Exploring the role of binarity in the origin of the bimodal rotational velocity distribution in stellar clusters

Abstract

Many young and intermediate age massive stellar clusters host bimodal distributions in the rotation rates of their stellar populations, with a dominant peak of rapidly rotating stars and a secondary peak of slow rotators. The origin of this bimodal rotational distribution is currently debated and two main theories have been put forward in the literature. The first is that all/most stars are born as rapid rotators and that interacting binaries brake a fraction of the stars, resulting in two populations. The second is that the rotational distribution is a reflection of the early evolution of pre-main sequence stars, in particular, whether they are able to retain or lose their protoplanetary discs during the first few Myr. Here, we test the binary channel by exploiting multi-epoch VLT/MUSE observations of NGC 1850, a ∼100 Myr massive cluster in the LMC, to search for differences in the binary fractions of the slow and fast rotating populations. If binarity is the cause of the rotational bimodality, we would expect that the slowly rotating population should have a much larger binary fraction than the rapid rotators. However, in our data we detect similar fractions of binary stars in the slow and rapidly rotating populations (5.9 ± 1.1% and 4.5 ± 0.6%, respectively). Hence, we conclude that binarity is not a dominant mechanism in the formation of the observed bimodal rotational distributions.

MUSE spectroscopic observations of the young massive cluster NGC 1850

“…Observations consist of a set of spectral cubes observed with the integral field spectrograph, MUSE (Bacon et al 2010), at the Very Large Telescope of the European Southern Observatory (Cerro Paranal, Chile) under the observing programme 0102.D-0268(A) (PI: Bastian). This data set has recently been used to investigate the relation between binarity and rotation in this cluster (Kamann et al 2021). The instrument was set up in widefield mode using the ground-layer adaptive optics system.…”

“…In particular, Fischer et al (1993) analysed echelle spectra of 36 supergiants measuring only their radial velocities. Similarly, Kamann et al (2021) derived multi-epoch radial velocities for more than 1000 stars in this cluster to investigate binarity and internal rotation among its members. Until recent years, all the metallicity estimates of NGC 1850 had been based on integrated spectroscopy (Usher et al 2019) or photometric indices (Piatti et al 2019).…”

MUSE spectroscopic observations of the Young Massive Cluster NGC1850

“…The role of rotation in producing the colour spread at the MSTO is supported by the link between line broadening and colour in MSTO stars. Stars on the red side of the MSTO have indeed broader line profiles, suggesting a higher degree of internal rotation with respect to stars on the blue side (see also Kamann et al 2021). This could be also the reason why the colour spread remains visible even excluding Be stars (Correnti et al 2017): these stars are indeed at the extreme of the distribution of rotational velocities (Ω ∼ Ω crit ), while a shift to-ward red colours can be present also in stars rotating with lower velocities.…”

“…Observations consist of a set of spectral cubes observed with the Integral Field Spectrograph MUSE (Bacon et al 2010) at the Very Large Telescope of the European Southern Observatory (Cerro Paranal, Chile) under the observing programme 0102.D-0268(A) (PI: Bastian). This dataset has been recently used to investigate the relation between binarity and rotation in this cluster (Kamann et al 2021). The instrument was set up in the widefield mode using of the ground-layer adaptive optics system.…”

“…In particular, Fischer et al (1993) analysed echelle spectra of 36 supergiants measuring only their radial velocities. Similarly, Kamann et al (2021) derived multi-epoch radial velocities for more than 1000 stars in this cluster to investigate binarity and internal rotation among its members. Until recent years, all the metallicity estimates of NGC1850 were based on integrated spectroscopy (Usher et al 2019) or photometric indices (Piatti et al 2019).…”