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\IBVShead{5667}{21 December 2005}

\IBVStitle{Five New {\large {\boldmath $\beta$}} Cephei Stars Revealed in ASAS Photometry}

\IBVSauth{Handler, G.} 

\IBVSinsto{Institut f\"ur Astronomie, Universit\"at Wien,
T\"urkenschanzstrasse 17, A-1180 Wien, Austria}

\SIMBADobj{HD 100495}
\SIMBADobj{CD -61 3543}
\SIMBADobj{ALS 2798}
\SIMBADobj{ALS 2877}
\SIMBADobj{HD 191531}
\IBVStyp{ beta Cep }
\IBVSkey{photometry}
\IBVSabs{We report the discovery of five new beta Cephei pulsators from}
\IBVSabs{an analysis of ASAS photometry. Three objects are multiperiodic}
\IBVSabs{with equal frequency splittings.}


\begintext

The $\beta$ Cephei stars are a group of massive early B-type pulsators 
that oscillate in radial and/or nonradial modes of low radial order. Some 
110 galactic representatives of this group are known (e.g.\ Stankov \& 
Handler 2005, Pigulski 2005) to date, but some open questions concerning 
their group behaviour still remain.

For instance, the theoretical boundaries of the $\beta$ Cephei instability
strip comprise all known pulsators, but some regions are not populated. In
particular, theory predicts the existence of O-type $\beta$ Cephei stars
which are not observed to date. Another interesting aspect is the apparent
existence of a few rapidly rotating stars with high photometric pulsation
amplitude (Stankov \& Handler 2005, Aerts et al.\ 2005). It is also not
known whether post-main sequence $\beta$ Cephei stars exist or not.  
Therefore, but also for unravelling interesting asteroseismic targets, the
discovery of new $\beta$ Cephei stars is important.

During the preparation of an observing programme, we noticed that the
ASAS-2 (see Pojma\'nski 1997 for a description of this project) periodic
variable ASAS J113318-6306.2, so far deemed to be a $\delta$ Scuti
candidate (Pojma\'nski 1998), had virtually the same coordinates as the
B1III star HD 100495. Published Str\"omgren photometry (Kaltcheva et al.\
2000) puts the star into the $\beta$ Cephei instability strip. We
therefore concluded that ASAS J113318-6306.2 and HD~100495 are the same
star, which is a $\beta$ Cephei pulsator.

Consequently, we searched the ASAS-2 data base of periodic variables for
similar objects, i.e. stars of O and (early) B spectral type that show
short-period variability indicative of $\beta$ Cephei pulsation, and we
performed frequency analyses for these data. Indeed, we found a total of
four previously unrecognised $\beta$ Cephei stars among these variables. 
As three of these were initially classified as eclipsing variables, we 
also searched the ASAS-3 data base for OB type variables classified as 
contact or semi-detached binaries, which resulted in the discovery of 
another new $\beta$ Cephei pulsator. In the following, we briefly report 
our findings for the individual new pulsating stars.

{\it ASAS J113318-6306.2 = HD~100495}

We performed a frequency analysis of the ASAS-2 photometry of this star,
after adjusting the zeropoints of the individual subsets of data, with the
program {\tt Period98} (Sperl 1998), applying both single-frequency power
spectrum analysis and simultaneous multi-frequency sine-wave fitting. The
Fourier amplitude spectrum of the data and some prewhitening thereof is
shown in Fig.\ 1. We find the same main period as Pojma\'nski (1998), but
we also see evidence for a secondary period in the residual amplitude
spectrum. Its value is however uncertain due to aliasing.

We estimated the luminosity, effective temperature and mass of the star 
from the published Str\"omgren photometry (Kaltcheva et al.\ 2000) 
following the procedure described by Stankov \& Handler (2005), resulting 
in $T_{\rm eff} = 27000 \pm 1500$~K, log $L=4.34 \pm 0.16$ and
$M\approx14$~M$_{\odot}$. With these parameters we obtain a pulsation 
constant $Q = 0.036\pm0.011$, perfectly consistent with the values for 
other $\beta$~Cephei stars (cf.\ Stankov \& Handler 2005). 

\IBVSfig{3.6cm}{5667-f1.eps}{Left panel: amplitude spectrum of the ASAS-2 photometry of
HD~100495. Right panel: amplitude spectrum after prewhitening the
strongest signal. A possible second periodicity is indicated.}
\IBVSfigKey{5667-f1.eps}{HD 100495}{periodogram}


{\it ASAS J123748-6219.4 = CD-61 3543}

This B star was originally classified as an eclipsing binary of the W~UMa
type (Pojma\'nski 1998) with a period near 10~hr. However, our frequency
analysis reveals it to be a multiperiodic variable (see Fig.\ 2). We find
four independent variations with periods near 5~hr and one combination
frequency (with evidence for more combinations). Three of the independent
frequencies form an (almost) equally spaced triplet. The multiperiodicity
of the star and its periods make it clear that it is a $\beta$~Cephei
pulsator.

\IBVSfig{8.2cm}{5667-f2.eps}{Original and prewhitened amplitude spectra of the 
ASAS-2 photometry of CD-61 3543.}
\IBVSfigKey{5667-f2.eps}{CD-61 3543}{periodogram}

\newpage

{\it ASAS J125319-6401.4 = ALS 2798}

The B type star ALS~2798 was also originally classified as a W~UMa binary
with a period near 10~hr (Pojma\'nski 1998). Again, we find multiperiodic
variability with periods near half this value (see Fig.\ 3). The length of
the periods and the fact that there are several also imply that we are
dealing with a new $\beta$~Cephei star. Interestingly, the four signals we
find are arranged in two close doublets that have the same frequency
spacing.

