A dearth of short-period massive binaries in the young massive star forming region M 17

Abstract

Aims. The formation of massive stars remains poorly understood and little is known about their birth multiplicity properties. Here, we aim to quantitatively investigate the strikingly low radial-velocity dispersion measured for a sample of 11 massive pre-and nearmain-sequence stars (σ 1D = 5.6 ± 0.2 km s −1 ) in the very young massive star forming region M 17, in order to obtain first constraints on the multiplicity properties of young massive stellar objects. Methods. We compute the radial-velocity dispersion of synthetic populations of massive stars for various multiplicity properties and we compare the obtained σ 1D distributions to the observed value. We specifically investigate two scenarios: a low binary fraction and a dearth of short-period binary systems. Results. Simulated populations with low binary fractions ( f bin = 0.12 +0.16 −0.09 ) or with truncated period distributions (P cutoff > 9 months) are able to reproduce the low σ 1D observed within their 68%-confidence intervals. Furthermore, parent populations with f bin > 0.42 or P cutoff < 47 d can be rejected at the 5%-significance level. Both constraints are in stark contrast with the high binary fraction and plethora of short-period systems in few Myr-old, well characterized OB-type populations. To explain the difference in the context of the first scenario would require a variation of the outcome of the massive star formation process. In the context of the second scenario, compact binaries must form later on, and the cut-off period may be related to physical length-scales representative of the bloated pre-main-sequence stellar radii or of their accretion disks. Conclusions. If the obtained constraints for the M 17's massive-star population are representative of the multiplicity properties of massive young stellar objects, our results may provide support to a massive star formation process in which binaries are initially formed at larger separations, then harden or migrate to produce the typical (untruncated) power-law period distribution observed in few Myr-old OB binaries.

Multiple star systems in the Orion nebula

“…Our finding of a large number of wide binaries with high mass ratios might reflect the fact that the binary population in the ONC did not have enough time to be altered by dynamical interactions (e.g., Moe & Kratter (2018) show different simulated scenarios of how close binaries can be formed, 60% form by unstable triples). Sana et al (2017) also notice a lack of close companions and conclude their findings may support a theory in which binaries form initially at large separations and then harden to closer systems. This could explain why the companion separations and masses in the ONC are different than the distributions in more evolved clusters.…”

Companion-driven evolution of massive stellar binaries

“…• Initial Period Gap: Sana et al (2017) suggest that massive binaries may form with large separations and tighten over time to match the parameter distributions of older populations. The EKL mechanism in concert with tidal dissipation represents a channel for producing hardened binaries.…”

“…certain simulations. Additionally, Sana et al (2017) present observations of eleven young (< 1 Myr) massive binaries in the open cluster M17 with a low radial-velocity dispersion. They offer multiple explanations for the peculiar velocity dispersion.…”

The multiplicity of massive stars: a 2016 view

“…The cut-off period may then be related to physical length-scales representative of the bloated pre-main-sequence stellar radii or of their accretion disks. The full discussion is to be found in Ramírez-Tannus et al (2017) and Sana et al (2017). 8 H. Sana…”