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\IBVShead{5787}{17 August 2007}

\IBVStitle{Discovery of 6-Minute Oscillations in HD 151878 }

\IBVSauth{Tiwari, S. K.; Chaubey, U. S.; Pandey, C. P.}

\IBVSinsto{Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital - 263129 India}


\SIMBADobj{HD 151878}

\begintext

\vspace*{-1cm}

The rapidly oscillating Ap (roAp) stars are cool, magnetic, chemically
peculiar A-type stars, pulsate with periods ranging from 4-21 minutes,
and have pulsation amplitudes $\leq 16$ mmag in Johnson B. 
Some of the roAp stars are of great significance to astrophysics
because they  allow us  to study pulsation  and chemical  diffusion in
presence of magnetic  fields.  Till 2006, of the  35 roAp stars known,
30 are in the southern  hemisphere, and thus inaccessible with most of
the  astronomers  from  the  northern  hemisphere.  To  discover 
northern roAp stars, we are  carrying out a survey programme entitled
``Search for pulsation in chemically peculiar stars''.

 HD  151878  is  classified  as  a  F2  star  in  HD  catalogue.   The
Str\"{o}mgren indices of  the star HD 151878 are $b-y  = 0.225$, $m_1$ =
0.234,  $c_1$ =  0.684, $\beta$  = 2.759  (Hauck \&  Mermilliod, 1998)
which indicate a strong metallicity which is generally found in Ap and
Am stars. On the basis of these peculiar colours, we observed the star
HD 151878 on May 30, 2007 with 104-cm Sampurnanand telescope of ARIES,
Nainital, equipped with high-speed fast photometer. We were rewarded
with the discovery of 6-min oscillations in the star. Further,  we 
observed  the star  HD  151878 on  June 01  and 03,  2007
(corresponding  JDs 2454253,  and 2454255)  and noted  the  same 6-min
oscillations.

As we were searching for variations in the 4-21 min
range and also due to the absence of any suitable comparison star in the 
field, we did not observe any comparison star.
The data were acquired as continuous single channel 10s integrations through
a Johnson B filter. A diaphragm of 2-mm in diameter which corresponds to 30
arcsec was used to minimize the light losses arising from seeing effect
and tracking drifts. The observations were interrupted, nearly every
20-30 minutes, for sky background measurements to take account of changes of
sky brightness during the night as well as to check the centering of the
programme star in the diaphragm.The observed data were  corrected 
for coincidence counting
losses due to the dead time of the photon counting electronics,
sky background and atmospheric extinction. Because of the absence of any
comparison star observations, the observed data have been normalized 
in the mean to zero on a nightly basis. There is always some degree of 
contamination of single channel high-speed photometry by sky 
transparency variations. The normalized nightly data
were prewhitened due to some mild sky transparency variations
on time scale $\geq$ 0.5~hr with caution, as they do not discriminate
between the sky transparency variations and real variations in the star.

The nightly observed light curves of HD 151878 are plotted in Figure 1.
Figure 2 shows the nightly amplitude spectrum of the light curve  
depicted in Figure 1. The amplitude spectrum of the light curve
peaks strongly at 2.78 mHz (Period = 6~min) for all the three dates.
It is evident from Figures 1 and 2 that the  nightly observed
mean  amplitude  of  the  oscillations  of all  the  three  dates  are
different  from each other. This amplitude modulation may be either 
due to excitation of  different  modes   or  due  to  rotation  of   
the  star.  Further observations   will   be  carried   out   to   
study  rotational   and multi-pulsational behaviour of this star.

\IBVSfig{7.5cm}{5787-f1.ps}{Discovery and confirming light curves
of  HD 151878  observed in Johnson B filter.}
\IBVSfigKey{5787-f1.ps}{HD 151878}{light curve}

\IBVSfig{7.7cm}{5787-f2.ps}{Amplitude  spectrum of  the nightly light 
curves depicted in Figure 1.}
\IBVSfigKey{5787-f2.ps}{HD 151878}{periodogram}

{\it Acknowledgments:}  Thanks  are  due  to  Prof. Ram  Sagar  for  the useful
suggestions. This survey programme is supported by DST
Govt.of India,  Grant No. SR/S2/HEP-20/2003.


\reference

Hauck, B., Mermilliod, M., 1998, {\it A\&AS}, {\bf 129}, 431     

\endreferences

\IBVSedata{5787-t1.txt}
\IBVSdataKey{5787-t1.txt}{HD 151878}{photometry}
   

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