湍流应力对AGB星物质损失率作用的初步探讨

通过分析8M⊙和2.8M⊙早期AGB星外部湍流应力与引力的比值在几个演化点上随质量的分布,发现在恒星外部存在一个湍流应力与引力的比值能达到几十的区域,该区域的位置随演化时间的变化而变化,此位置以外的壳层质量随时间的变化规律与Nieuwenhuijzen H.et.Al给出的恒星质量损失率的变化规律完全相同.在此基础上拟合出早期AGB星的质量损失率与出现最大湍流应力位置以外的壳层质量之间的关系式,提出湍流作用对星风的主要影响应体现在湍流作用大的区域在恒星外壳中的位置.     

大质量恒星的星风

大质量恒星活动的重要特征之一就是它们具有强烈的星风.恒星的质量是决定恒星结构和演化的主要参量之一.由于强烈星风带走恒星的大量物质,使恒星的质量不断减小,所以它严重影响大质量恒星的结构和演化.在过去半个多世纪中,无论在观测方面还是在理论方面,大质量恒星的星风研究取得了长足进展.在理论方面,已经建立了标准的星风加速模型,以及它的改良模型,包括利用有谱线覆盖效应的NLTE恒星大气模型进行光谱分析;在观测方面,地面望远镜和空间设备相结合,从射电波段一直到高能X射线波段,对大质量恒星进行了全波段的观测研究.理论研究和观测结果基本符合,但同时又引出了一些新问题.     

科学家首次观测到恒星风激烈碰撞

恒星风是恒星表面发出的物质流,是恒星质量流失的一种手段.恒星风在所有恒星中都普遍存在,但速度和强度有很大差别.太阳发出的星风通常称为太阳风,速度大约为每小时200～300km.从冕洞吹出的太阳风速度则要大一些,大约每小时700km.太阳通过星风损失质量的速率约为每年10-14倍太阳质量,在一生中通过星风大约会损失掉0.01%的质量,因此星风对其演化的影响可以忽略不计.     

金属丰度和对流超射对恒星演化影响的初步探讨

文章采用了pd90程序模拟了5 M⊙的恒星,在主序和主序后的演化情况。并且根据其程序给出的计算结果,绘出了5 M⊙恒星的赫罗图。选择金属丰度和对流超射的不问参数,形成六条赫罗图线。在赫罗图中,对5 M⊙的恒星其金属丰度和对流超射对恒星演化的影响进行讨论。从赫罗图中的lgL/L⊙和lgTeff,与演化时间序列图,可以直观的得出金属丰度和对流超射变化对恒星演化的影响。     

转动恒星引力昏暗效应及演化研究

转动和潮汐效应是影响恒星结构和演化非常重要的物理因素。根据Achernar的观测数据,用考虑转动和潮汐效应的单、双星模型,研究了Achernar引力昏暗现象。发现转动双星模型比单星模型更能符合Achernar的赤道和极半径之比观测值。计算结果表明,初始转动角速度快的恒星,其等价半径和中心密度在主序阶段的开始至40 M yr较大,随后变小。同时,由于初始转动角速度大的恒星,星风携带自转角动量损失多,造成后期演化角速度变小。另外,转动效应能增加恒星的中心集中度,但减少恒星的四极矩、回旋半径、中心温度、氢燃烧产能率,使转动恒星向赫罗图的低温和低光度演化。     

黄润乾 执著的星空守望者

<正>作为一名中国科学院院士,黄润乾在国内从事恒星物理研究50余年,建立了物质和角动量损失的非守恒双星演化理论,转动双星演化理论和星风碰撞激波理论,为恒星结构和演化理论的发展做出了重要贡献。那片蓝天中的收获自1958年从德国席勒大学学成归国,黄润乾先后在中科院兰州物理研究所和中科院云南天文台等单位工作。黄润乾对天     

潮汐转矩对转动恒星结构和演化的影响研究

潮汐效应是影响恒星结构和演化非常重要的物理因素.本文研究了影响潮汐转矩系数E_2的三个理论模型.根据转动恒星中的角动量传输和元素扩散方程,给出了潮汐转矩系数E_2对转动恒星内部结构和元素混合的影响.结果表明:潮汐转矩系数E_2与恒星质量、金属丰度、演化时间有密切关系.潮汐转矩系数E_2越大,双星系统轨道角动量转化为子星自转角动量和自转角动量在恒星内部传输的效率越高.强潮汐转矩造成恒星表面有较大的氦和氮元素的超丰和对流核质量,使恒星具有较高的光度.然而,对比模型M1和M3,充分的转动混合效应可以降低元素的不透明度和辐射温度梯度,压制转动动力学效应造成的辐射温度梯度的增加,使恒星中心对流核减少.强潮汐产生的转动混合效应使氢元素丰度和数密度增加,增强氢燃烧效率,提高中心核温度,使恒星膨胀,具有较大的转动惯量.因此,研究潮汐效应对元素混合效应的影响,对密近双星的演化具有重要意义.     

