A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained     

M.R. Burleigh  J.P. Pye  S.W. Poulton  K.B. Sohl  P.J. Wheatley  G.A. Wynn 《Monthly notices of the Royal Astronomical Society》2001,325(4):1458-1462

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ∼ 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT . The WFC observations began just ≈13 hr after the optical rise was detected. With a peak count rate of ∼ 4.5 count s⁻¹ in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ≈19 hr the measured EUV flux dropped to < 2 count s⁻¹, while the optical brightness remained essentially static at m _v∼11 . Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst, as reported by Mauche & Raymond. In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Mauche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that affect the accretion discs in these systems.  相似文献   

The Flared Disc Project: RXTE and ASCA observations of X 1822−371     

S. Heinz  M. A. Nowak 《Monthly notices of the Royal Astronomical Society》2001,320(2):249-260

We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M_⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M_⊙) black hole and is in a transitional state of duration shortward of 10⁷ yr.  相似文献   

The Geological-Geophysical-Geochemical Exploration Model of the Nancha Type Gold Deposit in Tonghua Area, Jilin Province     

《东北亚地学研究》2001,(2)

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Mid-infrared, Hα and radio continuum images of an unusual H ii region, G308.70 + 0.60     

Martin Cohen  Anne J. Green  Quentin A. Parker  Stacy Mader  Russell D. Cannon 《Monthly notices of the Royal Astronomical Society》2002,336(3):736-746

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On the Line Formation in Stellar Magnetized Atmospheres     

Zhong-Quan Qu  Xiao-Yu Zhang  Zhi Xu 《中国天文和天体物理学报》2001,1(2):161-175

1 INTRODUCTIONIn the previous paper (on et al. 1999, hereafter Paper I), we investigated the wavelength-dependence of four colltribution functions (CFs) derived from dmerent formal solutions andreferring to different emergellt quantities in the unpolarized case. Because one cannot generallyassign a single formation region to the whole line band in a real stellar atmosphere, e.g., the solaratmosphere, instead, the line formation region can be defined as the layers deviating farthestfrom t…  相似文献   

The pairwise velocity dispersion of galaxies: effects of non-radial motions     

A. Del Popolo 《Monthly notices of the Royal Astronomical Society》2001,326(2):667-674

I discuss the effect of non-radial motions on the small-scale peculiar pairwise velocity dispersions (PVD) of galaxies in a cold dark matter (CDM) model and calculate the PVD for the SCDM model by means of the refined cosmic virial theorem (CVT), taking account of non-radial motions by means of the Del Popolo & Gambera model. I compare the results of the present model with the data from Davis & Peebles, the IRAS value at 1  h ⁻¹ Mpc of Fisher et al. and Marzke et al. I show that while the SCDM model disagrees with the observed values, as pointed out by several authors, taking account of non-radial motions produces smaller values for the PVD. At r ≤1  h ⁻¹ Mpc the result is in agreement with Bartlett & Blanchard. In the light of this last paper, the result may be also read as a strong dependence of the CVT prediction on the model chosen to describe the mass distribution around galaxies, suggesting that the CVT cannot be taken as a direct evidence for a low-density Universe. Similarly to what is shown by Del Popolo & Gambera and Del Popolo et al., the agreement of our model to the observational data is because of a scale-dependent bias induced by the presence of non-radial motions. As the assumptions on which CVT is based have been questioned by several authors, I also calculated the PVD using the redshift distortion in the redshift-space correlation function, ξ _z( r _p, π), and compared it with the PVD measured from the Las Campanas Redshift Survey by Jing et al. The result confirms that non-radial motions influence the PVD making them agree better with the observed data.  相似文献   

Double starbursts triggered by mergers in hierarchical clustering scenarios     

P. B. Tissera  R. Domínguez-Tenreiro  C. Scannapieco  A. Sáiz 《Monthly notices of the Royal Astronomical Society》2002,333(2):327-338

We use cosmological smooth particle hydrodynamical (SPH) simulations to study the effects of mergers in the star formation history of galactic objects in hierarchical clustering scenarios. We find that during some merger events, gaseous discs can experience two starbursts: the first one during the orbital decay phase, owing to gas inflows driven as the satellite approaches, and the second one when the two baryonic clumps collide. A trend for these first induced starbursts to be more efficient at transforming the gas into stars is also found. We detect that systems that do not experience early gas inflows have well-formed stellar bulges and more concentrated potential wells, which seem to be responsible for preventing further gas inward transport triggered by tidal forces. The potential wells concentrate owing to the accumulation of baryons in the central regions and of dark matter as the result of the pulling in by baryons. The coupled evolution of the dark matter and baryons would lead to an evolutionary sequence during which systems with shallower total potential wells suffer early gas inflows during the orbital decay phase that help to feed their central mass concentration, pulling in dark matter and contributing to build up more stable systems. Within this scenario, starbursts triggered by early gas inflows are more likely to occur at early stages of evolution of the systems and to be an important contributor to the formation of stellar bulges. Our results constitute the first proof that bulges can form as the product of collapse, collisions and secular evolution in a cosmological framework, and they are consistent with a rejuvenation of the stellar population in bulges at intermediate z with, at least, 50 per cent of the stars (in SCDM) being formed at high z .  相似文献   

Accretion stream mapping of HU Aquarii     

Sonja Vrielmann  Axel D. Schwope 《Monthly notices of the Royal Astronomical Society》2001,322(2):269-279

We present a new mapping algorithm, the Accretion Stream Mapping (ASM), which uses the full phase-coverage of a light curve to derive spatially resolved intensity distributions along the accretion stream in magnetic cataclysmic variables of AM Herculis type (polars). The surface of the accretion stream is approximated as a 12-sided (duodecadon-shaped) tube. After successfully testing this method on artificial data we applied it to emission-line light curves of H β , H γ and He  ii λ 4686 of the bright eclipsing polar HU Aqr. We find hydrogen and helium line emission bright in the threading region of the stream where the stream couples on to magnetic field lines. It is particularly interesting that the stream is bright on the irradiated side facing the white dwarf, which highlights the interplay of collisional and radiative excitation/ionization.  相似文献   

An analytical expression for apsidal superhump precession and comparisons with numerical simulations and dwarf nova observations     

M.M. Montgomery 《Monthly notices of the Royal Astronomical Society》2001,325(2):761-766

We compare analytical expressions of precession rates from apsidal (positive) superhumps in close binary systems with numerical disc simulation results and observed values. In the analytical expressions, we include both the dynamical effects on the precession of the disc and effects caused by pressure forces that have been theorized to provide a retrograde effect (i.e. slowing) on the prograde disc precession. We establish new limits on density wave pitch angle to a normalized disc sound speed 60≥Ω_orb  d  tan  i / c >2.214 . Using average values for the density wave pitch angle i and speed of sound c , we find good correlation between numerical simulations and the analytical expression for the apsidal superhump period excess, which includes both the prograde and retrograde effects, for mass ratios of 0.025≤ q ≤0.33 . We also show good correlations with the four known eclipsing systems, OY Car, Z Cha, HT Cas, and WZ Sge. Our analytical expression for apsidal superhump period excess as a function of orbital period is consistent with the trend found in observed systems.  相似文献   

Erratum: Fluorescent H₂ in the reflection nebula NGC 2023 – I. Recent observations     

M. S. K. McCartney  P. W. J. L. Brand  M. G. Burton  A. Chrysostomou 《Monthly notices of the Royal Astronomical Society》2001,321(4):784-784

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