# How unique is Plaskett's star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems

Yael Nazé, Coralie Neiner, Jason Grunhut, Stefano Bagnulo, Evelyne Alecian, Gregor Rauw, Gregg Wade

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Yael Nazé, Coralie Neiner, Jason Grunhut, Stefano Bagnulo, Evelyne Alecian, et al.. How unique is Plaskett's star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems. Monthly Notices of the Royal Astronomical Society, 2017, pp.stx194. 10.1093/mnras/stx194. hal-02189634

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# How unique is Plaskett's star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems

Yaël Nazé,<fnref n="1" />★† Coralie Neiner,<fnref n="2" /> Jason Grunhut,<fnref n="3" />,<fnref n="4" /> Stefano Bagnulo,<fnref n="5" /> Evelyne Alecian,<fnref n="6" /> Gregor Rauw,<fnref n="1" /> Gregg A. Wade<fnref n="7" /> and the BinaMIcS collaboration<fnref n="7" />

Accepted 2017 January 19. Received 2017 January 17; in original form 2016 November 25

## ABSTRACT

Amongst O-type stars with detected magnetic fields, the fast rotator in the close binary called Plaskett's star shows a variety of unusual properties. Since strong binary interactions are believed to have occurred in this system, one may wonder about their potential role in generating magnetic fields. Stokes V spectra collected with the low-resolution FORS2 and high-resolution ESPaDOnS and Narval spectropolarimeters were therefore used to search for magnetic fields in 15 interacting or post-interaction massive binaries. No magnetic field was detected in any of them, with 0 G always being within 2σ of the derived values. For 17 out of 25 stars in the systems observed at high resolution, the 90 per cent upper limit on the individual dipolar fields is below the dipolar field strength of Plaskett's secondary; a similar result is found for five out of six systems observed at low resolution. If our sample is considered to form a group of stars sharing similar magnetic properties, a global statistical analysis results in a stringent upper limit of ~200 G on the dipolar field strength. Moreover, the magnetic incidence rate in the full sample of interacting or post-interaction systems (our targets + Plaskett's star) is compatible with that measured from large surveys, showing that they are not significantly different from the general O-star population. These results suggest that binary interactions play no systematic role in the magnetism of such massive systems.

**Key words:** stars: early-type – stars: individual: Plaskett's star, HD1337, HD25638, HD35652, HD35921, HD57060, HD100213, HD106871, HD115071, HD149404, HD152248, HD190967, HD209481, HD228854, LSS3074, XZ Cep – stars: magnetic field.

## 1 INTRODUCTION

Prior to the early 2000s, no direct detection of magnetic fields in O-type stars had been reported. However, such fields were strongly suspected to exist based on several indirect pieces of evidence, such as the presence of strong magnetic fields in the remnants of massive stars (pulsars and magnetars), of synchrotron radio emission in some massive binaries (for a review on non-thermal emitters; see Benaglia 2010) or of particular phenomena such as periodic modulations of line profiles (as e.g. in $\theta^1$ Ori C; Stahl et al. 1996).

Direct evidence of magnetism was finally acquired in the beginning of the twenty-first century. $\theta^1$ Ori C was the first O-type star found to be magnetic (Donati et al. 2002), HD 191612, an Of?p star, was the second (Donati et al. 2006), and many more have followed since then. Currently, about a dozen O-stars and several tens of early B-stars are known to be magnetic (see e.g. a summary by Petit et al. 2013), leading to an inferred incidence rate of such fields of about 6–8 per cent amongst massive stars (Fossati et al. 2015b; Grunhut et al. 2016).

Some of the first detections were made in binaries: For example, $\theta^1$ Ori C and HD 191612 are known to have companions. However, the orbital periods of these systems are long, implying that the binarity has little influence on the stellar properties. The case of Plaskett's star is very different: This 'star' actually is a

<footnote n="1">¹Groupe d'Astrophysique des Hautes Energies – STAR, Institut d'Astrophysique et de Géophysique – B5c, Université de Liège, 19c Allée du 6 Août, B-4000 Sart Tilman, Belgium</footnote>

<footnote n="2">²LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, F-92195 Meudon, France</footnote>

<footnote n="3">³European Southern Observatory (ESO), Karl Schwarzschild Strasse, D-85748 Garching bei München, Germany</footnote>

<footnote n="4">⁴Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Rm 101, 50 St. George Street, Toronto, ON M5S 3H4, Canada</footnote>

<footnote n="5">⁵Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK</footnote>

<footnote n="6">⁶Université Grenoble Alpes, IPAG, CNRS, F-38000 Grenoble, France</footnote>

<footnote n="7">⁷Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON, K7K 4B4, Canada</footnote>

<footnote>* E-mail: naze@astro.ulg.ac.be</footnote>

<footnote>† FNRS Research Associate.</footnote>
binary system harbouring two late-type O-stars in a rather tight or-
bit with a period of 14.4 d (as first demonstrated by Plaskett 1922).
It displays several peculiarities. The secondary star appears slightly
more massive than the primary and rotates more than four times
faster than its companion (Bagnuolo, Gies & Wiggs 1992; Lin-
der et al. 2008). The stars were classified O7.5I+O6I by Bagn-
uolo et al. (1992) and O8 III/I+O7.5III by Linder et al. (2008) –
but the latter authors noted that the secondary may actually be an
O7V star that shows an apparent O7.5III spectrum because of its
rapid rotation. A mismatch then exists between the spectroscopic
masses and the dynamical ones, the latter being too large (Linder
et al. 2008; Grunhut et al. 2013). Analysing the optical spectra with
atmosphere models further revealed that both stars are enriched in
helium, that the secondary surface is depleted in nitrogen and that
the nitrogen abundance of the primary star is about 16 times than
that of the Sun while its carbon abundance is depleted by a factor of 5
(Linder et al. 2008, the N enrichment was already detected in the
X-ray data by Linder et al. 2006). In order to explain the unusual
properties of the components, Bagnuolo et al. (1992) and Linder
et al. (2008) proposed that the system is a post-mass-transfer binary
in which the (currently) rapidly rotating secondary star received
mass and angular momentum from its companion. In parallel, the
Doppler maps derived from a tomographic analysis of the Hα and
He IIλ 4686 lines revealed an annular emission region, which was
interpreted in terms of a flattening of the secondary’s wind (Lin-
der et al. 2008). This was subsequently understood as a region of
magnetically confined winds when a strong magnetic field was de-
tected on this star (B<sub>d</sub> > 2.85 kG; Grunhut et al. 2013). It is worth
noting that Plaskett’s secondary is the only known rapidly rotating
magnetic O-star, hence the only O-star to appear in the centrifugally
supported magnetosphere zone of the confinement-rotation diagram
(Petit et al. 2013).

