Binarity at LOw Metallicity (BLOeM): a spectroscopic VLT monitoring survey of massive stars in the SMC

TL;DR

This paper introduces BLOeM, a VLT/FLAMES spectroscopic monitoring campaign targeting 929 massive stars in the SMC ($Z \approx 0.2 Z_\odot$) to probe multiplicity, orbital configurations, and dormant OB+BH binaries in a low-metallicity environment. It details sample selection from Gaia DR3, the data-reduction pipeline, spectral classification, and placement on an HR diagram using SMC-calibrated temperatures and Ks-based luminosities. Nine epochs (of 25) have been collected so far, with careful wavelength calibration, sky subtraction, and co-addition to yield high-S/N spectra for robust typing and RV work, flagging SB2s and OBe stars. The study reveals a diverse population from O to late-type supergiants, including eclipsing binaries and several X-ray sources, underscoring the survey’s potential to constrain massive-star evolution and binary statistics at low metallicity. This work lays the groundwork for subsequent analyses of intrinsic binary fractions, orbital distributions, and the identification of rare binary channels in the early-Universe analog environment of the SMC.

Abstract

Surveys in the Milky Way and Large Magellanic Cloud revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, which approaches the conditions of the Early Universe, remains sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign - an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the SMC - the lowest metallicity conditions in which multiplicity is probed to date (Z = 0.2 Zsun). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods P < 3 yr, (iii) dormant OB+BH binaries, and (iv) a legacy database of physical parameters of massive stars at low metallicity. The stars are observed with the LR02 setup of the giraffe instrument of the Very Large Telescope (3960-4570A, resolving power R=6200; typical signal-to-noise ratio S/N=70-100). This paper utilises the first 9 epochs obtained over a three-month time. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. The sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5<Teff/kK<45 and 3.7<log L/Lsun<6.1 and initial masses 8<Mini/Msun<80. It comprises 159 O-type stars, 331 early B-type (B0-3) dwarfs and giants (luminosity classes V-III), 303 early B-type supergiants (II-I), and 136 late-type supergiants. At least 82 stars are Oe/Be stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, it includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries.

Figures (13)

• Figure 1: The eight FLAMES pointings marked on a density map of the underlying Gaia source catalogue of the SMC (with $G < 19\,$mag) as a function of right ascension ($\alpha$) and declination ($\delta$) (darkest pixels correspond to $\approx 900$ stars). The green rings correspond to the FLAMES FoVs, which are 25' in diameter. The 929 targets are shown as blue and pink dots based on their estimated initial masses (see legend and text). We note that the regions most densely populated with stars in the SMC (e.g. the bar) are not rich in massive stars, and hence only a few fields were allocated there.
• Figure 2: Completeness of the BLOeM dataset with respect to the underlying Gaia catalogue. Left: CMD showing the underlying Gaia catalogue used to choose the BLOeM sample (black dots). Three evolution tracks computed by Schootemeijer2019 and Hastings2021 with the MESA stellar evolution code (see Sect. \ref{['subsec:HRD']}) for $M_{\rm ini} = 7.9, 13.8$, and $24\,M_\odot$ are plotted. The tracks were adjusted to the SMC distance and an average reddening and extinction (see text for details). BLOeM targets are encircled with blue ($M_{\rm ini}/M_\odot \gtrsim 14$; born as O-type stars) and pink ($8 \lesssim M_{\rm ini}/M_\odot \lesssim 14$; born as B-type stars) circles. Right: Magnitude distribution of the subsamples, along with completeness fractions for the two subsamples with respect to the underlying Gaia SMC catalogue.
• Figure 3: Histograms of the median S/N per physical 0.2 Å pixel across the nine available epochs, for each of the eight fields of the sample. Red dashed lines and labels denote the median of all median S/N values per field.
• Figure 4: Montage of the normalised co-added spectra formed from the nine available epochs for selected OB stars classified as dwarfs (luminosity class V) or sub-giants (IV). The spectra are shifted by a constant for clarity. BLOeM IDs and spectral types are noted above each spectrum, and diagnostic lines are identified.
• Figure 5: Montage of a selection of giant (luminosity class III) and bright giant (II) OB stars in the BLOeM sample, ordered from early (top) to late (bottom) type. BLOeM IDs and spectral types are noted above each spectrum. Diagnostic lines are identified.