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\IBVShead{6169}{26 May 2016}

\IBVStitle{Monitoring the Radial Velocity of H\lowercase{e}I 6678 of {\boldmath$\gamma$} C\lowercase{as}}

\IBVSauth{Pollmann, Ernst}

\IBVSinsto{International Working Group ASPA, Emil-Nolde-Str. 12, 51375 Leverkusen, Germany}

\SIMBADobj{gamma Cas}


\begintext

\section*{Introduction}
The Be star $\gamma$ Cas (27 Cas, HD 5394, HR 264) is a primary component of a spectroscopic binary and is the very first Be star known, discovered by Secchi (1887). Spectroscopically $\gamma$ Cas has been investigated mostly in the Balmer lines, mainly in H$\alpha$. Recent studies considered He and Fe II lines as well as the kinematics of the circumstellar shell (Hanuschik, 1994, Smith, 1995). The HeI 6678 line has an important diagnostic value of activity close to the star's surface. Investigations of Smith (1995), Harmanec et al. (2000), Harmanec (2002), Pollmann \& Stober (2007) and Pollmann (2009) give detailed information about the long-term behaviour of the equivalent widths of the HeI 6678  emission line.

In the context of the investigations concerning the periodic behaviour of the ratio $V/R$ of the relative intensities of the violet component $I_v$ to the red component $I_r$ of the HeI 6678 emission line in $\gamma$ Cas (Fig. 1), as it was described by Pollmann \& Guarro (2015), a clearly detectable and clear variation of its radial velocity with a spectral resolving power of 10000 to 20000 were noticed. The HeI 6678 emission is formed close to star’s photosphere and, therefore, it should be possible to measure RV without restructuring or turbulence effects, as in the outer region of the disk at H$\alpha$. Harmanec et al. (2000) (H2000 hereafter) argued that there are at different times ``migrating sub-features'' moving across the H$\alpha$ line profile which affect the blue and red wings. 

Against this background, the RV monitoring of the HeI 6678 absorption core of $\gamma$ Cas will offer an opportunity to observe an ``undisturbed'' process. The BeSS data base\footnote{\tt http://basebe.obspm.fr/basebe/} provides the use of corresponding $\gamma$ Cas HeI 6678 spectra of observers of the ARAS group\footnote{\tt http://www.astrosurf.com/aras} from the years 2000 to 2016. Altogether 112 spectra of that time period were usedfor the investigation presented here, 52 BeSS spectra (observers are mentioned the caption to Fig.~2) from Sept. 2000 (JD 2451810) to Feb. 2016 (JD 2457442), 15 spectra of the Pollmann \& Guarro (2015) paper, and 45 further spectra of the author (not all in BeSS). All RV data used in the analysis present here are available at: {\tt http://astrospectroscopy.de/media/files/RV\_data\_gamcas.txtt}.

\IBVSfig{10cm}{6169-f1.eps}{Example spectrum of the HeI 6678 emission of $\gamma$ Cas, taken by the author November 01, 2015 with a LHIRES III spectrograph (R ca. 17000)}
\IBVSfigKey{6169-f1.eps}{gammas Cas}{spectrum}

\section*{Observations}
The spectra for the RV measurements of the HeI 6678 line presented here, were taken at different locations with 20 cm Newtonian and 40 cm Schmidt-Cassegrain telescopes. Spectrographs with spectral resolving powers of 10000 to 20000 were used. The signal to noise ratio of these spectra were of the order of magnitude $S/N$ ca. 200-300. The spectra have been reduced with standard procedures (instr. response, normalisation, wavelength calibration) using the program VSPEC\footnote{\tt http://www.astrosurf.com/vdesnoux}.

The evaluation of the heliocentric RV was performed by the profile mirror method. This method measures the Doppler shift of spectra by correlation of the spectral lines with their mirroring around the laboratory wavelength, and is particularly suitable for the evaluation of asymmetrical lines within exactly specified profile ranges.

