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

57 Pages·2014·2.04 MB·English

Southern Massive Stars at High Angular Resolution:
Observational Campaign and Companion Detection
H. Sana1,2, J.-B. Le Bouquin3,4, S. Lacour5, J.-P. Berger6, G. Duvert3,4, L. Gauchet5,
B. Norris7, J. Olofsson8, D. Pickel5, G. Zins3,4, O. Absil9,16, A. de Koter10,11,
4
K. Kratter12,13, O. Schnurr14, H. Zinnecker15
1
0
2

p
e
ABSTRACT
S
MultiplicityisoneofthemostfundamentalobservablepropertiesofmassiveO-typestarsand
2
2 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
R
physicalseparationsof2-200AU.TheSouthernMAssiveStarsatHighangularresolutionsurvey
S
(smash+) was designed to fill this gap by providing the first systematic interferometric survey
.
h of Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162 O-type
p
stars with NACO/SAM, respectively probing the separation ranges 1-45 and 30-250 mas and
-
o brightness contrasts of ∆H < 4 and ∆H < 5. Taking advantage of NACO’s field-of-view, we
r further uniformly searched for visual companions in an 8(cid:48)(cid:48)-radius down to ∆H = 8. This paper
t
s describestheobservationsanddataanalysis,reportsthediscoveryofalmost200newcompanions
a
in the separation range from 1 mas to 8(cid:48)(cid:48) and presents the catalog of detections, including the
[
first resolved measurements of over a dozen known long-period spectroscopic binaries.
1
Excludingknownrunawaystarsforwhichnocompanionsaredetected,96objectsinourmain
v
sample (δ <0◦; H <7.5) were observed both with PIONIER and NACO/SAM. The fraction of
4
0 these stars with at least one resolved companion within 200 mas is 0.53. Accounting for known
3 but unresolved spectroscopic or eclipsing companions, the multiplicity fraction at separation
6 ρ < 8(cid:48)(cid:48) increases to f = 0.91 ± 0.03. The fraction of luminosity class V stars that have a
m
.
9 bound companion reaches 100% at 30 mas while their average number of physically connected
0 companions within 8(cid:48)(cid:48) is f = 2.2 ± 0.3. This demonstrates that massive stars form nearly
c
4 exclusively in multiple systems. The nine non-thermal radio emitters observed by smash+ are
1
all resolved, including the newly discovered pairs HD168112 and CPD−47◦2963. This lends
:
v strong support to the universality of the wind-wind collision scenario to explain the non-thermal
Xi emission from O-type stars.
r Subject headings: binaries: visual – stars: early-type – stars: imaging – techniques: high angular
a
resolution – techniques: interferometric – surveys
1EuropeanSpaceAgency/SpaceTelescopeScienceIn- Janssen,92195Meudon,France
stitute, 3700 San Martin Drive, Baltimore, MD 21218, 6EuropeanSouthernObservatory,Schwarzschild-Str. 2,
UnitedStates 85748GarchingbeiMu¨nchen,Germany
2E-mail: [email protected] 7SydneyInstituteforAstronomy, InstituteforPhoton-
3Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, ics and Optical Science, School of Physics, University of
France Sydney,NSW2006,Australia
4CNRS,IPAG,F-38000Grenoble,France 8Max-Planck-Institut fu¨r Astronomie, Koenigstuhl 17,
5LESIA,ObservatoiredeParis,CNRS,UPMC,Univer- 69117Heidelberg,Germany
sit´e Paris-Diderot, Paris Sciences et Lettres, 5 Place Jules 9D´epartement d’Astrophysique, G´eophysique et
1
1. Introduction (corresponding to periods of about 1 yr). Most
imaging techniques suffer from a brightness con-
One of the most striking properties of massive
trast vs. separation bias (Turner et al. 2008; Sana
starsistheirhighdegreeofmultiplicity. Inclusters
& Evans 2011) which limits the detection of mod-
and associations, 75% of the O-type objects have
erate brightness companions to separations larger
at least one companion detected either through than several 0.1(cid:48)(cid:48) at best. Separations below 0.1(cid:48)(cid:48)
spectroscopy or through imaging techniques (Ma-
have most successfully been probed through vari-
son et al. 2009). This rate has not been corrected
ousflavorsofinterferometry,suchasspeckle,aper-
forobservationalbiasessothatthetruemultiplic-
ture masking and long baseline interferometry, al-
ity fraction might very well come close to 100%.
though very few observations have been able to
Typically, the detected companions have a mass 1
probetheregimeofhighestcontrasts(∆mag>2)
to 5 times smaller than the primary mass and are
andclosestangularseparations(ρ<75mas; fora
mostlyOandBstars. Wecanthuspostulatethat
review, see Sana & Evans 2011).
the typical end product of massive star formation
This paper introduces the Southern MAssive
is not a single star but a multiple system, with at
StarsatHighangularresolutionsurvey(smash+),
leastoneandpossiblyseveralmassivecompanions
an interferometric survey of over 100 Galactic
(e.g. Kratter & Matzner 2006; Krumholz 2012).
