Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2023) / Chapter Skim
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Pages 31-80
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From page 31... ...
Solar physics is directly relevant to astronomy, as it is the study of our nearest star and interacts with stellar astrophysics, is input to studying the Earth-Sun connection and expanding to stellar-planetary interactions, and is vital to understanding Earth's climate and space weather. The survey committee concluded that an appropriate role for astronomy and astrophysics decadal surveys is to comment on the value of ground-based solar physics projects for astronomy and astrophysics priorities, with the solar and space physics decadal survey being the more appropriate body to prioritize and rank ground-based solar physics projects within the context of the full range of multi-agency activities in solar physics.
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From page 32... ...
32 PATHWAYS TO DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FOR THE 2020s 1.7 CONCLUSION The integrated program forwarded in this report advances a vision for discovery and progress for the coming decades. The content of the remaining chapters, together with the panel reports, represent an enormous effort that took years of preparation on the part of a large fraction of the astronomical community, and more than 2 years for the survey and its committees to complete.
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From page 33... ...
Newly discovered fossil structures from the formation of the Milky Way Galaxy open a window on the Milky Way's distant past, and observations take several steps closer to diagnosing the conditions present shortly after the Big Bang. The investments of previous decades bore fruit in this decade in the awarding of Nobel Prizes in Physics for six discoveries derived from astronomical measurements: dark energy, neutrino oscillations, gravitational waves, exoplanets, physical cosmology, and black holes (Figure 2.1)
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From page 34... ...
In 2011, Saul Perlmutter, Adam Riess, and Brian Schmidt received the prize "for the discovery of the accelerating expansion of the universe through observations of distant supernovae." The citation to Takaaki Kajita and Arthur McDonald in 2015 was "for the discovery of neutrino oscillations, which shows that neutrinos have mass." In 2017, Kip Thorne, Rainer Weiss, and Barry Barish were awarded the prize "for decisive contributions to the LIGO detector and the observations of gravitational waves." The year 2019 saw the awarding of the Nobel Prize in Physics to James Peebles "for theoretical discoveries in physical cosmology" and to Dider Queloz and Michael Mayor "for the discovery of an exoplanet orbiting a solar-type star." Most recently, in 2020, the topic of black holes received Nobel attention, with recognition to Roger Penrose "for the discovery that black hole formation is a robust theory of general relativity" and to Andrea Ghez and Reinhard Genzel "for the discovery of a supermassive compact object at the center of our galaxy." SOURCES: 2011: NASA/STScI/Ann Field; 2015: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research) , The University of Tokyo; 2017: R
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From page 35... ...
It soon became clear that most of the science questions and discovery areas could be organized into three broad thematic areas: Worlds and Suns in Context highlights the extraordinary advances over the past decade in the study of exoplanets, stars, and their associated planetary systems, and the opportunities for transformational advances in these areas, including the ultimate search for and characterization of habitable planets, in the decades ahead. Cosmic Ecosystems represents an integration and culmination of understanding the origins of galaxies, stars, planets, and massive black holes, and the realization that the life cycles of the universe over this billionfold range of scales are intimately connected, through feedback processes propagating through the gas within, surrounding, and between galaxies.
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From page 36... ...
2.1 WORLDS AND SUNS IN CONTEXT The planets in our solar system, and the Sun at the center of it, provide the most direct connection to the myriad other stars and planets in our galaxy and the universe. With the flowering of capabilities expected in the next decade, progress in both stellar astrophysics and planetary science will expand and provide a broader context with which to understand and appreciate our cosmic perspective.
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From page 37... ...
The cores of red giant stars appear to rotate faster than the surface, and oscillation frequencies differentiate red giant stars in which core helium burning is occurring, versus those only burning hydrogen in a shell. Latitudinal differential rotation in the convection zones of Sun-like stars, revealed through asteroseismic observations, indicate a shear much larger than predicted from numerical simulations.
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From page 38... ...
The detection of gravitational waves from astrophysical objects this decade leads to invigorated research into the endpoints of stellar evolution for the next decade. Stars in multiple systems can have very different evolutionary pathways compared to their single counterparts; detailed photometric and spectroscopic electromagnetic observations spanning infrared through X-ray wavelengths, coupled with gravitational wave measurements and attention from theory, will elucidate these multiple routes and their consequences.
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From page 39... ...
