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\begin{document}
\IBVShead{4573}{25 March 1998}

\IBVStitle{Spectroscopic and photometric variations of HR 5}


\IBVSauth{M. WEBER, K. G. STRASSMEIER}
\IBVSinsto{Institut f\"ur Astronomie, Universit\"at Wien, T\"urkenschanzstr. 17, A-1180 Wien, Austria}

\IBVSinsto{e-mail: weber@astro.univie.ac.at, strassmeier@astro.univie.ac.at}

\SIMBADobj{HR5}

\newcommand{\kms}{km\,s$^{-1}$}
\newcommand{\vsini}{$v\sin i$}

\begintext

We report on the discovery of radial velocity variations of the B component 
of the visual binary HR~5AB (=ADS~61, A-B separation: 1\farcs 455$\pm$0\farcs 004; 
ESA 1997) with an orbital period of 1.026 days.  Simultaneous 
photometric observations confirm the low-amplitude variability of similar 
periodicity originally discovered by Brettman et al. (1983). HR~5AB is a 
bright (V$\approx$ 6\mm) visual binary with a period of 106.83 years 
(Baize 1943, Lippincott 1963). Component A has been classified as dG4 and 
component B as dG8 (see Wilson 1953), but Lippincott (1963) found individual 
masses of 1.3~M$_\odot$ and 1.5~M$_\odot$, respectively and already noted 
that the fainter star is also the more massive.  

Our spectroscopic data were obtained at the National Solar Observatory
(NSO) with the McMath--Pierce telescope and the stellar spectrograph
with a 800$^2$ TI CCD (TI-4 chip, 15$\mu$ pixels). The resolving power 
was $R$=40,000 and covered a useful wavelength range of about 45~\AA . 
The observations were made within a two-month interval between October 31, 
1996 and January 8, 1997 and consist of a total of 23 spectra centered at 
6430~\AA . The radial velocities were obtained 
from cross correlating the HR\,5 spectra with nightly spectra 
of the IAU velocity standard $\alpha$\,Ari ($v_{\rm r}$=$-$14.3~\kms).

The photometric data were obtained with the Wolfgang 0.75-m automatic 
photoelectric telescope (APT), part of the Vienna Wolfgang-Amadeus twin APT 
at Fairborn Observatory in southern Arizona (Strassmeier et al. 1997). 
The observations were made differentially through Str\"omgren $b$ and $y$ filters 
with respect to HD\,663 as the comparison star (V=6.68 mag, B$-$V=1.212),  
and HD\,224784 as the check star (V=6.18 mag, B$-$V=1.032). Altogether, 48 
data points were gathered between JD\,2,450,395 and JD\,2,450,441 
and 7 additional points between JD\,2,450,625 and 2,450,630. 

\vskip 3mm
\centerline{Table 1. Preliminary orbital elements of HR~5B}
\vskip 3mm
\begin{center}
\begin{tabular}{ll}
  \hline
  Orbital element                  & Value \\
  \hline
  $P$\ (days)                      & 1.0260$\pm$0.0013  \\
  $T_0$ (HJD)                      & 2,450,393.95 \\
  $\gamma$ (\kms)                  & --12.1$\pm$0.6 \\
  $K_1$ (\kms)                     & 8.1$\pm$0.9 \\
  $e$                              & 0.0 (adopted)  \\
  $a_1 \sin i$ (km)                & 0.114$\pm$0.013 $\times$ 10$^6$ \\
  $f(M)$                           & 0.57$\pm$0.19 $\times$ 10$^{-4}$\\
  \hline
\end{tabular}
\end{center}

% -----------------------------------
\IBVSfig{7cm}{4573-f1.ps}{
Two representative spectra of HR~5 taken on Dec.~2, 1996 (JD 2,450,420, top 
panel) and on Dec. 14, 1996 (2,450,432, bottom panel). 
HR~5A is the stationary component and Ba the variable but always 
red-shifted component with respect to A. 
\label{spectrum}}