O-type stars designed to systematically explore
The properties of the binary population, for ex-
the separation range between 1 and 200 mas.
ample the period and mass ratio distributions,
TheSparseApertureMaskingmode(SAM,Lacour
can then serve as a useful diagnostics to discrim-
etal.2011b)ofNACOattheVeryLargeTelescope
inate between different formation models. Dif-
(VLT) has allowed us to resolve massive binaries
ferent massive star formation theories do indeed
with separations in the range of 30-250 mas (e.g.
have different expectations for multiplicity prop-
Sana et al. 2012a). Angular separations smaller
erties (for recent reviews, see Zinnecker & Yorke
than 30 mas require the use of long baseline in-
2007; Tan et al. 2014). Unfortunately, observa-
terferometry. Untilnow, ithasbeenimpossibleto
tionshavesofarfailedtoprovideacomprehensive
observe a sufficiently large sample because of the
viewoftheOstarmultiplicityoverthefullsepara-
lowmagnitudelimit,restrictingthenumberofob-
tionrangerelevantformassivestarformationand
servableobjects,andbecauseofthetypicallylarge
evolution, leaving us with a strongly biased view
execution time needed to achieve a reasonable de-
towards tight (physical separation d<1 AU) and
tection rate, i.e. to sufficiently cover the uv plane
wide (d>103 AU) companions.
(Sana & Le Bouquin 2010). The advent of the
Binary detection through spectroscopy is typi-
four beam combiner PIONIER (Le Bouquin et al.
cally limited to systems with mass ratios M /M
1 2 2011) at the VLT Interferometer (VLTI, Hague-
upto5to15(fordouble-andsingle-linedbinaries
nauer et al. 2008, 2010), that combines the light
respectively) and to separations up to a few AU
offourtelescopes,criticallychangedthesituation,
by opening the 1-45 mas angular resolution win-
Oc´eanographie,Universit´edeLi`ege,17All´eeduSixAouˆt, dow to a survey approach.
4000Li`ege,Belgium
In this paper, we report on the first observa-
10AstrophysicalInstituteAntonPannekoek,Universiteit
van Amsterdam, Science Park 904, 1098XH Amsterdam, tional results of our survey. Bias correction and
TheNetherlands detailed theoretical implications will be addressed
11Instituut voor Sterrenkunde, Universiteit Leuven, Ce- in subsequent papers in this series. This paper is
lestijnenlaan200D,3001,Leuven,Belgium organized as follows. Section 2 describes the sam-
12Hubble Fellow, JILA, 440 UCB, University of Col-
ple selection, observational campaign and instru-
orado,Boulder,CO80309-0440,UnitedStates
mental setups. Section 3 presents the data analy-
13StewardObservatory/Dept. ofAstronomy,University
ofArizona,933NCherryAve,Tucson,AZ,85721 sis and binary detection algorithms. The smash+
14Leibniz-Institut fu¨r Astrophysik Potsdam, An der constraints on the multiplicity properties of our
Sternwarte16,14482Potsdam,Germany sample stars are presented in Sect. 4. Section 5
15Deutsches SOFIA Institute, SOFIA Science Center, discusses our results and Sect. 6 summarizes our
NASAAmesResearchCenter,M.S,N232-12,MoffettField
main findings. Finally, Appendix A and B com-
CA94035,UnitedStates
pile notes on individual objects and provide find-
16F.R.S.-FNRSResearchAssociate
2
ing charts for systems with more than three com- indicate the spectral classification, coordinates
panions detected in the NACO field-of-view. (J2000.0), H-, Ks- and V-band magnitudes.
The bulk of the smash+ observations has been
2. Observations obtained in the course of a European Southern
Observatory (ESO) large program (189.C-0644)
2.1. Observational sample
whichwasgranted20VLTInightsovertheperiod
The sample selection has been driven by the April 2012 – March 2013 and three NACO/SAM
need to observe a sufficiently large number of O nights in June 2013. The NACO observations
stars to derive meaningful statistical constraints are complemented by a 2011 pilot program and
and by the observational constraints imposed by additional programs in 2012 and 2013 for a to-
PIONIER. The size of the sample defines the pre- tal of 13 VLT/UT4 nights (see Table 4 for an
cision at which one will constrain the multiplic- overview). Thirteen stars have further been ob-
ity rate. The statistical uncertainty (σ ) on the served as backup targets of various PIONIER
fm
measured multiplicity fraction (f ) in the consid- runs from Dec 2013 to Aug 2014. All in all,
m
eredrangedependsbothonf andonthesample 102 stars (81%) from our main target list have
m
size N (Sana et al. 2009). It is given by beenobservedwithPIONIERand120(95%)with
NACO/SAM.96stars(76%)havebothPIONIER
(cid:112)
σ (f ,N)= f (1−f )/N. (1) and NACO/SAM observations and only the run-
fm m m m
away star HD157857 has not been observed. Fig-
For a given sample size, σfm peaks at fm = 0.5, ures 1 and 2 provide an overview of the distribu-
so that σfm(fm,N) ≤ σfm(0.5,N). Observing a tions of spectral sub-types, luminosity classes and
sample of N = 100 is thus required to obtain a magnitudes for the stars in our main sample.
precision of σ <0.05 for any f .
