IAU GA Focus Meeting 4:
UV Insights to Massive Stars and Young Stellar Clusters

Dates: August 2022
Location: Busan, Republic of Korea
Coordinating Division: Division H
Danielle Berg (The University of Texas at Austin, USA; co-Chair)
Claus Leitherer (Space Telescope Science Institute, USA; co-Chair)
Angela Adamo (University of Stockholm, Sweden)
Ricardo Amorín (University of La Serena, Chile)
Randa Asa’d (American University of Sharjah, UAE)
JJ Eldridge (University of Auckland, New Zealand)
Ana Gomez de Castro (University Complutense of Madrid, Spain)
Lisa Kewley (Australian National University, Australia)
Young-Wook Lee (Yonsei University, South Korea)
Lucimara Martins (Universidade Cruzeiro de Sul, Brazil)
Daniel Schaerer (University of Geneva, Switzerland)
Aida Wofford (Universidad Nacional Autónoma de México, Mexico)
• New results from large UV HST programs beginning in the 2020s (i.e., ULLYSES, CLASSY, etc.)
• Star clusters as laboratories for testing massive star models
• Synergy between studies of individual massive stars and integrated cluster populations
• Interplay between stars/clusters and their host galaxy environment
• Very massive stars: existence, properties, role of environment
• Constraining the stellar ionizing continuum from cluster properties: models, observations, and implications for EoR
Massive stars and young stellar clusters are cosmic drivers of galaxy evolution and are key to understanding the formation and ionization of the early universe. While the light from massive stars influences all observable facets of star-forming galaxies, the interplay between stars and gas is not well understood. Gas is accreted onto galaxies from the cosmic web, settles into their gravitational wells, and is converted into young stellar clusters. The massive stars in these clusters ionize the surrounding gas, produce nebular emission, and drive outflows. Such feedback drives chemical evolution and can modulate or limit accretion processes, thereby regulating the subsequent growth of galaxies. The effects of massive stars are broad reaching. Therefore, answers to the critical questions surrounding galaxy formation and the early universe, as well as cosmic galaxy evolution, requires understanding the astrophysical properties and lifecycles of massive stars and stellar clusters.
       The rest-frame far-ultraviolet (FUV) offers the best observational window to study the properties of massive stars and the resulting physical conditions on the gas. In particular, FUV spectra contain features that characterize the ionizing stellar population, the imprint of their injected energy on the outflowing gas, and the physical conditions of the nebular gas within the same galaxies. This unparalleled diagnostic power is posed for rapid growth in utility, as it will be vital to the interpretation of the intermediate- and high-redshift galaxies that will be observed with future space- (JWST) and ground-based (ELTs) facilities in the next decades.
       Currently, we are unprepared to interpret the UV spectra produced by massive stars and stellar clusters, especially at low-metallicities, and are facing the impending loss of access to the observed-frame FUV. Many important unknowns remain that thwart our understanding of massive stars, and, as a result, high-redshift galaxies and galaxy evolution. Fortunately, several observing campaigns beginning in 2020 have been designed to specifically tackle these challenges, such as the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) initiative, The COS Legacy Archive Spectroscopic SurveY (CLASSY): A UV Treasury of Star-Forming Galaxies, and others. How we will use the existing FUV astronomical database and these coming programs to answer these key questions will be addressed in this Focus Meeting. Below we outline a program for this Focus Meeting to explore how these key questions can be addressed using upcoming programs and the existing FUV astronomical database, where the timeliness of this meeting will allow discussion of cutting-edge results from ULLYSES, CLASSY, and more. Within the proposed broad session topics, we will seek high-level discussions of the details of massive stars and clusters, including less understood phenomenon such as the rotation and mass-loss rates, enhanced alpha/Fe ratios, stripped stars and other remnants of binary star evolution, W-R stars, and more.