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1.
研究和比较了熊大闰的非局部对流和局部对流太阳包层模型的本征振动频率,观测与理论振动频率之间的差别小于1％．它们分为两个分立的群:对l≥60的模,其观测与理论本征振动频率的差完全分布在一条狭窄的倾斜带状区域之内,这说明理论太阳对流区模型大致反映了太阳在r=(0．70-0．95)R⊙这一区域的真实结构,理论与观测频率误差来自外层区域;对于l<60的模,理论的振动频率要比观测的振动频率小,这意味着在对流不稳定区上部的区域温度偏低．另一方面局部对流包层模型的频率差比非局部对流包层模型的频率差更为弥散,中低频端(v<3000)两者差别不大;而在高频端(v≥3000)局部对流包层模型的频率比非局部的频率要高,这意味着局部对流模型在对流区之下的辐射区的温度比非局部对流模型温度要高,非局部对流模型比局部对流模型更接近观测． 相似文献
2.
在Li&Yang (2 0 0 1 )所给出的局部对流理论的基础上 ,我们进一步采用梯度型方案给出了非局部对流理论 ,并将它用于太阳模型中。这一理论考虑了恒星对流区内的非局部效应 ,它得到了一个与原来用混合长理论或局部理论给出的结果有所区别的对流区 ,扩散效应很明显。但是 ,目前我们的理论还不能处理时间相关的对流以及对流超射等问题。这些问题将在后续工作中加以考虑。当把这一理论应用于太阳模型中时 ,我们发现它对标准太阳模型的改正非常微小。我们讨论了这一现象 ,并对其加以解释 相似文献
3.
锂在温度达到T＞2.5×106 K时即参与7Li(p,α)4He反应燃烧,经典恒星理论推测,当恒星演化到红巨星阶段时,由于向内非常延伸的对流包层的存在,锂的丰度A(Li)=12+ lg[n(Li)/n(H)]应为A(Li)＜0.5.但是,陆续有处于不同光谱型、不同演化阶段的富锂巨星被发现,其中一些巨星的锂丰度甚至远超出宇宙大爆炸的原初锂丰度,给经典恒星演化模型带来了难题.回顾了从恒星主序到巨星支的锂丰度观测研究历史,并详细介绍了我们对银河系矮星系中富锂巨星的最新发现,总结了所有富锂巨星的观测证据,以及为解决此难题所提出的理论解释. 相似文献
4.
根据化学非均匀恒星的非局部对流理论,计算了质量为0．7-1．15M(?)恒星主序演化模型锂和铍的衰减,并将理论和不同年龄疏散星团锂丰度的观测进行了比较．结果表明,贯穿对流混合能重现温度低于6400 K的晚型主序星锂丰度观测的一般性质．贯穿对流混合可能是晚型矮星锂衰减的一种重要机制． 相似文献
5.
依传统的局部混合长理论进行了对流包层模型计算,在黄红巨星和超巨星对流区的顶部出现超声速对流的情况,这与局部对流理论本身是不自恰的．文章详细分析了恒星大气超声速对流的成因,并按照星族Ⅰ恒星的演化轨迹,在赫罗图上给出了出现超声速对流的区域．其结果是。在演化过程中, (1)低质量星基本不出现超声速对流; (2)中等质量星的超声速对流主要出现在3．6相似文献
6.
熊大闰 《紫金山天文台台刊》2003,(2)
贯穿对流是恒星演化理论长期未获很好解决的理论问题。尽管它已有近五十年的研究历史 ,但至今人们对贯穿对流仍存在很多的错误的理解。究其原因是因为人们对贯穿对流的理解依然停留在唯象的混合长理论图象上。根据一种非局部对流理论 ,我们揭示了贯穿对流区的结构。在太阳下部的贯穿对流区 ,温度梯度是亚绝热的 ,但是是超辐射的。本文还对恒星对流理论给出了一个简短的评述。 相似文献
7.
