共查询到20条相似文献，搜索用时 23 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 & 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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Simultaneous X-ray and extreme ultraviolet (EUV) ( ROSAT XRT and WFC All-Sky Survey) observations of the highly active dMe flare stars YY Gem and AU Mic show that the two stars displayed an unusual type of flaring behaviour. We detect several X-ray and EUV flares superimposed on an enhanced and smoothly varying quiescent background. The two large impulsive-type X-ray flares on YY Gem reach peak X-ray luminosities of and we estimate that they had similar integrated luminosities (∼6–8×1033 erg). AU Mic also produced several X-ray and EUV flares, with one very impulsive flare producing a 10-fold increase in XRT count rate. This flare was even larger than the YY Gem flares (peak L X of and integrated L X of
The ratio for both stars is at the 'saturation' limit found in rapidly rotating dwarfs and the most active RS CVn stars. We suggest that the gradually varying components are the result of a period of continuous, unresolved flaring activity. Alternatively, they may be the result of the emergence and subsequent decay of a new magnetic active region on the stellar surface of these stars. 相似文献
The ratio for both stars is at the 'saturation' limit found in rapidly rotating dwarfs and the most active RS CVn stars. We suggest that the gradually varying components are the result of a period of continuous, unresolved flaring activity. Alternatively, they may be the result of the emergence and subsequent decay of a new magnetic active region on the stellar surface of these stars. 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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G. A. J. Hussain A. Collier Cameron M. M. Jardine N. Dunstone J. Ramirez Velez H. C. Stempels J.-F. Donati M. Semel G. Aulanier T. Harries J. Bouvier C. Dougados J. Ferreira B. D. Carter W. A. Lawson 《Monthly notices of the Royal Astronomical Society》2009,398(1):189-200
We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G- and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3–5 Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main-sequence stars. Brightness maps indicate the presence of dark polar caps and low-latitude spots – these brightness maps are very similar to those obtained for other pre-main-sequence and rapidly rotating main-sequence stars.
Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence. 相似文献
Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence. 相似文献
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M. Breger P. Lenz A. A. Pamyatnykh 《Monthly notices of the Royal Astronomical Society》2009,396(1):291-298
Only a fraction of the theoretically predicted non-radial pulsation modes have so far been observed in δ Scuti stars. Nevertheless, the large number of frequencies detected in recent photometric studies of selected δ Scuti stars allow us to look for regularities in the frequency spacing of modes. Mode identifications are used to interpret these results.
Statistical analyses of several δ Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies are not distributed at random, but that the excited non-radial modes cluster around the frequencies of the radial modes over many radial orders.
The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was proposed by Dziembowski & Królikowska and shown to be efficient for ℓ= 1 modes. New pulsation model calculations confirm the observed regularities.
We present the s – f diagram, which compares the average separation of the radial frequencies ( s ) with the frequency of the lowest frequency unstable radial mode ( f ). This provides an estimate for the log g value of the observed star, if we assume that the centres of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied δ Scuti variables in which radial modes were definitely identified. 相似文献
Statistical analyses of several δ Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies are not distributed at random, but that the excited non-radial modes cluster around the frequencies of the radial modes over many radial orders.
The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was proposed by Dziembowski & Królikowska and shown to be efficient for ℓ= 1 modes. New pulsation model calculations confirm the observed regularities.
We present the s – f diagram, which compares the average separation of the radial frequencies ( s ) with the frequency of the lowest frequency unstable radial mode ( f ). This provides an estimate for the log g value of the observed star, if we assume that the centres of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied δ Scuti variables in which radial modes were definitely identified. 相似文献
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G. Handler R. R. Shobbrook † M. Jerzykiewicz K. Krisciunas T. Tshenye E. Rodríguez V. Costa A.-Y. Zhou R. Medupe W. M. Phorah R. Garrido P. J. Amado M. Paparó D. Zsuffa L. Ramokgali R. Crowe N. Purves R. Avila R. Knight E. Brassfield P. M. Kilmartin P. L. Cottrell 《Monthly notices of the Royal Astronomical Society》2004,347(2):454-462
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Stuart Clark Alan McCall Antonio Chrysostomou Tim Gledhill Jeremy Yates James Hough 《Monthly notices of the Royal Astronomical Society》2000,319(2):337-349
Near-infrared linear imaging polarimetry of the young stellar objects R CrA and T CrA in the J , H and K n bands, and circular imaging polarimetry in the H band, is presented. The data are modelled with the Clark and McCall scattering model. The R CrA and T CrA system is shown to be a particularly complex scattering environment. In the case of R CrA there is evidence that the wavelength dependence of polarization changes across the nebula. MRN dust grain models do not explain this behaviour. Depolarization by line emission is considered as an alternative explanation. The dust grain properties could also be changing across the nebula.
Although surrounded by reflection nebulosity, there is a region of particularly low polarization surrounding R CrA that is best modelled by the canonical bipolar outflow being truncated by an evacuated spherical cavity surrounding the star. The symmetry axis of the nebula appears inclined by 50° to the plane of the sky.
The H -band circular polarimetry of R CrA clearly shows a quadrupolar structure of positive and negative degrees of circular polarization that reach peak magnitudes of ∼5 per cent within our limited map. It is shown that spherical MRN grains are incapable of producing this circular polarization given the observed linear polarization of the R CrA system. Instead, scattering from aligned non-spherical grains is proposed as the operating mechanism.
T CrA is a more archetypical bipolar reflection nebula, and this object is modelled as a canonical parabolic reflection nebula that lies in the plane of the sky. The wavelength independence of linear polarization in the T CrA reflection nebula suggests that the scattering particles are Rayleigh sized. This is modelled with the MRN interstellar grain size distribution. 相似文献
Although surrounded by reflection nebulosity, there is a region of particularly low polarization surrounding R CrA that is best modelled by the canonical bipolar outflow being truncated by an evacuated spherical cavity surrounding the star. The symmetry axis of the nebula appears inclined by 50° to the plane of the sky.
The H -band circular polarimetry of R CrA clearly shows a quadrupolar structure of positive and negative degrees of circular polarization that reach peak magnitudes of ∼5 per cent within our limited map. It is shown that spherical MRN grains are incapable of producing this circular polarization given the observed linear polarization of the R CrA system. Instead, scattering from aligned non-spherical grains is proposed as the operating mechanism.
T CrA is a more archetypical bipolar reflection nebula, and this object is modelled as a canonical parabolic reflection nebula that lies in the plane of the sky. The wavelength independence of linear polarization in the T CrA reflection nebula suggests that the scattering particles are Rayleigh sized. This is modelled with the MRN interstellar grain size distribution. 相似文献
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George Wallerstein † Sean Matt† Guillermo Gonzalez† 《Monthly notices of the Royal Astronomical Society》2000,311(2):414-422
We have used echelle spectra of resolving power 35 000 to derive chemical abundances and the 12 C/13 C ratio in the 1.9-d carbon Cepheid RT TrA and the Cepheid U TrA, employed as a comparison star. We confirm that RT TrA is very metal-rich with [Fe/H]=+0.4. In addition, C and N are substantially in excess, and a small deficiency in O is present. We interpret these anomalies as resulting from the appearance on the stellar surface of material enriched in 12 C by the 3- α process, followed by CNO cycling to convert 12 C to 13 C and 14 N. In addition, some 16 O has been processed to 14 N. The partial processing of 16 O to 14 N indicates that substantial 17 O may be present. Proton capture seems to have enhanced 23 Na from the Ne isotopes. 相似文献