三轴椭球体模型中星风物质损失率的分布

采用三轴椭球体模型和理论计算,数值模拟并分析了子星的星风物质损失率分布。结果表明三轴椭球体模型下受有效重力加速度和不透明度的影响,子星的星风物质损失率分布是非均匀的,在该分布和子星形变的共同影响下角动量损失率的分布也是非均匀的。     

RGB星分界点的不同对恒星演化的影响

提出恒星演化阶段分界点的不同会影响恒星在赫罗图中演化轨迹的观点．基于在恒星演化计算中质量损失率应是一连续的分段函数的考虑，使用改进后的恒星演化计算程序．采用2种不同的分界点分别计算了恒星的演化，基于质量损失率函数连续性的考虑，对于主序星与RGB星的分界点，选取Hayashi线光度最低处比选取核心H丰度为零处更合适．     

对流超射对5M_⊙恒星演化的影响

计算了5M⊙恒星从主序到红巨星阶段的演化,将考虑对流超射的演化结果与不考虑对流超射的演化结果进行了对比.结果发现:恒星在赫罗图中的演化轨迹向着低温度高光度的方向移动;氢主序与红巨星的寿命之和几乎不变;中心温度随其密度变化较为明显,并对其进行了讨论与分析.     

Massive star formation in 100,000 years from turbulent and pressurized molecular clouds

Massive stars (with mass m* > 8 solar masses Mmiddle dot in circle) are fundamental to the evolution of galaxies, because they produce heavy elements, inject energy into the interstellar medium, and possibly regulate the star formation rate. The individual star formation time, t*f, determines the accretion rate of the star; the value of the former quantity is currently uncertain by many orders of magnitude, leading to other astrophysical questions. For example, the variation of t*f with stellar mass dictates whether massive stars can form simultaneously with low-mass stars in clusters. Here we show that t*f is determined by the conditions in the star's natal cloud, and is typically about 105yr. The corresponding mass accretion rate depends on the pressure within the cloud--which we relate to the gas surface density--and on both the instantaneous and final stellar masses. Characteristic accretion rates are sufficient to overcome radiation pressure from about 100M middle dot in circle protostars, while simultaneously driving intense bipolar gas outflows. The weak dependence of t*f on the final mass of the star allows high- and low-mass star formation to occur nearly simultaneously in clusters.     

gamma Rays from close binaries in the quiet stage

THE presence of rotating neutron stars in binary systems is revealed by the existence of X-ray pulsators which are generally associated with massive companions. The evolutionary history of these systems is now relatively clear (see ref. 1 and refs therein). The X-ray phase, which corresponds to accretion on the neutron star of a strong stellar wind \M approximately 10(-6) M(\circ) yr(-1), is preceded by a much longer quiet state, where the primary is unevolved, possibly with a weak wind, \M approximately 10(-9) M(\circ) yr(-1) and the rotational energy loss of the neutron star inhibits accretion. However, with the only exception of PSR1913 + 16, radio pulsars are not found in binary systems. Here, motivated by the recent discovery of gamma-ray emission from slow pulsars(2,3), we suggest (gamma)-ray observations as a way of detecting binaries in the quiet state and compare the expected number with the COS B results. We refer to the review of van den Heuvel(1) where, as a representative case, a system of initial mass 20 + 8 M(\circ) and orbital period 4.7 d is considered. After the first stage of mass exchange and the supernova explosion, one has a neutron star with an unevolved companion of 22.7 M(\circ). The period is now P = 12.6 d. The primary remains on the main sequence (quiet stage) for t(ms) = 3.6 x 10(6)yr. The X-ray phase occurs after the star has left the main sequence, but before it fills its Roche lobe, as an excessive mass transfer absorbs the X-ray emission. Its duration is t(x) approximately 10(4)yr.     

Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing

In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars (the most common stars in our Galaxy), this model favours the formation of Earth-mass (M(o)) to Neptune-mass planets with orbital radii of 1 to 10 astronomical units (au), which is consistent with the small number of gas giant planets known to orbit M-dwarf host stars. More than 170 extrasolar planets have been discovered with a wide range of masses and orbital periods, but planets of Neptune's mass or less have not hitherto been detected at separations of more than 0.15 au from normal stars. Here we report the discovery of a 5.5(+5.5)(-2.7) M(o) planetary companion at a separation of 2.6+1.5-0.6 au from a 0.22+0.21-0.11 M(o) M-dwarf star, where M(o) refers to a solar mass. (We propose to name it OGLE-2005-BLG-390Lb, indicating a planetary mass companion to the lens star of the microlensing event.) The mass is lower than that of GJ876d (ref. 5), although the error bars overlap. Our detection suggests that such cool, sub-Neptune-mass planets may be more common than gas giant planets, as predicted by the core accretion theory.     