太阳模型的研究是了解太阳整体结构和性质的极为重要的手段。90年代以来太阳模型研究取得了进展。随着MHD及OPAL物态方程的引入,理论上的太阳振荡频率与观测值的差别已大为减小,而考虑湍流频谱分布的局域对流理论和三维流体动力学模拟结果可对太阳内部对流能量传输过程有更深刻的理解．以前所发现的理论模型与反演结果得到的初始氦丰度的差别已能由扩散过程加以解释,而太阳表面锂丰度亏损问题也可以由扩散过程或早期演化星风来加以解决,太阳中微子问题则似应由粒子物理而不是天体物理来解决。 相似文献
8.
本文对采用局部对流理论计算太阳稳定态对流层结构所得部分结果进行分析讨论。用实例说明,选用混合长,改变化学成份,估计对流元的辐射能损大小等方面对对流层结构和厚度均有影响。改变这些参数,可以得到10~4公里的薄模型和2×10~5公里的厚模型。 相似文献
9.
《天文学报》2014,(4)
捅姜利用一种非局部和非定常的恒星对流理论,计算了3种不同氦丰度(Y=0.28,0.13,0.00)、太阳金属丰度(Z=0.02)、质量1.4~3.0 M_☉恒星演化模型的径向和低球谐阶F-p8模的线性非绝热脉动.数值计算结果表明,理论的δScuti变星脉动不稳定区红边界几乎不随氦丰度变化而改变.随着氦丰度降低,δScuti变星脉动不稳定区蓝边界向低温方向推移,脉动不稳定区中热方的高温星变得脉动更稳定;而冷方的低温星则变得脉动更不稳定.这似乎表明不大可能用氦弥散去解释δScuti脉动不稳定带中的不变星.然而脉动不稳定带热方和冷方中的不变星与变星的比例可能作为是否存在氦弥散的一种观测证据. 相似文献
10.
11.
Guillermo Gonzalez 《Monthly notices of the Royal Astronomical Society》1999,308(2):447-458
The chemical-dynamical properties of stars with giant planets are compared to those of a nearby star sample within the framework of a stellar orbital diffusion model. The stars-with-planets sample includes recently discovered extrasolar planets and the Sun. We find that the planet-bearing stars, 14 Her, ρ 1 Cnc and τ Boo, are much more metal-rich than stars of similar age and this cannot be easily explained by orbital diffusion. We also confirm previous claims that the motion of the Sun relative to the local standard of rest is very small compared to other G dwarfs of similar age, and we offer a possible explanation for this apparent anomaly. 相似文献
12.
J. P. Phillips 《Monthly notices of the Royal Astronomical Society》2007,381(1):117-124
An analysis is undertaken of the relation between dust/gas mass ratios and elemental abundances within planetary nebulae (PNe). It is found that M DUST / M GAS is broadly invariant with abundance, and similar to the values observed in asymptotic giant branch (AGB)-type stars. However, it is noted that the masses of dust observed in low-abundance PNe are similar to the masses of heavy elements observed in the gas phase. This is taken to imply that levels of elemental depletion must be particularly severe, and extend to many more species than have been identified so far. In particular, given that levels of C and O depletion are likely to be large, then this probably implies that species such as Fe, S, Si and Mg are depleted as well. There is already evidence for depletion of Fe, Si and Mg in individual PNe. It follows that whilst quoted abundances may accurately reflect gas-phase conditions, they are likely to be at variance with intrinsic abundances in low Z N nebulae.
Finally, we note that there appears to be a variation in dust/gas mass ratios with galactocentric distance, with gradient similar to that observed for several elemental abundances. This may represent direct evidence for a correlation between dust/gas mass ratios and nebular abundances. 相似文献
Finally, we note that there appears to be a variation in dust/gas mass ratios with galactocentric distance, with gradient similar to that observed for several elemental abundances. This may represent direct evidence for a correlation between dust/gas mass ratios and nebular abundances. 相似文献
13.
M. A. Iatì C. Cecchi-Pestellini D. A. Williams F. Borghese P. Denti R. Saija S. Aiello 《Monthly notices of the Royal Astronomical Society》2001,322(4):749-756
Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities. 相似文献
14.
