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\IBVShead{5178}{21 September 2001}

\IBVStitletl{On the Orbital Periods of Two Bona-fide {\boldmath$\lambda$} Bootis Stars}{ HD~64491 and HD~141851}

\IBVSauth{Iliev, I.Kh.$^1$; Paunzen, E.$^2$; Barzova, I.$^1$; Andrievsky, S.M.$^3$; Chernyshova, I.V.$^4$;\\ \indent Kamp, I.$^5$}

\IBVSinst{Institute of Astronomy, National Astronomical Observatory,
P.O. Box 136, 4700 Smolyan, Bulgaria,\\ \indent e-mail: rozhen@mbox.digsys.bg}

\IBVSinst{Institut f\"ur Astronomie der Universit\"at Wien,
T\"urkenschanzstr.\ 17, 1180 Wien, Austria}

\IBVSinst{Instituto Astron\^{o}mico e Geof\'{\i}sico, Universidade de
S\~{a}o Paulo, Av.\ Miguel Stefano, 4200, S\~ao Paulo SP,\\ \indent
Brazil} \IBVSinst{Department of Astronomy, Odessa State University,
Shevchenko Park, 65014, Odessa, Ukraine}

\IBVSinst{Leiden Observatory, Niels Bohrweg 2, PO Box 9513, 2330 RA
Leiden, The Netherlands}

\SIMBADobj{HD 64491}
\SIMBADobj{HD 141851}

\begintext

We report about the first estimation of the orbital periods for two
apparent spectroscopic binary (SB hereafter) systems: HD 64491 ($V =
6\fmm22$, HR 3083, HIP 38723) and HD 141851 ($V = 5\fmm10$, HR 5895,
HIP 77660).  Both stars were classified as bona-fide  $\lambda$ Bootis
candidates in the literature because at this time they were believed to
be apparent single objects. This group is characterized as comprise of
late B to early F-type, Population I objects which exhibit a nearly
solar abundance of C, N, O and S whereas the Fe-peak elements are
significantly sub-solar. There are two SB systems in which both
components are true  $\lambda$ Bootis type objects established via a
detailed abundance analysis, namely HD 84948 and HD 171948 (Paunzen et
al.\ 1998a).

No detailed abundance analysis which takes the binarity nature into
account has been published for our two program stars up to now. Until
such an analysis has been done, a question mark has to be set behind
the apparent  $\lambda$ Bootis classification. Let us now discuss the
two objects in more detail.

HD 64491 was first classified as A9~Vp ($\lambda$ Boo) by Abt \&
Morrell (1995) and confirmed as kA3hF0mA3~V~($\lambda$ Boo) by
Paunzen \& Gray (1997). Note that Uesugi \& Fukuda (1982) give a
projected rotational velocity of 75~kms$^{-1}$ whereas Abt \& Morrell
(1995) list 15~kms$^{-1}$. Marchetti et al.\ (2001) performed speckle
interferometry for this star and found only an upper limit of 124~mas
for the separation of possible components. The first notification of
the SB nature for this object is given by Kamp et al.\ (2001) who
investigated high resolution spectra centered at 8670~\AA\@. Paunzen et
al.\ (1998b) reported $\delta$ Scuti type pulsation for this object with
a period of 71 minutes and an amplitude of 9~mmag in Str\"omgren $b$
which makes it especially interesting for further studies applying the
tools of asteroseismology.

HD 141851 is known as close visual binary with a separation of 0\farcs1
but without data about the luminosity of the components (Faraggiana \&
Bonifacio 1999). Abt (1984) classified this star as A3~Vnp (Mg~wk)
whereas Abt \& Morrell (1995) and Paunzen et al.\ (2001) list A3~Vp
(4481~wk) and A2~Va, respectively. Abt \& Morrell (1995) give a $v \sin
i$ value of 185~kms$^{-1}$.  This system consists probably of a hot and
a very cool component since it was identified as a X-ray source by
H\"unsch et al.\ (1998). No variability with an upper limit of 4.2~mmag
in Str\"omgren $b$ was found (Paunzen et al.\ 1998b).

