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\IBVShead{5773}{23 May 2007}

\IBVStitle{Long-term Spectroscopic Variability of Two O\lowercase{e} Stars}

\IBVSauth{Rauw, G.$^{1,2}$; Naz\'e, Y.$^{1,2}$; Marique, P.X.$^1$; De Becker, M.$^{1,2}$; Sana, H.$^3$; Vreux, J.-M.$^1$}

\IBVSinst{Institut d'Astrophysique et de G\'eophysique, Universit\'e de Li\`ege, All\'ee du 6 Ao\^ut, B\^at B5c, 4000
Li\`ege, Belgium, e-mail: rauw@astro.ulg.ac.be}

\IBVSinst{Fonds National de la Recherche Scientifique, Belgium}

\IBVSinst{European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago 19, Chile}

\SIMBADobjAlias{PZ Gem}{HD 45314}
\SIMBADobjAlias{BN Gem}{HD 60848}
\IBVSkey{star: spectroscopy -- stars: early-type -- stars: variables}
\IBVSabs{HD 45314 and HD 60848 are poorly studied Oe-type stars.}
\IBVSabs{Spectroscopic monitoring over 5 years reveals strong variations}
\IBVSabs{in the strength of the emission lines and for HD 45314 in the}
\IBVSabs{V/R ratio of the double-peaked emissions. Part of these long-term}
\IBVSabs{variations could be recurrent.}

\begintext

The Oe spectral category was first introduced by Conti \& Leep (1974) to classify those O-stars exhibiting emission in
the hydrogen Balmer lines, but not in He\,{\sc ii} $\lambda$\,4686 nor N\,{\sc iii} $\lambda\lambda$\,4634-40. These
objects are quite rare (see e.g.\ Negueruela et al., 2004) and most of them have not been studied in detail. Oe stars
have rather large rotational velocities and their emission lines frequently display a double-peaked morphology. As for
Be stars, these emission lines are interpreted as the signature of a circumstellar disk of matter expelled by the star.
Oe stars are thus believed to represent the earliest representatives of the Be phenomenon. Indeed, Negueruela et al.\
(2004) argued that many Oe stars had previously been classified too early because of the infilling of He\,{\sc i}
classification lines.

In this paper, we present the results of a spectroscopic monitoring of HD\,45314 and HD\,60848, which have been
reclassified as B0\,IVe and O9.5\,IVe respectively by Negueruela et al.\ (2004). Spectra of these stars were collected
with the Aur\'elie spectrograph at the 1.52-m telescope of the Observatoire de Haute Provence (OHP, France) and echelle
spectra were taken with the FEROS instrument at the 1.5 and 2.2-m telescopes at La Silla (ESO, Chile; see Table 1). All
the data were reduced with the MIDAS software developed at ESO and with private routines designed for the specific
reduction of Aur\'elie and FEROS data. Special attention was paid to ensure a homogeneous normalisation of the spectra.

\vskip 5mm
\begin{center}
\normalsize
Table 1. Journal of the observations of HD\,45314 and HD\,60848
\vskip 3mm
\begin{tabular}{llcccccrr}
\hline
Epoch   &  Instrument & Resolving power & Wavelength range & \multicolumn{2}{c}{ Number of spectra} \\
& & & & HD\,45314 & HD\,60848 \\
\hline
Feb.\ 1997 & Aur\'elie & 20000 & 6510--6710 \AA & \hss11 & \hss 6 \\
Nov.\ 1998 & Aur\'elie & 30000 & 6500--6620 \AA & \hss 7 & \hss 6 \\
Nov.\ 1998 & Aur\'elie & 30000 & 4795--4925 \AA & \hss 1 & \hss 1 \\
May   1999 & FEROS     & 48000 & 3900--7100 \AA & \hss10 & \hss10 \\
May   2000 & FEROS     & 48000 & 3900--7100 \AA & \hss 4 & \hss 6 \\
Sep.\ 2000 & Aur\'elie & 10000 & 4460--4900 \AA & \hss 3 & \hss 2 \\
May   2001 & FEROS     & 48000 & 3900--7100 \AA & \hss 3 & \hss 3 \\
Sep.\ 2001 & Aur\'elie & 10000 & 6350--6770 \AA & \hss 3 & \hss - \\
Sep.\ 2001 & Aur\'elie & 10000 & 4460--4900 \AA & \hss 1 & \hss - \\
Mar.\ 2002 & FEROS     & 48000 & 3900--7100 \AA & \hss 3 & \hss 3 \\
\hline
\end{tabular}
\end{center}

