Topics Included

1) Cosmic Distance Scale and the Hubble Constant
An epoch-making discovery of the H2O megamaser from an AGN (1995) provided the definite evidence of a super-massive black hole (SMBH). VLBI mapping of megamasers enabled us to measure SMBH distances from geometries of the maser features. Surveys of megamasers are establishing the Hubble Constant H0 in the local universe via an independent method with an accuracy better than 3%. The result of this Megamaser Cosmology Project is expected to play a decisive role in resolving the present H0  “tension”  between higher values determined in the local universe and lower ones resulting from CMB experiments. 

2) Black Hole Masses and the M-sigma Relation
Along with accurate distances used for the measurements of the Hubble Constant, VLBI mapping of megamasers can determine the accurate mass of SMBHs. The results can contribute to calibrate the M-σ relation. We will discuss various insights into physical properties of SMBHs, their formation and evolution, such as kinematics within the SMBH sphere of influence and mass accretion to SMBH. These studies will be an excellent synergy with the EHT and its future projects.

3) Structure of the Milky Way
VLBI astrometry has achieved annual parallax measurements of the Galactic masers at distances of up to 20 kpc from the Sun. They can be compared with data from Gaia (DR3 released in 2022 June) to explore the three-dimensional structure of the Milky Way. The Galactic fundamental parameters estimated by the maser astrometry can also be compared with those from the infrared astrometry with the Very Large Telescope Interferometer (VLTI) Gravity. We will review current studies on Galactic structure and discuss future extension toward the far outer Galaxy and southern hemisphere.

4) Dynamics of Formation of Massive Stars
Recent observational progress in disk-outflow systems around high-mass YSOs, in particular, episodic mass accretion events identified by the methanol maser flares, has provided a strong evidence for disk-mediated accretion in high-mass star-formation. At the same time, understanding of the episodic accretion encounters new challenges. Diversity in timescale, accretion rate, dependency on binarity/multiplicity of mass accretion, and consequent feedback via radiation and  jet/outflow launching within 100 au around YSOs are of great interest in this IAUS.

5) Pulsation and Outflows in Evolved Stars
Maser maps of various molecular species at the finest scales of stellar radii are useful diagnostics of three-dimensional dynamics and chemical properties of circumstellar envelopes of evolved stars. In spite of their great impact on the cosmic ecosystem ejecting heavy elements and dust, there still remain basic questions to be addressed in this IAUS, such as driving and acceleration mechanisms of stellar winds/outflows from stellar atmosphere to outer envelope, and origins of their diversity with shapes (isotropic and bipolar), mass, binarity/multiplicity and timescale.

6) Theory of Masers and Maser Sources
Maser pumping and amplification mechanisms are the most fundamental and general questions in this IAUS, which are essential for interpretation of the maser data. One of the main themes is origins of maser variability including periodic variability and sudden flares. In addition, maser polarization will be discussed intensively in terms of understanding of dynamical roles of magnetic fields in AGNs/SMBHs (Session 2), SFRs/YSOs (Session 3) and AGB stars (Session 4).

7) New Projects and Future Telescopes
Future large radio telescopes (SKA, ngVLA, regional/global VLBIs) will focus on science topics with masers, in particular, time-domain multi-wavelength studies through synergies with other wavelengths (JWST, ELT, GMT, TMT, etc.). The proposed IAUS will provide the ideal opportunity to start a discussion to maximize achievements of future large projects utilizing masers. New observational and calibration techniques to be employed by future radio telescopes will also be reviewed to optimize the astrometric outcomes at 10-times higher accuracy for all kinds of masers.