共查询到20条相似文献，搜索用时 608 毫秒
1.
Sylvain Turcotte 《Journal of Astrophysics and Astronomy》2005,26(2-3):231-239
Chemical composition is a good tracer of the hydrodynamical processes that occur in stars as they often lead to mixing and
particle transport. By comparing abundances predicted by models and those observed in stars we can infer some constraints
on those mixing processes. As pulsations in the stars are often very sensitive to chemical composition, we can use asteroseismology
to probe the internal chemical composition of stars where no direct observations are possible. In this paper I focus on main
sequence stars Am, λ Bootis, and HgMn stars and discuss what we can learn of mixing processes in these stars from seismology. 相似文献
2.
M. J. Seaton 《Astrophysics and Space Science》1996,237(1-2):107-123
Some stars have outer layers which are sufficiently quiescent for diffusion to occur and to modify the relative abundances
of the chemical elements. Levitation occurs when the forces due to radiation pressure are larger than those due to gravitation.
The paper describes some recent work on the calculation of the radiative forces using atomic data obtained in the course of
the work of the Opacity Project.
Large abundance anomalies are observed for the HgMn stars, which lie on or close to the main sequence and have effective temperatures
in the range 11000 to 15000 K. Some results are given for calculated abundances of elements of the iron group in the HgMn
stars.
levitate:
Torise or cause to rise and float in the air, usually attributed to supernatural intervention. (Collins Dictionary of the English Language) 相似文献
3.
L. S. Lyubimkov 《Bulletin of the Crimean Astrophysical Observatory》2014,110(1):9-16
The studies of the abundances of superheavy chemical elements in magnetic stars conducted at the Crimean Astrophysical Observatory were facilitated by the launch of the Astron space station in 1983. This spacecraft observed the ultraviolet spectra of such stars. The present brief review is focused on the abundances of superheavy elements (Pt, Au, Hg, Tl, Pb, Bi, Th, and U) in the atmospheres of magnetic Ap and HgMn stars. These results were obtained basing on the visible and ultraviolet spectra of stars. The data accumulated over more than 30 years show that these stars are characterized by significant overabundances (up to 6–7 dex) of such elements. The following important fact is noted: the superheavy element anomalies follow the trend in the anomalies that are characteristic of less heavy elements. Therefore, it may be assumed that all these anomalies (including the significant overabundances of superheavy elements) share a common explanation. Certain unresolved problems are discussed briefly. 相似文献
4.
In the present paper the abundance anomalies of mercury and its isotopes in the atmospheres of HgMn stars have been studied. Observations have shown strongly anomalous isotopic composition of Hg, Pt, Tl and He in the atmospheres of such CP stars. Generation of elemental abundance anomalies in quiescent atmospheres of CP stars can generally be explained by the mechanism of diffusive segregation of elements due to oppositely directed gravitational and radiative forces. It has been shown that the formation of the observed isotopic anomalies can be successfully explained by a diffusion mechanism called the light‐induced drift (LID). The observed ratios of isotopes also enable to estimate the evolutionary stages of CP stars. 相似文献
5.
We make new non-local thermodynamic equilibrium calculations to deduce the abundances of neon from visible-region echelle spectra of selected Ne i lines in seven normal stars and 20 HgMn stars. We find that the best strong blend-free Ne line that can be used at the lower end of the effective temperature T eff range is λ 6402, although several other potentially useful Ne i lines are found in the red region of the spectra of these stars. The mean neon abundance in the normal stars (log A =8.10) is in excellent agreement with the standard abundance of neon (8.08). However, in HgMn stars neon is almost universally underabundant, ranging from marginal deficits of 0.1–0.3 dex to underabundances of an order of magnitude or more. In many cases, the lines are so weak that only upper limits can be established. The most extreme example found is υ Her with an underabundance of at least 1.5 dex. These underabundances are qualitatively expected from radiative acceleration calculations, which show that Ne has a very small radiative acceleration in the photosphere, and that it is expected to undergo gravitational settling if the mixing processes are sufficiently weak and there is no strong stellar wind. According to theoretical predictions , the low Ne abundances place an important constraint on the intensity of such stellar winds, which must be less than 10−14 M⊙ yr−1 if they are non-turbulent. 相似文献
6.
