共查询到20条相似文献，搜索用时 78 毫秒
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We present numerical models based on realistic treatment of the intensity spectrum (from model atmospheres), and demonstrate that they are consistent with Kurtz and Medupe's recent formula in showing that limb darkening is too small an effect to explain the observed sharp decline of pulsation light amplitude with wavelength in rapidly oscillating Ap stars. Kurtz and Medupe's formula is shown to be a special form of Watson's earlier general formula for non-radial light variations of a star pulsating in any mode ( l m ). Using a technique suggested by Kurtz and Medupe we derive temperature semi-amplitude as a function of depth in the atmospheres of α Cir and HR 3831, assuming that we can neglect non-adiabatic effects. 相似文献
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C. Weidner P. Kroupa T. Maschberger 《Monthly notices of the Royal Astronomical Society》2009,393(2):663-680
The study of young stellar populations has revealed that most stars are in binary or higher order multiple systems. In this study, the influence on the stellar initial mass function (IMF) of large quantities of unresolved multiple massive stars is investigated by taking into account the stellar evolution and photometrically determined system masses. The models, where initial masses are derived from the luminosity and colour of unresolved multiple systems, show that even under extreme circumstances (100 per cent binaries or higher order multiples), the difference between the power-law index of the mass function (MF) of all stars and the observed MF is small (≲0.1). Thus, if the observed IMF has the Salpeter index α= 2.35 , then the true stellar IMF has an index not flatter than α= 2.25 . Additionally, unresolved multiple systems may hide between 15 and 60 per cent of the underlying true mass of a star cluster. While already a known result, it is important to point out that the presence of a large number of unresolved binaries amongst pre-main-sequence stars induces a significant spread in the measured ages of these stars even if there is none. Also, lower mass stars in a single-age binary-rich cluster appear older than the massive stars by about 0.6 Myr. 相似文献
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J. M. Oliveira R. D. Jeffries J. Th. van Loon 《Monthly notices of the Royal Astronomical Society》2009,392(3):1034-1050
NGC 6611 is the massive young cluster (2–3 Myr) that ionizes the Eagle Nebula. We present very deep photometric observations of the central region of NGC 6611 obtained with the Hubble Space Telescope and the following filters: ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to ground-based IZJH filters. This survey reaches down to I ∼ 26 mag . We construct the initial mass function (IMF) from ∼1.5 M⊙ well into the brown dwarf regime (down to ∼0.02 M⊙ ). We have detected 30–35 brown dwarf candidates in this sample. The low-mass IMF is combined with a higher-mass IMF constructed from the ground-based catalogue from Oliveira et al. We compare the final IMF with those of well-studied star-forming regions: we find that the IMF of NGC 6611 more closely resembles that of the low-mass star-forming region in Taurus than that of the more massive Orion Nebula Cluster. We conclude that there seems to be no severe environmental effect in the IMF due to the proximity of the massive stars in NGC 6611. 相似文献
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I. W. Roxburgh S. V. Vorontsov 《Monthly notices of the Royal Astronomical Society》2006,369(3):1491-1496
The basic properties of acoustic wave propagation in stellar interiors can be analysed from the autocorrelation function (ACF) of intensity (or velocity) observations without measuring the resonant p-mode frequencies. We show how the strength of acoustic wave refraction in the stellar core, or forward acoustic amplitude, can be measured from a modulation in the ACF. This is the basic physical quantity which governs the so-called 'small frequency separations', and its measurement from the ACF can be used for determining the small frequency separations when the data is of insufficient quality for a reliable identification of the stellar p-mode frequencies. 相似文献
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Slavek M. Rucinski 《Monthly notices of the Royal Astronomical Society》2006,368(3):1319-1322
The luminosity function for contact binary stars of the W UMa type is evaluated on the basis of the All Sky Automated Survey (ASAS) photometric project covering all stars south of δ=+ 28° within a magnitude range 8 < V < 13 . Lack of colour indices enforced a limitation to 3374 systems with P < 0.562 d (i.e. 73 per cent of all systems with P < 1 d ) where a simplified MV (log P ) calibration could be used. The spatial density relative to the main-sequence FGK stars of 0.2 per cent, as established previously from the Hipparcos sample to V = 7.5 , is confirmed. While the numbers of contact binaries in the ASAS are large and thus the statistical uncertainties small, derivation of the luminosity function required a correction for missed systems with small amplitudes and with orbital periods longer than 0.562 d; the correction, by a factor of 3, carries an uncertainty of about 30 per cent. 相似文献
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Q. Liu R. de Grijs L. C. Deng Y. Hu I. Baraffe S. F. Beaulieu 《Monthly notices of the Royal Astronomical Society》2009,396(3):1665-1674
We use deep Hubble Space Telescope photometry of the rich, young (∼20- to 45-Myr old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its stellar mass function (MF) down to ∼0.15 M⊙ . This represents the deepest robust MF thus far obtained for a stellar system in an extragalactic, low-metallicity ([Fe/H]≃−0.4 dex) environment. Combining our results with the published MF for masses above 1.0 M⊙ , we obtain a complete present-day MF. This is a good representation of the cluster's initial MF (IMF), particularly at low masses, because our observations are centred on the cluster's uncrowded half-mass radius. Therefore, stellar and dynamical evolution of the cluster will not have affected the low-mass stars significantly. The NGC 1818 IMF is well described by both a lognormal and a broken power-law distribution with slopes of Γ= 0.46 ± 0.10 and Γ≃−1.35 (Salpeter-like) for masses in the range from 0.15 to 0.8 M⊙ and greater than 0.8 M⊙ , respectively. Within the uncertainties, the NGC 1818 IMF is fully consistent with both the Kroupa solar neighbourhood and the Chabrier lognormal mass distributions. 相似文献