Using GALEX UV Excess to Search for Metal-poor Halo Stars

Authors

• Chase L. Smith, College of Engineering and Physical Sciences
• Maxwell Moe, College of Engineering and Physical Sciences
• Megan Frank, College of Engineering and Physical Sciences
• Raven Cilley, University of Michigan, Ann Arbor
• Javier Fregoso, California State University Channel Islands
• Alexander Gleason, The University of Oklahoma
• Grace Nelson, Michigan Technological UniversityFollow
• et al.

Document Type

Article

Publication Date

6-22-2026

Department

Department of Physics

Abstract

Metal-poor solar-type stars display a significant reduction in metal-line blanketing at short wavelengths, leading to an excess of near-ultraviolet (NUV) flux compared to their metal-rich counterparts. We utilize Galaxy Evolution Explorer Satellite (GALEX) NUV and Gaia DR3 photometry along with ground-based spectroscopy to establish a correlation between NUV excess and [Fe/H]. We construct a sample of 492 solar-type (F5-G9) halo stars with NUV excess and measured metallicities. We perform our own observations with the KOSMOS spectrograph at Apache Point Observatory’s 3.5 m telescope to measure the abundances of 13 halo stars, 11 of which did not have previous metallicity measurements. Our targeted 13 halo stars span −2.92 < [Fe/H] < −1.97 and are all α enhanced with [α/Fe] = 0.05–0.73. For our full sample of 492 objects, we find an anticorrelation between NUV excess and [Fe/H] that is statistically significant at the 8σ level. GALEX NUV excess can be used to distinguish very metal-poor (VMP) stars ([Fe/H] < −2) from their metal-rich counterparts. However, there is significant dispersion in the relation due to NUV chromospheric variability caused by rotational effects and magnetic cycle activity. The NUV chromospheric variability inhibits our ability to reliably distinguish extremely metal-poor (EMP) stars ([Fe/H] < −3) from VMP stars based on photometry alone. UV spectra of EMP halo stars are needed to better calibrate their atmospheric properties and variability.

Publisher's Statement

© 2026. The Author(s). Published by the American Astronomical Society. Publisher’s version of record: https://doi.org/10.3847/1538-4357/ae74c3

Publication Title

Astrophysical Journal

Recommended Citation

Smith, C., Moe, M., Frank, M., Cilley, R., Fregoso, J., Gleason, A., Nelson, G., & et al. (2026). Using GALEX UV Excess to Search for Metal-poor Halo Stars. Astrophysical Journal, 1005(1). http://doi.org/10.3847/1538-4357/ae74c3
Retrieved from: https://digitalcommons.mtu.edu/michigantech-p2/2733

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Version

Publisher's PDF