共查询到20条相似文献，搜索用时 62 毫秒
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Three spectra of Pleione (1981, 1987–88, 1994), obtained in the blue wavelength range, show strong long-term changes which indicate that the surrounding shell, first rather cool and close to the central star, then expands gradually in the equatorial plane of the star and towards the poles. From 1981 to 1987–88, the spectrum of Pleione is that of the central star, with a cool shell which globally mimics a late-type star. The year 1994 is marked by a pure and stronger Be spectrum, with no shell line. 相似文献
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P. Petit B. Dintrans S. K. Solanki J.-F. Donati M. Aurière F. Lignières J. Morin F. Paletou J. Ramirez C. Catala R. Fares 《Monthly notices of the Royal Astronomical Society》2008,388(1):80-88
From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars.
We reconstruct the large-scale magnetic geometry of the targets as a low-order (ℓ < 10) spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component. 相似文献
We reconstruct the large-scale magnetic geometry of the targets as a low-order (ℓ < 10) spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component. 相似文献
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Patrick Wils Stelios Kleidis Eric Broens 《Monthly notices of the Royal Astronomical Society》2008,387(2):783-787
The Blazhko effect in RR Lyrae stars is still poorly understood theoretically. Stars with multiple Blazhko periods or in which the Blazhko effect itself varies are particularly challenging. This study investigates the Blazhko effect in the RRc star LS Her. Detailed CCD photometry in the V , R C and I C band has been performed on 63 nights during six months. LS Her is confirmed to have a Blazhko period of 12.75 ± 0.02 d. However, where normally the side frequencies of the Blazhko triplet are expected, an equidistant group of three frequencies is found on both sides of the main pulsation frequency. As a consequence, the period and amplitude of the Blazhko effect itself vary in a cycle of 109 ± 4 d. LS Her is a unique object turning out to be very important in the verification of the theories for the Blazhko effect. 相似文献
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Antonino F. Lanza 《Monthly notices of the Royal Astronomical Society》2006,373(2):819-826
A model for the angular momentum transfer within the convection zone of a rapidly rotating star is introduced and applied to the analysis of recent observations of temporal fluctuations of the differential rotation on the young late-type stars AB Doradus (AB Dor) and LQ Hydrae (LQ Hya). Under the hypothesis that the mean magnetic field produced by the stellar dynamo rules the angular momentum exchanges and that the angular velocity depends only on the distance s from the rotation axis and the time, the minimum azimuthal Maxwell stress | Bs B φ | , averaged over the convection zone, is found to range from ∼0.04 to ∼0.14 T2 . If the poloidal mean magnetic field B s is of the order of 0.01 T, as indicated by the Zeeman–Doppler imaging maps of those stars, then the azimuthal mean field B φ can reach an intensity of several teslas, which significantly exceeds equipartition with the turbulent kinetic energy. Such strong fields can account also for the orbital period modulation observed in cataclysmic variables and RS Canum Venaticorum systems with a main-sequence secondary component. Moreover, the model allows us to compute the kinetic energy dissipation rate during the maintenance of the differential rotation. Only in the case of the largest surface shear observed on LQ Hya may the dissipated power exceed the stellar luminosity, but the lack of a sufficient statistic on the occurrence of such episodes of large shear does not allow us to estimate their impact on the energy budget of the convection zone. 相似文献
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T. R. Bedding Albert A. Zijlstra A. Jones & G. Foster 《Monthly notices of the Royal Astronomical Society》1998,301(4):1073-1082
We discuss visual observations spanning nearly 70 years of the nearby semiregular variable R Doradus. Using wavelet analysis, we show that the star switches back and forth between two pulsation modes having periods of 332 d and about 175 d, the latter with much smaller amplitude. Comparison with model calculations suggests that the two modes are the first and third radial overtone, with the physical diameter of the star making fundamental-mode pulsation unlikely. The mode changes occur on a time-scale of about 1000 d, which is too rapid to be related to a change in the overall thermal structure of the star and may instead be related to weak chaos. The Hipparcos distance to R Dor is 62.4 ± 2.8 pc which, taken with its dominant 332-d period, places it exactly on the period–luminosity (P–L) relation of Miras in the Large Magellanic Cloud. Our results imply first-overtone pulsation for all Miras which fall on the P–L relation. We argue that semiregular variables with long periods may largely be a subset of Miras and should be included in studies of Mira behaviour. The semiregulars may contain the immediate evolutionary Mira progenitors, or stars may alternate between periods of semiregular and Mira behaviour. 相似文献
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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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Ai-Ying Zhou E. Rodríguez Shi-Yang Jiang A. Rolland Víctor Costa 《Monthly notices of the Royal Astronomical Society》1999,308(3):631-639
A multifrequency analysis of the SX Phoenicis star BL Camelopardalis is presented on the basis of new high-speed photometry, along with fitting a total of 136 maxima. BL Cam is a multiple periodic pulsator. We find f 0 =25.5768, f 1 =25.2982, f 2 =25.8622, f 3 =31.5912, f 4 =25.1065, f 5 =25.5147 and f 6 =25.6188 cycle d−1 together with the harmonics 51.1513 and 76.7268 cycle d−1 and combination frequencies f 0 + f 1 , f 0 + f 2 and f 0 + f 3 . The new frequency solution represents the light curves of BL Cam quite well. The observed minus calculated (O-C) analysis indicates that the fundamental frequency is in good agreement with the results of Fourier analysis. 相似文献
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Hugh R. A. Jones R. Paul Butler C. G. Tinney Geoffrey W. Marcy Brad D. Carter Alan J. Penny Chris McCarthy Jeremy Bailey 《Monthly notices of the Royal Astronomical Society》2006,369(1):249-256
We report Doppler measurements of the stars HD 187085 and HD 20782 which indicate two high eccentricity low-mass companions to the stars. We find HD 187085 has a Jupiter-mass companion with a ∼1000-d orbit. Our formal 'best-fitting' solution suggests an eccentricity of 0.47, however, it does not sample the periastron passage of the companion and we find that orbital solutions with eccentricities between 0.1 and 0.8 give only slightly poorer fits (based on rms and χ2 ν ) and are thus plausible. Observations made during periastron passage in 2007 June should allow for the reliable determination of the orbital eccentricity for the companion to HD 187085. Our data set for HD 20782 does sample periastron and so the orbit for its companion can be more reliably determined. We find the companion to HD 20782 has M sin i = 1.77 ± 0.22 M Jup , an orbital period of 595.86 ± 0.03 d and an orbit with an eccentricity of 0.92 ± 0.03. The detection of such high-eccentricity (and relatively low-velocity amplitude) exoplanets appears to be facilitated by the long-term precision of the Anglo-Australian Planet Search. Looking at exoplanet detections as a whole, we find that those with higher eccentricity seem to have relatively higher velocity amplitudes indicating higher mass planets and/or an observational bias against the detection of high-eccentricity systems. 相似文献
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T. &#;ahin David L. Lambert 《Monthly notices of the Royal Astronomical Society》2009,398(4):1730-1741
An abundance analysis based on a high-resolution spectrum is presented for a newly discovered post-asymptotic giant branch (PAGB) star in the globular cluster M79. The surprising result is that the iron abundance of the star is apparently about 0.6 dex less than that of the cluster's red giants as reported by published studies including a recent high-resolution spectroscopic analysis by Carretta and colleagues. Abundances relative to iron appear to be the same for the PAGB star and the red giants for the 15 common elements. It is suggested that the explanation for the lower abundances of the PAGB star may be that its atmospheric structure differs from that of a classical atmosphere; the temperature gradient may be flatter than predicted by a classical atmosphere. 相似文献