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121.
T. Eisenbeiss M. Moualla M. Mugrauer T.O.B. Schmidt St. Raetz R. Neuhuser Ch. Ginski M.M. Hohle A. Koeltzsch C. Marka W. Rammo A. Reithe T. Roell M. Vako 《Astronomische Nachrichten》2009,330(5):439-458
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 MJ. 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) 相似文献
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M. Gieles 《Monthly notices of the Royal Astronomical Society》2009,394(4):2113-2126
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 τ0.6 . 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. 相似文献
124.
We report the identification, from a photometric, astrometric and spectroscopic study, of a massive white dwarf member of the nearby, approximately solar metallicity, Coma Berenices open star cluster (Melotte 111). We find the optical to near-infrared energy distribution of WD 1216+260 to be entirely consistent with that of an isolated DA and determine the effective temperature and surface gravity of this object to be T eff = 15 739+197 −196 K and log g = 8.46+0.03 −0.02 . We set tight limits on the mass of a putative cool companion, M ≳ 0.036 M⊙ (spatially unresolved) and M ≳ 0.034 M⊙ (spatially resolved and a ≲ 2500 au ). Based on the predictions of CO core, thick H layer evolutionary models we determine the mass and cooling time of WD 1216+260 to be M WD = 0.90 ± 0.04 M⊙ and τcool = 363+46 −41 Myr , respectively. For an adopted cluster age of τ= 500 ± 100 Myr we infer the mass of its progenitor star to be M init = 4.77+5.37 −0.97 M⊙ . We briefly discuss this result in the context of the form of the stellar initial mass–final mass relation. 相似文献
125.
F. Elias E. J. Alfaro J. Cabrera-Caño 《Monthly notices of the Royal Astronomical Society》2009,397(1):2-13
We perform a study of the spatial and kinematical distribution of young open clusters in the solar neighbourhood, discerning between bound clusters and transient stellar condensations within our sample. Then, we discriminate between Gould Belt (GB) and local Galactic disc (LGD) members, using our previous estimate of the structural parameters of both systems obtained from a sample of O-B6 Hipparcos stars. Single membership probabilities of the clusters are also calculated in the separation process. Using this classified sample, we analyse the spatial structure and the kinematic behaviour of the cluster system in the GB. The two star formation regions that dominate and give the GB its characteristic-inclined shape show a striking difference in their content of star clusters: while Ori OB1 is richly populated by open clusters, not a single one can be found within the boundaries of Sco OB2. This is mirrored in the velocity space, translating again into an abundance of clusters in the region of the kinematic space populated by the members of Ori OB1, and a marginal number of them associated with Sco OB2. We interpret all these differences by characterizing the Orion region as a cluster complex typically surrounded by a stellar halo, and the Sco-Cen region as an OB association in the outskirts of the complex. In the light of these results, we study the nature of the GB with respect to the optical segment of the Orion Arm, and we propose that the different content of star clusters, the different heights over the Galactic plane and the different residual velocities of Ori OB1 and Sco OB2 can be explained in terms of their relative position to the density maximum of the Local Arm in the solar neighbourhood. Although morphologically intriguing, the GB appears to be the result of our local and biased view of a larger star cluster complex in the Local Arm, that could be explained by the internal dynamics of the Galactic disc. 相似文献
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128.
Jonathan Irwin Simon Hodgkin Suzanne Aigrain Jerome Bouvier Leslie Hebb Estelle Moraux 《Monthly notices of the Royal Astronomical Society》2008,383(4):1588-1602
We report on the results of an I -band time-series photometric survey of NGC 2547 using the MPG/ESO 2.2-m telescope with Wide Field Imager, achieving better than 1 per cent photometric precision per data point over 14 ≲ I ≲ 18 . Candidate cluster members were selected from a V versus V − I colour–magnitude diagram over 12.5 < V < 24 (covering masses from 0.9 M⊙ down to below the brown dwarf limit), finding 800 candidates, of which we expect ∼330 to be real cluster members, taking into account contamination from the field (which is most severe at the extremes of our mass range). Searching for periodic variations in these gave 176 detections over the mass range 0.1 ≲ M /M⊙ ≲ 0.9 . The rotation period distributions were found to show a clear mass-dependent morphology, qualitatively intermediate between the distributions obtained from similar surveys in NGC 2362 and 2516, as would be expected from the age of this cluster. Models of the rotational evolution were investigated, finding that the evolution from NGC 2362 to 2547 was qualitatively reproduced (given the uncertainty in the age of NGC 2547) by solid body and core-envelope decoupled models from our earlier NGC 2516 study without need for significant modification. 相似文献
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130.