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On November 11, , Tycho Brahe noticed a star in the constellation Cassiopeia that was as bright as the planet Jupiter which was in the night sky in Pisces. No such star had ever been observed at this location before. It soon equaled Venus in brightness which was at For about two weeks the star could be seen in daylight. At the end of November it began to fade and change color, from bright white to yellow and orange to faint reddish light, finally fading away from visibility in March , having been visible to the naked eye for about 16 months.

Tycho's meticulous record of the brightening and dimming of the supernova now allows astronomers to identify its "light signature" as that of a Type Ia supernova. Tycho Brahe's supernova was very important in that it helped 16th-century astronomers abandon the idea of the immutability of the heavens. At the present time, Type Ia supernovae remain key players in the newest cosmological discoveries. To learn more about them and their explosion mechanism, and to make them even more useful as cosmological probes, a current Hubble Space Telescope project led by Filippenko is studying a sample of supernovae in other galaxies at the very time they explode.

Explosion Supports Supernova Theory. Stellar Survivor from A. On This Page. Supernova Explosion. Research Box Title An international team of astronomers is announcing today that they have identified the probable surviving companion star to a titanic supernova explosion witnessed in the year by the great Danish astronomer Tycho Brahe and other astronomers of that era.

The suspect star is breaking the speed limit for that particular region of the Milky Way Galaxy by moving three times faster than the surrounding stars. When the system was disrupted by the white dwarf's explosion, the companion star went hurtling off into space, like a stone thrown by a sling, retaining the velocity of its orbital motion. This is purely circumstantial evidence that the speeding star is the perpetrator because there are alternative explanations to its suspicious behaviour.

It could be falling into the region from the galactic halo that surrounds the Milky Way's disk at a high velocity. But spectra obtained with the 4. Keck telescopes in Hawaii show that the suspect has the high heavy-element content typical of stars that dwell in the Milky Way's disk, not the halo.

The star found by the Ruiz-Lapuente team is an aging version of our own Sun. The star has begun to expand in diameter as it progresses toward a red-giant phase the end stage of a Sun-like star's lifetime. The star turns out to fit the profile of the perpetrator in one of the proposed supernova conjectures. In Type Ia supernova binary systems, the more massive star of the pair will age faster and eventually becomes a white dwarf star. When the slower-evolving companion star subsequently ages to the point where it begins to balloon in size, it spills hydrogen onto the dwarf.

The hydrogen accumulates, gradually fusing into heavier elements until it reaches a critical and precise mass threshold, called the Chandrasekhar limit, where it explodes as a titanic nuclear fusion bomb. The energy output of this explosion is so well known that it can be used as a standard candle for measuring vast astronomical distances. An astronomical "standard candle" is any type of luminous object whose intrinsic power is so accurately determined that it can be used to make distance measurements based on the rate the light dims over astronomical distances.

A system called U Scorpii has a white dwarf and a star similar to the one found here.

Stellar survivor from 1572 A.D. explosion supports supernova theory

These results would confirm that such binaries will end up in an explosion like the one observed by Tycho Brahe, but that would occur several hundreds of thousands of years from now," says Ruiz-Lapuente. New multidimensional simulations are revealing once-elusive details of the explosion, such as how asymmetry contributes to the diversity seen in SNe Ia. Advances in understanding progenitors have been proceeding apace, led by new observations. The properties of SNe Ia are being correlated with the stellar populations in their host galaxies, yielding real progenitor constraints.

Dramatic new discoveries, like supernovae that appear to require a progenitor above the Chandrasekhar mass, are challenging pre-existing theories. Evidence has been mounting that the long-favoured scenario of accretion from a main sequence or red giant companion star cannot account for all SNe Ia. Instead, the WD merger and even the sub-Chandrasekhar mass explosion models have been experiencing a revival.

Finally, we conclude that empirical SN Ia cosmology remains solid, but is being continually refined by taking into account ever-more-subtle effects and new knowledge. But our understanding of the progenitors of SNe Ia and the explosion process may be undergoing an evolution, as the data from vast new surveys with thousands of supernovae are becoming available.

In a SN Ia, a wave of thermonuclear fusion rips through a degenerate WD star, synthesizing iron-peak elements Ni, Co, Fe in the dense inner regions, intermediate mass elements Si, S, Ca, Mg, O where burning is incomplete, and sometimes leaving unburned material C,O near the outer layers 9. Though the explosion provides the kinetic energy of the SN Ia and unbinds the WD, this is not what we see as the supernova. The lightcurve is powered by the radioactive decay of 56 Ni half-life 6. Figure 1 shows the quasi-bolometric ultraviolet-optical-infrared lightcurve evolution of a typical SN Ia, SN du ref.