fm m We also observed 37 O stars outside our main
Following PIONIER observational constraints, listoftargets. Table3summarizesthemainprop-
the smash+ survey has been designed as a erties of the supplementary targets in a format
magnitude- and declination-limited survey. The identical to that of Table 1. These supplemen-
practical limiting magnitude of PIONIER in its tary targets are either northern stars, stars just
small spectral dispersion mode is H = 7.5. The above our magnitude cutoff, or stars taken from
limiting magnitude from the fast guiding systems the GOSC-v2 supplements. Among these addi-
oftheauxiliarytelescopes(STRAP)allowingfora tional stars, BN Gem is a known runaway that
properinjectionofthebeamsintotheinstruments has H and V magnitudes within our magnitude
fibers is V =11. The practical range of accessible limitsbutisaNorthernstar. Welistitalongwith
declinations (δ < 0◦) is limited by observability theotherrunawayobjectsinTable2. Intotal, we
constraintsoftheauxiliarytelescopesintheinter- observed 174 different stars. 162 have NACO ob-
mediate and large configurations. The Galactic O servationsand117havePIONIERones. 105stars
StarCatalog(GOSC-v2,Sotaetal.2008)lists147 have both types of observations.
O-typestarsfulfillingthesecriteria. Rejectingthe
Orion stars that have already been observed by 2.2. Observational biases
theVLTI(Grellmannetal.2013),weareleftwith
As a consequence of our magnitude-limited ap-
138 possible targets. Twelve of these are flagged
proach,oursamplecontainsseveralbuilt-inbiases.
as runaway stars in the GOSC and are handled
While it is not our intent to perform detailed bias
separately from the main target list.
corrections in this initial paper, we describe here
Tables 1 and 2 list the properties of stars in
severalaspectsthatneedtobekeptinmindwhile
our main list of targets and in the runaway list.
directly interpreting the observational results of
Columns 1 and 2 indicate whether the object the smash+ survey.
has been observed with PIONIER and NACO.
As for all magnitude-limited surveys, the
Columns 3 and 4 provide the main identifier used
brightness selection criterion favors nearby stars
in our survey (HD number if available, BD/CPD
as well as intrinsically brighter objects. We used
identifiers otherwise) and alternative names com-
the absolute H-band magnitude of O stars listed
monly used in the literature. Columns 5 to 12
3
in Martins & Plez (2006) to estimate the maxi- 2.3. Long baseline interferometry
mum distance at which an isolated O star can be
2.3.1. Observational setup and calibration
located for its apparent magnitude to be brighter
than our cut-off of H =7.5. Neglecting the effect All long baseline interferometric data were ob-
of extinction, Fig. 3 shows the obtained maxi- tained with the PIONIER combiner (Le Bouquin
mum distances as a function of spectral sub-type etal.2011,2012)andthefourauxiliarytelescopes
for the various luminosity classes considered in of the VLTI. We used the widest configurations
Martins & Plez (2006). Early-type O dwarfs can offered by the auxiliary telescopes: A0-K0-GI-I1
be located up to 3.3 kpc away, while late-type O in period P89 (Apr - Sep 2012) and A0-K0-G1-
dwarfs need to be closer than 1.5 kpc to belong I3 in period P90 (Oct 2012 - Mar 2013). Data
to our sample. Early- and late-type giants need were dispersed over three spectral channels across
to be at a distance of 3.8 and 2.5 kpc at the most the H band (1.50 - 1.80µm), providing a spec-
while supergiants may reside up to 4.2 kpc away. tral resolving power of R ≈ 15. As discussed in
Extinction will probably not affect these distance Sect. 2.3.2, this is the best compromise between
estimates by much more than a few 100 pc given sensitivityandthesizeoftheinterferometricfield-
that its effect in the H-band is rather limited. of-view (FOV).
The GOSC catalog is complete down to B =8, Data were reduced and calibrated with the
roughly corresponding to V = 8.3 and H = 9.0 pndrs package described in Le Bouquin et al.
in the absence of reddening. Our initial target (2011). Eachobservationblock(OB)providesfive
lististhusdominatedbyourmagnitudecut-offat consecutive files within a few minutes. Each file
H = 7.5, but for stars that have a B-band ex- containssixsquaredvisibilitiesV2 andfourphase
tinction larger than 1.5 mag. GOSC further does closures φ dispersed over the three spectral chan-
not contain many stars more distant than the Ca- nels. Whenever possible, the five files were aver-
rina nebula, i.e. more than ≈ 3−3.5 kpc away. aged together to reduce the final amount of data
In that sense the sample of supergiants and, to to be analyzed and to increase the signal-to-noise
some extent, the sample of giant stars, are more ratio. The statistical uncertainties typically range
reminiscent of a volume limited sample. from 0.5 to 10 degrees for the phase closures and
Close to the magnitude cut-off, our approach 2.5 to 20% for the squared visibilities, depending
also favors multiple objects which receive an ap- on target brightness and atmospheric conditions.
