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\IBVShead{6058}{3 June 2013}

\IBVStitletl{Search for rapid oscillations among}{ seven northern CP stars}

\IBVSauth{Paunzen, E.$^{1,2}$; Handler, G.$^3$; Netopil, M.$^4$; Fossati, L.$^5$; Iliev, I. Kh.$^2$; \newline \indent Rode-Paunzen, M.$^4$; L{\"u}ftinger, T.$^4$; Ryabchikova, T.$^6$; Bo\v{z}i\'{c}, H.$^7$}


\IBVSinst{Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlarska 2, CZ-611~37 Brno, \linebreak \indent Czech Republic; e-mail: epaunzen@physics.muni.cz}
\IBVSinst{Rozhen National Astronomical Observatory, Institute of Astronomy of the Bulgarian Academy of Sciences, \linebreak \indent P.O. Box 136, BG-4700 Smolyan, Bulgaria}
\IBVSinst{Copernicus Astronomical Center, Bartycka 18, 00-716, Warsaw, Poland}
\IBVSinst{Institut f{\"u}r Astrophysik der Universit{\"a}t Wien, T{\"u}rkenschanzstr. 17, A-1180 Wien, Austria}
\IBVSinst{Argelander Institut f{\"u}r Astronomie, Auf dem H{\"u}gel 71, Bonn, 53121, Germany}
\IBVSinst{Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 119017 Moscow, Russia}
\IBVSinst{Hvar Observatory, Faculty of Geodesy, University of Zagreb, Ka\v{c}i\'{c}eva 26, HR-10000 Zagreb, Croatia}

\SIMBADobj{BD +25 4371}
\SIMBADobj{BD +35 3616}
\SIMBADobj{BC Cam}
\SIMBADobj{HD 94427}
\SIMBADobj{EP UMa}
\SIMBADobj{SV Crt}
\SIMBADobj{HD 115606}

\IBVSkey{variables, roAp}

\IBVSabs{We present the time series analysis of photoelectric }
\IBVSabs{photometry at the 0.75m APT at Fairborn Observatory, Arizona. }
\IBVSabs{We found no variability which exceeds an amplitude of 1.3 mmag for the }
\IBVSabs{seven targets. }

\begintext

Up to now, there are only about 45 rapidly oscillating Ap (roAp) 
stars known. They are located within an area
of pulsational instability in the Hertzsprung-Russell diagram,
at the main sequence, ranging in effective temperature 
from about 6600\,K to 8500\,K.  

Photometric investigations for these stars show a period 
range of five to twenty five minutes, which is consistent
with acoustic (p-mode) pulsations of low degree and high 
radial overtone (Kurtz et al. 2011).

The driving mechanism of their oscillation modes is still a 
matter of debate. But the most probable explanation
seems to be the ``classical'' $\kappa$-mechanism operating in the hydrogen 
ionisation zone. Many physical processes could
play a role in this context: for example the coupling with the magnetic field, 
the ability of the latter to freeze convection and allow the stratification of 
chemical elements (Balmforth et al. 2001). 

Paunzen et al. (2012) published a list of roAp candidates based on their location
in the Hertzsprung-Russell diagram which served us as a primary source. In addition,
we selected
targets from the catalogue of Ap, HgMn and Am stars
by Renson \& Manfroid (2009). Since most of the known roAp stars are classified as
SrCrEu, we searched for such objects in the aforementioned reference. The fundamental 
parameters of our targets listed in Table 1 were calibrated as described in Paunzen et al. (2012).
Luminosities are only derived if Hipparcos parallaxes were available.

The observations were done at the 0.75m APT at Fairborn Observatory (Arizona)
either in Str{\"o}mgren $v$ or Johnson $B$ depending
on the brightness of the target. The integration time was set to 20 seconds with apertures
of 30 and 45 arcseconds, respectively. No comparison stars were used because a good sampling
of the possible periods are needed. A polynomial function was fitted to the sky subtracted data
to account for the extinction and atmospheric variations. In Table 2 the observation log
is given. Because all data have an arbitrary zero point, the measurements for each individual
star were scaled and then merged. 

