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\IBVShead{5841}{14 July 2008}

\IBVStitle{Evidence for short-term variations in two O-type stars}

\IBVSauth{De Becker, M.$^{1,2}$; Linder, N.$^{1}$; Rauw, G.$^{1,2}$}

\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: debecker@astro.ulg.ac.be}

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

\SIMBADobjAlias{BD +55 534}{HD 13268}
\SIMBADobjAlias{BD +51 579}{HD 15137}
\IBVSkey{star: spectroscopy - stars: early-type - stars: variables}
\IBVSabs{Only a few O-type stars are known to display short-term}
\IBVSabs{(a fraction of a day) variations. Intense spectroscopic monitoring}
\IBVSabs{of the ON8V star HD 13268 revealed low amplitude variations}
\IBVSabs{with periods of several hours. In addition, observations}
\IBVSabs{of the SB1 system HD 15137 revealed variations on a time} 
\IBVSabs{scale of a few hours. We consider these stars to be a good candidates}
\IBVSabs{for non radial pulsations, even though variations related to}
\IBVSabs{inhomogeneities in a circumstellar disk can not be rejected.}

\begintext

The study of spectral variability of O-type stars led, in a few cases, to the discovery of variations with periods of a fraction of a day (see e.g. $\zeta$\,Pup, Baade 1991, and $\zeta$\,Oph, Kambe et al.\ 1997). These variations are often considered to be the signature of non-radial pulsations. More recently, Rauw et al.\ (2008) investigated in detail the case of HD\,93521 and reported on variations on time scales of 1.75 and 2.89\,h. As only a handful of O-type stars displaying short-term variations are known, any new detection constitutes as significant improvement of the catalog of short-term varying massive stars.\\



In this paper, we present the results of a spectroscopic monitoring of two O-type stars. HD\,13268 is an ON8V star that belongs to the Per\,OB1 association. HD\,15137 (O9.5V) is a runaway SB1 system (P\,$\sim$\,30\,d) whose preliminary orbital parameters have been published by Boyajian et al.\,(2005) and McSwain et al.\,(2007). The former authors suggested that this star may have been ejected from the nearby cluster NGC\,654 in the Perseus spiral arm. Both stars display broad absorption lines due to a rather high rotational velocity. Their projected rotational velocities are respectively estimated to be equal to 302\,km\,s$^{-1}$ (Penny\,1996) and 178\,km\,s$^{-1}$ (Conti \& Ebbets\,1977).

 Our analysis is based on data collected with the Aur\'elie spectrograph at the 1.52-m telescope of the Observatoire de Haute Provence (OHP, France). In the case of HD\,13268, the time series is constituted of 62 spectra obtained between October 2004 and November 2007, but most of the data were obtained during 4 nights in autumn 2007 (see Table\,1). We observed HD\,15137 44 times during the same epoch, with 12 spectra obtained on the same night with a timespan of about 6 hours (on 2006, October 26th, see Table\,2). Our spectra covered wavelengths between 4460 and 4890\,\AA\,, with a resolving power of about 8000. Exposure times were of the order of 25--45 minutes depending on the sky conditions. The signal-to-noise of our spectra -- estimated in regions devoid of spectral lines -- was higher than 200. The data were reduced following the procedure described by Rauw \& De Becker\,(2004).

We applied the Time Variance Spectrum (TVS) analysis technique as described by Fullerton et al.\,(1996) to our spectral time series. In the case of HD\,13268, we detected a significant -- although weak -- variability in the profile of He\,{\sc i} $\lambda$ 4471, He\,{\sc i} $\lambda$$\lambda$ 4542,4686, and H\,$\beta$. This variability was present both in the complete data set, and in data sets of individual nights during which several spectra were collected. We analyzed our time series using the generalized Fourier technique of Heck et al.\,(1985) and revised by Gosset et al.\,(2001). This technique is especially adapted to the case of unequally spaced data. The power spectra are shown in Fig.\,1 for He\,{\sc ii} $\lambda$ 4686 and H\,$\beta$. Our results point to potential variability time scales of 14.5 and 6.7\,h respectively for the two lines, even though significant residuals are still present after prewhitening (see middle panels of Fig.\,1). Several factors are likely to contribute to these residuals: (i) the reported time scales may be incorrect (aliasing, ...), (ii) the low amplitude of the variations makes our temporal analysis very sensitive to noise, and (iii) more than one unidentified time scale may contribute to the detected variations. The studies of the other few examples of short-term varying O-type stars suggest indeed that multiperiodic variations are occurring. 

