Evolution on the AGB and beyond: on the formation of H-deficient post-AGB stars     

Thomas Blöcker 《Astrophysics and Space Science》2001,275(1-2):1-14

The evolution on the AGB and beyond is reviewed with respect to the origin of Wolf-Rayet central stars. We focus on thermal pulses due to their particular importance for the evolution of hydrogen deficient stars. It is shown that overshoot applied to all convection regions is a key ingredient to model these objects leading to intershell abundances already close to the surface abundances of Wolf-Rayetcentral stars. In contrast to standard evolutionary calculations, overshoot models do show dredge up for very low envelope masses and efficient dredge up was found even during the post-AGB stage. Three thermal pulse scenarios for Wolf-Rayet central stars can nowbe distinguished: an AGB Final Thermal Pulse (AFTP) occurring at the very end of the AGB evolution, a Late Thermal Pulse (LTP) occurring during the post-AGBevolution when hydrogen burning is still on, and a Very Late Thermal Pulse (VLTP) occurring on the cooling branch when hydrogen burning has already ceased. All scenarios lead to hydrogen-deficient post-AGB stars with carbon and oxygen abundances as observed for Wolf-Rayet stars.Hydrogen is either diluted by dredge up (AFTP, LTP) or completely burnt (VLTP).  相似文献   

Space–VLBI observations of the OH maser OH 34.26+0.15: low interstellar scattering     

V. I. Slysh  M. A. Voronkov  V. Migenes  K. M. Shibata  T. Umemoto  V. I. Altunin  I. E. Val'Tts  B. Z. Kanevsky  M. V. Popov  A. V. Kovalenko  E. B. Fomalont  B. A. Poperechenko  YU. N. Gorshenkov  B. R. Carlson  S. M. Dougherty  J. E. Reynolds  D. R. Jiang  A. I. Smirnov  V. G. Grachev 《Monthly notices of the Royal Astronomical Society》2001,320(2):217-223

We report on the first space–VLBI observations of the OH 34.26+0.15 maser in two main-line OH transitions at 1665 and 1667 MHz. The observations involved the space radio telescope on board the Japanese satellite HALCA and an array of ground radio telescopes. The map of the maser region and images of individual maser spots were produced with an angular resolution of 1 mas, which is several times higher than the angular resolution available on the ground. The maser spots were only partly resolved and a lower limit to the brightness temperature     was obtained. The maser seems to be located in the direction of low interstellar scattering, an order of magnitude lower than the scattering of a nearby extragalactic source and pulsar.  相似文献   

Near Infrared Observation of Carbon and S Stars with Silicate Feature     

P.S. Chen  G.Z. Xiong  X.H. Wang 《Astrophysics and Space Science》1997,249(2):227-234

We apply the concept of wave quantum theory in an astronomical scale and discuss the periodic distribution of the galaxies red shift. A generalization of the commutation relation is used to write down the wave equation and the redshift spectrum is calculated. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

1998 UBV Light Curves of Eclipsing Binary AI Draconis and Absolute Parameters     

D.M.Z. Jassur  M.S. Khaledian  M.H. Kermani 《Astrophysics and Space Science》2001,278(4):433-440

New UBV photometry of Algol-Type eclipsing binary star AI Dra andthe absolute physical parameters of this system have been presented.The light curve analysis carried out by the method of differentialcorrections indicates that both components are inside their Roche-Lobes.From combining the photometric solution with spectroscopic data obtainedfrom velocity curve analysis, it has been found that the system consistof a main sequence primary and an evolved (subgiant) secondary.  相似文献   

THE IMPACT-FLOOD CONNECTION: DOES IT EXIST?*     

Alexander Deutsch  Christian Koeberl  Joel D. Blum  Bevan M. French  Billy P. Glass  Richard Grieve  Peter Horn  Elmar K. Jessberger  Gero Kurat  Wolf Uwe Reimold  Jan Smit  Dieter Stöffler  Stuart Ross Taylor 《地学学报》1994,6(6):644-650

In a recent article in Terra Nova, Kristan-Tollmann and Tollmann (1994) suggested that the Biblical Flood can be explained by seven fragments of a comet that impacted the ocean at seven locations on Earth at 03.00h (C.E.T.) on 23 September, 9545 yr BP. We demonstrate that all the ‘geological proofs’ that allegedly support their conclusions are not supported by the available data on impact cratering. Their hypothesis is based on insufficient and ambiguous data, selective citation, and incomplete comprehension of previous research.  相似文献   

