Betelgeuse, Betelgeuse, Betelgeuse, Betel-buddy? Constraints on the dynamical companion to $\alpha$ Orionis from HST

TL;DR

This study used HST UV spectroscopy to search for a binary companion to Betelgeuse, aiming to explain its long secondary period, but found no spectral evidence of such a companion within certain mass and emission limits.

Contribution

The paper provides the first targeted UV observational constraints on the hypothesized Betelgeuse companion, ruling out companions above 1.5 solar masses during specific orbital phases.

Findings

No spectral features of the companion detected in UV data.

Companion mass likely below 1.5 solar masses based on non-detection.

Constraints on companion emission set at approximately 10^{-14} erg s^{-1} cm^{-2} Å^{-1}.

Abstract

Recently, two independent analyses have asserted that the cause of the Long Secondary Period (LSP) observed in the variability spectrum of our nearest red supergiant, Betelgeuse ($\alpha$ Ori), is an as-yet undetected, low-mass binary companion dubbed $\alpha$ Ori B. In this paper, we present the results of a far-UV observational campaign using the STIS echelle spectrograph on the Hubble Space Telescope aimed at detecting spectral signatures of the companion. The four-quadrant tiling pattern and timing of the observations were optimized to isolate the companion, with observations taking place during a period of maximum angular and velocity separation between Betelgeuse and the putative companion. Spectral differencing between quadrants recovers no spectral features at the companion's velocity in excess of the background or Betelgeuse's chromosphere, i.e. a non-detection. Having determined that $\alpha$ Ori B is most likely a Young Stellar Object (YSO) thanks to constraints from a complementary X-ray campaign with the Chandra X-ray Observatory in a companion paper, comparison of our data against canonical spectra from YSOs in the ULLYSES database allows us to confidently exclude masses above $\gtrsim1.5M_\odot$ and companion continuum or line emission in excess of $\approx10^{-14}$ erg s$^{-1}$ cm$^{-2}$ angstrom$^{-1}$ in the FUV ($\approx1200-1700$ angstroms). Future observational campaigns aware of the LSP phase are needed to place deeper constraints on the spectroscopic nature of $\alpha$ Ori B.