\documentstyle[twoside,epsf]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{5193}{6 November 2001}

\IBVStitletl{Emission activity of the B\lowercase{e} star 28~CM\lowercase{a}:}{ entering a new cycle?}

\IBVSauth{\v{S}tefl, Stanislav$^1$; Baade, Dietrich$^2$; Rivinius, Thomas$^2$; Otero, Sebastian$^3$; Setiawan, Johny$^4$}

\IBVSinst{Astronomical Institute, Academy of Sciences of the Czech Republic, 
CZ-25165 Ond\v{r}ejov, Czech Republic,\\ \indent e-mail: sstefl@sunstel.asu.cas.cz}
\IBVSinst{European Southern Observatory, Karl-Schwarzschild-Str.\ 2, 
D-85748 Garching bei M\"unchen, Germany,\\ \indent e-mail: dbaade@eso.org, triviniu@eso.org}
\IBVSinst{Liga Iberoamericana de Astronomia, Buenos Aires, Argentina,
     \\ \indent e-mail: varsao@fullzero.com.ar }
\IBVSinst{Kiepenheuer-Institut f\"ur Sonnenphysik, Sch\"oneckstr.\ 6-7,
     D-79104 Freiburg, Germany, \\ \indent e-mail: setiawan@uni-freiburg.de }

\SIMBADobjAlias{28 CMa}{HD 56139}
\IBVSkey{photometry}
\IBVSkey{spectroscopy}
\IBVSkey{Be stars}

\IBVSabs{A new major emission phase of the southern Be star 28 CMa is} 
\IBVSabs{reported.  It manifests itself as a brightening by more than}
\IBVSabs{0.4m in the V band and a drop of the Balmer line emission.}
\IBVSabs{The spectroscopic behaviour fits the outburst scheme derived}
\IBVSabs{for mu Cen, but takes place on a much longer time scale,}
\IBVSabs{thereby making 28 CMa an ideal target for detailed studies of} 
\IBVSabs{Be star outbursts.  On this basis, some spectroscopic phenomena}
\IBVSabs{expected to appear within a few months are mentioned.} 

\begintext

The bright southern Be star 28~CMa (HR\,2740, HD\,56139) is
proto-typical of the spectroscopic and photometric variability of Be
stars.  New modeling techniques have been tested on it.  Very
conspicuous line profile variations ($lpv$) with a period of 1.37~d
were discovered by Baade (1982a).  A detailed description of the $lpv$
of He{\sc i}\,6678 was presented by \v{S}tefl et al.\ (1999), and
advanced non-radial pulsation models for different spectral lines were
computed by Maintz et al.\ (2000).  Although rapid light variations
have been known almost equally early (Baade, 1982b), the derived
periods were questionable and inconsistent with the spectroscopic one.
\v{S}tefl et al.\ (1999)
analysed the Str\"omgren and Geneva photometry obtained during 16
years and isolated the 1.37~d periodic component of the light
variations, whose amplitude is variable from season to season
and reaches only a few mmag.  These rapid periodic variations are
combined with much larger variations on time scales of weeks to years.
The latter are probably connected with the photospheric activity of
the star as well as with restructuring in the circumstellar disk.
A large brightening by about 0.$\!^m$4 was observed by Hipparcos in 
1992.  New observations show that an emission outburst of a comparable 
intensity started in 2001.
\medskip

Visual observations since 1997 April
and the 1990-1993 Hipparcos database reveal that the photometric
ground state of 28\,CMa corresponds to a magnitude of $ \sim $4.$\!^m$05
visually or $\sim$3.$\!^m$98 in the Hipparcos broad photometric band.  
Superimposed
on this plateau are fluctuations, mainly in the form of brightenings
by up to 0.1 mag.  The most recent brightening, which started in 2001 March
and reached about 3.$\!^m$8 in June, has a significantly larger
amplitude.  First visual observations revealing the strength of this new
outburst were obtained by E.\ De Bernardini (Buenos Aires) on Oct.\
13, 2001 and confirmed by SO on Oct.\ 19.  The star reached
V$_{vis}$~=~3.$\!^m$67.  The rising branch of the 2001 light curve
resembles the one during the 1992 Hipparcos outburst (see Fig.\ 1).  
The most recent observations from the end of October 2001 seem to 
indicate that the outburst has already reached the descending branch.

