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Context. For Galactic open clusters, fundamental parameters such as age or reddening are usually determined independently of their integrated colours. For extragalactic clusters, on the other hand, they are derived by comparing their integrated colours with predictions of simple stellar population (SSP) models.

Aims. We determine the integrated magnitudes and colours of 650 clusters in optical (BV) and the near-infrared (JHKs) passbands and construct the luminosity functions of the Galactic open clusters in these passbands.

Context. Ellipticities have been determined for only a few tens of open clusters.

Aims. We aim to identify the origins of field O-stars in the nearest 2 to 3 kpc around the Sun using the best presently available kinematic data on O-stars and on young open clusters. We investigate the question of whether the present-day data are consistent with the assumption that O-stars have formed in groups (clusters, associations), or in isolation.

Aims. We build a catalogue PPM-Extended (PPMX) on the ICRS system which is complete down to a well-defined limiting magnitude and contains the best presently available proper motions suited for kinematic studies in the Galaxy.

We aim at the construction of luminosity and mass functions for Galactic open clusters, based on integrated magnitudes and tidal masses. We also aim at studying the evolution of these functions, with the ultimate purpose of deriving the initialluminosity and mass distributions of star clusters, independent of model assumptions regarding the cluster mass-to-light ratio. Finally we aim at a new determination of the percentage of field stars that have originated in open clusters.
The integrated magnitudes are computed from individual photometry of cluster members selected from the ASCC-2.5catalogue. The cluster masses we assumed to be the estimated tidal mass recently published by us elsewhere. Analysis of the cluster brightness distribution as a function of apparent integrated magnitudes shows that the cluster sample drawn from the ASCC-2.5is complete down to apparent integrated magnitude IV= 8, with 440 clusters and compact associations above this completeness limit. This, on average, corresponds to a completeness area in the solar neighbourhood with an effective radius of about 1 kpc.
The observed luminosity function can be constructed in a range of absolute integrated magnitudes $I_{M_V}= [-10,\,-0.5]$mag, i.e. about 5 mag deeper than in the most nearby galaxies. It increases linearly from the brightest limit to a turnover at about $I_{M_V}\approx-2.5$. The slope of this linear portion is $a=0.41\pm0.01$, which agrees perfectly with the slope deduced for star cluster observations in nearby galaxies. The masses of the Galactic clusters span a range from a few $M_\odot$to $\log M_{\rm c}/M_\odot \approx 5.5$. The mass function of these clusters can be fit as a linear function with log mass for $\log M_{\rm c}/M_\odot > 2.5$, and shows a broad maximum between $\log M_{\rm c}/M_\odot=1.5$and 2.5. For $\log M_{\rm c}/M_\odot >2.5$, the linear part of the upper cluster mass function has a slope $\alpha=2.03\pm0.05$, again in agreement with data on extragalactic clusters. We regard this agreement as indirect evidence that the tidal masses for Galactic clusters and the luminosity-based masses for extragalactic clusters are on the same scale.
Considering the evolution of the cluster mass function now reveals a slight but significant steepening of the slope with increasing age from $\alpha=1.66\pm0.14$at $\log t\leq 6.9$to $\alpha=2.13\pm0.08$at $\log t\leq 8.5$. This indicates that open clusters are formed with a flatter (initial) mass distribution than the overall observed cluster mass distribution averaged over all ages. Interestingly, the luminosity function of open clusters does not show the same systematic steepening with age as the mass function does.
We find that the initial mass function of open clusters (CIMF) has a two-segment structure with the slopes $\alpha=1.66\pm0.14$in the range $\log M_{\rm c}/M_\odot=3.37\dots4.93$and $\alpha=0.82\pm0.14$in the range $\log M_{\rm c}/M_\odot=1.7\dots3.37$. The average mass of open clusters at birth is $4.5\times 10^3\,M_\odot$, which should be compared to the average observed mass of about $700\,M_\odot$. The average cluster formation rate derived from the comparison of initial and observed mass functions is $\overline{\upsilon}=0.4\,\,\mathrm{kpc}^{-2}\,\mathrm{Myr}^{-1}$. Multiplying by the age of the Galactic disc ($T = 13$Gyr) the predicted surface density of Galactic disc field stars originating from dissolved open clusters amounts to $22\,M_\odot\,\mathrm{pc}^{-2}$which is about 40% of the total surface density of the Galactic disc in the solar neighbourhood. Thus, we conclude that almost half of all field stars were born in open clusters, a much higher fraction than previously thought.