流夸克质量对混合星性质的影响

利用强子物质的相对论平均场理论和夸克物质的有效质量口袋模型, 研究流夸克质量的选取对混合星性质的影响. 结果表明, 随着流夸克质量的增大, 强子\|夸克相变使密度变大, 超子种类变多, 混合相区域的状态方程变硬, 中子星的最大质量及半径变大. 当奇异流夸克质量由90 MeV增到200 MeV时, 混合星最大质量由1.4 M⊙增至1.63 M⊙（M⊙=1.99×1030kg为太阳质量）, 半径由10.24 km增至11.64 km.      

核子“直接URCA”过程发生的条件和中子星的冷却机制

讨论了核子的“直接ＵＲＣＡ”过程发生的条件，发现在ＵＶ１４＋ＵＶⅡ态式描述的中子星物质中，核子的“直接ＵＲＣＡ”过程发生的条件是质子数分数必须超过０．１４；强调在中子昨丙“标准冷却”机制和“非标准冷却”机械可能同时存在；对几个没质量中子星的冷却过程的计算结果表明，引力 质量小于约１．６Ｍ的中子星核内应发生“标准冷却”过程，而在引力质量大于约１．６Ｍ的中子星核内可能存在２种冷却机制。     

A neutron-star-driven X-ray flash associated with supernova SN 2006aj

Supernovae connected with long-duration gamma-ray bursts (GRBs) are hyper-energetic explosions resulting from the collapse of very massive stars ( approximately 40 M\circ, where M\circ is the mass of the Sun) stripped of their outer hydrogen and helium envelopes. A very massive progenitor, collapsing to a black hole, was thought to be a requirement for the launch of a GRB. Here we report the results of modelling the spectra and light curve of SN 2006aj (ref. 9), which demonstrate that the supernova had a much smaller explosion energy and ejected much less mass than the other GRB-supernovae, suggesting that it was produced by a star whose initial mass was only approximately 20 M\circ. A star of this mass is expected to form a neutron star rather than a black hole when its core collapses. The smaller explosion energy of SN 2006aj is matched by the weakness and softness of GRB 060218 (an X-ray flash), and the weakness of the radio flux of the supernova. Our results indicate that the supernova-GRB connection extends to a much broader range of stellar masses than previously thought, possibly involving different physical mechanisms: a 'collapsar' (ref. 8) for the more massive stars collapsing to a black hole, and magnetic activity of the nascent neutron star for the less massive stars.     

A minimum column density of 1 g cm(-2) for massive star formation

Massive stars are very rare, but their extreme luminosities make them both the only type of young star we can observe in distant galaxies and the dominant energy sources in the Universe today. They form rarely because efficient radiative cooling keeps most star--forming gas clouds close to isothermal as they collapse, and this favours fragmentation into stars of one solar mass or lower. Heating of a cloud by accreting low-mass stars within it can prevent fragmentation and allow formation of massive stars, but the necessary properties for a cloud to form massive stars-and therefore where massive stars form in a galaxy--have not yet been determined. Here we show that only clouds with column densities of at least 1 g cm(-2) can avoid fragmentation and form massive stars. This threshold, and the environmental variation of the stellar initial mass function that it implies, naturally explain the characteristic column densities associated with massive star clusters and the difference between the radial profiles of Halpha and ultraviolet emission in galactic disks. The existence of a threshold also implies that the initial mass function should show detectable variation with environment within the Galaxy, that the characteristic column densities of clusters containing massive stars should vary between galaxies, and that star formation rates in some galactic environments may have been systematically underestimated.     

A 15.65-solar-mass black hole in an eclipsing binary in the nearby spiral galaxy M 33

Stellar-mass black holes are found in X-ray-emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses more than ten times that of the Sun (>10; ref. 4), which is consistent with the fact that the most massive stellar black holes known so far all have masses within one standard deviation of 10. Here we report a mass of (15.65 +/- 1.45) for the black hole in the recently discovered system M 33 X-7, which is located in the nearby galaxy Messier 33 (M 33) and is the only known black hole that is in an eclipsing binary. To produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45-day orbit about its (70.0 +/- 6.9) companion, there must have been a 'common envelope' phase of evolution in which a significant amount of mass was lost from the system. We find that the common envelope phase could not have occurred in M 33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.     

Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun

Theory predicts and observations confirm that low-mass stars (like the Sun) in their early life grow by accreting gas from the surrounding material. But for stars approximately 10 times more massive than the Sun (approximately 10M(o)), the powerful stellar radiation is expected to inhibit accretion and thus limit the growth of their mass. Clearly, stars with masses >10M(o) exist, so there must be a way for them to form. The problem may be solved by non-spherical accretion, which allows some of the stellar photons to escape along the symmetry axis where the density is lower. The recent detection of rotating disks and toroids around very young massive stars has lent support to the idea that high-mass ( > 8M(o)) stars could form in this way. Here we report observations of an ammonia line towards a high-mass star forming region. We conclude that the gas is falling inwards towards a very young star of approximately 20M(o), in line with theoretical predictions of non-spherical accretion.     

RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system

RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies. There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system, because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses M[symbol see text] and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with M[symbol see text] and 0.8M[symbol see text]stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung-Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.