Initially, the time for photons to diffuse out of the dense ejecta is high, so that the rate of deposition of energy by radioactive decay exceeds the energy radiated by the SN. Thus, at early times, spectra show only absorption lines probing the outer layers of the SN. As the SN expands and thins, the light-emitting region the photosphere recedes in mass or velocity coordinates , and spectra start to probe deeper layers.

Eventually, the ejecta reach a point where the rate of energy deposition by radioactive decay is equal to the radiated luminosity of the SN, and the SN starts to decline in luminosity Around, or after maximum light, spectra show P-Cygni profiles—emission at the rest wavelength initially weak , and blueshifted absorption. Weeks after maximum light, the spectra start becoming dominated by scattering from permitted lines at least in the blue 14 and ultimately become dominated by emission features as the ejecta start to become optically thin, and the SN makes the transition from the photospheric to the nebular phase.

At late times hundreds of days , gamma rays can freely escape, but positrons may be trapped 15 , and the lightcurve slope may match the slope expected from the decay of 56 Co, or can be steeper than this in the case of incomplete trapping Late time spectra are dominated by emission lines from iron-peak elements, synthesized in the deepest regions where the WD was densest. Orange points mark the dates of spectra shown in panel b. SN ejecta thin with time, and we see to deeper layers.

As SNe leave the photospheric phase and enter the nebular phase, emission features start to dominate. The top two panels show the remarkable improvements in w made using SNe Ia over the past decade assuming a flat universe , the bottom two show the importance of improving systematics in the early years of the next decade Judged by the area of the inner SNe Ia can show a factor of ten or more difference in peak luminosity, but the luminosity is correlated with the time it takes the supernova to rise and fall in brightness Therefore, the width of the lightcurve is measured and used to correct the peak luminosity.

A correction must also be made for colour, since redder SNe are dimmer, both intrinsically, and due to dust 18 , Various techniques are used to determine these parameters by fitting lightcurve models to the data, but leading fitters include MLCS2k2 ref. To determine cosmological parameters, an observed Hubble diagram distance versus redshift is constructed, and cosmological parameters are varied in a model that is fitted to the data. Note that since relative magnitudes are used, neither the absolute magnitude of the SN, nor the value of the Hubble constant must be known.

Cosmological studies using SNe Ia, have reached a point where over most redshift ranges systematic errors those that affect many measurements simultaneously in a correlated way dominate statistical errors that is, those that are reduced by 24 , 25 , 26 —see Figure 2. Many systematic uncertainties can be lowered with new methods, improved statistics, by comparing subsamples of supernovae, or breakthroughs in understanding. The primary systematic uncertainties affecting SNe Ia are survey-dependent, but there is general consensus that the largest are calibration to the historic Landolt photometric system, treatment of the ultraviolet, reddening due to dust, the differences supernovae show with respect to environment, and the possible evolution of SNe with redshift.

As supernovae are observed at high redshift, the region of the spectrum seen through a given broadband filter changes.

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A correction must be made for this, the k-correction, which requires knowledge of the spectral energy distribution of an average supernova 27 , and precise knowledge of the filter transmission curves and calibration system used for each supernova. Unfortunately, many historical low-redshift supernovae were transformed to the Landolt photometric standard system, which is poorly understood and no longer reproducible, as the filter and instrument transmission curves used to establish it are not known to modern precision and no longer exist. The fact that the low- z SN sample is on the Landolt system has forced many high redshift surveys to transform to it, incurring a systematic uncertainty in zero points and colour terms These systematic uncertainties will be reduced once a large new low-redshift sample of SNe is assembled, calibrated onto a new system for example, Sloan.

Unfortunately, there is a great deal of scatter in restframe U observations. Some surveys rely heavily on the restframe U , so uncertainties from this region can translate into dominant systematic uncertainties In fact, differences seen between the distances determined by different lightcurve fitters, such as MLCS2k2 ref. The Supernova Legacy Survey SNLS has shown that when longer-wavelength high-redshift observations from a well-calibrated single survey are k-corrected to restframe U -band, the dispersion is more than three times lower than for low-redshift data where the U -band is observed directly Therefore, discrepancies are most likely due to problematic low-redshift observations where many factors conspire to make U observations difficult, including atmospheric variation, extinction, nonstandard filters, and poor calibrators This systematics can be dealt with by building a new, better-calibrated low-redshift sample, and possibly by avoiding the restframe U -band.

Lightcurve fitters must correct for the fact that redder supernovae are dimmer.

