A Catalog of Galactic Multiple Systems with a Red Supergiant and a B Star

Binary stars are useful to study many aspects of stellar structure and evolution and the systems where the two objects have very different spectral types are especially so, as (barring interactions between them or with third objects) they signal coeval evolutionary phases. In this context, Neugent et al. (2019), from now on N19, indicate that "until a year ago (2018), the total number of known Galactic binary RSGs was 11" (RSG = red supergiant) and then go on to wonder about where the missing binaries are and to point toward 87 new RSG+B binary systems in M31, M33, the SMC, and the LMC. In this work we examine that quoted text.

We have searched the literature and Simbad to find Galactic RSG+B binary systems. More specifically, we define RSGs as objects with spectral type G, K, or M and luminosity classes II to Ia for consistency with N19, but we note that criterion includes some Cepheids, which can be of spectral type G-K0 in some phases. Results are listed in Table 1 along with some useful information. The separation column is populated if a visual component with Δm small enough to contribute to the spectral type at short wavelengths is listed in the Washington Double Star Catalog (Mason et al. 2001).

We have found 108 Galactic RSG+B binary systems in the literature, with 61% of the sample already present in two references alone (Ginestet & Carquillat 2002; Samus' et al. 2017). A significant number of the objects have two entries in the HD catalog, a likely consequence of the composite nature of the spectra. The total number in the sample has to be taken with care, as literature spectral classifications can include gross errors (see e.g., Maíz Apellániz et al. 2016). Also, RSGs in the spectral classification sense we are using here should not be identified with RSGs in the stellar evolution sense: some in our list are Cepheids and others may be bright AGB stars or even more peculiar objects (e.g., β Cyg A, Bastian & Anton 2018, or V838 Mon, Munari et al. 2007). Nevertheless, the number of known Galactic RSG+B systems is much larger than the one claimed by N19 and indeed it is not that different from the value found by those authors for the other four large galaxies in the Local Group. Nine of the eleven stars in N19 are in our sample.AL Vel and Algol are missing because they do not satisfy the luminosity class requirement.

Obviously, there must be many more Galactic RSG+B systems to be discovered. Two biases are a sign of this. 71% of the sample is brighter than V = 8 and 67% is in the northern hemisphere (despite the southern hemisphere containing many more Galactic disk stars). Future studies are required to find them and improve our statistics on these important systems. Interestingly, there are no known Galactic RSG+O binary systems. There are, however, known Galactic BSG+O systems (e.g.,HD 115 071, Sota et al. 2014, BSG = B-type supergiant) and at least one ASG+O system (6 Cas, Bartaya et al. 1994, ASG = A-type supergiant). This is a likely manifestation of the H-D limit (Humphreys & Davidson 1979).

(a) M.P.G. and J.M.A. and (b) I.N. acknowledge support from the Spanish Government Ministerio de Ciencia, Innovación y Universidades through grants (a) PGC2018-095 049-B-C22 and (b) PGC2018-093 741-B-C21, respectively. R.H.B. acknowledges support from DIDULS Project 18 143 and the ESAC visitors program. This research has made extensive use of the SIMBAD database, operated at CDS, Strasbourg, France.