# Unicorns and Giraffes in the binary zoo: stripped giants with subgiant companions

**Authors:** Kareem El-Badry, Rhys Seeburger, Tharindu Jayasinghe, Hans-Walter Rix, Silvia Almada, Charlie Conroy, Adrian M. Price-Whelan, Kevin Burdge  
**arXiv:** [2203.06348](https://arxiv.org/abs/2203.06348)  
**Date:** 2022-03-12  
**Summary page:** https://www.summarizepaper.com/en/paper/2203.06348v1/

## Key points

- The study analyzes two binary systems containing giant stars: V723 Mon and 2M04123153+6738486.
- Both giants orbit more massive but less luminous companions that were previously proposed to be mass-gap black holes, but spectral disentangling reveals that both systems actually contain luminous companions with star-like spectra.
- By jointly modeling the spectra, light curves and spectral energy distributions, the authors are able to constrain the masses, temperatures and radii of both components.
- The primaries in both systems are found to be luminous cool giants with exceptionally low masses ($M_{\rm giant} \approx 0.4\,M_{\odot}$) that likely fill their Roche lobes.
- The effective temperatures of these giants are $T_{\rm eff,\,giant} = 3,800\,\rm K$ and $4,000\,\rm K$, with radii of $R_{\rm giant}= 22.5\ R_{\odot}$ and $25 R_{\odot}$ for V723 Mon and the Giraffe respectively.
- The secondaries in both systems are only slightly warmer subgiants with temperatures of $T_{eff ,2 } = 5800 \ \text{K}$ and $5150 \ \text{K}$, and radii of $R_2= 8.3 R_{\odot}$ and $9 R_{\odot}$ for V723 Mon and the Giraffe respectively.
- These subgiants are consistent with observed UV limits that would rule out main-sequence stars with similar masses ($M_2 \approx 2.8 M_ {\odot }$
- and $\approx 1.8 M_ {\odot }$).
- Rapid rotation in the Unicorn blurs the spectral lines of the subgiant making it challenging to detect even at wavelengths where it dominates the total light.
- Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping.
- The properties of both systems can be reproduced by binary evolution models in which a primary between 1–2 solar masses is stripped by a companion as it ascends the giant branch.
- The fact that these companions are also evolved implies either that initial mass ratio was very near unity or that they are temporarily inflated due to rapid accretion.
- These two binary systems offer a window into a rarely

## Summary

In this study, the authors analyze two binary systems containing giant stars: V723 Mon (also known as "the Unicorn") and 2M04123153+6738486 (referred to as "the Giraffe"). These giants orbit more massive but less luminous companions that were previously proposed to be mass-gap black holes. However, spectral disentangling reveals that both systems actually contain luminous companions with star-like spectra. By jointly modeling the spectra, light curves and spectral energy distributions, the authors are able to constrain the masses, temperatures and radii of both components. The primaries in both systems are found to be luminous cool giants with exceptionally low masses ($M_{\rm giant} \approx 0.4\,M_{\odot}$) that likely fill their Roche lobes. The effective temperatures of these giants are $T_{\rm eff,\,giant} = 3,800\,\rm K$ and $4,000\,\rm K$, with radii of $R_{\rm giant}= 22.5\ R_{\odot}$ and $25 R_{\odot}$ for V723 Mon and the Giraffe respectively. The secondaries in both systems are only slightly warmer subgiants with temperatures of $T_{eff ,2 } = 5800 \ \text{K}$ and $5150 \ \text{K}$, and radii of $R_2= 8.3 R_{\odot}$ and $9 R_{\odot}$ for V723 Mon and the Giraffe respectively. These subgiants are consistent with observed UV limits that would rule out main-sequence stars with similar masses ($M_2 \approx 2.8 M_ {\odot }$ 
and $\approx 1.8 M_ {\odot }$). The authors note that rapid rotation in the Unicorn blurs the spectral lines of the subgiant making it challenging to detect even at wavelengths where it dominates the total light. Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping. The properties of both systems can be reproduced by binary evolution models in which a primary between 1–2 solar masses is stripped by a companion as it ascends the giant branch. The fact that these companions are also evolved implies either that initial mass ratio was very near unity or that they are temporarily inflated due to rapid accretion. Overall these two binary systems offer a window into a rarely observed phase of binary evolution preceding formation wide-orbit helium white dwarfs or eventually compact binaries containing two helium white dwarfs.