Exploring the origin of magnetic fields in massive stars: A survey of O-type stars in clusters and in the field

S.~Hubrig$^1$, M.~Sch"oller$^2$, N.~V.~Kharchenko$^{1,3}$, N.~Langer$^4$,
W.~J.~de Wit$^5$, I.~Ilyin$^1$, A.~F.~Kholtygin$^6$, A.~E.~Piskunov$^{1,7}$,
N.~Przybilla$^8$, the MAGORI collaboration

$^1$ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482~Potsdam, Germany
$^2$ European Southern Observatory, Karl-Schwarzschild-Str.~2, 85748~Garching, Germany
$^3$ Main Astronomical Observatory, 27~Academica Zabolotnogo Str., 03680~Kiev, Ukraine
$^4$ Argelander-Institut f"ur Astronomie, Universit"at Bonn, Auf dem H"ugel~71, 53121~Bonn, Germany
$^5$ European Southern Observatory, Alonso de Cordova~3107, Santiago, Chile
$^6$ Astronomical Institute, Saint-Petersburg State University, Saint-Petersburg, Russia
$^7$ Institute of Astronomy of the Russian Acad. Sci., 48 Pyatnitskaya Str., 109017 Moscow, Russia
$^8$ Dr.-Karl-Remeis-Sternwarte Bamberg & ECAP, Universit"at Erlangen-N"urnberg, Sternwartstr.~7, 96049~Bamberg, Germany

Although important effects of magnetic fields in massive stars are suggested by
recent models and observations, only a small number of massive O-type stars have
been investigated for magnetic fields until now. Additional observations are of
utmost importance to constrain the conditions which enable the presence of
magnetic fields and give first trends about their occurrence rate and field
strength distribution.

To investigate statistically whether magnetic fields in massive stars are
ubiquitous or appear in stars with specific spectral classification, certain
ages, or in a special environment, we acquired 41 new spectropolarimetric
observations for 36 stars. Among the observed sample roughly half of the stars
are probable members of clusters at different ages, whereas the remaining stars
are field stars not known to belong to any cluster or association.

Spectropolarimetric observations were obtained during three different nights
using the low-resolution spectropolarimetric mode of FORS,2 (FOcal Reducer low
dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. To assess
the membership in open clusters and associations, we used astrometric catalogues
with the best currently available kinematic and photometric data.

A field at a significance level of 3$sigma$ was detected in ten O-type stars.
Importantly, the largest longitudinal magnetic fields were measured in two Of?p
stars: $langle$$B_z$$rangle$,=,$-$381$pm$122,G for CPD$-$28,2561 and
$langle$$B_z$$rangle$,=,$-$297$pm$62,G for HD,148937, previously detected
by us as magnetic. The obtained observations of HD,148937 on three different
nights indicate that the magnetic field is slightly variable. Our new
measurements support our previous conclusion that large-scale organized magnetic
fields with polar field strengths in excess of 1,kG are not widespread among
O-type stars. Among the stars with a detected magnetic field, only one star,
HD,156154, belongs to an open cluster at high membership probability. According
to previous kinematic studies, four magnetic O-type stars in the sample are
well-known candidate runaway stars.

Reference: to be published in A&A
Status: Manuscript has been accepted

Weblink: http://arxiv.org/abs/1102.2503


Email: mschoell@eso.org