parent brightness boost through their unresolved Eachobservationsequenceofoneofoursmash+
companions, while similar isolated objects may targets was immediately followed by the observa-
have been left out of the sample, falling short of tionofacalibrationstarinordertomasterthein-
themagnitudecut-off(forfurtherdiscussionofthe strumentalandatmosphericresponse. LeBouquin
effectsofmagnitudecutoffonthemeasuredbinary et al. (2012) have shown that this calibration star
fraction, see Sana et al. 2013b). Equal brightness should be chosen close to the science object both
binaries can be observed up to a distance larger in terms of position (within a few degrees) and
by 600 pc compared to distances shown in Fig. 3. magnitude (within ±1.5mag). We were unable
Becauseoftheeffectsdescribedabove,oursam- to use the pre-computed JMMC Stellar Diame-
ple contains a larger fraction of supergiants, a ters Catalog (JSDC1) to look for calibration stars
larger fraction of hot stars and a larger fraction as this catalog only contains a suitable calibrator
of multiple systems than a distance limited sam- density down to a magnitude H ≈ 6. Instead
ple would. The first two effects can be mitigated we used the tool SearchCal2 in its FAINT mode
by discussing our observational results as a func- (Bonneauetal.2011)toidentifyatleastonesuit-
tion of spectral type and luminosity class. Given able calibration star within a radius of 3◦ of each
proper bias corrections, the latter aspect may be object observed within our sample.
viewedasadvantageousasitimpliesthattelescope Mostofourobjectsaregroupedintoclustersin
timeisspentonobjectsthatwehavemorechance the sky. Consequently the instrumental response
to resolve as multiple.
1http://www.jmmc.fr/catalogue jsdc
2http://www.jmmc.fr/searchcal
4
could be cross-checked between various calibra- pairsarebetterstudiedwithcomplementarytech-
tion stars. This allowed us to unveil a few pre- niques, such as speckle interferometry, aperture
viously unknown binaries among the calibration masking or adaptive optics.
stars. These have been reported to the bad cali- Inaddition,thesingle-modeopticalfibersofPI-
brator list3 maintained by the IAU and the Jean ONIERtheoreticallyrestricttheFOVtotheAiry
Marie Mariotti Center4. We estimated the typi- diskoftheindividualapertures. Thiscorresponds
cal calibration accuracy to be 1.5◦ for the phase to 180 mas when using the auxiliary telescopes.
closures and 5% for the squared visibilities. However, this limit is much less clear when con-
A critical point for the final accuracy on the sidering the effect of the atmospheric turbulence.
binary separation is the calibration of the effec- Forsure, ourPIONIERsurveyisblindtobinaries
tive wavelengths. In PIONIER this calibration with separation larger than 500 mas.
is performed routinely in the course of the ob- Even within the range 1.5−45 mas, the depth
servation using the optical path modulation as to which a companion can be detected depends
a Fourier transform spectrometer of the internal on the relative orientation of the companion and
source. The typical accuracy is 2% (Le Bouquin the interferometric baselines. This is due to the
et al. 2011). Finally, the on-the-sky orientation of sparsestructureofthepointspreadfunctionasso-
PIONIER has been checked several times and is ciated with the diluted aperture of an interferom-
consistentwiththedefinitionofPaulsetal.(2005). eter. Consequently, thesensitivitylimitshouldbe
defined for a given completeness level. Consider-
2.3.2. PIONIER field-of-view and dynamics ing an accuracy of 1.5◦ on the phase closures and
three OBs per target, we found that our survey
Long baseline interferometric observations are
should provide a 90% coverage of the separation
onlysensitivetobinariesinaspecificrangeofsep-
regime between 1.5 and 45 mas for a flux ratio
arations,thatcanbeapproximatelydefinedbyan
dynamics of 1:20, equivalent to a magnitude dif-
innerandanouterworkingangle. Theinnerwork-
ference of ∆H = 3.25 (see the middle panel of
ing angle (IWA), i.e. the maximum angular reso-
figure 2 in Le Bouquin & Absil 2012).
lution, is defined by the typical length B of the
interferometric baselines and the wavelength λ of
2.3.3. PIONIER observations
the observations:
Thebulkoftheobservationswereobtaineddur-
λ
IWA= ≈1.5mas. (2) ing 20 nights of visitor-mode spread over ESO pe-
2B
riods 89 and 90 (Table 4). Thirteen stars were
The spatial frequency smearing across one spec- further observed as backup targets from Dec 2013
tral channel induced by the low spectral resolving to Apr 2014. Raw and reduced data in OIFITS
powerR≈15ofthePIONIERobservationsisthe formatarestoredinthePIONIERarchiveandare
main limiting factor for the outer working angle available upon request. The time lost to weather
(OWA): amounted to approximately 20%, and is largely
λ due to wind speeds larger than 10 m s−1. The
OWA=R ≈45mas. (3)
B amountoftechnicallosseswasapproximately10%,
dominated by issues on the auxiliary telescopes
We checked that neither the temporal averag-
and the delay lines. Two nights in August 2012
ing over several minutes nor the spatial frequency
have been used to unveil and characterize the po-
smearing over the telescope pupil impact the ex-
larization behavior of the VLTI optical train.