S. Vauclair 《Journal of Astrophysics and Astronomy》2000,21(3-4):323-329
The inversion of helioseismic modes leads to the sound velocity inside the Sun with a precision of about 0.1 per cent. Comparisoons
of solar models with the &#x201C;seismic sun&#x201D; represent powerful tools to test the physics: depth of the convection
zone, equation of state, opacities, element diffusion processes and mixing inside the radiative zone. We now have evidence
that microscopic diffusion (element segregation) does occur below the convection zone, leading to a mild helium depletion
in the solar outer layers. Meanwhile this process must be slowed down by some macroscopic effect, presumably rotation-induced
mixing. The same mixing is also responsible for the observed lithium depletion. On the other hand, the observations of beryllium
and helium 3 impose specific constraints on the depth of this mildly mixed zone. Helioseismology also gives information on
the internal solar rotation: while differential rotation exists in the convection zone, solid rotation prevails in the radiative
zone, and the transition layer (the so-called &#x201C;tachocline&#x201D;) is very small. These effects are discussed,
together with the astrophysical constraints on the solar neutrino fluxes. 相似文献
15.
Previous studies using observations made at low spatial and spectral resolution showed that the resonance lines of He i and He ii are anomalously strong in the quiet Sun when compared with other transition region lines formed at similar temperatures. Here, the higher spatial and spectral resolution provided by the Coronal Diagnostic Spectrometer ( cds ) instrument on board the Solar and Heliospheric Observatory ( SOHO ) is used to re-examine the behaviour of the He i and He ii lines and other transition region lines, in quiet regions near Sun centre. Supergranulation cell boundaries and cell interiors are examined separately. Near-simultaneous observations with the sumer instrument provide information on the lower transition region and the electron pressure. While the lines of He i and He ii have a common behaviour, as do the other transition region lines, the behaviour of the helium lines relative to the other transition region lines is significantly different. The emission measure distributions that account for all transition region lines, except those of helium, fail to produce sufficient emission in the He i and He ii resonance lines by around an order of magnitude, in both supergranulation cell boundary and cell interior regions. The electron pressure appears to be higher in the cell interiors than in the average cell boundaries, although the uncertainties are large. While the VAL-D model gives a closer match to the He i 584.3-Å line, it does not successfully reproduce other transition region lines. 相似文献
16.
Using a self-consistent dynamic theory of non-local convection, the atmospheric lithium-abundance depletion values of a series of stellar evolutionary models with are calculated, and the results show that the general observed properties of lithium abundance in lower-main-sequence stars can be reproduced by overshooting mixing and gravitational settling. After a careful study of the mechanisms of lithium depletion in stars, it is concluded that overshooting mixing and microdiffusion induced by gravitational settling and radiative acceleration are the two primary depletion mechanisms: for warm stars with (or ) microdiffusion dominates, while it is the other way round for cooler objects with (or ). 相似文献
17.
C. Jordan K.P. Macpherson G.R. Smith 《Monthly notices of the Royal Astronomical Society》2001,328(4):1098-1114
Observations made at the quiet Sun-centre with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments on the Solar and Heliospheric Observatory ( SOHO ) have shown that the intensities of the resonance lines of He i and He ii are significantly larger than predicted by emission measure distributions found from other transition region lines. The intensities of the helium lines are observed to be lower in coronal holes than in the quiet Sun. Any theory proposed to account for the behaviour of the helium lines must explain the observations of both the quiet Sun and coronal holes. We use observations made with SOHO to find the physical conditions in a polar coronal hole. The electron pressure is found using the C iii 1175-Å and N iii 991.5-Å lines, as the C iii line at 977.0 Å becomes optically thick in some regions at high latitudes. The mean electron pressure is a factor of ≃2 lower than that at the quiet Sun-centre. The mean coronal electron temperature is . The helium lines are enhanced with respect to other transition region lines but by factors which are ≃ 30 per cent smaller than at the quiet Sun-centre. The mean ratios of the intensities of the He i 537.0- and 584.3-Å lines and of the He i and He ii 303.8-Å lines vary little with the type of region studied. These ratios are compared with those predicted by models of the transition region, taking into account the radiative transfer in the helium lines. No significant variation is found in the relative abundances of carbon and silicon. 相似文献
18.
