Studying the formation of massive stars with VLT/X‐shooter     

L. Kaper  L.E. Ellerbroek  B.B. Ochsendorf  R.N. Caballero Pouroutidou 《Astronomische Nachrichten》2011,332(3):232-237

The birth process and (early) evolution of massive stars is still poorly understood. Massive stars are rare, their birthplaces are hidden from view and their formation timescale is short. So far, our physical knowledge of these young massive stars has been derived from near‐IR imaging and spectroscopy, revealing populations of young OB‐type stars, some still surrounded by a (remnant?) accretion disk, others apparently “normal” main sequence stars powering H II regions. The most important spectral features of OB‐type stars are, however, located in the UV and optical range. With VLT/X‐shooter it is possible to extend the spectral coverage of these young massive stars into the optical range, to better determine their photospheric properties, to study the onset of the stellar wind, and to characterize the physical structure of the circumstellar disk (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

X‐shooting Herbig Ae/Be stars: Accretion probed by near‐infrared He I emission     

R.D. Oudmaijer  M.E. van den Ancker  D. Baines  P. Caselli  J.E. Drew  M.G. Hoare  S.L. Lumsden  B. Montesinos  S. Sim  J.S. Vink  H.E. Wheelwright  W.J. de Wit 《Astronomische Nachrichten》2011,332(3):238-241

The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Spectral lag of gamma‐ray bursts caused by the intrinsic spectral evolution and the curvature effect     

Z.Y. Peng  Y. Yin  X.W. Bi  Y.Y. Bao  L. Ma 《Astronomische Nachrichten》2011,332(1):92-101

Assuming an intrinsic ‘Band’ shape spectrum and an intrinsic energy‐independent emission profile we have investigated the connection between the evolution of the rest‐frame spectral parameters and the spectral lags measured in gamma‐ray burst (GRB) pulses by using a pulse model. We first focus our attention on the evolution of the peak energy, E_0,p, and neglect the effect of the curvature effect. It is found that the evolution of E_0,p alone can produce the observed lags. When E_0,p varies from hard to soft only the positive lags can be observed. The negative lags would occur in the case of E_0,p varying from soft to hard. When the evolution of E_0,p and the low‐energy spectral index α₀ varying from soft to hard then to soft we can find the aforesaid two sorts of lags. We then examine the combined case of the spectral evolution and the curvature effect of fireball and find the observed spectral lags would increase. A sample including 15 single pulses whose spectral evolution follows hard to soft has been investigated. All the lags of these pulses are positive, which is in good agreement with our theoretical predictions. Our analysis shows that only the intrinsic spectral evolution can produce the spectral lags and the observed lags should be contributed by the intrinsic spectral evolution and the curvature effect. But it is still unclear what cause the spectral evolution (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Oligocene–Miocene thrusting in central Aegean: insights from the Cycladic island of Amorgos     

V. Chatzaras  P. Xypolias  S. Kokkalas  I. Koukouvelas 《Geological Journal》2011,46(6):619-636

In the central Aegean, the Cycladic island of Amorgos consists of two high‐pressure (HP) units, the marble‐rich Amorgos unit, which is correlated to the Mesozoic ‘cover’ sequence of the Menderes Massif, and the Cycladic Blueschist unit. New structural data show that the deformation history of the Amorgos HP‐rocks was principally governed by early Oligocene (or late Eocene)–early Miocene ductile to brittle thrusting (D₁–D₃) followed by middle–late Miocene oblique contractional movements (D₄–D₅). The D₁ phase caused syn‐blueschist‐facies ductile thrusting of the Cycladic Blueschist unit over the Amorgos unit, with ambiguous kinematics. Progressive deformation under continuous NW–SE compression produced a sequence of imbricate NW‐directed thrusts (D_2/3) characterized by a stratification of fault‐related rocks, with mylonitic zones (D₂) giving way downwards to cataclastic zones (D₃). Ductile D₂ thrusting synchronous to greenschist‐facies retrogression, was accompanied by mega‐sheath folding during constrictional and general shear deformation. Brittle D₃ thrusting was associated with NW‐verging F₃ folds trending at a high‐angle to the transport direction. Orthogonal contraction gave way to transpression during which the compression orientation changed from NW–SE (D₄) to NE–SW (D₅). Back‐arc related NW–SE pure extension (D₆) seems to have been established in post‐late Miocene times and related high‐angle normal faulting affected HP‐rocks only after they had already reached the uppermost crustal levels. Oligocene–early Miocene deformation history is interpreted to indicate syn‐compressional exhumation of HP‐rocks possibly in an extrusion wedge. In this case, Amorgos HP‐rocks should have occupied the base of the extrusion wedge. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Neoproterozoic calc‐alkaline peraluminous granitoids of the Deleihimmi pluton,Central Eastern Desert,Egypt: implications for transition from late‐ to post‐collisional tectonomagmatic evolution in the northern Arabian–Nubian Shield     