The peculiar properties and evolutionary history of this system
lead to speculations regarding the origin of the magnetic field of
Plaskett’s secondary, with two main possibilities. On one hand,
Plaskett’s secondary is a massive star, and global (i.e. not local-
ized) magnetic fields in massive stars are generally thought to be
fossil (e.g. Moss 2001, for a review see Neiner et al. 2015b and
references therein). This could be the case of Plaskett’s secondary
and, in this case, it would simply be amongst the few per cent of
magnetic objects in the massive star population. Its peculiarities –
linked to binary interactions (stellar tides, see Palate & Rauw 2014,
and previous mass transfer event, Linder et al. 2008) – would then
be a coincidence. On the other hand, binarity could have played a
key role, being directly responsible for the generation of a global
magnetic field. This could occur through the triggering of differen-
tial rotation (e.g. Spruit 2002; Braithwaite 2006). Such a scenario
was proposed by several authors, in particular to explain magnetism
in massive stars as a result of stellar mergers (Ferrario et al. 2009;
Langer 2014). Magnetism acquired after a merging event would
naturally concern (currently) single magnetic objects or magnetic
stars in long-period systems, but this idea cannot be applied di-
rectly to Plaskett’s star as it is a short-period system. However,
‘almost-merging’ systems, after a common-envelope phase or a
mass-transfer episode, should also experience strong shear, though
discussions arose on whether such processes would also constitute
a viable road towards magnetism. Indeed, some Be stars in binary
systems may be the products of a past mass transfer (van Bever &
Vanbeveren 1997; McSwain & Gies 2005; de Mink et al. 2013, and
references therein) but no large-scale magnetic field was detected
for those objects (see Wade et al. 2016b and section 3.3 of Rivinius,
Carciofi & Martayan 2013). Nevertheless, Plaskett’s star clearly

appears as a potential candidate for this scenario (e.g. Langer 2014;
Schneider et al. 2016). If true, it would have important implications
for understanding the general pathways leading to the production
of magnetic fields in massive stars.

In this study, we set out to directly assess the validity of this sce-
nario, which has never before been done, by verifying the magnetic
status of ‘twins’ of Plaskett’s star, defined as short-period massive
binaries that interact or have interacted in the recent past. If binary
interactions, in particular mass-transfer events, constitute a domi-
nant process for the generation of magnetic fields of massive stars,
then most (if not all) of these systems would possess a magnetic
component. In that case, we would thus have identified a specific
group of magnetic O-stars, the second such category after the group
of Of?p stars that were all found to be magnetic since the pio-
neering detection of 2006 (HD 191612, Donati et al. 2006; Wade
et al. 2011; HD 148937, Hubrig et al. 2008; Wade et al. 2012a;
HD 108, Martins et al. 2010; NGC 1624-2, Wade et al. 2012b;
CPD −28°2561, Hubrig et al. 2012; Wade et al. 2015). On the
contrary, if mass-transfer events are not a source of strong, large-
scale magnetic fields, then the magnetic incidence in these systems
would not differ significantly from that found for large stellar sam-
ples (Fossati et al. 2015b; Grunhut et al. 2016). In this paper, we
report the results of such a campaign. Sections 2 and 3 present the
targets and the observations used in this study, respectively, while
Section 4 provides the results and Section 5 reports our conclusions.

## 2 THE SAMPLE

The first step of the project was to select interacting or post-
interaction massive binaries. The key features to identify such sys-
tems are the sizes of the stars (e.g. filling their Roche lobes or close
to doing so), but also the detection of non-solar abundances and/or
of rotational asynchronicity, as well as a mismatch between predic-
tions from evolutionary models of isolated stars and the observed
stellar properties (age, masses, abundances and luminosities). In
addition, these binaries need to be bright enough to be observable
with the current generation of spectropolarimeters, but they also
ought to be well known, i.e. to have reliable physical parameters, so
that there is no ambiguity on their evolutionary status. Gies (2003)
provided lists of well-studied double-lined spectroscopic (SB2) bi-
nary systems located in the Galaxy and the Magellanic Clouds,
among which he separately listed unevolved cases (his table 1) and
evolved cases (his table 2), the latter ones being subdivided between
systems before contact, in contact and after mass transfer. Penny,
Ouzts & Gies (2008) also listed binaries in which one star fills its
Roche lobe, complementing the list. Eliminating the complex high
multiplicity systems δ Ori and Cyg OB2 5, which could blur the
picture, while adding HD 149404 and LSS 3074, two other post-
Roche Lobe Overflow systems not listed in these references but
whose interactions were recently analysed (see details below), we
ended up with 15 relatively bright targets.

(i) *HD1337=AO Cas* (O9.7III+O9.5V) consists of two stars in a circular orbit of 3.5 d period (Stickland 1997; Linder 2008). The analysis of the eclipses yields a semidetached configuration, with a ‘hotspot’ on the secondary possibly corresponding to the impact of an accreting stream (Linder 2008). Furthermore, stellar masses and radii appear too small compared to typical values for massive stars of the same spectral types, reinforcing the mass-transfer scenario (Linder 2008).

(ii) *HD25638=SZ Cam* (O9.5V-9IV+B0-0.5V) comprises an eclipsing, circular SB2 system with a period of 2.7 d (Gorda 2015,
and references therein). The derived radius of the primary is too large for a main-sequence star while the mass of the secondary is too small for its spectral type (Lorenz, Mayer & Drechsel 1998). Furthermore, the derived masses, luminosities and temperatures could not be fitted by theoretical evolutionary tracks, ruling out an 'interaction-free evolution' (Harries, Hilditch & Hill 1998; Lorenz et al. 1998; Tamajo et al. 2012).

(iii) HD35652=IU Aur (O9.5V+B0.5IV-V) comprises an eclipsing, semidetached binary with a circular orbit and a period of P = 1.8 d (Harries et al. 1998; Özdemir et al. 2003, and references therein). The derived masses and gravities of the stars cannot be explained by the separate evolution of massive stars born at the same time, suggesting a past mass-transfer event (Harries et al. 1998, see in particular their fig. 9).

(iv) HD35921=LY Aur (O9III+O9.5III) is a circular binary with a period of 4.0 d (Stickland et al. 1994; Zhao et al. 2014, and references therein). The analysis of the eclipses suggests a semidetached or slightly overcontact configuration (Li & Leung 1985; Drechsel, Lorenz & Mayer 1989; Mayer et al. 2013) and the derived masses appear slightly too low (Stickland et al. 1994; Mayer et al. 2013), favouring an evolutionary scenario where mass transfer has occurred.

(v) HD57060=29 CMa (O8.5I+O9.7V) is a slightly eccentric binary with a period of 4.4 d (Bagnuolo et al. 1994; Stickland 1997; Linder 2008). The photometric eclipses suggest some interaction between the stars (from semidetached to overcontact configuration; Bagnuolo et al. 1994; Linder 2008; Antokhina, Srinivasa Rao & Parthasarathy 2011) and the anomalous luminosities also seem to indicate an ongoing mass transfer. Furthermore, a tomographic analysis of the Hα and He II 4686 lines (Linder 2008) and discrepancies in the mass-ratio determination (Antokhina et al. 2011) suggest a flattened, disc-like secondary wind, similar to that found in Plaskett's star. Finally, a wind-wind collision is also present in the system (Linder 2008).

(vi) HD100213=TU Mus (O7.5V+O9.5V) is a binary with *e* = 0 and *P* = 1.4 d (Linder et al. 2007; Penny et al. 2008). Mutual irradiation of the stars in this very short-period system leads to the observation of line strength variations along the orbit (Linder et al. 2007; Palate et al. 2013). The eclipses of the system indicate that the stars are in contact while the masses derived from the orbital solution are lower than expected from evolutionary models, suggesting a past episode of mass transfer (Qian et al. 2007; Linder et al. 2007; Penny et al. 2008, and references therein).

(vii) HD106871=AB Cru (O8V+B0.5) appears as a semidetached, eclipsing binary with a circular orbit and a period of 3.4 d (Lorenz, Mayer & Drechsel 1994). A consistent fit to the derived properties of the stars (effective temperatures, radii, masses, luminosities) cannot be found with evolutionary tracks considering the absence of interaction while anomalous hydrogen and helium abundances are detected, both facts suggesting the system to have undergone mass transfer (Lorenz et al. 1994).

