Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011) / Chapter Skim
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5 Report of the Panel on Stars and Stellar Evolution
5 Report of the Panel on Stars and Stellar Evolution
Pages 207-246
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From page 207... ...
Because astronomers understand stars well, they have the confidence to use them as cosmic probes to trace the history of cosmic expansion; but because this understanding is not complete, there is much to learn about the subtle interplay of convection, rotation, and magnetism or the not-so-subtle violent events that destroy stars or transform them into neutron stars or black holes. Although the topics of stars and their changes over time comprise great chunks of introductory astronomy textbooks, and although the tools for these investigations are tested and sharp, many of the simplest assertions about the formation of white dwarfs, mass loss from giant stars, and the evolution of binary stars are based on conjecture and a slender foundation of facts.
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From page 208... ...
How do rotation and magnetic fields affect stars?
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From page 209... ...
Because this result points to a profound lack in the understanding of gravitation, a problem right at the heart of modern physics, completing the astronomical story of Type Ia supernovae is a pressing priority for the coming decade. First of all, the provenance of the exploding white dwarfs seen in other galaxies is not known with certainty.
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From page 210... ...
, and collapse to form neutron stars or black holes. The elements that they synthesize and eject become the stuff of other stars, planets, and life.
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From page 211... ...
Unanswered questions about the magnetic fields and rotation of stars carry through to similar questions about the exotic remnants that they leave behind as neutron stars and black holes. These are exceptional places in the universe where understanding of physics is extended beyond the reach of any laboratory.
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From page 212... ...
Newly discovered radio pulsars in binaries, includ ing the unique double-pulsar system, provided some of the most stringent tests of general relativity. Advances in X-ray astronomy have led to new discoveries related to accreting neutron stars and black holes, compact remnants of massive stars, and energetic phenomena, such as coronae and flares, on normal stars.
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From page 213... ...
Current and ATST, SDO, HST, 4-m PTF, PanStarrs-1, KAIT, PTF, PanStarrs-1, EVLA, ALMA, LOFAR, expected and 8-m telescopes, PAIRITEL, JWST, Swift, Swift, NuSTAR, EVLA, Gaia FAST, LIGO, FRIB, facilities Kepler, CoRoT, Gaia HST, Chandra, 8- to 8- to 10-m telescopes, Chandra, XMM, Suzaku, 10-m telescopes HST, GALEX, Chandra RXTE, Fermi New High spatial and OIR time-domain, Multiwavelength (radio Large-area decimeterfacilities synoptic solar large-FOV, high- to X-ray) time-domain, wavelength telescope; needed magnetometry; helio- cadence observations; large-FOV, high- large-effective-area X-ray and asteroseismology; precise IR follow-up; cadence observations; timing and spectroscopy; OIR interferometry; X-ray spectroscopy; post-Swift GRB gravitational wave OIR time-domain, 20- to 30-m telescope studies; X-ray observatory large-FOV spectroscopy; neutrino observations; high- and gravitational wave resolution multiobject observatories; 20- to OIR spectroscopy; 30-m telescope plasma physics experiments Crucial Detailed solar and Panchromatic Large-scale three- High-sensitivity X-ray capabilities stellar studies of spectroscopy of a dimensional timing and spectral internal rotation and representative sample; simulations; discovery observations of known magnetism; surveys large sample for of broad range neutron stars and of stellar surface finding diverse objects of transients and black holes; laboratory rotation, activity, and correlations with multiwavelength measurements of nuclear magnetism, and mixing environment; advanced follow-up; nuclear equation of state; deep diagnostics; three- three-dimensional data and oscillator radio pulsar searches; dimensional MHD simulation capability; strengths; abundance star-cluster white-dwarf simulations; pulsation progenitor surveys; studies of extremely searches; gravitational theory; UV and X-ray nuclear cross sections low metallicity stars wave detection of spectroscopy compact binary inspirals Other Dedicated follow-up for inferring fundamental stellar properties; progress on abundance priorities determinations; laboratory measurements of opacities; support for basic theory and computational astrophysics NOTE: Acronyms are defined in Appendix C
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From page 214... ...
