star clusters and associations is one of the simplest and most easily applied methods for determining cluster ages; but there are some limitations in its application.     

Study of the galactic cluster NGC 6823

PhotoelectricUBV magnitudes have been determined for 41 stars in NGC 6823. The reddening across the cluster field is determined and found that it varies fromE(B-V)=0.60 to 1.16 mag. The true distance modulus to the cluster is estimated at 12.7±0.3 mag. It is found that cluster stars are not co-equal in age.     

Proper motions and CCD photometry of stars in the region of the open cluster NGC 6866

We present the results of our comprehensive study of the Galactic open star cluster NGC 6866. The positions of stars in the investigated region have been obtained with the “Fantasy” automatic measuring machine from 10 plates of the normal astrograph at the Pulkovo Astronomical Observatory. The size of the investigated field is 40′ × 40′, the limiting magnitude is B ∼ 16_· ^m 6, and the maximum epoch difference is 79 yr. For 1202 field stars, we have determined the relative proper motions with an rms error of 2.5 mas yr⁻¹. Out of them, 423 stars may be considered cluster members with a probability P > 70% according to the astrometric criterion. Photometric diagrams have been used as an additional criterion. We have performed two-color BV CCD photometry of stars with the Pulkovo ZA-320M mirror astrograph. The U magnitudes from the literature have also been used to construct the two-color diagrams. A total of 267 stars have turned out to be members of NGC 6866 according to the two criteria. We present refined physical parameters of the cluster and its age estimate (5.6 × 10⁸ yr). The cluster membership of red and blue giants, variable, double, and multiple stars is considered. We have found an almost complete coincidence of the positions of one of the stars in the region (a cluster nonmember) and a soft X-ray source in the ROSAT catalog. The “Fantasy” automatic measuring machine is described in the Appendix.     

Age determination of the open cluster NGC 2287 (M41)

We have estimated the age of the open cluster NGC 2287 (M41) by taking into account the rotation of the member stars.TheUBV photoelectric measurement values were taken from Eggen (1974). Besides the Main-Sequence stars this cluster has seven red giants, all of them above the turn-off point.Having obtained the theoretical Hertzsprung-Russell diagram from observed colour-magnitude diagram, we have discussed two methods, given in the literature, to estimate the age of the cluster under consideration. The first method we have followed is to determine the turn-off point on the cluster Main-Sequence. The other is fitting the position of the red giants with the evolutionary tracks of rotating stars with different masses on the Hertzsprung-Russell diagram.We have estimated the cluster age based on the turn-off point as 3.10×10⁷ yr. From the fitting of the evolutionary tracks of rotating stars to the cluster Hertzsprung-Russell diagram, we infer that the age of the cluster is about 8.00×10⁷ yr.     

Schmidt survey in the Galactic anticentre direction. 1. Investigation of open clusters

A study of four open clusters in the direction of the Galactic anticentre (l = 186°, b = +2°) is presented. In a field of 8.32 square degrees proper motions and B magnitudes for about 79 000 stars down to 19.5 were determined on Tautenburg Schmidt plates. For more than 15 500 of them U magnitudes down to 17.3 could be obtained. Additionally, OCA Schmidt plates were used to determine V, R magnitudes in a larger field of 24.45 square degrees for 271 000 stars down to V = 18.2. For stars brighter than V = 15.5 an accuracy of about 1.5 mas/yr has been estimated for proper motions. The rms errors of stellar magnitudes and colour indices are 0.09 – 0.12 mag. Several open clusters have been already known in this direction of the sky, e.g. NGC 2168 or M 35 (C 0605+243), NGC 2158 (C 0604+241) and IC 2157 (C 0601+240). Inspecting the plates and analysing the colour-magnitude diagrams and published data, we could identify an additional anonymous cluster C 0605+242 with a projection on the sky near the centre of M 35 but at a larger distance from the Sun. The cluster membership determination was carried out using information on spatial and proper motion distributions of stars in the field. The colour-magnitude diagrams were derived down to the limiting stellar magnitude. For each cluster the interstellar extinction A_v, the diameters of the core and corona, the ages and spatial velocity components (V,W) relative to the LSR in the Y,Z – Galactic directions were determined. The distances to the clusters of 960 pc, 2 600 pc, 2 520 pc and 3 700 pc were obtained for M 35, IC 2157, C 0605+242 and NGC 2158. They show the loci of the clusters in the Local and Perseus spiral arms and at external border of Perseus arm, respectively.     

