SUMS Catalog & Stripped Star Candidates

UV Photometric Catalog and Stripped Star Candidate Selection

This project presents a new ultraviolet photometric catalog of the Magellanic Clouds, created to identify candidate intermediate-mass stripped stars: stars that have lost their hydrogen-rich envelopes through binary interaction. This page gives a brief overview of the catalog and the lore behind it. For more details, please check out our paper.

Stripped-Star Ultraviolet Magellanic Cloud Survey

The Origin of SUMS

SUMS (the Stripped star Ultraviolet Magellanic Clouds Survey) began in 2018 as a collaboration between myself (Bethany Ludwig), Maria Drout, and Ylva Götberg. Our goal was ambitious: to uncover the first observed population of massive stars that had been stripped of their outer layers by a companion star. Although this population of so-calledstripped stars has been predicted since the 1960s, and plays a key role in everything from stellar evolution to supernovae and gravitational wave sources, no unambiguous example had ever been confirmed.

The audacity to search for what had not yet been found came from the conviction that the absence of observed systems did not mean the theory of binary evolution was flawed or that these stars didn’t exist–it meant the challenge was observational. Inspired by the predictions of Götberg et al. (2018), which showed that some stripped stars should reveal themselves through excess ultraviolet light in binary systems, we turned to the UV. At the time, however, no wide-field UV surveys offered the necessary depth, resolution, and coverage to detect them in regions rich with massive stars. So we built one. The SUMS catalog is our custom UV photometric survey, designed from the ground up to uncover these elusive products of binary interaction.

Building a photometric pipeline using the Tractor

The UV Photometry

To build the SUMS catalog, we started with archival images taken by the Swift-UVOT detector and predominantly sourced from theSUMaC project (Swift Ultraviolet Survey of the Magellanic Clouds). SUMaC had collected thousands of UV images of the Magellanic Clouds over three years, giving us an unprecedented dataset to work with. In total, we processed nearly 2,500 Swift-UVOT images across the UVW1, UVM2, and UVW2 filters. To extract reliable photometry from this crowded and complex field, we developed a custom pipeline using The Tractor, a forward-modeling image fitting tool. The Tractor allowed us to model point sources directly on the UVOT images using prior positions from the MCPS optical catalog. This approach helped us deblend sources in crowded regions and recover clean photometry even in challenging areas.

A Diversity of Known Stars — and a Glimpse into the Unknown

The SUMS Catalog

This plot showcases the SUMS catalog in ultraviolet (UVM2) and optical light (V) for the Large and Small Magellanic Clouds (LMC and SMC). Each panel shows a color-magnitude diagram using UVM2 and V-band magnitudes, where different symbols and colors represent a wide variety of known stellar populations--ranging from O- and B-type stars to RR Lyrae, planetary nebulae, red supergiants, and more. The dense cloud of gray points in the background marks the full photometric sample, revealing the richness and complexity of UV-bright sources. SUMS not only captures well-known stellar types, but also illuminates a vast population of sources that have never been studied in detail before--offering new ground for discovery. Among them are UV-bright, optically faint stars that stand out with a clear ultraviolet excess. These are the systems where we expect stripped stars to reside--making this catalog the first step in our odyssey to uncover them.

The Pathway to Discovering Intermediate-Mass Stripped Stars

The SUMS Candidates

This figure shows how candidate stripped stars from the SUMS catalog compare to theoretical predictions and previously known systems across multiple ultraviolet–optical color–magnitude diagrams. The top row shows evolutionary tracks for stripped stars at LMC metallicity (dark blue line), as well as Main Sequence stars (gray lines moving to the right of the thick zero-age main sequence line) and a combination of the two as the web between them. WR, WN3/03, and subdwarf stars are shown for reference. Each column moves from more to less UV-excess with three different color–magnitude spaces: UVM2–V, UVW1–B, and U–V. The middle and bottom rows show our final stripped star candidates, identified by their UV excess and reduced by a number of cuts related to the data-quality and theoretical expectations of what their overall photometry (SEDs) should look like. We divide our 820 candidates into four categories based on their UV excess-- Very Blue (VB) or Blue (B) and their observability-- Excellent (E) or Good (G). While this catalog was being perfected, we were simultaneously taking spectroscopic observations on a subset of these candidates in order to confirm them as bonafide stripped stars. That effort led to the first confirmed population of stripped stars in Drout, Götberg & Ludwig (2023) marked here with green stars for comparison. SUMS is already opening a new window into this hidden population of binary stars--and this is just the beginning.