The formation of stars, galaxies, and the intracluster medium in galaxy clusters is strongly influenced by the heavy elements, ionizing photons, and explosions produced by massive stars. The Sun continues to be a working template for understanding magnetohydrodynamics and plasma physics "in practice" -- physics that is crucial in many other arenas, including that of compact objects.
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From page 215... ...
. Other problems in which a full three-dimensional understanding is crucial are the thermonuclear explosions of white dwarfs in Type Ia supernovae, and the core collapse and explosion of massive stars (science questions SSE 2 and SSE 3)
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From page 216... ...
Strong magnetic fields are also detected in hot, higher-mass stars without strong surface convection, and in their stellar remnants. Their origin and evolution, possibly from relic fields, is a mystery.
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From page 217... ...
The complementary approaches of large surveys and detailed studies of smaller samples promise new observational constraints on the origin, nature, and consequences of stellar rotation and magnetic fields. Gaia will provide precise astrometric data and spectroscopic information for an unprecedented sample, which will be invaluable for characterizing stellar properties.
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From page 218... ...
search includes exploring dynamos in the cores of massive stars and examining a range of rotation rates, masses, and evolutionary states, including fully convective stars and giants and supergiants. Precise radius measurements in stars with differ ing rotation and starspot properties, along with detailed seismic studies, will allow the measurement of how interior stellar structure is affected by magnetic activity.
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From page 219... ...
Rotation samples of early-type stars will require moderate-resolution, multiobject spectroscopy. Direct tests of internal stellar rotation through interferometry (resolved imaging of distorted massive stars)
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From page 220... ...
Rotation in massive stars is even observed to approach the critical rate, generating distortion and the associated equatorial gravity dark ening that can now be directly measured interferometrically (Figure 5.3)
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From page 221... ...
; the still-inexplicable presence of strong, long-lived magnetic fields and magnetic activity in fully convective stars; and the appearance of chromospheres in objects as disparate as red giants and brown dwarfs. All of these diagnostics will help constrain the dynamo mechanism(s)
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From page 222... ...
There is also a significant population of neutron stars with magnetic fields ~1014-1015 G The birthrate of these "magnetars" is uncertain, but they probably represent at least 10 percent, and perhaps up to 50 percent, of all neutron stars.
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From page 223... ...
SN Ia explosions triggered by merging white dwarfs are not ruled out. Type Ia supernovae are an important area of stellar astrophysics and cosmic evolution: they are violent end points of stellar evolution, and they create much of the iron in the universe.
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From page 224... ...
Matching the observed populations with the evolutionary paths and the observed rates of stel lar death requires large, intensive, and carefully characterized surveys to find and count the stars at each stage. In the next decade, surveys in the galaxy should aim to discover binaries that contain white dwarfs approaching the Chandrasekhar mass, on their way to becoming SNe Ia.
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From page 225... ...
Major advances are also expected in the understanding of important nuclear physics model input such as the 12C+12C fusion rate and electron capture rates. Progress depends on the development of experimental facilities as well as of nuclear theory, because not all relevant transitions can be measured in the laboratory.
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From page 226... ...
Rolling searches that have a cadence of a few days will provide excellent sampling of thousands of SNe Ia light curves. Spectroscopic follow-up remains a high scientific priority: queue observing on Gemini has been effective, and improved access would be useful.
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From page 227... ...
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.
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From page 228... ...
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 10 M◉ explode with neutrino heating alone but that more massive stars are more tightly bound and are harder to blow up this way.
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From page 229... ...
One possibility is that rotation and magnetic fields become a bigger factor in the explosion as the mass increases. Some ultrabright supernovae such as SN 2007bi suggest that the most massive stars may not even die by core collapse at all, but by an electron-positron "pair instability." Binary evolution can also affect the appearance of a supernova through mass transfer to or mass loss from the progenitor star.