Figure 1 shows two representative spectra where the AB components 
are either clearly resolved (top panel) or blended (bottom panel). 
The stronger line is identified with the stationary A component while the 
weaker line is due to the variable Ba component. The Bb component is 
invisible. To measure precise radial velocities, we fitted two Gaussians  
to the doubled cross-correlation peaks. The resulting velocities for the A 
and Ba component are plotted in Figure 2.  
The standard error of a measure of unit weight is 1.1~\kms \ 
for component A and 4.0~\kms \ for component Ba.    
To estimate the projected rotational velocities of the two components,  
we also fitted two Gaussians to the blended spectral lines and obtained  
\vsini(A) = 4.7$\pm$1.7 \kms \ and \vsini(Ba) = 4.7$\pm$1.3 \kms \ for 
component A and Ba, respectively. A radial-tangential macroturbulence model 
of 3~\kms \ has been adopted for both components according to Gray (1992). 

A period analysis of the radial velocities of component B showed a strong 
peak at 1.0260 days (Figure~2) and this was adopted to be the orbital 
period of Ba and Bb. We then used a modified version of the 
differential-correction program of Barker et al. (1967) to compute a 
SB-1 orbit for Bab. The elements are listed in Table~1.
Note that a circular orbit was finally assumed since the error in $e$ was 
larger than its optimized value. Figure~2 also shows the radial 
velocities of the A-component. Its standard deviation is 1.1~\kms \ while 
the B-component's are much larger (4~\kms ).  
% -----------------------------------
\IBVSfig{7cm}{4573-f2.ps}{
Periodogram and radial-velocity curve for component Ba. The best-fit period 
is indicated as $f_1$ (1.026 days) and its $1-f_1$ alias as $f_2$ (39.5 days).
The observed velocities are plotted as dots with error bars, while the  
line is the orbit from Table~1.  Also shown are the radial velocities of 
component A. 
\label{p-orb}}

The photometric data show a marginally significant period of 1.127$\pm$0.008 
day (Figure 3) which is -- formally -- not consistent with the  
1.082$\pm$0.002-day period found by Brettman et al. (1983). Due to the small 
amplitude of just $\approx$10 mmag in $b$ and $y$, and because our time 
coverage was just two months with one measure per night, we believe that the 
two periods are still consistent.  

The Hipparcos catalog (ESA 1997) lists HR\,5 with a parallax of 49.3$\pm$0.87 
milli-arcseconds corresponding to a distance of 20.28$\pm$0.43 pc and with 
magnitudes in the Hipparcos system of 6.488$\pm$0.004 and 7.427$\pm$0.009 
for the A and B component, respectively. The respective Johnson UBV 
magnitudes are $V({\rm A})$=6.352 and $V({\rm B})$=7.291 mag, 
which add up to 5.97 mag for the whole A+Ba+Bb system. The magnitude 
difference B--A is 0.94 mag, i.e. larger by almost 0.2 mag compared to 
Wallenquist's (1954) 0.75-mag value from photographic plates.  The fact that 
the photometric period is almost equal to the orbital period of Ba+b 
(different by just 9\%) 
suggests that the weaker of the two visual components, the Ba component, is 
the photometric variable. Of course, our photometry contains both visible 
components but we may subtract the (presumably constant) A-component from 
the total light and derive the maximum V magnitude for B of  
$V_{\rm max}({\rm B})$=7.181 mag (transformed from Str\"omgren $y$ to 
Johnson $V$ with the relation of Olsen 1983). The corrected amplitude of 
Ba is then 0.015 mag. 

