Does the HR 6819 triple system contain a dormant black hole? Not necessarily

TL;DR

This paper questions the evidence for a dormant black hole in HR 6819, proposing an alternative model where the companion is a short-period binary of two A0 stars, emphasizing the need for spectral analysis to distinguish models.

Contribution

It challenges the previous black hole hypothesis by proposing an alternative binary star model and highlights the importance of spectral analysis to confirm the system's nature.

Findings

The evidence for a black hole in HR 6819 is inconclusive.

An alternative model with a binary of two A0 stars is plausible.

Spectral analysis can help distinguish between the models.

Abstract

A recent paper by Rivinius et al. proposed that HR 6819 is a triple system, with a distant Be star and a binary of 40-day orbit, composed of a B3 III giant and a dormant black hole (BH). We suggest that the evidence for this model is not conclusive. In an alternative model, the companion of the giant is by itself a short-period binary in, say, a $\sim$$4$-day orbit, consisting, for example, of two A0 stars. Each of the two A0 stars should contribute $\sim$$4\%$ of the total brightness of the system in the $V$ band, and their spectral lines are moving due to their assumed orbital motion with an unknown period. Therefore, only a careful analysis of the observed spectra can exclude such a model. Before such an analysis is presented and upper limits for the depths of the hypothetical A0 star absorption lines are derived, the model of a hidden close pair is more probable than the BH model.