Classification of planetary nebulae by their departure from axisymmetry     

NoamSoker  RonHadar† 《Monthly notices of the Royal Astronomical Society》2002,331(3):731-735

We propose a scheme to classify planetary nebulae (PNe) according to their departure from axisymmetric structure. We consider only departure along and near the equatorial plane, i.e. between the two sides perpendicular to the symmetry axis of the nebula. We consider six types of departure from axisymmetry: (1) PNe where the central star is not at the centre of the nebula; (2) PNe having one side brighter than the other; (3) PNe having unequal size or shape of the two sides; (4) PNe where the symmetry axis is bent, e.g. the two lobes in a bipolar PN are bent toward the same side; (5) PNe where the main departure from axisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe that show no departure from axisymmetry, i.e. any departure, if it exists, is on scales smaller than the scale of blobs, filaments and other irregularities in the nebula. PNe that possess more than one type of departure are classified by the most prominent type. We discuss the connection between departure types and the physical mechanisms that may cause them, mainly resulting from the influence of a stellar binary companion. We find that ∼50 per cent of all PNe in the analysed sample possess large-scale departure from axisymmetry. This number is larger than that expected from the influence of binary companions, namely ∼25–30 per cent. We argue that this discrepancy comes from many PNe where the departure from axisymmetry, mainly unequal size, shape or intensity, results from the presence of long-lived and large (hot or cool) spots on the surface of their asymptotic giant branch progenitors. Such spots locally enhance the mass-loss rate, leading to a departure from axisymmetry, mainly near the equator, in the descendent PN.  相似文献   

Time-dependent photoionization opacities in dense gamma-ray burst environments     

Davide Lazzati  Rosalba Perna  Gabriele Ghisellini 《Monthly notices of the Royal Astronomical Society》2001,325(4):L19-L23

The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.  相似文献   

Post-glitch relaxation in pulsars     

P. B. Jones 《Monthly notices of the Royal Astronomical Society》2002,335(3):733-740

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Revisiting the magnification of type Ia supernovae with SDSS     

Brice Ménard  Neal Dalal † 《Monthly notices of the Royal Astronomical Society》2005,358(1):101-104

We cross-correlate the sample of type Ia supernovae from Riess A. G. et al. with the SDSS DR2 photometric galaxy catalogue. In contrast to recent work, we find no detectable correlation between supernova magnitude and galaxy overdensity on scales ranging between 1 and 10 arcmin. Our results are in accord with theoretical expectations for gravitational lensing of supernovae by large-scale structure. Future supernova surveys such as SNAP will be capable of detecting unambiguously the predicted lensing signal.  相似文献   

Antonov problem and quasi-equilibrium states in an N-body system     

Atsushi Taruya  Masa-aki Sakagami 《Monthly notices of the Royal Astronomical Society》2005,364(3):990-1010

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Common Proper Motion Search for Faint Companions Around Early-Type Field Stars – Progress Report     

Valentin D. Ivanov  G. Chauvin  C. Foellmi  M. Hartung  N. Huélamo  C. Melo  D. Nürnberger  M. Sterzik 《Astrophysics and Space Science》2006,304(1-4):247-249

The multiplicity of early-type stars is still not well established. The derived binary fraction is different for individual star forming regions, suggesting a connection with the age and the environment conditions. The few studies that have investigated this connection do not provide conclusive results. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of early-type field stars to derive their multiplicity in a homogeneous way. The sample has been extracted from the Hipparcos Catalog and consists of 341 BA-type stars within ∼300 pc from the Sun. We report the current status of the survey and describe a Monte-Carlo simulation that estimates the completeness of our companion detection.  相似文献   

On the highly reddened members in six young galactic star clusters – a multiwavelength study     

Brijesh Kumar  Ram Sagar  B. B. Sanwal  M. S. Bessell 《Monthly notices of the Royal Astronomical Society》2004,353(3):991-1014

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Constraints on a quintessence model from gravitational lensing statistics     

M. Sereno 《Monthly notices of the Royal Astronomical Society》2005,356(3):937-943

Constraints on an exact quintessence scalar-field model with an exponential potential are derived from gravitational lens statistics. An exponential potential can account for data from both optical quasar surveys and radio-selected sources. Based on the Cosmic Lens All-Sky Survey (CLASS) sample, lensing statistics provides, for the pressureless matter density parameter, an estimate of  Ω_M0= 0.31^+0.12_−0.14  .  相似文献   

Cosmic metal production and the mean metallicity of the Universe     

F. Calura  F. Matteucci 《Monthly notices of the Royal Astronomical Society》2004,350(1):351-364

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The old open cluster Trumpler 5: a reddened, metal-poor anticentre cluster     

Andrés E. Piatti  Juan J. Clariá  Andrea V. Ahumada 《Monthly notices of the Royal Astronomical Society》2004,349(2):641-648

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