S. Roser

Astrophysical parameters of galactic open clusters

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.

Global survey of star clusters in the Milky Way - II. The catalogue of basic parameters

Context. Although they are the main constituents of the Galactic disk population, for half of the open clusters in the Milky Way reported in the literature nothing is known except the raw position and an approximate size. Aims. The main goal of this study is to determine a full set of uniform spatial, structural, kinematic, and astrophysical param eters for as many known open clusters as possible. Methods. On the basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing pipeline to determine kinematic and photometric membership probabilities for stars in a cluster region. Results. For an input list of 3784 targets from the literature, we confi rm that 3006 are real objects, the vast majority of them are open clusters, but associations and globular clusters are also p resent. For each confirmed object we determined the exact pos ition of the cluster centre, the apparent size, proper motion, distance , colour excess, and age. For about 1500 clusters, these basic astrophysical parameters have been determined for the first time. For the bu lk of the clusters we also derived the tidal radius. We estimated additionally average radial velocities for more than 30% of the confirmed clusters. The present sample (called MWSC) reaches both the central parts of the Milky Way and its outer regions. It is almost complete up to 1.8 kpc from the Sun and also covers neighbouring spiral arms. However, for a small subset of the oldest open clusters (log t & 9) we found some evidence of incompleteness within about 1 kpc from the Sun.

109 new Galactic open clusters

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.

Astrophysical supplements to the ASCC‐2.5: Ia. Radial velocities of ∼55000 stars and mean radial velocities of 516 Galactic open clusters and associations

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)

Tidal radii and masses of open clusters

Context. In a previous paper we obtained King’s parameters for 236 of 650 Galactic open clusters identified in the ASCC-2.5. Aims. Estimating tidal radii by use of observable parameters available for all clusters. Bias-free results are required. Methods. We use methods of stellar statistics and develop a semi-empirical model of open clusters. Results. We check two effects impacting the determination of tidal radii from a fitting of King’s profiles to the observed density distribution, i.e., ellipticity of open clusters and a bias depending on distances. Though a typical cluster has an elliptical form, the effect is rather weak to produce a prominent bias in the resulting tidal radii. In contrast, a distance dependent bias is not negligible and can cause a systematic underestimation of tidal radii computed with ASCC-2.5 data by a factor of two for the most distant clusters of our sample. This finding is used to correct the original results for 236 clusters and to extend the system of tidal radii and masses to all 650 clusters. We found that the semi-major axis of the projected distribution of cluster members on the sky is a parameter suited to estimate tidal radii of open clusters of our sample. No systematic differences are found between measured and calibrated tidal radii. From the comparison with mass estimates based on star counts and on the assumption of the Salpeter IMF, empirical evidence is obtained for an evolution of cluster mass functions starting in young clusters. Conclusions. The set of homogeneous parameters available for all clusters of our sample is extended by tidal radius and mass. Within 850 pc where our sample is complete, the distributions of tidal radii and masses peak at rt ≈ 6p c and logMc/m� ≈ 2.5, respectively. In young open clusters, the mass distributions show differences to the Salpeter IMF, and this discrepancy increases with cluster age.

On the origin of field O-type stars

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. Methods. We apply the epicycle theory to back-trace the orbits of O-type stars and of candidate parent open clusters. Results. From the 370 O-stars in the “Galactic O star catalog v 2.0” (GOSV2) we have investigated 93 stars classified as field ,a nd found the origin for 73 of them in 48 open clusters younger than 30 Myr. Only for 32 stars or about 9% of all O-stars from this catalogue is the question of their origin in groups not solved; some of them may have originated in isolation or may have disintegrated the group in which they formed. Fifty percent of the young open clusters (age < 30 Myr) in the “Catalogue of Open Cluster Data” (COCD) have O-stars as members, or have ejected at least one O-star in the first 10 Myr of their life, or both. During this period the