I. I. Romanyuk E. A. Semenko I. A. Yakunin D. O. Kudryavtsev 《Astrophysical Bulletin》2013,68(3):300-337
A total of 85 CP stars of various types are identified among 814 members of the OriOB1 association. The fraction of CP stars decreases with age for different cluster subgroups: from 15.1% in the youngest subgroup (b) to 7.7% in the oldest one (a). Individual comments are provided for each of the 85 stars, where we analyze the physical parameters and distance of the objects. All the 23 Am stars identified as a result of this study are found to have heliocentric distances between 100 and 300 pc and appear not to be members of the OrionOB1 association, but foreground objects. We identified 59 Bp stars, which account for 13.4% of the total number of B-type stars in the association. The fraction of peculiar B-type stars in the OriOB1 association is found to be twice higher than that of peculiar A-type stars. The same is true for field stars. The association contains 22 magnetic stars, out of which 21 are Bp stars and only one is an Ap star. Seventeen of these stars are objects with anomalous helium lines. Magnetic stars show a well-defined tendency to concentrate in the central region of the association (in Orion’s Belt), which contains most of these objects. No significant differences are found between the field strengths in the B-type stars of the association and Bp-type field stars, although there is a noticeable trend for He-rich stars to have stronger fields compared to He-weak stars. We identified 17 binaries, which make up 20% of the total number of peculiar stars studied, which is the standard ratio for CP stars. Except for one HgMn star (HD35548), the radial velocities and proper motions of our identified objects are consistent with the corresponding parameters of normal B-type stars. 相似文献
7.
Keith C. Smith 《Astrophysics and Space Science》1996,237(1-2):77-105
The upper main sequence is home to a diverse family of chemically peculiar stars, including λ Boo, Am-Fm, Bp-Ap, HgMn, He-weak,
and He-rich varieties. This paper presents an informal review of the physical properties of these objects, including their
location in the H-R diagram, frequency of incidence, rotation, binarity, magnetic fields, and variability. Part of the discussion
is devoted to describing the bizarre surface compositions encountered in chemically peculiar stars, with an emphasis on insights
provided by a generation of ultraviolet observations obtained with theIUE satellite. The paper concludes with an overview of the radiative diffusion mechanism which has been proposed as an explanation
of the chemically peculiar phenomenon. 相似文献
8.
The magnetic chemically peculiar (CP2) stars of the upper main sequence are well‐suited for investigating the impact of magnetic fields on the surface layers of stars, which leads to abundance inhomogeneities (spots) resulting in photometric variability. The light changes are explained in terms of the oblique rotator model; the derived photometric periods thus correlate with the rotational periods of the stars. CP2 stars exhibiting this kind of variability are classified as α2 Canum Venaticorum (ACV) variables. We have analysed around 3850000 individual photometric WASP measurements of magnetic chemically peculiar (CP2) stars and candidates selected from the catalogue of Ap, HgMn, and Am stars, with the ultimate goal of detecting new ACV variables. In total, we found 80 variables, from which 74 are reported here for the first time. The data allowed us to establish variability for 23 stars which had been reported as probably constant in the literature before. Light curve parameters were obtained for all stars by a least‐squares fit with the fundamental sine wave and its first harmonic. Because of the scarcity of Strömgren uvbyβ measurements and the lack of parallax measurements with an accuracy better than 20%, we are not able to give reliable astro‐physical parameters for the investigated objects. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
Yu. V. Glagolevskij 《Astrophysical Bulletin》2017,72(4):418-446
The paper argues in favor of the assumption that magnetic and non-magnetic protostars, from which CP stars were formed, are the objects that had rotation velocities of the parent cloud V smaller than a critical value V c . At V greater than the critical value, differential rotation emerges in the collapsing protostellar cloud, which twists magnetic lines of force into an’ invisible’ toroidal shape and disturbs the stability of the atmosphere. In magnetic protostars, the loss of angular momentum is due to magnetic braking, while in metallic protostars, the loss of rotation momentum occurs due to tidal interactions with a close component. HgMn stars are most likely not affected by some braking mechanism, but originated from the slowest protostellar rotators. The boundary of V c where the differential rotation occurs is not sharp. The slower the protostar rotates, the greater the probability of suppressing the differential rotation and the more likely the possibility of CP star birth. 相似文献
10.
Anharmonic oscillations of rotating stars have been studied by various authors in literature to explain the observed features of certain variable stars. However, there is no study available in literature that has discussed the combined effect of rotation and tidal distortions on the anharmonic oscillations of stars. In this paper, we have created a model to determine the effect of rotation and tidal distortions on the anharmonic radial oscillations associated with various polytropic models of pulsating variable stars. For this study we have used the theory of Rosseland to obtain the anharmonic pulsation equation for rotationally and tidally distorted polytropic models of pulsating variable stars. The main objective of this study is to investigate the effect of rotation and tidal distortions on the shapes of the radial velocity curves for rotationally and tidally distorted polytropic models of pulsating variable stars. The results of the present study show that the rotational effects cause more deviations in the shapes of radial velocity curves of pulsating variable stars as compared to tidal effects. 相似文献
11.
In the context of the MIT bag model we compute the mass defect and the gravitational packing factor for three models of strange stars and study the contribution of gravitational and internal energy to the total energy of the system. For strange stars it is possible to realize a model with negative internal energy, leading to a greater binding energy of these stars compared to neutron stars. This is the reason for the absence of configurations with negative mass defect for the models in question. We analyze the question of identifying the remnants of supernovae with neutron or strange stars.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
12.
