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\IBVShead{5797}{27 September 2007}

\IBVStitletl{Physical Parameters of the Components}{ of the Visual Binary CCDM 11289--6256}

\IBVSauth{Khaliullin, Kh.F.$^1$; Khaliullina, A.I.$^1$; Antipin, S.V.$^{1,2}$; Samus, N.N.$^{2,1}$}

\IBVSinst{Sternberg Astronomical Institute, 13, University Ave.,
119992 Moscow, Russia}

\IBVSinst{Institute of Astronomy, Russian Academy of Sciences,
48, Pyatnitskaya Str., Moscow 119017, Russia; \\
e-mail: antipin@sai.msu.ru, samus@sai.msu.ru}

\SIMBADobj{NSV 18773}
\GCVSobj{NSV 18773}

\begintext

Recently (Fabricius et al., 2002), the star HD~99898 was
discovered to be a visual binary (CCDM~11289--6256) with V$_{\rm
A}$=9\fmm9, V$_{\rm B}$=10\fmm3, and $\rho=0\farcs8$. Somewhat
earlier, it was found to be an eclipsing system with
$P=5\fday048912$ (Pojmanski, 2000). Otero and Wils (2006) reported
fast apsidal motion of the eclipsing binary's elliptic orbit,
with the period $U_{\rm obs}=135\pm10$~years.

Earlier the star, which is a member of the young association
Cru~OB1, was considered a single object. Its brightness
variability was first noticed from outside-atmosphere ultraviolet
observations by Wesselius et al. (1982), and it entered the
Supplement to the NSV catalog as NSV~18773. From Str\"omgren and
H$_\beta$ photometry, Kaltcheva and Georgiev (1994) estimated the
star's absolute parameters. However, the discovery of the star
being a visual binary and of its eclipsing variability makes it
necessary to revise the parameters determined earlier.

Figures~1 and~2 present the V-band and I-band light curves of
NSV~18773, respectively from ASAS-3 (Pojmanski, 2002) and ASAS-2
(Pojmanski, 2000) data. To plot the curves, the phases of the
observations near MinI and MinII were calculated with the same
epoch, MinI = HJD 2452068.1717(22), which corresponds to the
primary minimum epoch in the middle of the available observations,
but with different periods, $P_{\rm I}$ and $P_{\rm II}$
respectively for MinI (phases between 0.75 and 0.25) 
and MinII (phases between 0.25 and 0.75), derived from our analysis of all
the observations:
$$P_{\rm I}=5\fday049164(10), \ \ \ P_{\rm II}=5\fday049833(12).$$

It appears from the figures that the V-band and I-band light
curves are very similar and that the primary minimum is twice
wider than the secondary one, evidencing a large orbital
eccentricity. We determined the photometric elements from our
analysis of these light curves applying the iterative method of
differential corrections (Khaliullina and Khaliullin, 1984), they
are presented in the figures using the standard notation. Table~1
contains the V- and I-band magnitudes of all the components of the
system, found from the derived $L_1$, $L_2$, and $L_3$ and the
combined outside-eclipse V and I magnitudes of the system. The
physical parameters of the components computed from our
photometric elements are collected in Table~2.

The following remarks to the tables are needed.

1. The contribution of the third light to the V-band and I-band
light curves is the same, $L_3\equiv L_{\rm A}=0.61$ of the
visual system's combined brightness. Thus, it is the fainter
B~component of the visual system that is the eclipsing binary,
$L_{\rm B}\equiv L_1+L_2=0.39$.

2. The minima being shallow, the light curves do not permit to
find the components' radius ratio precisely enough without
additional assumptions. Thus we used the natural assumption that
the components are of equal age. The ages of the components were
determined by comparison of $\log g_1$ and $\log g_2$ to the
stellar evolutionary models from Claret and Gimenez (1992).

3. Since it is the A~component that mainly contributes to the
system's light, the spectral type estimate Sp = O9V (Jaschek,
1978) must refer to this particular component. In such a case, we
are able to estimate the spectral type of the primary of the
eclipsing binary as Sp$_1$ = B0V and of its secondary, as Sp$_2$ =
B1V from the V-magnitude differences of all the components
(equivalent to the differences of their absolute magnitudes,
$M_V$) and the ratio of surface brightnesses, $J_2/J_1$.

4. No radial velocity curves were published for the system. We
thus adopted $M_1=(20\pm1.5)M_\odot$ and
$T_1=(31\,500\pm1\,500)$~K for Sp$_1$ = B0V, in agreement with the
known empirical relations between stellar parameters. The rest of
the absolute parameters in Table~2 are derived from $M_1$, $T_1$,
and the photometric elements.

5. The color excess, $E_{B-V}=0\fmm65$, and the extinction,
$A_V=R\cdot E_{B-V}=2\fmm02$ for $R=3.1$, were calculated using
the UBV magnitudes of HD~99898: V = 9\fmm35, B--V = 0\fmm34, U--B
= --0\fmm63 (Nicolet, 1978) and (B--V)$_0$ = --0\fmm31 for Sp =
O9V. IR photometry of HD 99898 is known from the 2MASS Point
Source Catalog: J = 8\fmm421, H = 8\fmm352, and K = 8\fmm287. The
value $R=3.1$ used to calculate $A_V$ is based on the agreement
of (V--K)$_0^{\rm obs}$ with the mean (V--K)$_0=-0\fmm90$ for OV
and B0V stars.