SOURCE: Adapted from R.M. Roettenbacher et al., 2013, Imaging starspot evolution on Kepler target KIC 5110407 using light curve inversion, The Astrophysical Journal 767:60, © AAS.
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From page 40... ...
The sunspot is sculpted by a convergence of intense magnetic fields and hot gas roiling up from below. This image uses a warm palette of red and orange, but the context viewer took this sunspot image at the wavelength of 530 nanometers -- in the greenish-yellow part of the visible spectrum.
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From page 41... ...
The importance of magnetic fields in a stellar context was expected but is now unequivocal. Magnetism in massive stars has been confirmed (but not entirely explained)
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42 PATHWAYS TO DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FOR THE 2020s 2010 (Minimum) Mass (Earth Masses)
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From page 43... ...
planets have been discovered within the habitable zone of low-mass stars -- neither too close nor too far, but receiving enough energy to allow liquid water on their surface. A key question that Kepler and other projects have tried to address is the frequency of potentially habitable planets -- the average number of Earth-sized planets within the habitable zone of their star, particularly around Sun-like stars.
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From page 44... ...
Understanding this is crucial to placing potentially habitable planets in context, including questions as to how water and atmospheric volatiles are delivered to Earth, whether giant planets affect the evolution of terrestrial worlds, and whether "mini-Neptune" planets can evolve toward habitability. More sensitive observations of planetforming disks to understand the astrochemistry, dynamics, and role of water in the formation of habitable planets through radio, mm, and far-infrared (IR)
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. Already several dozen planetary atmospheres have been characterized by the Spitzer and Hubble space telescopes and by large ground-based telescopes.
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, to examine the atmospheres of potentially Earth-like planets around Sun-like stars will require further development of a specialized space telescope for high-contrast imaging to measure a reflection spectrum that could show oxygen, methane, water vapor, and carbon dioxide. SOURCE: Courtesy of N
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This cross-disciplinary approach will be key to interpreting exoplanet transit spectra taken by the James Webb Space Telescope. SOURCE: Courtesy of T.D.
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Directly imaging non-transiting, potentially habitable planets of the nearest low-mass stars requires extreme angular resolution but is only moderately demanding in terms of relative brightness, and hence is feasible from the ground with high-performance adaptive optics. Achieving this is a key science goal of the proposed ground-based extremely large telescopes.
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From page 49... ...
Time-domain surveys such as the Vera Rubin Observatory's Legacy Survey of Space and Time will greatly expand the number of known small bodies in the outer solar system, and provide information about its early evolution. Results from recent studies analyzing dynamics of small bodies in the Kuiper Belt provide tantalizing hints, to be confirmed, about the possibility of additional planets.
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From page 50... ...
The star's properties and evolution influence the evolution and habitability of the planets, particularly of terrestrial planets. The end stages of star formation provide the initial conditions for planet formation.
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From page 51... ...
Magnetic fields of low-mass stars may prevent some eruptive events from ejecting mass, lending complexity to a blind application of the solar analog, and there are currently few observational constraints on stellar
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, and with an aperture of at least 6 m and coronagraphic imaging capability, should be capable of observing approximately 100 nearby stars and of successfully detecting potentially habitable planets around at least a quarter of the systems. Such an observatory would also provide valuable information on other extrasolar planets, and be versatile enough to carry out groundbreaking observations of stars, galaxies, black holes, and the gases and baryons within and between galaxies, with a scientific impact rivaling that of previous "great observatories" such as HST.
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From page 53... ...
In doing so, X-ray observations revealed the dynamic coronae of the Sun and other stars, accreting neutron stars and black holes, and the hot plasma that pervades interstellar and circumgalactic environments. Radio observations revealed the existence of neutron stars, whose remarkably stable rotation rates have since been used to discover planets and confirm the theory of general relativity's prediction of orbital decay via the emission of gravitational waves.
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From page 54... ...
In 2013, the south pole IceCube observatory detected a diffuse high-energy neutrino flux of astrophysical, but unknown, origin. Starting in 2015, LIGO opened up the gravitational wave view of the universe by detecting merging binary black holes.
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From page 55... ...
While physicists attempt to identify dark matter through ambitious and excruciatingly careful laboratory experiments, astronomers in the coming decade will wield the threefold tools of theory, simulation, and observations to search in parallel. Dark matter could leave detectable traces of its potentially more complex interactions through deviations from the simplest version of the cold dark matter paradigm or through emission of unexpected particles (gamma rays, positrons, narrow radio frequency lines)
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From page 56... ...