13.
Martin Beech 《Astrophysics and Space Science》1989,161(1):133-143
Two related problems are discussed in this article: The width of the Main-Sequence of massive stars and sensitivity peatures introduced into the evolutionary tracks of massive stars by mass loss and core-overshooting. It is suggested that core-overshooting may not necessarily be implied by the observations of the width of the Main-Sequence band. It is also noted that models evolved with both mass loss and/or core-overshooting reveal the presence of a large and unexplained expansion of the stellar models under certain conditions. This sensitivity feature would seem to be a fundamental feature inherent to the structure of massive stars. 相似文献
14.
We calculate NLTE models of stellar winds of hot compact stars (central stars of planetary nebulae and subdwarf stars). The studied range of subdwarf parameters is selected to cover a large part of these stars. The models predict the wind hydrodynamical structure and provide mass-loss rates for different abundances. Our models show that CNO elements are important drivers of subdwarf winds, especially for low-luminosity stars. We study the effect of X-rays and instabilities on these winds. Due to the line-driven wind instability, a significant part of the wind could be very hot. 相似文献
15.
《New Astronomy》2007,12(4):346-352
We monitored the light variations of 16 solar-type stars recently discovered in the X-ray wave-length range during the ROSAT all-sky survey. We find that 9 out of 16 stars showed appreciable light variability with amplitudes of a few hundredths of a magnitude. They are all proved to be in periodic variations. Using the methods of the phase dispersion minimization (PDM) and Fourier Analysis (PERIOD04), we derive the photometric periods for these stars. The rotational periods are found range from 0.471 to 17.31 days and the period of stars most (of 7 stars) being shorter than 3 days. Apart from binaries system, the results give further evidence for the spin up of solar-type stars as predicted by models of angular momentum evolution of pre-main sequence stars. 相似文献
16.
Philip Massey 《New Astronomy Reviews》2013,57(1-2):14-27
The star-forming galaxies of the Local Group act as our laboratories for testing massive star evolutionary models. In this review, I briefly summarize what we believe we know about massive star evolution, and the connection between OB stars, Luminous Blue Variables, yellow supergiants, red supergiants, and Wolf–Rayet stars. The difficulties and recent successes in identifying these various types of massive stars in the neighboring galaxies of the Local Group will be discussed. 相似文献
17.
Be单星和Be/X射线双星作为一类特殊早型天体和特殊的大质量X射线双星 ,在各个波段都有与其它相同光谱型的B型天体显著不同的特征 ,因此长期以来引起中外天文学者的关注。首先在可见光波段发射线的存在 ,就是对仅产生吸收线的经典大气的挑战 ;其次Be星作为一类早型带有包层天体 ,研究Be星包层的性质 ,对研究原恒星包层性质和进一步了解早型星其它光谱型的性质是非常重要的 ;再者可以研究Be星的存在与星际磁场或湍流的星际介质是否有关 ;最后研究Be/X射线双星 ,对双星的演化模型也有很重要的作用。随着天文观测手段的不断完善和理论模型的发展 ,我们对Be星的现象有了更深的了解 ,并且产生了一些模型。研究内容包括Be星包层和星风的性质 ,以及包层形成机制 ,Be/X射线双星物质相互作用等。本文共分五章 ,第一章主要概要介绍Be和Be/X射线双星历史和目前已经取得的成就 ;第二章介绍Be单星多波段观测结果 ;第三章介绍Be/X射线双星的观测结果 ;第四章介绍目前主要的Be单星和Be/X射线双星模型 ;第五章给出Be/X射线双星XPer/ 4U0 352 30的分光观测结果 ,并结合单臂振动盘模型给出定性解释。 相似文献
18.
The properties of such phenomena as the appearance of humps at the ascending branch or double peaks in the light curve of
long-period pulsating stars are examined. The characteristics of humps and the average light curves are obtained for a group
of Mira-type stars and semiregular (SRa) variable stars. Correlations between the parameters of the light curves and the characteristics
of the humps for these stars are discussed. The results obtained here are compared with published theoretical models.
__________
Translated from Astrofizika, Vol. 50, No. 1, pp. 99–105 (February 2007). 相似文献
19.
P. G. Jonker G. Nelemans Z. Wang A. K. H. Kong D. Chakrabarty M. Garcia P. J. Groot M. van der Klis T. Kerr B. Mobasher M. Sullivan T. Augusteijn B. W. Stappers P. Challis R. P. Kirshner J. Hjorth A. Delsanti 《Monthly notices of the Royal Astronomical Society》2003,344(1):201-206
We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of R > 23.1, I > 21.6, Z > 20.6, J > 19.6 and K > 19.2 . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system. 相似文献
20.
Jeremy S. Heyl & Lars Hernquist 《Monthly notices of the Royal Astronomical Society》1998,300(2):599-615
Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B ∼ 1015 − 1016 G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources. 相似文献