Letters of Intent received in 2019

LoI 2021-2068
Focus Meeting (GA): UV Insights to Massive Stars and Young Stellar Clusters

Topics

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

Rationale

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 recent observing campaigns 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. Specifically, we must use the existing FUV astronomical database and these coming programs to answer:

• Can we trust models for the ionizing continuum of massive stars?
• What is the frequency of massive interacting binaries, and how do binaries affect population properties?
• Do observations of clusters suggest deficiencies in our understanding of massive star evolution?
• Do very massive stars exist, and what are their properties?
• Constraining the EUV ionizing continuum from FUV stellar population synthesis models
• The UV nebular HeII problem: Are the ionizing continua from massive stars / clusters sufficient to reproduce observed UV emission line features?
• Do stellar cluster metallicities trace nebular metallicities?
• Do the stellar cluster populations provide the mechanical energy to reproduce the observed outflows in the ISM?
• How does stellar feedback modify the ISM and regulate star formation?
• Can stellar cluster properties, such as mass distributions, multiple populations, ages, survival rates, formation efficiency provide important diagnostics to probe star formation in their host galaxies?
• Does the alpha/Fe ratio of massive stars evolve with redshift?

This focus meeting will bring together astronomers who study resolved massive stars, integrated light stellar clusters, and their effects on their host galaxies, combining the best rest-frame UV spectra and photometric observations to date with state-of-the-art models. This meeting will have a significant interdisciplinary component, focusing on the interplay between massive stars and their host galaxies (e.g. the consequences of environment on the formation and evolution of massive stars and clusters, and their subsequent feedback in terms of ionizing photon production, mass-loss due to winds, and properties of the ISM).