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Register a new userWhite Paper: Exoplanetary Microlensing from the Ground in the 2020s
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Microlensing can access planet populations that no other method can probe: cold wide-orbit planets beyond the snow line, planets in both the Galactic bulge and disk, and free floating planets (FFPs). The demographics of each population will provide unique constraints on planet formation. Over the past 5 years, U.S. microlensing campaigns with Spitzer and UKIRT have provided a powerful complement to international ground-based microlensing surveys, with major breakthroughs in parallax measurements and probing new regions of the Galaxy. The scientific vitality of these projects has also promoted the development of the U.S. microlensing community. In the 2020s, the U.S. can continue to play a major role in ground-based microlensing by leveraging U.S. assets to complement ongoing ground-based international surveys. LSST and UKIRT microlensing surveys would probe vast regions of the Galaxy, where planets form under drastically different conditions. Moreover, while ground-based surveys will measure the planet mass-ratio function beyond the snow line, adaptive optics (AO) observations with ELTs would turn all of these mass ratios into masses and also distinguish between very wide-orbit planets and genuine FFPs. To the extent possible, cooperation of U.S. scientists with international surveys should also be encouraged and supported.
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[abridged] WFIRST is uniquely capable of finding planets with masses as small as Mars at separations comparable to Jupiter, i.e., beyond the current ice lines of their stars. These planets fall between the close-in planets found by Kepler and the wide separation gas giants seen by direct imaging and ice giants inferred from ALMA observations. Furthermore, the smallest planets WFIRST can detect are smaller than the planets probed by RV and Gaia at comparable separations. Interpreting planet populations to infer the underlying formation and evolutionary processes requires combining results from multiple detection methods to measure the full variation of planets as a function of planet size, orbital separation, and host star mass. Microlensing is the only way to find planets from 0.5 to 5M_E at 1 to 5au. The case for a microlensing survey from space has not changed in the past 20 yrs: space allows wide-field diffraction-limited observations that resolve main-sequence stars in the bulge, which allows the detection and characterization of the smallest signals including those from planets with masses at least as small as Mars. What has changed is that ground-based (GB) microlensing is reaching its limits, underscoring the scientific necessity for a space-based survey. GB microlensing has found a break in the mass-ratio distribution at about a Neptune, implying that these are the most common microlensing planet and that planets smaller than this are rare. However, GB microlensing reaches its detection limits only slightly below the observed break. WFIRST will measure the shape of the mass-ratio function below the break by finding numerous smaller planets: 500 Neptunes, 600 gas giants, 200 Earths, and planets as small as 0.1M_E. Because it will also measure host masses and distances, WFIRST will also track the behavior of the planet distribution as a function of separation and host star mass.
Asteroseismology is the only observational tool in astronomy that can probe the interiors of stars, and is a benchmark method for deriving fundamental properties of stars and exoplanets. Over the coming decade, space-based and ground-based observations will provide a several order of magnitude increase of solar-like oscillators, as well as a dramatic increase in the number and quality of classical pulsator observations, providing unprecedented possibilities to study stellar physics and galactic stellar populations. In this white paper, we describe key science questions and necessary facilities to continue the asteroseismology revolution into the 2020s.
This White Paper summarizes the discussions from a five-day workshop, involving 50 people from 18 countries, held in Cape Town, South Africa in February 2020. Convened by the International Astronomical Unions Executive Committee Working Group on Global Coordination of Ground and Space Astrophysics and sponsored by the Kavli Foundation, we discussed existing and potential bottlenecks for transient and multi-messenger astronomy, identifying eight broad areas of concern. Some of these are very similar to the challenges faced by many astronomers engaging in international collaboration, for example, data access policies, funding, theoretical and computational resources and workforce equity. Others, including, alerts, telescope coordination and target-of-opportunity implementation, are strongly linked to the time domain and are particularly challenging as we respond to transients. To address these bottlenecks we offer thirty-five specific recommendations, some of which are simply starting points and require development. These recommendations are not only aimed at collaborative groups and individuals, but also at the various organizations who are essential to making transient collaborations efficient and effective: including the International Astronomical Union, observatories, projects, scientific journals and funding agencies. We hope those involved in transient research will find them constructive and use them to develop collaborations with greater impact and more inclusive teams.
The blossoming field of joint gravitational wave and electromagnetic (GW-EM) astronomy is one of the most promising in astronomy. The first, and only, joint GW-EM event GW170817 provided remarkable science returns that still continue to this day. Continued growth in this field requires increasing the sample size of joint GW-EM detections. In this white paper, we outline the case for using some percentage of LSST survey time for dedicated target-of-opportunity follow up of GW triggers in order to efficiently and rapidly identify optical counterparts. We show that the timeline for the LSST science survey is well matched to the planned improvements to ground based GW detectors in the next decade. LSST will become particularly crucial in the later half of the 2020s as more and more distant GW sources are detected. Lastly, we highlight some of the key science goals that can be addressed by a large sample of joint GW-EM detections.
The past four years have seen a scientific revolution through the birth of a new field: gravitational-wave astronomy. The first detection of gravitational waves---recognised by the 2017 Nobel Prize in Physics---provided unprecedented tests of general relativity while unveiling a previously unknown class of massive black holes, thirty times more massive than the Sun. The subsequent detection of gravitational waves from a merging binary neutron star confirmed the hypothesised connection between binary neutron stars and short gamma-ray bursts while providing an independent measurement of the expansion of the Universe. The discovery enabled precision measurement of the speed of gravity while shedding light on the origin of heavy elements. At the time of writing, the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner, Virgo, have published the detection of eleven gravitational-wave events. New, not-yet-published detections are announced on a nearly weekly basis. This fast-growing catalogue of gravitational-wave transients is expected to yield insights into a number of topics, from the equation of state of matter at supra-nuclear densities to the fate of massive stars. The science potential of 3G observatories is enormous, enabling measurements of gravitational waves from the edge of the Universe and precise determination of the neutron star equation of state. Australia is well-positioned to help develop the required technology. The Mid-term Review for the Decadal plan for Australian astronomy 2016-2025 should consider investment in a scoping study for an Australian Gravitational-Wave Pathfinder that develops and validates core technologies required for the global 3G detector network.
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