Towards absolute scales for the radii and masses of open clusters

Aims. In this paper we derive tidal radii and masses of open clusters in the nearest kiloparsecs around the Sun. Methods. For each cluster, the mass is estimated from tidal radii determined from a fitting of three-parameter King profiles to the observed integrated density distribution. Different samples of members are investigated. Results. For 236 open clusters, all contained in the catalogue ASCC-2.5, we obtain core and tidal radii, as well as tidal masses. The distributions of the core and tidal radii peak at about 1.5 pc and 7–10 pc, respectively. A typical relative error of the core radius lies between 15% and 50%, whereas, for the majority of clusters, the tidal radius was determined with a relative accuracy better than 20%. Most of the clusters have tidal masses between 50 and 1000 m� , and for about half of the clusters, the masses were obtained with a relative error better than 50%.

A Strategy for Identifying the Grid Stars for the Space Interferometry Mission

We present a strategy to identify several thousand stars that are astrometrically stable at the microarcsecond level for use in the Space Interferometry Mission (SIM) astrometric grid. The requirements on the grid stars make this a rather challenging task. Taking a variety of considerations into account, we argue for K giants as the best type of stars for the grid, mainly because they can be located at much larger distances than any other type of star owing to their intrinsic brightness. We show that it is possible to identify suitable candidate grid K giants from existing astrometric catalogs. However, double stars have to be eliminated from these candidate grid samples, since they generally produce much larger astrometric jitter than tolerable for the grid. The most efficient way to achieve this is probably by means of a radial velocity survey. To demonstrate the feasibility of this approach, we repeatedly measured the radial velocities for a preselected sample of 86 nearby Hipparcos K giants with precisions of 5-8 m s–1. The distribution of the intrinsic radial velocity variations for the bona fide single K giants shows a maximum around 20 m s–1, which is small enough not to severely affect the identification of stellar companions around other K giants. We use the results of our observations as input parameters for Monte Carlo simulations on the possible design of a radial velocity survey of all grid stars. Our favored scenario would result in a grid which consists to 68% of true single stars and to 32% of double or multiple stars with periods mostly larger than 200 years, but only 3.6% of all grid stars would display astrometric jitter larger than 1 μas. This contamination level is probably tolerable.

The initial luminosity and mass functions of the Galactic open clusters

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 initial luminosity 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-2.5 catalogue. 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-2.5 is 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 IMV = [−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 IMV ≈− 2.5. The slope of this linear portion is a = 0.41 ± 0.01, which agrees perfectly with the slope deduced for star cluster observations in ⎛ ⎜ �

A deep all-sky census of the Hyades

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. Methods. We use the proper motions from PPMXL in the convergent point method to determine probable kinematic members. From 2MASS photometry and CMC14r ′ -band photometry, we derive empirical colour-absolute magnitude diagrams and, finally, determine photometric membership for all kinematic candidates. Results. This is the first deep (r ′ ≤ 17) all-sky survey of the Hyades allowing a full three-dimensional analysis of the cluster. The survey is complete down to at least MKs = 7.3 or 0.25 m⊙. We find 724 stellar systems co-moving with the bulk Hyades sp ace velocity, which represent a total mass of 435 m⊙. The tidal radius is about 9 pc, and 275 m⊙ (364 systems) are gravitationally bound. This is the cluster proper. Its mass density profile is perfectly fitted b y a Plummer model with a central density of 2.21 m⊙pc −3 and a core radius of rco = 3.10 pc, while the half-mass radius is r h = 4.1 pc. There are another 100 m⊙ in a volume between one and two tidal radii (halo), and another 60 m⊙ up to a distance of 30 pc from the centre. Strong mass segregation is inherent in the cluster. The present-day luminosity and mass functions are noticeably different in various parts of the cluster (core, corona, halo, an d co-movers). They are strongly evolved compared to presently favoured initial mass functions. The analysis of the velocity dispersion of the cluster shows that about 20% of its members must be binaries. As a by-product, we find that presently available theoretical isochrones a re not able to adequately describe the near-infrared colour-absolute magnitude relation for those cluster stars that are less mas sive than about 0.6 m⊙.