Detecting planets in planetary nebulae     

Noam Soker 《Monthly notices of the Royal Astronomical Society》1999,306(4):806-808

We examine the possibility of detecting signatures of surviving Uranus/Neptune-like planets inside planetary nebulae. Planets that are not too close to the stars (orbital separation larger than ∼5 au) are likely to survive the entire evolution of the star. As the star turns into a planetary nebula, it has a fast wind and strong ionizing radiation. The interaction of the radiation and wind with a planet may lead to the formation of a compact condensation or tail inside the planetary nebula, which emits strongly in H α , but not in [O  iii ]. The position of the condensation (or tail) will change over a time-scale of ∼10 yr. Such condensations might be detected with currently existing telescopes.  相似文献   

Evidence for evolving accretion disc structure in 4U1957+115     

P. J. Hakala  P. Muhli  G. Dubus 《Monthly notices of the Royal Astronomical Society》1999,306(3):701-707

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Brown dwarfs and the cataclysmic variable period minimum     

U. Kolb  I. Baraffe 《Monthly notices of the Royal Astronomical Society》1999,309(4):1034-1042

Using improved, up-to-date stellar input physics tested against observations of low-mass stars and brown dwarfs, we calculate the secular evolution of low-mass donor cataclysmic variables (CVs), including those that form with a brown-dwarf donor. Our models confirm the mismatch between the calculated minimum period ( P _min70 min) and the observed short-period cut-off (80 min) in the CV period histogram. We find that tidal and rotational corrections applied to the one-dimensional stellar structure equations have no significant effect on the period minimum. Theoretical period distributions synthesized from our model sequences always show an accumulation of systems at the minimum period, a feature absent from the observed distribution. We suggest that non-magnetic CVs become unobservable as they are effectively trapped in permanent quiescence before they reach P _min, and that small-number statistics may hide the period spike for magnetic CVs.  相似文献   

Orbital period of the dwarf nova HL Canis Majoris     

M. D. Still  †  D. Steeghs  V. S. Dhillon  D. A. H. Buckley 《Monthly notices of the Royal Astronomical Society》1999,310(1):39-42

We present a small sample of time-resolved optical spectroscopy of the dwarf nova HL CMa during an outburst state. By combining radial velocity measurements with published data we show that the previously quoted value is not the only candidate for the orbital period of this system. We reduce the significance of daily aliasing but cannot distinguish between two periods at 0.2146±0.0004 and 0.2212±0.0005 d. We show that the low-excitation emission lines are composites from an accretion disc and the companion star, and that high-excitation emission originates in the disc or outflowing material associated with the accreting white dwarf.  相似文献   

A study of the new X-ray transient RXTE J2123−058 during its post-outburst state     

Roberto Soria  Kinwah Wu  Duncan K. Galloway 《Monthly notices of the Royal Astronomical Society》1999,309(2):528-532

We carried out I , R , V and B photometric observations of the neutron star X-ray binary RXTE J2123−058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low-mass binary model to interpret our observations. After folding our data on the 0.24 821‐d orbital period, and correcting for the steady brightness decline following the outburst, we observed sinusoidal oscillations with hints of ellipsoidal modulations which became progressively more evident. Our data also show that the decline in brightness was faster in the V band than in the R and I bands. This suggests both the cooling of an irradiation-heated secondary star and the fading of an accretion disc over the nights of our observations.  相似文献   

Infrared spectroscopic variability of Cygnus X-3 in outburst and quiescence     

R. P. Fender  M. M. Hanson †  G. G. Pooley ‡ 《Monthly notices of the Royal Astronomical Society》1999,308(2):473-484

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Time‐series photometric spot modelling – VI. A new computer code and its application to 23 years of photometry of the active giant IM Pegasi     

