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Building a Roadmap for Hubble science into the 2030s: Crucial UV spectroscopy of Oe stars in nearby galaxies

This paper advocates for a strategic roadmap to utilize Hubble's unique UV spectroscopic capabilities in the 2030s to characterize Oe stars in nearby galaxies, thereby advancing our understanding of massive binaries, galaxy evolution, and future UV missions like the Habitable Worlds Observatory.

Original authors: A. Wofford, D. Pauli, A. A. C. Sander, O. Aranguré, M. S. Oey, O. G. Telford, P. A. Crowther, J. S. Vink, Tomer Shenar, M. Gull, L. P. Martins, S. Simón-Díaz, S. Zharikov, L. M. Oskinova, L. J. Smith

Published 2026-06-05

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CC BY 4.0

Original authors: A. Wofford, D. Pauli, A. A. C. Sander, O. Aranguré, M. S. Oey, O. G. Telford, P. A. Crowther, J. S. Vink, Tomer Shenar, M. Gull, L. P. Martins, S. Simón-Díaz, S. Zharikov, L. M. Oskinova, L. J. Smith

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Why We Need to Look at "Super-Spinners"

Imagine the universe is a giant construction site. The massive stars (called O-type stars) are the heavy-duty cranes and bulldozers that shape the neighborhood. They blast out radiation, create heavy elements, and eventually explode, seeding the cosmos with the ingredients for new stars and planets.

Among these massive stars, there is a special, rare group called Oe stars. Think of them as the "Formula 1 race cars" of the stellar world. They are massive, but they are also spinning incredibly fast. Because they spin so fast, they fling material off their surfaces, creating a swirling disk of gas around them (like a figure skater spinning so fast their arms blur).

Why do we care?

They are the "Spin-Up" Artists: Scientists believe these stars got their super-speed by stealing mass from a partner star in a binary system (a cosmic dance of two stars). Understanding them helps us understand how stars interact and evolve.
They are the "Low-Metal" Specialists: These stars are much more common in "metal-poor" galaxies (galaxies that are younger or less chemically evolved, like the Small Magellanic Cloud). Since the early universe was metal-poor, studying these stars today is like looking at a time machine to see how the first generations of stars behaved.

The Problem: The "Optical Blindfold"

Here is the tricky part. When we look at these stars with standard optical telescopes (the kind that see visible light, like our eyes), we hit a wall.

The swirling gas disk around an Oe star is so bright and messy that it fills in the "fingerprint" lines we usually use to identify the star. It's like trying to read a book where someone has spilled thick, colorful paint all over the pages. You can see the book, but you can't read the words. Because of this, we can't easily tell how hot the star is, how fast it's losing mass, or how much ionizing radiation it's pumping out.

The Solution: Hubble's "UV Flashlight"

This is where the Hubble Space Telescope (HST) becomes the hero. Hubble has special instruments (COS and STIS) that can see Ultraviolet (UV) light.

Think of UV light as a special "X-ray vision" or a "flashlight" that cuts through the paint.

The Magic Lines: In the UV spectrum, there are specific lines of light (like C III and N III) that are not affected by the messy gas disk. They come straight from the star's surface.
The Result: By looking at these UV lines, astronomers can finally read the "book" again. They can accurately measure the star's temperature, its chemical makeup, and how much energy it is blasting into space.

Why is this essential?
Without Hubble's UV view, we are guessing. With it, we get the hard data needed to understand how these stars influence their galaxies.

The Plan: A Roadmap for the 2030s

The paper proposes a specific plan to keep Hubble working on this mission into the 2030s. Here is the strategy:

The Target List: They want to observe a specific list of these "super-spinners" in nearby galaxies (like the Milky Way, the Magellanic Clouds, and others).
The Team-Up: This isn't just about looking at the stars in isolation. The team is coordinating with a massive new optical survey called the Local Volume Mapper (LVM).
- The Analogy: Imagine the LVM is a high-resolution map of a city's streets (the gas clouds around the stars). Hubble is the satellite taking a close-up photo of the specific buildings (the stars).
- The Goal: By combining the street map (LVM) with the building photo (Hubble UV), scientists can verify exactly how much energy the stars are pumping into the gas clouds. This helps us understand how stars "feed" and shape their galaxies.
The Future Connection: This work is a training ground for the future. The data gathered now will help scientists design and plan for the Habitable Worlds Observatory (HWO), a future telescope that will eventually take over this job. We need to learn the rules of the game now so we can play it better later.