Context.In a previous paper we obtained King's parameters for 236 of 650 Galactic open clusters identified in the ASCC-2.5.

Aims.In this paper we derive tidal radii and masses of open clusters in the nearest kiloparsecs around the Sun.

Aims.Based on our well-determined sample of open clusters in the all-sky catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, and investigate the effect of mass segregation of stars in open clusters.

We present results of a study of the galactic open cluster population based on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2, Hipparcos and other catalogues. The sample of optical clusters from ASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry plane of the clusters' distribution is determined to be at $Z_0=-22\pm4$pc, and the scale height of open clusters is only $56\pm3$pc. The total surface density and volume density in the symmetry plane are $\Sigma= 114$kpc-2and $D(Z_0)=1015$kpc-3, respectively. We find the total number of open clusters in the Galactic disk to be of order of 105at present. Fluctuations in the spatial and velocity distributions are attributed to the existence of four open cluster complexes (OCCs) of different ages containing up to a few tens of clusters. Members in an OCC show the same kinematic behaviour, and a narrow age spread. We find, that the youngest cluster complex, OCC 1 ($\log t<7.9$), with 19 deg inclination to the Galactic plane, is apparently a signature of Gould's Belt. The most abundant OCC 2 complex has moderate age ($\log t\approx8.45$). The clusters of the Perseus-Auriga group, having the same age as OCC 2, but different kinematics are seen in breaks between Perseus-Auriga clouds. The oldest ($\log t\approx8.85$) and sparsest group was identified due to a large motion in the Galactic anticentre direction. Formation rate and lifetime of open clusters are found to be $0.23\pm0.03$kpc-2Myr-1and $322\pm31$Myr, respectively. This implies a total number of cluster generations in the history of the Galaxy between 30 to 40. We estimate that less than about 10% of the total Galactic stellar disk population has ever passed an open cluster membership.

We present a list of 130 Galactic Open Clusters, found in the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). For these clusters we determined a homogeneous set of astrophysical parameters such as size, membership, motion, distance and age. In a previous work, 520 already-known open clusters out of a sample of 1700 clusters from the literature were confirmed in the ASCC-2.5 using independent, objective methods. Using these methods the whole sky was systematically screened for new clusters. The newly detected clusters show the same distribution over the sky as the known ones. It is found that without the a priori knowledge about existing clusters our search lead to clusters which are, on average, brighter, have more members and cover larger angular radii than the 520 previously-known ones.

We present a catalogue of astrophysical data for 520 Galactic open clusters. These are the clusters for which at least three most probable members (18 on average) could be identified in the ASCC-2.5, a catalogue of stars based on the Tycho-2 observations from the Hipparcos mission. We applied homogeneous methods and algorithms to determine angular sizes of cluster cores and coronae, heliocentric distances, mean proper motions, mean radial velocities, and ages. For the first time we derive distances for 200 clusters, radial velocities for 94 clusters, and ages of 196 clusters. This homogeneous new parameter set is compared with earlier determinations, where we find, in particular, that the angular sizes were systematically underestimated in the literature.

