# Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars

## TL;DR

This study used archival IUE spectra and Doppler tomography to detect and characterize the hot subdwarf companion of 60 Cygni, providing estimates of its temperature, mass ratio, and flux contribution, and setting upper limits for other Be star companions.

## Contribution

First detection of a hot subdwarf companion in 60 Cygni using UV spectra and Doppler tomography, with detailed parameter estimation and analysis of non-detections.

## Key findings

- Detected hot companion in 60 Cygni with T_eff = 42,000 K.
- Estimated flux ratio of the companion to the primary near 1525 Å.
- Set upper limits on the UV flux contribution of companions in other Be stars.

## Abstract

We used archival International Ultraviolet Explorer (IUE) high-dispersion, short wavelength spectra data to search for evidence of the spectra of hot subdwarf companions of six rapidly rotating Be stars in binary systems. We searched for the signature of a hot companion through an analysis of the cross-correlation functions of observed and model spectra that were separated into primary and secondary components using a Doppler tomography algorithm and adopted spectroscopic orbital solutions. A positive detection of the flux from a hot companion was made for the reconstructed secondary cross-correlation function of just one target, 60 Cygni (B1 Ve). We estimate that the companion of the Be star in 60 Cygni has $T_{\rm eff} = 42 \pm 4$ kK, mass ratio $M_2/M_1 = 0.15 \pm 0.02$, and monochromatic flux ratio ${f_2}/{f_1} = 0.034 \pm 0.002$ in the spectral region near 1525 \AA. If the companions of the other target Be stars are also hot, then they must be faint and contribute less than $\approx 1\%$ of the UV flux ($<0.6 \%$ in the case of $\gamma$ Cas). We also discuss in an appendix a shell episode of Pleione (28 Tau) recorded in the IUE spectra.