Summary

In short, this paper argues that Oe stars are crucial cosmic engines, but they are currently "blinded" by their own fast-spinning gas disks when viewed in normal light. Hubble is the only tool we have right now that can see through the noise using UV light to reveal their true nature. By observing them in the 2030s, we will not only solve the mystery of these fast-spinning stars but also validate our models for how galaxies evolve and prepare for the next generation of space telescopes.

Technical Summary: Building a Roadmap for Hubble Science into the 2030s – Crucial UV Spectroscopy of Oe Stars in Nearby Galaxies

Problem Statement
Massive O- and B-type stars drive galaxy evolution through feedback mechanisms (ionizing radiation, mechanical energy, metal production) and serve as progenitors for multi-messenger phenomena (supernovae, gravitational waves). A significant subset of these stars, known as Oe stars (O-type stars with emission lines), are believed to be spun-up mass gainers in binary systems. These stars are particularly prevalent at low metallicity, where they may constitute 12–30% of the O-star population.

However, characterizing Oe stars using conventional optical analysis is extremely challenging. The emission from their decretion disks fills in key optical hydrogen Balmer and He I absorption lines, obscuring the spectral features necessary to determine fundamental properties such as spectral type, effective temperature ( $T_{eff}$ ), and ionizing spectra. Consequently, the stellar and wind parameters of Oe stars, especially at low metallicity, remain poorly constrained, creating a gap in our understanding of massive binary evolution and galactic feedback.

Methodology and Instrumentation
The paper argues that Far-Ultraviolet (FUV) spectroscopy is the only viable route to constrain the fundamental properties of Oe stars. The proposed methodology relies on utilizing the unique capabilities of the Hubble Space Telescope (HST):

COS (Cosmic Origins Spectrograph): Specifically the G130M and G160M medium-resolution gratings. G130M is required to cover temperature-sensitive photospheric lines (e.g., C III 1176, N III 1183/84, relative to C IV 1169), while G160M covers stellar wind lines (e.g., C IV 1550, He II 1640, N IV 1718). G140L is noted for use with highly reddened stars.
STIS (Space Telescope Imaging Spectrograph): The E140M and E230M modes are identified as necessary for targets that are too bright for COS or located in densely populated regions where COS aperture contamination is a risk.

The study proposes obtaining UV spectra for a community-driven sample of optically identified Oe stars and candidates across the Milky Way, the Magellanic Clouds (LMC and SMC), and other Local Group galaxies (e.g., IC 1613, M33, NGC 6822).

Key Contributions and Results
The paper synthesizes existing knowledge and preliminary data to highlight the following:

Diagnostic Power of FUV: Unlike optical spectra, FUV spectra of Oe stars are not contaminated by disk emission features. This allows for the direct fitting of synthetic stellar atmosphere models to derive $T_{eff}$ , CNO abundances, and mass-loss rates ( $\dot{M}$ ).
Metallicity Dependence: Oe stars are more common at low metallicity. The paper notes that current FUV samples are small and often located in complex regions (e.g., crowded fields or faint nebulae), preventing the validation of ionizing fluxes using associated nebulae.
Synergy with IFU Surveys: The paper outlines a critical synergy between HST UV spectroscopy and upcoming wide-field Integral Field Unit (IFU) optical surveys, specifically the "Local Volume Mapper" (LVM) from SDSS-V. LVM will provide spatially resolved data of ionized gas in nearby galaxies (e.g., SMC) with ~10 pc resolution.
- Validation: LVM data can validate the ionizing fluxes inferred from HST stellar FUV spectra by comparing them to the luminosity of H II regions in Balmer lines.
- Limitation: LVM data alone cannot calibrate stellar atmosphere models without the UV constraints provided by HST.
Binary Evolution: The paper emphasizes that Oe stars are likely spun-up mass gainers. Understanding their properties is essential for tracing the binary evolution path to compact binaries and gravitational wave events, particularly in low-metallicity environments where empirical constraints are lacking.

Significance and Future Outlook
The paper positions this work as a necessary precursor to future UV astronomy.

Immediate Impact: Observations in the 2030s with HST are deemed crucial to inform the era of ultra-wide-field IFU and transient astronomy surveys. Without HST UV data, the ionizing spectra of Oe stars in these surveys cannot be accurately modeled, hindering the understanding of H II region dynamics, radiation pressure, and the leakage of ionizing photons into the circumgalactic medium.
Long-term Legacy: The proposed observations are intended to inform the design and planning of the Habitable Worlds Observatory (HWO). By characterizing a representative sample of local Oe stars via HST, the community will be better prepared to obtain UV spectra of similar populations in Local Group galaxies and beyond with future observatories.

The authors propose a community-driven "Hubble Legacy Program" to execute this roadmap, ensuring a comprehensive dataset of Oe stars across varying metallicities to refine models of massive star evolution and binary interactions.