Multiplicity of Massive stars in the Milky Way (M 3 W)

Context. Multiplicity is ubiquitous among massive stars and our knowledge of its processes is constrained by the limited sample of well-determined orbits, hampering progress across astronomical fields ranging from the initial mass function to gravitational-wave sources. Aims. The immediate goal of Multiplicity of Massive stars in the Milky Way (hereafter, M 3 W) is to significantly increase the number of massive multiple systems with accurately determined orbits and masses. With that information in hand, we are equipped to address issues such as multiplicity statistics, the mass function in clusters and the field, the properties of binaries with compact companions and gravitational-wave progenitors, the origin and characteristics of runaways and their 3D motions, the use of apsidal motion as a probe of stellar interiors, and the mass discrepancy between different methods (i.e. evolutionary, spectroscopic, and Keplerian). Methods. In this first paper, we present the project, describe the data and tools we plan to use, along with an explanation of how to apply the UNWIND tool for disentangling and analysing the very massive twin binary system GLS 11 448. We describe different issues related to massive-star multiplicity that will be analysed in the series. At the start, GLS 11 448 has been chosen as the first object to be analysed for three reasons: the extreme high mass of its components, the difficulties associated with its high extinction that can be addressed with our technique, and its status as an standard star for the interstellar medium (ISM). Results. We present a new orbital solution for GLS 11 448 using UNWIND to obtain, for the first time, disentangled spectra for the full 3820–11 000 Å range for an OB spectroscopic binary. Having derived the stellar parameters, we have the ability to explain how UNWIND makes new stellar lines available for the study of O stars. The Aa and Ab components of GLS 11 448, both classified as O3.5 II(f*), are the two most massive O stars ever detected according to the evolutionary masses of 70 ± 10 M ⊙ and 76 ± 11 M ⊙ determined in this paper. We also report the first-ever detection of the interstellar He I λ 10 830 triplet in absorption in an OB-star sightline. As a byproduct of the interstellar-medium model derived for UNWIND using GLS 11 448 and five other standard stars, we present the most detailed diffuse-interstellar-band (DIB) library ever built, with a total of 631 DIBs in the 4000–17 100 Å range, 37 of which have been fitted with multiple-Gaussian profiles and 116 had never been identified before.