\hspace*{-3cm}
\IBVSfig{8.2cm}{5193-f1.eps}{The strongest emission outbursts of 28~CMa observed during the past decade: the 1992 event covered by Hipparcos
  (lower panel, Perryman 1997) and the 2001 outburst
  documented by visual observations by SO (upper panel).  The uncertainty
  of the visual magnitudes is 0.05-0.10 mag before HJD 2451900
  and 0.03-0.05 mag thereafter. The times of H$\alpha$ observations by
  Hanuschik et al. (1996) are indicated in the upper part of the lower
  panel.  Intervals of the spectroscopic observing runs used in 
  Fig.\ 2 are marked in a similar way in the upper panel. }

Three high-resolution spectra obtained
on October 23 and 24, 2001 with the {\sc FEROS} echelle spectrograph and
fiber link to the ESO 1.5-m telescope on La Silla confirm a strong
outburst event.  The total equivalent width of the Balmer emission
lines dropped significantly (which is partly due to the strong
increase in the continuum flux).  But the strength of the wings
increased with respect to previous years.  The H$\alpha$ peak height,
E/C, of $\approx$~3.0 is among the lowest values ever observed (see
Fig.\ 2 and Harmanec, 1998).  The shoulders in the emission profile
have disappeared and the previous typical wine-bottle shape is no
longer there.  The other Balmer lines, particularly H$\beta$ and
H$\delta$, show an asymmetric structure in their cores.  By contrast,
intensity of metal emission lines increased only little and their peak 
separation did not change compared to January 2000 spectra (see Rivinius
et al., 2001; Fig. 1). No emission is detectable in the wings of He {\sc i} 
lines.

\hspace*{-3cm}
\IBVSfig{7.2cm}{5193-f2.eps}{Comparison of mean emission  profiles of 
  selected spectral lines in the 1996, 1997, 1999, 2000 and 2001 (from
  \v{S}tefl et al., in preparation).  A more careful inspection shows
  a higher emission in the wings of H$\beta$ and H$\delta$ lines in
  1996 and 2001, when outbursts took place.  Due to the large number
  of spectra, the strong 1.37-d $lpv$, which causes the asymmetry in
  the 2001 H\,{\sc i} lines, is averaged out in the 1996~-~2000 mean
  spectra.  The sharp features to the left of the He\,{\sc i} line in
  the 2000 and 2001 spectra are due to a detector blemish. Dotted
  lines indicate the values of the systemic velocity $\pm$
  $v$\,sin\,$i$.  The bars to the right provide the flux scales in
  units of the local continuum; note the differences between
  individual panels. }

The two panels of Fig.\ 1 indicate that emission
outbursts appear on a time scale of 200-350 days, in agreement with
Hubert and Floquet (1998).  Nevertheless, the outbursts around
JD2448800 (1992) and 2452000 (2001) differ substantially in their
dimensions.  Both photometry and spectroscopy show that the recent
outburst lasts several times longer and has a larger amplitude.  There
is intriguing evidence that another strong outburst took place on
JD2445200-300.  It is indicated by a 0.$\!^m$4 brightening in the
Str\"omgren $b$-band (see, e.g., Fig.\ 7 of Harmanec, 1998) and an
accompanying state of low H$\alpha$ emission (Hanuschik et al., 1996).
Considering also the Hipparcos outburst, it appears that such major
events take place once in 10 years; possibly they are even repetitive 
on a time scale of 3400-3600 days.

Analysis of the 1996 outburst (\v{S}tefl et al., in preparation) shows
that 28~CMa follows the general scheme derived from line emission
outbursts of $\mu$~Cen (Rivinius et al., 1998; Baade et al., 2001) but
on a time scale, which is longer by at least one order of magnitude.
This scheme consists of four phases: relative quiescence, precursor,
outburst proper and relaxation, which are so far defined only
spectroscopically.  Because no spectra of 28\,CMa are available for the time
between February, 2000 and October, 2001, it is difficult to determine
the present phase of the 2001 outburst.  Based on the line emission
being close to a minimum, on the missing emission in He~{\sc i} lines,
and on the relatively high separation of emission peaks in metal lines, 
one may crudely guess that the outburst is in the late precursor or early
outburst phase.  Provided that it started in 2001 May or earlier, it
develops very slowly in comparison with smaller outbursts in 28\,CMa
itself but also in other Be stars such as $\mu$\,Cen.

