Abstract
The era of all-sky space astrometry began with the Hipparcos mission in 1989 and provided the first very accurate catalogue of apparent magnitudes, positions, parallaxes and proper motions of 120 000 bright stars at the milliarcsec (or milliarcsec per year) accuracy level. Hipparcos has now been superseded by the results of the Gaia mission. The second Gaia data release contained astrometric data for almost 1.7 billion sources with tens of microarcsec (or microarcsec per year) accuracy in a vast volume of the Milky Way and future data releases will further improve on this. Gaia has just completed its nominal 5-year mission (July 2019), but is expected to continue in operations for an extended period of an additional 5 years through to mid 2024. Its final catalogue to be released sim 2027, will provide astrometry for sim 2 billion sources, with astrometric precisions reaching 10 microarcsec. Why is accurate astrometry so important? The answer is that it provides fundamental data which underpin much of modern observational astronomy as will be detailed in this White Paper. All-sky visible and Near-InfraRed (NIR) astrometry with a wavelength cutoff in the K-band is not just focused on a single or small number of key science cases. Instead, it is extremely broad, answering key science questions in nearly every branch of astronomy while also providing a dense and accurate visible-NIR reference frame needed for future astronomy facilities.
Highlights
For almost 2 billion common stars the combination of two all-sky space observatories would provide an astrometric foundation for all branches of astronomy – from the Solar System and stellar systems, including exoplanet systems, to compact galaxies, quasars, neutron stars, binaries and dark matter (DM) substructures
The addition of NIR will result in up to 8 billion newly measured stars in some of the most obscured parts of our Galaxy, and crucially reveal the very heart of the Galactic bulge region. In this White Paper we argue that rather than improving on the accuracy to answer specific science questions, a greater overall science return can be achieved by going deeper than Gaia and by expanding the wavelength range to the NIR
A new NIR astrometry mission will revolutionize the field of exoplanetary science in two ways. It will establish the presence of planetary-mass companions orbiting classes of stellar and sub-stellar primaries that cannot be observed with sufficient sensitivity using other techniques and that appear bright in the NIR, enabling a jump of typically a factor of several in achievable astrometric precision with respect to Gaia. These include two relevant samples: 1) hundreds of ultra-cool early-L through mid-T dwarfs in the Sun’s backyard (d 40 pc, see e.g. [130]), around which GaiaNIR could complete the census of any existing population of cold giant planets out to ∼ 4 − 5 AU (e.g., [222]), with the possibility to access the regime of cold Neptunes and Super Earths orbiting the nearest, brightest L dwarfs; 2) a statistical sample of maybe thousands of heavily reddened young stars all the way to the bottom of the main sequence in the nearest star-forming regions (t ∼ 3– 10 Myr, d 200 − 300 pc, see Section 3.10), around which only GaiaNIR might be sensitive to intermediate-separation gas giants
Summary
For almost 2 billion common stars the combination of two all-sky space observatories would provide an astrometric foundation for all branches of astronomy – from the Solar System and stellar systems, including exoplanet systems, to compact galaxies, quasars, neutron stars, binaries and dark matter (DM) substructures. The addition of NIR will result in up to 8 billion newly measured stars in some of the most obscured parts of our Galaxy, and crucially reveal the very heart of the Galactic bulge region (see Fig. 1 for a note of caution!) In this White Paper we argue that rather than improving on the accuracy to answer specific science questions, a greater overall science return can be achieved by going deeper than Gaia and by expanding the wavelength range to the NIR. NIR opens up a new wavelength range which allows us to probe the dusty obscured regions of the Galactic disc with high-precision astrometry and broad-band high-resolution photometry, while out of the Galactic plane a new mission will go deeper to enhance the halo science cases and provide complementary legacy data to ground based surveys such as the Rubin Observatory. As the US is the world leader in detector technology we recently proposed [109, 154] a collaboration with the US on this project, for the detectors, which would make the mission feasible and allow it to remain within the ESA M-class mission budget provided the US, Japanese, and Australian partners contribute significantly
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