X-Shooting ULLYSES: Massive stars at low metallicity: I. Project description

Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational wave events involving spectacular black hole mergers indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observing ∼250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES programme. The complementary X-Shooting ULLYSES (XShootU) project provides an enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESOa's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates as a function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of astrophysics, the data and modelling of the XShootU project is expected to be a game changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope spectra of the first stellar generations, the individual spectra of low-Z stars need to be understood, which is exactly where XShootU can deliver

Compositional layering in io driven by magmatic segregation and volcanism

The compositional evolution of volcanic bodies like Io is not well understood. Magmatic segregation and volcanic eruptions transport tidal heat from Io's interior to its surface. Several observed eruptions appear to be extremely high temperature (≥1600 K), suggesting either very high degrees of melting, refractory source regions, or intensive viscous heating on ascent. To address this ambiguity, we develop a model that couples crust and mantle dynamics to a simple compositional system. We analyze the model to investigate chemical structure and evolution. We demonstrate that magmatic segregation and volcanic eruptions lead to stratification of the mantle, the extent of which depends on how easily high temperature melts from the more refractory lower mantle can migrate upwards. We propose that Io's highest temperature eruptions originate from this lower mantle region and that such eruptions act to limit the degree of compositional stratification

Architecture and emplacement of flood basalt flow fields: case studies from the Columbia River Basalt Group, NW USA

The physical features and morphologies of collections of lava bodies emplaced during single eruptions (known as flow fields) can be used to understand flood basalt emplacement mechanisms. Characteristics and internal features of lava lobes and whole flow field morphologies result from the forward propagation, radial spread, and cooling of individual lobes and are used as a tool to understand the architecture of extensive flood basalt lavas. The features of three flood basalt flow fields from the Columbia River Basalt Group are presented, including the Palouse Falls flow field, a small (8,890 km2, ∼190 km3) unit by common flood basalt proportions, and visualized in three imensions. The architecture of the Palouse Falls flow field is compared to the complex Ginkgo and more extensive Sand Hollow flow fields to investigate the degree to which simple emplacement models represent the style, as well as the spatial and temporal developments, of flow fields. Evidence from each flow field supports emplacement by inflation as the predominant mechanism producing thick lobes. Inflation enables existing lobes to transmit lava to form new lobes, thus extending the advance and spread of lava flow fields. Minimum emplacement timescales calculated for each flow field are 19.3 years for Palouse Falls, 8.3 years for Ginkgo,and 16.9 years for Sand Hollow. Simple flow fields can be traced from vent to distal areas and an emplacement sequence visualized, but those with multiple-layered lobes present a degree of complexity that make lava pathways and emplacement sequences more difficult to identify

Rootless cone eruption processes informed by dissected tephra deposits and conduits

Rootless cones result from the explosive interaction between lava flows and underlying water-saturated sediment or volcaniclastic deposits. Rootless explosions can represent a significant far-field hazard during basaltic eruptions, but there are few detailed studies of their deposits. A rootless cone field in the 8.5 Ma Ice Harbor flow field of the Columbia River Basalt Province, NW USA, is revealed by sections through rootless conduit and cone structures. The Ice Harbor lava flow hosting the rootless cones was emplaced across a floodplain or lacustrine environment that had recently been mantled by a layer of silicic volcanic ash from a major explosive eruption. Our observations indicate a two-stage growth model for the rootless cones: (1) initial explosions generated sediment-rich tephra emplaced by fallout and pyroclastic density currents and (2) later weaker explosions that generated spatter-rich fountains. Variable explosive activity resulted in a wide range of pyroclast morphologies and vesicularities. Cross-sections through funnel-shaped conduits also show how the conduits were constructed and stabilised. The growth model is consistent with decreasing water availability with time, as inferred for rootless cones described in Iceland. The Ice Harbor rootless cones provide further lithological data to help distinguish between rootless cone-derived tephra and tephra generated above an erupting dyke