An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.     

An ancient nova shell around the dwarf nova Z Camelopardalis

Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, by way of an accretion disk, to its white dwarf companion. In dwarf novae, an instability is believed to episodically dump much of the accretion disk onto the white dwarf. The liberation of gravitational potential energy then brightens these systems by up to 100-fold every few weeks or months. Thermonuclear-powered eruptions thousands of times more luminous occur in classical novae, accompanied by significant mass ejection and formation of clearly visible shells from the ejected material. Theory predicts that the white dwarfs in all dwarf novae must eventually accrete enough mass to undergo classical nova eruptions. Here we report a shell, an order of magnitude more extended than those detected around many classical novae, surrounding the prototypical dwarf nova Z Camelopardalis. The derived shell mass matches that of classical novae, and is inconsistent with the mass expected from a dwarf nova wind or a planetary nebula. The shell observationally links the prototypical dwarf nova Z Camelopardalis with an ancient nova eruption and the classical nova process.     

An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula

Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of the X-ray emission is uncertain. Although many (or perhaps most) recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor approximately 50 increase in the X-ray flux from a young star that is at present undergoing a spectacular optical/infrared outburst (this star illuminates McNeil's nebula). The outburst seems to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/infrared eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion-enhanced X-ray emission from erupting young stars may be short-lived, because intense star-disk magnetospheric interactions are quenched rapidly by the subsequent flood of new material onto the star.     

Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae

Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage. A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape. A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hitherto been unclear, and some previous work has even suggested that a field could not be generated. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, having as its origin a dynamo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly slow rotation of most white dwarf stars.     

Discovery of the progenitor of the type Ia supernova 2007on

Type Ia supernovae are exploding stars that are used to measure the accelerated expansion of the Universe and are responsible for most of the iron ever produced. Although there is general agreement that the exploding star is a white dwarf in a binary system, the exact configuration and trigger of the explosion is unclear, which could hamper their use for precision cosmology. Two families of progenitor models have been proposed. In the first, a white dwarf accretes material from a companion until it exceeds the Chandrasekhar mass, collapses and explodes. Alternatively, two white dwarfs merge, again causing catastrophic collapse and an explosion. It has hitherto been impossible to determine if either model is correct. Here we report the discovery of an object in pre-supernova archival X-ray images at the position of the recent type Ia supernova (2007on) in the elliptical galaxy NGC 1404. Deep optical images (also archival) show no sign of this object. From this we conclude that the X-ray source is the progenitor of the supernova, which favours the accretion model for this supernova, although the host galaxy is older (6-9 Gyr) than the age at which the explosions are predicted in the accreting models.     

An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10

Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first 'seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize?2-10 (refs 5 and 6) contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize?2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize?2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.     

X-ray clusters of galaxies as tracers of structure in the Universe

Clusters of galaxies are visible tracers of the network of matter in the Universe, marking the high-density regions where filaments of dark matter join together. When observed at X-ray wavelengths these clusters shine like cosmic lighthouses, as a consequence of the hot gas trapped within their gravitational potential wells. The X-ray emission is linked directly to the total mass of a cluster, and so can be used to investigate the mass distribution for a sizeable fraction of the Universe. The picture that has emerged from recent studies is remarkably consistent with the predictions for a low-density Universe dominated by cold dark matter.     

Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star

Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.     

Light echoes reveal an unexpectedly cool η Carinae during its nineteenth-century Great Eruption

η?Carinae is one of the most massive binary stars in the Milky Way. It became the second-brightest star in our sky during its mid-nineteenth-century 'Great Eruption', but then faded from view (with only naked-eye estimates of brightness). Its eruption is unique in that it exceeded the Eddington luminosity limit for ten years. Because it is only 2.3 kiloparsecs away, spatially resolved studies of the nebula have constrained the ejected mass and velocity, indicating that during its nineteenth-century eruption, η?Car ejected more than ten solar masses in an event that released ten per cent of the energy of a typical core-collapse supernova, without destroying the star. Here we report observations of light echoes of η?Carinae from the 1838-1858 Great Eruption. Spectra of these light echoes show only absorption lines, which are blueshifted by -210?km?s(-1), in good agreement with predicted expansion speeds. The light-echo spectra correlate best with those of G2-to-G5 supergiants, which have effective temperatures of around 5,000?kelvin. In contrast to the class of extragalactic outbursts assumed to be analogues of the Great Eruption of η?Carinae, the effective temperature of its outburst is significantly lower than that allowed by standard opaque wind models. This indicates that other physical mechanisms such as an energetic blast wave may have triggered and influenced the eruption.     