With the new Hipparcos distance, we first obtain absolute magnitudes for the 
two visual components of $M_V({\rm A})$=+4.82$\pm$0.05 mag and 
$M_V({\rm Ba+b})$=+5.65$\pm$0.05 mag, 
and also revise the total mass of 2.8$\pm$0.6 M$_\odot$ from the visual orbit 
(Lippincott 1963) to 2.25$\pm$0.14 M$_\odot$ (the semi-major axis  
$a$ is revised to 29.5$\pm$0.6 A.U.). According to Gray (1992), the  
magnitudes match G3V and $\approx$G8V stars for A and Ba and, in case we 
neglect interstellar absorption, suggest individual masses of 1.04 M$_\odot$ 
and $\approx$0.88 M$_\odot$ and radii of 0.99 R$_\odot$ and 0.87 R$_\odot$, 
respectively.
The relative mass ratio $M_{\rm B}/(M_{\rm A}+M_{\rm B}$) of 0.546$\pm$0.006 
was computed by Lippincott (1963) from the brightness difference 
$\Delta m_{\rm pg}=0.75$ (Wallenquist 1954; note that Wilson (1953) listed a 
$\Delta m$ of 0.9). The luminosity ratio $L_{\rm B}/L_{\rm A}$  
from the Hipparcos brightness difference of $\Delta m_{\rm Hip}=0.94$ is 
$\approx$0.42 and agrees with the line ratio of 0.69$\pm$0.07 (1$\sigma$) 
measured from our red-wavelength spectra corresponding to 
$\Delta m_{\rm R}=0.40$ in the R-band. 
% -----------------------------------
\IBVSfig{7cm}{4573-f3.ps}{
Periodogram and light curves from the Str\"omgren $by$ APT data from 
Nov.--Jan. 1996/97. A peak at $P$=1.127 days near the orbital 
period is seen but is of marginal significance with respect to the average 
noise 
level of around 10 mmag. Since the standard error of a single datapoint is 
just 2.5~mmag for Wolfgang, the large noise level indicates that our 
sampling did not maintain phase coherence.    
\label{p-phot}}

Finally, with the revised total mass of 2.25~M$_\odot$, we obtain  
$M_{\rm Ba+b}$=1.23$\pm$0.09 M$_\odot$ and $M_{\rm A}$=1.02$\pm$0.23 
M$_\odot$ for the two visual components.  
The former value would leave just 0.35~M$_\odot$ for M$_{\rm Bb}$ 
in case the tabulated mass of a G8-dwarf is used for M$_{\rm Ba}$. The 
mass function from Table~1 further constrains the minimum Bb mass to 
$M_{\rm Bb}\sin i\approx$0.044 M$_\odot$. 
If we assume that the light-curve variability is due to starspots, which is 
very likely for a G8-dwarf and was already suggested by Brettman et al. 
(1983), and thus the photometric period is equal to the rotation period 
of the Ba component, our measured \vsini \ would correspond to a minimum 
radius for Ba of 
$R\sin i({\rm Ba})$=0.10$\pm$0.04. This would require the unusual low 
inclination of the rotation axis of 6\fdeg 6$\pm$3\fdeg 5 to match a typical 
radius for a late-G dwarf star of $\approx$0.87 R$_\odot$. If we further 
assume that the rotation axis of Ba is perpendicular to the orbital plane 
of Ba--Bb, the minimum mass for Bb corresponds to $\approx$0.38 M$_\odot$, 
in good agreement with the mass estimated from the visual orbit. The spectral 
type for Bb is then most likely M2 or M3. Note that the orbital plane of the 
spectroscopic binary Bab would then be inclined to the orbital plane of 
the visual components AB by $\approx$40$^\circ$.  
\vskip 3mm
\noindent We acknowledge support from the Austrian Fond zur F\"orderung der 
wissenschaftlichen Forschung (FWF) through grants S7301-AST and S7302-AST.


\references

Barker, E.~S., Evans, D.~S., Laing, J.~D.: 1967, {\em R. Obs. Bull.} 130

Baize P.: 1943, {\em Astronomie\/}, {\bf 57}, 44

Brettman, O.~H., Fried, R.~E., Duvall, W.~M., Hall, D.~S., Poe, C.~H., 
Shaw, J.~S.: 1983, {\em IBVS}, No.\,2389

ESA: 1997, {\em The Hipparcos Catalogue}, ESA SP-1200

Gray, D.~F.: 1992, {\em The observation and analysis of stellar photospheres},
Cambridge University Press, 2nd edition

Lippincott, S. L.: 1963, {\em AJ}, {\bf 68}, 45

Olsen, E. H.: 1983, {\em A\&AS}, {\bf 54}, 55

Strassmeier, K.~G., Boyd, L.~J., Epand, D.~H., Granzer, T.: 1997,
{\em PASP}, {\bf 109}, 697

Wallenquist, A.: 1954, {\em Uppsala Astron. Obs. Ann.\/}, {\bf 4}, 2

Wilson, R. E.: 1953, {\em General catalogue of stellar radial velocities}

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