(viii) HD115071 (O9.5V+B0.2III) consists of a semidetached binary with a circular orbit and a period of 2.7 d (Penny et al. 2002). The ellipsoidal variations of its light curve indicate that the secondary star fills its Roche lobe. The derived masses are smaller than expected, suggesting mass transfer to have occurred in a very recent past (Penny et al. 2002, and references therein).

(ix) HD149404 (O7.5If+ON9.7I) is a non-eclipsing, detached binary with a circular orbit and a period of 9.8 d (Rauw et al. 2001; Thaller et al. 2001). Emission lines such as Hα and He II 4686 display phase-locked variations indicating the presence of a wind-wind collision (Rauw et al. 2001; Thaller et al. 2001). In addition, the sec-

ondary star is of the ON type, i.e. it presents anomalous abundances that hint at a past interaction (Rauw et al. 2001; Thaller et al. 2001). A recent, more detailed study further reveals an asynchronous rotation of the two stars, a significant nitrogen overabundance of the secondary star and peculiar positions of the stars in the HR diagram (Raucq et al. 2016). All of these properties clearly disagree with expectations of single star evolutionary models, yielding further support to the past mass-exchange scenario (Raucq et al. 2016).

(x) HD152248 [O7.5III(f)+O7III(f)] is a slightly eccentric and eclipsing binary with a period of 5.8 d (Sana, Rauw & Gosset 2001; Mayer et al. 2008). Phase-locked variations in Hα and He II 4686 lines as well as in the X-ray emission arise from a wind-wind collision located between the two stars (Sana et al. 2001, 2004). The derived masses are lower than expected from evolutionary tracks and both components appear close to filling their Roche lobes, suggesting a past mass-transfer episode (Sana et al. 2001). Note that tidal effects are reinforced by a small, non-zero, eccentricity.

(xi) HD190967 (O9.5V+B1I-II) consists of an eclipsing binary in a semidetached configuration and with a period of 6.5 d and *e* = 0 (Harries, Hilditch & Hill 1997; Kumsiashvili, Kochiashvili & Djurasevi 2005). In a mass-gravity diagram, the derived values are at odds with predictions of evolutionary models for stars of the same age, indicating a past episode of rapid mass transfer (Harries et al. 1997; Kumsiashvili et al. 2005). Note that, to best fit the light curve, either bright spots or an accretion disc are needed (Kumsiashvili et al. 2005; Djurasevi et al. 2009).

(xii) HD209481=LZ Cep (O9III+ON9.7V) is a binary with a circular orbit and a period of 3.1 d. The ellipsoidal variations of its optical light curve indicate that both stars almost fill their Roche lobes (Mahy et al. 2011; Palate, Rauw & Mahy 2013). Several lines of evidence point towards a past mass-transfer episode (Harries et al. 1998; Mahy et al. 2011): The photosphere of the secondary star appears strongly enriched in helium and nitrogen but depleted in carbon and oxygen, and there is a slight asynchronicity between the rotation periods, and the evolutionary masses are higher than the masses inferred from the orbital solution.

(xiii) HD228854 (O7.3V+O7.7V) is a system with a circular orbit and a period of 1.9 d (Yaşarsoy & Yakut 2013, and references therein). The analysis of the eclipses indicates filling of the Roche lobe(s) (either overcontact, Değirmenci et al. 1999; Qian et al. 2007, or contact/semidetached, Mayer, Lorenz & Drechsel 2002; Yaşarsoy & Yakut 2013) and the derived physical properties (in particular masses and gravities) disagree with theoretical expectations for (single) massive star evolution (Harries et al. 1997), pointing towards a post mass-transfer situation.

(xiv) LSS3074 (O4If+O7.5f) is a circular binary with a period of 2.2 d (Niemela et al. 1992; Haefner, Simon & Fiedler 1994, and Raucq et al. 2017). Hα and He II 4686 lines present line profile modulations that are reminiscent of those observed in HD 149404, suggesting wind-wind collision to be present (Raucq et al. 2017). Photometric, spectroscopic and polarization measurements led to the derivation of the physical parameters for the system (Niemela et al. 1992, and Raucq et al. 2017). The anomalous abundances as well as the mismatch between evolutionary tracks and observed masses and luminosities suggest a mass transfer in the system (Raucq et al. 2017).

(xv) XZ Cep (O9.5V+B1III) is a non-eccentric, eclipsing system with a period of 5.1 d (Harries et al. 1997). The secondary star fills its Roche lobe while the derived stellar properties are at odds with predictions from evolutionary models (incompatible ages for the derived masses and gravities; Harries et al. 1997), suggesting a previous episode of mass transfer.
<table><caption>**Table 1.** Summary of the properties of our targets and of Plaskett's star.</caption><thead><tr><th>ID</th><th>V</th><th>Sp. types</th><th>P (d)</th><th>e</th><th>C/SD?</th><th>AA?</th><th>Mass gainer</th><th>v sin (i) (km s<sup>-1</sup>) prim-sec-tert</th></tr></thead><tbody><tr><td>Plaskett's star</td><td>6.06</td><td>O8III/I+O7.5III</td><td>14.4</td><td>0</td><td>N</td><td>Y</td><td>sec.</td><td>77-370*</td></tr><tr><td>HD 1337</td><td>6.14</td><td>O9.7III+O9.5V</td><td>3.5</td><td>0</td><td>Y</td><td></td><td>sec.</td><td>118-82*</td></tr><tr><td>HD 25638</td><td>6.93</td><td>O9.5V-9IV+B0-0.5+B0III-III</td><td>2.7</td><td>0</td><td>N</td><td></td><td>prim.</td><td>137-69-40*</td></tr><tr><td>HD 35652</td><td>8.39</td><td>O9.5V+B0.5IV-V+B</td><td>1.8</td><td>0</td><td>Y</td><td></td><td>prim.</td><td>172-159-71*</td></tr><tr><td>HD 35921</td><td>6.85</td><td>O9III+O9.5III+B</td><td>4.0</td><td>0</td><td>Y</td><td></td><td>prim.?</td><td>202-122-27*</td></tr><tr><td>HD 57060</td><td>4.95</td><td>O8.5I+O9.7V</td><td>4.4</td><td>0.1</td><td>Y</td><td></td><td>sec.</td><td>107-193*</td></tr><tr><td>HD 100213</td><td>8.41</td><td>O7.5V+O9.5V</td><td>1.4</td><td>0</td><td>Y</td><td></td><td></td><td>222-191</td></tr><tr><td>HD 106871</td><td>8.48</td><td>O8V+B0.5:</td><td>3.4</td><td>0</td><td>Y</td><td>Y</td><td>prim.</td><td></td></tr><tr><td>HD 115071</td><td>7.97</td><td>O9.5V+B0.2III</td><td>2.7</td><td>0</td><td>Y</td><td></td><td>prim.</td><td>101-132</td></tr><tr><td>HD 149404</td><td>5.47</td><td>O7.5If+ON9.7I</td><td>9.8</td><td>0</td><td>N</td><td>Y</td><td>prim.</td><td>56-80*</td></tr><tr><td>HD 152248</td><td>6.05</td><td>O7.5III(f)+O7III(f)</td><td>5.8</td><td>0.1</td><td>N</td><td></td><td></td><td>139-128*</td></tr><tr><td>HD 190967</td><td>8.16</td><td>O9.5V+B1I-II</td><td>6.5</td><td>0</td><td>Y</td><td></td><td>prim.</td><td>230-115*</td></tr><tr><td>HD 209481</td><td>5.55</td><td>O9III+ON9.7V</td><td>3.1</td><td>0</td><td>N</td><td>Y</td><td>prim.</td><td>127-93*</td></tr><tr><td>HD 228854</td><td>8.65</td><td>O7.3V+O7.7V</td><td>1.9</td><td>0</td><td>Y</td><td></td><td></td><td>223-203*</td></tr><tr><td>LSS 3074</td><td>11.7</td><td>O4If+O7.5</td><td>2.2</td><td>0</td><td>N</td><td>Y</td><td>sec.</td><td>110-127</td></tr><tr><td>XZ Cep</td><td>8.51</td><td>O9.5V+B1III</td><td>5.1</td><td>0</td><td>Y</td><td></td><td>prim.</td><td>104-172*</td></tr></tbody></table>

