ALBERT

Description: Dear readers: We are sad to report that, soon after submitting her draft manuscript for this prefatory chapter, Nancy Grace Roman passed away on December 25, 2018. This final version of her memoir has been lightly edited but remains very true to the original. However, an Abstract was missing. Rather than trying to synthesize one in Nancy Grace's inimitable style, we take this opportunity to comment briefly on her life and its significance. Nancy Grace Roman was born in 1925 and came of age scientifically in the United States during the 1940s and 1950s. Together with the equally fascinating prefatory by Vera Rubin ( ARAA, Vol. 49), which we also recommend to you, these two memoirs give us intimate insight into the obstacles faced by women astronomers trying to rise in the field during those years. Roman's memoir is bitingly candid, recounting numerous snubs by teachers, insultingly small salaries, and attempts by her thesis advisor to simultaneously exploit her scientific findings and smother her role in them. Discouragement at every turn from doing forefront research is what drove Roman into government service, where she found a niche and blossomed as one of the visionary founders of the US civilian space program. We do not know what impact Roman might have had as a researcher with access to the world's largest telescopes, but we do know that her influence as an enabler of other people's science was vast. Her sobriquet as the “Mother of Hubble,” bestowed by admirer Ed Weiler, is well deserved. Nancy Grace granted an audio interview to Joss Bland-Hawthorn on August 4, 2018, just a few months before her passing. It captures her persona more vividly than mere words on paper, and we recommend the online recording to you at https://www.annualreviews.org/r/nancy-grace-roman-interview .

Description: The recent advancements in far-infrared (far-IR) astronomy brought about by the Herschel, SOFIA, and ALMA observatories have led to technological advancements in millimeterwave and submillimeterwave laboratory spectroscopy that is used to support molecular observations. This review gives an overview of rotational spectroscopy and its relationship with observational astronomy, as well as an overview of laboratory spectroscopic techniques focusing on both historical approaches and new advancements. Additional topics discussed include production and detection techniques for unstable molecular species of astrochemical interest, data analysis approaches that address spectral complexity and line confusion, and the current state of and limitations to spectral line databases. Potential areas for new developments in this field are also reviewed. To advance the field, the following challenges must be addressed: ▪ Data acquisition speed, spectral sensitivity, and analysis approaches for complex mixtures and broadband spectra are the greatest limitations—and hold the greatest promise for advancement—in this field of research. ▪ Full science return from far-IR observatories cannot be realized until laboratory spectroscopy catches up with the data rate for observations. ▪ New techniques building on those used in the microwave and IR regimes are required to fill the terahertz gap.

Description: Star clusters stand at the intersection of much of modern astrophysics: the ISM, gravitational dynamics, stellar evolution, and cosmology. Here, we review observations and theoretical models for the formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle including the following several phases: ▪ Clusters form in hierarchically structured, accreting molecular clouds that convert gas into stars at a low rate per dynamical time until feedback disperses the gas. ▪ The densest parts of the hierarchy resist gas removal long enough to reach high star-formation efficiency, becoming dynamically relaxed and well mixed. These remain bound after gas removal. ▪ In the first ∼100 Myr after gas removal, clusters disperse moderately fast, through a combination of mass loss and tidal shocks by dense molecular structures in the star-forming environment. ▪ After ∼100 Myr, clusters lose mass via two-body relaxation and shocks by giant molecular clouds, processes that preferentially affect low-mass clusters and cause a turnover in the cluster mass function to appear on ∼1–10-Gyr timescales. ▪ Even after dispersal, some clusters remain coherent and thus detectable in chemical or action space for multiple galactic orbits. In the next decade, a new generation of space– and adaptive optics–assisted ground-based telescopes will enable us to test and refine this picture.

Description: Exoplanetary science is on the verge of an unprecedented revolution. The thousands of exoplanets discovered over the past decade have most recently been supplemented by discoveries of potentially habitable planets around nearby low-mass stars. Currently, the field is rapidly progressing toward detailed spectroscopic observations to characterize the atmospheres of these planets. Various surveys from space and the ground are expected to detect numerous more exoplanets orbiting nearby stars that make the planets conducive for atmospheric characterization. The current state of this frontier of exoplanetary atmospheres may be summarized as follows. ▪ We have entered the era of comparative exoplanetology thanks to high-fidelity atmospheric observations now available for tens of exoplanets. ▪ Recent studies reveal a rich diversity of chemical compositions and atmospheric processes hitherto unseen in the Solar System. ▪ Elemental abundances of exoplanetary atmospheres place important constraints on exoplanetary formation and migration histories. ▪ Upcoming observational facilities promise to revolutionize exoplanetary spectroscopy down to rocky exoplanets. ▪ The detection of a biosignature in an exoplanetary atmosphere is conceivable over the next decade. In the present review, we discuss the modern and future landscape of this frontier area of exoplanetary atmospheres. We start with a brief review of the area, emphasising the key insights gained from different observationalmethods and theoretical studies. This is followed by an in-depth discussion of the state of the art, challenges, and future prospects in three forefront branches of the area.

