Binarity at LOw Metallicity (BLOeM): Projected rotational velocities

TL;DR

The BLOeM survey provides multi-epoch spectroscopy for 929 massive stars in the SMC to quantify multiplicity and rotation in a low-metallicity environment. Using a Fourier-transform approach on multiple lines, the study derives $v\sin i$ for singles, SB1s, and SB2s, revealing a robust main-sequence peak at $v\sin i \approx 30$–$60$ km s$^{-1}$ and a separate SB2 population around $\sim$140 km s$^{-1}$, with OBe stars occupying higher speeds. A HRD locus near the terminal-age main sequence corresponds to a decline in $v\sin i$, while supergiants are mostly slow and unresolved; nitrogen enhancements appear in a substantial fraction of slowly rotating stars, implying mixed pre- and post-main-sequence evolutionary paths. Overall, the results suggest that population differences, rather than metallicity alone, drive rotation distributions, with tidal synchronization, mass transfer, and mergers shaping the observed $v\sin i$ landscape. The work provides a framework for linking rotation to binary evolution and will benefit from future orbital solutions and surface abundance analyses.

Abstract

The Binarity at LOw Metallicity (BLOeM) survey is an ESO large programme designed to obtain multi-epoch spectroscopy for 929 massive stars in the Small Magellanic Cloud (SMC). It will provide binary fractions and orbital configurations of binary systems, and search for dormant black-hole binary candidates (OB+BH). Here we present projected rotational velocities (vsini) of all sources and, using the multiplicity properties presented in previous papers, we derive the vsini distributions of apparent single stars, single lined spectroscopic (SB1) binaries, and SB2 systems. We identify a locus in the Hertzsprung-Russell diagram where rotational velocities decrease significantly; we interpret this feature as broadly corresponding to the terminal-age main sequence. The main sequence cohort is distinguished by a broad range of vsini values, but with a strong peak in the distribution in the range 30-60 km/s, close to the resolution limit of 30 km/s. Sources in this low vsini peak are distributed throughout the main sequence, and are also present in the SB1 sample, though less prominent than in the single star distribution. A preliminary analysis of the lowest vsini cohort, that includes SB1 systems, implies that roughly one third may be nitrogen rich and we speculate that this cohort is a mix of pristine single stars, long period binaries, and merger products. The SB2 systems appear to be mostly short period binaries in synchronous rotation and have vsini estimates distributed around a mean value of approximately 140 km/s . Higher vsini sources are also present in the single and SB1 systems, all of which have tail to higher vsini values, consistent with tidal and mass-transfer effects. The supergiants, with a few exceptions, have low vsini, the bulk of these systems being essentially unresolved at current spectral resolution.

Figures (18)

• Figure 1: Distribution of BLOeM targets (white circles) compared with those of some other major $\varv\sin i$ surveys; FSMS hunter2008b, Martayan martayan2007, Ramachandran ramachandran2019, Dufton dufton2019, RIOTS4 dorigojones2020.
• Figure 2: Two Hei 4387 Å line profiles for the source BLOeM 1-020 which is classified as an SB1 system, and has a peak-to-peak velocity of $\sim$100 km s$^{-1}$. The full and dashed lines show the profiles obtained at each quadrature, showing that a faint secondary is visible on opposing sides of each profile.
• Figure 3: Comparison of pipeline bestenlehner2025 results with present measurements (black dots). Note the banding of the former results which is a consequence of their adoption of a discrete grid of $\varv\sin i$ values. SB1 and SB2 systems are flagged as indicated in the legend, and component A $\varv\sin i$ values were adopted for the SB2 sources. The outlier at position (280,81) is the SB2 system BLOeM 5-057 which has very different line widths of 280 and 56 km s$^{-1}$ for components A and B. Clearly the pipeline value is appropriate for component B.
• Figure 4: Comparison of $\varv\sin i$ measurements for designated A and B components of SB2 systems. Orange circles denote eclipsing or ellipsoidal binaries already with well determined periods from the OGLE project pawlak2016.
• Figure 5: Normalized histograms for the total sample and each of the BLOeM groups, split into two panels for clarity. Groups are identified as indicated by the insets, that also indicate the number of sources in each group.