Notes. Evidence for current or past interaction (for references, see text): 'C/SD' = in a contact or semi-detached configuration, 'AA' = abundances anomalies detected – note that there are also other anomalies detected (e.g. too low masses, asynchronous rotation, etc.), see text for details. The projected rotational velocities v sin (i) are given for primary and then secondary then tertiary, they come from our analysis of high-resolution spectra when an asterisk is added and from literature otherwise (Howarth et al. 1997 for HD 100213, otherwise see dedicated items in text for literature references).

Evidence for past or current interactions (abundance anomalies, Roche lobe filling, etc.) in the chosen systems has just been mentioned above (see Table 1 for a summary), but the presence of such interactions does not necessarily imply that they were actually efficient in modifying the stellar structure. In particular, Plaskett's secondary displays a high rotation rate (~300 km s⁻¹; Bagnulo et al. 1992; Linder et al. 2008; Grunhut et al. 2013), demonstrating that momentum exchange was very efficient for that object. For the stars in our sample, projected rotational velocities are also rather high and generally amount to 100–200 km s⁻¹, with components in four systems (HD 35921, HD 100213, HD 190967 and HD 228854) reaching even higher velocities [v sin (i) >200 km s⁻¹, see Table 1]. This confirms that our sample is well suited to find out whether magnetic fields are triggered in massive binary interactions.

## 3 OBSERVATIONS

For the northern targets, high-resolution spectropolarimetry was acquired with ESPaDOnS at Canada—France—Hawaii Telescope (CFHT; Donati 2003) and Narval at TBL (Aurière 2003). For the southern targets, low-resolution spectropolarimetry was acquired with FORS2 at European Southern Observatory (ESO; Appenzeller & Rupprecht 1992; Szeifert et al. 1998). Two targets, HD 152248 and HD 149404, were observed at both low and high resolutions, as they are visible from Chile as well as from Hawaii.

### 3.1 Low-resolution FORS2 spectropolarimetry

Low-resolution spectropolarimetric data of six targets were obtained with the Very Large Telescope equipped with FORS2 in Spring 2015 (ESO 095.D-0075, PI Nazé, see Table 2). These data were taken in service mode with the red CCD (a mosaic composed of two 2k × 4k MIT chips), no binning, a slit of 1 arcsec and the 1200B grating (R ~ 1400). The observing sequence consisted of eight subexposures with retarder waveplate positions of +45°, -45°, -45°, +45°, +45°, -45°, -45°, +45°. We reduced these spectropolarimetric data with IRAF as explained by Nazé et al.

(2012): The aperture extraction radius was fixed to 20 pixels, the nearby sky background was subtracted and wavelength calibration was performed from 3675 to 5128 Å (with pixels of 0.25 Å) considering arc lamp data taken at only one retarder waveplate position (in our case, -45°). There was no indication of variation of the Stokes I spectra between subexposures. We then constructed the normalized Stokes V/I profile, as well as a diagnostic ‘null’ profile N/I (see Donati et al. 1997; Bagnulo et al. 2009, for details on the procedure and Fig. 1 for an illustration in our case). Finally, the associated longitudinal magnetic field was estimated by minimizing $\chi^2 = \sum_i \frac{(y_i - B_i x_i - a)^2}{\sigma_i^2}$ with $y_i$ either V/I or the null profile at the wavelength $\lambda_i$ and $x_i = -g_{eff} 4.67 \times 10^{-13} \lambda_i^2 1/I_i (dI/d\lambda)_i$ (Bagnulo et al. 2002). This was done for $x_i$ in the interval between $-10^{-6}$ and $+10^{-6}$, after discarding edges and deviant points and after selecting spectral windows centred on stellar lines (see Nazé et al. 2012, for further discussion). These windows comprise both primary and secondary absorption lines, but avoid emission lines (as some of them arise in wind–wind collision regions and are thus not representative of the stellar photosphere). The values reported in Table 2 were obtained after rectifying the Stokes profiles, but similar values are found if no rectification is applied.

### 3.2 High-resolution ESPaDOnS and Narval spectropolarimetry

Most high-resolution spectropolarimetric data of our targets were collected in the context of the MiMeS (Wade et al. 2016a) and the BinaMiCS (Aleian et al. 2015) large programmes. They were acquired with ESPaDOnS, an echelle spectropolarimeter installed at the CFHT. In addition, archival data from dedicated programmes (PIs Neiner, Bouret) were obtained for HD 209481 in 2006 and 2009 using Narval, a similar spectropolarimeter installed at Pic du Midi (France). Biases, flat-fields and ThAr calibrations were obtained at the beginning and at the end of each night. The data reduction was performed using LIBRE-ESPRIT, the dedicated reduction software based on ESPRIT (Donati et al. 1997). There was no indication of
<table><caption>**Table 2.** Results from the low-resolution spectropolarimetry obtained with FORS2.</caption><thead><tr><th>ID</th><th>L<sub>1</sub>/L<sub>2</sub></th><th>Exp. (s)</th><th>SNR</th><th>Date</th><th>HJD-2450000</th><th>B<sub>z</sub> (G)</th><th>N<sub>z</sub> (G)</th></tr></thead><tbody><tr><td rowspan="2">HD 100213</td><td rowspan="2">~2</td><td>8 × 70</td><td>2470</td><td>2015-04-03</td><td>7115.687</td><td>34 ± 48</td><td>2 ± 47</td></tr><tr><td>8 × 70</td><td>2365</td><td>2015-04-05</td><td>7117.732</td><td>24 ± 50</td><td>37 ± 49</td></tr><tr><td rowspan="2">HD 106871</td><td rowspan="2">~2.5</td><td>8 × 80</td><td>2210</td><td>2015-04-03</td><td>7115.733</td><td>-31 ± 45</td><td>3 ± 44</td></tr><tr><td>8 × 80</td><td>2380</td><td>2015-04-05</td><td>7117.754</td><td>32 ± 39</td><td>60 ± 38</td></tr><tr><td rowspan="2">HD 115071</td><td rowspan="2">~0.9</td><td>8 × 60</td><td>2350</td><td>2015-04-05</td><td>7117.837</td><td>-22 ± 34</td><td>-44 ± 33</td></tr><tr><td>8 × 60</td><td>2520</td><td>2015-05-01</td><td>7143.618</td><td>-42 ± 32</td><td>16 ± 33</td></tr><tr><td rowspan="2">HD 149404</td><td rowspan="2">~0.7</td><td>8 × 7</td><td>1650</td><td>2015-05-16</td><td>7158.907</td><td>214 ± 119</td><td>-234 ± 117</td></tr><tr><td>8 × 7</td><td>2130</td><td>2015-05-17</td><td>7159.585</td><td>-64 ± 80</td><td>3 ± 69</td></tr><tr><td rowspan="2">HD 152248</td><td rowspan="2">0.8–1.</td><td>8 × 10</td><td>2100</td><td>2015-05-02</td><td>7144.620</td><td>-25 ± 69</td><td>36 ± 63</td></tr><tr><td>8 × 10</td><td>1845</td><td>2015-05-17</td><td>7159.570</td><td>29 ± 59</td><td>90 ± 59</td></tr><tr><td rowspan="2">LSS 3074</td><td rowspan="2">1.–2.5</td><td>8 × 536</td><td>1400</td><td>2015-04-03</td><td>7115.809</td><td>-591 ± 619</td><td>200 ± 427</td></tr><tr><td>8 × 536</td><td>1450</td><td>2015-04-05</td><td>7117.796</td><td>-452 ± 391</td><td>187 ± 362</td></tr></tbody></table>