The Evolution of Compact Binary Star Systems

This is due to a combination of an intrinsic colour-luminosity relation faint supernovae are intrinsically red 18 , and reddening due to dust. Although there is consensus that the two effects ought to be corrected for independently, there is disagreement over whether this can be practically achieved given current data limitations. MLCS2k2 attempts to separate intrinsic and dust reddening and make different corrections for each This may result from the conflation of dust and intrinsic SN reddening, and would indicate that the intrinsic SN Ia reddening-luminosity relation has a shallower slope than the dust relation.

However, since the lightcurve shape-luminosity relation is already factored out in this method 23 ; it means there must be a component of intrinsic colour that does not correlate with lightcurve shape An alternative is that the dust along the line of sight to SNe Ia is intrinsically different, or that scattering effects in the circumstellar environment result in a different apparent reddening law The most reliable is using u to infrared data to determine the reddening law red stars , but this can only be done for a few individual SNe Ia.

Data is from K06 ref. The average from the Carnegie Supernova Program F10 infrared data, excluding their most reddened events, is shown as a red circle, the only determination consistent with Milky Way dust dotted line. However, NG08 ref. Blue squares show reddening behaviour determined by solving for the reddening law for ensembles of SNe that would minimize their residual on the Hubble diagram.

There are some indications that solving for reddening by minimizing Hubble residuals is incompatible with infrared determinations Finally, W09 ref. However, this is opposite to earlier findings from optical data 34 , and the small sample size makes it difficult to draw reliable conclusions.

There are indications that supernovae with higher velocity ejecta have lower inferred R V values 35 Fig. As the colour is correlated with physical SN features, this may indicate that sometimes an intrinsic colour difference in SNe is wrongly ascribed to dust reddening though this can be mitigated by making a colour cut , or perhaps that there are different progenitor scenarios, with different circumstellar dust-scattering properties, that produce SNe with different velocities.

Various authors have exploited the low sensitivity to dust in the infrared to make infrared Hubble diagrams. This is theoretically expected, in part because less luminous SNe are cooler and radiate a larger fraction of their luminosity in the infrared Although heroic attempts have been made to produce high redshift infrared Hubble diagrams of SNe Ia 39 , the massive amount of observing time required, and the fact that the restframe infrared is redshifted to even longer wavelengths, has made serious cosmological constraints from this method elusive.

Even before SNe Ia were used for cosmology, it was known that the most luminous SNe Ia those with the broadest lightcurves , occur only in late-type galaxies Likewise, subluminous SNe Ia are preferentially found in galaxies with a significant old population SN lightcurve width and luminosity have now been shown to correlate with host galaxy star formation rate, galaxy mass, and metallicity 10 , 42 , 43 , Because star formation increases by a factor of 10 up to redshift 1. The spectra of high-redshift supernovae also show fewer intermediate mass elements, consistent with the idea that they make more iron-peak elements to power their luminosity A changing mix of supernovae with redshift is not necessarily problematic for cosmology, if lightcurve shape and colour corrections allow all supernovae to be corrected to the same absolute magnitude.

Unfortunately, they do not 44 , 47 —as is shown in Figure 4 , supernovae in high- and low-mass galaxies each correct to an absolute magnitude different by 0. This trend is also present though weaker , if SNe are split by host star formation rate, or metallicity. Therefore, aside from colour and lightcurve shape, a third correction, one for host galaxy properties, must be applied to SNe Ia to avoid systematic residuals with respect to the Hubble diagram.

Data are from Sullivan et al. Red points are averages. Horizontal lines show the average residual for each histogram. SNe Ia in high- and low-mass hosts correct to an absolute magnitude different by 0. Supernovae in low-mass galaxies are, on average, brighter than those in high mass galaxies before correction, but are dimmer after s - and c -correction. This galaxy-dependent residual can be corrected by taking host galaxy information into account. Reproduced with permission from John Wiley and Sons.

The physical origins for the differences in supernova properties in spiral and elliptical galaxies have been elusive 40 , 41 , 42 —are they related to progenitor age, metallicity or entirely different progenitors? The recently discovered trend that galaxy mass may have the most significant role in determining SN properties 10 , 44 would seem to implicate metallicity, as higher mass galaxies retain more metals in their deeper potential wells. Still, the age-metallicity degeneracy precludes firm conclusions on this evidence alone Theoretically, there are reasons that high metallicity progenitors should produce subluminous SNe Ia 49 : increased 22 Ne in high metallicity WDs provides more neutrons during the nucleosynthesis that occurs during the explosion, and thus yields more stable 58 Ni and less of the radioactive lightcurve-powering 56 Ni.