pected OWA. Contrary to the IWA, the OWA is
notahardlimit. Pairswithwiderseparationsstill As described earlier, VLTI observations were
leave a strong signature in the interferometric ob- obtainedfor117objectsfromtheinitialtargetse-
servables. However,estimatingproperlytheirsep- lection and for six supplementary targets. Of the
arationandfluxratiobecomeschallenging. These observed sample, 73% of the objects have magni-
tude H >6.0 (Fig. 1), which is the limiting mag-
3http://apps.jmmc.fr/badcal nitudeoftheVLTI/AMBERinstrumentinservice
4http://www.jmmc.fr mode. Observing such a large number of faint ob-
5
jects was only made possible thanks to the sensi- the stellar brightness. Each data cube was taken
tivity and efficiency offered by the PIONIER in- withtheobjectatdifferentpositionsonthedetec-
strument. tor(dithering). Thestandardreductioncomprises
flat fielding, bad pixel correction and background
2.4. Aperture masking and AO observa- subtraction. The background was estimated us-
tions ing the median value of the eight data cubes. An
example of reduced and stacked NACO images is
2.4.1. Observational setup and calibration
shown in Fig. 6 and further images are provided
All aperture masking data have been obtained in the appendix.
withtheNACOinstrumentontheVLT/UT4tele-
scope. In most cases, four to eight targets were 2.4.2. NACO/SAM field-of-view and dynamics
grouped by magnitude and angular proximity in
As seen in Fig. 6, the point spread function
the sky in a single observing sequence. Targets
(PSF) of a star appears as a complex fringe pat-
in a given group were observed sequentially using
ternthatresultsfromFizeauinterferencebetween
the star hopping mode (Lacour et al. 2011b). In
the holes of the aperture mask. The size of the
this approach, we froze the adaptive optics (AO)
PSF is given by the Airy disk of a single hole
configuration on the first target and fast switched
(≈400 mas in the Ks-band).
between targets using telescope offsets, without
The NACO/SAM data result from the combi-
neither AO re-acquisition nor optimization on the
nation of aperture masking and adaptive optics
subsequent targets in the series. For long science
techniques. Theyallowustoinvestigatetwocom-
sequences, no calibrators were observed. Instead,
plementary separation regimes. At small work-
we used the scientific objects that turned out to
ing angles, the analysis of the Fizeau interference
be point sources as calibrators. The advantage of
pattern produced by the masked aperture enables
thestarhoppingmodeliesinitshighefficiency. It
us to search for companions within each object’s
approximately doubles the observing time spent
PSF.
on scientific targets compared to the classical ap-
proachofusingscience-calibratorsequencesofob- The IWA of this technique is obtained from
servations. Whenever stars could not be grouped Eq. (2) with B taken to be the maximum separa-
together, a K III stellar calibrator, with similar tion between holes, i.e. ≈ 7 m. This yields about
magnitude and a nearby position on the sky, was 30 mas.
observedimmediatelybeforeorafterthescientific The OWA is limited by the size of the PSF as
object. we fit the data with sines and cosines weighted
TheNACO/SAMobservationsmadeuseofthe by the Airy pattern. The weighting is the culprit,
7-hole mask (Tuthill et al. 2010) and, for the vast however necessary to have a good fit of the data
majority of our targets, were repeated using at and avoid being hampered by detector noise out-
least two different broadband filters. Most of our side the diffraction pattern. The OWA is there-
targetswereobservedwiththeH andKsfilterand fore 1.22λ/dhole ≈300 mas, in the H-band, where
the visible wave-front sensor. Depending on the dhole = 1.2 m is the diameter of a hole in our
weather conditions and instrumental/operational adopted aperture mask.
constraints, some targets were observed with the AsforPIONIERobservationstheNACO/SAM
L(cid:48) broadband filter and/or the AO correction OWA is not clear cut, but a progressive decrease
made use of the near-infrared (NIR) wave-front in sensitivity down to zero outside the first Airy
sensor. lobe. The maximum brightness contrast that can
We used either the S27 camera and a 512×512 be achieved depends on the signal-to-noise of our
pixel windowing or the S13 camera in full frame observations, and typically reaches 5 mag.
mode. These choices result in an effective FOV
of 13(cid:48)(cid:48) × 13(cid:48)(cid:48). For a given object, a typical ob- 2.4.3. NACO FOV and dynamics
servation consists generally of a set of eight data
At larger working angles, i.e. outside the PSF
cubes of 100 frames with individual integration oftheindividualobjects(ρ>300mas),the13(cid:48)(cid:48) ×
times ranging from 100 to 250 ms, depending on 13(cid:48)(cid:48) FOV of our NACO observations provides us
6
with an AO-corrected image of the surrounding correlation technique to search for (mostly faint)
field (Fig. 6). The IWA is limited by the blurring companions in a 8(cid:48)(cid:48)-radius from the central object
of the extended aperture masking PSF, hence to (Sect. 3.3).
1.22λ/d ≈0.3(cid:48)(cid:48). We further limited our search
hole
to a field of 8(cid:48)(cid:48) around the target. 3.1. PIONIER data analysis
ThePSFofeachcompanionintheNACOFOV
The calibrated interferometric data were ana-
also results from the Fizeau interference pattern
lyzedfollowingtheapproachdetailedinsection3.2
producedbythemaskedaperture,sothatourAO
of Absil et al. (2011). The underlying idea is to
images are not as deep as one could expect from
test whether an observation is compatible with
similar exposure images obtained on an 8m-class
that of a single star model. The main differences
telescope. Yet they are often the deepest AO-
with Absil et al. are:
corrected images ever obtained around our stars
and allow us to search for companions with sepa- - Wedonotre-normalizetheχ2withthebest-
rationsbetween0.3(cid:48)(cid:48) and8(cid:48)(cid:48) andwithabrightness fit binary model. This is because of the lim-
contrast of up to 8 mag. ited size of the data set obtained for each
individual object (typically 2 OBs).