In parallel with lensing studies on large scales, more detailed, focused studies of individual lensed galaxies, supernovae, and quasars (accreting black holes) , and even lensed stars at cosmological distances, will test the predictions of the cold dark matter paradigm on the smallest scales, where we expect deviations to be most evident.
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From page 57... ...
The fluctuations seen in the CMB are believed to have been imprinted in the earliest phases of the Big Bang during a period of cosmological inflation in which extraordinarily rapid expansion established the large-scale homogeneity and flatness of the universe while also causing quantum fluctuations that subsequently grew into the fluctuations we observe. One of the most exciting opportunities in the coming decade is that CMB measurements may reveal remnant gravitational waves from this early epoch, as depicted in Figure 2.16, later in this chapter.
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From page 58... ...
This led to the identification of time-domain astronomy as a key discovery area in Astro2010. These seemingly separate advances in observational techniques are in fact intimately related: most of the known and anticipated sources of gravitational waves, neutrinos, and cosmic rays are also time variable or transient electromagnetic sources (e.g., neutron star mergers, gamma-ray bursts, black hole jets, and stellar explosions)
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From page 59... ...
Source parameters and basic inferences measurement with GW170817 is not as precise as other techniques, multi-messenger cosmology will increase The physical properties of GW sources are inferred by matching the observed data with in importance in the coming decade as we detect ever more binary neutron star and black hole mergers. waveforms generated following the prescriptions of general relativity, which makes detailed Observations of SN predictions for 1987A andand the inspiral the Sun in light coalescence signaland neutrinos of merging andBHs.
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From page 60... ...
As similar supernovae explode throughout the universe, they collectively produce a diffuse background of MeV neutrinos, one that should be within reach of forthcoming experiments if current theories are correct. Detection of either a galactic supernova or the diffuse background would test models of the formation of neutron stars and black holes and the core-collapse explosion mechanism.
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From page 61... ...
Are the TeV–PeV neutrinos and UHECRs produced in the same sources? They are widely surmised to be accelerated in the relativistic jets of accreting supermassive black holes or GRBs, but this has yet to be tested observationally.
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From page 62... ...
. The importance of gravitational waves lies in part with the central role that black holes and neutron stars play in many areas of astronomy, from stellar evolution to galaxy formation.
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From page 63... ...
The population of neutron star and black hole mergers with masses of ~2–5 Msun4 in the "mass gap" between neutron stars and black holes will provide key constraints on our understanding of massive stellar evolution, the maximum mass of neutron stars, and core-collapse explosion physics. Combined gravitational wave and electromagnetic observations have the potential to finally crack the long-standing puzzle of the origin and growth of massive black holes, one that lies at the intersection of the understanding of stars, galaxies, accretion disks, and cosmology.
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From page 64... ...
Some of these are cataclysmic events that herald the formation of neutron stars and black holes in stellar core-collapse, the thermonuclear explosions of white dwarfs, or the mergers of stars or compact objects. Others are repeating phenomena, such as stellar flares or the explosions of the surface layers of white dwarfs.
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From page 65... ...
Roman's microlensing survey will allow characterization for the first time of the mass function of the majority of neutron stars and black holes in the galaxy. The extended Roentgen Survey with an Imaging Telescope Array (eROSITA)
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From page 66... ...
Priority Science Area: New Windows on the Dynamic Universe The combination of new multi-messenger probes of astronomical phenomena with the maturation of time-domain observations opens up tremendous discovery spaces across nearly all areas of astrophysics. Within this discovery landscape, driven by improvements in gravitational wave and neutrino detection, and upcoming facilities such as the Rubin Observatory, one priority area stands out: the application of these new tools to the formation, evolution, and nature of compact stellar remnants such as white dwarfs, neutron stars, and black holes, as probed by the gravitational wave signatures of their mergers, together with rare explosive events that can be explored by the unique cadence and multi-color sensitivity of the Rubin Observatory.
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From page 67... ...
2.3 COSMIC ECOSYSTEMS Processes on a wide range of time and length scales together drive the formation, evolution, and interaction of the remarkable diversity of objects we observe, from exoplanets and stars to black holes and galaxies. A confluence of advances in theory, computational modeling, and observational capabilities expected in the next decade will transform our understanding by identifying the key mechanisms shaping this web of interconnected systems.