\section*{Results }
With the HeI 6678 RV of these spectra, along with Fig. 2 of the H2000 paper, it was possible to design a total overview of the RV time behaviour since 1993 (Fig. 2). The time base of 22 years in that long-term overview with the BeSS data starting from September 2000 demonstrates the continuation of the RV process from where the H2000 measurements ended. 

\IBVSfig{8.5cm}{6169-f2.eps}{The compound long-term monitoring of RV measurements of H2000 (open circles), and the monitoring of observers of the ARAS group: Pollmann (open squares), Guarro (magenta squares), Lester (turquoise squares), Montier (yellow squares), Houpert (red squares), Buil (blue squares), Thizy (green squares), Berardi (open triangle)}
\IBVSfigKey{6169-f2.eps}{gammas Cas}{radial velocity}

The first surprise is that after the RV (H2000) maximum with +90~km/s (approx. JD 2451300, May 1999), our campaign clearly shows the RV at a more or less constant level between 0.5 and $-$20~km/s. This overall RV behaviour leads to the question of the physical causes within this ring-like Helium zone, the answers to which cannot be given here. But at least it was interesting to see, whether the periodic behaviour of the HeI 6678 RV between Sept. 1993 and Sept. 2000, found by H2000, has changed. In order to find this out, a PDM (Phase Dispersion Minimization) analysis (Stellingwerf, 1978) of our data from Sept. 2000 to Dec. 2015 has been performed. The result with a clearly detectable period of 202 d is shown in Fig. 3. The corresponding phase diagram is shown in Fig. 4.

\IBVSfig{9cm}{6169-f3.eps}{The PDM analysis of the ARAS RV data from Sept. 2000 to Dec. 2015 with a clearly detectable period of 202.33 days.}
\IBVSfigKey{6169-f3.eps}{gammas Cas}{radial velocity}


\IBVSfig{8.5cm}{6169-f4.eps}{Phase diagram of the found period of the PDM analysis in Fig. 3}
\IBVSfigKey{6169-f4.eps}{gammas Cas}{radial velocity}

Table 1 shows the comparison of the period, semi-amplitude, epoch $T_0$ and RMS, measured as result of the ARAS analysis and the results of H2000.


\begin{table}
\centering
\begin{tabular}{lcccc}
\hline
& Period & Semi-amplitude & Epoch $T_0$ (JD) & RMS \\
\hline
H2000 & $203.59\pm ?$~d & $7.0\pm 1.5$~km/s & $2450578.7\pm ?$ & 8.95~km/s\\
ARAS & $202.2\pm 0.6$~d & $4.44\pm 0.51$~km/s & $2451740.89\pm 14.3$ & 3.6~km/s\\
\hline

\end{tabular}
\caption{Result comparison of the analysis of H2000 and ARAS. }
\end{table}

\section*{Discussion}
With our instruments nowadays we were able to achieve primarily a much better signal to noise ratio ($S/N$) than H2000. In addition the application of the spline filtering interpolation of the program VSPEC enables in small limits by adjustment of very carefully adapted spline coefficients, to smooth the line profile (ca. factor 1.1 to 1.2 of the normalized continuum), without any modification of the spectral information itself. The use of that tool leads to an improvement of the accuracy of the RV determination. With the results in Figs. 3 \& 4, this campaign confirms a clearly detectable phase behaviour with a period very close to the period of H2000, in spite of very different RV time behaviour, shown in Fig. 2. It is difficult to judge, whether the small difference of 1.39~km/s established by H2000 and the period found in this analysis, is really significant. Maybe the small difference between the period is caused by the very strong change of the RV amplitude (Fig. 2) during the observation period of H2000. I want to emphasize that with our spectra we achieved a much better RMS (3.6 km/s) than H2000 (8.95~km/s).

\section*{Acknowledgements}
I am grateful to Sara and Carl Sawicki (Alpine, Texas, USA) for their helpful improvements and suggestions in language. 


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