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\IBVShead{5881}{23 March 2009}

\IBVStitle{HD~190336 a new $\beta$ C\lowercase{ep} star}

\IBVSauth{Jurcsik,~J.$^1$; S\'odor,~\'A.$^1$; Hajdu, G.$^2$}

\IBVSinst{Konkoly Observatory of the Hungarian Academy of Sciences, P.O. Box 67, H-1525 Budapest, Hungary; e-mail: name@konkoly.hu}
\IBVSinst{E\"otv\"os Lor\'and University, Department of Astronomy, P.O. Box 32, H-1518 Budapest, Hungary}

\SIMBADobj{HD 190336}

\IBVStyp{ beta Cep }
\IBVSkey{Variable stars, beta Cep}
\IBVSabs{HD 190336 (B0.7 II-III) is shown to be a multiperiodic beta Cep variable}
\IBVSabs{based on the publicly available HAT-5, Integral/OMC, and Hipparcos data.}

\begintext
HD~190336 ($m_V \approx 8\fmm6, \alpha_{\rm{2000.0}}=20\hr03\mm18\fsec68, \delta_{\rm{2000.0}}=+33\deg26\arcm59\farcs7 $) was found to be an unsolved new variable with $f=4.45732\,\mathrm{cd}^{-1}$ frequency and 0.0247 mag amplitude by Koen and Eyer (2002) during the revision of the epoch photometry data of the Hipparcos catalogue. This star lies in the field that was observed by the HAT-5 telescope in June and July 2003 (Hartman et al. 2004). Based on these data the HATNET variability survey classified HD~190336 as a pulsating variable with $P=0.2244$\,d period ($f=4.45633 \,\mathrm{cd}^{-1}$). Although the HAT-5 light curve of  HD~190336  shows it definitely that the star is not monoperiodic (Fig.~1), no detailed analysis of the available photometric data of HD~190336 has been performed previously. Photometric time series of HD~190336 was also observed with the Optical Monitoring Camera (OMC, Mas-Hesse et al., 2003) on board the Integral satellite (Winkler et al., 2003). The FITS format OMC data were converted into ASCII tables using the OMC2ASCII program as described in Sokolovsky (2007).

Table~1 summarises the log of the photometric data available for HD~190336.


\begin{table}[!h]
\centerline{{\bf Table~1.} Photometric observations of HD~190336}

\begin{center}
\begin{tabular}{lccc}
\hline
Source & Observation period [JD]  & number of data$^*$  & filter  \\

\hline
Hipparcos& 2\,447\,894--2\,449\,046&138& $Hip$   \\
OMC & 2\,452\,595--2\,453\,961&296& $V$    \\
HAT-5    & 2\,452\,800--2\,452\,830&756& $I_C$  \\
\hline
\multicolumn{4}{l}{\footnotesize{The most deviant data points are omitted}}\\
\end{tabular}
\end{center}
%\vspace*{-0.6cm}
\end{table}


The spectral type of HD~190336 was classified to be B0.7 II-III (Walborn, 1971), its empirical temperature calibrations gave $\log T_{\mathrm{eff}}=4.37$ (Gulati et al., 1989). Based on its period, amplitude, shape and variability of the light curve, the star can be classified as a new $\beta$ Cep variable. Although all these data were available previously, HD~190336 was not listed as a candidate $\beta$ Cep star in the recent catalogue of galactic  $\beta$ Cep stars (Stankov \& Handler, 2005).


The Fourier spectrum of the HAT-5 data shows 6 distinct frequencies in the  4--5\,cd$^{-1}$ frequency range, forming two equidistant triplets with $\Delta f= 0.135 \,\mathrm{cd}^{-1}$ spacing. Three combination frequency components can also be detected. Cleaned spectra of the HAT-5 observations of HD~190336 are shown in Fig.~2. (See Roberts et al., 1987, on the application of the Clean algorithm on Fourier amplitude spectra of variable star time series.) The positions of the equidistant triplets and their linear combination frequencies are marked in Fig.~2. The detected frequencies and their Fourier amplitudes and phases are listed in Table~2. Frequency triplets and doublets with the same frequency spacing were detected in other $\beta$ Cep variables also (see e.g. Handler, 2005). A trivial explanation of equidistant triplets is rotational splitting, however e.g., Handler et al. (2006) interpreted the equidistant spacing of the triplet frequencies detected in the spectrum of 12 Lac as accidental because the components were found to belong to different order $l$ modes.

\IBVSfig{9.7cm}{5881-f1.eps}{HAT-5 light curve of HD~190336. }
\IBVSfigKey{5881-f1.eps}{HD~190336}{light curve}

\IBVSfig{9.5cm}{5881-f2.eps}{Clean spectra of the HAT-5 observations of HD~190336. Marks show the positions of equidistant triplets with $0.135 \,\mathrm{cd}^{-1}$ spacing, and the exact positions of the linear combination frequencies.}
\IBVSfigKey{5881-f2.eps}{HD~190336}{}

\begin{table}[!h]
\centerline{{\bf Table~2.} Frequencies detected in the HAT-5 data of HD~190336}

\begin{center}
\begin{tabular}{lccc}
\hline
&frequency & amplitude / error & phase$^*$  / error \\
& cd$^{-1}$& mag&rad\\
\hline
$f_1\approx f_2-\Delta f$ &4.1894& 0.0083 \,\,  0.0003   & 4.04 \,\,  0.08\\
$f_2$              &4.3269& 0.0134 \,\,  0.0003   & 0.93 \,\,  0.05\\
$f_3\approx f_2+\Delta f$ &4.4576& 0.0245 \,\,  0.0003   & 3.94 \,\,  0.03\\
$f_4\approx f_5-\Delta f$ &4.5347& 0.0064 \,\,  0.0003   & 3.89 \,\,  0.10\\
$f_5$              &4.6665& 0.0125 \,\,  0.0003   & 4.37 \,\,  0.05\\
$f_6\approx f_5+\Delta f$ &4.8074& 0.0054 \,\,  0.0003   & 6.17 \,\,  0.12\\

