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\IBVShead{4538}{15 December 1997}

\IBVStitletl{Photospheric and chromospheric activity of the}{ bright and single G5 dwarf HR~4864 = HD\,111395}

\IBVSauth{K.~G.~STRASSMEIER, Th.~GRANZER, M.~WEBER}

\IBVSinsto{Institut f\"ur Astronomie, Universit\"at Wien, 
T\"urkenschanzstr. 17, A-1180 Wien, Austria}
\vskip -1mm
\IBVSinsto{e-mail: strassmeier@astro.ast.univie.ac.at, wooz@mozart.ast.univie.ac.at, weber@astro.ast.univie.ac.at}

\SIMBADobj{HR4864}
\IBVStyp{ G5V }
\IBVSkey{photometry, spectroscopy}
\IBVSabs{We present new photometric and spectroscopic data for HR 4864} 
\IBVSabs{and conclude that the star is a spotted and chromospherically} 
\IBVSabs{active solar-type star. }

\begintext
Stars similar to our Sun are an important test field for the deeper 
understanding of solar physics. So it is no surprise that much effort has 
been put into the study of magnetic activity of these stars culminating 
in the still ongoing, long-term Mt. Wilson H\&K project initiated 
by the late Olin C. Wilson in the sixties. 
However, there is a real problem finding solar twins, i.e. stars that exactly 
match the Sun's astrophysical parameters (see, e.g., the annual Lowell 
workshops) and even if stars are found with only approximate solar parameters  
they are usually faint and not easily accessible for high-resolution 
spectroscopy. In this paper we draw attention to a single and bright G5 
dwarf that deserves further high-resolution monitoring. 

% ------------------  Table 1
\vskip 3mm
\centerline{Table 1. NSO and KPNO radial velocities 
(for HJD 2,450,000+, in km\,s$^{-1}$)}
\begin{center}
\begin{tabular}{cccc|cccc|cccc}
\hline
HJD   &  $v_{\rm r}$ & $\sigma$ & Obs. & 
HJD   &  $v_{\rm r}$ & $\sigma$ & Obs. & 
HJD   &  $v_{\rm r}$ & $\sigma$ & Obs. \\
\hline
 432.037 & --8.6 & 1.3 & NSO & 439.022 & --6.4 & 1.3 & NSO & 541.815 
& --10.8 & 0.75 & KP \\
 433.022 & --9.6 & 1.2 & NSO & 441.032 & --8.7 & 1.2 & NSO & 548.729 
& --9.2  & 0.37 & KP \\
 434.028 & --8.4 & 1.2 & NSO & 442.036 & --5.7 & 1.7 & NSO & 549.881 
& --8.9  & 0.39 & KP \\
 435.040 & --8.5 & 1.2 & NSO & 447.016 & --7.2 & 1.3 & NSO & 550.903 
& --9.3  & 0.39 & KP \\
 436.057 & --10.2& 1.5 & NSO & 448.024 & --9.1 & 1.4 & NSO & 553.824 
& --9.0  & 0.48 & KP \\
 437.062 & --10.0& 1.5 & NSO & 451.027 & --8.4 & 1.5 & NSO & 554.831 
& --9.1  & 0.49 & KP \\
 438.023 & --9.5 & 1.6 & NSO & 458.018 & --7.0 & 1.3 & NSO & 
& & & \\
\hline
\end{tabular}
\end{center}
\vskip 3mm