The study of a sky area with a 20 degree diameter centered on the nearby association Per OB2 has been carried out on the basis of the “Compiled Catalogue of Astronomical Data in the Per OB2 Field” presented in a previous paper. The catalogue contains about $30\,000$stars with accurate proper motions in the Hipparcos system as well as magnitudes supplemented with spectral classes and other relevant data. We applied the reduced proper motion technique to estimate individual distances for almost all catalogue stars. The distributions of reddening and interstellar extinction were determined on the basis of the spectral classification of stars in the area. Proper motions were corrected for differential rotation of the Galactic disk and parallactic motion effects and used for the evaluation of association membership. The analysis of a uniform subset of main sequence (MS) stars earlier than A7 – which is complete within 500 pc – has clearly revealed the association as an area of enhanced density with an angular size of about $7^\circ$. In comparison to the previous results on Per OB2, we show that the presently determined area extends the association from its classical position towards the California nebula and the Auriga dark cloud. There are 1025 MS proper motion members residing in this region. The distance to the association centre derived from a kinematic calibration is found to be about 300 pc i.e., in perfect agreement with the Hipparcos data. The shape of the association is almost spherical, and its diameter found from stellar counts is about 40 pc. An examination of internal systematic motions from proper motion data has shown that there is no evidence of an expansion or a rotation of the association around the line of sight. The average tangential and spatial velocity vectors of Per OB2 with respect to the LSR are determined as $(V_l,\,V_b)=(+7.2\pm0.4,+2.0\pm0.3)$km s-1and $(U,\,V,\,W)=(+12.7\pm 1.6,-3.0\pm 0.6,-0.9\pm 0.8)$km s-1.

In this paper we describe the construction of a Compiled Catalogue (CC) of about 30 000 stars in a nearby region of star formation including the Per OB2 association. The CC is a photometric and astrometric survey with the center at $(\alpha,\delta)_{\rm J2000}=(3^\mathrm{h}51^\mathrm{m}36^\mathrm{s}, 34^\circ36\arcmin)$and radius of 10 degrees. The CC is based on recently published astronomical catalogues, e.g. Hipparcos and Tycho-2, and supplemented by relevant astrophysical data from numerous data sources. The CC is complete down to $V=11.6$mag, in general, and to $V=18.5$mag in the one square degree field centered around the IC 348 cluster. We describe the reduction procedures allowing us to put the data onto one homogeneous system, i.e., the coordinates and proper motions on the Hipparcos system and the photometry on the Johnson system. Typical accuracies of 1... 20 mas for coordinates, 1... 3 mas/yr for proper motions and 0.01... 0.05 mag for B, Vmagnitudes were achieved for the bright CC stars ($V<12$mag). For ≈7000 stars we also collected parallaxes and spectral classes. Down to the completeness limit of the CC, a preliminary proper motion selection yielded more than 1000 probable members of Per OB2. The catalogue is available in electronic form from the CDS at Strasbourg.

The complete data set of Mira variables from the 4th edition of the General Catalog of Variable Stars was analyzed and supplemented by the proper motions and radial velocities presently available for Miras. The resulting sample of 724 Miras with periods between 78 and 612 days contains proper motions reduced to the Hipparcos system, radial velocities and Vmagnitudes. For each of 10 subgroups of Miras divided according to their periods and spectral types, statistical parallaxes were determined by application of five different methods. The mean absolute magnitudes, the spatial velocities and their dispersions as well as the elements of the Galactic orbits were computed as functions of the periods. The ($M_{\rm bol}-\log P$) relation obtained was found to be considerably steeper than the PLR usually assumed for LMC Miras. For Miras in the period range $145{-} 200$days, no significant net motion radially outwards in the Galaxy as suggested by Feast & Whitelock (2000) was found. The predicted frequency of Miras was computed as a function of the visual magnitude.

A compiled catalogue of astrometric and photometric data for about 2200 stars down to $V=16.8^{\rm m}$is presented. The stars are located in an area with a radius of 22.8 arcmin around the extremely young open cluster NGC 6611 with the cluster center at ($\alpha,\delta)_{2000.0}=18^{\rm h}18^{\rm m}40^{\rm s},-13^\circ 47.1'$. The catalogue is based on new reductions of positions, proper motions, and of photometric data that have been published in this region and includes, among others, Hipparcos, AC/GSC and Tautenburg Schmidt plate observations. For the membership determination, spatial and proper motion distributions of stars in the region were analysed. An extinction map of the area and the cluster color-magnitude diagram down to $M_V=+1^{\rm m}$were constructed. The distance of NGC 6611 was determined as $2.14 \pm 0.10$kpc.