20世纪中国大旱的一个重要根源

本文研究了新星爆发和中国干旱的关系。从演绎推理和归纳推理两种不同的思维方法得到了相同的结论:新星爆发是中国发生严重干旱和大旱的一个重要根源。具体内容有三点。第一,新星爆发是20世纪中国全部10个最严重旱年的一个根源。第二,最亮视星等3等及更亮的新星爆发后,中国都要发生严重的干旱。这种严重的干旱绝大多数要连续两年,时间多数发生在新星爆发后的第2年和第3年。第三,最亮视星等3等及更亮的新星爆发后,黄河流域都要发生严重的干旱,时间是新星爆发后的第2年或第3年。新星爆发引起中国干旱的程度和最亮视星等有关,大体是新星越亮,引起中国干旱的程度也越重,但并非严格的正比关系。超新星也是引起中国干旱的根源。本文指出,古人畏惧异常天象、害怕它引起自然灾害的观点是正确的,不应被指责为迷信。本文的发现为预警我国今后的大旱提供了参考。     

The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star

The accelerating expansion of the Universe, and the need for dark energy, were inferred from observations of type Ia supernovae. There is a consensus that type Ia supernovae are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that have accreted matter from a companion star, although the nature of this companion remains uncertain. These supernovae are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger: they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass of 1.4 solar masses (M(o)). Here we show that the high-redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar-mass progenitor. Super-Chandrasekhar-mass supernovae should occur preferentially in a young stellar population, so this may provide an explanation for the observed trend that overluminous type Ia supernovae occur only in 'young' environments. As this supernova does not obey the relations that allow type Ia supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events.     

An asymptotic-giant-branch star in the progenitor system of a type Ia supernova

Stars that explode as supernovae come in two main classes. A type Ia supernova is recognized by the absence of hydrogen and the presence of elements such as silicon and sulphur in its spectrum; this class of supernova is thought to produce the majority of iron-peak elements in the Universe. They are also used as precise 'standard candles' to measure the distances to galaxies. While there is general agreement that a type Ia supernova is produced by an exploding white dwarf star, no progenitor system has ever been directly observed. Significant effort has gone into searching for circumstellar material to help discriminate between the possible kinds of progenitor systems, but no such material has hitherto been found associated with a type Ia supernova. Here we report the presence of strong hydrogen emission associated with the type Ia supernova SN2002ic, indicating the presence of large amounts of circumstellar material. We infer from this that the progenitor system contained a massive asymptotic-giant-branch star that lost several solar masses of hydrogen-rich gas before the supernova explosion.     

年青行星状星云NGC7027 HCO^＋谱线发射的一个模型

对在年青行星状星云 NGC7027中新发现的 HCO~+分子离子谱线进行了研究.采用具有不同速度的双壳层膨胀模型,计算了 NGC7027 HCO~+,J=1-0,v=89.2GHz 的谱线发射.将模型结果与观测资料进行比较,导出了双壳层的膨胀速度和半径.发现谱线的理论轮廓与观测轮廓符合甚好.它说明,双壳层的运动学模型对年青行星状星云离子分子谱线的形成可能有重要的作用.     

A blast wave from the 1843 eruption of eta Carinae

Very massive stars shed much of their mass in violent precursor eruptions as luminous blue variables (LBVs) before reaching their most likely end as supernovae, but the cause of LBV eruptions is unknown. The nineteenth-century eruption of eta Carinae, the prototype of these events, ejected about 12 solar masses at speeds of 650 km s(-1), with a kinetic energy of almost 10(50) erg (ref. 4). Some faster material with speeds up to 1,000-2,000 km s(-1) had previously been reported but its full distribution was unknown. Here I report observations of much faster material with speeds up to 3,500-6,000 km s(-1), reaching farther from the star than the fastest material in previous reports. This fast material roughly doubles the kinetic energy of the nineteenth-century event and suggests that it released a blast wave now propagating ahead of the massive ejecta. As a result, eta Carinae's outer shell now mimics a low-energy supernova remnant. The eruption has usually been discussed in terms of an extreme wind driven by the star's luminosity, but the fast material reported here indicates that it may have been powered by a deep-seated explosion rivalling a supernova, perhaps triggered by the pulsational pair instability. This may alter interpretations of similar events seen in other galaxies.     

Gravitational detection of a low-mass dark satellite galaxy at cosmological distance

The mass function of dwarf satellite galaxies that are observed around Local Group galaxies differs substantially from simulations based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard. A massive dark satellite in an early-type lens galaxy at a redshift of 0.222 was recently found using a method based on gravitational lensing, suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low-mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a (1.9?±?0.1)?×?10(8) M dark satellite galaxy in the Einstein ring system JVAS B1938+666 (ref. 11) at a redshift of 0.881, where M denotes the solar mass. This satellite galaxy has a mass similar to that of the Sagittarius galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be 1.1(+0.6)(-0.4), with an average mass fraction of 3.3(+3.6)(-1.8) per cent, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.     

