Theia: Faint objects in motion or the new astrometry frontier

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.

Overview of Theia: Faint Objects in Motion or the New Astrometry Frontier

The Theia satellite mission proposal, situated within the European Space Agency's M5 call, represents a significant advance in astrometric techniques aimed at exploring the Universe at both macroscopic and microscopic scales. The mission is distinguished by its application of relative astrometry to achieve unparalleled sub-microarcsecond precision. This precision level marks a substantial improvement, nearly 1000 times better than ESA's Gaia mission for the brightest objects and an order of magnitude better for the faintest stars. Theia's mission ambitiously targets several fundamental areas: dark matter distribution in nearby galactic structures, potentially habitable Earth-like exoplanets, and the detailed physics of compact astrophysical objects, such as neutron stars and black holes.

Scientific Objectives

Theia is structured to address critical scientific inquiries that align with ESA's cosmic vision:

• Dark Matter: A primary focus of the mission is the examination of local dark matter properties. Theia seeks to validate or challenge current models of dark matter distribution, particularly the cold dark matter (CDM) hypothesis, through precise analysis of dwarf spheroidal galaxies and hyper-velocity stars. The anticipated outcomes include discerning whether dark matter halos are cuspy or cored, understanding the triaxial shape of the Milky Way halo, and probing the existence and impacts of dark matter subhalos.
• Exoplanets: The ambition to detect and characterize Earth and super-Earth exoplanets in habitable zones around the nearest stars holds promise for advancing exobiological studies. Theia will uniquely provide direct measurements of planetary masses and orbital inclinations, crucial for transitioning from detection to comprehensive characterization.
• Compact Objects: Theia aims to enhance our understanding of fundamental physical laws by analyzing the compositions and equations of state of neutron stars and the dynamics involved in black hole formation.

Instrumentation and Technological Approach

The mission design features an optical telescope employing a Korsch on-axis three-mirror anastigmat model, complemented by CCD detectors. The advanced metrology system integrated into Theia enables the precise measurement of stellar coordinates by mitigating distortive factors down to the micropixel level. This equipment configuration is projected to optimize photon detection efficiency between the observational periods required for maintaining astrometric precision.

Implications and Future Directions

The implications of Theia’s research are multifaceted, extending both practical and theoretical domains:

• Cosmology: Insights into the small-scale properties of dark matter may definitively confirm or refute the CDM model, impacting broader cosmological paradigms. The detection or absence of ultra-compact dark matter halos could alter prevailing inflationary models.
• Astrobiology: The census of nearby habitable exoplanets could significantly refine future observational goals and methodologies in the search for extraterrestrial life signatures.
• Astrophysics: For compact objects, the mass-radius constraints provided by Theia may offer tangible progress in understanding the state of matter under extreme conditions, potentially influencing particle physics and general relativity applications.

Conclusion

Theia stands poised to serve as a pivotal mission in the continuum of astrometric advancements. Its ambitious objectives and technological innovations are likely to yield substantial contributions to cosmology, exoplanet science, and our comprehension of compact objects. Looking ahead, Theia's legacy may well influence subsequent astronomical research and missions, including advancements in analytical methodologies, observational strategies, and even spacecraft design, underscoring its potential to enhance the scientific rigor of astrometry and allied fields.