Spatial variability of ontogenetic patterns in habitat associations by coral reef fishes (Moorea lagoon – French Polynesia)

The present study explored ontogenetic shifts in habitat associations by coral reef fishes between recently settled juvenile and adult life stages (Moorea Island: Tiahura and Papetoai sites). Visual censuses highlighted four ontogenetic patterns in habitat associations: (1) no change in habitat associations between the juvenile and adult stages; (2) a decrease in the number of habitats used by adults compared to juveniles; (3) an increase in the number of habitats used during the adult stage; and (4) use of nursery areas by juveniles followed by an extensive movement to an entirely different adult habitat. The comparative analysis of spatial distribution of fish at Tiahura and Papetoai highlighted no-spatial variability in ontogenetic patterns (i.e., 10 of the 15 recorded species have spatial consistency in ontogenetic patterns). Overall, the shifts in habitat associations are of interest in the perspective of understanding flexibility and adaptation capability of coral reef fish, at least at the settlement time.     

Surface distribution of the luminous and early-type emission stars in the OB Vulpecula associations

Die Flächenverteilung von hellen und frühen Emissionssternen in der Milchstraße in Vulpecula wurde erhalten. Die Ergebnisse deuten auf Inkonsistenz der Gebiete größter Konzentration dieser Sternarten mit den veröffentlichten Positionen von OB-Assoziationen.     

Flare star activity in the open cluster Alpha Persei

The results from a flare star investigation in the open cluster Alpha Persei are presented. Photographic flare star monitoring and CCD photometry of the discovered 4 new flare stars are made. The flare star activity phenomenon is restricted to the classical flare stars (UV Ceti type) as classified in the GCVS. The V/V‐I diagramme of the members of the cluster with the locations of the considered flare stars is given. Most of the flare stars are probable cluster members. The flare frequency determined from the Rozhen flare star monitoring is very low–one flare event occurs for 38.5 hours effective observing time. Comparison with the flare activity of the Pleiades is made because of the small difference in the age and distance of the clusters.     

DBSCAN Clustering Algorithm for the Detection of Nearby Open Clusters Based on Gaia-DR2two

In this paper, we attempt to use the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) clustering algorithm to detect nearby open clusters based on the Gaia Data Release 2 (Gaia-DR2). We select 594284 stars (within a distance of 100 pc to the Sun) from the Gaia-DR2 catalog, and construct a five-dimensional phase space (three-dimensional spatial position and two-dimensional proper motion) in order to obtain reliable cluster members. At the data preprocessing stage, we normalize each dimension of data to the [0, 1] interval in order to avoid the effect of inconsistent units. Then, we use the k-dist graphs to determine the input parameters of the DBSCAN Algorithm. Finally, we obtain 133 reliable members using the DBSCAN algorithm, which correspond to two open clusters—Hyades and Coma. According to these cluster members, the distances to the Hyades and Coma clusters are determined to be (6.5 ± 0.3) pc and (4.9 ± 0.4) pc, respectively.     

The distribution of bright OB stars in the Canis Major–Puppis–Vela region of the Milky Way

The picture of the young stellar groups in the Canis Major–Puppis–Vela (215°< l <275°) section of the Milky Way is studied and updated utilizing uvbyβ photometry of intrinsically luminous OB stars. We use all data from the literature to create a sample with 98 per cent completeness to 9.5 mag.
The very dense low reddened OB association CMa OB1 is confirmed at a distance of 0.99 (±0.05 s.e. ) kpc. Towards Puppis the brightest intrinsically luminous stars do not reveal Pup OB1 and Pup OB2. In the same direction, we separate two small groups, previously related to the association surrounding NGC 2439 at 3.5–4.5 kpc. The first one contains four highly reddened B-type supergiants situated in front of the cluster at 1.03 (±0.14 s.e.) kpc – much closer to the Sun than has been estimated before. The second one lies north-west from the cluster at 3.2 (±0.23 s.e.) kpc according to our estimate. In the direction to Vela, the bright OB stars are apparently embedded in a dust cloud and spread out between 0.3 and 2.5 kpc, forming clumps over this distance range.
In general, the prominent apparent young structures delineated by the brightest intrinsically luminous OB stars in the directions of Canis Major and Vela are some 20–25 per cent closer to the Sun than has previously been thought. This is in agreement with the Hipparcos results for the Galactic OB associations, and is highly likely to be caused by the overestimation of the spectroscopic distances used in the previous studies.     

Dense star clusters as the sources of gamma-ray-burst progenitors

Two independent sets of arguments lead us to conclude that the progenitors of superintense bursts (with an energy yield larger than that for ordinary supernovae by one or two orders of magnitude) are born in massive dense star clusters, but generally flare up only after they have left the cluster; these are the same objects that are the progenitors of gamma-ray bursts (GRBs). Each of the giant stellar arcs which are grouped into multiple systems of stellar complexes in the LMC and NGC 6946 could only be produced by a single powerful energy release near its center. The progenitors of these systems of arc-shaped stellar complexes must have had a common source nearby, and it could only be a massive star cluster. Such clusters are actually known near both systems. On the other hand, calculations of the dynamical evolution of star clusters show that close binary systems of compact objects are formed in the dense central parts of the clusters and are then ejected from them during triple encounters. Mergers of the components of such systems are believed to be responsible for GRBs. Since their progenitors are ejected from the cluster before merging, the arc-shaped stellar complexes produced by GRBs are observed near (but not around) the parent clusters. If a considerable fraction of the GRB progenitors are formed as a result star encounters in massive star clusters, and if the GRBs themselves trigger star formation near the parent clusters, then observations of GRBs in star-forming regions are consistent with their origin during mergers of pairs of compact objects.     

疏散星团M67成员星的两点相关分析

对著名老年疏散星团M67的成员星进行了两点相关函数分析,讨论了团星的空间分布情况及质量分层现象.结果表明:M67成员星的两点相关函数能很好地用幂律形式来描述,其成员星在空间分布上具有显著的成团性和自相似性.对M67不同光度成员星两点自相关函数的分析表明,亮星相比暗星具有更强的相关强度和更大的相关指数,说明在星团内部出现了明显的空间质量分层现象,亮星比暗星具有更强的成团性和更大的中心聚度.不同光度成员星间两点交叉相关函数的分析进一步表明,不同光度成员星之间并没有表现出绝对的空间质量分层现象,亮星和暗星在空间分布上相互交织、相互渗透.