In addition to the strong hydrogen Balmer emission lines (mainly H$\gamma$, H$\beta$ and H$\alpha$), the optical
spectrum of HD\,45314 displays double-peaked emission in many Fe\,{\sc ii} lines (e.g.\ $\lambda\lambda$\,5169,
5198, 5235, 5275, 5319, 5363, 6318, 6346, 6370, 6384\dots) as well as some He\,{\sc i} lines ($\lambda\lambda$\,5876,
6678, 7065 being the strongest ones). We further note the existence of weak (but definite) He\,{\sc ii} absorption lines
at $\lambda\lambda$\,4200, 4542, 4686 and 5412, but also some lines of C\,{\sc iii}, N\,{\sc iii} and Si\,{\sc iv}.
These features are broadly consistent with an O9.5-B0 spectral type. We note that Fremat et al.\ (2006) inferred $T_{\rm
eff} = 31092 \pm 557$~K and $\log{g} = 3.97 \pm 0.05$ for HD\,45314 which corresponds to an O9.5\,V spectral type, but
does not rule out a B0 classification.

The spectrum of HD\,60848 is dominated by emissions in H$\alpha$, H$\beta$, He\,{\sc i} $\lambda\lambda$\,5876,
6678 and 7072. During some campaigns, the emission lines (with the exception of H$\alpha$) appear shell-like with a
strong central absorption that reaches below the continuum level. There are a number of strong absorption lines,
including amongst others He\,{\sc i} $\lambda$\,4471 and He\,{\sc ii} $\lambda\lambda$\,4200, 4542, 4686 and 5412, as
well as lines of C\,{\sc iii}, C\,{\sc iv}, N\,{\sc iii}, O\,{\sc ii}, O\,{\sc iii}, Si\,{\sc iii} and Si\,{\sc iv}.
There is no indication of Fe\,{\sc ii} emissions with a strength comparable to those seen in the spectrum of HD\,45314.
 
We have analysed the variability of the various spectral features using the tools described by Rauw et al.\ (2001). All
emission lines were found to display significant variations. Here, we focus on the changes seen in the hydrogen Balmer
lines (see Figs.~1, 2) as well as the Fe\,{\sc ii} lines.

\IBVSfig{6.5cm}{5773-f1.ps}{Line profile variations of the H$\alpha$ and H$\beta$ emission lines of HD\,45314}
 \IBVSfigKey{5773-f1.ps}{HD 45314}{spectrum}

\IBVSfig{6.5cm}{5773-f2.ps}{Same as Fig.~1 but for the H$\alpha$ and H$\beta$ emission lines of HD\,60848}
 \IBVSfigKey{5773-f2.ps}{HD 60848}{spectrum} 


\IBVSfig{8.1cm}{5773-f3.ps}{Variations of the spectral characteristics of HD\,45314. {\it Left, top panel}: radial
velocities of the He\,{\sc ii} $\lambda$\,4686 absorption and average of the RVs of the violet and red peaks of the
H$\beta$ and Fe\,{\sc ii} $\lambda$\,5319 emissions. {\it Left, bottom panel}: equivalent width of the H$\alpha$ line as
a function of time as measured on our spectra. The filled square corresponds to the January 2002 measurement of
Negueruela et al.\ (2004). {\it Right}: $V/R = (I_V - I_c)/(I_R - I_c)$ ratio (where $I_V$ and $I_R$ are the intensities
of the violet and red peaks respectively and $I_c$ is the intensity of the continuum) of the H$\alpha$ and H$\beta$
lines. The top panels zoom in on those campaigns where significant trends were observed}
 \IBVSfigKey{5773-f3.ps}{HD 60848}{}

