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Register a new userA dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?
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The formation of massive stars remains poorly understood and little is known about their birth multiplicity properties. Here, we investigate the strikingly low radial-velocity dispersion measured for a sample of 11 massive pre- and near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive star forming region M17 to obtain first constraints on the multiplicity properties of young massive stellar objects. Methods: We compute the RV dispersion of synthetic populations of massive stars for various multiplicity properties and we compare the simulated sigma_rv 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 sigma_rv observed within their 68%-confidence intervals. Parent populations with f_bin > 0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level. Both constraints are contrast with the high binary fraction and plethora of short-period systems found in few Myr-old, OB-type populations. To explain the difference, the first scenario requires a variation of the outcome of the massive star formation process. In the 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 are representative of the overall properties of massive young stellar objects, our results may provide support to a 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.
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Almost all massive stars have bound stellar companions, existing in binaries or higher-order multiples. While binarity is theorized to be an essential feature of how massive stars form, essentially all information about such properties is derived from observations of already formed stars, whose orbital properties may have evolved since birth. Little is known about binarity during formation stages. Here we report high angular resolution observations of 1.3 mm continuum and H30alpha recombination line emission, which reveal a massive protobinary with apparent separation of 180 au at the center of the massive star-forming region IRAS07299-1651. From the line-of-sight velocity difference of 9.5 km/s of the two protostars, the binary is estimated to have a minimum total mass of 18 solar masses, consistent with several other metrics, and maximum period of 570 years, assuming a circular orbit. The H30alpha line from the primary protostar shows kinematics consistent with rotation along a ring of radius of 12 au. The observations indicate that disk fragmentation at several hundred au may have formed the binary, and much smaller disks are feeding the individual protostars.
We study the formation of massive Population III binary stars using a newly developed radiation hydrodynamics code with the adaptive mesh refinement and adaptive ray-tracing methods. We follow the evolution of a typical primordial star-forming cloud obtained from a cosmological hydrodynamics simulation. Several protostars form as a result of disk fragmentation and grow in mass by the gas accretion, which is finally quenched by the radiation feedback from the protostars. Our code enables us, for the first time, to consider the feedback by both the ionizing and dissociating radiation from the multiple protostars, which is essential for self-consistently determining their final masses. At the final step of the simulation, we observe a very wide ($gtrsim 10^4,mathrm{au}$) binary stellar system consisting of $60$ and $70,M_odot$ stars. One of the member stars also has two smaller mass ($10,M_odot$) companion stars orbiting at $200$ and $800,mathrm{au}$, making up a mini-triplet system. Our results suggest that massive binary or multiple systems are common among Population III stars.
Protoplanetary disks form through angular momentum conservation in collapsing dense cores. In this work, we perform the first simulations with a maximal resolution down to the astronomical unit (au) of protoplanetary disk formation, through the collapse of 1000 solar mass clumps, treating self-consistently both non-ideal magnetohydrodynamics with ambipolar diffusion as well as radiative transfer in the flux-limited diffusion approximation including stellar feedback. Using the adaptive mesh-refinement code RAMSES, we investigate the influence of the magnetic field on the disks properties with three models. We show that, without magnetic fields, a population dominated by large disks is formed, which is not consistent with Class 0 disk properties as estimated from observations. The inclusion of magnetic field leads, through magnetic braking, to a very different evolution. When it is included, small < 50 au disks represent about half the population. In addition, about ~ 70% of the stars have no disk in this case which suggests that our resolution is still insufficient to preserve the smaller disks. With ambipolar diffusion, the proportion of small disks is also prominent and we report a flat mass distribution around 0.01-0.1 solar mass and a typical disk-to-star mass ratios of ~0.01-0.1. This work shows that the magnetic field and its evolution plays a prominent role in setting the initial properties of disk populations.
We analyzed high angular resolution 45.5 GHz images of the W49 North massive star forming region obtained in 1998 and 2016 with the Very Large Array. Most of the ultracompact HII regions show no detectable changes over the time interval of the observations. However, subcomponents B1, B2, G2a and G2c have increased its peak flux densities by values in the range of 3.8 to 21.4 %. Most interestingly, the cometary region C clearly shows proper motions that at the distance of the region are equivalent to a velocity of 76$pm$6 km s$^{-1}$ in the plane of the sky. We interpret this region as the ionized bowshock produced by a runaway O6 ZAMS star that was ejected from the eastern edge of Welchs ring about 6,400 years ago.
A significant fraction of massive stars are found in multiple systems. The effect of binarity on stellar evolution is poorly constrained. In particular, the role of tides and mass transfer on surface chemical abundances is not constrained observationally. The aim of this study is to investigate the effect of binarity on the stellar properties and surface abundances of massive binaries. We perform a spectroscopic analysis of six Galactic massive binaries. The spectra of individual components are obtained from a spectral disentangling method and are subsequently analyzed by means of atmosphere models. The stellar parameters and CNO surface abundances are determined. Most systems are made of main-sequence stars. Three systems are detached, two are in contact and no information is available for the sixth system. For eleven out of the twelve stars studied the surface abundances are only mildly affected by stellar evolution and mixing. They are not different from those of single stars, within the uncertainties. The secondary of XZ~Cep is strongly chemically enriched. Considering previous determinations of surface abundance in massive binary systems suggests that the effect of tides on chemical mixing is limited, whereas mass transfer and removal of outer layers of the mass donor leads to the appearance of chemically processed material at the surface, although this is not systematic. The evolutionary masses of the components of our six systems are on average 16.5% higher than the dynamical masses. Some systems seem to have reached synchronization, while others may still be in a transitory phase.
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