Structure and Evolution of Single and Binary Stars
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Red, green, and blue colors are assigned to bands 0. Warren, J. Hughes, C. Badenes, P. Ghavamian, C. McKee, D. Moffett, P. Plucinsky, C. Rakowski, E.
Reynoso, and P. In principle, a detailed map of the speed and abundance of radioactive 56 Ni and 56 Co would sharply constrain the explosion physics. To get a reasonable rate of events, the sensitivity of a useful gamma-ray instrument needs to be much better than has been achieved so far. The detection of the integrated gamma-ray background from all the SNe Ia that have ever occurred over cosmic time would be a major accomplishment and warrants further study.
Instrument development is needed in the coming decade for this form of astronomy to realize its potential. What they lack in numbers, massive initial mass stars make up for in energy and extreme conditions. In their deaths, they produce spectacular fireworks—supernovae and gamma-ray bursts—that leave behind exotic objects: neutron stars and black holes. They create the elements necessary for life. The neutrino burst that announces the central core collapse is one of the most powerful events in the universe, and the jet that makes a GRB is one of the fastest flows of matter.
Yet no one is really sure how it all works. Most massive stars end their lives when their inner cores of heavy elements collapse to form neutron stars or black holes. This releases an enormous amount of energy, chiefly radiated away as neutrinos. As already noted in the late s, if a small fraction—approximately 1 percent—of this binding energy could be tapped, this could account for the comparatively modest kinetic energy of common supernovae.
For nearly 70 years, however, finding a robust mechanism to drive the explosion has proven elusive. Today, the picture is both complicated and made more tractable by the realization that probably not every star blows up or collapses in the same way. In particular, stars that die with rapidly rotating cores have an additional source of energy to tap, and the conduit may involve magnetic fields. In the next decade, driven especially by advances in computation and large observational surveys, it is likely that major advances will occur in the understanding of how massive stars of all masses, metallicities, and rotation rates end their lives.
The outcome of core collapse depends on the properties of the pre-supernova star. The greatest uncertainties lie at the extremes of mass, metallicity, and rotation rate. On the low-mass end , what is the main-sequence mass that separates white-dwarf remnants from supernovae?
Might some progenitor stars even have a white dwarf carbon and oxygen or oxygen and magnesium core? At the high-mass end, does the well-established metallicity dependence of line-driven winds for lower-mass stars carry over to the episodic ejections that now appear to dominate for very massive stars?
A deeper understanding of mass loss is essential to extend what is learned from relatively close Type II SNe those with observed hydrogen to those that exploded when the universe was young. It is also important for determining what main-sequence mass separates stars that form neutron stars from those that form black holes. Recent models suggest that stars of only explode with neutrino heating alone but that more massive stars are more tightly bound and are harder to blow up this way. About a dozen Type II supernovae currently have reliably determined progenitors based on pre-supernova archival imaging, especially with HST.
These progenitors generally have properties consistent with main-sequence masses around 10 to , as expected for average core-collapse SNe, although a Type IIn narrow line SN has been found to have a massive, luminous blue variable progenitor Figure 5.
The strong episodic mass loss in LBVs affects the appearance of the. Using multicolor imaging of nearby galaxies with a space telescope or a large ground-based telescope, many more progenitor stars should be identified. Gal-Yam and D. Leonard, A massive hypergiant star as the progenitor of the supernova SN gl, Nature , , copyright Observations of young massive star clusters show that highly magnetized neutron stars, magnetars, form from stars with main-sequence masses above 40 to.
One possibility is that rotation and magnetic fields become a bigger factor in the explosion as the mass increases. Better understanding of binary-star populations is needed to discern these possibilities. In the next decade, large-volume optical and near-IR time-domain surveys will increase the observational database of supernovae by orders of magnitude.
In particular, surveys with approximately 5-day cadence and volumes 10 to times larger than current surveys could double the known SNe population in about 2 years. These surveys will find the very rare events that are often keys to progress.
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These discoveries will help to constrain directly the white dwarf—neutron star transition mass of the progenitor star, the maximum stellar mass for main-sequence stars, the uncertain stellar evolution prior to core collapse e. The light curves provide physical constraints on the explosion parameters.
Smith, W. Li, R. Foley, J. Wheeler, D. Pooley, R. Chornock, A.
Filippenko, J. Silverman, R. Quimby, J. Bloom, and C.
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Radio and X-ray observations will also probe shock interactions in the circumstellar medium and mass loss in the hundreds to thousands of years leading up to the explosion. The EVLA will allow detailed radio observations of all types of core-collapse supernovae. For young galactic supernova remnants, the Chandra X-ray observatory is a powerful tool and has yielded useful imaging and composition information on objects such as Cassiopeia A.
