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\IBVShead{5397}{27 March 2003}

\IBVStitle{A New Bright Helium Variable B Star: HR\,2949}

\IBVSauth{Rivinius, Th.$^1$; Stahl, O.$^1$; Baade, D.$^2$; Kaufer, A.$^3$}

\IBVSinst{Landessternwarte Heidelberg, D-69117 Heidelberg, Germany; e-mail: T.Rivinius@lsw.uni-heidelberg.de}
\IBVSinst{European Southern Observatory, Karl-Schwarzschild-Str.~2, D-85748 Garching bei M\"{u}nchen, Germany; e-mail: dbaade@eso.org}
\IBVSinst{European Southern Observatory, Casilla 19001, Santiago 19, Chile; e-mail: akaufer@eso.org}

\SIMBADobj{HR 2949}
\SIMBADobj{HIP 37229}

\IBVSabs{We report high-resolution spectroscopic observations of HR 2949,}
\IBVSabs{revealing the star as Helium variable B star with P=1.9093d. }

\begintext

During a search for pulsational line profile variability in Bn stars,
both components of the visual pair HD\,61555/6 have been observed with
{\sc Feros} at La Silla, Chile.  While the results about Bn star
variability will be reported elsewhere, the component with the lower
$v\sin i$, HD\,61556, was found to be spectroscopically variable.

A variability search in the photometric Hipparcos database
(Koen\,\&\,Eyer, 2002) revealed a period of $P=1.9093$\,d with an
amplitude of $A=0.0063$\,mag.  However, the Hipparcos identifier
HIP\,37229 corresponds to the combined light of both objects
($V=3\fmm83$, Hauck\,\&\,Mermilliod, 1998), while the components in
fact have $V=4\fmm53$ (HD\,61555, B6\,V) and $V=4\fmm78$
(HD\,61556, B5\,IV), also taken from Hauck\,\& Mermilliod (1998). The
latter star has been suspected to be variable already by Kukarkin\,\&
Kholopov (1982, NSV\,3673).  For the folded Hipparcos light curve
(Figure~1), we adopt
\[
T_{\rm min~light} = {\rm JD}\,2448001.1 + E\times 1.9093
\]


HD\,61556 was observed spectroscopically four times with an exposure
  time of 300~seconds (Table~1). {\sc Feros} covers the wavelength
  range from 370 to 920\,nm with a resolving power
  $R=\Delta\lambda/\lambda = 48\,000$ (Kaufer et al., 1997).

The equivalent widths of most lines, particularly the He{\sc i} lines,
were found to be variable. Although the four spectra do not allow to
derive the period independently, the two last spectra were taken one
day apart.  Since these latter two spectra roughly bracket the full
observed range of variability, this supports a two day timescale in
the equivalent width variations. In the following, we will, therefore,
assume the photometric period also for the spectroscopic variations.


While the Balmer lines do not vary at all, the {He}{\sc i} lines are
highly variable. When the star is bright the He{\sc i} lines are
strong. Sorted by decreasing relative amplitude, lines of Fe{\sc ii}, C{\sc
ii}, and Mg{\sc ii} vary in phase with {He}{\sc i}. Lines of Si{\sc
ii} and Si{\sc iii} vary in antiphase w.r.t.~He{\sc i}.


Judging from the period and the nature of the spectral variations
(Figs.\ 2 and 3), the star is likely a magnetic variable with a strong
surface field having produced surface abundance inhomogeneities. Thus the
period of 1.9093\,d is the period of rotation. For a B5\,IV star with
a typical radius of 5\,R$_\odot$, this would imply $v_{\rm rot}\approx
130$\,km\,s$^{-1}$.  Since the equivalent widths of metal lines (Table
1) indicate this classification to be probably slightly too cool,
which is typical for undetected He-weak stars (Jaschek\,\&\,Jaschek,
1987), the derived velocity should be a robust lower limit.

