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\IBVShead{5589}{7 January 2005}

\IBVStitletl{ {\large \boldmath $\delta$} Scuti Like Pulsation of H254 Using ROTSE3d CCD}{ Observations}


\IBVSauth{Kizilo\u{g}lu, \"U.$^1$; Kizilo\u{g}lu, N.$^{1,2}$; Baykal, A.$^{1,3}$}

\IBVSinst{Physics Dept., Middle East Technical University, Ankara 06531, Turkey, e-mail: umk@astroa.physics.metu.edu.tr}
\IBVSinst{e-mail: nil@astroa.physics.metu.edu.tr}
\IBVSinst{e-mail: altan@astroa.physics.metu.edu.tr}

\SIMBADobj{Cl* IC 348 H254}

\begintext

H254 is a pre-main sequence F0 spectral type star with T$_{eff}$= 7200 K
and L$_{bol}$=31.4~L$_{\odot}$ (Luhman et al. 1998) 
in the young cluster IC 348.  
Ripepi et al. (2002) identified four frequencies for this source
by using  their eleven days observations. One of these frequencies was at
7.406 \hbox{d$^{-1}$} which is typical of {$\delta$ Scuti } type pulsators. 
They reported that the other three frequencies result from the
 long term behavior associated with a daily variation of H254
 and partially with the similar variability in the comparison star H20.  

We attempted to detect the light variations of H254 
 in our 95 days of observation span obtained
 with ROTSE3d telescope located at Bak{\i}rl{\i}tepe, Turkey.
ROTSE3 systems were described in detail in Akerlof et al. (2003). 
It operates without filters and has a wide passband which peaks at
550 nm. It is equipped with a CCD, 2048$\times$2048 pixel, the pixel scale
is 3.3 arcsec per pixel for a total field of view
 1\fdeg85$\times$1\fdeg85.

The observations were obtained between the nights MJD 53232 (August)
 and MJD~53327 (November). 
 We were able to obtain 3 to 40 frames for IC 348 at each night,
because of the other scheduled observations.
The exposure time was 5 seconds. 
A total of about 1600 CCD frames were analysed.
Aperture photometry by SExtractor (Bertin$\&$Arnouts, 1996) were
applied to the observed CCD frames to obtain the instrumental magnitudes.
 Then, ROTSE magnitudes were calculated by comparing all the
 field stars to the USNO A2.0 R-band catalog.
All the processes were done in sequential automated mode.
Barycentric corrections were made to the times of each observation
by using JPL DE200 ephemerides.
As a comparison star we chose H261  
which is  at a distance of 4\arcm from H254. 
Its spectral type (F2) is  not too
 different from the spectral type of H254.

\IBVSfig{10cm}{5589-f1.eps}{ROTSE3d light curve}
\IBVSfigKey{5589-f1.eps}{H254}{light curve}

Fig.1 shows ROTSE3d light curve ($\delta$m$_{ROTSE}$= m$_{ROTSE}$$^{254}$-
m$_{ROTSE}$$^{261}$)
obtained from CCD observations.

 A Fourier analysis of data led to the detection of a signal
 for a frequency 7.406~\hbox{d$^{-1}$}. We used a period search programme
written by M. Sperl (Period98: available at \hfill\linebreak
{\tt www.astro.univie.ac.at/$^{\sim}$dsn/}). 
Fig.2 (upper panel) shows the power spectrum
 of H254 which displays the frequency
7.406~\hbox{d$^{-1}$} with one day alias pattern. 
When this frequency is removed from the spectrum  
no other significant frequency is seen.

\IBVSfig{10cm}{5589-f2.eps}{ Power spectra for H254 (upper panel: Period98,
lower panel: Scargle algorithm, \hbox{MJD = JD -- 2400000.5}). Solid line represents $2\sigma$ confidence level.}
\IBVSfigKey{5589-f2.eps}{H254}{spectrum}

We also employed the method of Scargle (Scargle, 1982) for period search
in order to evaluate the confidence levels of oscillations 
(see Fig.2 lower panel).
We estimated the noise level of periodogram by fitting a constant line.
 The probability of
a signal above this level has an exponential probability
distribution 
which is essentially a $\chi ^{2}$ distribution for two
degrees of freedom (Scargle 1982).  For given parameters,
the confidence level of the signal for the maximum power
at 7.406 \hbox{d$^{-1}$} is
$ \sim 0.997 $.
 This confidence level is close to the
$\sim 5\sigma$ level signal detection. As seen from Fig.2
all other detected powers are below the $2\sigma $ detection level 
which indicate that 0.157, 0.283 and 0.931 \hbox{d$^{-1}$} frequencies
 detected by Ripepi et al. (2002) 
are not present in our light curve.

\IBVSfig{9.9cm}{5589-f3.ps}{ Light curve phased with the frequency 7.406 \hbox{d$^{-1}$} }
\IBVSfigKey{5589-f3.eps}{H254}{light curve}

Fig.3 shows the light curve phased with the frequency 7.406 \hbox{d$^{-1}$}.   

As a conclusion, ROTSE3d data allowed us to identify the frequency 7.406
\hbox{d$^{-1}$}. No other significant frequency was detected.

\leftline{\bf Acknowledgements:} This study was supported by TUG (Turkish
National Observatory), TUBITAK ( Turkish Scientific and Research Councile).


\references

Akerlof, C.W., Kehoe, R.L., McKay, T.A., Rykoff, E.S., Smith, D.A., et al., 2003, {\it PASP}, {\bf 115}, 132
%%(2003PASP...115..132A)

Bertin, E., Arnauts, S., 1996, {\it A\&AS}, {\bf 117}, 393
%%(1996A\&AS...117..393)

Luhman, K.L., Rieke, G.H., Lada, C.J., Lada, E.A., 1998, {\it ApJ}, {\bf 508}, 347 
%%(1998ApJ...508..347L)

Ripepi, V., Palla, F., Marconi, M., Bernabei, S., Arellano Ferro, A., Terranegra, L., Alcal\'a, J. M., 2002, {\it A\&A}, {\bf 391}, 587
%%(2002A\&A...391..587R)

Scargle, J.D., 1982, {\it ApJ}, {\bf 263}, 835 
%%(1982ApJ...263..835S)


\endreferences


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