A new proper motion study for about 1400 stars with $R < 18$mag in a one square degree region around the young open cluster IC 348 based on Schmidt plates is presented. With an overall accuracy of about 3 mas/yr (2.5 mas/yr, respectively for all stars with $R < 17$mag) we have obtained membership probabilities dividing our sample into three groups: foreground stars, cluster stars and background stars. This separation is also supported by the proper motion distribution with respect to the direction of the standard antapex and an increase of mean proper motion cluster membership probabilities with smaller cluster radii. 240 stars of our sample have cluster membership probabilities larger than 80%. The results are compared with the highly accurate proper motion study of Fredrick (1956) and with other catalogues (PPM, ACT, Hipparcos), all containing only bright stars. From PPM and ACT proper motions a cluster radius of about 30 arcmin can be assumed. Hipparcos proper motions and parallaxes allow the separation of foreground stars, cluster stars and background stars in a somewhat larger region around IC 348 (with distances from the cluster centre of up to 85 arcmin). On the basis of Hipparcos data we calculate a mean distance of $261_{-23}^{+27}$pc for the cluster stars with common proper motion. This is nearly the same distance as obtained by de Zeeuw et al. (1999) for the Per OB 2 association covering more than $15 \times 10$square degrees. The mean proper motion of the cluster IC 348 obtained in our study is in good agreement with that of the highly probable members of the Per OB 2 association according to de Zeeuw et al. (1999). Therefore, we conclude that the cluster IC 348 is embedded in the Per OB 2 association. From our proper motion membership probabilities we found a cluster radius of 10-15 arcmin. There is a concentration of Fredrick's highly probable cluster stars just in between our cluster stars and the distant field stars proper motion distributions. On the other hand, two groups of stars in the proper motion diagram of Fredrick (1956) also seem to be indicated if all his possible cluster members are considered. The X-ray sources from Preibisch et al. (1996) identified with optical counterparts from our proper motion sample show a strong concentration in the proper motion diagram. For these objects which are likely T Tauri stars we obtained high membership probabilities so that we conclude that they belong to the cluster IC 348 and to the Per OB 2 association. The proper motion foreground stars and cluster members which are counterparts of near-infrared (NIR) sources (Lada & Lada 1995) are located in different parts of the $(J-H)-(H-K)$two colour diagram, respectively.

It is textbook knowledge that open clusters are conspicuous members of the thin disk of our Galaxy, but their role as contributors to the stellar population of the disk was regarded as minor. Starting from a homogenous stellar sky survey, the ASCC2.5, we revisited the population of open clusters in the solar neighbourhood from scratch. In the course of this enterprise we detected 130 formerly unknown open clusters, constructed volume and magnitudelimited samples of clusters, redetermined distances, motions, sizes, ages, luminosities and masses of 650 open clusters. We derived the presentday luminosity and mass functions of open clusters not the stellar mass function in open clusters, the cluster initial mass function CIMF and the formation rate of open clusters. We find that open clusters contributed around 40 percent to the stellar content of the disk during the history of our Galaxy. Hence, open clusters are important building blocks of the Galactic disk © 2010 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

