On the distance scale of open clusters

We discuss the state of the distance scale of open clusters in view of various capabilities provided by optical and near-IR observations of galactic objects. Various checks lead to the conclusion that open cluster distance scale requires no appreciable correction. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

On the two-dimensional maps of the distribution of interstellar extinction

A modified version of the method for determining the mean interstellar extinction and constructing the maps of its distribution based on JHK s photometry is proposed. The extinction for each star of the sample is estimated from the Q -index by linearly approximating the line of unreddened stars on the QJHK - (J - H) diagram. The J -band extinction can be calculated by the equation AJ = 2.68 [(J - H) + 0.03 - 1.18 QJHK]. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Study of possible regularities of the metallic distribution of open cluster stars according to the LAMOST catalog

В ходе работы были изучены возможности каталога LAMOST для статистических исследований спектров звезд рассеянных звездных скоплений (РЗС). Были рассмотрены скопления Плеяды и Ясли. Для членов этих скоплений рассмотрены зависимости абсолютная звездная величина — металличность. Вопреки ожиданиям у звезд главных последовательностей исследуемых скоплений обнаружена заметная зависимость металличности от абсолютной звездной величины, которая может быть связана с систематическими ошибками измерения металличностей для визуально слабых звезд.In this study were discussed the LAMOST catalog opportunities in the statistical studies of the spectra of stars in open star clusters (OSC). We considered the Pleiades and Praesepe clusters. We examined the absolute stellar magnitude — metallicity relationships for the members of these clusters. Contrary to expectations, a noticeable dependence of the metallicity on the absolute magnitude of the stars in the main sequences of the studied clusters was found, which may be associated with systematic errors in measuring the metallicities for visually faint stars

Gaia and Open Clusters

We show the results of our first attempts to use Gaia DR2 data for estimation of the parameters of open clusters using on the base of 42 objects.На примере 42 рассеянных звездных скоплений показаны результаты наших первых попыток использования данных DR2 Gaia при оценивании параметров этих объектов.Работа выполнена при финансовой поддержке государства в лице Министерства науки Российской Федерации (базовая часть госзадания, РК № AAAA-A17-117030310283-7)

On the Metallicity Gradient in the Galaxy

Based on open clusters data, the problem of the metallicity gradient in the galactic disk was studied. A saltatory variation in the average metallicity of clusters with galactocentric distance was obtained. No significant dependence of the average metallicity on the age of the clusters was revealed.По данным о рассеянных звездных скоплениях исследован вопрос о градиенте металличности в диске Галактики. Получено скачкообразное изменение средней металличности скоплений с ростом галактоцентрического расстояния. Не выявлено значимой зависимости средней металличности от возраста скоплений.Работа проведена при финансовой поддержке государства в лице Министерства науки Российской Федерации (базовая часть госзадания, РК № AAAA-A17-117030310283-7), а также при финансовой поддержке Правительства Российской Федерации, постановление № 211, контракт № 02.A03.21.0006

Galactic Rotation Parameters from Data on Open Star Clusters

Currently available data on the field of velocities Vr, Vl, Vb for open star clusters are used to perform a kinematic analysis of various samples that differ by heliocentric distance, age, and membership in individual structures (the Orion, Carina--Sagittarius, and Perseus arms). Based on 375 clusters located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the Galactic rotation parameters Wo =-26.0+-0.3 km/s/kpc, W'o = 4.18+-0.17 km/s/kpc^2, W''o=-0.45+-0.06 km/s/kpc^3, the system contraction parameter K = -2.4+-0.1 km/s/kpc, and the parameters of the kinematic center Ro =7.4+-0.3 kpc and lo = 0+-1 degrees. The Galactocentric distance Ro in the model used has been found to depend significantly on the sample age. Thus, for example, it is 9.5+-0.7 kpc and 5.6+-0.3 kpc for the samples of young (50 Myr) clusters, respectively. Our study of the kinematics of young open star clusters in various spiral arms has shown that the kinematic parameters are similar to the parameters obtained from the entire sample for the Carina-Sagittarius and Perseus arms and differ significantly from them for the Orion arm. The contraction effect is shown to be typical of star clusters with various ages. It is most pronounced for clusters with a mean age of 100 Myr, with the contraction velocity being Kr = -4.3+-1.0 km/s.Comment: 14 pages, 4 figures, 2 table

Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Here we present a survey of projected rotational velocities for a large sample of mainly B-type stars in young clusters to study the time evolution of the rotational properties of massive stars. The survey is based upon moderate resolution spectra made with the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs, and the target stars are members of 19 young open clusters with an age range of approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387, 4471 and Mg II 4481 using model theoretical profiles to find projected rotational velocities for a total of 496 OB stars. We find that there are fewer slow rotators among the cluster B-type stars relative to nearby B stars in the field. We present evidence consistent with the idea that the more massive B stars (M > 9 solar masses) spin down during their main sequence phase. However, we also find that the rotational velocity distribution appears to show an increase in the numbers of rapid rotators among clusters with ages of 10 Myr and higher. These rapid rotators appear to be distributed between the zero age and terminal age main sequence locations in the Hertzsprung-Russell diagram, and thus only a minority of them can be explained as the result of a spin up at the terminal age main sequence due to core contraction. We suggest instead that some of these rapid rotators may have been spun up through mass transfer in close binary systems.Comment: 33 pages, 11 figures, accepted by Ap

Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance

We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the H-gamma profile in their spectra. We use synthetic model profiles for rotating stars to develop a method to estimate the polar gravity for these stars, which we argue is a useful indicator of their evolutionary status. We combine these results with projected rotational velocity measurements obtained in a previous paper on these same open clusters. We find that the more massive B-stars experience a spin down as predicted by the theories for the evolution of rotating stars. Furthermore, we find that the members of binary stars also experience a marked spin down with advanced evolutionary state due to tidal interactions. We also derive non-LTE-corrected helium abundances for most of the sample by fitting the He I 4026, 4387, 4471 lines. A large number of helium peculiar stars are found among cooler stars with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M < 16 solar masses) shows that the helium enrichment process progresses through the main sequence (MS) phase and is greater among the faster rotators. This discovery supports the theoretical claim that rotationally induced internal mixing is the main cause of surface chemical anomalies that appear during the MS phase. The lower mass stars appear to have slower rotation rates among the low gravity objects, and they have a large proportion of helium peculiar stars. We suggest that both properties are due to their youth. The low gravity stars are probably pre-main sequence objects that will spin up as they contract. These young objects very likely host a remnant magnetic field from their natal cloud, and these strong fields sculpt out surface regions with unusual chemical abundances.Comment: 50 pages 18 figures, accepted by Ap

Blue Stragglers in Galactic Open Clusters and the Integrated Spectral Energy Distributions

Synthetic integrated spectral properties of the old Galactic open clusters are studies in this work, where twenty-seven Galactic open clusters of ages >= 1Gyr are selected as the working sample. Based on the photometric observations of these open clusters, synthetic integrated spectrum has been made for the stellar population of each cluster. The effects of blue straggler stars (BSSs) on the conventional simple stellar population (SSP) model are analyzed on an individual cluster base. It is shown that the BSSs, whose holding positions in the color-magnitude diagrams (CMDs) cannot be predicted by the current single-star evolution theory, present significant modifications to the integrated properties of theoretical SSP model. The synthesized integrated spectral energy distributions (ISEDs) of our sample clusters are dramatically different from the SSPs based on isochrone only. The BSSs corrected ISEDs of stellar populations show systematic enhancements towards shorter wavelength in the spectra. When measured with wide-band colors in unresolvable conditions, the age of a stellar population can be seriously under-estimated by the conventional SSP model. Therefore, considering the common existence of BSS component in real stellar populations, a considerable amount of alternations on the conventional ISEDs should be expected when applying the technique of evolutionary population synthesis (EPS) to more complicated stellar systems.Comment: 45 pages, 21 figures Accepted for publication in ApJ (Feburary 1, 2005 issue

Global survey of star clusters in the Milky Way I. The pipeline and fundamental parameters in the second quadrant

Aims: On the basis of the PPMXL star catalogue we performed a survey of star clusters in the second quadrant of the Milky Way. Methods: From the PPMXL catalogue of positions and proper motions we took the subset of stars with near-infrared photometry from 2MASS and added the remaining 2MASS stars without proper motions (called 2MAst, i.e. 2MASS with astrometry). We developed a data-processing pipeline including interactive human control of a standardised set of multi-dimensional diagrams to determine kinematic and photometric membership probabilities for stars in a cluster region. The pipeline simultaneously produced the astrophysical parameters of a cluster. From literature we compiled a target list of presently known open and globular clusters, cluster candidates, associations, and moving groups. From established member stars we derived spatial parameters (coordinates of centres and radii of the main morphological parts of clusters) and cluster kinematics (average proper motions and sometimes radial velocities). For distance, reddening, and age determination we used specific sets of theoretical isochrones. Tidal parameters were obtained by a fit of three-parameter King profiles to the observed density distributions of members. Results: We investigated all 871 objects in the 2nd Galactic quadrant, of which we successfully treated 642 open clusters, 2 globular clusters, and 8 stellar associations. The remaining 219 objects (24%) were recognised by us to be nonexistent clusters, duplicate entries, or clusters too faint for 2MAst. We found that our sample is complete in the 2nd quadrant up to a distance of 2 kpc, where the average surface density is 94 clusters per kpc$^{2}$. Compared with literature values we found good agreement in spatial and kinematic data, as well as for optical distances and reddening. Small, but systematic offsets were detected in the age determination.Comment: published in Astronomy and Astrophysics, 10 pages, 7 figures (plus 3 pages of appendices incl. 2 more figures), catalogues will be available at the CDS, all the machine-readable online data described in appendices A, B, and C are also available at: http://www.aip.de/People/rdscholz/kharchenko_etal_2012