2.4.4. NACO/SAM observations
- The analysis is performed using the phase
We observed a total of 162 targets during five
closures and the squared visibilities jointly.
observing runs spread from March 2011 to July
2013 (Table 4), for a total of almost 12 nights. - Thestellarsurfacesareconsideredtobeun-
Onethirdofthetimewaslostduetobadweather. resolved, which is a realistic assumption for
DetectorissuesduringourFeb2012runrestricted our early-type objects observed with 100m
the effective FOV to a 6.5(cid:48)(cid:48) × 13(cid:48)(cid:48) area but had baselines.
no further impact on the aperture masking ob-
servations. Most of the run was anyway lost to Consequently in our analysis, the probability P1
poor weather, with only six objects observed in for the data to be compatible with the single-star
the course of three nights. model is:
P =1−CDF (χ2) (4)
1 ν
3. Companion detection
with
(cid:88)(V2−1)2 (cid:88) φ2
In this section, we describe the algorithms χ2 = + . (5)
σ2 σ2
adopted to search for companions. Owing to the V2 φ
different nature of data collected by the smash+ CDF is the χ2 cumulative probability distribu-
ν
survey, different approaches were used for the PI- tion function with ν degrees of freedom (ν being
ONIER, the NACO/SAM and the NACO FOV the total number of V2 and φ minus the number
data. of parameters in the model). The distribution of
For the long baseline interferometric data ob- the computed χ2 values is shown in Fig. 4.
tainedbyPIONIER(Sect.3.1),wefitbothasingle If the probability P in Eq. (4) is higher than
1
star and a binary model to the squared visibility an adopted threshold, the dataset is considered
andphaseclosuresandcomparetheobtainedχ2to to be compatible with the single-star model. For
decide which model fits best. The analysis of the these objects, we derived a two-dimensional map
NACO data is split in two parts, according to the of sensitivity limits as detailed in section 3.3 of
separation regime considered. At small working Absiletal.(2011). Wethencomputedanannular
angles(ρ∼<250mas,NACO/SAM),i.e.withinthe sensitivity limit for a completeness of 90%. That
diffractionpatternoftheNACOPSF,weperform is, for each radius, we identified the dynamic for
an interferometric analysis of the Fizeau interfer- whichacompanionwouldhavebeendetectedover
ence pattern produced by the aperture mask to 90% of the annular region.
searchforcompanionsinFourierspace(Sect.3.2). If the probability P in Eq. (4) is below the
1
At larger working angles (ρ ∼> 250 mas, NACO adopted threshold, the detection of spatial com-
FOV), i.e. outside the object PSF, we use a cross- plexity in the object is considered significant and
7
we reject the single-star model. In this case we instrumentsetup. Column4givestheepochofob-
performaleast-squarefitofthedatawithabinary servations in Besselian years (b.y.). Columns 5 to
model. We incorporate in the model a first-order 7 provide the position angle, projected separation
correctiontoaccountforthebandwidthsmearing. andH-bandmagnitudedifferencebetweenthetwo
The complex visibility V, hence the squared visi- companions. Columns 8 (∆Ks) and 9 (∆L(cid:48)) are
bilities and phase closures, of our binary model is not used for the PIONIER detections.
defined as :
3.2. SAM interferometric data analysis
1 + f exp(−2iπx)sinc(πxR)
V = , (6)
As mentioned in Sect. 2.4.2, the interferomet-
1 + f
ric analysis of the NACO/SAM data corresponds
with to a search for a stellar companion within the
x=ρ(usinθ+vcosθ), (7) diffractionpatternofthePSF.WeusedtheSAMP
pipeline presented in Lacour et al. (2011a). In
where f, ρ and θ are the flux ratio, the angular
short, the PSF is modeled as a sum of spatial fre-
separation and the position angle of the binary.
quencies, modulated by the Airy pattern caused
The latter is defined as the orientation of the sec-
by the diffraction of a single hole. To each pair of
ondary measured from North to East. The vector
holes corresponds a baseline vector and a spatial
(u,v) is the spatial frequency of the observation
frequency. The individual frames are projected
(Pauls et al. 2005) and R is the spectral resolv-
onto that set of spatial frequencies. The bispec-
ingpower. Forthefewobjectsthatwereobserved
trum is obtained by multiplication of the complex
severaltimes,weperformedthefitwiththebinary
values extracted from a triangle of holes, hence
model independently for each epoch.
three spatially closing frequencies. The phase clo-
In some cases, the best-fit binary model still
suresarethenextractedfromtheargumentofthe
does not provide a satisfactory reduced χ2. This
bispectrum. The final calibration is made by sub-
indicates that the object shows some additional
tracting the average value of all point-like stars
spatialcomplexitythatisnotproperlyreproduced
observed within the same OB.
