Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection

H. Sana [1,2],
J.-B. Le Bouquin [3,4],
S. Lacour [5],
J.-P. Berger [6],
G. Duvert [3,4],
L. Gauchet [5],
B. Norris [7],
J. Olofsson [8],
D. Pickel [5],
G. Zins [3,4],
O. Absil [9],
A. de Koter [10,11],
K. Kratter [12,13],
O. Schnurr [14]
H. Zinnecker [15]

1. European Space Agency / Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, United States
2. E-mail:
3. Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France
4. CNRS, IPAG, F-38000 Grenoble, France
5. LESIA, Observatoire de Paris, CNRS, UPMC, Universit'e Paris-Diderot, Paris Sciences et Lettres, 5 Place Jules Janssen, 92195 Meudon, France
6. European Southern Observatory, Schwarzschild-Str. 2, 85748 Garching bei M"unchen, Germany
7. Sydney Institute for Astronomy, Institute for Photonics and Optical Science, School of Physics, University of Sydney, NSW 2006, Australia
8. Max-Planck-Institut f"ur Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany
9. D'epartement d'Astrophysique, G'eophysique et Oc'eanographie, Universit'e de Li`ege, 17 All'ee du Six Ao^ut, 4000 Li`ege, Belgium
10. Astrophysical Institute Anton Pannekoek, Universiteit van Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands
11. Instituut voor Sterrenkunde, Universiteit Leuven, Celestijnenlaan 200 D, 3001, Leuven, Belgium}
12. Hubble Fellow, JILA, 440 UCB, University of Colorado, Boulder, CO 80309-0440, United States
13. Steward Observatory / Dept. of Astronomy, University of Arizona, 933 N Cherry Ave, Tucson, AZ, 85721
14. Leibniz-Institut f"ur Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
15. Deutsches SOFIA Institute, SOFIA Science Center, NASA Ames Research Center, M.S, N232-12, Moffett Field CA 94035, United States

Multiplicity is one of the most fundamental observable properties of massive O-type stars and offers a promising way to discriminate between massive star formation theories.
Nevertheless, companions at separations between
1 and 100 milli-arcsec (mas) remain mostly unknown due to intrinsic observational limitations. At a typical distance of 2~kpc, this corresponds to projected physical separations of 2-200~AU.
The Southern MAssive Stars at High angular resolution survey (smash) was designed to fill this gap by providing the first systematic interferometric survey of Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162 O-type stars with NACO/SAM, respectively probing the separation ranges 1-45 and 30-250~mas and brightness contrasts of $Delta H < 4$ and $Delta H < 5$. Taking advantage of NACO's field-of-view, we further uniformly searched for visual companions in an 8arcsec-radius down to $Delta H = 8$.
This paper describes the observations and data analysis, reports the discovery of almost 200 new companions in the separation range from 1~mas to 8arcsec and presents the catalog of detections, including the first resolved measurements of over a dozen known long-period spectroscopic binaries.

Excluding known runaway stars for which no companions are detected, 96 objects in our main sample ($delta < 0$degr; $H<7.5$) were observed both with PIONIER and NACO/SAM. The fraction of these stars with at least one resolved companion within 200~mas is 0.53. Accounting for known but unresolved spectroscopic or eclipsing companions, the multiplicity fraction at separation $rho < 8$arcsec increases to $f_mathrm{m}=0.91pm0.03$. The fraction of luminosity class V stars that have a bound companion reaches 100% at 30~mas while their average number of physically connected companions within 8arcsec is $f_mathrm{c}=2.2pm0.3$.
This demonstrates that massive stars form nearly exclusively in multiple systems. The nine non-thermal radio emitters observed by smash are all resolved, including the newly discovered pairs HD,168112 and CPD$-$47degr2963. This lends strong support to the universality of the wind-wind collision scenario to explain the non-thermal emission from O-type stars.

Reference: ApJS, in press
Status: Manuscript has been accepted