\vspace*{-0.4cm}

\IBVSfig{9cm}{5397-f1.eps}{ The photometry sorted with $P=1.9093$\,d. The
Hipparcos magnitude differs somewhat from that in the V band. Note the
outliers towards lower magnitudes are the result of partial
disentanglement of the visual pair. For this reason also only
measurements with a standard error of 0.01\,mag or less are plotted.}
%%\IBVSfigKey{5397-f1.eps}{HR 2949}{light curve}

The width of the lines is about $v \sin i = 70$\,km\,s$^{-1}$,
which gives a relatively polar inclination of $i\approx30^\circ$ for
the rotational axis.

Because the Hipparcos amplitude of 0.0063\,mag corresponds to the
variations of the combined light (around 3.83\,mag), but the variable
component is a star of only 4.78\,mag, the real amplitude is higher by
a factor of $10^{-0.4\times(3.83-4.78)}=2.42$, i.e.\ about 0.015\,mag.

These observations put HD\,61556 among the brighter chemically
peculiar B stars in the sky. Due to the photometric variability, it
should be removed from the catalog of $uvby\beta$ standard stars (Perry et
al., 1987).

\vfill

\centerline{Table 1. Spectroscopic observations and measured equivalent width for several lines.}  
\centerline{Typical uncertainties are in the order of 5\,\%.}
\vskip 3mm
\begin{center}
\begin{tabular}{rrrrrcccc}
\hline
\multicolumn{1}{c}{Phase}& \multicolumn{1}{c}{Julian Date}&
\multicolumn{5}{c}{$W_\lambda$ [m\AA ]}
\\
& & \multicolumn{1}{c}{He{\sc i}\,4026}& \multicolumn{1}{c}{He{\sc i}\,4471}
&\multicolumn{1}{c}{He{\sc i}\,6678}& \multicolumn{1}{c}{Fe{\sc ii}\,4549}
& \multicolumn{1}{c}{Si{\sc iii}\,4553}
\\
%
\hline
0.04   & 2451151.770 & 700  & 680  & 230  & 64 & 91  \\
0.28   & 2452686.511 & 740  & 590  & 220  & 61 & 96  \\
0.61   & 2451196.764 & 1020 & 960  & 260  & 81 & 75  \\
0.80   & 2452687.507 & 1000 & 890  & 250  & 81 & 77  \\
%
\hline
\end{tabular}
\end{center}

\IBVSfig{10cm}{5397-f2.eps}{Spectroscopic line variations at phase 0.80
  (full line) vs.\ phase 0.04 (dashed line). Most species behave like
  He{\sc i}, only Si lines vary in antiphase. Balmer lines are invariant.
}
%%\IBVSfigKey{5397-f2.eps}{HR 2949}{spectrum}

\IBVSfig{10cm}{5397-f3.eps}{Variations of He{\sc i} and Si{\sc ii} lines.
Note that not only the strength, but also the shape of the profile
varies, as in the centre of the Si{\sc ii} lines.
}
%%\IBVSfigKey{5397-f3.eps}{HR 2949}{spectrum}

\clearpage

\references

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

Jaschek, C.,~\& Jaschek, M., 1987, {\it ``The classification of stars''},
Cambridge: University Press  
%%\BIBCODE{1987clst.book.....J}

Kaufer, A., Wolf, B., Andersen, L., Pasquini, L., 1997, {\it The ESO
Messenger}, {\bf 89}, 1

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

Kukarkin, B.~V.,~\& Kholopov, P.~N.,\ 1982, {\it ``New catalogue of suspected
variable stars''}, Moscow: Publication Office ``Nauka''
%%\BIBCODE{1982ncsv.book.....K}

Perry, C.~L., Olsen, E.~H., \& Crawford, D.~L.,\ 1987, {\it PASP}, {\bf
  99}, 1184 
%%\BIBCODE{1987PASP...99.1184P}

\endreferences



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