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\IBVShead{5747}{9 January 2007}

\IBVStitletl{Remarkable Absorption Strength Variability of the}{ \boldmath{$\varepsilon$} Aurigae H\boldmath{$\alpha$} Line outside Eclipse}

\IBVSauth{Schanne, L.}

\IBVSinsto{Hohlstrasse 19, D-66333 V\"olklingen (Germany); e-mail: schanne@t-online.de}

\SIMBADobj{eps Aur}

\begintext

In April and May 2005 the H$\alpha$ line of $\varepsilon$~Aur was observed in an exceptional `weak absorption
phase'. In the period October~2005 to November 2006 the normal line profile was registered again, with a variable
absorption and a weak red-shifted emission component. The time variations of the line profile and a comparison with
former observations outside eclipse are presented.

$\varepsilon$~Aur is a binary system, consisting of a yellow supergiant (F0Ia) and an enormous dusty gas disk, that
eclipses every 27 years the primary component for approximately two years. From the eclipsing light curve it is
concluded that within the dust disk one or two (B?) stars exist, which have so far never been observed directly
(Stencel, 1985). The first contact of the next eclipse is expected in August~2009.  Castelli (1978) lists the
characteristic parameters of the primary component (F0Ia).  The H$\alpha$ line of $\varepsilon$~Aur is reported in the
literature to be variable, but the line always shows a strong photospheric absorption and mostly weak emission
components on the edges of the absorption.

The observations cover the period from 1 April, 2005 to 15 November~15, 2006 (out of eclipse, phase $\approx 0.9$). The
used amateur equipment consists of a Maksutov Newton telescope (127 mm of aperture, f 1/8) and a slitless reflecting
grating spectrograph (grating 25~mm${}\times{}$25~mm, 1200 lines/mm, collimator $f=135$ mm, camera objective $f=135$
mm). The CCD camera (Audine, KAF 401E) is water-cooled. The chip temperature was, depending on the ambient temperature,
between $-10$ and $-30$~\deg{}C{}. The dispersion is 41 \AA/mm or 0.38 \AA/pixel within the range of the H$\alpha$ line.
The resolution was measured from the FWHM of terrestrial lines to 0.8 \AA\ ($R=8,000$). The quality of the adjustment
and the mechanical stability of the system limit the exposure times for a single exposure between 30 and 60 sec. For
each sum spectrum, between 10 and 80 single photographs were taken. The data were reduced using ESO MIDAS and the OPA
scripts of G. Gebhardt (www.spektros.de). The single photographs are corrected by the median of 10 darks and the
background of the sky before extraction of the spectra and their registering. No flatfield correction is performed. The
final S/N of the continuum is between 120 and 400 (Table~1). The slitless spectra were wavelength calibrated by using 3
to 6 photospheric absorption lines from the following list: Fe\,{\sc ii}: 6416.90~\AA, 6430.84~\AA, 6456.38~\AA,
6516.05~\AA, Si\,{\sc ii}: 6347.10~\AA, 6371.36~\AA\ as reference lines. The quality of EW measurements is demonstrated
by comparison of the EW-integration results of this lines with the data given by Castelli (1978) and the integrations of
a reference spectrum of $\varepsilon$~Aur (20031101) given in ELODIE (Table~2).

\begin{table}[t]
\begin{center}
\normalsize
\caption{List of spectra and equivalent widths of components of $\varepsilon$~Aur H$\alpha$ line}
\begin{tabular}{lccc|rcr}
\hline
\multicolumn{4}{c|}{{H$\alpha$ line measurements of $\varepsilon$ Aur}} & \multicolumn{3}{c}{Equivalent width 
[{\AA}]}\\
Date                & JD    & Exposure  & S/N & Blue wing & Central & Red wing \\
                    &       & time [min]&     &           & absorption &       \\
\hline
April 1, 2005       & 2,453,462.42 & 5  & 140 & $-$0.13 & 0.01 & $-$0.15 \\
April 11, 2005      & 2,453,472.40 & 10 & 170 & $-$0.18 & 0.06 & $-$0.17 \\
April 21, 2005      & 2,453,482.40 & 10 & 150 & $-$0.07 & 0.06 & $-$0.15 \\
May 10, 2005        & 2,453,501.40 & 10 & 120 & $-$0.14 & 0.06 & $-$0.24 \\
May 11, 2005        & 2,453,502.40 & 10 & 270 & $-$0.06 & 0.08 & $-$0.15 \\
October 30, 2005    & 2,453,674.48 & 30 & 300 &    0.00 & 0.66 &    0.00 \\
December 10, 2005   & 2,453,715.50 & 18 & 280 &    0.00 & 1.07 & $-$0.05 \\
January 23, 2006    & 2,453,759.31 & 27 & 300 &    0.00 & 0.99 &    0.00 \\
January 24, 2006    & 2,453,760.32 & 25 & 400 &    0.00 & 1.04 &    0.00 \\
January 30, 2006    & 2,453,766.36 & 30 & 350 &    0.00 & 1.02 &    0.00 \\
February 1, 2006    & 2,453,768.32 & 30 & 320 &    0.00 & 1.02 & $-$0.04 \\
March 12, 2006      & 2,453,807.33 & 42 & 270 &    0.00 & 0.78 & $-$0.07 \\
March 13, 2006      & 2,453,808.42 & 25 & 390 &    0.00 & 0.82 & $-$0.04 \\
April 7, 2006       & 2,453,833.42 & 15 & 160 & $-$0.06 & 0.69 & $-$0.04 \\
April 19, 2006      & 2,453,844.34 & 15 & 160 &    0.00 & 0.60 & $-$0.10 \\
May 2, 2006         & 2,453,858.40 & 50 & 370 &    0.00 & 0.55 & $-$0.12 \\
September 10, 2006  & 2,453,988.48 & 30 & 200 &    0.00 & 0.70 &    0.00 \\
September 21, 2006  & 2,454,000.53 & 27 & 190 &    0.00 & 0.66 &    0.00 \\
October 7, 2006     & 2,454,016.42 & 60 & 360 &    0.00 & 0.54 & $-$0.01 \\
November 15, 2006   & 2,454,055.46 & 52 & 210 & $-$0.02 & 0.49 & $-$0.06 \\
\hline
\end{tabular}
\end{center}
\end{table}

