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\IBVShead{6019}{10 April 2012}

\IBVStitletl{Magnetic field and spectral variability}{ of the O\lowercase{f}?\lowercase{p} star CPD$-$28\,2561}

\IBVSauth{Hubrig, S.$^1$; Kholtygin, A.$^2$; Sch\"oller, M.$^3$; Langer, N.$^4$; Ilyin, I.$^1$; Oskinova, L.$^5$}

\IBVSinst{Leibniz-Institut f\"ur Astrophysik Potsdam (AIP), An der Sternwarte~16, 14482~Potsdam, Germany}
\IBVSinst{Astronomical Institute, Saint-Petersburg State University, Saint-Petersburg, Russia}
\IBVSinst{European Southern Observatory, Karl-Schwarzschild-Str.~2, 85748~Garching, Germany}
\IBVSinst{Argelander-Institut f\"ur Astronomie, Universit\"at Bonn, Auf dem H\"ugel~71, 53121~Bonn, Germany}
\IBVSinst{Institut f\"ur Physik und Astronomie, Universit\"at Potsdam, 14476~Potsdam, Germany}

\SIMBADobjAlias{CPD-28 2561}{CD-28 5104}
\IBVSkey{massive stars, magnetic field, spectropolarimetry}

\IBVSabs{European South Observatory Very Large Telescope observations using}
\IBVSabs{FORS 2 in spectropolarimetric mode reveal the presence of a variable}
\IBVSabs{magnetic field and spectral variability in the Of?p star CPD-28 2561}
\IBVSabs{Only five Galactic Of?p stars are presently known: HD 108, NGC 1624-2,}
\IBVSabs{CPD-28 2561, HD 148937, and HD 191612. After our previous detection}
\IBVSabs{of a magnetic field in the Of?p star HD 148937 using FORS 1 at the}
\IBVSabs{VLT, it is the second Of?p star with a detected magnetic field of} 
\IBVSabs{the order of a few hundred Gauss. The obtained low-resolution FORS 2}
\IBVSabs{polarimetric spectra allowed us to detect clear variability of several}
\IBVSabs{hydrogen and helium lines on a rather short time scale.}

\begintext

\vspace*{-1cm}

Walborn (1973)
introduced the Of?p category for massive O stars displaying recurrent spectral variations in 
certain spectral lines, sharp emission or P~Cygni profiles in He\,I and the Balmer lines, and strong C\,III 
emission lines around 4650\,\AA{}.
Only five Galactic Of?p stars are presently known: HD\,108, NGC\,1624-2, CPD$-$28\,2561, HD\,148937, 
and HD\,191612 
(Walborn et al.\ 2010). Using the high-resolution ESPaDOnS spectropolarimeter, 
installed on the 3.6-m Canada-France-Hawaii Telescope, and the NARVAL spectropolarimeter
at the Bernard Lyot telescope, mean longitudinal magnetic fields 
of the order of a few hundred gauss were detected in two Galactic Of?p stars, HD\,191612 (Donati et al.\ 2006)
and HD\,108 (Martins et al.\ 2010).
The detection of a mean longitudinal magnetic field $\langle$$B_z$$\rangle$\,=\,$-$254$\pm$81\,G
in the third Of?p star HD\,148937 using FORS\,1 at the VLT was previously 
reported by Hubrig et al.\ (2008) and was later confirmed by additional observations
by Hubrig et al.\ (2011) and Wade et al.\ (2012).
Our observations of the fourth Of?p star, CPD$-$28\,2561, in 2010 with FORS\,2 (Hubrig et al.\ 2011) 
enabled us to detect a magnetic field at a significance level of more than 3$\sigma$ in a single observation.
A successful attempt has been made to measure the magnetic field in the fifth  Of?p star, 
NGC\,1624-2 (Wade et al., in preparation).

Due to the relative faintness of the Of?p star CPD$-$28\,2561 with $m_{\rm V}=10.1$,
it was only scarcely studied in the past. Levato et al.\ (1988) acquired radial velocities
of 35 OB stars with carbon, nitrogen, and oxygen anomalies and found variability 
of a few emission lines with a probable period of 17 days.  
Walborn et al.\ (2010) mentioned that CPD$-$28\,2561 undergoes extreme spectral transformations very similar 
to those of HD\,191612, on a timescale of weeks, inferred from the variable emission intensity 
of the C\,III $\lambda\lambda$,~4647-4650-4652 triplet. 

