IY Lyr: A Thick-Disk first-overtone RR Lyrae Star with a Possible Neutron Star Companion

Abstract

IY Lyr, historically misclassified as an eclipsing binary, has been previously identified as a first-overtone RR Lyrae star (RRc star). Using multiband photometry (All-Sky Automated Survey for Supernovae, Zwicky Transient Facility, TESS, and our BVRI data), Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, and Gaia astrometry, we investigate its pulsation, binarity, and Galactic population. From O-C analysis, we detect a long-term period decrease and a light-travel time effect with an orbital period of 3.94 $\pm$ 0.09 years, eccentricity of 0.46 $\pm$ 0.15, and a mass function of 0.65 $\pm$ 0.14 M${}_{\odot }$. The companion is independently supported by radial velocity residuals and Gaia proper motions. Combined constraints yield an orbital inclination of 94.2${}^{\circ }$ $\pm$ 1.1${}^{\circ }$ and a companion mass of 1.37 $\pm$ 0.19 M${}_{\odot }$. Chemical abundances ([Fe/H] $\simeq$ -1.0 $\pm$ 0.1, [$\hat{I}\pm$/Fe] $\simeq$ +0.27 $\pm$ 0.03, Xiang et al. 2019) and dynamics ($L_{\mathrm{z}}$ $\simeq$ 1287 $\pm$ 35 kpc km s${}^{-1}$, $Z_{\max }$ $\simeq$ 1.17 $\pm$ 0.10 kpc) identify IY Lyr as likely an old, high-$\hat{I}\pm$, thick-disk star. The companion mass lies at the peak of the neutron star mass distribution, and the system's age excludes a main-sequence star; we conclude the companion is most likely a typical neutron star, although a massive white dwarf near the Chandrasekhar limit cannot be ruled out. IY Lyr is among the few RRc binaries with a compact companion supported by multiple methods, and it has important implications for thick-disk binary evolution and neutron star formation.

20 pages, 6 figures, 5 tables