Notes. From left-hand to right-hand side, the columns show the binaries' names, the light ratios (see Section 2 for references), the exposure lengths, the peak signal-to-noise ratio in the 4000–5000 Å range, the observing times (both in YYYY-MM-DD format and in heliocentric Julian dates) and finally the derived magnetic field values and their associated errors for both the system's Stokes V profile and from the null diagnostic profile. These $B_z$ and $N_z$ values were found using rectification and within selected spectral windows.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Top left panel: the spectrum of HD 100213 taken on the second night of observation with FORS2, with the selected windows shown by a green thick line. Top right panel: associated V/I (top blue) and N/I (bottom red) as a function of wavelength. Bottom panel: V/I (left-hand panel) and N/I (right-hand panel) as a function of $x_i$, with their best-fitting straight lines shown in cyan. Rejected values, not considered for the fitting, appear in red. Note the null slopes of the best-fitting straight lines that indicate the absence of a significant magnetic field.</figcaption></figure>

variation of the Stokes I spectra between subexposures. Normalization of the individual orders was then performed using low-degree polynomial fits.

After the Stokes spectra were calculated, we followed the same procedure as Grunhut et al. (2016). First, the Least-Squares Deconvolution technique (LSD; Donati et al. 1997; Kochukhov, Makaganiuk & Piskunov 2010) was applied to all polarimetric spectra to increase the signal to noise, thereby helping to detect weak magnetic Zeeman signatures (see Fig. 2 for an illustration). We first constructed a line mask from the VALD2 data base (Piskunov et al. 1995; Kupka et al. 1999) using the appropriate temperature

and log(g) values for each star, assuming solar abundances and considering only lines deeper than 1 per cent of the continuum. We then excluded all hydrogen lines and lines blended with hydrogen. This yielded masks with between 430 and 1220 lines, depending on the star, over the 3700–9800 Å interval. The line depths were then adjusted to provide the best fit to the observed Stokes I spectrum of each target. Note that regularization (see Kochukhov et al. 2010) and clipping of the deviant points were both applied to improve the final signal to noise of the LSD profiles.

Since our targets are binaries, the second step was to derive the individual properties of each component (see Fig. 2 and Grunhut et al. 2016 for details on the procedure). To this aim, multiple absorption components with profiles derived from the convolution of a rotationally broadened profile with a radial-tangential macroturbulence profile were fitted to the Stokes I profiles. Since this disentangling process often leads to degenerate solutions, relevant information [e.g. $v \sin(i)$] from the literature was used to constrain the fits, whenever needed. The derived RVs were checked for compatibility with the known orbital solutions of these systems, i.e. we verified that each observed pair of RVs agreed with the solution at some phase during the orbit (accurate orbital phases at current epoch cannot be computed due to the large accumulated phase uncertainties). Disentangled I profiles were then obtained for each system component, and used to estimate individual V and N contributions. Note that, if line profiles overlap and one component is magnetic, this procedure does not account for the possible magnetic contamination caused by the Stokes V signal of the companion.

Finally, the magnetic properties were derived from the Stokes V profiles. We calculated in each case the probability that deviations of the observed $V/I_c$ signal from the normalized value of 0 occur by chance (i.e. because of noise only – see Donati, Semel & Rees 1992, but note that this reference reports the complement probability). This was done using an optimal velocity grid (with a minimum resolution of 1.8 km s⁻¹) representing the best compromise between increasing the signal to noise and not smearing the Zeeman signature. Next, the longitudinal field values $B_z$ were computed from the first-order moment of the unbinned Stokes V profiles (Mathys 1989; Donati et al. 1997), with errors derived by propagating the known uncertainties for each pixel. Similar measurements were made for the diagnostic null profiles N. Note that the field values for the MiMeS targets (HD 1337, HD 35921 and HD 209481) were already reported by Grunhut et al. (2016).
<figure><img src="image_4.png" /><figcaption>**Figure 2.** LSD profiles derived for the BinaMIcS sample (coming from Grunhut et al. 2016 for the three MiMeS targets – HD 1337, HD 35921, HD 209481). Each panel provides, from top to bottom, the system name and the Stokes V, N and I profiles, respectively. The individual components of the I profiles, derived from the disentangling procedure described in Section 3.2, are added with colours (yellow, blue or green solid lines) along with their sum (dashed red lines).</figcaption></figure>

Since the significance level (or false-alarm probability) was larger than $10^{-3}$ in each case, corresponding to clear non-detections, upper limits on the dipolar field values were derived using Monte Carlo simulations as described by Neiner et al. (2015a) and Blazère et al. (2015). For both the primary and secondary, we simulated 1000 oblique dipole models for various values of the polar magnetic field strength with random inclination angles $i$, obliquity angles $\beta$ and rotational phases. White Gaussian noise with a null average and a variance corresponding to the signal to noise of each observed profile was added. Using the best-fitting disentangled LSD I profiles, we calculated local Stokes V profiles assuming the weak-field case, which were then integrated over the visible hemisphere of the star. The derived synthetic Stokes V profiles were normalized to the intensity of the continuum, and the Neyman-Pearson likelihood ratio test was then applied to estimate the probability of detecting a dipolar oblique magnetic field (with a threshold of $10^{-3}$ for the false alarm probability). A 90 per cent detection rate was required to consider that the simulated field would statistically be detected in the data. This translates into upper limits for the possible undetected dipolar field strength for each star and each observation. For

HD 152248 and HD 209481, multiple observations were available and since none of them resulted in a magnetic detection, single-observation statistics were combined to extract a stricter upper limit on the non-detected field (see section 4.2 of Neiner et al. 2015a for details).

