共查询到20条相似文献，搜索用时 417 毫秒
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Saul J. Adelman 《Monthly notices of the Royal Astronomical Society》1999,310(1):146-156
Elemental abundances of the moderately rotating B9–A3 stars λ UMa, 59 Her, 14 Cyg and 29 Cyg have been derived in a consistent manner with previous studies of this series from spectrograms obtained with Reticon and CCD detectors. The derived elemental abundances show that λ UMa is a mild Am star, while 59 Her is slightly metal-rich. Although 14 Cyg has values closer to solar than these stars, its subsolar Ca and Sc abundances indicate that it might be the hottest known hot-Am star. 29 Cyg is a metal-poor λ Boo star. 相似文献
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Saul J. Adelman 《Monthly notices of the Royal Astronomical Society》1998,296(4):856-862
Elemental abundances of the superficially normal early and middle B starsζ Dra, ε Lyr, 8 Cyg and 22 Cyg are derived, consistent with previous studies in this series, using spectrograms obtained with Reticon and CCD detectors. Almost all of the derived metal abundances are found to be solar within the errors of the analysis. However, the He/H ratios are slightly greater than solar. 相似文献
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Anthony B. Kaye & Klaus G. Strassmeier 《Monthly notices of the Royal Astronomical Society》1998,294(2):L35-L39
In an investigation of the starspot hypothesis as it applies to the 'slowly variable' F-type dwarfs, we spectroscopically observed eight promising γ Doradus candidates to search for Ca ii H&K emission. We found that there are no significant emission reversals in the cores of these resonance lines. Based on the ceiling flux calculations of the Ca ii K line and on calculations of the Rossby number, we conclude that there is no support for the presence of strong magnetic activity and the starspot hypothesis in these objects. 相似文献
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M. Pozzo R. D. Jeffries T. Naylor E. J. Totten † S. Harmer‡ M. Kenyon§ 《Monthly notices of the Royal Astronomical Society》2000,313(2):L23-L27
We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf–Rayet/O-star binary system γ 2 Vel and the Vela OB2 association. We argue that γ 2 Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 σ level with the recent Hipparcos parallax of γ 2 Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ 2 Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings. 相似文献
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Saul J. Adelman & Berahitdin Albayrak 《Monthly notices of the Royal Astronomical Society》1998,300(2):359-372
Elemental abundances of the early A stars ε Ser, 29 Vul and σ Aqr are derived consistently with previous studies of this series using spectrograms obtained with Reticon and CCD detectors. The derived abundances confirm that ε Ser is a definite Am star. 29 Vul shows evidence for a weakly operating Am star phenomenon. σ Aqr, a hot Am star prototype, has abundances similar to those of o Peg, another class prototype. 相似文献
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We have earlier investigated the surface structures of a late‐type, single, giant FK Com for the years 1994–1998 using Doppler imaging. These surface temperature maps revealed long‐lived active regions at high latitudes. Long‐term photometric observations also show that these active regions tend to occur at two permanent active longitudes which are 180 degrees apart from each other, and that the activity switches the longitude with an average period of about 3 years (the “flip‐flop” phenomenon). In this work we present new Doppler maps of FK Com obtained 1998‐2003 and light‐curve maps obtained 2002–2003. These new maps are investigated together with the earlier temperature maps and light‐curve maps, with an aim of further studying the active longitudes, “flip‐flop” phenomenon and surface differential rotation on FK Com. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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The results of nine CCD photometric observing runs on KPD 0629–0016 are presented. During six of the runs measurements were obtained alternately through B and V filters. Four periodicities, for which there is good agreement between the frequencies identified in the B and V data, were extracted: these lie in the range 46–81 min. A fifth lower frequency appears to be definitely present, but is very uncertain due to aliasing. The ratios of the mode amplitudes measured in B and V , and the phase differences between variations in the two colours, are compatible with pulsation theory. 相似文献
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P. J. Amado † C. J. Butler P. B. Byrne‡ 《Monthly notices of the Royal Astronomical Society》1999,310(4):1023-1032
