The Magnetic Field and Confined Wind of the Star
&theta 1 Orionis C
Wade, G.A.; Fullerton, A.W.; Donait, J.-F.; Landstreet, J.D.; Petit, P.; and Strasser, S.
Abstract
Aims. In this paper we confirm the presence of a globally-ordered, kG-strength magnetic field in the photosphere of the young O star
&theta 1 Orionis C, and examine the properties of its optical line profile variations.
Methods. A new series of high-resolution MuSiCoS Stokes V and I spectra has been acquired which samples approximately uniformly the
rotational cycle of &theta 1 Orionis C. Using the Least-Squares Deconvolution (LSD) multiline technique, we have succeeded in detecting variable
Stokes V Zeeman signatures associated with the LSD mean line profile. These signatures have been modeled to determine the magnetic field
geometry. We have furthermore examined the profile variations of lines formed in both the wind and photosphere using dynamic spectra.
Results. Based on spectrum synthesis fitting of the LSD profiles, we determine that the polar strength of the magnetic dipole component
is 1150 <=
Bd <=
1800 G and that the magnetic obliquity is 27o <~ &beta <~
68o, assuming i = 45 ± 20o. The best-fit values for i = 45o are
Bd = 1300 ± 150 (1&sigma) G and &beta = 50o +/- 6o (1&sigma). Our data confirm the previous detection of a magnetic field in this star, and furthermore
demonstrate the sinusoidal variability of the longitudinal field and accurately determine the phases and intensities of the magnetic extrema.
The analysis of "photospheric" and "wind" line profile variations supports previous reports of the optical spectroscopic characteristics, and
provides evidence for infall of material within the magnetic equatorial plane.
Summary
This paper is really just a confirmation of previous work by Donati et al of the existence of a kG
magnetic field on &theta 1 Ori C. The most interesting thing to take from this paper is
that they confirm the field using line variations at much longer wavelengths than previously.
Before, this work was done almost exclusively in the X-ray. This paper explores the visible and
UV ranges, with the spectra reaching down to &lambda = 6000 Angstroms (this corresponds to a peak
blackbody temperature of log(T) = 5.68 for &lambda = 6000 Angstroms).
Another useful note from this paper is that it appears customary to set the phase angle equal to
zero when the magnetic equator is face on. This should be useful for comparing data later on.
Note from David:
It's important to realize that the actual magnetic field measurements
are separate from the line-profile variability related to circumstellar
matter (these lines generally can't be the same, since lines like
H-alpha that are related to circumstellar matter will be broad and
magnetic field measurements require narrow - photospheric - lines -- see
Figs. 5 and 6 vs. Fig. 7). The magnetic field, Zeeman splitting
measurements always involve optical spectropolarimetry (spectroscopy on
the different components of polarized light) - they're not made
(primarily) from the "Stokes I" profiles shown in Fig. 7. I agree with
your sentiment that the magnetic field info in this paper is really just
confirmation and minor tweaking (though the field is now stronger). To
me, the more interesting part is the evidence for more or less
continuous infall, and the very sensitive measurements of
phase-dependent structures in the lines that show this infall. So,
these represent very stringent quantitative measurements that our MCWS
model should be able to reproduce, if it's right. The purpose of your
project is to make this comparison as realistically and quantitatively as possible.
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