Spectroscopic data were obtained in April 1997 at KPNO using the coud\'e 
feed telescope 
with camera 5, grating A, the long collimator and the 3k$\times$1k F3KB 
CCD. This set up allows a resolution of $R=35,000$ in the red wavelength 
regions and $R=30,000$ in the blue. The summation of several short exposures 
enables a S/N ratio of close to 800:1 for the red spectra and approximately 
300 for the Ca\,{\sc ii} H\&K spectra centered at 3950~\AA . Earlier 
6420-\AA \ spectra were obtained in December 1996 with the 
NSO McMath telescope and the stellar spectrograph at a resolution of 40,000 
and varying S/N of 150--200:1. 
Table~1 presents 20 new radial velocities for HR~4864 that were obtained from 
cross correlations of the entire HR~4864 spectra with spectra of the IAU 
radial velocity standards $\beta$~Gem, 16~Vir, and $\alpha$~Ari. These data 
indicate a constant velocity of --8.7$\pm$1.3~km\,s$^{-1}$ 
in good agreement with earlier 
data by Beavers \& Eitter (1986) and Duquennoy et al. (1991) who 
found average velocities of --8.6 and --9.1~km\,s$^{-1}$, respectively. It  
seems now safe to say that we can exclude any binarity for HR\,4864.  

Figure~1 shows three interesting spectral regions of HR\,4864; the 
6420-\AA \ region with several unblended photospheric absorption lines (a 
region that is frequently used for Doppler imaging), a spectrum of H$\alpha$,   
and a spectrum including the two Ca\,{\sc ii} resonance lines at around 
3950~\AA . Cross correlations 
of the many weak and moderately-strong photospheric lines (residual 
intensity above 0.6) showed an average FWHM of 0.123$\pm$0.002 \AA ,  
and with the calibration of Fekel (1997) this value relates to a 
projected rotational velocity $v\sin i$ of 2.9$\pm$0.4~km\,s$^{-1}$  
(adopting a radial tangential macroturbulence velocity of 3~km\,s$^{-1}$). 
A comparison of the overall spectral appearance of HR\,4864 in the 6340--6600 
\AA \ wavelength region is in very good agreement with its G5V 
classification from prism spectra at 75~\AA/mm by Harlan \& Taylor (1970).  
The only significant disagreement is the 
Fe\,{\sc i}\,6430/Fe\, {\sc ii}\,6432 line ratio (Figure 1, upper panel) that 
indicates a more solar-like temperature (G1 to G2) rather than G5. However, 
the singly-ionized line may overlap with an 
(unresolved) water vapor line at exactly the same wavelength. Also, 
Eggen (1978) mentioned a slight overabundance of iron relative 
to the Sun, [Fe/H]=+0.2, which could affect the observed B$-$V color and thus 
the line ratio versus $T_{\rm eff}$ calibration. 

\IBVSfig{10cm}{4538-f1.ps}{KPNO spectra of HR~4864 for three spectral 
regions of interest. From top to bottom: photospheric lines near 6420 \AA , 
H$\alpha$, and Ca\,{\sc ii} H\&K. Note the core emission at the bottom of 
the Ca resonance lines typical for chromospherically active stars}

The H$\alpha$ line appears like a normal absorption feature with line wings 
typical for a G-dwarf slightly cooler than the Sun. A comparison with 
spectra of other G dwarfs (not obtained with the same equipment though) 
indicates very small filling in of the line core, but this needs to be 
confirmed with higher resolution data. 

Our single Ca\,{\sc ii} spectrum in Fig.~1 clearly shows emission in the 
cores of the H and K lines. In a G5V star we interpret these due to an 
active chromosphere. Their emission strengths significantly exceed the  
line strength seen in solar plage regions of the active Sun. Using the 
calibration of Linsky et al. (1979) and V$-$R=0.54 we measure an absolute 
emission surface flux in the H and K lines of 2.1$\times$10$^6$ and 
2.2$\times$10$^6$ erg\,cm$^{-2}$s$^{-1}$, respectively.  