The age we derive for the system, $t=(2.8\pm0.5)\cdot10^6$ years,
is twice lower than that found by Kaltcheva and Georgiev (1994),
whereas our distance to the system, $d=(3.3\pm0.3)$~kpc, is
larger by a factor of 1.5. This is obviously due to multiplicity
of HD~99898 not taken into account in the cited paper.

With the derived parameters of the system, we can use the known
theoretical relations (Kopal, 1978) and models of stellar
evolution (Claret and Gimenez, 1992) to compute the theoretically
expected apsidal-motion period:
$$U_{\rm th}=169 \pm15 \ {\rm years.}$$ $U_{\rm th}$ somewhat
exceeds $U_{\rm obs}=135\pm10$~years, as found in Otero and Wils
(2006). To improve the system parameters, spectroscopic
observations permitting to obtain the radial velocity curves and
to determine the axial-rotation angular velocities of the
components are needed.

\medskip

This study was supported, in part, by a grant from the Russian
Foundation for Basic Research (grant No.~05-02-16289) and by a
grant from the ``Origin and Evolution of Stars and Galaxies''
Program of the Presidium of the Russian Academy of Sciences.

\references

Claret, A. and Gimenez, A., 1992, {\it Astron. and Astrophys.
Suppl. Ser.}, {\bf 96}, 255

Fabricius, C., H\o g, E., Makarov, V.V., et al., 2002, {\it
Astron. and Astrophys.}, {\bf 384}, 180

Jaschek, M., 1978, {\it Bull. Inform. CDS}, {\bf 15}, 121

Kaltcheva, N.T. and Georgiev, L.N., 1994, {\it MNRAS}, {\bf 269},
289

Khaliullina, A.I. and Khaliullin, Kh.F., 1984, {\it Soviet
Astronomy}, {\bf 28}, 228

Kopal, Z., 1978, {\it Dynamics of Close Binary Systems},
Dordrecht: D.~Reidel Publishing Co.

Nicolet, B., 1978, {\it Astron. and Astrophys. Suppl. Ser.}, {\bf
34}, 1

Otero, S., Wils, P., 2006, {\it IBVS}, No.~5680

Pojmanski, G., 2000, {\it Acta Astron.}, {\bf 50}, 177

Pojmanski, G., 2002, {\it Acta Astron.}, {\bf 52}, 397

Wesselius, P.R., van Duinen, R.J., de Jonge, A.R.W., et al., 1982,
{\it Astron. and Astrophys. Suppl. Ser.}, {\bf 49}, 427

\endreferences


{\small
\begin{center}
{\normalsize Table 1. Magnitudes and spectral types of the components
of the visual binary CCDM 11289--6256 (A + B), the eclipsing binary NSV~18773
(B = Pr + Sec), and the whole system of HD~99898 (A + Pr + Sec)} \vskip2mm
\begin{tabular}{llcllc}
\hline
 &A&B = Pr + Sec&Primary&Secondary&HD 99898\\
 & &            &       &         &A + Pr + Sec\\
\hline
V&9\fmm89&10\fmm37&10\fmm80&11\fmm58&9\fmm35\\
I&9\fmm37& 9\fmm85&10\fmm27&11\fmm09&8\fmm83\\
Sp&O9V&--&B0V&B1V&--\\
\hline
\end{tabular}
\end{center}
}

{\small
\begin{center}
{\normalsize Table 2. Physical parameters for the eclipsing binary NSV 18773} \vskip2mm
\begin{tabular}{lll}
\hline
Parameter&Primary&Secondary\\
\hline
Mass $M/M_\odot$&$20\pm1.5$&$14\pm1.0$\\
Radius $R/R_\odot$&$6.5\pm0.2$&$5.0\pm0.2$\\
Effective temperature $T_e$, K&$31\,500\pm1\,500$&$27\,000\pm1\,000$\\
Luminosity $\log L/L_\odot$&$4.57\pm0.08$&$4.08\pm0.06$\\
Gravity $\log g$&$4.11\pm0.03$&$4.18\pm0.03$\\
Abs. visual magnitude $M_V$&$-3\fmm79\pm0\fmm21$&$-3\fmm00\pm0\fmm17$\\
\hline
\end{tabular}
\end{center}
}



\IBVSfig{10cm}{5797-f1.eps}{The ASAS-3 V-band light curve of NSV 18773.
The solid curve is the theoretical light curve with the
photometric elements given in the figure}
\IBVSfigKey{5797-f1.eps}{CCDM 11289-6256}{light curve}

\IBVSfig{10cm}{5797-f2.eps}{The ASAS-3 I-band light curve of NSV 18773.
The solid curve is the theoretical light curve with the
photometric elements given in the figure}
\IBVSfigKey{5797-f2.eps}{CCDM 11289-6256}{light curve}


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