\vskip 5mm
\begin{center}
\normalsize
Table 1. Journal of the observations of HD\,13268. The heliocentric Julian date at mid-exposure is given as HJD - 2\,450\,000, and the date (yyyy/mm/dd) is that of the beginning of the night.
\vskip 3mm
\begin{tabular}{lcc|lcc|lcc}
\hline
\# &  HJD & Date & \# &  HJD & Date & \# &  HJD & Date \\
\hline
1 & 3286.512 & 2004/10/07 & 22 & 4396.484 & 2007/10/22 & 43 & 4421.454 & 2007/11/16 \\
2 & 3289.595 & 2004/10/10 & 23 & 4396.505 & 2007/10/22 & 44 & 4421.475 & 2007/11/16 \\
3 & 3290.490 & 2004/10/11 & 24 & 4396.551 & 2007/10/22 & 45 & 4421.499 & 2007/11/16 \\
4 & 3294.634 & 2004/10/15 & 25 & 4396.574 & 2007/10/22 & 46 & 4421.549 & 2007/11/16 \\
5 & 3295.482 & 2004/10/16 & 26 & 4396.595 & 2007/10/22 & 47 & 4421.573 & 2007/11/16 \\
6 & 3295.672 & 2004/10/16 & 27 & 4407.364 & 2007/11/02 & 48 & 4421.598 & 2007/11/16 \\
7 & 3296.597 & 2004/10/17 & 28 & 4407.381 & 2007/11/02 & 49 & 4422.356 & 2007/11/17 \\
8 & 3648.620 & 2005/10/04 & 29 & 4407.399 & 2007/11/02 & 50 & 4422.373 & 2007/11/17 \\
9 & 3652.586 & 2005/10/08 & 30 & 4407.418 & 2007/11/02 & 51 & 4422.388 & 2007/11/17 \\
10 & 3654.456 & 2005/10/10 & 31 & 4407.436 & 2007/11/02 & 52 & 4422.402 & 2007/11/17 \\
11 & 3982.621 & 2006/09/03 & 32 & 4407.454 & 2007/11/02 & 53 & 4422.439 & 2007/11/17 \\
12 & 3984.575 & 2006/09/05 & 33 & 4407.473 & 2007/11/02 & 54 & 4422.454 & 2007/11/17 \\
13 & 4034.408 & 2006/10/25 & 34 & 4407.490 & 2007/11/02 & 55 & 4422.469 & 2007/11/17 \\
14 & 4034.466 & 2006/10/25 & 35 & 4407.553 & 2007/11/02 & 56 & 4422.484 & 2007/11/17 \\
15 & 4034.524 & 2006/10/25 & 36 & 4407.570 & 2007/11/02 & 57 & 4422.516 & 2007/11/17 \\
16 & 4034.585 & 2006/10/25 & 37 & 4407.691 & 2007/11/02 & 58 & 4422.533 & 2007/11/17 \\
17 & 4035.386 & 2006/10/26 & 38 & 4421.613 & 2007/11/16 & 59 & 4422.557 & 2007/11/17 \\
18 & 4396.369 & 2007/10/22 & 39 & 4421.350 & 2007/11/16 & 60 & 4422.570 & 2007/11/17 \\
19 & 4396.392 & 2007/10/22 & 40 & 4421.370 & 2007/11/16 & 61 & 4422.594 & 2007/11/17 \\
20 & 4396.414 & 2007/10/22 & 41 & 4421.393 & 2007/11/16 & 62 & 4422.608 & 2007/11/17 \\
21 & 4396.462 & 2007/10/22 & 42 & 4421.433 & 2007/11/16 &  &  &  \\
\hline
\end{tabular}
\end{center}