\vspace{-36pt}

HD\,45314 presents important variations of the strengths of its emission features: the equivalent width (EW) of the
H$\alpha$ emission increased from $\sim -20$ to $\sim -35$\,\AA\ between 1997 and 2002 (Fig.\,3). During our campaign,
the H$\alpha$ emission was hence much stronger than the EWs of $-7.4$ and $-4.7$\,\AA\ reported by Andrillat et al.\
(1982) and Andrillat (1983) from observations obtained in February 1981 and October 1981 respectively. The EW variations
obviously occur on time scales of more than five years and our data do not allow to detect any periodicity.
Simultaneously, we note prominent variations of the V/R ratio (see Fig.\,3). Significant variations of this ratio
sometimes occur over the typical duration of our observing campaigns (see the top panels of Fig.\,3) tentatively
suggesting a time scale of order a few months. The V/R variations of the H$\beta$ line are less clear cut, though they
qualitatively agree with the trends seen in H$\alpha$.  We have also measured the radial velocity of the He\,{\sc ii}
$\lambda$\,4686 absorption line. On average, we obtain $-2.9 \pm 11.2$\,km\,s$^{-1}$ with the RV increasing
progressively from a minimum of $-22.1$ to a maximum of $+22.3$\,km\,s$^{-1}$ between May 1999 and March 2002. The
violet and red peaks of the H$\beta$ and Fe\,{\sc ii} emissions also shift in RV with time, although it is not fully
clear whether these RV variations are correlated with those of the absorption line (see Fig.\,3).

HD\,60848 also displays strong variations of the strengths of its emission features. The EW of the H$\alpha$ emission
varies between $\sim -5.5$ and $\sim -14.5$\,\AA, with a maximum occurring between May 2000 and May 2001 (Fig.\,4). The
EW apparently increased at a rather slow rate between 1998 and 2001 and subsequently decreased dramatically back to its
initial level in 2002. It is interesting to note that a similar decrease in the H$\alpha$ EW from about $-17$ to
$-7$\,\AA\ was observed between early 1981 and early 1983 (Divan et al.\ 1983, Andrillat et al.\ 1982). This suggests
that the EW variations might be cyclic with a recurrence time of order five years. Contrary to HD\,45314, the V/R ratio
remains close to unity and displays no large variations (see Fig.\,4).  The radial velocity of the He\,{\sc ii}
$\lambda$\,4686 absorption line is found to be $22.7 \pm 6.2$\,km\,s$^{-1}$ on average with a minimum of $+13.1$ and a
maximum of $+41.3$\,km\,s$^{-1}$ with no clear trend during our campaign.

\IBVSfig{4.5cm}{5773-f4.ps}{Variations of the spectral characteristics of HD\,60848. {\it Left:} equivalent width of the
H$\alpha$ line as a function of time. The filled square corresponds to the May 2002 measurement of Negueruela et al.\
(2004). {\it Right:} V/R ratio of the H$\alpha$ and H$\beta$ lines}
 \IBVSfigKey{5773-f4.ps}{HD 60848}{}

\vspace{-12pt}

In summary, HD\,45314 and HD\,60848 both display strong long-term spectroscopic variations. Part of these variations
could be recurrent. Monitoring these stars over several months and/or several years could help to specify the origin of
the Oe phenomenon.

{\bf Acknowledgements.} The authors acknowledge the support from the FNRS (Belgium), the `Communaut\'e Fran\c caise'
(Belgium), as well as through the XMM and INTEGRAL PRODEX contract (Belspo).


\references

Andrillat, Y., 1983, {\it A\&AS}, {\bf 53}, 319

Andrillat, Y., Vreux, J.-M., Dennefeld, M., 1982, {\it IAU Symp.}, {\bf 98}, 229, Be Stars, eds.\ M.\ Jaschek \& 
H.-G.\ Groth

Conti, P.S., Leep, E.M., 1974, {\it ApJ}, {\bf 193}, 113

Divan, L., Zorec, J., Andrillat, Y., 1983, {\it A\&A}, {\bf 126}, L8

Fremat, Y., Zorec, J., Hubert, A.-M., Floquet, M., 2005, {\it A\&A}, {\bf 440}, 305

Negueruela, I., Steele, I.A., Bernabeu, G., 2004, {\it Astron.\ Nachr.}, {\bf 325}, 749

Rauw, G., Morrison, N.D., Vreux, J.-M., Gosset, E., Mulliss, C.L., 2001, {\it A\&A}, {\bf 366},
585

\endreferences

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