Higher sensitivity by a factor of approximately 10 is required to obtain spectra of extragalactic supernovae. Wide-field X-ray and UV surveys should yield observations of shock breakout from core-collapse supernovae, which can be used to determine the radii of the progenitor stars and to tie down the times of the explosions. To date, this has been observed only for the Type Ib supernova SN D, which had a relatively compact progenitor and produced fainter, harder shock-breakout emission. The more common Type II supernovae emit longer, brighter transients, chiefly in the ultraviolet.
The Evolution of Compact Binary Star Systems
On the theoretical front, it is not currently understood which stars leave behind neutron stars and which leave behind black holes, nor is it understood what determines the observed masses, spins, and magnetic fields of these compact objects see discussion of science question SSE 4. In the next decade, the exponential increase in computer power, plus the wide availability of multidimensional codes that scale well on these big machines, will lead to major advances in the understanding of how.
It is just now becoming feasible to do three-dimensional simulations of stellar core collapse with multigroup neutrino transport coupled to the hydrodynamics, an essential for a first-principles model. By the middle of the decade, such calculations will be commonplace and should, at a minimum, answer the long-standing problem of whether the simplest non-rotating neutrinotransport model works for any range of stellar masses. What about the rest? Driven in part by the interest in gamma-ray bursts, codes that realistically couple magnetic fields, rotation, and general relativity in the context of the core-collapse problem are also in an advanced stage of development.
Neutrinos have not yet been included in these codes in a realistic way, but before the end of the decade they will. By the end of the decade there should be a quantitative theory of how massive stars of all masses die—given the pre-supernova characteristics. On the observational front, spectroscopic and spectropolarimetric observations of supernovae will ensure major advances in studies of the composition, kinematics, and asphericity of the explosion at a time when numerical models will become increasingly predictive.
When combined with time-domain surveys that will discover new and rare events, these observations will significantly advance the understanding of core-collapse explosion physics. As multidimensional simulations become, it is hoped, increasingly successful at producing supernovae, comparison to the observed polarimetry is likely to become particularly productive. Estimates of supernova energy are useful constraints on the supernova mechanism. The observation of neutron-star kicks received during a supernova provides another constraint on the symmetry of the explosion mechanism.
Such a detection would provide stringent and unique tests of theoretical models as well as a laboratory for studying the properties of the neutrino itself. Unfortunately, a neutrino detection can only be made for a very nearby e. Only about 1 in massive-star deaths end in a GRB.
The distinguishing properties of the stars that end this way are not known, although a high central angular momentum is likely to play a role. A combination of theoretical studies. Gamma-ray line signals from radioactive decays offer a way of sampling SN nucleosynthesis and dynamics in a most direct way, if the signals can be seen.
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The strongest gamma-ray line expected from galactic supernova remnants that are centuries old comes from the decay of 44 Ti. The only remnant detected to date is Cas A, which is about years old. Determining the amount of 44 Ti in an explosion is important for determining the mass cut between the central compact object and the ejected material. More generally, nuclear physics is a crucial ingredient in core-collapse SN calculations.
This includes the nuclear equation of state, rates for electron capture on nuclei, properties of neutron-rich nuclei produced in the r-process, and, potentially, the physics of neutrino oscillations in the core-collapse environment. In the next decade, advances in observations, theory, and laboratory nuclear physics will likely determine whether some core-collapse SNe can produce r-process nuclei approximately half of the nuclei above the iron peak ; this would solve one of the major puzzles regarding the origin of heavy elements.
The most distant point source in the universe is now a GRB at redshift 8. GRBs can be used as tools for exploring stellar evolution throughout cosmic time because of their brightness and the penetrating power of gamma rays. GRBs may be the deepest direct probe of first-generation stars.
In addition, observations of the composition of nearby metal-poor stars provide information on the chemical traces of early supernovae and their evolution, and thus provide a critical piece of information for understanding supernovae. Currently, the number of very metal-poor stars known is extremely small, and larger samples are critical for progress. This requires large-scale surveys and high-resolution spectroscopic follow-up with optical telescopes up to 30 m in diameter.
The deaths of stars give rise to compact stellar remnants—neutron stars, black holes, and white dwarfs—that produce the most exotic and energetic phenomena in the universe, from the brightest known sources of radiation to the steadiest astrophysical clocks.
The properties of compact stellar remnants provide not only unique information about the late stages of stellar evolution, but also a testing. Many fundamental questions about compact stellar remnants center on understanding, both empirically and theoretically, their basic physical properties, including mass, radius, spin, and magnetic field. Here several key questions in which major progress can be made in the coming decade are highlighted. The basic properties of neutron stars are closely coupled to the physics of their interiors.