\vskip 5mm
\begin{center}
\normalsize
Table 2. Journal of the observations of HD\,15137.
\vskip 3mm
\begin{tabular}{lcc|lcc|lcc}
\hline
\# &  HJD & Date & \# &  HJD & Date & \# &  HJD & Date \\
\hline
1 & 3652.642 & 2005/10/08 & 16 & 4035.545 & 2006/10/26 & 31 & 4409.528 & 2007/11/04 \\
2 & 3654.538 & 2005/10/10 & 17 & 4035.568 & 2006/10/26 & 32 & 4410.475 & 2007/11/05 \\
3 & 3980.585 & 2006/09/01 & 18 & 4035.588 & 2006/10/26 & 33 & 4411.551 & 2007/11/06 \\
4 & 3982.552 & 2006/09/03 & 19 & 4035.611 & 2006/10/26 & 34 & 4412.504 & 2007/11/07 \\
5 & 3984.545 & 2006/09/05 & 20 & 4035.632 & 2006/10/26 & 35 & 4413.584 & 2007/11/08 \\
6 & 4033.406 & 2006/10/24 & 21 & 4035.655 & 2006/10/26 & 36 & 4414.530 & 2007/11/09 \\
7 & 4033.495 & 2006/10/24 & 22 & 4396.438 & 2007/10/22 & 37 & 4415.546 & 2007/11/10 \\
8 & 4033.602 & 2006/10/24 & 23 & 4397.516 & 2007/10/23 & 38 & 4416.453 & 2007/11/11 \\
9 & 4034.670 & 2006/10/25 & 24 & 4400.646 & 2007/10/26 & 39 & 4417.467 & 2007/11/12 \\
10 & 4035.413 & 2006/10/26 & 25 & 4401.598 & 2007/10/27 & 40 & 4418.383 & 2007/11/13 \\
11 & 4035.437 & 2006/10/26 & 26 & 4402.588 & 2007/10/28 & 41 & 4419.412 & 2007/11/14 \\
12 & 4035.458 & 2006/10/26 & 27 & 4405.599 & 2007/10/31 & 42 & 4421.525 & 2007/11/16 \\
13 & 4035.480 & 2006/10/26 & 28 & 4406.590 & 2007/11/01 & 43 & 4422.421 & 2007/11/17 \\
14 & 4035.502 & 2006/10/26 & 29 & 4407.532 & 2007/11/02 & 44 & 4423.322 & 2007/11/18 \\
15 & 4035.524 & 2006/10/26 & 30 & 4408.591 & 2007/11/03 & & & \\
\hline
\end{tabular}
\end{center}

In the case of HD\,15137, our sampling of high frequencies is rather poor as this star was intensively observed during only one night. However, the TVS indicates a significant line profile variability of at least He\,{\sc i} $\lambda$ 4471 and H\,$\beta$, i.e. the strongest absorption lines in the blue spectrum, during that particular night. The comparison of the line profiles obtained during a same night reveals indeed variations from one spectrum to the other, with time intervals of the order of 30--40 minutes between two consecutive observations. These variations can be seen in the line profiles plotted in the left part of Fig.\,2. We note that it is unlikely that these variations be due to the orbital motion of the SB1 as its period is of the order of 30\,d, i.e. much longer than the time scales investigated during a single night. The right part of Fig.\,2 shows the power spectrum of the complete time series for H\,$\beta$. The wavelength position of the line profile has been corrected for the orbital motion before computing the power spectrum. The corrections were calculated on the basis of a SB1 orbital solution computed from our data and the radial velocities published by Boyajian et al.\,(2005) and McSwain et al.\,(2005), using the same method as De Becker et al.\,(2006). We note the presence of a family of peaks close to 2\,d$^{-1}$. Some power is also found at higher frequencies (see for instance a low amplitude family of peaks whose presence is suggested around 10\,d$^{-1}$). A much better sampling of high frequencies is however needed in order to clarify the situation and propose valuable values for the short variability time scale(s). We note that Boyajian et al.\,(2005) already suggested the occurrence of short-term variations for this star.

