WIYN Open Cluster Study – XVI. Optical/infrared photometry and comparisons with theoretical isochrones

We present combined optical/near-infrared photometry ( BVIK ) for six open clusters – M35, M37, NGC 1817, NGC 2477, NGC 2420 and M67. The open clusters span an age range from 150 Myr to 4 Gyr and have metal abundances from  [Fe/H]=−0.27  to +0.09 dex. We have utilized these data to test the robustness of theoretical main sequences constructed by several groups as denoted by the following designations – Padova, Baraffe, Y², Geneva and Siess. The comparisons of the models with the observations have been performed in the  [ M_V , ( B − V )₀], [ M_V , ( V − I )₀]  and  [ M_V , ( V − K )₀]  colour–magnitude diagrams as well as the distance-independent  [( V − K )₀, ( B − V )₀]  and  [( V − K )₀, ( V − I )₀]  two-colour diagrams. We conclude that none of the theoretical models reproduces the observational data in a consistent manner over the magnitude and colour range of the unevolved main sequence. In particular, there are significant zero-point and shape differences between the models and the observations. We speculate that the crux of the problem lies in the precise mismatch between theoretical and observational colour–temperature relations. These results underscore the importance of pursuing the study of stellar structure and stellar modelling with even greater intensity.     

Parameters of the spiral structure of the galaxy from data on open star clusters

We use data on open star clusters (OSCs) from the Homogeneous Catalog of OSC Parameters to determine some of the parameters of the spiral structure of our Galaxy: the pitch angle of the spiral arms \(i = 21\mathop .\limits^ \circ 5\), the pattern speed Ω_p = 20.4 ± 2.5 km s^?1 kpc^?1, and the initial phase of the spiral θ₀ = 206°. The spiral pattern of the Galaxy proves to have been virtually unchanged over the last billion years, and signatures of the concentration of objects toward the spiral arms can be traced back to this age. However, the number of spiral arms in the structure cannot be determined from OSCs.     

开放性地质空间数据库的设计与应用

使用ArcGIS及Access软件设计了开放性地质空间数据库,高效地管理沉积盆地中的海量地质数据,将地质属性数据、地理空间数据、地质图件等在空间位置上有机地结合起来。实践证明,开放性空间数据库可以有效地管理、交流、分析、提取、更新地质信息,在古地理研究中具有重要的辅助作用。随着地质数据的逐渐增加,其适用性、效率性、直观性愈加显著,古地理研究成果逐步接近准确。     

开源GIS支持下的气象灾害风险区划分析

近年来气象灾害频繁发生,给我国人民生活、经济发展带来严重威胁。灾害风险区划分析作为防灾减灾的基础工作,在我国逐渐展开。为探索一种基于开源GIS的县级区域气象灾害风险区划分析技术方案,首先分析开源GIS的发展状况,探讨了开源GIS的质量、性能及应用中需注意的法律因素等,而后以东营市河口区为例,选择QGIS、ILWIS等开源软件,基于区域灾害系统论构建不同气象灾害的分析模型及综合气象灾害分析模型,进行县级区域气象灾害风险区划分析,最终编制实验区气象灾害风险区划分级图。     

The Cepheid impostor HD 18391 and its anonymous parent cluster

New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in order to confirm the nature of the variability previously detected in the star, which lies off the hot edge of the Cepheid instability strip. Small‐amplitude variability at a level of δV = 0.016 ± 0.002 is indicated, with a period of P = 123^d.04 ± 0^d.06. A weaker second signal may be present at P = 177^d.84 ± 0^d.18 with δV = 0.007 ± 0.002, likely corresponding to fundamental mode pulsation if the primary signal represents overtone pulsation (123.04/177.84 = 0.69). The star, with a spectroscopic reddening of E_B–V = 1.02 ± 0.003, is associated with heavily‐reddened B‐type stars in its immediate vicinity that appear to be outlying members of an anonymous young cluster centered ∼10′ to the west and 1661 ± 73 pc distant. The cluster has nuclear and coronal radii of r_n = 3.5′ and R_c = 14′, respectively, while the parameters for HD 18391 derived from membership in the cluster with its outlying B stars are consistent with those implied by its Cepheid‐like pulsation, provided that it follows the semi‐period‐luminosity relation expected of such objects. Its inferred luminosity as a cluster member is M_V = –7.76 ± 0.10, its age (9 ± 1) × 10⁶ years, and its evolutionary mass ∼19 M_⊙. HD 18391 is not a classical Cepheid, yet it follows the Cepheid period‐luminosity relation closely, much like another Cepheid impostor, V810 Cen (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New brown dwarf candidates in the Pleiades

We have performed deep, wide‐field imaging on a ∼0.4 deg² field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R ∼ 22 mag and I ∼ 20 mag, sufficient to detect brown dwarf candidates down to 40 M_J. We found 197 objects, whose location in the (I, R – I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co‐add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two‐dimensional χ² fitting (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The early evolution of the star cluster mass function

Several recent studies have shown that the star cluster initial mass function (CIMF) can be well approximated by a power law, with indications for a steepening or truncation at high masses. This contribution considers the evolution of such a mass function due to cluster disruption, with emphasis on the part of the mass function that is observable in the first ∼1 Gyr. A Schechter type function is used for the CIMF, with a power-law index of −2 at low masses and an exponential truncation at M _*. Cluster disruption due to the tidal field of the host galaxy and encounters with giant molecular clouds flattens the low-mass end of the mass function, but there is always a part of the 'evolved Schechter function' that can be approximated by a power law with index −2. The mass range for which this holds depends on age, τ, and shifts to higher masses roughly as  τ^0.6  . Mean cluster masses derived from luminosity-limited samples increase with age very similarly due to the evolutionary fading of clusters. Empirical mass functions are, therefore, approximately power laws with index −2, or slightly steeper, at all ages. The results are illustrated by an application to the star cluster population of the interacting galaxy M51, which can be well described by a model with   M _*= (1.9 ± 0.5) × 10⁵ M_⊙  and a short (mass-dependent) disruption time destroying M _* clusters in roughly a Gyr.