日本早白垩世非海相软体动物群组合

日本具有4个标志早白垩世的半咸水和淡水软体动物群组合:欧特里沃期—早巴列姆期的立川和菖蒲动物群组合,晚巴列姆期的濑林动物群组合和早阿普特期动物群组合。纯非海相下白垩统大量分布于日本东北部(本州岛北部)的部分地区,以及日本中部、西南部的内带。在日本东北部的部分地区以及日本西南部的外带(四国岛与本州岛中部),从巴列姆阶下部往上有一些海相地层夹在非海相层中,这为研究非海相动物群提供了准确的年代依据。生物组合之间在种一级水平上的区别,主要由环境的差异而引起。1)立川动物群组合(欧特里沃期—早巴列姆期),含有半咸水Hayamina naumanni组合和淡水Megasphaerioides okurodaniensis组合;手取群的Okurodani、Kuwajima和Izuki三个组内的淡水软体动物可与本动物组合相对比;2)菖蒲动物群组合(欧特里沃期—早巴列姆期),含有Eomiodon nipponicus组合;3)濑林动物群组合(晚巴列姆期),含有Costocyrena radiatostriata组合,可与手取群的Kitadani组和日本东北部关门群的Sengoku组以及Monomiyama组的淡水软体动物组合相对比;4)日比原动物群组合(早阿普特期),含有Costocyrena minor组合,关门群Wakamiya组的淡水软体动物可与本动物组合相对比。     

A study of the young open cluster Collinder 359

We present the first deep, optical, wide‐field imaging survey of the young open cluster Collinder 359, complemented by near‐infrared follow‐up observations. This study is part of a large programme aimed at examining the dependence of the mass function on environment and time. We have surveyed 1.6 square degree in the cluster in the I and z filters with the CFH12K camera on the Canada‐France‐Hawaii 3.6m telescope down to completeness and detection limits in both filters of 22.0m and 24.0m, respectively. Based on their location in the optical (I‐z ,I ) colour‐magnitude diagram, we have extracted new cluster member candidates in Collinder 359 spanning 1.3‐0.04 M_⊙, assuming an age of 100 Myr and a distance of 450 pc for the cluster.We have used the 2MASS database as well as our own near‐infrared photometry to confirm the membership of the optically‐selected cluster candidates. Additionally, we have obtained optical spectroscopy and employed chromospheric activity as a further criterion to assess the membership of candidates. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New stellar and substellar candidate members of the Coma Berenices open star cluster

We present the results of a survey of the open star cluster Melotte 111 in Coma Berenices, undertaken using the USNO‐B1.0 and 2MASS Point Source catalogues. On the basis of their astrometric and photometric properties, we have identified 60 new candidate members with masses in the range 1.007 < M < 0.269 M_⊙. We estimate a membership probability for each by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being cluster members, which if they are confirmed as members, more than doubles the number of known cluster members. We also have I and Z photometry for 100 low mass candidate members of the cluster, 13 of which we suggest may be brown dwarfs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The lower mass function of young open clusters: clues to (sub)stellar formation

We report the present day mass functions (PDMFs) of 4 young open clusters over a mass range from 30 Jupiter masses to 3M_ . Three of these clusters have been chosen to have a similar age of ∼100 Myr. Their PDMFs are remarkably similar and are comparable to the field mass function. This suggests little impact of initial conditions (stellar density, metallicity) on the mass distribution and raises some issues concerning the currently debated star and brown dwarf formation theories. The fourth cluster is older (600 Myr) which allow us to investigate the effect of the cluster dynamical evolution on the shape of the mass function. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Praesepe – two merging clusters?

A membership catalogue for Praesepe was compiled and split into four mass bins. A contour plot indicates the presence of a subcluster some 3 pc from the centre of the cluster, of approximately 30 M_⊙. A tidally truncated King profile was fitted to the remainder of the cluster and the tidal radius is found to be 12.1 pc; the mass of the cluster (excluding the subcluster) is 630 M_⊙. From the calculated velocity dispersions we find that the cluster appears to have too much kinetic energy and should be rapidly disintegrating. X-ray data suggest that there may be an age spread between the main core stars and the subcluster stars. This leads us to the conclusion that Praesepe is two merging clusters.     

The region of Collinder 121

The distribution of bright B-type stars in a field with a radius of 5° centred at the Galactic open cluster Cr 121 is studied utilizing Strömgren and H β photometry. All PPM stars earlier than spectral type A0 are used, revealing a loose nearby structure at a distance of 660–730 pc, and a compact more distant group, which appears to be a genuine cluster: Cr 121. Based on similar coordinates, distances and positions on the colour–magnitude (CM) and Hertzsprung–Russell (HR) diagrams, 11 photometric cluster members are selected at a mean distance of 1085(±41 standard error) pc. The results are discussed in the light of both classical and Hipparcos points of view.     