by the binary model. In particular, this situ-
Detection is then obtained as in Lacour et al.
ation occurs for the seven detected pairs whose
(2011a), similarly to what we have done for the
separations are larger than the PIONIER OWA
PIONIER data: the phase closures are adjusted
(see Fig. 5). For these objects, our model does
by models for either an unresolved object or a
not hold anymore because of the limited valid-
resolved binary system. All SAM data sets of a
ity of the bandwidth smearing correction. Fortu-
given target are fitted simultaneously. Combining
nately, most of these objects were observed with
data obtained with the different filters allows us
NACO/SAM,allowingustoconfirmthetentative
toliftthedegeneracyonthepositionoftheglobal
PIONIER detection in each case.
optimum in the χ2 map that results from the pe-
For the PIONIER companion detection, we
riodic sampling of the uv-plane. In a few cases,
adopted a P threshold of 0.9973 (corresponding
1 data were obtained at different epochs. These
to 3σ for a Gaussian distribution). The proba-
were still combined together given that we do not
bility of false detection is thus lower than 0.27%
expect significant changes in the position of the
(Eq.4), hencelessthanoneobjectgivenoursam-
companions over the 2.5 yr maximum baseline of
ple size. A total of 42 objects were flagged with
ourobservations. Theoneexceptiontothisruleis
positivedetectionandseparationswithinthePIO-
HD93129AaAb,forwhichourthreeobservational
NIEROWA,i.e.45mas. Wevisuallyinspectedall
epochs are handled separately.
datasets(detectionsandnon-detections). Oneob-
We distinguish three outcomes of the fitting
ject with positive detection was removed (µ Nor)
procedure: (i) non-detection: the phase closures
because it shows an inconsistent signal between
arecompatiblewithzerowithintheuncertainties;
epochs as well as a poor fit with a binary model.
(ii)cleardetection: thephaseclosuresarecompat-
Thepropertiesoftheresolvedsystemsaresum-
ible with a binary model; (iii) tentative detection:
marizedinTable5. Column1indicatesthetarget
thephaseclosuresarenotcompatiblewithapoint
name. Columns 2 and 3 identify the pair and the
source, but the binary model does not fit well ei-
8
ther. In the following we only report the clear byNACO/SAMuptothementionedseparationof
detections (case ii). 120 mas. PIONIER is hardly sensitive to any bi-
Thepropertiesoftheresolvedsystemsaresum- narieswithρ>150mas. Thesepropertieslineup
marized in Table 5 to allow for a direct compar- very well with the expected sensitivity range dis-
ison with PIONIER measurements. Columns 8 cussedinSect.2,illustratingtheexcellentinternal
and 9 indicates Ks and L(cid:48) magnitude differences consistencyofthesmash+detectionsatsmallan-
between the central object and the detected com- gular resolutions.
panion(s). Thepositionsofthedetectedcompanionsinthe
separation vs. brightness-contrast plane are dis-
3.3. NACO field-of-view analysis played in Fig. 7, together with the median sensi-
tivitylimitofallourobservations. Thelattershow
The second analysis of the NACO data aims to
that we have an excellent coverage of the parame-
search for stellar companions outside the diffrac-
ter space except in the 200-500 mas range, corre-
tion pattern of the SAM PSF. After correction
sponding to the transition between NACO/SAM
of the detector defects, each frame and each data
and NACO-FOV detections. These results will be
cube is shifted to center the target on a reference
discussed more extensively in Sect. 5, but two in-
point. Eachcubeisthencollapsed,andthecentral
teresting comments can already be made: (i) the
PSF is extracted for reference. This PSF is then
densityofsimilarbrightnesspairs(∆H <1)drops
cross-correlated over the entire detector. Last, all
significantly at separations larger than 50 mas;
the cross-correlated images – one for each data
(ii) there seems to be a lack of fainter compan-
cube – are derotated according to the parallactic
ions (∆H > 3) in the range 10-30 mas and 50-
angle (SAM observations are done in pupil track-
150 mas even though our first estimate of the de-
ing) and averaged. For each image we looked for
tectionlimitextendsdownto∆H =4and5mag,
companions by searching for local maxima in the
respectively. Further investigations on the accu-
cross-correlation function, independently of filters
racy of our detection limit estimates will allow to
or epochs.
verify this result.
Properties of the detected companions are
listed in Table 6, in a layout similar to that of Ta-
4. Constraints on the multiplicity proper-
ble 5. Column 8 gives the probability of spurious
ties
detection P obtained in Sect. 4.1. Whenever
spur
several companions are detected, their properties In this section, we present the statistical con-
are listed in Cols. 2-8 on subsequent lines. Each straints on the multiplicity properties of massive
companioninTable6eithercorrespondstoaclear stars. Section 4.1 investigates spurious associa-
detection, or to a detection confirmed at several tions. Section 4.2 compares our new detections
epochs and/or in several filters. with previous knowledge in the regime of sepa-
rations investigated by the smash+ survey. The
3.4. Detected companions and internal observedmultiplicityfractionandaveragenumber
consistency of companions per star are described in Sect. 4.3.
