Three-body treatment of the penetration through the Coulomb field of a two-fragment nucleus

On the basis of the Faddeev integral equations method and the Watson- Feshbach concept of the effective (optical) interaction potential, the first fully consistent three-body approach to the description of the penetration of a charged particle through the Coulomb field of a two-particle bound complex (composed of one charged and one neutral particles) has been developed. A general formalism has been elaborated and on its basis, to a first approximation in the Sommerfeld parameter, the influence of the nuclear structure on the probability of the penetration of a charged particle (the muon, the pion, the kaon and the proton) through the Gamow barrier of a two-fragment nucleus (the deuteron and the two lightest lambda hypernuclei, lambda hypertriton and lambda hyperhelium-5, has been calculated and studied.Comment: LaTeX, 30 pages, 4 eps figure

Formation of adsorbate structures induced by external electric field in plasma-condensate systems

We present a new model of plasma-condensate system, by taking into account an anisotropy of transference reactions of adatoms between neighbor layers of multi-layer system, caused by the strength of the electric field near substrate. We discuss an influence of the strength of the electric field onto first-order phase transitions and conditions for adsorbate patterning in plasma-condensate systems. It is shown that separated pyramidal-like multi-layer adsorbate islands can be formed in the plasma-condensate system if the strength of the electric field near substrate becomes larger tan the critical value, which depends on the interaction energy of adsorbate and adsorption coefficient.Comment: 8 pages, 8 figure

Global survey of star clusters in the Milky Way: III. 139 new open clusters at high Galactic latitudes

Context. An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (t � 1 Gyr) open clusters in the solar neighbourhood (d � 1 kpc). Aims. To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes | b | > 20°. Methods. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters. Results. In total we discovered 782 density enhancements, 524 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [yr]) < 9.7, distances d< 3 kpc, and distances from the Galactic plane 0.3 <Z< 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size

Population analysis of open clusters: radii and mass segregation

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. Methods: Using statistical methods, we study the distribution of linear sizes as a function of spatial position and cluster age. We also examine statistically the distribution of stars of different masses within clusters as a function of the cluster age. Results: No significant dependence of the cluster size on location in the Galaxy is detected for younger clusters (< 200 Myr), whereas older clusters inside the solar orbit turned out to be, on average, smaller than outside. Also, small old clusters are preferentially found close to the Galactic plane, whereas larger ones more frequently live farther away from the plane and at larger Galactocentric distances. For clusters with (V - M_V) < 10.5, a clear dependence of the apparent radius on age has been detected: the cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age of 1 Gyr. A detailed analysis shows that this observed effect can be explained by mass segregation and does not necessarily reflect a real decrease of cluster radii. We found evidence for the latter for the majority of clusters older than 30 Myr. Among the youngest clusters (between 5 and 30 Myr), there are some clusters with a significant grade of mass segregation, whereas some others show no segregation at all. At a cluster age between 50 and 100 Myrs, the distribution of stars of different masses becomes more regular over cluster area. In older clusters the evolution of the massive stars is the most prominent effect we observe.Comment: 14 pages, 12 figures, accepted for publication in Astronomy & Astrophysic

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

On the determination of age and mass functions of stars in young open star clusters from the analysis of their luminosity functions

Based on the CCD observations of remote young open clusters NGC 2383, NGC 2384, NGC 4103, NGC 4755, NGC 7510 and Hogg 15, we constructed their observed luminosity functions (LFs). The observed LFs are corrected for field star contamination determined with the help of galactic star count model. In the case of Hogg 15 and NGC 2383 we also considered the additional contamination from neighbouring clusters NGC 4609 and NGC 2384 respectively. These corrections provided the realistic pattern of cluster LF in the vicinity of the MS turn on point and at fainter magnitudes, revealed the so called H-feature arising due to transition of the Pre-MS phase to MS, which is dependent on the cluster age. The theoretical LFs were constructed representing a cluster population model with continuous star formation for a short time scale and a power law Initial Mass Function (IMF) and these were fitted to the observed LF. As a result we are able to determine for each cluster a set of parameters, describing cluster population (the age, duration of star formation, IMF slope and percentage of field star contamination). It was found that in spite of the non-monotonic behaviour of observed LFs, cluster IMFs can be described as the power law functions with slopes similar to Salpeter's value. The present MS turn on cluster ages are several times lower than those derived from the fitting of theoretical isochrones to the turn off region of the upper Main Sequences.Comment: 17 pages, 5 figures, To appear in MNRA