In the first part of this work, the empirical correlation of stellar surface brightness F V with ( I c − K ) broad-band colour is investigated by using a sample of stars cooler than the Sun. A bilinear correlation is found to represent well the brightness of G, K and M giant stars. The change in slope occurs at ( I c − K )∼2.1 or at about the transition from K to M spectral types. The same relationship is also investigated for dwarf stars and found to be distinctly different from that of the giants. The dwarf star correlation differs by an average of −0.4 in ( I c − K ) or by a maximum in F V of ∼−0.1, positioning it below that of the giants, with both trends tending towards convergence for the hotter stars in our sample. The flux distribution derived from the F V −( I c − K ) relationship for the giant stars, together with that derived from an F V −( V − K ) relationship and the blackbody flux distribution, is then utilized to compute synthetic light V and colour ( V − R )c , ( V − I )c and ( V − K ) curves of cool spotted stars. We investigate the effects on the amplitudes of the curves by using these F V –colour relations and by assuming the effective gravity of the spots to be lower than the gravity of the unspotted photosphere. We find that the amplitudes produced by using the F V −( I c − K ) relationship are larger than those produced by the other two brightness correlations, meaning smaller and/or warmer spots. 相似文献
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M. Desmet M. Briquet † A. Thoul ‡ W. Zima P. De Cat G. Handler I. Ilyin E. Kambe J. Krzesinski H. Lehmann S. Masuda P. Mathias D. E. Mkrtichian J. Telting K. Uytterhoeven S. L. S. Yang C. Aerts 《Monthly notices of the Royal Astronomical Society》2009,396(3):1460-1472
We present the results of a spectroscopic multisite campaign for the β Cephei star 12 (DD) Lacertae. Our study is based on more than thousand high-resolution high S/N spectra gathered with eight different telescopes in a time span of 11 months. In addition, we make use of numerous archival spectroscopic measurements. We confirm 10 independent frequencies recently discovered from photometry, as well as harmonics and combination frequencies. In particular, the slowly pulsating B-stars (SPB)-like g -mode with frequency 0.3428 d−1 reported before is detected in our spectroscopy. We identify the four main modes as (ℓ1 , m 1 ) = (1, 1), (ℓ2 , m 2 ) = (0, 0), (ℓ3 , m 3 ) = (1, 0) and (ℓ4 , m 4 ) = (2, 1) for f 1 = 5.178 964 d−1 , f 2 = 5.334 224 d−1 , f 3 = 5.066 316 d−1 and f 4 = 5.490 133 d−1 , respectively. Our seismic modelling shows that f 2 is likely the radial first overtone and that the core overshooting parameter αov is lower than 0.4 local pressure scale heights. 相似文献
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We present temporal and spectral characteristics of X-ray flares observed from six late-type G–K active dwarfs (V368 Cep, XI Boo, IM Vir, V471 Tau, CC Eri and EP Eri) using data from observations with the XMM–Newton observatory. All the stars were found to be flaring frequently and altogether a total of 17 flares were detected above the 'quiescent' state X-ray emission which varied from 0.5 to 8.3 × 1029 erg s−1 . The largest flare was observed in a low-activity dwarf XI Boo with a decay time of 10 ks and ratio of peak flare luminosity to 'quiescent' state luminosity of 2. We have studied the spectral changes during the flares by using colour–colour diagram and by detailed spectral analysis during the temporal evolution of the flares. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. We have derived the semiloop lengths of flares based on the hydrodynamic flare model. The size of the flaring loops is found to be less than the stellar radius. The hydrodynamic flare decay analysis indicates the presence of sustained heating during the decay of most flares. 相似文献
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We investigate how the frequencies of gravity modes depend on the detailed properties of the chemical composition gradient that develops near the core of main‐sequence stars and, therefore, on the transport processes that are able to modify the μ profile in the central regions. We show that in main‐sequence models, similarly to the case of white dwarfs, the periods of high‐order gravity modes are accurately described by a uniform period spacing superposed to an oscillatory component. The periodicity and amplitude of such a component are related, respectively, to the location and sharpness of the μ gradient. We briefly discuss and interpret, by means of this simple approximation, the effect of turbulent mixing near the core on the periods of both high‐order and low‐order g modes, as well as of modes of mixed pressure‐gravity character. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献