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.     

An absence of ex-companion stars in the type Ia supernova remnant SNR 0509-67.5

A type Ia supernova is thought to begin with the explosion of a white dwarf star. The explosion could be triggered by the merger of two white dwarfs (a 'double-degenerate' origin), or by mass transfer from a companion star (the 'single-degenerate' path). The identity of the progenitor is still controversial; for example, a recent argument against the single-degenerate origin has been widely rejected. One way to distinguish between the double- and single-degenerate progenitors is to look at the centre of a known type Ia supernova remnant to see whether any former companion star is present. A likely ex-companion star for the progenitor of the supernova observed by Tycho Brahe has been identified, but that claim is still controversial. Here we report that the central region of the supernova remnant SNR 0509-67.5 (the site of a type Ia supernova 400?±?50 years ago, based on its light echo) in the Large Magellanic Cloud contains no ex-companion star to a visual magnitude limit of 26.9 (an absolute magnitude of M(V) = +8.4) within a region of radius 1.43 arcseconds. (This corresponds to the 3σ maximum distance to which a companion could have been 'kicked' by the explosion.) This lack of any ex-companion star to deep limits rules out all published single-degenerate models for this supernova. The only remaining possibility is that the progenitor of this particular type Ia supernova was a double-degenerate system.     

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.     

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.     

The binary progenitor of Tycho Brahe's 1572 supernova

The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion. Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties. Tycho's supernova is one of only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the surviving companion. Here we report a survey of the central region of its remnant, around the position of the explosion, which excludes red giants as the mass donor of the exploding white dwarf. We found a type G0-G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which appears to be the surviving companion of the supernova.     

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.     

The massive binary companion star to the progenitor of supernova 1993J

The massive star that underwent a collapse of its core to produce supernova (SN)1993J was subsequently identified as a non-variable red supergiant star in images of the galaxy M81 taken before explosion. It showed an excess in ultraviolet and B-band colours, suggesting either the presence of a hot, massive companion star or that it was embedded in an unresolved young stellar association. The spectra of SN1993J underwent a remarkable transformation from the signature of a hydrogen-rich type II supernova to one of a helium-rich (hydrogen-deficient) type Ib. The spectral and photometric peculiarities were best explained by models in which the 13-20 solar mass supergiant had lost almost its entire hydrogen envelope to a close binary companion, producing a 'type IIb' supernova, but the hypothetical massive companion stars for this class of supernovae have so far eluded discovery. Here we report photometric and spectroscopic observations of SN1993J ten years after the explosion. At the position of the fading supernova we detect the unambiguous signature of a massive star: the binary companion to the progenitor.     

An X-ray-emitting blast wave from the recurrent nova RS Ophiuchi

Stellar explosions such as novae and supernovae produce most of the heavy elements in the Universe. The onset of a nova is well understood as driven by runaway thermonuclear fusion reactions on the surface of a white dwarf in a binary star system; but the structure, dynamics and mass of the ejecta are not well known. In rare cases, the white dwarf is embedded in the wind nebula of a red-giant companion, and the explosion products plough through the nebula and produce X-ray emission. Here we report X-ray observations of such an event, from the eruption of the recurrent nova RS Ophiuchi. The hard X-ray emission from RS Ophiuchi early in the eruption emanates from behind a blast wave, or outward-moving shock wave, that expanded freely for less than 2 days and then decelerated owing to interaction with the nebula. The X-rays faded rapidly, suggesting that the blast wave deviates from the standard spherical shell structure. The early onset of deceleration indicates that the ejected shell had a low mass, the white dwarf has a high mass, and that RS Ophiuchi is therefore a progenitor of the type of supernova (type Ia) integral to studies of the expansion of the Universe.     

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.     

A giant outburst two years before the core-collapse of a massive star

The death of massive stars produces a variety of supernovae, which are linked to the structure of the exploding stars. The detection of several precursor stars of type II supernovae has been reported (see, for example, ref. 3), but we do not yet have direct information on the progenitors of the hydrogen-deficient type Ib and Ic supernovae. Here we report that the peculiar type Ib supernova SN 2006jc is spatially coincident with a bright optical transient that occurred in 2004. Spectroscopic and photometric monitoring of the supernova leads us to suggest that the progenitor was a carbon-oxygen Wolf-Rayet star embedded within a helium-rich circumstellar medium. There are different possible explanations for this pre-explosion transient. It appears similar to the giant outbursts of luminous blue variable stars (LBVs) of 60-100 solar masses, but the progenitor of SN 2006jc was helium- and hydrogen-deficient (unlike LBVs). An LBV-like outburst of a Wolf-Rayet star could be invoked, but this would be the first observational evidence of such a phenomenon. Alternatively, a massive binary system composed of an LBV that erupted in 2004, and a Wolf-Rayet star exploding as SN 2006jc, could explain the observations.     