## 4 RESULTS AND DISCUSSION

Table 2 reports the resulting longitudinal magnetic field values<fnref n="1" /> for the low-resolution spectropolarimetric data, while Table 3 provides

<footnote n="1">¹ They are weighted averages of the contributions of all components in each system, i.e. each value is valid for the entire system. The light ratios $L_1/L_2$ of the systems are known (Table 2). However, deriving individual limits from the system's values is not straightforward, since longitudinal fields are derived from the analysis of sets of lines, whose strengths for primary and secondary do not necessarily reflect the light ratios between components and furthermore vary with ion under consideration. We therefore refrain from deriving individual limits.</footnote>
<table><caption>**Table 3.** Results from the high-resolution ESPaDOnS and Narval spectropolarimetry.</caption><thead><tr><th>ID</th><th>Exp. /SNR</th><th>Date</th><th>HJD -2.45e6</th><th>Obj</th><th>V<sub>r</sub> (km s<sup>-1</sup>)</th><th>B<sub>z</sub> (G)</th><th>N<sub>z</sub> (G)</th><th>σ (G)</th><th>up. lim. (kG)</th></tr></thead><tbody><tr><td rowspan="3">HD 1337</td><td rowspan="3">4 × 720 1280</td><td rowspan="3">2009-10-09</td><td rowspan="3">5113.845</td><td>prim</td><td>178</td><td>46</td><td>-37</td><td>39</td><td>1.0</td></tr><tr><td>sec</td><td>-160</td><td>-46</td><td>-134</td><td>74</td><td>1.9</td></tr><tr><td>system</td><td></td><td>35</td><td>40</td><td>60</td><td></td></tr><tr><td rowspan="4">HD 25638</td><td rowspan="4">8 × 840 1557</td><td rowspan="4">2013-11-20</td><td rowspan="4">6616.786</td><td>prim</td><td>193</td><td>-62</td><td>-7</td><td>69</td><td>1.7</td></tr><tr><td>sec</td><td>-272</td><td>-112</td><td>-154</td><td>241</td><td>6.8</td></tr><tr><td>tert</td><td>17</td><td>-12</td><td>-62</td><td>55</td><td>2.0</td></tr><tr><td>system</td><td></td><td>-55</td><td>85</td><td>101</td><td></td></tr><tr><td rowspan="4">HD 35652</td><td rowspan="4">8 × 840 942</td><td rowspan="4">2016-02-27</td><td rowspan="4">7445.812</td><td>prim</td><td>-198</td><td>-122</td><td>-42</td><td>267</td><td>7.1</td></tr><tr><td>sec</td><td>315</td><td>216</td><td>120</td><td>205</td><td>4.8</td></tr><tr><td>tert</td><td>124</td><td>189</td><td>-368</td><td>329</td><td>8.7</td></tr><tr><td>system</td><td></td><td>37</td><td>-161</td><td>249</td><td></td></tr><tr><td rowspan="4">HD 35921</td><td rowspan="4">4 × 940 1215</td><td rowspan="4">2011-11-13</td><td rowspan="4">5878.872</td><td>prim</td><td>130</td><td>-102</td><td>-44</td><td>75</td><td>1.7</td></tr><tr><td>sec</td><td>-287</td><td>76</td><td>-48</td><td>104</td><td>2.8</td></tr><tr><td>tert</td><td>-18</td><td>16</td><td>-45</td><td>74</td><td>2.0</td></tr><tr><td>system</td><td></td><td>74</td><td>-15</td><td>98</td><td></td></tr><tr><td rowspan="3">HD 57060</td><td rowspan="3">12 × 540 2942</td><td rowspan="3">2013-03-03</td><td rowspan="3">6354.835</td><td>prim</td><td>160</td><td>17</td><td>-5</td><td>15</td><td>0.6</td></tr><tr><td>sec</td><td>-141</td><td>173</td><td>286</td><td>196</td><td>3.3</td></tr><tr><td>system</td><td></td><td>92</td><td>102</td><td>47</td><td></td></tr><tr><td rowspan="3">HD 149404</td><td rowspan="3">16 × 390 3038</td><td rowspan="3">2014-06-08</td><td rowspan="3">6816.897</td><td>prim</td><td>-108</td><td>14</td><td>-10</td><td>25</td><td>0.7</td></tr><tr><td>sec</td><td>7</td><td>34</td><td>4</td><td>16</td><td>0.3</td></tr><tr><td>system</td><td></td><td>35</td><td>9</td><td>16</td><td></td></tr><tr><td rowspan="3">HD 152248</td><td rowspan="3">4 × 540 794</td><td rowspan="3">2014-04-13</td><td rowspan="3">6761.045</td><td>prim</td><td>-135</td><td>-175</td><td>-72</td><td>155</td><td>4.1, comb: 1.5</td></tr><tr><td>sec</td><td>111</td><td>416</td><td>-148</td><td>206</td><td>5.4, comb: 1.9</td></tr><tr><td>system</td><td></td><td>-9</td><td>-39</td><td>148</td><td></td></tr><tr><td rowspan="3">HD 152248</td><td rowspan="3">12 × 540 1358</td><td rowspan="3">2014-04-14</td><td rowspan="3">6762.060</td><td>prim</td><td>129</td><td>112</td><td>88</td><td>89</td><td>2.2</td></tr><tr><td>sec</td><td>-192</td><td>-153</td><td>-135</td><td>104</td><td>2.5</td></tr><tr><td>system</td><td></td><td>-46</td><td>7</td><td>97</td><td></td></tr><tr><td rowspan="3">HD 190967</td><td rowspan="3">8 × 840 989</td><td rowspan="3">2016-05-18</td><td rowspan="3">7527.097</td><td>prim</td><td>-32</td><td>-15</td><td>-15</td><td>87</td><td>2.1</td></tr><tr><td>sec</td><td>80</td><td>-3</td><td>21</td><td>32</td><td>0.9</td></tr><tr><td>system</td><td></td><td>-14</td><td>-13</td><td>46</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 600 867</td><td rowspan="3">2006-12-14</td><td rowspan="3">4084.274</td><td>prim</td><td>74</td><td>14</td><td>60</td><td>74</td><td>1.7, comb: 0.4</td></tr><tr><td>sec</td><td>-226</td><td>-36</td><td>51</td><td>120</td><td>3.1, comb: 0.6</td></tr><tr><td>system</td><td></td><td>-34</td><td>107</td><td>91</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 667</td><td rowspan="3">2009-07-20</td><td rowspan="3">5033.483</td><td>prim</td><td>27</td><td>-73</td><td>124</td><td>97</td><td>2.3</td></tr><tr><td>sec</td><td>-100</td><td>160</td><td>-168</td><td>134</td><td>3.4</td></tr><tr><td>system</td><td></td><td>-46</td><td>-10</td><td>65</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 825 1191</td><td rowspan="3">2009-07-24</td><td rowspan="3">5037.485</td><td>prim</td><td>-98</td><td>85</td><td>-32</td><td>48</td><td>1.1</td></tr><tr><td>sec</td><td>212</td><td>213</td><td>24</td><td>83</td><td>2.1</td></tr><tr><td>system</td><td></td><td>99</td><td>-87</td><td>63</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 916</td><td rowspan="3">2009-07-25</td><td rowspan="3">5038.492</td><td>prim</td><td>26</td><td>59</td><td>2</td><td>66</td><td>1.5</td></tr><tr><td>sec</td><td>-95</td><td>-51</td><td>60</td><td>107</td><td>2.7</td></tr><tr><td>system</td><td></td><td>-34</td><td>33</td><td>45</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 804</td><td rowspan="3">2009-07-26</td><td rowspan="3">5039.492</td><td>prim</td><td>46</td><td>192</td><td>67</td><td>62</td><td>1.4</td></tr><tr><td>sec</td><td>-154</td><td>-225</td><td>9</td><td>90</td><td>2.3</td></tr><tr><td>system</td><td></td><td>16</td><td>0</td><td>54</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 315</td><td rowspan="3">2009-07-27</td><td rowspan="3">5040.490</td><td>prim</td><td>-97</td><td>142</td><td>-112</td><td>180</td><td>4.2</td></tr><tr><td>sec</td><td>212</td><td>-459</td><td>-307</td><td>322</td><td>8.3</td></tr><tr><td>system</td><td></td><td>-12</td><td>-80</td><td>238</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 1058</td><td rowspan="3">2009-07-28</td><td rowspan="3">5041.480</td><td>prim</td><td>10</td><td>26</td><td>-9</td><td>54</td><td>1.2</td></tr><tr><td>sec</td><td>-66</td><td>5</td><td>1</td><td>76</td><td>1.9</td></tr><tr><td>system</td><td></td><td>-19</td><td>-2</td><td>29</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 1199</td><td rowspan="3">2009-07-29</td><td rowspan="3">5042.496</td><td>prim</td><td>54</td><td>23</td><td>-62</td><td>48</td><td>1.1</td></tr><tr><td>sec</td><td>-174</td><td>-21</td><td>128</td><td>70</td><td>1.8</td></tr><tr><td>system</td><td></td><td>26</td><td>-19</td><td>46</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 1037</td><td rowspan="3">2009-07-30</td><td rowspan="3">5043.476</td><td>prim</td><td>-96</td><td>85</td><td>-17</td><td>60</td><td>1.4</td></tr><tr><td>sec</td><td>201</td><td>-62</td><td>-49</td><td>100</td><td>2.5</td></tr><tr><td>system</td><td></td><td>76</td><td>25</td><td>69</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 1105</td><td rowspan="3">2009-07-31</td><td rowspan="3">5044.496</td><td>prim</td><td>-4</td><td>2</td><td>-39</td><td>57</td><td>1.3</td></tr><tr><td>sec</td><td>-40</td><td>40</td><td>-17</td><td>86</td><td>2.2</td></tr><tr><td>system</td><td></td><td>-19</td><td>-32</td><td>30</td><td></td></tr></tbody></table>
<table><caption>**Table 3** – continued</caption><thead><tr><th>ID</th><th>Exp. /SNR</th><th>Date</th><th>HJD -2.45e6</th><th>Obj</th><th>V<sub>r</sub> (km s<sup>-1</sup>)</th><th>B<sub>z</sub> (G)</th><th>N<sub>z</sub> (G)</th><th>σ (G)</th><th>up. lim. (kG)</th></tr></thead><tbody><tr><td rowspan="3">HD 209481</td><td rowspan="3">4 × 675 968</td><td rowspan="3">2009-08-03</td><td rowspan="3">5047.461</td><td>prim</td><td>-24</td><td>39</td><td>-69</td><td>69</td><td>1.6</td></tr><tr><td>sec</td><td>6</td><td>71</td><td>5</td><td>89</td><td>2.2</td></tr><tr><td>system</td><td></td><td>46</td><td>-2</td><td>32</td><td></td></tr><tr><td rowspan="3">HD 209481</td><td rowspan="3">12 × 400 2417</td><td rowspan="3">2011-06-22</td><td rowspan="3">5735.027</td><td>prim</td><td>-60</td><td>3</td><td>2</td><td>22</td><td>0.5</td></tr><tr><td>sec</td><td>108</td><td>-3</td><td>-6</td><td>39</td><td>1.0</td></tr><tr><td>system</td><td></td><td>1</td><td>-16</td><td>17</td><td></td></tr><tr><td rowspan="3">HD 228854</td><td rowspan="3">8 × 840 975</td><td rowspan="3">2016-05-17</td><td rowspan="3">7526.067</td><td>prim</td><td>-206</td><td>-142</td><td>-146</td><td>188</td><td>5.7</td></tr><tr><td>sec</td><td>275</td><td>-122</td><td>142</td><td>256</td><td>7.6</td></tr><tr><td>system</td><td></td><td>27</td><td>208</td><td>269</td><td></td></tr><tr><td rowspan="3">XZ Cep</td><td rowspan="3">8 × 840 609</td><td rowspan="3">2013-08-19</td><td rowspan="3">6523.930</td><td>prim</td><td>-265</td><td>-30</td><td>5</td><td>45</td><td>1.1</td></tr><tr><td>sec</td><td>103</td><td>621</td><td>-473</td><td>488</td><td>11.1</td></tr><tr><td>system</td><td></td><td>37</td><td>-180</td><td>184</td><td></td></tr></tbody></table>