弹片与爆炸冲击波耦合作用分析

弹片和冲击波是武器战斗部毁伤效应的两个重要方面,两者具有一定的相关性.通过研究弹片与爆炸冲击波的耦合作用,分析并推导了弹片在空气中的运动规律和运动方程,以及弹壳对武器爆炸冲击波的影响.通过实例分析,验证了建立的模型和计算方法的合理性和正确性.弹片的最大速度和壳体厚度对爆炸冲击波能量的影响因子随着弹壳厚度的增加而降低.弹壳增厚会导致在爆轰气体与弹壳相互作用的阶段,有更多的能量损耗于壳体膨胀和破碎过程,从而使爆炸冲击波的能量减弱.在实际计算中,必须考虑弹壳的存在以及壳体厚度对爆炸冲击波能量的影响作用.     

A characteristic size of approximately 10 Mpc for the ionized bubbles at the end of cosmic reionization

The first galaxies to appear in the Universe at redshifts z > 20 created ionized bubbles in the intergalactic medium of neutral hydrogen left over from the Big Bang. The ionized bubbles grew with time, surrounding clusters of dwarf galaxies and eventually overlapped quickly throughout the Universe over a narrow redshift interval near z approximately 6. This event signalled the end of the reionization epoch when the Universe was a billion years old. Measuring the size distribution of the bubbles at their final overlap phase is a focus of forthcoming programmes to observe highly redshifted radio emission from atomic hydrogen. Here we show that the combined constraints of cosmic variance and light travel time imply an observed bubble size at the end of the overlap epoch of approximately 10 physical Mpc, and a scatter in the observed redshift of overlap along different lines-of-sight of approximately 0.15. This scatter is consistent with observational constraints from recent spectroscopic data on the farthest known quasars. This implies that future radio experiments should be tuned to a characteristic angular scale of 0.5 degrees and have a minimum frequency bandwidth of approximately 8 MHz for an optimal detection of 21-cm flux fluctuations near the end of reionization.     

一个关于白矮星力学平衡的新理论

将一个新理论——正、负能谱(PNES)热力学理论应用于白矮星系统的力学平衡问题，对白矮星初期淡化阶段的力学平衡进行了详细的分析．虽然应用PNES热力学理论所得出的结论与原有的理论结果一致．但是PNES理论的基本原理和思路与原有的理论完全不同，按照原有理论，白矮星系统只是一种电子系统，氮离子系统只被视为约束电子系统的外部约束条件．显然原有理论不能反映白矮星的实际状态，因为白矮星并不是一个电子系统而是一个由电子子系和氯离子子系组成的复合系统．因此，对白矮星的力学平衡问题和演化问题都必须以由电子子系和氮离子子系组成的复合系统来进行研究．本文应用这一新理论(PNES热力学理论)，以复合系统的观点研究了白矮的力学平衡问题．     

Sublimation from icy jets as a probe of the interstellar volatile content of comets

Comets are some of the most primitive bodies left over from the Solar System's early history. They may preserve both interstellar material and material from the proto-solar nebula, and so studies of their volatile components can provide clues about the evolution of gases and ices, as a collapsing molecular cloud transforms into a mature planetary system. Previous observations of emission from rotational transitions in molecules have averaged over large areas of the inner coma, and therefore include both molecules that sublimed from the nucleus and those that result from subsequent chemical processes in the coma Here we present high-resolution observations of emission from the molecules HNC, DCN and HDO associated with comet Hale-Bopp. Our data reveal arc-like structures-icy jets-offset from (but close to) the nucleus. The measured abundance ratios on 1-3" scales are substantially different from those on larger scales, and cannot be accounted for by models of chemical processes in the coma; they are, however, similar to the values observed in the cores of dense interstellar clouds and young stellar objects. We therefore propose that sublimation from millimetre-sized icy grains ejected from the nucleus provides access to relatively unaltered volatiles. The D/H ratios inferred from our data suggest that, by mass, Hale-Bopp (and by inference the outer regions of the early solar nebula) consists of > or =15-40% of largely unprocessed interstellar material.     

爆炸作用下薄壁柱壳结构动力响应实验研究

为获得薄壁圆柱壳结构在可燃气体爆炸作用下的动力响应特性,该文进行了乙炔/空气混合气体爆炸冲击波对缩比薄壁柱壳模型的冲击实验,测得模型壁面的超压荷载、动态应变和振动加速度时程曲线。通过对时程曲线的变化特征分析,研究了爆炸荷载作用下薄壁柱壳结构的响应特性及其破坏机理。实验结果表明,在爆炸荷载作用下,正反射迎爆区域壁面超压和应变峰值始终最高;柱壳模型抵抗变形破坏的能力与其径高比D/H有关,径高比接近1的模型抵抗变形破坏的能力较强;柱壳模型内部液体能够吸收和耗散爆炸冲击能量。