\IBVSfig{6.5cm}{5841-f1.eps}{Fourier analysis of He\,{\sc ii} $\lambda$ 4686 between 4682 and 4685\,\AA\, ({\it left part}) and H\,$\beta$ between 4857 and 4860\,\AA\, ({\it right part}) in the case of HD\,13268. {\it Upper panels:} mean power spectrum over the specified wavelength domain. {\it Middle panels:} prewhitened power spectrum using the specified frequency corresponding to the highest peak in the upper panel. Significant residual power is still present, but a much better sampling of higher frequencies is requested in order to determine accurately the variability time scales. {\it Lower panels:} spectral window related to the data sampling.}
 \IBVSfigKey{5841-f1.eps}{HD 13268}{periodogram}

\IBVSfig{7.0cm}{5841-f2.eps}{{\it Left part:} Line profiles of He\,{\sc i} $\lambda$ 4471 and H\,$\beta$ in the case of HD\,15137. The selected spectra are those obtained during a single night (number \#10 to \#21 in Table\,2, from the top to the bottom). {\it Right part:} Fourier analysis of the complete time series (44 spectra) for H\,$\beta$ after correction for the SB1 orbital motion. We note that some residual power due to the orbital motion may still be present at low frequencies. The power spectrum suggests however the presence of frequencies likely related to time scales of a fraction of a day.}
 \IBVSfigKey{5841-f2.eps}{HD 15137}{periodogram}

Considering (i) the low amplitude of the variations and (ii) their rather high frequency, intensive monitoring with large collecting area telescopes is really needed if one wants to investigate the short term behaviour of these stars. Typically, several complete nights on 4-m class telescopes, using rather high resolving power (at least 20000), should be devoted to these targets in order to characterize their short term variations in a way similar to that of HD\,93521 (Rauw et al.\,2008) or $\zeta$\,Oph (Kambe et al.\,1997).

In summary, we report on the detection of significant variations on time scales of a fraction of a day in the blue spectrum of two late-type main-sequence O stars: HD\,13268 and HD\,15137. The frequency sampling of our time series did not allow us to determine the variability time scale, but we claim that these stars should be considered as very valuable targets for future studies aiming at investigating rapid variations in O-type stars. Such variations may be the signature of non-radial pulsations, or of structures related to circumstellar rotating material. Intensive high spectral resolution spectroscopic campaigns are needed to investigate such a behaviour.


{\bf Acknowledgements.} MD would like to thank Dr Hugues Sana for preparing and providing the orbital solution package. 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

Baade, D., 1991, in {\it ESO Workshop on Rapid Variability of OB--Stars: Nature and Diagnostic Value}, ed. D. Baade, 21 \BIBCODE{1991rvos.conf...21B}

Boyajian, T.S., Beaulieu, T.D., Gies, D.R., et al., 2005, {\it ApJ}, {\bf 621}, 978

Conti, P.S.\& Ebbets, D., 1977, {\it ApJ}, {\bf 213}, 438

De Becker, M., Rauw, G., Manfroid, J., Eenens, P., 2006, {\it A\&A}, {\bf 456}, 1121

Kambe, E., Hirata, R., Ando, H., et al., 1997, {\it ApJ}, {\bf 481}, 406

McSwain, M.V., Boyajian, T.S., Grundstrom, E.D., Gies, D.R., 2007, {\it ApJ}, {\bf 655}, 473

Penny, L.R., 1996, {\it ApJ}, {\bf 463}, 737

Rauw, G. \& De Becker, M., 2004, {\it A\&A}, {\bf 421}, 693

Rauw, G., De Becker, M., van Winkel, H., et al., 2008, {\it A\&A}, in press

\endreferences

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\IBVSetable{5841-t1.tex}

\IBVSecomment{Electronic version of Table 1., with data file links}

\end{document}