\vspace*{-2pt}
In Fig.~1 the observed spectra and the reference spectrum are plotted. Between 1~April (JD 2453462) and 11 May,
2005 (JD 2453502), the H$\alpha$ line shows a nearly symmetrical shell spectrum with small variations of the V/R ratio
of the emission components and an exceptionally small absorption component in the line core. On 30 October, 2005 (JD
2453674) the H$\alpha$ line was detected in pure absorption. Until the end of the 2006 observing season, the line was
observed in normal absorption, with an occasional variable red shifted emission component. Two types of line profiles
can be distinguished: The `weak absorption phase' from the beginning of the observations (1 April to 11 May, 2005), and
the `normal absorption phase' later. The emission components of the `weak absorption phase' are symmetrically shifted
towards the blue and red, respectively, by about 80 km/s relative to the absorption minimum. In the `normal absorption
phase' the red wing maximum is red-shifted by about 100 to 160 km/s. The equivalent widths of the blue wing, the red
wing and the absorption core in the spectra were calculated (F/Fc${}> 1$ emission, F/Fc${}< 1$ absorption, Table~1).
Fig.~2 shows these EW's as time series. The variability of the absorption component is the most dominant effect.

\IBVSfig{22cm}{5747-f1.ps}{H$\alpha$ line profiles of $\varepsilon$~Aur (measurements April 2005--November 2006 and a 
reference spectrum ELODIE of November 2003}
\IBVSfigKey{5747-f1.ps}{epsilon Aur}{spectrum}

\IBVSfig{10cm}{5747-f2.ps}{Equivalent widths of $\varepsilon$~Aur H$\alpha$ line components outside eclipse April 
2005--November 2006}
\IBVSfigKey{5747-f2.ps}{epsilon Aur}{}

\IBVSfig{10cm}{5747-f3.ps}{Equivalent widths of $\varepsilon$~Aur H$\alpha$ line components outside eclipse, including
data of Castelli (1978), Ferro (1985), Cha et al.\ (1995) and ELODIE (20031101)}
\IBVSfigKey{5747-f3.ps}{epsilon Aur}{}

\begin{table}
\normalsize
\begin{center}
\caption{Comparison of EW differences of measured spectra and reference spectra of $\varepsilon$~Aur }
\begin{tabular}{cc|cc|ccc|cr}
\hline
\multicolumn{2}{c|}{} & \multicolumn{2}{|c|}{Reference spectra [\AA]} & \multicolumn{3}{|c|}{Measurements [\AA]}& 
\multicolumn{2}{|c}{Differences [\AA]} \\
Line & Ion & Castelli & ELODIE & Average & Std.\ dev.  & No.\ of meas. & Castelli & ELODIE \\
\hline
6347 & SiII & 0.694 & 0.627 & 0.596 & 0.027 & 5  & $-$0.098 & $-$0.031 \\
6371 & SiII & 0.531 & 0.538 & 0.529 & 0.024 & 7  & $-$0.002 & $-$0.009 \\
6416 & FeII & 0.245 & 0.191 & 0.190 & 0.030 & 14 & $-$0.055 & $-$0.001 \\
6432 & FeII & 0.178 & 0.158 & 0.170 & 0.027 & 17 & $-$0.008 &    0.012 \\
6456 & FeII & 0.539 & 0.533 & 0.513 & 0.034 & 17 & $-$0.026 & $-$0.020 \\
6613 & ?    &   $-$ & 0.120 & 0.120 & 0.012 & 13 &      $-$ & 0.000 \\
\hline
\end{tabular}
\end{center}
\end{table}

