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

Evidence for a large orbital separation at birth?

¹ Institute of astrophysics, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium
e-mail: hugues.sana@kuleuven.be
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: m.c.ramireztannus@uva.nl
³ European Space Astronomy Centre (ESAC), Camino bajo del Castillo, s/n Urbanizacion Villafranca del Castillo, Villanueva de la Canada, 28 692 Madrid, Spain
⁴ Department of Astronomy, Stockholm University, Oskar Klein Center, 106 91 Stockholm, Sweden

Received: 18 November 2016
Accepted: 30 December 2016

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 near-main-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_-0.09^+0.16) 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.