[Paper Review] Theia: Faint objects in motion or the new astrometry frontier

The Theia Collaboration, Céline Bœhm+97 more|Edinburgh Research Explorer|Jul 2, 2017

Astronomy and Astrophysical ResearchPhysics and Astronomy4 references51 citations

TL;DR

Theia proposes an ultra-precise space astrometry mission (sub micro-arcsecond) to study dark matter, nearby habitable exoplanets, and compact objects, with a 15% open observatory time and Gaia-level reference frame.

ABSTRACT

In the context of the ESA M5 (medium mission) call we proposed a new satellite mission, Theia, based on relative astrometry and extreme precision to study the motion of very faint objects in the Universe. Theia is primarily designed to study the local dark matter properties, the existence of Earth-like exoplanets in our nearest star systems and the physics of compact objects. Furthermore, about 15 $\%$ of the mission time was dedicated to an open observatory for the wider community to propose complementary science cases. With its unique metrology system and "point and stare" strategy, Theia's precision would have reached the sub micro-arcsecond level. This is about 1000 times better than ESA/Gaia's accuracy for the brightest objects and represents a factor 10-30 improvement for the faintest stars (depending on the exact observational program). In the version submitted to ESA, we proposed an optical (350-1000nm) on-axis TMA telescope. Due to ESA Technology readiness level, the camera's focal plane would have been made of CCD detectors but we anticipated an upgrade with CMOS detectors. Photometric measurements would have been performed during slew time and stabilisation phases needed for reaching the required astrometric precision.

Motivation & Objective

• Address the nature of dark matter through high-precision astrometry of dwarf spheroidal galaxies and the Milky Way halo.
• Measure orbital properties and proper motions to determine dark matter halo shapes and subhalo perturbations.
• Detect and characterize Earth-like exoplanets around nearby stars via precise mass and inclination measurements.
• Constrain the equation of state of matter in extreme environments by observing neutron stars and black holes.
• Provide 15% of observing time for an open community program to enable additional science cases.

Proposed method

• Propose a 350–1000 nm on-axis Korsch three-mirror anastigmat telescope with 0.8 m primary and f=32 m.
• Employ a metrology system on the focal plane and telescope to achieve sub-microarcsecond relative astrometry.
• Use a ‘point and stare’ differential astrometry strategy to reach unprecedented precision for faint objects.
• Include a focal plane of 24 detectors (CCD or CMOS) with metrology-based pixel positioning at the micron/pixel level.
• Calibrate astrometric solutions and absolute reference frames using Gaia-era reference frames and background quasars.
• Schedule observations to optimize photometric measurements during slews and stabilization phases.

Experimental results

Research questions

• RQ1 Can Theia determine whether dark matter halos in dwarf spheroidal galaxies are cuspy or cored using precise proper motions?
• RQ2 What is the outer shape (triaxiality) of the Milky Way dark matter halo and how can hypervelocity stars constrain it?
• RQ3 Can the orbital distribution of dark matter in halos be inferred from the orbits of halo stars observed by Theia?
• RQ4 To what extent can dark matter subhalos perturb the Galactic disk, and can such perturbations be detected astrometrically?
• RQ5 What are the masses and orbital characteristics of nearby Earth-like exoplanets through high-precision astrometry?

Key findings

• Theia’s micro-arcsecond astrometry enables distinguishing cuspy vs cored dark matter profiles in dwarf spheroidal galaxies when stellar proper motions are included.
• Proper motions of stars in dwarf galaxies lift the degeneracy between density profiles and orbital anisotropy, enabling stronger DM constraints.
• Theia can constrain Milky Way halo triaxiality to about 5% on axis ratios using precise proper motions of hypervelocity stars, outperforming Gaia.
• Theia’s measurements can map disc perturbations caused by dark matter subhalos in the 10^6–10^8 solar mass range, via observable bending modes.
• The mission aims to measure masses and orbital inclinations of nearby Earth-like exoplanets and provide true orbital architectures for planetary systems.
• Astrometric data will contribute to the cosmic distance ladder via precise photometry during observations and the absolute astrometric reference frame.