\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{4853}{23 February 2000}

\IBVStitle{Discovery of Rapid Oscillations in HD~12098}

\IBVSauth{Martinez, Peter$^1$; Girish, V.$^2$; Joshi, S.$^3$; Kurtz, D.W.$^4$; Ashoka, B.N.$^2$;\\ \indent Chaubey, U.S.$^3$; Gupta, S.K.$^3$; Sagar, R.$^3$; Seetha, S.$^2$}

\IBVSinst{South African Astronomical Observatory, P.O. Box 9, Observatory
7935, South Africa, \\ \indent e-mail: peter@saao.ac.za}

\IBVSinst{ISRO Satellite Applications Centre, Airport Rd, Bangalore 560 034, India, email: girishv@isac.ernet.in}

\IBVSinst{Uttar Pradesh State Observatory, Manora Peak -- 263 129, Naini Tal, India, email: santosh@upso.ernet.in}

\IBVSinst{Dept of Astronomy, University of Cape Town, Rondebosch 7700, South Africa, email: dkurtz@saao.ac.za}

\SIMBADobjAlias{HD 12098}{BD+57 447}
\IBVStyp{ F(p) }
\IBVSkey{spectroscopy}

\begintext

The rapidly oscillating Ap (roAp) stars are cool, magnetic, chemically
peculiar A-type stars that pulsate with periods in the range 6--16
minutes and Johnson $B$ semi-amplitudes $\le 0.008$ mag. In 1998 we
initiated the {\it Naini Tal -- Cape roAp Star Survey\/} to find
northern roAp stars. Candidates are selected on the basis of an Ap
spectral classification and/or peculiar photometric colours. HD~12098
is classified as an F0 star in the HD catalogue, but it has Str\"omgren
indices indicative of strong metallicity found in the Am and Ap stars,
{\it viz.}\ $b-y=0.191$, $m_1=0.328$, $c_1=0.517$ and $\beta=2.796$
(Hauck \& Mermilliod 1998).

On the basis of these peculiar colours we decided to search for rapid
oscillations in HD~12098 on night 21/22 November 1999, JD~2451504. Our
observations comprised continuous 10-s integrations in Johnson $B$
light acquired with the ISRO high-speed photometer attached to the
1.0-m Sampurnanand telescope of the Uttar Pradesh State Observatory in
Naini Tal.

We were rewarded with the discovery of 7-minute oscillations. These
observations were observed again on nights JD2451505, 51534 and 51535.
Fig.~1 shows the light curve obtained on night JD~2451534. The data
shown here were corrected for coincidence counting losses, sky
background and extinction, and were then binned to 40-s integrations.
Finally, some mild sky transparency variations on time scales $\ge
1/2$~hr were prewhitened.

Fig.~2 shows the amplitude spectrum of the light curve depicted in
Fig.~1. The amplitude spectrum peaks strongly at 2.19~mHz
($P=7.61$~min). Fig.~2 also underscores the excellent photometric
quality attainable on a good night at Naini Tal -- the scintillation
noise is $\le 0.25$~mmag in this 2-hr run.

Inspection of the available light curves indicates the presence of
amplitude modulation, which may be caused by beating among several
frequencies and/or non-radial pulsations being seen from variable
aspect as the star rotates. An exploratory single-site study of
HD~12098 spanning several weeks will reveal the frequencies responsible
for this amplitude modulation. At 2.2 mHz, the oscillations in HD~12098
are well resolved from the sky transparency variations at good
photometric sites, so the prospects for detailed studies of the
oscillations are quite good.

\IBVSfig{3.75cm}{4853-f1.ps}{}

\IBVSfig{4.25cm}{4853-f2.ps}{}

The discovery or rapid oscillations in HD~12098 is consistent with the
high metallicism indicated by the peculiar colours, and implies an Ap
(rather than Am) nature for this star.

\reference

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

\end{document}


