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\IBVShead{5506}{5 March 2004}

\IBVStitletl{UBVRI CCD Observations of HM Monocerotis}{ and Sixty Two Year Period Study}

\IBVSauth{Samec, Ronald G.$^{1,2}$; Duddles, Candace$^1$; Hussey, Mathew$^1$; Hawkins, Nathan$^1$; Loflin, Trevor$^1$; Miller, Jess$^1$; Faulkner, Danny R.$^{2,3}$}

\IBVSinst{Astronomy program, Department of Physics Bob Jones University, Greenville, SC 29614 USA; e-mail: rsamec@bju.edu}
\IBVSinst{Visiting Astronomer, Cerro Tololo InterAmerican Observatory, Chile}
\IBVSinst{U of South Carolina, Lancaster}

\SIMBADobj{HM Mon}

\begintext

\vspace*{-1cm}

As a part of our search for solar type binaries with gas streams we observed
the neglected variable, HM Monocerotis (GSC 162 265,$\alpha$(2000) = 07\hr 03\mm 2\fsec89, 
$\delta$(2000) = $-$00\deg 13\arcm 49\farcs1).  Wachmann (1968) gave 48 times of minimum light and 
a starting ephemeris (recalculated by us),

\begin{equation}
         {\rm HJD\ Tmin\ I} = 2430110.195\ (\pm 0.004) + 0.4076573\ (\pm 0.0000005){\rm d} \times {\rm E}.
\end{equation}

His photographic light curves show a large difference in minima, yet a smoothly changing slope 
in the out-of-eclipse portion, both marks of a near contact system.

Our present UBVRI light curves of HM Mon were taken at CTIO in
Chile with the 0.9-m reflector on 18, 19, 20, 23 May 2001, by RGS and
DRF. The CFIM 2K$\times$2K T2K CCD camera operating in a 1K$\times$1K quad amplifier
mode for fast readouts. Standard UBV$R_c$$I_c$ Johnson-Cousins filters were
used.  Over 180 observations were taken in each pass band.  
The stars (GSC 162 1551, $\alpha$(2000) = 07\hr 02\mm 59\fsec60,
$\delta$(2000) = 0\deg 14\arcm 32\farcs8) and (GSC 162 
1709, $\alpha(2000)$ = 07\hr 03\mm 12\fsec16, 
$\delta(2000)$ = 00\deg 14\arcm 31\farcs1) 
were used as comparison and check stars,
respectively. A finding chart of HM Mon (V), the comparison (C) and
check star (K) are given in Figure 1.                     
The light curves and color curves of the variable are given in Figure 2 as normalized flux versus phase.

We were able to perform a period study with 89 times of minimum light spanning
 an amazing 62 years (Kreiner, Kim and Nah, 1999; BBSAG 1994, 1995, 1997).
Our three mean epochs of minimum 
light were determined from U,B,V,R,I
eclipse timings:
\vspace*{-0.4cm}
\begin{table}[!hb]
\begin{center}
\begin{tabular}{c@{=}c@{$\pm$}c}
HJD MIN I & 2452632.7926 & 0.0004 \\
          & 2452637.6842 & 0.0005 \\
HJD MIN II& 2452636.6638 & 0.0008 \\
\end{tabular}
\end{center}
\end{table}
\vspace*{-0.6cm}

using parabola fits. We calculated the following linear ephemeris from recent epochs (after JD 2444500) which we used to phase our data:
\begin{equation}
                 {\rm HJD\ Tmin\ I} = 2452632.7895{\rm d} (0.0041) + 0.40765554 (0.00000037){\rm d} \times {\rm E} .
\end{equation}

A weighted quadratic fit was applied to all available timings of minimum light gave:

\vbox{
\begin{equation}
             {\rm HJD\ Tmin\ I} = 2452632.7888 +  0.40765508{\rm d}\times {\rm E}  -0.000000000018 \times {\rm E}^2. \\[6pt]
\end{equation}
\vspace*{-0.7cm}
\begin{displaymath}
            \qquad \qquad \qquad \qquad    \pm 0.0013\quad  \pm 0.00000014\quad \qquad      \pm 0.000000000002
\end{displaymath}
}

Figure 3 shows the $O-C$'s calculated from the linear part of equation 3 overlain
by the quadratic fit (the last term). This demonstrates that equation 3
provides a good representation of the period behavior of the system. 
The steady period decrease it indicates is not unusual for solar type binaries 
undergoing magnetic breaking via stellar winds. 
Our $U-B,\ B-V,\ V-I,\ R-I$ color indices 
indicated a spectral type of about
G2V $\pm$ 2  for the system, so solar type activity is expected. The check star 
was found to be F8V with a Vmag of
13.03 and the comp star a K2V star with V = 12.25.

