Intermediate-Mass Stripped Stars in the Magellanic Clouds: Forward Modeling the Observed Population Discovered Via UV Excess

Abstract

Stripped stars are hot, helium-rich stars formed when binary interactions remove a star's hydrogen envelope. While low-mass ($\lesssim 1 M_\odot$) and high-mass ($\gtrsim 8 M_\odot$) stripped stars are well studied as hot subdwarfs and Wolf-Rayet stars, their intermediate-mass counterparts ($1-8 M_\odot$) have only recently been discovered. The Stripped-Star Ultraviolet Magellanic Cloud Survey (SUMS) identified UV-excess sources (i.e., sources lying blueward of the main sequence) in the Magellanic Clouds using Swift-UVOT photometry and selected 820 photometric stripped-star candidates. However, the completeness and purity of this sample remain poorly understood. We forward model the population of stripped stars in the Magellanic Clouds using a binary population synthesis model combined with spatially resolved star formation histories and simulated UV photometry. To assess survey sensitivity, we inject simulated sources into real Swift-UVOT images and reproduce the SUMS selection process, including crowding, extinction, and photometric quality cuts. For the SMC and LMC, we respectively recover 31\% and 15\% of sources with intrinsic UV excess, and $11\%$ and $7\%$ of all stripped stars. The rest are missed due to dilution by luminous companions, crowding, high extinction, and limited survey coverage. The observed population is biased toward systems with low-mass companions formed by common envelope evolution and systems with compact object companions. We predict contamination of the observed stripped-star candidates by main-sequence stars with spurious UV excess due to crowding and provide guidelines for selecting higher-purity subsamples. Our population synthesis model matches the number and properties of observed stripped star candidates with masses of $(2-5) M_\odot$ well. It underpredicts the number of lower- and higher-mass candidates, perhaps due to contamination.