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\IBVShead{4650}{11 November 1998}

\IBVStitletl{Rapid radial velocity variations in roAp star $\gamma$ E\lowercase{qu}}{ from lines of N\lowercase{d}III and P\lowercase{r}III}

\IBVSauth{ V.~Malanushenko$^{1,2}$, I.~Savanov$^{1,2}$, T. Ryabchikova $^{3}$}

\IBVSinst{Crimean Astrophysical Observatory, 334413 Nauchny, Crimea,
Ukraine\\
victor@crao.crimea.ua, savanov@crao.crimea.ua} %email


\IBVSinst{Isaac Newton Institute of Chile, Crimean Branch, 334413 Nauchny,
Crimea, Ukraine}

\IBVSinst{Institute of Astronomy, Russian Academy of Sciences,
Pyatnitskaya str.\ 48, 109017 Moscow, Russia \\
ryabchik@inasan.rssi.ru} %email


\SIMBADobj{gamma Equ}
\IBVStyp{ Ap }
\IBVSkey{radial velocities}
\IBVSkey{elements}

\begintext

\IBVSfig{12.5cm}{4650-f1.eps}{}

\IBVSfig{6.18cm}{4650-f2.eps}{}

Cool, magnetic, chemically peculiar stars which exhibit low-amplitude
($\Delta B \le$ 16 mmag) photometric variations with the typical periods
of 6 to 16 minutes form the group of rapidly oscillating Ap (roAp) stars.
Detailed description of the observed properties of roAp stars is given in
a review by Martinez and Kurtz (1995).

SrCrEu star $\gamma$ Equ (HD~201601) is one of the best-studied among
roAp stars. The analysis of its photometric oscillations revealed the
existence of at least four high-overtone pulsational modes (Martinez et al.\ 1996).  Practically zero rotational velocity of this star
(Ryabchikova et al.\ 1997) makes it the best candidate for the
measurements of radial velocity (RV) variations due to pulsations. The
first RV monitoring was made by Libbrecht (1988) who found a
peak-to-peak amplitude of 42 m/s. Recently, Kanaan and Hatzes (1998)
performed new observations of $\gamma$ Equ with the iodine absorption
cell and found peak-to-peak amplitudes of RV variations ranging from
less than 100 m/s to about 1000 m/s. Kanaan and Hatzes showed that the weaker chromium and titanium lines have the largest amplitudes in
comparison with the stronger lines of the same elements as well as with
the iron lines.

We performed new observations of $\gamma$  Equ in the red spectral
region looking for high amplitude radial velocity variations with the
classical technique of RV measurements.

All observations have been performed with the Photometrics CCD camera
installed in the coud\'{e} spectrograph of the 2.6--meter Shajn
reflector of the Crimean Astrophysical Observatory during 14 nights in
August--October of 1998.  The exposure time was 90 seconds. This value
is a reasonable compromise that allows to obtain sufficient
signal-to-noise ratio, and is still short enough to minimize phase
smearing for the 12 minutes pulsation period (Martinez et al.\ 1996).
The interval between the two consecutive exposures (including readout
and data acquisition time) was equal to 128--130 seconds.  Typical
signal-to-noise ratios per pixel are in the range 60--120 (depending on
the weather conditions and seeing).  Spectral resolution is about 35000
for the observations in the second order of grating (regions
$\lambda\lambda$~6690, 6340, 6140~\AA).  For the region centered
at $\lambda$ 6530 \AA\ (the first order of the grating) it is equal to
18000.  A calibration spectrum (ThAr lamp) was obtained for each
stellar spectrum.  Typical set of observations consists of 20 to 90
exposures. The estimated error of the RV measurements for a single line
is within 160--240 m/s.

Data reduction was done with the SPE package written by S.G. Sergeev.
Continuum fitting was made with the help of synthetic spectrum calculated
for each spectral region with SYNTH code (Piskunov 1992) using  abundances
from Ryabchikova et al.\ (1997).