Esam S. Farahat  Rafat Zaki  Christoph Hauzenberger  Mabrouk Sami 《Geological Journal》2011,46(6):544-560

The widely distributed late‐collisional calc‐alkaline granitoids in the northern Arabian–Nubian Shield (ANS) have a geodynamic interest as they represent significant addition of material into the ANS juvenile crust in a short time interval (∼630–590 Ma). The Deleihimmi granitoids in the Egyptian Central Eastern Desert are, therefore, particularly interesting since they form a multiphase pluton composed largely of late‐collisional biotite granitoids enclosing granodiorite microgranular enclaves and intruded by leuco‐ and muscovite granites. Geochemically, different granitoid phases share some features and distinctly vary in others. They display slightly peraluminous (ASI = 1–1.16), non‐alkaline (calc‐alkaline and highly fractionated calc‐alkaline), I‐type affinities. Both biotite granitoids and leucogranites show similar rare earth element (REE) patterns [(La/Lu)_N = 3.04–2.92 and 1.9–1.14; Eu/Eu* = 0.26–0.19 and 0.11–0.08, respectively) and related most likely by closed system crystal fractionation of a common parent. On the other hand, the late phase muscovite granites have distinctive geochemical features typical of rare‐metal granites. They are remarkably depleted in Sr and Ba (4–35 and 13–18 ppm, respectively), and enriched in Rb (381–473 ppm) and many rare metals. Moreover, their REE patterns show a tetrad effect (TE_1,3 = 1.13 and 1.29) and pronounced negative Eu anomalies (Eu/Eu* = 0.07 and 0.08), implying extensive open system fractionation via fluid–rock interaction during the magmatic stage. Origin of the calc‐alkaline granitoids by high degree of partial melting of mafic lower crust with subsequent crystal fractionation is advocated. The broad distribution of late‐collisional calc‐alkaline granitoids in the northern ANS is related most likely to large areal and intensive lithospheric delamination subsequent to slab break‐off and crustal/mantle thickening. Such delamination caused both crustal uplift and partial melting of the remaining mantle lithosphere in response to asthenospheric uprise. The melts produced underplate the lower crust to promote its melting. The presence of microgranular enclaves, resulting from mingling of mantle‐derived mafic magma with felsic crustal‐derived liquid, favours this process. The derivation of the late‐phase rare‐metal granites by open system fractionation via fluid interaction is almost related to the onset of extension above the rising asthenosphere that results in mantle degassing during the switch to post‐collisional stage. Consequently, the switch from late‐ to post‐collisional stage of crustal evolution in the northern ANS could be potentially significant not only geodynamically but also economically. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Genetic type,distribution patterns and controlling factors of beach and bars in the second member of the Shahejie formation in the Dawangbei Sag,Bohai Bay,China     

Deng Hongwen  Xiao Yi  Ma Lixiang  Jiang Zhenglong 《Geological Journal》2011,46(4):380-389

