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\begin{document}

\IBVShead{4755}{26 August 1999}

\IBVStitle{Absolute Properties of ZZ Ursae Majoris}

\IBVSauth{CLAUD H. SANDBERG LACY AND JEFFREY A. SABBY}

\IBVSinsto{Department of Physics, University of Arkansas, Fayetteville, AR 72701 USA}

\SIMBADobj{ZZ UMa}

\begintext

Radial velocities have been measured on 27 spectrograms obtained with the
Kitt Peak National Observatory coude-feed CCD spectrometer between May
1989 and May 1999. A typical spectrogram is shown in Fig.~1. Radial
velocities were obtained by cross-correlation with suitably broadened
spectra of the radial velocity standard $\beta$~Vir (RV = 4.3~km/s, Mayor \& Maurice 1985). The radial velocities are listed in Table~1.

We have adopted an eclipse ephemeris based on the dates of minima of Mallama (1980) and Hanzl (1991):
$$\begin{array}{r@{}r}
{\rm Min~I} = 2\fday2992599 & n + {\rm JD~}2447967.4134 \\
\pm 0.0000005 & \pm 0.0004 \\
\end{array}$$
We have fitted a circular spectroscopic orbit, given in Table~2. The fitted orbit is displayed in Fig.~2. The residuals from both the primary and secondary orbits were 1.1~km/s. We have combined our spectroscopic orbits with the photometric orbit of Clement et al.\ (1997); the results are shown in Table~2.

\bigskip

\begin{center}
{\normalsize Table 1: Heliocentric radial velocities of ZZ UMa.}
\vskip2mm
\begin{tabular}{rrr|rrr}
\hline
\raise-6pt\hbox{${\rm HJD}-2400000$} & \multicolumn{2}{c|}{RV (km/s)} & \raise-6pt\hbox{${\rm HJD}-2400000$} & \multicolumn{2}{c}{RV (km/s)} \\[-6pt]
& Primary & Secondary & & Primary & Secondary \\
\hline
47651.6661 & 18.7 & $-$165.2 & 51246.8659 & $-$154.6 & 38.0 \\
47652.7547 & $-$144.8 & 24.7 & 51246.9082 & $-$149.5 & 33.8 \\
47655.6915 & $-$109.7 & $-$14.4 & 51247.9057 & 28.8 & $-$176.7 \\
47656.7073 & 5.3 & $-$149.0 & 51248.9040 & $-$154.2 & 40.1 \\
48013.6395 & $-$124.1 & 3.8 & 51249.9436 & 9.2 & $-$153.4 \\
48017.7156 & 2.8 & $-$141.0 & 51309.7210 & 9.1 & $-$151.3 \\
48018.7101 & $-$154.2 & 39.4 & 51309.8099 & 21.6 & $-$165.0 \\
49485.7416 & $-$140.7 & 23.3 & 51312.6475 & $-$18.6 & $-$120.2 \\
49486.7165 & 27.6 & $-$175.0 & 51313.6480 & $-$140.6 & 25.1 \\
49488.7593 & 17.3 & $-$162.1 & 51313.6900 & $-$133.3 & 16.7 \\
50938.7165 & $-$157.9 & 44.0 & 51313.7331 & $-$124.9 & 8.0 \\
50939.6885 & 26.4 & $-$172.4 & 51314.7072 & 22.7 & $-$169.3 \\
50940.7404 & $-$147.1 & 31.4 & 51315.7079 & $-$160.5 & 47.2 \\
50944.7106 & $-$7.4 & $-$133.0 & \\
\hline
\end{tabular}
\end{center}

\newpage

\IBVSfig{9.25cm}{4755-f1.ps}{Typical spectrogram of ZZ UMa near 643 nm. Lines of the secondary component are displaced to the left (blueward) at the phase shown.}

\IBVSfig{9.25cm}{4755-f2.ps}{Spectroscopic orbits of the components of ZZ UMa. The primary (hotter and more massive) component is represented by open circles, the secondary by filled circles.}

