The discussion on winds and magnetic fields of active OB stars was carried out by S. Owoki, G. Wade, M. Cantiello, O. Kochukhov, M. Smith, C. Neiner, T. Rivinius, H. Henrichs and R. Townsend. The topics were the ability to detect small and large scale magnetic fields in massive stars and the need to consider limits on photometric variability of the star surface brightness.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Bouret, Jean Claude

Cidale, Lydia Sonia

Proceedings of the International Astronomical Union

English

SEDICI - Repositorio de la UNLP

Active OB stars: structure, evolution, mass loss, and critical limits
Proceedings IAU Symposium No. 272, 2010
C. Neiner, G. Wade, G. Meynet & G. Peters, eds.
c° International Astronomical Union 2011
doi:10.1017/S1743921311010209
Discussion – Winds and magnetic fields
of active OB stars
Jean-Claude Bouret1,2 and Lydia Cidale3,4
1Laboratoire d’Astrophysique de Marseille,
38 rue Joliot-Curie, 13388 Marseille Cedex 13, France
2NASA Goddard Space Flight Center, Greenbelt, 20771 MD, USA
email: jean-claude.bouret@oamp.fr
3Facultad de Ciencias Astrono´micas y Geof´ısicas, Universidad Nacional de La Plata
4 Instituto de Astrof´ısica La Plata, CCT La Plata-CONICET-UNLP
Paseo del Bosque S/N, La Plata, Argentina
email: lydia@fcaglp.unlp.edu.ar
Abstract. The discussion on winds and magnetic fields of active OB stars was carried out by
S. Owoki, G. Wade, M. Cantiello, O. Kochukhov, M. Smith, C. Neiner, T. Rivinius, H. Henrichs
and R. Townsend. The topics were the ability to detect small and large scale magnetic fields
in massive stars and the need to consider limits on photometric variability of the star surface
brightness.
Keywords. stars: early-type, stars: magnetic fields, stars: mass loss
Discussion
The discussion of session 2 was first driven by the invited talk about observations of
magnetic fields by V. Petit, who reviewed modern spectropolarimetric methods used to
diagnose magnetic fields of massive stars. The discussion was mostly focused on large
and small scale magnetic fields, their strengths, and on the possibility of detecting great
complex fields.
Stan Owocki compared the large complex magnetic fields that are actually detected
on massive stars with spectropolarimetry with those strong fields seen at small scale in
the Sun. He explained that in the case of very strong fields well spread over the star,
the rotational modulation allowed us to see a modulation-signal. However, he wondered
what would happen if some stars present small complex magnetic fields, as solar-like
spots, or have large and complex fields with different scales and different polarization,
and the two fields canceled on the disc. How could we see those? The answer to that
issue is really important because people, for a long time, have been wondering whether
magnetic fields might play an important role in the Be phenomena. Brown & Cassinelli
proposed for instance to use a magnetic field moment arm to have a sub-critical rotation
and that moment arm then swept up the matter to reach Keplerian orbits. On the other
hand, numerical simulations (Owocki and collaborators) rather suggest that large scale
magnetic fields are really incompatible with Keplerian discs because these discs have
shear. This is in stark contrast with observational results (presented in some posters
at this symposium) bringing strong evidence that the disc of Be stars are generally
Keplerian. We note however that, at the same time, very strong complex field might play
an important role in ejecting material from the star into the orbit.
Another issue that was mentioned was the long-term V/R variations observed in many
Be discs. If they really are interpreted as one-arm modes, they imply the precession of
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Discussion – Winds and magnetic fields 173
features as the material is going around the star every day while the one-arm actually
goes around the star every few years. The precession of orbits thus indicates the existence
of very small perturbation of 1/r potential. Since the perturbation amplitude is on the
scale of the rotation period over the precession period, it means that there could have
very large magnetic forces and large scale forces, other than the 1/r gravity as a potential
acting on the material. Therefore, it is clear that it is of great importance to make the
distinction between what could be observed at large scale versus small scale or in other
words to state why fields cannot be detected if/because they are too complex.
From the observational point of view, G. Wade answered that in upper main sequence
stars, and perhaps to some extent in evolved stars, it is possible to establish the existence
of strong stable large scale magnetic fields, which are oblique to stellar rotation axes and
do not appear to correlate strongly with the physical properties of the stars. This phe-
nomenon was well-established in the ’40 by Babcock, and strengthened by people like
J. Landstreet and the Russian team during the ’70 and ’80. G. Wade also stated that
nowadays this phenomenon was globally understood and details were being investigated.
However, it is clear that although the measured quantity is a vector, for sufficiently com-
plex fields or sufficiently weak fields, it is possible that magnetic fields could remain below
the detection threshold. Therefore, the field could be certainly present but still remain
undetectable. Those kind of fields, that had been illustrated in the talks by Kochukhov
and Petit (this volume), are examples of the kind of complex dynamo generated mag-
netic fields observed routinely in active type stars. It is worth mentioning that the activity
expressed by these stars is significantly greater than the one seen in the Sun.
G. Wade made the point that, at the present time, the state-of-art is that “we are
barely able to detect a magnetic field similar to that of the Sun in a Sun-like star and,
probably in most cases, barely might be highly stretch. However, we are certainly not
able to detect a similar field in a B-type star”. While thousands of lines can be worked
with when multi-line-techniques are used in late-type stars, like G-type stars, the reality
is that only tens of lines can be used in the case of a B star.
G. Wade’s conclusion is that it is absolutely possible that fields of that sorts could
be highly below the detection threshold. He also commented that M. Cantiello and him-