Survival of a brown dwarf after engulfment by a red giant star

Many sub-stellar companions (usually planets but also some brown dwarfs) orbit solar-type stars. These stars can engulf their sub-stellar companions when they become red giants. This interaction may explain several outstanding problems in astrophysics but it is unclear under what conditions a low mass companion will evaporate, survive the interaction unchanged or gain mass. Observational tests of models for this interaction have been hampered by a lack of positively identified remnants-that is, white dwarf stars with close, sub-stellar companions. The companion to the pre-white dwarf AA Doradus may be a brown dwarf, but the uncertain history of this star and the extreme luminosity difference between the components make it difficult to interpret the observations or to put strong constraints on the models. The magnetic white dwarf SDSS J121209.31 + 013627.7 may have a close brown dwarf companion but little is known about this binary at present. Here we report the discovery of a brown dwarf in a short period orbit around a white dwarf. The properties of both stars in this binary can be directly observed and show that the brown dwarf was engulfed by a red giant but that this had little effect on it.     

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.     

A test of general relativity from the three-dimensional orbital geometry of a binary pulsar.

Binary pulsars provide an excellent system for testing general relativity because of their intrinsic rotational stability and the precision with which radio observations can be used to determine their orbital dynamics. Measurements of the rate of orbital decay of two pulsars have been shown to be consistent with the emission of gravitational waves as predicted by general relativity, but independent verification was not possible. Such verification can in principle be obtained by determining the orbital inclination in a binary pulsar system using only classical geometrical constraints. This would permit a measurement of the expected retardation of the pulse signal arising from the general relativistic curvature of space-time in the vicinity of the companion object (the 'Shapiro delay'). Here we report high-precision radio observations of the binary millisecond pulsar PSR J0437-4715, which establish the three-dimensional structure of its orbit. We see the Shapiro delay predicted by general relativity, and we determine the mass of the neutron star and its white dwarf companion. The determination of such masses is necessary in order to understand the origin and evolution of neutron stars.     

A runaway collision in a young star cluster as the origin of the brightest supernova

Supernova SN 2006gy in the galaxy NGC 1260 is the most luminous recorded. Its progenitor might have been a very massive (>100 Mo, where is the mass of the Sun) star, but that interpretation is incompatible with hydrogen in the spectrum of the supernova; stars >40 Moare believed to have shed their hydrogen envelopes several hundred thousand years before the explosion. Alternatively, the progenitor might have arisen from the merger of two massive stars. Here we show that the collision frequency of massive stars in a dense and young cluster (of the kind to be expected near the centre of a galaxy) is sufficient to provide a reasonable chance that SN 2006gy resulted from such a bombardment. If this is the correct explanation, then we predict that when the supernova fades (in a year or so) a dense cluster of massive stars will become visible at the site of the explosion.     

The association of GRB 060218 with a supernova and the evolution of the shock wave

Although the link between long gamma-ray bursts (GRBs) and supernovae has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a gamma-ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5) and its connection to supernova SN 2006aj (ref. 6). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor. We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf-Rayet star.     

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.     

The signature of supernova ejecta in the X-ray afterglow of the gamma-ray burst 011211

Now that gamma-ray bursts (GRBs) have been determined to lie at cosmological distances, their isotropic burst energies are estimated to be as high as 1054 erg (ref. 2), making them the most energetic phenomena in the Universe. The nature of the progenitors responsible for the bursts remains, however, elusive. The favoured models range from the merger of two neutron stars in a binary system to the collapse of a massive star. Spectroscopic studies of the afterglow emission could reveal details of the environment of the burst, by indicating the elements present, the speed of the outflow and an estimate of the temperature. Here we report an X-ray spectrum of the afterglow of GRB011211, which shows emission lines of magnesium, silicon, sulphur, argon, calcium and possibly nickel, arising in metal-enriched material with an outflow velocity of the order of one-tenth the speed of light. These observations strongly favour models where a supernova explosion from a massive stellar progenitor precedes the burst event and is responsible for the outflowing matter.     

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.     

白矮星是负熵系统

应用正、负能谱热力学理论(PNES热力学理论)，通过白矮星熵的演化对白矮星的初期演化阶段作了仔细分析，得出了白矮星在其初期收缩过程中是一个负熵系统，白矮星的演化由其正熵(电子子系的熵)Se和负熵(氮离子子系的熵)SHe之问的竞争决定，当SHe比Se增长得快时，则系统的总熵将更负，它必将更趋于有序态；反之，当SHe比Se增长得慢时，系统的负熵将减少，于是系统的有序程度将减少．     

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．