\newpage

\IBVSfig{9cm}{4755-f3.ps}{The components of ZZ UMa (solid circles with error bars) in the mass-radius plane. Open circles correspond to accurately-known components of eclipsing binaries. The curve is the theoretical ZAMS of Schaller et al.\ (1992).}

\IBVSfig{9cm}{4755-f4.ps}{The components of ZZ UMa in the mass-luminosity plane. The symbols are the same as in Fig.~3.}

\newpage

\begin{center}
{\normalsize Table 2: Absolute properties of ZZ UMa.}
\vskip2mm
\begin{tabular}{lcr@{${}\pm{}$}lr@{${}\pm{}$}l}
\hline
Parameter & Symbol & \multicolumn{2}{c}{Primary} & \multicolumn{2}{c}{Secondary} \\
\hline
Relative radius & $r$ & 0.161 & 0.002 & 0.123 & 0.001 \\
Orbital inclination (degrees) & $i$ & \multicolumn{4}{c}{$88.01 \pm 0.07$} \\
Visual central surface brightness & $J_{\rm v}$ & \multicolumn{2}{c}{1.0} & 0.556 & 0.006 \\
Intrinsic color index (mag.) & $(b-y)_0$ & 0.374 & 0.012 & 0.479 & 0.012 \\
Temperature (K) & $T$ & 5960 & 70 & 5270 & 90 \\
Radial velocity semi-amp. (km/s) & $K$ & 95.1 & 0.3 & 111.8 & 0.3 \\
Center-of-mass radial vel (km/s) & $\gamma$ & $-$65.7 & 0.2 & $-$64.8 & 0.2 \\
Observed rotational vel (km/s) & $v \sin i$ & 32 & 2 & 26 & 2 \\
Mass (solar units) & $M$ & 1.142 & 0.007 & 0.972 & 0.007 \\
Radius (solar units) & $R$ & 1.51 & 0.02 & 1.16 & 0.01 \\
Surface gravity (cm/s$^2$) & $\log g$ & 4.135 & 0.011 & 4.299 & 0.007 \\
Synchronous rotational vel (km/s) & $v \sin i$ & 33.3 & 0.4 & 25.5 & 0.2 \\
Luminosity (solar units) & $\log L$ & 0.42 & 0.02 & $-$0.03 & 0.03 \\
Absolute visual mag. & $M_{\rm v}$ & 3.69 & 0.07 & 4.94 & 0.07 \\
Distance (pc) & & \multicolumn{4}{c}{$195 \pm 10$} \\
\hline
\end{tabular}
\end{center}

\bigskip

The results are displayed in Figs.~3 \&~4. Our results differ from those of Clement et al.\ (1997) due to improved accuracy of the spectroscopic orbits, and due to a significantly redder adopted color index for the secondary component. Our color index is based on the visual central surface brightness of the secondary, which is a parameter of the photometric orbit, through the equation in Lacy et al.\ (1987): $\delta F_{\rm v} = 1/4 \log J_{\rm c}$. Popper's (1980) calibration (his Table 1) was used to convert from surface brightness parameters into effective temperatures. The primary component parameters are well fitted by theoretical models such as those of Schaller et al.\ (1992) or Claret \& Gimenez (1992) for solar composition at an age of about 5--$6 \times 10^9$~yr, but the secondary component can not be well fitted by any of the current models because it is cooler than predicted by the models. The origin of this discrepancy is unknown.


\references

Claret, A., \& Gimenez, A. 1992, A\&AS, 96, 255

Clement, R., Garcia, M., Reglero, V., Suso, J., \& Fabregat, J. 1997, A\&AS, 125, 529

Hanzl, D. 1991, IBVS, No.\,3615

Lacy, C.H., Frueh, M.L., \& Turner, A.E. 1987, AJ, 94, 1035

Mallama, A.D. 1980, ApJS, 44, 241

Mayor, M., \& Maurice, E. 1985, in Stellar Radial Velocities, ed.\ A.G. Davis Philip \& David W. Latham, (Davis Press), p.\ 299

Popper, D.M. 1980, ARAA, 18, 115

Schaller, G., Schaerer, D., Meynet, G., \& Maeder, A. 1992, A\&AS, 96, 269

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