During our preliminary analysis we try to identify the lines with the
largest RV amplitudes.  The Vienna Atomic Line Database (VALD, Piskunov
et al.\ 1995) was extensively used for line identification. A few
rather strong features at $\lambda\lambda$ 6145, 6160, 6327, 6690 \AA, which show the largest RV variations, were identified with the
lines of Nd III and Pr III.  Data for the identification of the second
ions of rare-earth elements were taken from Mathys and Cowley (1992),
and from Cowley and Bord (1998). Fig.\ 1 shows the curve of the RV
variations (upper panel) and its amplitude spectrum (lower panel) for
Nd III $\lambda$ 6145.07 \AA, and for Pr III $\lambda$~6160.24 \AA\
lines for one night -- August 27, 1998 (94 spectra).  The oscillations
clearly seen in the RV curve, also appear in the Fourier power
spectrum.  The amplitude spectrum has a strong peak at a frequency of
1.35 $\pm$ 0.05~mHz, which corresponds to the pulsation period of
12.35 minutes.  The RV data were analysed using the period-finding
package ISDA by Pelt (1992) and the program PERDET (Breger, 1990).
Combining RV measurements obtained in different nights (the data were
rectified by subtracting a mean value for each night) we get the
resulting periodogram in the frequency range of interest $1180 \le \nu
\le 1530~\mu$Hz which is shown in Fig.\ 2 together with the spectrum
of the window function of observations.  The largest amplitude is found
at the frequency of 1352.6 $\mu$Hz. The pulsation frequencies detected
in the photometric data (Martinez et al.\ 1996) are indicated by dotted
lines.

The analysis of the RV variations indicates a presence of the amplitude
modulation, which may be caused by beating among several frequencies.
Kanaan and Hatzes (1998) obtained the highest RV variations in
$\lambda\lambda$~5828--5852 \AA\ spectral region. This region
contains Nd III $\lambda$ 5845.07 line. Perhaps this line rather than
Cr I $\lambda$ 5844.30 is responsible for high RV amplitude.

The preliminary analysis of the radial velocity data for $\gamma$ Equ
shows that:
\begin{itemize}
\item  the highest amplitudes of RV pulsations are associated with
the lines of Pr III and Nd III.
\item the spectral lines of other species in the observed spectral regions
do not show  regular RV variations.
\item Two out of four photometric
frequencies (Martinez et al.\ 1996) appear in the RV periodograms.
\end{itemize}

\references

Breger, M., 1990, {\it Communications in Asteroseismology}, No.\,6

Cowley C.R., Bord D.J. 1998, {\it ASP Conf. Ser.}, {\bf 143}, 346, in 
{\it The Scientific Impact of the Goddard High Resolution Spectrograph}, 
eds.\ J.C. Brandt, T.B. Ake, and C.C. Petersen
%Changed: Fri Jun  3 09:19:53 MEST 2005

Kanaan, A., Hatzes, A.P., 1998, {\it Astrophys.\ J.}, {\bf 503}, 848

Libbrecht K.G., 1988, {\it Astrophys.\ J.}, {\bf 330}, L51

Martinez, P., Kurtz, D., 1995, {\it ASP Conf. Ser.}, {\bf 83}, 58, in {\it Astrophysical 
Applications of Stellar Pulsation\/}, eds.\ R. Stobie and P. Whitelock
%Corrected: Fri Jun  3 09:19:53 MEST 2005
%Original: Kurtz, D., 1995,

Martinez P., Weiss W.W., Nelson M.J., Kreidl T.J., Roberts G.R.,
Mkrtichian D.E., Dorokhov N.I., Dorokhova T.N., Birch P.V., 1996, {\it MNRAS}, {\bf 282}, 243

Mathys G., Cowley  C.R., 1992, {\it Astron.\ Astrophys.}, {\bf 253}, 199

Pelt, J., 1992, {\it Irregularly Spaced Data Analysis}, User manual,
Helsinki

Piskunov, N., 1992, in {\it Stellar Magnetism}, eds.\ Yu.V. Glagolevskij
and I.I. Romanyuk, Nauka, Sankt--Peterburg, p.\ 92

Piskunov, N.E., Kupka, F., Ryabchikova, T.A., Weiss, W.W., Jeffery, C.S.,
1995, {\it Astron.\ Astrophys.\ Supp.}, {\bf 112}, 525

Ryabchikova  T.A.,  Adelman  S.J.,  Weiss  W.W.  and Kuschnig R., 1997,
{\it Astron.\ Astrophys.}, {\bf 322}, 234


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