\IBVSfig{8.2cm}{5667-f3.eps}{Original and prewhitened amplitude spectra of the 
ASAS-2 photometry of ALS~2798. The series of peaks discernible in the 
lower panel of the right-hand side are integer multiples of 1 cycle per 
sidereal day and are therefore unlikely to be intrinsic to the star.} 
\IBVSfigKey{5667-f3.eps}{ALS 2798}{periodogram}


{\it ASAS J130221-6328.4 = ALS 2877}

This B star was also classified as a W~UMa binary, but with a period near
9~hr (Pojma\'nski 1998). Once more, multiperiodic variability with periods
near half this value is in fact present. This is shown in Fig.\ 4, where 
we only used the first two seasons of ASAS measurements since the third 
was of considerably lower quality. We find three frequencies in the light 
variations of the star that are (almost) equally spaced.

\IBVSfig{5.8cm}{5667-f4.eps}{Original and prewhitened amplitude spectra of the 
ASAS-2 photometry of ALS~2877.}
\IBVSfigKey{5667-f4.eps}{ALS 2877}{periodogram}


{\it ASAS J200939+2104.8 = HD 191531}

The variability of this B0.5III-IV star (sometimes also mentioned as a Be
star) has already been suggested by Koen \& Eyer (2002) on the basis of an
analysis of its Hipparcos photometry. We find the same frequency as these 
authors in the ASAS-3 data, and detect a secondary periodicity as well 
(Fig.\ 5). The latter is also present in the residual Hipparcos photometry 
amplitude spectrum, as our re-analysis shows.

\IBVSfig{3.7cm}{5667-f5.eps}{Original and prewhitened amplitude spectra of the 
ASAS-3 photometry of HD 191531.}
\IBVSfigKey{5667-f5.eps}{HD 191531}{periodogram}


\begin{table}[!hr]
\caption{Pulsation frequencies and amplitudes of the new $\beta$ Cephei stars}
\begin{center}
%\label{elements}
\begin{tabular}{ccc}
\hline
Star & Frequency & Amplitude\\
 & [c/d]    & [mmag]     \\
\hline
HD 100495 & $5.9353\pm0.0004$ & $18.6\pm0.7$ (I)\\
\hline
CD-61 3543 & 4.598600 $\pm$ 0.000014 &  41.6 $\pm$ 1.1 (I)\\
& 4.54019 $\pm$ 0.00004 &  16.9 $\pm$ 1.1 (I)\\
& 4.55647 $\pm$ 0.00003 &  17.9 $\pm$ 1.1 (I)\\
& 4.57811 $\pm$ 0.00004 &  13.5 $\pm$ 1.1 (I)\\
& 9.13879 &   8.2 $\pm$ 1.1 (I)\\
\hline
ALS~2798  &  4.69541 $\pm$ 0.00002 &   56.3 $\pm$ 1.7 (I)\\
  &  4.68776 $\pm$ 0.00003 &   31.4 $\pm$ 1.7 (I)\\
  &  4.44148 $\pm$ 0.00006 &   15.9 $\pm$ 1.7 (I)\\
  &  4.44909 $\pm$ 0.00008 & 10.8 $\pm$ 1.7 (I)\\
\hline
ALS~2877 &      5.28745  $\pm$  0.00002  &    42.8 $\pm$ 1.3 (I)\\
 &      5.30263  $\pm$  0.00004  &    20.3 $\pm$ 1.3 (I)\\
 &     5.27222  $\pm$  0.00007  &    12.2 $\pm$ 1.3 (I)\\
\hline
HD 191531 &    6.08589 $\pm$ 0.00004 &  26.1 $\pm$ 1.5 (V)\\
  &  5.91437 $\pm$ 0.00011 &  8.7 $\pm$ 1.5 (V)\\
\hline
\end{tabular}
\end{center}
\end{table}

Table 1 summarises our findings on the new $\beta$ Cephei stars.  
HD~100495 is a new pulsator because of the time scale of its variability
and because of its position in the HR diagram. All others are revealed to
be pulsators because of their multiperiodicity. The three stars with the 
most periods appear very attractive for asteroseismology because of their 
frequency multiplets. Most notably, ALS 2798 seems suitable for a search 
for differential interior rotation.


\references

Aerts, C., De Cat, P., De Ridder, J., Van Winckel, H., Raskin, G., 
Davignon, G., Uytterhoeven, K., 2005, {\it A\&A}, in press 
(astro-ph/0511306)

Kaltcheva, N.\ T., Olsen, E.\ H., Clausen, J.\ V., 2000, {\it A\&AS}, {\bf
146}, 365. 
%%\BIBCODE{2000A&AS..146..365K}

Koen, C., Eyer, L., 2002, {\it MNRAS}, {\bf 331}, 45.
%%\BIBCODE{2002MNRAS.331...45K}

Pigulski, A., 2005,  {\it Acta Astr.}, {\bf 55}, 219. 
%%\BIBCODE{2005AcA....55..219P}

Pojma\'nski, G., 1997, {\it Acta Astr.}, {\bf 47}, 467. 
%%\BIBCODE{1997AcA....47..467P}

Pojma\'nski, G., 1998, {\it Acta Astr.}, {\bf 48}, 35. 
%%\BIBCODE{1998AcA....48..35P}

Sperl M., 1998, Master's Thesis, University of Vienna

Stankov, A., Handler, G., 2005, {\it ApJS}, {\bf 158}, 193. 
%%\BIBCODE{2005ApJS..158..193S}

\endreferences

\end{document}