In modeling the light curve we first used Binary Maker 2.0 (Bradstreet, 1992) 
to fit the light curves.  We tried contact and near contact models. None 
provided satisfactory fits even with a spot. Later, I tried a V1010~Oph type 
with a stream spot near the L1 point on the smaller star with a large dark 
spot facing it on the larger component. This immediately provided an excellent
 fit to these otherwise difficult curves. 

Using averaged starting values from Binary Maker as our input values, we 
calculated complete simultaneous 5 color synthetic light curve solutions with 
the Wilson Code (Wilson \& Devinney, 1971; Wilson, 1990, 1994). Our best 
solution indicates that the secondary component is slightly under-filling its 
Roche~lobe (fill-out = 99.39$\pm$0.03\%) while the secondary component is 
filling. Other values calculated include the temperature difference, $\delta T = 2345$~K, mass ratio $m_2$/$m_1$ = 0.590 and an inclination of 
75.4 degrees. The two modelled spots are as follows: a stream spot 
with a temperature factor of 1.46 and a radius of 
13.1~degrees very near the L1 point of the secondary component and a 
solar-type dark spot of radius
33.1~degrees with a T factor of 0.92. Another solution 
was obtained in a contact mode (presented at the January 2004 AAS meeting, Dudles et al. 2003) 
with a cool third light. But the goodness of fit parameters were about 3 times
 worse and the fit was questionable.

Our best solution is shown overlaying the data in Figure 1. A geometrical representation of HM Mon with the two spots is given in Figure 4 as viewed from the 
pole.

Only after many trails did we understand the correct modeling configuration. The difficulties in the modeling undoubtedly arose from the dominating character of the spots. The stream spot persisted through the entire set of differential corrections iterations (hundreds) and is undoubtedly real. This system is probably undergoing rapid evolution into contact. Good quality spectroscopic data is need to verify this these important results.


\IBVSfig{6cm}{5506-f1.eps}{}
\IBVSfigKey{5506-f1.eps}{HM Mon}{finding chart}


\IBVSfig{10cm}{5506-f2.eps}{\vspace*{-2cm}}
\IBVSfigKey{5506-f2.eps}{HM Mon}{light curve}

\IBVSfig{10cm}{5506-f3.eps}{}
\IBVSfigKey{5506-f3.eps}{HM Mon}{light curve}

\clearpage

\IBVSfig{8cm}{5506-f4.eps}{\vspace*{-1.4cm}}
\IBVSfigKey{5506-f4.eps}{HM Mon}{O-C diagram}

\IBVSfig{6cm}{5506-f5.eps}{}
\IBVSfigKey{5506-f5.eps}{HM Mon}{}

\medskip


We wish to thank CTIO for their allocation of observing time, and a small
research grant from the American Astronomical Society which supported this
run.


\references 

{\it BBSAG}, {\bf 106}, 110, 114.

Bradstreet, D. H., 1992, {\it BAAS}, {\bf 24}, 1125

Kreiner, J. M., Kim, C-H, Nha, I-S, 1992, {\it An Atlas of O-C Diagrams of Eclipsing Binary Stars}, Wydawnictwo Naukowe AP, Krakow, ul. Studencka 5, POLAND 

Wachmann, A. A., 1968, {\it Bergd Abh7}, {\bf 8} 
%Maybe 1968AAHam...7..381W ?

Wilson, R. E. \& Devinney, E. J., 1971, {\it ApJ}, {\bf 166}, 605

Wilson, R. E., 1990, {\it ApJ}, {\bf 356}, 613

Wilson, R. E., 1994, {\it PASP}, {\bf 106}, 921

\endreferences

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