Notes. From left-hand to right-hand side, the columns provide the binaries' names, the exposure lengths and peak signal-to-noise ratios in the 5000–6500 Å range, the observing times (both in YYYY-MM-DD format and in heliocentric Julian dates), the considered component, its heliocentric velocity, the magnetic field values and their associated errors ($\sigma$) derived from Stokes V profiles ($B_z$) and from the null diagnostics profiles ($N_z$), and finally the upper limits on the dipolar field strength with 90 per cent chance of being detected. When several observations of the same target are available (as for HD 152248 and HD 209481), the upper limit on the undetected field from the combined statistics is quoted in italics in the last column of the first observation line, after a 'comb' prefix.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Cumulative (top panel) or simple (bottom panel) distribution of the $B_z$ values normalized to their error bars for our sample (left-hand side, for $B_z$ values of individual stars – labelled ‘primary’, ‘secondary’ or ‘tertiary’ in Table 3; right-hand side, for $B_z$ values estimated for the whole system – from Table 2 for FORS measurements and under ‘system’ label in Table 3 for high-resolution observations). The dotted red lines are for the $N_z$ values, while the dashed vertical lines correspond to a $3\sigma$ detection level. Note how the measured values are all within $2\sigma$ from zero.</figcaption></figure>

the field values derived from each high-resolution spectropolarimetric observation for individual components and for each system as well as upper limits on non-detected magnetic fields per component from individual and combined observations. It is clear from these tables (see also Fig. 3) that (1) no problem due to spurious signals affects the data (the null diagnostics being always compatible with zero within $2\sigma$ at most – and generally well within $1\sigma$) and (2) no significant magnetic field is detected for any of the systems or their components.

One caveat must however be mentioned: When using low-resolution spectropolarimeters like FORS2, even strong fields may become unobservable, depending on the magnetic geometry of the star and the observing phase. Indeed, as the sinusoidal $B_z$ curve of magnetic oblique rotators reaches or nears a value of 0 G at cross-over phases (i.e. when the dipole is seen equator-on), the field would become undetectable in low-resolution observations. However, this occurs during a small fraction of the rotational period only, an interval depending on the error bars. For example, we may consider Plaskett’s $B_z$ values (Grunhut et al. 2013) and assume a sinusoidal $B_z$ varying from -810 G to 680 G. Non-detections imply $B_z \sim 0$, i.e. $|B_z| < N\sigma$ with $N$ generally taken to be 3 (though a value of 5 appears better suited for FORS; see Bagnulo et al. 2002). Since the typical error on the 10 FORS measurements amounts to ~55 G, excluding the more uncertain case of LSS 3074, non-detections would then occur during 15 per cent of the orbit if we consider $3\sigma$, or 25 per cent for a $5\sigma$ threshold. Missing a detection is less probable when several observations of the same target are acquired, as was done for all systems observed at low resolution. Indeed, it is unlikely to have two random exposures sampling cross-over phases for one target. Moreover, if it is already unlikely for one target, it is even more so for several independent objects. Considering a 15 per cent chance to miss detecting a Plaskett-like field in one observation, the probability to miss field detections in all 10 observations is $(0.15)^{10} \sim 10^{-8}$ (or $10^{-6}$ if 25 per cent is considered). Of course, there is a high chance (~80 per cent) to miss one detection over 10 observations, but since we have two observations of each target and two targets in common with the high-resolution sample, then this risk may be mitigated. One could object that some of the observation pairs are separated by only one or two days, so that there would be no large phase change over that interval if the rotational period is long. However, it must be remembered that, contrary to the ‘usual’ magnetic massive stars, our targets are relatively fast rotators in short-period systems so that $B_z$ should change rapidly, with significant variations expected on the time-scale of one day, as observed for Plaskett’s star. Moreover, even if we do consider that the two observations of a target were taken at the same rotational phase, the probability to miss a field in all five cases is still low: $(0.15)^5 \sim 10^{-4}$ considering a 15 per cent chance of missing
the field detection, or 10⁻³ if a 25 per cent chance is used instead. Therefore, statistically, we could have missed detecting the magnetic field of one star in the FORS2 sample, but not more. Note that for high-resolution data, this problem does not exist as a single random observation has proven to be efficient to detect magnetism (when it exists) in a massive star during the MiMeS, BinaMlcS and other magnetic surveys (see also a more theoretical discussion by Petit & Wade 2012). Indeed, except in very rare cases,<fnref n="2" /> even if the derived longitudinal field is zero, the high-resolution Stokes V profile of a magnetic object is not flat and the field can be detected whatever the phase. Missing the detection of a strong field in the high-resolution sample thus is unlikely.