As an important reservoir type in the Bohai Bay Basin, China, lacustrine beach and bar sands which refer to the shallow water complex deposited mainly by nearshore, delta‐rim and buried hill‐related beaches as well as longshore bars were developed in a particular stage in the evolution of those faulted Cenozoic continental depressions. In the Chezhen Depression, for example, the Second Member of the Oligocene Shehejie Formation (abbr. as Es2 hereafter) formed during the rifting‐to‐thermal subsidence transitional stage. Although well developed in Es2, beach and bar sands are difficult to recognize owing to their relative thinness. The paper summarizes sedimentary characteristics of lacustrine beach and bar sands on cores and logs. Low‐angle cross‐stratification, swash stratification, as well as occasional small‐scale hummocky cross‐stratification resulted from storms can be observed in beach and bar sands. The paper distinguishes bars and beaches from each other in Es2 mainly based on the grain‐size, bed thickness, facies succession and log responses. In order to predict the distribution of beach and bars, a chrono‐stratigraphic correlation framework of Es2 in the study strata is established using a high‐resolution sequence stratigraphic approach. Es2 strata are sub‐divided into six medium‐scale cycles and the mapping of the high‐frequency cycles allows the geographic and stratigraphic distribution of both beach and bar sands to be predicted. The study shows that beach and bars are better developed in times of base‐level fall than in base‐level rise. Factors such as lake‐level fluctuation, sediment supply, palaeogeomorphology and palaeowind direction have exerted control on the formation and distribution of beach and bar sands. Finally, the genetic pattern of beach and bar sands in the Es2 unit has been constructed, which provides a foundation for the prediction of beach and bars reservoir in continental basins in general. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Charcoal as evidence of palaeo‐wildfires in the Late Triassic of SW Germany     

Dieter Uhl  Michael Montenari 《Geological Journal》2011,46(1):34-41

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Culture and morphology of some New Zealand barnacles (Crustacea: Cirripedia)     

M. F. Barker 《新西兰海洋与淡水研究杂志》2013,47(1):139-158

The larval stages of the acorn barnacles Chamaesipho columna, Chamaesipho brunnea, Elminius plicatus, Elminius modestus, Balanus trigonus, and Tetraclita purpurascens are described from larvae reared in the laboratory. A key and a table are included for separating larval stages and species.

Skeletonema costatum appears to have a wide application as a food in the rearing of larvae of the Balanidae.

The two Chamaesipho species show typical chthamalid characters of hispid antennal setae and unilobed labrum. The similarity between the larvae of Elminius plicatus and Tetraclita purpurascens supports the classification of E. plicatus as a tetraclitid.  相似文献   

Three sipunculan species (two new) from New Zealand     

S. J. Edmonds 《新西兰海洋与淡水研究杂志》2013,47(1):217-224

Two new species of Sipuncula arc described from New Zealand; Phascolion temporariae from, the empty tubes of the polychaete Temporaria inexpectata (Mestayeri), and Phascolion tortum from the shells of four species of molluscs. Specimens of Golfingia improvisa (Theel) are also reported from the empty frustules of the foraminiferan Ammodiscoides mestayeri (Cushman) and the sandy tubes of the foraminifera Rhizammina sp. All the species were dredged at depths of 370–660 m from Taiaroa and Papanui Canyons, off the Otago Peninsula, New Zealand. The body cavity of several specimens of G. improvisa contained the larval stage of a nematode.  相似文献   

Removal of intertidal algae by herbivores in experimental frames and on shores near Auckland     

Penelope A. Luckens 《新西兰海洋与淡水研究杂志》2013,47(4):637-654

When grazing animals are excluded from any intertidal area algae grow unchecked. In screened frames on the shore where predation experiments are in progress such growth is undesirable. A number of shore herbivores were tested for their ability to remove such algal growth and prevent its reformation without interfering with the main experiments. Only the browsing species Lunella smaragda and Melagraphia aethiops were capable of removing large algal growths, and these species were acceptable food for some of the predatory species. The complex inter‐relationship between grazing species and shore zonation at several places near Auckland is discussed.  相似文献