The VLT-FLAMES Tarantula Survey

Abstract

Context. The Tarantula Nebula in the Large Magellanic Cloud is our closest view of a starburst region and is the ideal environment to investigate important questions regarding the formation, evolution and final fate of the most massive stars. Aims. We analyze the multiplicity properties of the massive O-type star population observed through multi-epoch spectroscopy in the framework of the VLT-FLAMES Tarantula Survey. With 360 O-type stars, this is the largest homogeneous sample of massive stars analyzed to date. Methods. We use multi-epoch spectroscopy and variability analysis to identify spectroscopic binaries. We also use a Monte-Carlo method to correct for observational biases. By modeling simultaneously the observed binary fraction, the distributions of the amplitudes of the radial velocity variations and the distribution of the time scales of these variations, we constrain the intrinsic current binary fraction and period and mass-ratio distributions. Results. We observe a spectroscopic binary fraction of 0.35±0.03, which corresponds to the fraction of objects displaying statistically significant radial velocity variations with an amplitude of at least 20 km s −1 . We compute the intrinsic binary fraction to be 0.51±0.04. We adopt power-laws to describe the intrinsic period and mass-ratio distributions: f (log 10 P/d) ∼ (log 10 P/d) π (with log 10 P/d in the range 0.15−3.5) and f (q) ∼ q κ with 0.1 ≤ q = M 2 /M 1 ≤ 1.0. The power-law indexes that best reproduce the observed quantities are π = −0.45 ± 0.30 and κ = −1.0 ± 0.4. The period distribution that we obtain thus favours shorter period systems compared to an Öpik law (π = 0). The mass ratio distribution is slightly skewed towards low mass ratio systems but remains incompatible with a random sampling of a classical mass function (κ = −2.35). The binary fraction seems mostly uniform across the field of view and independent of the spectral types and luminosity classes. The binary fraction in the outer region of the field of view (r > 7.8 , i.e. ≈117 pc) and among the O9.7 I/II objects are however significantly lower than expected from statistical fluctuations. The observed and intrinsic binary fractions are also lower for the faintest objects in our sample (K s > 15.5 mag), which results from observational effects and the fact that our O star sample is not magnitude-limited but is defined by a spectral-type cutoff. We also conclude that magnitude-limited investigations are biased towards larger binary fractions. Conclusions. Using the multiplicity properties of the O stars in the Tarantula region and simple evolutionary considerations, we estimate that over 50% of the current O star population will exchange mass with its companion within a binary system. This shows that binary interaction is greatly affecting the evolution and fate of massive stars, and must be taken into account to correctly interpret unresolved populations of massive stars. Full Tables 1-3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via

Hubble Tarantula Treasury Project. V. The Star Cluster Hodge 301: The Old Face of 30 Doradus*

“…Although our findings do not critically depend on the adopted binary fraction and mass ratio, a very high binary fraction (above 50%; Sana et al 2013) and flatter mass ratio q (our binary prescription favors low-q binaries), as found for O-stars, can affect the recovered SFH. In practice, the effect of equal mass binaries is to make stars appear 0.75 mag brighter, and not taking it into account leads to an underestimate of the age.…”

“…Dunstall et al (2015) studied the multiplicity of 408 B-type stars observed in different regions (NGC 2070, NGC 2060, Hodge 301, SL 639) of 30Doradus with multi-epoch spectroscopy from the VLT-FLAMES Tarantula Survey (VFTS). Although they found an average binary fraction of about 58%, close to the O-stars multiplicity by Sana et al (2013), the intrinsic binary fraction in H301 was found to be remarkably low and around 20% (8% detected, with a detection probability of 40±10%). Concerning the mass ratio, they found a distribution of q favoring lowmass companions ( f (q)∝q −2.8 ).…”

The VLT-FLAMES Tarantula Survey

“…This value was an appropriate compromise between minimising the incompleteness of the detections of binarity, while ensuring that pulsations (and other sources of line-profile variations) did not lead to too many false positives. The O-type stars have been analysed by Sana et al (2013) using similar criteria 1 .…”

Massive stars, successes and challenges

“…Many massive stars are in multiple systems and part of those may follow a different evolution because of interactions with their close companions (Sana et al 2012(Sana et al , 2013. This has triggered many recent works exploring close binary evolution and their consequences for explaining the origin of various stellar populations (see e.g.…”