The luminosity function for the globular cluster M15

We obtained the luminosity function from the Main-Sequence turnoff region to the subgiants for M15 (V=17.0–19.6), using plates taken by the Kitt Peak 4 m reflector. The measured region isr=2.5–8.0 zone. We extrapolated the star counts to the cluster center and to the tidal limit, using King's (1966) photometry, Kinget al.'s (1968) counts, Peterson and King's (1975) result, and Simoda and Iben's (1970) theoretical results.The comparison of the observed function with the theoretical ones by Simoda and Iben (1970) indicates that the observation is consistent with the theory forX0.9±0.1, thus marginally inconsistent with the Big Bang cosmology value (X=0.76). The comparison with Bertelliet al. (1970) theoretical ones gives contradictory results. Possible causes of this problem are the improper extrapolation of star counts to the cluster center and/or the inaccuracies of the theoretical luminosity functions so far published, and/or the impropriety of the canonical structure and evolution theory of Population II stars. In the last two cases, the ages and the helium abundances of globular clusters may vary.We also obtained the luminosity function for blue horizontal branch stars fromV=17.0 to 19.6.Kitt Peak National Observatory, U.S.A., which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Search for HH Objects and Emission Stars in Star Formation Regions. III. PMS Stars in NGC 7129

Slitless spectroscopy is used to discover 22 emission stars in the central and northwest regions of the cluster located in the NGC 7129 nebula. 16 of them are found for the first time. This sample is essentially complete up to V 20.0. The emission stars are distributed nonuniformly over the field of the cluster and are concentrated toward its center. Photometry in the V, R, and I bands is conducted on more than a hundred stars in the cluster. This yields an average absorption coefficient A _V=1.7±0.27 for this region. Based on their positions in color diagrams for the optical and near IR ranges, most of the emission stars can be regarded as T Tau objects.     

Separation of dwarf and giant stars with ROTSE–IIId

136 stars which were known to be members of the open cluster NGC 752 were observed at the R band with the ROTSE–IIId telescope located at the Turkish National Observatory (TUG) site. The data had been evaluated together with BV and 2MASS photometric data. A new practical method for separating dwarf and giant was described and applied. Evaluating the colour magnitude–diagrams with Padova isochrones revealed for the open cluster NGC 752 a metallicity similar to the Sun and an age of 1.41 ± 0.20 Gyr. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Possibility of mass loss in the red-giant stage from H-R diagrams of galactic clusters

A statistical research on evolved stars beyond hydrogen exhaustion is performed by comparing the H-R diagrams of about 60 open clusters with a set of isochronous curves without mass loss derived from Iben's evolutionary tracks and time scales for Population I stars. Interpreting the difference in magnitude between the theoretical positions thus calculated and the observed ones as due to mass loss, when negative, the results indicate that this loss may be conspicuous only for very massive and red stars. However, a comparison with an analogous work of Lindoff reveals that the uncertainties connected with the bolometric and colour corrections may invalidate by a large amount the conclusions which might be drawn from such research.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

The outer-halo globular cluster NGC 6229 – IV. Variable stars

We present time-series B , V photometry of NGC 6229, obtained with the purpose of providing the first extensive CCD variability study of this cluster. As a result, we were able to obtain periods for all NGC 6229 variables, with the exception of five stars from the candidate list of Borissova et al. located very close to the cluster centre. Two stars suspected to be variables by Carney, Fullton and Trammell are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates of Borissova et al. are confirmed variables – three of them being fundamental RR Lyrae (RRab) pulsators, two first-overtone pulsators, one an eclipsing binary (most likely an Algol system) and one a bright star whose variability status could not be satisfactorily determined. A new image subtraction method (ISM) suggested by Alard has been employed which, together with the Welch–Stetson technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemerides are provided. Ten of these are RRabs, whereas the other two are RRcs. As originally suggested by Mayer, NGC 6229 is clearly an Oosterhoff type I globular cluster. We also confirm that V8 is a Population II Cepheid of the W Virginis type, and suspect that this is the case for V22 as well. The physical properties of the NGC 6229 RR Lyrae population are contrasted with those of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.     

Be phenomenon in open clusters: results from a survey of emission-line stars in young open clusters

Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour–magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.     

Investigation of the open star cluster NGC 2323 (M50) based on the proper motions and photometry of its constituent stars

The results of a comprehensive study of the Galactic open cluster NGC 2323 (M50) are presented. The positions of stars to a limiting magnitude {ie74-1} in a {ie74-2} area centered on the cluster were measured on six plates from the Pulkovo normal astrograph with a maximum epoch difference of 60 yr. The measurements were performed with the Pulkovo “Fantasy” automated measuring system upgraded in 2010. The corresponding areas from the USNO-A2.0, USNO-B1, and 2MASS catalogues were used as additional plates. As a result, the relative proper motions of stars were obtained with a root-mean-square error of 5.85 mas yr⁻¹. A catalogue of UBV and JHK magnitudes for objects in the investigated area was compiled from available published resources. The astrometric selection of cluster members was made by the maximum likelihood method. A high individual cluster membership probability of a star (P ≥ 80%) served as the first selection criterion. The position of a star on the photometric color-magnitude (V ∝ (B-V), J ∝ (J-K)) diagrams of the cluster was considered as the second criterion. The position of an object on the color-color ((U-B)-(B-V), (J-H)-(J-K)) diagrams served as the third criterion. On the basis of these criteria, it was established that 508 stars are members of NGC 2323. These data were used to refine the physical parameters of the cluster: the mean reddening {ie74-3}, the true distance modulus {ie74-4}, and the cluster age of about 140 Myr from the grid of isochrones computed by the Padova group for solar chemical composition. Two tables contain the catalogues of proper motions and photometry for stars in the area. The luminosity and mass functions were constructed. The cluster membership of red and blue giants, variable, double, and multiple stars was considered. The position of the cluster center was improved: {ie74-5}, δ = −08°20′16″(2000.0).     