The final light curves were examined in more detail 
using the Period04 program (Lenz \& Breger 2005), which performs a discrete 
Fourier transformation. Significant peaks with periods of more than one hour were subtracted.
In Figure 1, the Fourier spectra of all light curves are shown. There is no peak, which exceeds
1.3 mmag. For BD +35 3616, we notice a quite prominent peak at about 5\,mHz which corresponds to
about 200 seconds. Such a high frequency was never detected in any roAp star, so far. However, we 
are not able find to a possible explanation for this peak.

\begin{table}[!ht]
\centerline{\bf Table 1: Fundamental data of our target stars.}
\begin{center}
\begin{tabular}{lclcl}\hline
Star ID & $V$ & Spec   & $T_{\rm eff}$ & $\log(L/L_{\odot})$  \\
        &     &        &  [K]      & \\
\hline
BD +25 4371 & 9.45 & F0 Sr Cr Eu           & 8040(160)       & -- \\
BD +35 3616 & 9.47 & F0 Sr Eu              & 6830(410)       & -- \\
HD 62140    & 6.47 & A8 Sr Eu              & 7800(140)       & 1.30(7) \\
HD 94427    & 7.36 & A5 Sr Eu Cr           & 7500(250)       & 1.29(10) \\
HD 96707    & 6.08 & A8 Sr                 & 7820(230)       & 1.55(6) \\
HD 98088    & 6.11 & A8 Sr Cr Eu           & 8030(120)       & 1.66(12) \\
HD 115606   & 8.55 & A2 Sr                 & 7880(250)       & 1.20(21) \\
\hline
\end{tabular}
\end{center}
\end{table}

\begin{table}[!ht]
\centerline{\bf Table 2: Observation log.}
\begin{center}
\begin{tabular}{lrrr|lrrr}\hline
Star ID     & HJD(start)  & HJD(end)       & $N$ & Star ID     & HJD(start) & HJD(end)      & $N$ \\
(BD ...)    & 2456200+    & 2456200+       &     & (HD ...)    & 2456200+   & 2456200+      &     \\
\hline
+25 4371 &  61.5687 &  61.6422    & 254 & 62140    & 64.0183 & 64.0465    & 103 \\ 
         &  62.5603 &  62.6340    & 256 &          & 66.0085 & 66.0244    &  62 \\
         &  64.5695 &  64.5695    & 224 &          & 86.9491 & 86.9840    &  69 \\
+35 3616 &  21.6505 &  21.6682    &  48 & 94427    & 73.9817 & 74.0476    & 224 \\
         &  23.5663 &  23.6548    & 242 &          & 74.9755 & 75.0449    & 235 \\
         &  25.6500 &  25.6570    & 260 & 96707    & 69.9945 & 70.0507    & 189 \\
         &  27.5633 &  27.6461    & 232 &          & 71.9850 & 72.0516    & 224 \\
         &  28.5686 &  28.6459    & 216 &          & 90.9760 & 90.9946    &  66 \\
         &  29.5620 &  29.6451    & 232 & 98088    & 82.0011 & 82.0535    & 138 \\
         &  30.5615 &  30.6446    & 234 &          & 82.9552 & 83.0515    & 309 \\
         &  31.5763 &  31.6405    & 175 & 115606   & 86.9870 & 87.0141    &  93 \\
         &  32.5628 &  32.6338    & 138 & \\
         &  33.5605 &  33.6331    & 205 & \\
         &  34.5589 &  34.6325    & 204 & \\
\hline
\end{tabular}
\normalsize
\end{center}
\end{table}

\IBVSfig{10cm}{6058-f1.eps}{Fourier spectra of the target star light curves.}
\IBVSfigKey{6058-f1.eps}{BD +25 4371; BD +35 3616; D 62140; HD 94427; HD 96707; HD 98088; HD 115606}{periodogram}

\bigskip

{\bf Acknowledgements:} This work was supported by grant GA \v{C}R P209/12/0217, DO 02-85 of Bulgarian NSF, 
the Austrian Research Fund via the project FWF P22691-N16
and the financial contributions of the Austrian Agency for International 
Cooperation in Education and Research (CZ-10/2012).

\newpage
\references

Balmforth, N.J., Cunha, M.S., Dolez, N., Gough, D.O., Vauclair, S. 2001, {\it MNRAS}, {\bf 323}, 362

Kurtz, D.W., Cunha, M.S., Saio, H., et al. 2011, {\it MNRAS}, {\bf 414}, 2550

Lenz, P., Breger, M. 2005, {\it CoAst}, {\bf 146}, 53

Paunzen, E., Netopil, M., Rode-Paunzen, M., et al. 2012, {\it A\&A}, {\bf 542}, A89

Renson, P., Manfroid, J. 2009, {\it A\&A}, {\bf 498}, 961

\endreferences

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