\IBVSfig{6cm}{4538-f2.ps}{Seasonal V and B$-$V light and color curves of 
HR\,4864. The 1996 light curve combines 53 consecutive nights starting at 
JD 2,450,212 while the 1997 data cover 92 consecutive nights beginning at 
JD 2,450,429}
\vskip -1.5cm

\IBVSfig{7.5cm}{4538-f3.ps}{Periodogram from the combined 1996 and 1997 
V-band photometry. The data indicate a best-fit frequency at $f_1=0.059$ 
cycles/day corresponding to a period of 16.95$\pm$0.50 days. }

Continuous photometry of HR\,4864 has been carried out with one of the 
University of Vienna twin APTs at Washington Camp in southern Arizona 
(Strassmeier et al. 1997) since early 1996 and a first light curve was 
presented in that paper. Here we analyse this data and present additional 
BV-photometry from 1997 (Figure 2). The comparison and check stars were 
HD\,111469 (V=5.78, B$-$V=0.03 mag) and HD\,111812 (V=4.94, B$-$V=0.67 mag), 
respectively. All readings incorporated a 2.5-mag neutral-density filter. 
Figure~3 shows the periodogram and the 
window function for all available V data of HR\,4864. A single period of 
16.95$\pm$0.50 days gives the largest reduction of the squared residuals 
and we interpret this period as the rotation period of the star. In 1996 
the light curve was nearly sinusoidal with a full peak to peak V amplitude 
of just 0.019$\pm$0.003 mag, while in 1997 the light curve appeared to have 
significantly changed in amplitude at around JD 2,450,500 from 0.025 mag 
before to 0.015 mag thereafter. 

The Hipparcos catalog (ESA 1997) lists HR\,4864 with a V magnitude of 6.29, 
and B$-$V=0.703 mag and a parallax which implies a distance of 17.2 pc. The 
absolute visual brightness is thus +5.1 mag in perfect agreement with the 
tabulated brightness of a G5V star according to Gray (1992). The expected 
B$-$V color, on the other hand, is 0.672 mag and our measured B$-$V was 
0.690$\pm$0.003 (std) mag in 1996 and 0.680$\pm$0.003 (std) mag in 1997. 
The decrease of 0.01 mag is barely significant but still noticeable in Figure 2 
and could be attributed to overall changes of the starspot activity. 

Our new measure of $v\sin i=2.9\pm0.4$ km\,s$^{-1}$ and 
$P_{\rm rot}=16.95\pm0.50$ days determines the minimum stellar radius to  
$R\sin i=0.97$~R$_\odot$. The nominal radius of a G5V star is tabulated 
as 0.96~R$_\odot$ (e.g. Gray 1992). HR\,4864 is thus seen almost 
equator-on, i.e. $\sin i\approx0.988$ or $i\approx81^\circ$. 

With all these parameters known, HR\,4864 might also be a good candidate to  
search for further extra-solar planets by means of high-precision radial 
velocities or eclipse techniques and could eventually help to resolve 
the 51~Peg debate. \\

{\bf Acknowledgements.} We thank the Austrian Science Fond (FWF) for 
support under grants S7301-AST and S7302-AST.

\vskip 1cm
\references

Beavers W. I., Eitter J. J., 1986, {\it ApJS}, {\bf 62}, 147 

Duquennoy A., Mayor M., Halbwachs J.-L., 1991, {\it A\&AS}, {\bf 88}, 281

Eggen O. J., 1978, {\it ApJ}, {\bf 222}, 191 

ESA-Hipparcos, 1997, {\it The Hipparcos and Tycho catalog}, ESA SP-1200 

Fekel F. C., 1997, {\it PASP}, {\bf 109}, 514 

Gray D. F., 1992, {\it The observation and analysis of stellar photospheres}, 
Cambridge Univ. Press, appendix 2 

Harlan E. A., Taylor D. C., 1970, {\it AJ}, {\bf 75}, 507 

Linsky J. L., Worden S. P., McClintock W., Robertson R. M., 1979, 
{\it ApJS}, {\bf 41}, 47 

Strassmeier K. G., Boyd L. J., Epand D. H., Granzer Th., 1997, 
{\it PASP}, {\bf 109}, 697 

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