It is important to note that the error bars on $B_\zeta$ are small ($\sigma$ smaller than 100 G), except for a few cases – for FORS data, only LSS 3074 has large error bars ($\sigma > 100$ G) because it is a faint object ($B \sim 13$ mag compared to $B = 5-9$ for the others); for ESPaDOnS/Narval data, large errors are only found for all components of HD 35652 and HD 228854, as well as the secondary stars of HD 25638, HD 57060 and XZ Cep. Hence our survey is generally very sensitive. In the high-resolution data, this is also reflected by the determination of the upper limits on the dipolar fields that could have remained hidden in the noise for each target. The limits are well below the field strength of Plaskett's star ($B_d > 2.85$ kG; Grunhut et al. 2013) for the vast majority of the cases (17 out of 25 limits on individual components).

The objective of this paper is to test the plausibility of binary interactions as efficient processes at generating stable magnetic fields in massive systems. If this were true, we would expect all our targets to share similar properties, different from the usual massive star population. In such a case, we can further statistically characterize our sample by following the Bayesian approach of Kolenberg & Bagnulo (2009, see in particular their equation 7, later generalized by Asensio Ramos, Martínez González & Manso Sainz 2015). It assumes that all stars of our sample are similar, i.e. they have a dipolar field with identical field strength, but their magnetic axes are randomly oriented with respect to the line of sight, and it combines the observational values into a constraint on the common field strength. We note that equation (7) of Kolenberg & Bagnulo (2009) is valid only for independent measurements on different stars, therefore for those objects that have been observed more than once we have considered only the field measurement with the smallest error bar. Note also that equation (7) has been applied twice (see Fig. 3): first to measurements of individual stars (results from high-resolution data for stars labelled ‘primary’, ‘secondary’ or ‘tertiary’ in Table 3), second to field estimates for the systems (combining results from Table 2 for low-resolution observations and those listed under the ‘system’ label in Table 3 for high-resolution observations). We then derive that there is a 90 per cent chance that the dipolar field strength at the pole is $B_d \le 140-220$ G, a very stringent limit well below Plaskett’s secondary field. Hence, our sample and Plaskett’s star, while having all underwent binary interactions, do not seem to form a specific and coherent group of magnetic stars.

To shed more light on to our results, we can compare them to the current knowledge of large-scale magnetic fields in massive stars. Such fields are generally considered to be (mostly) dipolar. In addition, except for $\zeta$ Ori A (a supergiant O-star with an extremely low field; Blazère et al. 2015) and $\beta$ CMa (Fossati et al. 2015a), the detected fields are strong, with $B_d$ larger than 0.5 kG for O and B0.5–1 stars (e.g. Petit et al. 2013). This is notably the case of Plaskett’s secondary. The detection rate of such fields, found in large samples composed of single massive stars and/or long-period massive binaries, amounts to 6–8 per cent (Fossati et al. 2015b; Grunhut et al. 2016). How does that compare to the case of massive interacting or post-interaction binaries? Overall, we have one magnetic detection (Plaskett’s star) in 16 investigated multiple systems (our sample + Plaskett’s star, comprising 35 individual stars in total). Excluding the targets with less stringent constraints, then the high-resolution data yield one magnetic detection (Plaskett’s secondary) on 19 stars if we keep only individual measurements with field limits lower than Plaskett’s field (2.85 kG), one on 17 stars for a limit of 2 kG, and one on nine stars for a limit of 1 kG. Focusing only on the mass gainers (Plaskett’s secondary + those of our sample – when known, see Table 1), there is one magnetic object out of eight stars measured at high resolution with small error bars. In summary, whatever the case under consideration, these incidence levels appear entirely compatible with that derived from general samples, considering the small number statistics. The incidence rate in the interacting/post-interaction binary sample is thus not much larger than for the general O-star population. Therefore, there is no need to include additional processes linked to binary interactions to explain the presence of a magnetic field in Plaskett’s secondary.

## 5 CONCLUSION

Amongst magnetic O-stars, one object is particularly remarkable: The secondary of Plaskett’s star, a binary system that has very likely undergone mass transfer in the recent past. This situation led to the intriguing possibility that binary interactions could be a source of magnetism in massive stars. To test the validity of this scenario, we have observed a sample of 15 interacting and post-interaction systems. Dedicated spectropolarimetry did not lead to any magnetic detection in these systems. In fact, for the vast majority of our targets, the longitudinal fields are <300 G and the individual upper limits on the dipolar fields are below the dipolar field strength of Plaskett’s secondary for 17 out of 25 binary components. Considering our targets as a homogeneous group of similar objects, a global statistical analysis of the derived field values leads to a 90 per cent upper limit on the (common) dipolar field strength of only 140–220 G, underlining once again the dissimilarity between our targets and Plaskett’s star. Moreover, the low rate of magnetic detection in these interacting and post-interaction systems is compatible with the rates found from general surveys of O-stars. Together with the lack of magnetic detection in Be stars, some of which are also likely products of mass transfer, this suggests that binary interactions do not systematically trigger stable, strong magnetic fields in such systems, and that a fossil origin is still the best scenario for explaining the magnetic fields of massive stars.

## ACKNOWLEDGEMENTS

This paper is based on observations collected at the European Organization for Astronomical Research in the Southern hemisphere under ESO programme 095.D-0075. YN and GR acknowledge support

<footnote n="2">² When both components are magnetic, when the observation has been obtained at conjunction, and when the signatures of both components are of opposite shape and cancel each other either partially or totally, the magnetic signal could be diluted and not be detected if present. Note, however, that not only is this case very unlikely but none of our targets was observed at conjunction, as demonstrated by the very different velocities in Table 3 (see also Fig. 2 for a visual inspection of the components' separations).</footnote>
from the Fonds National de la Recherche Scientifique (Belgium), the Communauté Française de Belgique, the PRODEX XMM contract (Belspo) and an ARC grant for concerted research actions financed by the French community of Belgium (Wallonia-Brussels Federation). GAW acknowledges Discovery Grant support from the Natural Science and Engineering Research Council (NSERC) of Canada.

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