\vspace{-36pt}
Because of the unusual eclipsing behaviour, which is caused by a dusty cloud every 27.08 years, the star has been
observed intensively.  The investigations focus on those approximately 2 years of the eclipsing events. Castelli (1977,
1978) also published two measurements out of eclipse (1971). The variable H$\alpha$ lines consisted of a central
absorption (F/Fc 0.45 and 0.55) and two weak emission components which are shifted relative to the core of the
absorption by $-72$ km/s and $+61$ km/s, respectively. Radial outward flows are attributed to instabilities in the star
producing the blue-shifted emission component.  Gas from behind the star causes the red-shifted emission component.  
The last eclipse of 1982 to 1984 is summarized by Stencel (1985). The H$\alpha$ line profiles of 1984 (Ferluga \& Heck
in Stencel, 1985) resemble the normal absorption phase, whereby partly also more intensive blue wings were registered.
However, one year earlier the spectra showed the absorption with stable red wings and variable blue wings (Boehm \&
Ferluga, 1983).  15 spectra, measured between September 1980 and May 1981 by Ferro (1985), just one year before the
eclipse of 1982 to 1984, showed a `normal absorption phase' similar to Fig.~1 with stable red wing and variable blue
wing. H$\alpha$ line profiles measured by Cha et al.\ (1994) in November 1989 until April 1992 also resemble the
profiles of the normal absorption phase in Fig.~1.  The radial velocities of the absorption centers vary between $+0.4$
and $-39.1$ km/s, the emission components vary parallel to it around $-60$ and $+60$ km/s, respectively. The equivalent
widths of the absorption move between 296 and 650 m\AA, the emission components between 0 and 343 m\AA\ (blue wing) and
0 and 295 m\AA\ (red wing). Additional measurements of Cha et al.\ (1995) in the year 1993 show absorption with a clear
blue emission wing (EW approx.\ 200 to 300 m\AA), but only a weak red wing. The absorption line has an EW of approx. 550
m\AA\ in this period. The authors discuss their observations using a model, which explains the emissions with a rotating
inhomogenous gas ring around the primary F0Ia component. UV-spectroscopy with the HST taken on 16 February, 1996 are
described by Sheffer \& Lambert (1999). The split resonance lines are attributed to a gas disk rotating in the orbit
around the invisible secondary component. The rotation speed of the disk was determined from the distance of the
emission maxima to 103 km/s. The origin of the emissions from a gas disk around the secondary component is not
confirmed, however.

Published spectra could be digitized (Castelli: spectrum February~1971; Ferro: spectra 1980-1981)). The calculated
component equivalent widths of the published spectra, of the ELODIE reference spectrum (2003) and the results of Cha et
al.\ (1994, Table 2) are shown in Fig.~3 together with the equivalent widths of Table~1. The time series demonstrate the
exceptionally small central absorption in spring 2005. The star shows a remarkable variability in absorption strength of
the core of the H$\alpha$ line outside eclipse, also in former observations.

It remains to conclude:
 \begin{itemize}
 \item H$\alpha$ line in predominant emission and vanishing core absorption - like in spring 2005 is an exceptional
 phenomenon of $\varepsilon$~Aur.
 \item The absorption components EW of the H$\alpha$ line show a remarkable variability outside eclipse.
 \end{itemize}
The line profile variations in the optical spectrum outside of the eclipsing phase, e.g.\ the presented observation of
an exceptionally weak absorption phase in H$\alpha$, are still not satisfactorily explained. The interpretation of the
H$\alpha$ line in eclipse has to take the out-of-eclipse variations into account. Further observations, also far from
eclipse, are needed.

\newpage

{\bf Acknowledgements:} I thank Dr.\ Petr Harmanec, Dr.\ Otmar Stahl and Dr.\ Andreas Kaufer for helpful discussions and
assistance during the preparation of the manuscript.


\references

Boehm, C., Ferluga, S., 1983, {\it IBVS}, No.\ 2326

Castelli, F., 1977, {\it Astrophysics and Space Science}, 49, 179

Castelli, F., 1978, {\it Astron. Astrophys.}, 69, 23

Cha, G., Li, Y., Xu J., 1995, {\it IBVS}, No.\ 4149

Cha, G., Tan, H., Xu, J., Li, Y., 1994, {\it Astron.\ Astrophys.}, 284, 874

Ferro, A.A., 1985, {\it Rev.\ Mexicana Astron.\ Astrof.}, 11, \nobibcode 113 \ADSBIBCODE{1985RMxAA..11..113A}

Sheffer, Y., Lambert, D.L., 1999, {\it PASP}, 111, 829

Stencel, R.E., 1985,  {\it NASA Conference Publication}, No.\ 2384, 1982--1984 eclipse of $\varepsilon$ Aur \ADSBIBCODE{1985eepa.rept.....S}

\endreferences



\end{document}