$f_7\approx f_1+f_6=f_2+f_5=f_3+f_4$     &8.9908& 0.0028 \,\,  0.0004 &  3.50  \,\, 0.24\\
$f_8\approx f_3+f_5=f_2+f_6      $     &9.1361& 0.0017 \,\,  0.0004 &  3.37  \,\, 0.39\\
$f_9\approx f_4+f_5  $               &9.2040& 0.0018 \,\,  0.0004 &  3.44  \,\, 0.36\\

\hline
\multicolumn{4}{l}{\footnotesize{$^*$ Initial epoch $ T_0=2\,452\,800.0$ }}\\
\end{tabular}
\end{center}
%\vspace*{-0.6cm}
\end{table}



\IBVSfig{9.5cm}{5881-f3.eps}{Hipparcos, Integral OMC and HAT-5 light curves of HD~190336 folded with the three main frequencies found in the data. In each plot the the signals belonging to the other frequencies has been removed.}
\IBVSfigKey{5881-f3.eps}{HD 190336}{light curve}


The Hipparcos and OMC data of HD~190336 make it also possible to investigate the stability of the detected frequencies on longer time scale. Pigulski \& Pojmanski (2008) detected both period and amplitude changes of the frequencies of multiperiodic $\beta$ Cep variables using ASAS-3 observations spanning over 6 years.

Hipparcos, OMC and HAT-5 light curves of HD~190336 folded with the three main frequencies found in the data are plotted in Fig.~3. In each panel of this figure, the the signals belonging to the other frequencies has been removed.


In the Hipparcos data the three largest amplitude signals, $f_3, f_5$ and $f_2$ also appear. Their amplitudes
are 0.026, 0.022 and 0.019 mag, respectively, indicating either that the amplitudes of these frequencies are changing, or that there are significant differences between their amplitude ratios in different bands.
The frequencies of the three signals found in the Hipparcos data agree within the error ranges with the frequencies of the three largest amplitude frequencies detected in the HAT-5 data.

The OMC $V$ band data span over four years, overlapping with the 30 days interval of the HAT-5 observations. Although both systematic and random noise are the largest in this data set, $f_3$ and $f_5$ can be detected in the OMC data too. Their amplitudes are close the same; 0.021 mag. For comparison, the amplitudes of these frequencies are 0.025 and 0.013 mag in the HAT-5 data ($I$ band). Removing $f_3$ and $f_5$ from the Integral data, the residual shows a frequency at $4.3354 \,\mathrm{cd}^{-1}$, close to the frequency of $f_2$ detected in the HAT-5 and Hipparcos data. The difference between the position of this frequency and $f_2$ is, however, much larger than it could be explained by the uncertainties of the frequencies. Therefore, most probably it is a different frequency.


The available photometric observations of HD~190336 do not allow to perform a more detailed asteroseismic study of the star and mode identification of the detected frequencies. They show, however, that HD~190336 is an ideal, interesting asteroseismic target which definitely deserves further attention. Both observational and theoretical efforts are needed in order to interpret its frequencies and their spacings.




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\noindent{\it Acknowledgements}\ \ This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. The financial support of OTKA grant T-068626 is acknowledged. The Clean algorithm, as implemented by J. Leh\'ar, has been used. These results are partially based on data from the OMC Archive at LAEFF, pre-processed by ISDC.

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\references


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Hartman J. D., Bakos, G., Stanek, K. Z., Noyes, R. W. 2004, {\it AJ}, {\bf 128}, 1761

Handler, G. 2005, {\it IBVS}, 5667

Handler, G. et al. 2006, {\it MNRAS}, {\bf 365}, 327

Koen, C., Eyer, L. 2002, {\it MNRAS}, {\bf 331}, 45

Mas-Hesse, J. M., Gimenez, A., Culhane, J.L., et al., 2003, {\it A\&A}, {\bf 411}, L261

Pigulski, A., Pojmanski, G. 2008, {\it A\&A}, {\bf 477}, 907

Roberts, D. H., Leh\'ar, J., Dreher, J. W. 1987, {\it AJ}, {\bf 93}, 968

Sokolovsky, K. V., 2007, {\it PZP}, {\bf 7}, 30

Stankov, A., Handler, G. 2005, {\it ApJ Suppl. Ser.}, {\bf 158}, 193

Walborn, N. R. 1971,  {\it ApJ Suppl. Ser.}, {\bf 23}, 257

Winkler, C., Courvoisier, T.J-L., Di Cocco, G., et al., 2003, {\it A\&A}, {\bf 411}, L1

\endreferences

%Corrected (Sokolovsky): Tue Apr  7 14:37:15 MEST 2009

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