If this large outburst does indeed fit the same scheme, the spectral
evolution over the next weeks to months might be as follows:  In the
Balmer emission lines, the peak height will steadily increase while
the wings will fade.  At the same time, the emission will increase in
the wings of He\,{\sc i} and metal lines.  
The separation of emission peaks of metal lines will decrease. A double 
structure of the blue and red peaks may develop for a limited time
until the inner part of the disk is formed.  The coexistence of two 
pairs of emission peaks may reflect a double-ring structure of the 
disk as suggested recently by Rivinius et al.\ (2001). One may also expect
the appearance of transient periods (\v{S}tefl et al.  1998, 2000),
which were already observed after the weaker 1996 outburst.  They may
be echos of the photospheric oscillations in the inner disk.  Finally, 
the visual magnitude will asymptotically approach its base value near
$4.^m0$. 

28\,CMa is a pole-on star (e.g., Maintz et al., 2000) and therefore
provides a nice illustration of the rule (e.g., Harmanec, 1983; Hubert 
\& Floquet, 1998) that during outbursts such stars brighten whereas
equator-on Be stars get fainter. 

The present strong outburst of this bright star offers considerable
opportunities:  (a) If it can be confirmed that such major and the
more frequent minor outbursts mainly differ in their dimensions, this
might place important constraints on the unknown physics of
outbursts.  (b) The slow evolution permits the outburst to be studied
in much detail.  (c) The most exciting aspect of the latter would
be to see whether during the course of an outburst the nonradial
pulsation exhibits any changes.  (d) Contemporaneous photometry and
spectroscopy can be arranged for, which could be important to
elucidate the photometric aspects of Be star outbursts. 
\medskip

{\bf Acknowledgements.} We thank Dr. Luca Pasquini, for the kind 
permission to take the October, 2001 spectra within the framework of his
ESO project No.\ 68.D-0334. S.~\v{S} appreciates the support of the Grant
Agency of the Academy of Sciences of the Czech Republic (grant no. AA3003001).

\references

Baade, D.; 1982a, {\it A\&{A}}, {\bf 105}, 65

Baade, D.; 1982b, {\it A\&{A}}, {\bf 110}, L15

Baade, D.; Rivinius, Th.; {\v S}tefl, S.; 2001, {\it IAU Circular} {\bf 7658}
% Original: Baade, D.; Rivinius, Th.; {\v S}tefl, S.; 2001, {\it IAU Circular} {\bf No.~7658}
% Corrected: Thu Aug 14 12:41:40 MET DST 2003

Harmanec, P.; 1983, {\it Hvar Obs. Bull.} {\bf 7}, 55

Harmanec, P.; 1998, {\it A\&{A}}, {\bf 334}, 558

Hanuschik, R. W.; Hummel, W.; Satorius, E.; Dietle, O.; Thimm, G.; 1996, 
        {\it A\&{A}S}, {\bf 116}, 309

Hubert, A. M.; \& Floquet, M.; 1998, {\it A\&{A}}, {\bf 335}, 565

Maintz, M.; Rivinius, Th.; Tubbesing, S.; Wolf B.; \v{S}tefl; Baade, D.; 2000,
      {\it IAU Coll.} {\bf 175}, p. 244

Perryman, M. A. C.; and the Hipparcos Science Team, 1997, {\it The Hipparcos
and Tycho Catalogues}, ESA SP 1200

Rivinius, Th.; Baade, D.; \v{S}tefl, S.; Stahl, O.; Wolf, B.; Kaufer, A.;
   1998, {\it A\&{A}}, {\bf 333}, 125

Rivinius, Th.; Baade, D.; \v{S}tefl, S.; Maintz, M.; 2001, {\it A\&{A}}, {\bf 379}, 257 

\v{S}tefl, S.; Baade, D.; Rivinius, Th.; Kaufer, A.; Stahl, O.; Wolf, B.; 
  1998, {\it ASP Conf.\ Series}, {\bf 135}, 348, in: {\it A Half Century of 
  Stellar Pulsation Interpretation: A Tribute to Arthur N. Cox}, eds.~P.A. 
  Bradley and J.A. Guzik
%Corrected : Tue May 31 13:29:15 MEST 2005

\v{S}tefl, S.; Aerts, C.; Balona, L. A.; 1999, {\it MNRAS}, {\bf 305}, 505

\v{S}tefl, S.; Budovi\v{c}ov\'{a}, A.; Baade, D.; Kaufer, A.; Rivinius, Th.; 
    Stahl, O.; Wolf, B.; 2000, in {\it IAU Coll.} {\bf 175}, p. 240


\end{document}