The evolution of luminosity, colour and the mass-to-luminosity ratio of Galactic open clusters: comparison of discrete vs. continuous IMF models

(abridged) 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 NIR spectral range. (...) We construct a numerical SSP-model, with an underlying Salpeter IMF, valid within an upper $m_u$ and lower $m_l$ stellar mass range, and with total masses $M_c=10^2...10^4\,m_\odot$ typical of open clusters. We assume that the mass loss from a cluster is provided by mass loss from evolved stars and by the dynamical evaporation of low-mass members due to two-body relaxation. The data for the latter process were scaled to the models from high-resolution N-body calculations. We also investigate how a change of the $m_l$-limit influences magnitudes and colours of clusters of a given mass and derive a necessary condition for a luminosity and colour flash. The discreteness of the IMF leads to bursts in magnitude and colour of model clusters at moments when red supergiants or giants appear and then die. The amplitude of the burst depends on the cluster mass and on the spectral range; it is strongly increased in the NIR compared to optical passbands. In the discrete case, variations of the parameter $m_l$ are able to substantially change the magnitude-age and $M/L$-age relations. For the colours, the lowering of $m_l$ considerably amplifies the discreteness effect. The influence of dynamical mass loss on colour and magnitude is weak, although it provides a change of the slopes of the considered relations, improving their agreement with observations. For the Galactic open clusters we determined luminosity and tidal mass independent of each other. The derived mass-to-luminosity ratio shows, on average, an increase with cluster age in the optical, but gradually declines with age in the NIR. The observed flash statistics can be used to constrain $m_l$ in open clusters.Comment: 15 pages, 13 figures, accepted for publication in Astronomy and Astrophysic

An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters

We present a simple analytical description of the disruption of star clusters in a tidal field, which agrees excellently with detailed N-body simulations. The analytic expression can be used to predict the mass and age histograms of surviving clusters for any cluster initial mass function and any cluster formation history. The method is applied to open clusters in the solar neighbourhood, based on the new cluster sample of Kharchenko et al. From a comparison between the observed and predicted age distributions in the age range between 10 Myr to 3 Gyr we find the following results: (1) The disruption time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr. This is a factor 5 shorter than derived from N-body simulations of clusters in the tidal field of the galaxy. (2) The present starformation rate in bound clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7 10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher than before and after. (4) The present star formation rate in bound clusters is half as small as that derived from the study of embedded clusters. The difference suggests that half of the clusters in the solar neighbourhood become unbound within 10 Myr. (5) The most massive clusters within 600 pc had an initial mass of 3*10^4 M_sun. This is in agreement with the statistically expected value based on a cluster initial mass function with a slope of -2, even if the physical upper mass limit is as high as 10^6 M_sun.Comment: 14 pages, 15 figures, to appear in Astronomy & Astrophysic

Towards absolute scales of 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-parametric King's 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 $m_\odot$, and for about half of the clusters, the masses were obtained with a relative error better than 50%.Comment: 11 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Global survey of star clusters in the Milky Way IV. 63 new open clusters detected by proper motions

AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey, Schmeja et al. applied photometric filters to the 2MASS to find new cluster candidates that were subsequently confirmed or rejected by the MWSC pipeline. To further extend the MWSC census, we aimed at discovering new clusters by conducting an almost global search in proper motion catalogues as a starting point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4 for comparison and verification of the proper motions. For 441 circular proper motion bins (radius 15 mas/yr) within $\pm$50 mas/yr, the sky outside a thin Galactic plane zone ($|b|$$<$5$^{\circ}$) was binned in small areas ('sky pixels') of 0.25$\times$0.25 deg$^2$. Sky pixels with enhanced numbers of stars with a certain common proper motion in both catalogues were considered as cluster candidates. After visual inspection of the sky images, we built an automated procedure that combined these representations of the sky for neighbouring proper motion subsamples after a background correction. RESULTS: About half of our 692 candidates overlapped with known clusters (46 globular and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open clusters confirmed by the MWSC pipeline. They occupy predominantly the two inner Galactic quadrants and have apparent sizes and numbers of high-probable members slightly larger than those of the typically small MWSC clusters, whereas their other parameters (ages, distances, tidal radii) fall in the typical ranges. As our search aimed at finding compact clusters, we did not find new very nearby (extended) clusters. (abridged)Comment: 14 pages, 14 figures, accepted for publication in Astronomy and Astrophysic