If progenitor age is the predominant effect controlling SN luminosity, this has implications for SN progenitors. Therefore, in this model, the delay time is a direct indication of the secondary star's mass. In the double degenerate DD model, where two WDs merge, gravitational wave radiation ultimately brings the stars together, but they may have a head start if they have drawn closer, by orbiting in the common envelope of one of the stars as it is evolving.

Age may still have a role in determining luminosity, because at early times the only WDs will be massive ones derived from the more massive stars.

In what might be called the standard model for a SN Ia, a carbon-oxygen CO WD accretes matter until it compresses to the point that carbon is ignited just before the Chandrasekhar limit note that the common misconception that the WD goes over the Chandrasekhar limit is wrong—this would lead to collapse to a neutron star. The evidence that the exploding star is a WD is strong, albeit circumstantial: neither hydrogen nor helium is seen in the spectrum of a SN Ia 50 , SNe Ia can happen long after star formation has ceased, the explosive process may implicate degenerate matter, the energy obtained by the thermonuclear burning of a WD minus the binding energy roughly matches the kinetic energy of SNe Ia, and simulations of the process have been successful at reproducing SN Ia lightcurves and spectra 9 , 51 , The WD may have a 'simmering' phase of order a thousand years following unstable carbon ignition, where thermonuclear runaway is prevented by convection Ultimately, however, explosive burning is ignited and the WD is incinerated in seconds.

If a WD near the Chandrasekhar mass is detonated that is, the burning occurs supersonically , then the WD burns at such a high density that the fusion products consist almost entirely of iron-peak elements This does not match the spectra or lightcurves of SNe Ia. A deflagration subsonic burning , on the other hand, gives the SN time to pre-expand. Burning at a lower density can produce intermediate mass elements and reproduce many of the observational features of SNe Ia However, pure deflagrations fail to produce the high velocity material Fig.

Therefore, the consensus is that in a Chandrasekhar mass explosion, the flame must start out subsonically, but at some point become supersonic Though the physics remains poorly understood, models that start as a deflagration but impose a transition to a detonation under certain conditions have been successful at reproducing normal SN Ia lightcurves and spectra, and even lightcurve width-luminosity relations and metallicity effects Explosions dominated by deflagration produce more intermediate mass elements dimmer SNe Ia , while those dominated by detonation produce brighter and more 56 Ni-rich SNe Ia Fig.

The simulations, from Kasen et al. Blue shows intermediate mass elements Si, S, Ca , green is stable-iron group elements, and red is 56 Ni. An initial deflagration wave produces turbulent instabilities, but a later detonation wave burns much of the remaining fuel to 56 Ni. In low density outer regions the detonation produces only intermediate mass elements.

Weak detonations produce the lower 56 Ni mass, less luminous SNe Ia. It is possible to gain insight into the explosion physics with spectropolarimetry or other techniques that reveal asphericity—recent work indicates that some of the dispersion in SN properties results from broken symmetries SN asymmetry can be measured via spectropolarimetry, since asymmetric electron scattering leads to polarization vectors that do not cancel.

Most normal SNe Ia are found to be spherically symmetric 58 , The first convincing evidence for significant deviations from spherical symmetry was seen in a subluminous SN Ia 60 , which may make sense if they are deflagration-dominated. However, supernovae with high velocity features often show even stronger spectropolarimetric signatures of asymmetry, possibly due to clumpy ejecta 58 , Strangely, SNe Ia whose spectroscopic features start off with high velocity and evolve rapidly, often show nebular lines that appear redshifted, while SNe Ia with slower velocity evolution show blueshifted nebular lines This probably indicates asymmetry in the explosion, in qualitative agreement with models where a deflagration burns off-center, and is followed by a detonation.

Even if there is agreement that the primary star in a SN Ia is a WD, the identity of the secondary star in the binary system is uncertain. There are three broad classes of models: 1 single degenerate 6 , where the companion is a main sequence or red giant star that loses mass via either Roche lobe overflow, or a wind symbiotic star , 2 DD, where two WDs merge and explode 63 , 64 , and 3 sub-Chandra 65 , where a layer of helium builds up on the surface of a WD below the Chandrasekhar mass until it detonates.

In the single degenerate model, a WD ignites carbon burning, causing it to explode, by accreting matter from a nondegenerate companion to near the Chandra mass. If it accretes faster, it will form a red giant-like envelope. Slower accretion is thought to lead to the build up of matter that results in novae, and mass being lost from the system.