Aims. We determine the integrated magnitudes and colours of 650?clusters in optical (BV) and the near-infrared (JHKs) passbands and construct the luminosity functions of the Galactic open clusters in these passbands. Methods. The magnitudes are based on accurate and uniform cluster membership parameters derived from the ASCC-2.5?catalogue and were computed by adding the individual luminosities of the most secure cluster members. To put the computed magnitudes into a uniform and unbiased system, they were corrected for the effect of unseen stars in the ASCC-2.5?. Comparison of the derived parameters with published optical data shows that our integrated magnitudes and colours are accurate within 0.6 and 0.2 mag, respectively. Comparison of cluster distributions over apparent integrated magnitudes with the prediction of a model of cluster counts shows that the sample can be regarded as magnitude-limited down to 8.1, 7.7, 6.3, 5.3, and 5.3?mag in BVJHKs. Results. Out of 650?clusters, 422 (or about 2/3) have received optical integrated magnitudes for the first time. This increases the data bank of BV integrated magnitudes to about 780?clusters, compared to about 350?clusters with BV-magnitudes available in the literature. In the near-infrared, data on cluster-integrated parameters were not available before this study. The cluster sample is found to be magnitude-limited both in the optical and in the near-infrared. This enabled us to construct cluster luminosity functions in five (BVJHKs) photometric passbands. We find that both in the optical and in the NIR the luminosity functions show similar behaviour: a linear increase to fainter magnitudes, which stops at about -2.5?mag in the optical and at -4.0?mag in the NIR. At the brightest magnitudes the luminosity functions exhibit a deficiency with respect to the linear relation. The youngest clusters have flatter luminosity functions in all five passbands with a slope of about 0.2-0.3, while the total cluster sample (all ages) produces luminosity functions with significantly steeper slopes of the order of 0.35-0.50.

Aims. We measure absolute trigonometric parallaxes and proper motions with respect to many background galaxies for a sample of ten ultracool subdwarfs.Methods. Observations were taken in the H-band with the OMEGA2000 camera on the 3.5?m-telescope at Calar Alto, Spain during a time period of 3.5?years. For the first time, the reduction of the astrometric measurements was carried out directly with respect to background galaxies. We obtained absolute parallaxes with mean errors ranging between 1 and 3?mas.Results. With six completely new parallaxes we more than doubled the number of benchmark ultracool (>sdM7) subdwarfs. Six stars in the MKs?vs. J-Ksdiagram fit perfectly to model subdwarf sequences from?M7 to L4 with [M/H] between -1.0 and -1.5, whereas?4 are consistent with a moderately low metallicity ([M/H]=-0.5) from M7 to?T6. All but one of our objects have large tangential velocities between 200 and 320?km?s-1typical of the Galactic halo population.?Our results are in good agreement with recent independent measurements for three of our targets and confirm the previously measured parallax and absolute magnitude MKsof the nearest and coolest (T-type) subdwarf 2MASS 0937+29 with higher accuracy.?For all targets, we also obtained infrared J,H,Ksphotometry at a level of a few milli-magnitudes relative to 2MASS standards.

Aims. We build a catalogue PPM-Extended?(PPMX) on the ICRS system which is complete down to a well-defined limiting magnitude and contains the best presently available proper motions suited for kinematic studies in the Galaxy.Methods. We perform a rigorous weighted least-squares adjustment of individual observations, spread over more than a century, to determine mean positions and proper motions. The stellar content of PPMX is taken from GSC?1.2 supplemented by catalogues like ARIHIP, PPM and Tycho-2 at the bright end. All observations have been weighted according to their individual accuracy. The catalogue has been screened towards rejecting false entries in the various source catalogues.Results. PPM-Extended (PPMX) is a catalogue of 18?088?919?stars containing astrometric and photometric information. Its limiting magnitude is about 15.2 in the GSC photometric system. PPMX consists of three parts: a) a survey complete down to RU= 12.8 in the magnitude system of UCAC2; b) additional stars of high-precision proper motions, and c) all other stars from GSC?1.2 identified in 2MASS. The typical accuracy of the proper motions is 2 mas/y for 66?percent of the survey stars (a) and the high-precision stars?(b), and about 10?mas/y for all other stars. PPMX contains photometric information from ASCC-2.5 and 2MASS.