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From page 230... ...
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 deter mines the observed masses, spins, and magnetic fields of these compact objects (see discussion of science question SSE 4)
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From page 231... ...
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.
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From page 232... ...
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
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From page 233... ...
Radio pulsars are the most commonly observed neutron stars known, with almost 2,000 cataloged to date. Precise neutron-star-mass determinations can be obtained in "recycled" binary millisecond pulsars by the measurement of relativistic orbital effects, but only a handful of suitable systems are known.
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From page 234... ...
:109-165, copyright 2007, with permission from Elsevier. large-area decimeter-wavelength radio telescope could find approximately 20,000 new radio pulsars, including thousands of new millisecond pulsars.
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From page 235... ...
What Is the Spin Distribution and Maximum Spin of Neutron Stars and Black Holes? Millisecond pulsars (MSPs)
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From page 236... ...
New constraints on neutron-star physics would come from the detection of a neutron star spinning more rapidly than 1,000 Hz. Astrophysical black holes are completely described by just two quantities, their mass and spin.
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From page 237... ...
What Determines the Initial-Final Mass Relation Connecting Progenitors to White Dwarfs? Observational constraints on the initial-final mass relation come primarily from white dwarfs in open clusters and require accurate main-sequence turnoff ages plus white-dwarf cooling ages (to infer the initial mass)
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From page 238... ...
With the detection of many more white dwarfs in clusters, enough information to constrain mass-loss models and perhaps enough to extrapolate to unobservable populations (low metallicity, high mass -- as in the early universe) may become available.
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From page 239... ...
The formation of He white dwarfs is understood only in the context of binaries, so further understanding of other ways of forming them potentially by single stars or through disrupted binaries or ejections from dense star clusters is needed. Stars that ignite C in their degenerate cores before losing their envelopes to mass loss may also produce unusual thermonuclear supernovae.
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From page 240... ...
Thomas, and N Yasuda for the Supernova Cosmology Project, Discovery of an unusual optical transient with the Hubble Space Telescope, Astrophysical Journal 690(2)
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From page 241... ...
Supernovae and GRBs Optical/IR, n, gravitational Milky Way supernova Multiwavelength photometry, spectroscopy; waves, γ-ray, IR, radio rapid response Supernova searches including Optical/IR Multiwavelength photometry, spectroscopy rare forms, optical transients Shock breakout in supernovae UV, X-Ray Multiwavelength photometry, spectroscopy Types II and Ibc Electromagnetic counterparts to Optical/IR, UV, X-ray Photometry, spectroscopy -- radio; rapid gravitational wave sources response γ-ray, X-ray Gamma-ray bursts Multiwavelength photometry, spectroscopy; rapid response Orphan afterglows of GRBs Optical/IR, radio Multiwavelength photometry NOTE: Acronyms are defined in Appendix C
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From page 242... ...
to form a neutron star, rather than blowing up as an SN Ia? An understanding of this question is essential for understanding the evolution of white dwarfs in binary systems and would dramati cally constrain the allowed progenitors of SNe Ia.
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From page 243... ...
. Recently, several relatively smallscale radio surveys have uncovered new forms of transients from known sources, such as extremely rare millisecond-duration pulses from rotating neutron stars (the so-called RRATs, or rotating radio transients)
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From page 244... ...
Carpenter, P Challis, et al., Two-Micron All-Sky Survey J01542930+0053266: A new eclipsing M dwarf binary system, Monthly Notices of the Royal Astronomical Society 386:416, 2008, copyright 2008 Royal Astronomical Society.
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From page 245... ...
These events have the potential to tell about particle acceleration, stellar magnetic fields and rotation, strong-field gravity, the interstellar and intergalactic media, the violent deaths of stars, and possibly physics beyond the standard model. Summary of SSE Discovery Area In summary, the time domain represents great discovery potential well matched to the timescales that are relevant for stellar phenomena during their lifetimes and their death throes.
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