Star formation in the Cepheus Flare region – II. A-type stars associated with IRAS sources

In order to reveal the star-forming history of the molecular cloud complex we studied the intermediate mass stellar population in the Cepheus Flare region. (Paper I dealt with the distance and the young stellar object candidates of the region.) Correlating the IRAS Point Source Catalogue and Faint Source Catalogue positions with those of 1214 B8–A8 and 1760 F0–F5 type stars brighter than     and classified during an objective prism survey, we identified 19 stars showing far-infrared excess emission in the Cepheus Flare region. In addition to the 16 stars whose counterparts are given in the IRAS catalogues, we found three more stars with infrared excess not recognized before. In order to identify the young medium-mass stars associated with the Cepheus Flare molecular clouds we observed the optical spectra of the IR-excess stars, and using published optical photometry and the IRAS data we examined their spectral energy distributions (SEDs) and IRAS two-colour diagram. The observations resulted in the discovery of a new Herbig Ae/Be star, BD +68°1118, coinciding with IRAS 21169+6842. More evolved HAe/Be stars may be SAO 19953, BD +67°1314 and BD +69°1231, whose H α lines showed weak emission components. Possible β Pictoris- or Vega-type stars may be HD 203854, HD 212826 and HD 216486, whereas the far-infrared fluxes at the positions of BD +72°1018, HD 210806 and HD 217903 can be attributed to the heating of the interstellar environment. We used distances and radial velocities of the stars derived from the spectroscopy and published optical photometry as indicators of their relations to the clouds. Information on the environment of the observed stars deduced from the diffuse interstellar band at λ 6613 is briefly discussed.     

Period–luminosity relations for red supergiant variables – I.The calibration

We present CCD photometry of red supergiant long-period variables (LPVs) in the Per OB1 association, the Large Magellanic Cloud (LMC) and M33. The photometry was obtained in the Kron–Cousins R and I bandpasses and in a narrow bandpass ( λ ₀=8250 Å, FWHM=300 Å) chosen to avoid TiO bands in the spectral energy distribution of the LPVs. Because the strength of the TiO bands varies greatly with temperature, which varies with the phase of an LPV, avoiding TiO reduces the amplitude of the photometric variations seen in LPVs. The result is a lower dispersion and a well defined period–luminosity (PL) relation.
For the LMC sample we find an rms dispersion of 0.27 mag in the narrow-band PL relation and slightly larger dispersions for the LPVs in Per OB1 and M33. This dispersion is comparable to that of the Cepheid PL relation at similar wavelengths. Adopting a distance modulus of 18.5±0.1 mag for the LMC, we obtain distance moduli of 11.68±0.15 mag for Per OB1 and 24.85±0.13 mag for M33. These distances agree well with those based on main sequence fitting for Per OB1 and the Cepheid distance for M33. Since LPVs are ∼ 5 times more common than Cepheids and have a well defined PL relation, LPVs provide a promising method for estimating Galactic and extra galactic distances.     

A population of very young brown dwarfs and free-floating planets in Orion

We describe the results of a very deep imaging survey of the Trapezium cluster in the IJH bands, using the UKIRT high-resolution camera UFTI. Approximately 32 per cent of the 515 point sources detected are brown dwarf candidates, including several free-floating objects with masses below the deuterium-burning (planetary) threshold at 0.013 M_⊙, which are detectable because of their extreme youth. We have confidence that almost all the sources detected are cluster members, since foreground contamination is minimal in the 33-arcmin² area surveyed, and the dense backdrop of OMC-1 obscures all background stars at these wavelengths. Extinction is calculated from the ( J − H ) colours, permitting accurate luminosity estimates, and temperatures are derived from the dereddened ( I − J ) colours. There is some evidence for a cut-off in the luminosity function below the level corresponding to several Jupiter masses, which may represent the bottom end of the initial mass function . Since star formation is complete in the Trapezium, this limit could have wide significance, if confirmed. However, it could well be an effect of the dispersal of the molecular cloud by the central O-type stars, a process for which the time-scale will vary between star formation regions.     

Abundance analysis of the long period southern cepheid RZ Vel

The long period classical cepheid RZ Vel (HD 73502) is known to be a member of an OB association, Vel OB1 in Vela, and a high metallicity is ascribed to it by the photometric work of Eggen (1982). We have done an abundance analysis for this long period (P = 20.4 days) and hence young (age ≈ 1.80×10⁷ yr) classical cepheid using high resolution CCD spectra with good S/N ratio. We have used a detailed model atmosphere method to derive the abundances of the light elements C, O, A1, S and of many Fe-peak elements and a few s-process elements. Our present work indicates near solar abundance for most of the elements for RZ Vel and hence we do not confirm the high metallicity derived photometrically by Eggen (1982) for this star