# A Spectroscopic Orbit for the late-type Be star $\beta$ CMi

## TL;DR

This study analyzes spectroscopic data of the Be star β CMi, revealing evidence of a low-mass binary companion through periodic Doppler shifts, and discusses implications for binary evolution and disk truncation.

## Contribution

It provides the first spectroscopic detection of a potential low-mass companion to β CMi and links binary evolution to its disk properties.

## Key findings

- Detected 170-day periodic Doppler shifts in Hα line wings.
- Found weak correlation between V/R variations and orbital motion.
- Suggested binary evolution scenario with a possible hot subdwarf or white dwarf companion.

## Abstract

The late-type Be star $\beta$ CMi is remarkably stable compared to other Be stars that have been studied. This has led to a realistic model of the outflowing Be disk by Klement et al. These results showed that the disk is likely truncated at a finite radius from the star, which Klement et al.~suggest is evidence for an unseen binary companion in orbit. Here we report on an analysis of the Ritter Observatory spectroscopic archive of $\beta$ CMi to search for evidence of the elusive companion. We detect periodic Doppler shifts in the wings of the H$\alpha$ line with a period of 170 d and an amplitude of 2.25 km s$^{-1}$, consistent with a low-mass binary companion ($M\approx 0.42 M_\odot$). We then compared the small changes in the violet-to-red peak height changes ($V/R$) with the orbital motion. We find weak evidence that it does follow the orbital motion, as suggested by recent Be binary models by Panoglou et al. Our results, which are similar to those for several other Be stars, suggest that $\beta$ CMi may be a product of binary evolution where Roche lobe overflow has spun up the current Be star, likely leaving a hot subdwarf or white dwarf in orbit around the star. Unfortunately, no direct sign of this companion star is found in the very limited archive of {\it International Ultraviolet Explorer} spectra.