self were discussing about the degree of complexity in fields that could be predicted with
Cantiello’s sub-surface convection models. It is very likely that fields like these are proba-
bly not detectable in B-type stars either, using today techniques. Furthermore, Cantiello
gave orders of magnitude for the amplitude of complex magnetic fields. He explained
that when looking at the equipartition fields in the convective regions the magnetic field
strength got up to Kilogauss, but if the field got to the surface by buoancy instability,
the density had basically to be rescaled and the magnetic fields got up to hundreds,
maybe two hundreds gauss (unless there was something special, like erupting like-tubes
where the fields are stronger). He remarked that the average field would have been up
to hundreds of gauss, but not bigger than that and could present very complex scales.
In addition, G. Wade commented, for example, that in the case of our Sun the field was
complex, and the strongest regions had fields of the order of Kilogauss. He explained
that since those magnetic fields could not be possibly detected, there was no reason for
believing that such fields did exist. He stated that it was certainly true that it wouldn’t
have been detectable using today techniques. Nevertheless, this point can be regarded as
over-pessimistic as magnetic fields in solar-type stars have already been detected by P.
Petit.
In order to give an answer to the magnetic field scale, O. Kochukhov invited theoreti-
cians to provide observers with a power spectrum because it would help to calculate the
Stoke parameters. He explained that if the scale of the magnetic fields was small, smaller
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174 J.-C. Bouret & L. Cidale
than the mean photon path-length, magnetic fields would not be observable. However, if
the spectrum contained features at that scale, of a few degrees, it would be possible to
do something. In relation to this last comment, T. Rivinius brought in mind the case of γ
Cas which is a rapid rotator. He speculated that if it just was a matter of a few degrees,
or how quick the star rotates, γ Cas should be easily resolved in a few degrees. To this
particular problem, C. Neiner explained that if the magnetic fields were of the order of a
Kilogauss, like those fields found in M. Smith’s model, they would had been detected by
now, even if the scale of the field was small because of the very big v sin i of that star. She
added that they had observed γ Cas with stellar high-resolution spectropolarimeters, as
NARVAL and ESPaDOnS, during many nights and they were not able to detect any field
in the photospheric lines. Still, she stated that there might be very marginal magnetic
signatures in the Fe II lines. So, if there is a magnetic field in that star it is in the disc
and not in the star. Moreover, G. Wade pointed out that there were families of field
topologies, that were going to be undetectable in any data set and so that came down to
absence of evidence being not evidence of absence. Even though, theoreticians would be
able to provide with a model, the field could be hidden in the noise and certainly remain
undetectable.
On the other hand, M. Smith mentioned that, particularly for γ Cas, they had four lines
of independent evidence of magnetic field. One of them obtained under the bolometric
light-curve, which was a very robust reproducible result. The real issue with this star
would be then to explain the production of X-ray of 108 degrees, 12 Kev at least, without
a magnetic field! There are now eight stars sharing the same global properties, one of
which gives plasma temperature twice that of γ Cas. Therefore, M. Smith argued that
we are looking in a fairly, rare, a very small subgroup of stars and might not be easy
to extrapolate σ Ori’s and even the HD 37776 successes other than by the photometric
signature that went out to 2 or 3 Kilogauss.
Rich Townsend remarked that rotation modulation and surface spot leading to pho-
tometric variability were not necessary signs of magnetic field. It is worth mentioning
that some works presented in this volume, showing also spectroscopic variability due to
surface inhomogeneities, are not associated with magnetic fields. In contrast to the pre-
vious examples, H. Henrichs mentioned the particular case of σLup, a B1 V star with a
magnetic field that phases exactly with the 3 days photometric period, and the maximum
field coincides with the maximum bright.
The need to consider the limits on photometric variability of the surface brightness of
massive stars was highlighted by Owocki. He argued, in particular, that the photometric
variation is directly related to the presence of CIRs in the wind of these stars and the best
way to make a CIR is to have a bright spot on the stellar surface. Therefore, he suggested
that high-precision photometric data like those delivered by MOST (or CoRoT) could
put tight constraints on Cantiello’s model and wondered if that model could predict a
bright magnetic spot, versus the case of the Sun where the magnetic spots are actually
dark.
Cantiello explained that the envelope calculation gave spots in excess of 200 K respect
to the rest of the photosphere, but of course, it would also depend on how many spots
were there. It was difficult to define the filling factor of the magnetic field to determine
the factor of fluctuation in the luminosity.
Another important reason to look at photometric variations is related to the topology
of the wind. In order for a magnetic field to channel the wind, a large scale magnetic
field is needed and it must have a large η∗ (as defined in a series of papers by Owocki
and Ud-doula) but in order to have an effect in perturbing the base of the wind, it
should be on smaller scales and more complex. In any case, the signature would be in
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Discussion – Winds and magnetic fields 175
brightness variations, thus the importance of having very small limits for the detection
of photometric variations.
R. Townsend indicated that, probably, the limit for periodical photometric variations
that we could get from ground based observations is around 10 millimagnitudes. He also
remarked that with space satellite (MOST, CoRoT) we could see all sort of interesting
variations that showed off nowadays.
The session on winds and magnetic fields ended on these comments.
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