We present the 2nd version of the Catalogue of Radial Velocities with Astrometric Data (CRVAD-2). This is the result of the cross-identification of stars from the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5) with the General Catalogue of Radial Velocities and with other recently published radial velocity lists and catalogues. The CRVAD-2 includes accurate J2000 equatorial coordinates, proper motions and trigonometric parallaxes in the Hipparcos system, B, V photometry in the Johnson system, spectral types, radial velocities (RVs), multiplicity and variability flags for 54907 ASCC-2.5 stars. We have used the CRVAD-2 for a new determination of mean RVs of 363 open clusters and stellar associations considering their established members from proper motions and photometry in the ASCC-2.5. For 330 clusters and associations we compiled previously published mean RVs from the literature, critically reviewed and partly revised them. The resulting Catalogue of Radial Velocities of Open Clusters and Associations (CRVOCA) contains about 460 open clusters and about 60 stellar associations in the Solar neighbourhood. These numbers still represent less than 30% of the total number of about 1820 objects currently known in the Galaxy. The mean RVs of young clusters are generally better known than those of older ones. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a catalogue (CSOCA) of stars residing in 520 Galactic open cluster sky areas which is the result of the kinematic (proper motion) and photometric member selection of stars listed in the homogeneous All-sky Compiled Catalogue of 2.5Million Stars (ASCC-2.5).We describe the structure and contents of the catalogue, the selection procedure applied, and the proper motion and photometric membership constraints adopted. In every cluster area the CSOCA contains the complete list of the ASCC-2.5 stars regardless of their membership probability. For every star the CSOCA includes accurate J2000 equatorial coordinates, proper motions in the Hipparcos system, BV photometric data in the Johnson system, proper motion and photometric membership probabilities, as well as angular distances from the cluster centers for about 166 000 ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types, multiplicity and variability flags from the ASCC-2.5, and radial velocities with their errors from the Catalogue of Radial Velocities of Galactic Stars with high precision Astrometric Data (CRVAD) are also given. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In order to study a model-dependence of the results achieved in the previous investigations of the Pleiades luminosity function using D'Antona and Mazzitelli (1994) evolutionary tracks, we repeated the computations with the new track system by D'Antona and Mazzitelli (1997). The following main conclusions can be drawn: the new models agree better with observations; the helium abundance needed to fit the Hipparcos distance modulus is reduced to a more reasonable value of Y = 0.31; the cluster age becomes slightly higher and the slope of the initial mass function somewhat lower. The conclusions on the fine structure of the luminosity function do not change significantly due to the application of the new models.

The observations and the plate reduction technique for the determination of positions and absolute proper motions which is used in Potsdam are described. Recent results have shown that an accuracy of about 0.?1 for positions and 0.?7 cent.-1 for proper motions can be achieved both for bright (8m–12m) and faint (16m–18m) stars. Three astrometric programmes using the Tautenburg plates are presented.

Aims.On the basis of the PPMXL catalogue we perform an all-sky census of the Hyades down to masses of about 0.2 m⊙in a region up to 30 pc from the cluster centre.

We report the discovery of a bow shock around the high-mass X-ray binary (HMXB) 4U 1907+09 using the Spitzer Space Telescope 24 μm data (after Vela X-1 the second example of bow shocks associated with HMXBs). The detection of the bow shock implies that 4U 1907+09 is moving through space with a high (supersonic) peculiar velocity. To confirm the runaway nature of 4U 1907+09, we measured its proper motion, which for an adopted distance to the system of 4 kpc corresponds to a peculiar transverse velocity of  ≃ 160 ± 115   km   s-1, meaning that 4U 1907+09 is indeed a runaway system. This also supports the general belief that most HMXBs possess high space velocities. The direction of motion of 4U 1907+09 inferred from the proper motion measurement is consistent with the orientation of the symmetry axis of the bow shock, and shows that the HMXB is running away from the Galactic plane. We also present the Spitzer images of the bow shock around Vela X-1 (a system similar to 4U 1907+09) and compare it with the bow shock generated by 4U 1907+09.

Context.We found in previous studies that standard simple stellar population (SSP) models are unable to describe or explain the colours of Galactic open clusters both in the visible and in the near-infrared (NIR) spectral range. The reason for this disagreement is the continuous nature of the stellar initial mass function (IMF) in clusters, which is the underlying assumption in the SSP models. In reality, the Galactic open clusters are sparsely populated with the brightest stars responsible for integrated fluxes.