Finally, Sect. 4.4 investigates how the multiplicity
PIONIER resolved 53 companions in the sam-
properties change with the luminosity class.
ple of 117 objects (42 have ρ < 45 mas and 11
have larger separations). 48 of these companions
4.1. Spurious associations
are resolved for the first time. Their separations
range from ≈ 1 mas to > 100 mas. 22 of the Given the detection limits adopted in the pre-
pairsfallinthesensitivityregimeofNACO/SAM. vious section, all the companions that we report
In practice, all the companions detected by PI- are, to a very large degree of confidence, real ob-
ONIER with 24 < ρ < 120 mas are also de- jects. The components of some of the detected
tected by NACO/SAM. The PIONIER accuracy pairs may however not have any physical relation
remains higher than that of SAM up to its OWA, with one another. In this section, we estimate
i.e. about 45 mas. Interestingly, all tentative PI- the probability P of spurious association that
spur
ONIERdetectionsoutsideitsOWAareconfirmed would result from background or foreground ob-
9
jects or from line-of-sight alignment in a cluster against those of the Washington Double Star cat-
environment. For each central object, we queried alog (WDS Mason et al. 2001). The SB status
the2MASScatalogtolookforthenumberN of from the GOSC-v3 was further complemented by
obj
stars brighter than ∆H < 5 mag within a radius results of various published spectroscopic surveys
ofr =120(cid:48)(cid:48). Thiscontrastthresholdisrepresenta- (Sanaetal.2008a,2009,2011a,2012b;Chinietal.
tive of our SAM observations and a conservative 2012), byearlyresultsfromthespectroscopicsur-
value for PIONIER since our faintest PIONIER vey of Galactic O and WN stars (OWN, Barb´a
detection is at ∆H = 3.2 mag. The local den- et al. 2010) described in Sota et al. (2014) and by
sity was then converted into a probability of spu- individual papers on various objects (see individ-
rious detection due to chance alignment by con- ualnotesinAppendixA).Regardingtheresultsof
servatively assuming that all our PIONIER and Chinietal.(2012),weonlyacceptedSBstatusfor
SAM observations are sensitive to separations up V-IIIclassstars. Radialvelocitymeasurementsof
to ρ=0.2(cid:48)(cid:48): II-I stars may indeed be affected by atmospheric
variabilityand, unlessanorbitalperiodwasavail-
(cid:16)ρ(cid:17)2
P =N × . (8) able for these objects, we conservatively ignore a
spur obj r
potential spectroscopic companion.
We found that Pspur is always smaller than In Fig. 9, we compare the cumulative number
0.001%. Consequently our interferometric sur- distribution of companion separations before and
vey is virtually free from spurious detections. All after smash+. As expected, our survey is the
the companions detected by PIONIER and SAM first to resolve a significant number of systems
are very likely to be physically related to their with separations smaller than 50 mas (only two
central object. companions were known out of 52 detected now).
We performed a similar test for the compan- Moreover smash+ contributes to the companion-
ions detected in the NACO FOV, but using the ship census at larger separations. In total, our
actual magnitude difference to retrieve N from survey has increased the number of resolved com-
obj
the 2MASS catalog as well as the measured pro- panions within 100 mas roughly by a factor of 17
jected separation ρ in Eq. (8). Results are pre- (from 4 to 66) and within 8(cid:48)(cid:48) roughly by a factor
sented in Fig. 8 and listed in Table 6. of 4 (from 64 to 260).
All companions with ρ < 1(cid:48)(cid:48), 2(cid:48)(cid:48) and 5(cid:48)(cid:48) have a Fig. 9 also shows two clear trends, although
probability1−P ofphysicalconnectionbetter thephysicalinterpretationremainsunclear. First,
spur
than 99, 90 and 50% respectively. At large sepa- there is an apparent concentration of companions
rations, only the brightest companions are likely at separations of 30-50 mas, which corresponds
physically connected to their central object while to the transition between the PIONIER and the
atclosestseparations(ρ<1(cid:48)(cid:48)),allcompanionsare SAM samples. The larger sample and the higher
likely bound. This is in line with the conclusions sensitivity of the SAM observations seems to only
reached by Ma´ız Apell´aniz (2010) for AstraLux account for about half the increase in the cumu-
observations. lative number density, while the other half seems
to be genuine (Fig. 10); however, an appropriate
4.2. The smash+ observational window correction for observational biases is needed for
confirmation. Second, thecompaniondistribution
Most of the objects in our sample have been
function increases linearly with the logarithm of
previously observed by various high-resolution
the separation above 50-60 mas, but this increase
imaging campaigns. We compiled the astromet-
is almost entirely due to relatively faint compan-
ric results of Mason et al. (1998; 2009), Nelan
ions (∆H > 5). At closer separations, these faint
etal.(2004),Turneretal.(2008),Tokovininetal.
companions are below the detection threshold of
(2010) and Ma´ız Apell´aniz (2010) together with
all the previous surveys, including smash+. It
thespectroscopicstatusfromtheGOSC-v3(Ma´ız
is thus not possible to provide observational con-
Apell´anizetal.2013)inasingledatabaseinorder
straintsastowhethersuchfaintcompanionsexist
toobtainthemostcompleteviewofthemultiplic-
at smaller separations.
itypropertiesofoursamplestars. Theastrometric
data and naming conventions were cross checked
10