\begin{table}[h]
\caption{The observations for HD 64491 (upper section) and HD 141851
(lower section)}
\begin{center}
\begin{tabular}{crrl}
\hline
\lower6pt\hbox{HJD}  &  RV & $\sigma$(RV) & \lower6pt\hbox{Obs.} \\[-4pt]
& [kms$^{-1}$] & [kms$^{-1}$] \\
\hline
   2450925.6351  & +22.2  & 0.6    &   KPNO; 0.9~m; Coud\'e \\
   2451238.4985  &  +6.3  & 0.8    &   BNAO; 2.0~m; Coud\'e  \\
   2451238.5257  &  +7.5  & 0.7    &   BNAO \\
   2451888.5126  & +40.2  & 0.7    &   BNAO \\
   2451891.4220  & +36.1  & 1.1    &   BNAO \\
   2451913.3713  & +22.3  & 1.2    &   BNAO \\
   2451914.4005  & +19.6  & 0.8    &   BNAO \\
   2451920.4512  & +22.6  & 0.6    &   BNAO \\
   2451921.3880  & +21.5  & 0.7    &   BNAO \\
   2451971.3864  & +11.9  & 0.9    &   BNAO \\
   2451973.3791  & +12.6  & 0.6    &   BNAO \\
   2451977.2216  & +11.7  & 1.0    &   BNAO \\
   2452003.2919  &  +7.4  & 0.8    &   BNAO \\
   2452004.2442  &  +8.9  & 0.8    &   BNAO \\
   2452152.5931  & +15.4  & 0.7    &   BNAO \\
\hline
   2449885.5946  &  $-$95  &   7    &     LNA; 1.6~m, Coud\'e \\
   2449885.5946  & $-$115  &  15    &     LNA \\
   2450495.6312  &  $-$26  &  10    &     ASIAGO; 1.8~m, Echelle \\
   2450664.4413  &  $-$39  &   5    &     CASLEO; 2.15~m, Echelle\\
   2451234.5377  &  $-$10  &   3    &     BNAO \\
   2451236.4845  &   $-$8  &   3    &     BNAO \\
   2451238.5538  &  $-$12  &   3    &     BNAO \\
   2451284.4546  &  $-$17  &   3    &     BNAO \\
   2451296.4247  &  $-$20  &   5    &     BNAO \\
   2451307.4360  &  $-$19  &   4    &     BNAO \\
   2452003.4438  &  $-$12  &   2    &     BNAO \\
   2452069.3324  &   $-$9  &   2    &     BNAO \\
   2452090.2856  &   $-$7  &   3    &     BNAO \\
   2452123.2754  &   $-$9  &   2    &     BNAO \\
   2452150.2469  &   $-$9  &   2    &     BNAO \\
\hline
\end{tabular}
\end{center}
\end{table}


Table 1 lists the Heliocentric Julian Dates, the average radial velocity, its
mean error and the observatories where the measurements were done.
The average radial velocities were calculated from individual measurements of several  
lines with the correction for the mean heliocentric velocity.
The errors are much larger for HD 141851 than for the moderate rotator HD 66491
because there is one hot, fast rotating, component superposed with a very
cool slow rotating one with very weak lines in the used spectral
domain (mainly the Na~D doublet). Figure 1 shows the measurements graphically. 

\clearpage

\IBVSfig{10cm}{5178-f1.eps}{The radial velocity curves for HD 64491 (upper panel)
and HD 141851 (lower panel) as listed in Table~1}

Due to the temporal distribution of our observations, classical time
series algorithm such as Fourier techniques, sine fits or the
Phase-Dispersion-Minimization does not result in a reasonable
solution.  From an examination of Table~1 we are able to conclude that
the orbital period for HD 64491 is between 230 days (taking the
measurements at HJD 2451888.5126 and 2452003.2919 as half the period)
and 760 days (taking the two ``minima'' at HJD 2451238.4985 and
2452003.2919 as hypothetical real period).

The orbital period for HD 141851 is significant longer than the time
base of our available observations (2265 days). Due to the shape of the
radial velocity curve (Figure 1) we think that the period is at least
ten times longer than the actual time base of our observations.

Further radial velocity measurements and a detailed abundance analysis
for both objects taking into account the binarity are needed to shed
more light on the true nature of these systems.

\bigskip

{\it Acknowledgements:} SMA would like to express his gratitude to
FAPESP for the visiting professor fellowship (No.\ 2000/06587-3). Use
was made of the SIMBAD, operated at CDS, Strasbourg, France.


\references

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Faraggiana, R., Bonifacio, P., 1999, {\it A\&A}, {\bf 349}, 521 
%\BIBCODE{1999A&A...349..521F}

H\"unsch, M., Schmitt, J.H.M.M., Voges, W., 1998, {\it A\&AS}, {\bf 132}, 155
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Paunzen, E., Gray, R.O., 1997, {\it A\&AS}, {\bf 126}, 407
%\BIBCODE{1997A&AS..126..407P}

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%\BIBCODE{1998A&A...329..155P}

Paunzen, E., Weiss, W.W., Kuschnig, R., et al., 1998b, {\it A\&A}, {\bf 335}, 533
%\BIBCODE{1998A&A...335..533P}

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%\BIBCODE{2001A&A...373..625P}

Uesugi, A., Fukuda, I., 1982, {\it Revised Catalogue of Stellar Rotational
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\endreferences

\end{document}