In this work we report on our new observations obtained in 2011 using FORS\,2 at the VLT
indicating the presence of a variable
mean longitudinal magnetic field.
Polarimetric spectra were obtained with the GRISM~600B and 
the narrowest slit width of 0$\farcs$4 to achieve 
a spectral resolving power of $R\sim2000$. The use of the mosaic detector made of 
blue optimized E2V chips and a  pixel size of 15\,$\mu$m allowed us to cover a large
spectral range, from 3250 to 6215\,\AA{}, which includes all hydrogen Balmer lines 
from H$\beta$ to the Balmer jump. 
More details on the observing technique with FORS\,1/2 and
the data reduction can be found for example in Hubrig et al.\ (2004) and
references therein. Using the method described in the same place,
we obtained the mean longitudinal magnetic field
$\left<B_z\right>$, which is
the component of the magnetic field parallel to the line of sight,
averaged over the visible stellar hemisphere, weighted by the local emergent spectral line
intensity. It is diagnosed from the slope of a linear regression
of $V/I$ versus the quantity $-\frac{g_{\rm eff}e}{4\pi{}m_ec^2}
\lambda^2 \frac{1}{I} \frac{{\mathrm d}I}{{\mathrm d}\lambda}
\left<B_z\right> + V_0/I_0$, where $V$ is the Stokes parameter
that measures the circular polarization, $I$ is the intensity
observed in unpolarized light, $g_{\rm eff}$ is the effective
Land\'e factor, $e$ is the electron charge, $\lambda$  is the
wavelength, $m_e$ is the electron mass, $c$ is the speed of light,
${{\rm d}I/{\rm d}\lambda}$ is the derivative of Stokes~$I$,
$V_0/I_0$ is a constant, and $\left<B_z\right>$ is the mean
longitudinal magnetic field.
The results of our measurements are presented in Table~\ref{tab:fields}.
Longitudinal magnetic fields were measured in two ways: using the entire spectrum including 
all available absorption lines (Column 2) and
using only the absorption hydrogen Balmer lines (Column 3).
The lines that show evidence of emission were not used to determine the magnetic field strength. 

\begin{table*}
\caption{
Longitudinal magnetic fields measured with FORS\,2 in the Of?p star CPD$-$28\,2561.
All quoted errors are 1$\sigma$ uncertainties.
}
\label{tab:fields}
\vspace*{0.1cm}
\centering
\begin{tabular}{cr @{$\pm$} lr @{$\pm$} l}
\hline
\hline
\multicolumn{1}{c}{MJD} &
\multicolumn{2}{c}{$\left< B_z\right>_{\rm all}$} &
\multicolumn{2}{c}{$\left< B_z\right>_{\rm hydr}$} \\
 &
\multicolumn{2}{c}{[G]} &
\multicolumn{2}{c}{[G]} \\
\hline
55338.969 &  $-$381 & 122 & $-$534 & 167 \\
55685.982 &      99 &  82 &     65 & 108 \\
55686.984 &   $-$44 &  80 &      6 & 102 \\
55687.980 &     269 &  81 &    281 &  90 \\
\hline
\end{tabular}
\end{table*}

Our spectropolarimetric observations of this star indicate that the magnetic field is variable,
but owing to the small number of 
measurements it is not possible to estimate the magnetic/rotation period. The new measurements
obtained on May 5 and May 6 2011 are close to zero, while for the measurement carried out
on May 7 2011
we achieve a 3~$\sigma$ detection. This behaviour of the magnetic field can probably be explained
by the strong geometric dependence of
the longitudinal magnetic field, i.e. various orientation
of the field at different rotation phases.
 
Spectropolarimetric observations
on three consecutive nights in 2011 reveal strong variations in both the longitudinal
magnetic field strength and several hydrogen and helium line profiles.
The variation of He\,I $\lambda$4026, He\,I $\lambda$5876, and of 
H$\beta$ on such a short time scale were not reported in the literature yet.
A few examples of the detected spectral line profile variations 
are presented in Fig.~1.
Apart from  the He\,I $\lambda$4026 line, the contribution of a variable emission component
is well visible in the other three lines.
The  profile of the He\,I $\lambda$4026 line appears only weakly variable. These variations 
have an amplitude of about 1--2\% of the continuum level and can likely be explained by 
rotation modulation.

The line He\,II $\lambda$4686 exhibited strong emission during our observations in 2010,
which transforms to a much weaker P Cygni line profile with a weak blue shifted absorption in 2011.
A contribution of the emission component is clearly seen in the profile of the He\,I $\lambda$5876
line in 2010, but its presence can only be suspected in the variable line depth observed
in 2011.
A strong emission component is well visible in the line center of H$\beta$ in 2010. 
The shape of the line profile of H$\beta$ is similar to the behaviour of the 
H$\alpha$ line profile in the spectra
of 19\,Cep presented in Fig.~13 of Kaper et al.\ (1997) and in Fig.~1
of Kholtygin et al.\ (2003).

 
{\normalsize {\bf Acknowledgments:} Based on VLT observations in visitor mode (ESO programme 087.D-0049(A)).}

\vspace*{-0.5cm}
\newpage
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\endreferences

\IBVSfig{0.6\textwidth}{6019-f1.eps}{ 
Distinct profile variability observed in the He\,I $\lambda$4026 (top left),  He\,II $\lambda$4686
(top right), H$\beta$ (bottom left), and  He\,I $\lambda$5876 (bottom right) lines.
}

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