The apparently required steady burning generates supersoft X-rays, and as a result, systems in this phase of their evolution are observable as supersoft X-ray sources The less massive WD is disrupted into a disk that will eventually accrete onto the more massive WD In this case, carbon may be ignited on the surface of the more massive WD during the accretion process, resulting in non-explosive carbon burning, converting the star to an O-Ne WD. This should ultimately undergo accretion induced collapse to a neutron star rather than producing a SN Ia 72 , though it may be possible to avoid this fate under certain conditions The simulated merger of two equal mass 0.

More massive equal-mass mergers may produce normal, or even overluminous SNe Ia, but would be more rare.


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Lower-mass mergers should be common, though it is not clear if they lead to SNe Ia The merger of two WDs is a very complicated three dimensional process necessitating approximations in current models , and very few simulations have been completed. The WD merger scenario has a natural explanation for greater SN luminosity in young environments: younger and more massive stars produce more massive WDs. Massive WD mergers have more potential fuel than less massive mergers.

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Although approaching the Chandra mass is a convenient method for triggering carbon burning, there is no hard evidence that the Chandra mass is required. When the pressure at the base of the helium layer reaches a critical threshold, it detonates, driving a shock into the core of the WD. This causes a second detonation, resulting in a flame propagating outward from the core or near it , destroying the WD. Because the sub-Chandra WD has a lower density throughout, a simple detonation does not burn the entire star to iron-peak elements. This model thus has the advantage that an ad hoc deflagration to detonation transition is avoided.

However, the significant outer helium layer is efficiently burned to 56 Ni, resulting in early time spectra that should be rich in 56 Ni, which does not match observations If helium can detonate with a smaller layer, which some studies hint 77 , then sub-Chandra simulations can reproduce many of the observed properties of SNe Ia Different progenitor scenarios lead to different delay times between the birth of the binary system and the explosion as a SN Ia So there is hope of constraining the progenitors by measuring the delay time distribution DTD; Fig.

One approach is to measure the lag between the cosmic star formation rate and the SN Ia rate as a function of redshift. However, large uncertainties in each make this particularly difficult, leading authors to different conclusions even when using largely the same data 80 , Adapted from Maoz, Sharon, and Gal-Yam The first solid circle results from a constraint on the observed iron-to-stellar mass ratio in clusters. Blue stars are derived from SN rates and galaxy stellar populations in the Lick observatory supernova survey Reproduced by permission of the AAS.

Another approach is to use the relative SN rates in different types of galaxies to constrain the DTD. Whereas there has been much argument about whether the SN Ia DTD is bimodal 83 , a leap forward came with the application of spectral energy distribution fitting techniques to SN host galaxies by modelling their stellar populations It is difficult to explain the power law DTD results using the single degenerate scenario alone 79 , 84 , though there is at least one claim Host galaxy spectroscopy contains even more information to constrain progenitors.

New studies using different methods and different data sets 87 , 88 , but the same galaxy fitting code, have determined the relative rates of supernovae in 0—0. They find that 'prompt' and 'delayed' supernovae are required at several sigma.

The Evolution of Compact Binary Star Systems

They also confirm earlier findings that bright, broad-lightcurve supernovae favour a prompt population, while dim, narrow-lightcurve SNe Ia favour a delayed population. Pinpointing the locations of SNe Ia relative to the stellar populations in which they reside can give some information on progenitors, even if it is imprecise due to the probability that the SN has migrated from its birth site.

Comparing the distributions of SNe Ia to the blue light in galaxies reveals that even 'prompt' SNe Ia are significantly delayed, with delay times — Myr Additional constraints have been obtained by comparing SN Ia remnants in the Magellanic clouds triangles in Fig. However, despite decades of searching, no such smoking gun has been found.

This disfavours the symbiotic star hypothesis, where the WD accretes matter from the stellar wind of its companion.

If, on the other hand, the WD is accreting from Roche lobe overflow of a companion, the outer hydrogen layers of the secondary ought to become stripped and entrained in the SN ejecta, where it will show up at low velocities when the ejecta become optically thin However, in a couple of well-observed SNe Ia, upper limits on the amount of hydrogen detected are 0. In a few cases, apparent SNe Ia have been seen interacting with pre-existing hydrogen, as in SN 93 , although it is not clear these are really SNe Ia 94 , and even if they are, these systems are the exception rather than the rule.

Arguments on the left strongly disfavour a certain scenario, whereas arguments on the right strongly favour that model. Note that an argument against a given scenario need not be an equally strong argument for another scenario that is, the diagram need not be symmetric. Though all evidence is subject to theoretical interpretation, italics indicate exclusively theoretical arguments. Relative rankings reflect an effort to distill community consensus, but are subjective.