Joseph D. Meiring Varsha P. Kulkarni James T. Lauroesch Celine Péroux Pushpa Khare Donald G. York Arlin P. S. Crotts 《Monthly notices of the Royal Astronomical Society》2008,384(3):1015-1033
We present chemical abundance measurements from medium-resolution observations of eight subdamped Lyα (sub-DLA) absorber and two strong Lyman-limit systems at z ≲ 1.5 observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in z abs ≲ 1.5 quasar absorption line systems focusing on sub-DLA systems. These observations increase the sample of Zn measurements in z abs ≲ 1.5 sub-DLAs by ∼50 per cent. Lines of Mg i , Mg ii , Al ii , Al iii , Ca ii , Mn ii , Fe ii and Zn ii were detected and column densities were determined. Zn ii , a relatively undepleted element and tracer of the gas-phase metallicity is detected in two of these systems, with [Zn/H]=−0.05 ± 0.12 and [Zn/H] > +0.86 . The latter one is however a weak system with N H I < 18.8 , and therefore may need significant ionization corrections to the abundances. Fe ii lines were detected in all systems, with an average Fe abundance of 〈[Fe/H]〉=−0.68 , higher than typical Fe abundances for DLA systems at these redshifts. This high mean [Fe/H] could be due to less depletion of Fe on to dust grains, or to higher abundances in these systems. We also discuss the relative abundances in these absorbers. The systems with high metallicity show high ratios of [Mn/Fe] and [Zn/Fe], as seen previously in another sub-DLA. These higher values of [Mn/Fe] could be a result of heavy depletion of Fe on to grains, unmixed gas, or an intrinsically non-solar abundance pattern. Based on cloudy modelling, we do not expect ionization effects to cause this phenomenon. 相似文献
19.
L. S. Lyubimkov S. I. Rostopchin T. M. Rachkovskaya D. B. Poklad D. L. Lambert 《Monthly notices of the Royal Astronomical Society》2005,358(1):193-201
From high-resolution spectra a non-local thermodynamic equilibrium analysis of the Mg ii 4481.2-Å feature is implemented for 52 early and medium local B stars on the main sequence (MS). The influence of the neighbouring line Al iii 4479.9-Å is considered. The magnesium abundance is determined; it is found that log ɛ(Mg) = 7.67 ± 0.21 on average. It is shown that uncertainties in the microturbulent parameter Vt are the main source of errors in log ɛ(Mg) . When using 36 stars with the most reliable Vt values derived from O ii and N ii lines, we obtain the mean abundance log ɛ(Mg) = 7.59 ± 0.15 . The latter value is precisely confirmed for several hot B stars from an analysis of the Mg ii 7877-Å weak line. The derived abundance log ɛ(Mg) = 7.59 ± 0.15 is in excellent agreement with the solar magnesium abundance log ɛ⊙ (Mg) = 7.55 ± 0.02 , as well as with the proto-Sun abundance log ɛ ps (Mg) = 7.62 ± 0.02 . Thus, it is confirmed that the Sun and the B-type MS stars in our neighbourhood have the same metallicity. 相似文献
20.
We obtain the chemical abundances of six barium stars and two CH subgiant stars based on the high signal-to-noise ratio and high resolution Echelle spectra. The neu- tron capture process elements Y, Zr, Ba, La and Eu show obvious overabundances relative to the Sun, for example, their [Ba/Fe] values are from 0.45 to 1.27. Other elements, in- cluding Na, Mg, A1, Si, Ca, Sc, Ti, V, Cr, Mn and Ni, show comparable abundances to the Solar ones, and their [Fe/H] covers a range from -0.40 to 0.21, which means they belong to the Galactic disk. The predictions of the theoretical model of wind accretion for bi- nary systems can explain the observed abundance patterns of the neutron capture process elements in these stars, which means that their overabundant heavy-elements could be caused by accreting the ejecta of AGB stars, the progenitors of present-day white dwarf companions in binary systems. 相似文献