Description: The solar chromosphere forms a crucial, yet complex and until recently poorly understood, interface between the solar photosphere and the heliosphere. ▪ Advances in high-resolution instrumentation, adaptive optics, image reconstruction techniques, and space-based observatories allow unprecedented high-resolution views of the finely structured and highly dynamic chromosphere. ▪ Dramatic progress in numerical computations allows 3D radiative magnetohydrodynamic forward models to take the place of the previous generation of 1D semiempirical atmosphere models. These new models provide deep insight into complex nonlocal thermodynamic equilibrium chromospheric diagnostics and enable physics-based interpretations of observations. ▪ This combination of modeling and observations has led to new insights into the role of shock waves, transverse magnetic waves, magnetic reconnection and flux emergence in the chromospheric energy balance, the formation of spicules, the impact of ion-neutral interactions, and the connectivity between chromosphere and transition region. ▪ During the next few years, the advent of new instrumentation (integral-field-unit spectropolarimetry) and observatories (ALMA, DKIST), coupled with novel inversion codes and expansion of existing numerical models to deal with ever more complex physical processes (including multifluid approaches), is expected to lead to major new insights into the dominant heating processes in the chromosphere and beyond.

Description: Stars lose a significant amount of angular momentum between birth and death, implying that efficient processes transporting it from the core to the surface are active. Space asteroseismology delivered the interior rotation rates of more than a thousand low- and intermediate-mass stars, revealing the following: ▪ Single stars rotate nearly uniformly during the core-hydrogen and core-helium burning phases. ▪ Stellar cores spin up to a factor of 10 faster than the envelope during the red giant phase. ▪ The angular momentum of the helium-burning core of stars is in agreement with the angular momentum of white dwarfs. Observations reveal a strong decrease of core angular momentum when stars have a convective core. Current theory of angular momentum transport fails to explain this. We propose improving the theory with a data-driven approach, whereby angular momentum prescriptions derived frommultidimensional (magneto)hydrodynamical simulations and theoretical considerations are continuously tested against modern observations. The TESS and PLATO space missions have the potential to derive the interior rotation of large samples of stars, including high-mass and metal-poor stars in binaries and clusters. This will provide the powerful observational constraints needed to improve theory and simulations.

Description: We review the use of emission lines for understanding galaxy evolution, focusing on excitation source, metallicity, ionization parameter, ISM pressure, and electron density. We discuss the physics, benefits, and caveats of emission line diagnostics, including the effects of theoretical model uncertainties, diffuse ionized gas, and sample selection bias. In anticipation of upcoming telescope facilities, we provide new self-consistent emission line diagnostic calibrations for complete spectral coverage from the UV to the IR. These diagnostics can be used in concert to understand how fundamental galaxy properties have changed across cosmic time. We conclude the following: ▪ The UV, optical, and IR contain complementary diagnostics that can probe the conditions within different nebular ionization zones. ▪ Accounting for complex density gradients and temperature profiles is critical for reliably estimating the fundamental properties of Hii regions and galaxies. ▪ Diffuse ionized gas can raise metallicity estimates, flatten metallicity gradients, and introduce scatter in ionization parameter measurements. ▪ New 3D emission line diagnostics successfully separate the contributions from star formation, AGN, and shocks using integral field spectroscopy. We summarize with a discussion of the challenges and major opportunities for emission line diagnostics in the coming years.

Description: There has been an incredibly large investment in obtaining high-resolution stellar spectra for determining chemical abundances of stars. This information is crucial to answer fundamental questions in astronomy by constraining the formation and evolution scenarios of the Milky Way as well as the stars and planets residing in it. We have just entered a new era, in which chemical abundances of FGK-type stars are being produced at industrial scales, and in which the observations, reduction, and analysis of the data are automatically performed by machines. Here, we review the latest human efforts to assess the accuracy and precision of such industrial abundances by providing insights into the steps and uncertainties associated with the process of determining stellar abundances. We also provide a description of current and forthcoming spectroscopic surveys, focusing on their reported abundances and uncertainties. This allows us to identify which elements and spectral lines are best and why. Finally, we make a brief selection of main scientific questions the community is aiming to answer with abundances. ▪ Uncertainties in abundances need to be disentangled into random and systematic components. ▪ Precision can be increased by applying differential or data-driven methods based on accurate data. ▪ High-resolution and signal-to-noise spectra provide fundamental data that can be used to calibrate lower-resolution and signal-to-noise spectra of millions of stars. ▪ Different survey calibration strategies must agree on a common set of reference stars to create data products that are consistent. ▪ Data products provided by individual groups must be published using standard formats to ensure straightforward applicability.

Description: Cosmological observations are beginning to reach a level of precision that allows us to test some of the most fundamental assumptions in our working model of the Universe. One such assumption is that gravity is governed by the theory of general relativity. In this review, we discuss how one might go about extending general relativity and how such extensions can be described in a unified way on large scales. This allows us to describe the phenomenology of modified gravity in the growth and morphology of the large-scale structure of the Universe. On smaller scales, we explore the physics of gravitational screening and how it might manifest itself in galaxies, clusters, and, more generally, in the cosmic web. We then analyze the current constraints from large-scale structure and conclude by discussing the future prospects of the field in light of the plethora of surveys currently being planned. Key results include the following: ▪ There are a plethora of alternative theories of gravity that are restricted by fundamental physics considerations. ▪ There is now a well-established formalism for describing cosmological perturbations in the linear regime for general theories of gravity. ▪ Gravitational screening can mask modifications to general relativity on small scales but may, itself, lead to distinctive signatures in the large-scale structure of the Universe. ▪ Current constraints on both linear and nonlinear scales may be affected by systematic uncertainties that limit our ability to rule out alternatives to general relativity. ▪ The next generation of cosmological surveys will dramatically improve constraints on general relativity, by up to two orders of magnitude.