[Paper Review] The young massive SMC cluster NGC 330 seen by MUSE II. Multiplicity properties of the massive-star population

J. Bodensteiner, H. Sana+11 more|UvA-DARE (University of Amsterdam)|Apr 27, 2021

Stellar, planetary, and galactic studies87 references4 citations

TL;DR

This study presents the first homogeneous radial velocity (RV) survey of B-type stars in a low-metallicity environment using multi-epoch VLT/MUSE integral-field spectroscopy of the 40 Myr-old NGC 330 cluster in the Small Magellanic Cloud. After correcting for observational biases—particularly the SB2 detection bias—it finds a bias-corrected close binary fraction of 34⁺⁸₋₇% for massive stars, with a significant decline in binary fraction among fainter, lower-mass B-type stars, suggesting evolutionary or mass-dependent effects on multiplicity.

ABSTRACT

Observations of massive stars in young open clusters (< ~8 Myr) have shown that a majority of them are in binary systems, most of which will interact during their life. Populations of massive stars older than ~20 Myr allow us to probe the outcome of such interactions after many systems have experienced mass and angular momentum transfer. Using multi-epoch integral-field spectroscopy, we investigate the multiplicity properties of the massive-star population in NGC 330 (~40 Myr) in the Small Magellanic Cloud to search for imprints of stellar evolution on the multiplicity properties. From six epochs of VLT/MUSE observations supported by adaptive optics we extract spectra and measure radial velocities for stars brighter than F814W = 19. We identify single-lined spectroscopic binaries through significant RV variability as well as double-lined spectroscopic binaries, and quantify the observational biases for binary detection. The observed spectroscopic binary fraction is 13.2+/-2.0 %. Considering period and mass ratio ranges from log(P)=0.15-3.5, and q = 0.1-1.0, and a representative set of orbital parameter distributions, we find a bias-corrected close binary fraction of 34 +8 -7 %. This seems to decline for the fainter stars, which indicates either that the close binary fraction drops in the B-type domain, or that the period distribution becomes more heavily weighted towards longer orbital periods. Both fractions vary strongly in different regions of the color-magnitude diagram which probably reveals the imprint of the binary history of different groups of stars. We provide the first homogeneous RV study of a large sample of B-type stars at a low metallicity. The overall bias-corrected close binary fraction of B stars in NGC 330 is lower than the one reported for younger Galactic and LMC clusters. More data are needed to establish whether this result from an age or a metallicty effect.

Motivation & Objective

• To investigate the multiplicity properties of massive stars in the 40 Myr-old NGC 330 cluster in the Small Magellanic Cloud, a low-metallicity environment.
• To measure the spectroscopic binary fraction among B-type stars and correct for observational biases, especially the SB2 detection bias in low-resolution MUSE data.
• To assess how binary evolution imprints on the observed multiplicity across different evolutionary stages using color-magnitude diagram (CMD) regions.
• To compare the derived binary fraction with those from younger, higher-metallicity clusters to disentangle age and metallicity effects on initial multiplicity.
• To lay the groundwork for future analysis of stellar parameters and rotational velocities from stacked RV data.

Proposed method

• Multi-epoch VLT/MUSE observations in wide-field mode with adaptive optics, covering six epochs with extended wavelength coverage.
• Spectral extraction and radial velocity (RV) measurement for stars brighter than m_F814W = 19, corresponding to ~5.5 M⊙ on the main sequence.
• Identification of single-lined spectroscopic binaries (SB1s) via RV variability with peak-to-peak amplitude >20 km s⁻¹.
• Visual inspection of spectra to detect composite lines and line profile variations indicative of double-lined binaries (SB2s).
• Simulation of observational biases: SB1 bias based on primary RV amplitude and SB2 bias due to similar component line strengths, using synthetic binary populations.
• Bias correction applied to the observed spectroscopic binary fraction using representative orbital parameter distributions (log P = 0.15–3.5, q = 0.1–1.0).

Experimental results

Research questions

• RQ1 What is the true close binary fraction of massive stars in NGC 330 after correcting for observational biases, particularly the SB2 detection bias?
• RQ2 How does the spectroscopic binary fraction vary across different evolutionary stages in the color-magnitude diagram, and what does this reveal about binary interaction history?
• RQ3 Is the observed binary fraction in NGC 330 lower than in younger, higher-metallicity clusters due to age or metallicity effects?
• RQ4 How does the multiplicity of Be stars compare to that of B-type stars in a low-metallicity environment?
• RQ5 Does the binary fraction decline with decreasing stellar mass, as suggested by the magnitude-dependent trend in the data?

Key findings

• The observed spectroscopic binary fraction among stars brighter than m_F814W = 19 is f_SB^obs = 13.2 ± 2.0%.
• After correcting for observational biases, the bias-corrected close binary fraction (log P < 3.5) for the massive-star population is f_cl = 34⁺⁸₋₇%.
• The bias-corrected close binary fraction decreases from ~55% to ~25% across the magnitude range corresponding to B5 to earlier spectral types, indicating a potential drop in multiplicity with decreasing mass.
• The observed spectroscopic binary fraction is significantly higher (~30%) for stars near and above the cluster turnoff than for main-sequence stars (~10%) or Be stars (~6%).
• The observed spectroscopic binary fraction of Be stars is f_SB^obs = 2 ± 2%, significantly lower than that of B-type stars (f_SB^obs = 9 ± 2%), suggesting different evolutionary pathways or formation mechanisms.
• The overall bias-corrected binary fraction in NGC 330 is lower than in younger Galactic and LMC clusters, possibly due to age-related evolution or metallicity effects, though disentangling these requires further data.