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From page 68... ...
In this way, the first stars and black holes were able to cause a global phase transition over scales of hundreds of Megaparsecs, in which most of the hydrogen in the universe was converted from a neutral to an ionized state, during what is referred to as the "Epoch of Reionization." Identifying the sources of cosmic reionization, and better understanding how photons escape from their gas-rich and dust-enshrouded sources, will be an important science area over the coming decade. Numerical simulations of the propagation of radiation through galaxies (radiative transport)
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From page 69... ...
Even in the local universe, for example, there are significant gaps in the understanding of stellar winds. This leads to large uncertainties about which massive stars become neutron stars and which become black holes.
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and local analogs in the coming decade may finally resolve this long-standing puzzle. Regions of high-mass star formation are often buried behind huge layers of dusty gas, so improved long-wavelength observations (far-infrared, sub-mm, radio)
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From page 71... ...
And higher sensitivity and spectral resolution opticalUV and X-ray spectroscopy of broad emission and absorption line outflows from AGN are needed to better diagnose the physical properties of accretion disk winds and determine how much mass and energy they actually carry. In addition to measuring the jet and wind properties, better constraints on the masses and spins of the supermassive black holes themselves will play an important role in understanding how these objects formed and how they grow, as well as their role in the feedback processes described above.
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From page 72... ...
Of all the observational diagnostics at our disposal, the SZ effect most directly constrains the energy content of gas in galactic halos produced by the combined effects of gravitational collapse and stellar and black hole feedback. Higher-sensitivity and higher-resolution CMB observations, motivated to a significant extent by cosmology (see Section 2.2)
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From page 73... ...
Spectral-line observations indicate velocity structures in the gas and reveal flows similar to what has been predicted for the early stages of planet formation. In each image, the lower-left icon indicates the beam size and the lower-right icon is a 10-AU scalebar.
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From page 74... ...
Or does an entirely different set of processes seed galactic nuclei with massive black holes at high redshift? A combination of gravitational wave measurements of black hole mergers across cosmic time (see Section 2.2)
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From page 75... ...
It is unclear how much of the colder gas is launched from the galaxy and how much forms in situ in the outflow. Recent far-infrared dust polarization maps showing smooth vertical magnetic fields over large portions of galaxy disks argue that the cold gas in which this field is embedded is influenced by the wind; future infrared data will test these ideas and provide direct information on feedback processes in and around the cold ISM.
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From page 76... ...
Within a virialized halo, the CGM may comprise 80 percent or more of the baryons. All processes that drive galaxy evolution, from the evolution of individual stars and accretion onto black holes to the galactic-scale winds that are powered by them, take place against the backdrop of the large-scale structure of the universe, which feeds and sustains galaxies.
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From page 77... ...
Box These are all major frontiers highlighted by the science panels. 2.3 continued FIGURE 2.3.1 Illustration of the circumgalactic medium, where intense outflows from galaxies -- driven by FIGURE 2.3.1 Illustration ofsupernovae the circumgalactic medium, where intense outflows from galaxies -- driven by supernovae and black holes -- interact with inflowing pristine gas from intergalactic space.
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From page 78... ...
These are the sites where baryons accrete on to galaxies, star formation is triggered, central black holes accrete and grow, and the feedback processes regulating galaxy growth are manifested. The key observational probes of all of these processes are emission and absorption-line spectroscopy of the diffuse gas, which contains a wealth of diagnostic information on the physical conditions, compositions, and dynamics of the gas.
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From page 79... ...
The study of radio jets in galaxies informs the mechanical and radiative inputs to galaxy structure and details the large-scale consequence of supermassive black holes at a galaxy's center, itself amenable to study at high energies, ultraviolet, optical, and infrared wavelengths. Research into aspects of star formation requires a panchromatic approach: deeply embedded sources appear only in the infrared,
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From page 80... ...
In the gravitational wave arena, space-based detection of inspiraling intermediate mass black hole mergers can signal the need for ground-based gravitational wave observation of the final merger, with searches for possible electromagnetic counterparts. Transients in electromagnetic emission will come by the millions per night from the Rubin Observatory once it is operational, with follow-up in other ground- and space-based telescopes needed for further characterization of the most interesting transient events.
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