Benthic foraminiferal distribution and sedimentary structures suggest tectonic erosion at the Costa Rica convergent plate margin     

Martin Meschede  Regine Weiss  Gerhard Schmiedl  Christoph Hemleben 《地学学报》2002,14(5):388-396

ABSTRACT Distribution patterns of benthic foraminiferal faunas from ODP Leg 170 Sites 1041 and 1042 show that the Costa Rican convergent margin subsided from coastal to abyssal depth from Middle Miocene to Present. This favours the model of a margin undergoing active subduction erosion. We propose that subduction erosion leads to the removal of material from the base of the forearc wedge and, as a consequence, to progressive subsidence of the forearc. A mean subsidence rate is estimated to be approximately 0.4 mm yr⁻¹.  相似文献   

Radio Observations at 232 MHz and Multifrequency Spectral Studies of SNR HB21     

X.Z. Zhang  S.J. Qian  L.A. Higgs  T.L. Landecker  X.J. Wu 《Astrophysics and Space Science》2002,279(4):355-366

Radio observational results at 232 MHz and multifrequency studies of supernova remnant (SNR) HB21 are presented. Its integrated flux density at 232 MHz is about 390 ± 30 Jy. Both the integrated spectral index and the spatial variations of spectral index of the remnant were calculated by combining the new map at 232 MHz with previously published maps made at 408, 1420, 2695, and 4750 MHz. The SNR has an integrated spectral index of about α = -0.43(S _ν ∝ ν^α) between 232 and 4750 MHz. In general the spectral index varies from –0.5 in southeast and west regions of the remnant to –0.3 in the central region and near the northwest edge. The new data of 232 MHz reveals that there is interaction between the remnant and the surrounding gas along the east edge of the remnant which causes the spectrum flattening at low frequency, while the very good agreement between the structure of X-ray emission and the central flat spectrum area suggests that the existence of thermal emission is the reason of spectrum flattening in the area. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Light variations and flares on V471 tauri     

Z. Tunca  V. Keskin  S. Evren  C. Ibanoglu  M. C. Akan 《Astrophysics and Space Science》1993,204(2):297-304

Two-colour photometric observations of the white-dwarf and K-dwarf eclipsing binary V471 Tau were made during the last four years. The resulting light curves and thirteen timings of mid-eclipse are presented. During the observations two flares were obtained on September 23 and December23, 1992. Both events occurred just after the fourth contact. The increment in the brightness was measured as 0 _. ^m 067. The energy liberated by the flare was computed to be 5.43 × 10³² erg s^–1 in the JohnsonB band, which amounts 1.13 times that of the total radiation of the K-dwarf.  相似文献   

Hydrodynamical Modelling of the Evolution of Dusty Outflows from AGB-Stars     

D. Schönberner  M. Steffen  R. Szczerba 《Astrophysics and Space Science》1997,255(1-2):459-467

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Aluminum-26, sodium-22 and cobalt-60 in two drill cores and some other samples of the Jilin chondrite     

G. HEUSSER  Z. OUYANG  J. OEHM  W. YI 《Meteoritics & planetary science》1996,31(5):657-665

Abstract— Two drill cores have been taken from the main mass of the Jilin chondrite, Jilin I. One was bored perpendicular to the 2π surface of the first exposure stage, the other parallel to this surface, and both intersected at the presumed 4π center of Jilin's second exposure stage. The probed depths were 65 cm and 105 cm, respectively, with a diameter of 26.5 mm. Fifteen drill core samples, 2 documented samples and 14 strewn field samples have been analysed for ²⁶Al, ⁶⁰Co and ²²Na. The reconstruction of the preatmospheric Jilin via ⁶⁰Co resulted in slight deviations for the center position and for the radius (now R = 80⁺¹⁰_?5 cm) compared to an earlier study in 1985. The ⁶⁰Co production rate profile is now almost completely reproduced. A constant production rate of ²²Na and of ²⁶Al during the 4π irradiation is confirmed also for the center region. The good linear correlation of ²⁶Al and ²¹Ne yields slightly revised irradiation ages for both stages of t₁ = 7 ± 1 Ma and t₂ = 0.32^+0.06-_0.03 Ma.  相似文献