G. Ribrik  K. Olh  K.G. Strassmeier 《Astronomische Nachrichten》2003,324(3):202-214

We present and apply a new computer program named SpotModeL to analyze single and multiple bandpass photometric data of spotted stars. It is based on the standard analytical formulae from Budding and Dorren. The program determines the position, size, and temperature of up to three spots by minimizing the fit residuals with the help of the Marquardt‐Levenberg non‐linear least‐squares algorithm. We also expand this procedure to full time‐series analysis of differential data, just as real observations would deliver. If multi‐bandpass data are available, all bandpasses can be treated simultaneously and thus the spot temperature is solved for implicitly. The program may be downloaded and used by anyone. In this paper, we apply our code to an ≈23 year long photometric dataset of the spotted RS CVn giant IM Peg. We extracted and modelled 33 individual light curves, additionally, we fitted the entire V dataset in one run. The resulting spot parameters reflect the long term light variability and reveal two active longitudes on the substellar point and on the antipode. The radius and longitude of the dominant spot show variations with 29.8 and 10.4 years period, respectively. Our multicolour data suggests that the spot temperature is increasing with the brightening of the star. The average spot temperature from V, I_C is 3550 ± 150 K or approximately 900 K below the effective temperature of the star.  相似文献   

近距离上层隧道施工对下层隧道的扰动影响          下载免费PDF全文

唐强森  刘涛 《地震工程学报》2018,40(3):591-596,603

针对近距离上层隧道施工时给下层隧道稳定性造成扰动影响的现状,基于简支土体梁力学模型,研究下层隧道顶部在周期性扰动力作用下的系统振动特性,得到顶部垂直位移的动态响应数学表达式,为定量掌握最大位移峰值的大小与出现时刻提供理论依据,指出考虑振动因素时上下层隧道安全距离的确定方法。结合具体算例探索扰动力等因素对最大位移的作用规律,结果表明:扰动力振幅和隧道跨度分别增加1倍和2倍时,最大位移峰值相应增大1倍和23.9倍;隧道层间垂距和土体弹性模量分别增加2倍和3倍时,最大位移峰值相应减小88.2%和70%。当主要影响因素变化时,最大位移的波动大周期延长、波动震荡性加剧,特别是峰值处的震荡幅值增大,隧道顶部稳定性受到明显影响。  相似文献   

A ‘superoutburst’ in XTEJ1118+480     

E. Kuulkers 《Astronomische Nachrichten》2001,322(1):9-14

I propose that the properties of the two outbursts observed in the X‐ray transient XTEJ1118+480 in 2000 are akin to superoutbursts of SU UMa stars. In these systems a ‘normal’ outburst immediately precedes a 5–10 times longer (‘super’) outburst. The optical light curve of the outbursts of XTEJ1118+480 is remarkably similar to that seen in some SU UMa stars, such as UVPer and TLeo, where the precursor outburst is distinct from the superoutburst, but the time scales are a factor of ∼15 different. The first outburst of XTEJ1118+480 was relatively short (∼1 month) while the second outburst was ∼5 times longer. During the second outburst superhumps were seen, a feature characteristic for superoutbursts. The gap of about a month between the two outbursts is longer in X‐rays with respect to the optical, a feature not previously recognized for X‐ray transients. Also in SU UMa stars the precursor outburst becomes more distinct at shorter wavelengths. Finally, I show that the time of appearance of the superhumps in XTEJ1118+480 is consistent with the expected superhump growth time, if the superhump mechanism was triggered during the first outburst. I conclude that the similarity in outburst behaviour in the two types of systems provides further support that a common mechanism is at work to start the long (‘super’) outbursts.  相似文献   

实景三维精细建模优化方法探索与实践     

吴昌 《测绘与空间地理信息》2022,(2):130-131,134

对比分析目前常用的几种实景三维建模方法及适用性,剖析倾斜摄影三维建模技术在几何结构和模型纹理等方面存在的问题,提出了实景三维精细建模的优化方法和技术路线,着重阐述了高低空航摄相结合的关键技术,明显改善了模型效果,提高了建模的质量与效率,为实景三维数字城市建设提供了技术参考.  相似文献