Constraining the origin of split main sequences: binaries in the young cluster NGC1755

Young and intermediate-age star clusters in the Magellanic Clouds exhibit extended main sequence turn-off points (eMSTOs) and, at ages under ∼ 600 Myr, a split main sequence (MS). Both photometry and spectroscopy studies have demonstrated that stellar rotation plays a major role in shaping these features. Specifically, the split MSs turn out to be consistent with multiple populations with different rotation rates; the blue MS is reproduced by non-rotating stars whereas the red MS by fast rotators. However, the underlying physical origin of the two MSs remains a subject of ongoing investigation. Three theoretical scenarios have emerged to explain the presence of slow-rotating blue main sequence stars: binary-induced braking, binary merger events, and magnetic braking mechanisms. These three frameworks predict qualitatively different binary fractions among the blue and red main sequences. Consequently, analyzing the binary populations across the sequences offers a valuable diagnostic to discriminate between these formation models. This work is focused on studying the binaries of the ∼ 80 Myr old cluster NGC1755 in the Large Magellanic Cloud, using high-precision photometry from the Hubble Space Telescope. In particular, we analyzed the photometry in the F336W, F814W and F555W filters to identify the two sequences, introduce a new pseudo color magnitude diagram (CMD) to select the equal-mass binaries, and derived the cumulative distribution of the observed binaries. This distribution was later compared with the corresponding simulated CMDs of artificial stars to estimate the fraction of blue and red MS binaries, resulting in fbin,bMS = 0.47 ± 0.04 and fbin,rMS = 0.53 ± 0.04. This result is discussed in the context of the formation scenarios for split MSs in young star clusters, where it aligns with the binary-induced braking model.