Spectroscopy of B and Be stars in the diffuse stellar clusters NGC 6871 and NGC 6913

This is a study of the population of B and Be stars in the young, relatively poor, diffuse stellar clusters NGC 6871 and NGC 6913. High resolution spectra are used to study the Hα line of eleven stars in order to detect emission. Emission profiles were found for three stars in the cluster NGC 6871; one of these is a known WR-star and the Be-star BD +35°3956 demonstrates the transition from the B to the Be phase. Spectra of seven of the B stars revealed no traces of emission in the Hα line. During the time of our observations, the Be star V1322 Cyg in the cluster NGC 6913 had a strong emission Hα line profile with substantial variability in intensity and equivalent width. Moderate resolution spectra of seven stars in the cluster NGC 6871 over wavelengths of 4420-4960 ? and ten stars in the cluster NGC 6913 over wavelengths of 4050-5100 ? are used to classify the series of B and Be stars spectrally and to estimate their T _eff and log g. It was found that three of the stars are not members of the clusters NGC 6871 or NGC 6913. Translated from Astrofizika, Vol. 52, No. 2, pp. 257–274 (May 2009).     

BVI CCD photometry of the distant open star clusters Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044

We present CCD observations for the distant northern open star clusters Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044 in B V I photometric passbands. A total of 9 900 stars have been observed in fields of about 6 × 6 arcmin² of the sky around the clusters. Colour–magnitude diagrams in V , ( B  −  V ) and V , ( V  −  I ) have been generated down to V  = 22 mag and, for the first time, such diagrams have been produced for the clusters Berkeley 81 and Berkeley 99. The data serve as a base for the study of mass functions and for comparison with theoretical models. Analysis of the radial distribution of stellar surface density indicates that the radius values for Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044 are 2.7, 2.8, 2.8 and 2.2 arcmin respectively. By fitting the latest convective core overshooting isochrones to the colour–magnitude diagram and using its morphological features, reddenings, distances and ages of the star clusters have been determined. Broad but well-defined main sequences with stellar evolutionary effects in the brighter stars are clearly visible in colour–magnitude diagrams of all the clusters under study. Some blue stragglers along with well-developed giant branches and red giant clumps are also clearly seen in all of them. The clusters studied here are located at a distance of ∼ 3 kpc, except for Berkeley 99 which is located at a distance of 4.9 kpc. Their linear sizes lie between 3.8 and 8.0 pc; E ( B  −  V ) values range from 0.3 to 1.0 mag, while their ages are between 0.5 and 3.2 Gyr. Thus the star clusters studied here are of intermediate and high age but are compact and distant objects.     

Ages of globular clusters. II

Evolutionary model sequences for (X, Z)=(0.7,4×10^–3) are constructed by using the same input physics and programming code as those of Saioet al. (1977). From these results the ages of globular clusters are estimated under the assumption of constant helium abundance (Y=0.3). The results suggest that there is a correlation between age and metal abundance for the globular clusters and that metal enrichment in the Galaxy slowly proceeded in several billion years from the value of the extreme Population II stars to that of the Population I stars. Comparison with some models of chemical evolution of galaxies is briefly made.     

Stars and clusters of the coma galaxies NGC 4921 and NGC 4923

Based on deep Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for the spiral and lenticular galaxies NGC 4921 and NGC 4923 in the Coma cluster. The derived Hertzsprung-Russell diagrams show that, apart from the stellar population, NGC 4921 has several thousand globular clusters. The asymmetry of their apparent distribution proves that NGC 4921 is in close spatial proximity to NGC 4923. We have determined the positions of the peaks for the luminosity functions of globular clusters in the two galaxies, which has allowed us to measure the distance to this pair (D = 97±5 Mpc). In many cases, compact star-forming regions at the periphery of NGC 4921 are located at the ends of extended (up to 1.5 kpc) radial gas and dust filaments. The metallicity of young stars in these star-forming regions is close to that of the Sun (Z = 0.02).     

Photometry of open cluster NGC 1931

PhotoelectricUBV magnitudes and colours have been determined for stars in the field of NGC 1931. The reddening across the cluster varies from 0 _. ^m 33 to 1 _. ^m 20. A distance of 2.16 kiloparsecs has been estimated for the cluster. It is concluded that the age of the cluster lies between the ages of NGC 6231 and NGC 2362 groups.