\documentstyle[twoside, epsf]{article}

\def\kms{km\,s$^{-1}$}			% km/s
\def\Msun{${\rm M_{\odot}}$}		% Msun
\def\Rsun{${\rm R_{\odot}}$}		% Rsun


\input ibvs2.sty

\begin{document}

\IBVShead{4809}{26 November 1999}

\IBVStitletl{The Nature of the Bright Early-Type Eclipsing Binary}{ Theta 1 O\lowercase{ri} A = V1016 Orionis}

\IBVSauth{LLOYD, C.; STICKLAND, D.J.}

\IBVSinsto{Space Science \& Technology Department, Rutherford Appleton
Laboratory, Chilton, Didcot, Oxon.\\ \indent OX11 0QX, UK, e-mail: {\tt cl@ast.star.rl.ac.uk, ds@ast.star.rl.ac.uk}}

\SIMBADobjAlias{HD37020}{V1016 Ori}
\IBVStyp{ B1V+A0 }
\IBVSkey{early-type, eclipsing binary, pre-main-sequence}
\IBVSabs{Analysis of the published photometry of theta 1 Ori A yields an}
\IBVSabs{improved ephemeris of}
\IBVSabs{JD 2442752.022(+/-0.004) + 65.43280(+/-0.00008)xE.}
\IBVSabs{Modelling the light curve suggests that the primary component is a}
\IBVSabs{early B-type main-sequence star and the secondary is a pre-}
\IBVSabs{main-sequence star about spectral type A0.}


\begintext

The bright ($V = 6.7$) westernmost member of the Trapezium,
$\theta^{1}$~Ori~A (41~Ori~A, HR~1893, HD~37020, V1016~Ori) is a
long-period eclipsing binary with magnitude-deep declines (Lohsen 1975).
Despite the brightness of the system, uncertainty surrounds the nature of
the secondary component, which is most probably a pre-main-sequence star
(Stickland \& Lloyd 2000). The purpose of this note is to highlight some
new observations that could clarify the issue. Most of the observational
difficulty with this system stems from the unusually long period for an
eclipsing system of 65.43 days, resolved by Baldwin (1976). The primary
eclipse is too long to observe continuously in one night (FWHM $\sim$ 10
hours), but it occupies barely one per cent of the cycle, and in an
observing season there may be only three cycles. As a consequence, the
light curve of the primary eclipse is composed of a series of sections
covering only parts of the eclipse, ingress, egress or the central part
of the minimum. The most complete eclipses are those of Lohsen (1976) and
more recently, Vitrichenko (1998), and visually by the AAVSO (Mattei
1977). Multi-colour photometry shows very little change in colour during the 
primary eclipse.

Spectroscopically the system is well, if not particularly accurately,
observed and several orbital solutions have been published, most recently
by Stickland \& Lloyd (2000), who provide a detailed review of all the
radial velocity data. The system is quite eccentric, with $e = 0.624$ and
the secondary eclipse occurs at photometric phase $0.130\pm0.015$. The
uncertainty on the time of secondary eclipse corresponds to $\pm1.0$
days. It is likely that none of the published spectroscopic or
photometric observations, even the visual ones, have been made during the
secondary eclipse.

A range of spectral types from O8 -- B3 have been given for the primary
with the majority around very early B type. Stickland \& Lloyd (2000) give B1V 
from the cross correlation of ultraviolet spectra while Vitrichenko
{\rm et~al.}\ (1998) give B0V on the basis of an adopted temperature of $T_{1} 
= 30\,000$~K derived from the photometry. There is no convincing
spectroscopic signature of the secondary component, even at primary
minimum (Vitrichenko {\rm et~al.}\ 1998), which implies that the secondary
component is at least 2--3 magnitudes fainter than the primary. However,
observations in the near infrared show an excess over and above what is
expected for an early B-type star Vitrichenko (1999). Attempts to model
the system have not led to any clear consensus. Bossi {\rm et~al.}\ (1989) could
not model the system with main-sequence stars and suggested a B3III--IV
primary component with a pre-main-sequence secondary. Vitrichenko (1998,
1999) used a star-dust model to fit the light curve and the
optical-to-infrared colours, with components of B0--0.5V and B8--A0V, a
dust shell at a temperature of $T_{D} = 1600$~K and a separate infrared
source. A recurring theme in many of the analyses is the
pre-main-sequence nature of the secondary component.

\begin{center}
\normalsize
{\bf Table 1.} Times of primary minimum of $\theta^{1}$ Ori A
\vskip 2mm
\begin{tabular}{crcl} \hline
JD & $O-C$ (days) & Note & Reference \\ \hline
2436863.073 &    0.003~~~ & 1 & Strand 1975  \\
2441966.813 & $-$0.015~~~ & 2, 11 & Lohsen 1975 \\
2441966.827 & $-$0.001~~~ & 3 & Lohsen 1976 \\
2442359.421 & $-$0.004~~~ & 3 & Lohsen 1976 \\
2442751.946 & $-$0.076~~~ & 4, 11 & Walker 1976 \\
2442752.015 & $-$0.007~~~ & 3 & Lohsen 1976 \\
2442752.010 & $-$0.012~~~ & 5 & Caton {\rm et~al.}\ 1977 \\
2442817.545 &    0.090~~~ & 6, 11 & Baldwin 1976 \\
2443144.613 & $-$0.006~~~ & 2, 7 & Mattei 1977 \\
2443144.639 &    0.020~~~ & 8 & Walker 1977 \\
2443210.033 & $-$0.019~~~ & 9 & Franz 1977 \\
2443537.235 &    0.019~~~ & 2 & Zakirov 1979 \\
2444191.552 &    0.008~~~ & 10 & Sowell \& Hall 1982 \\
2450080.494 & $-$0.002~~~ & 2 & Agerer \& Huebscher 1997 \\ \hline
\end{tabular}
\end{center}

{\normalsize Notes: 
1. Mean of timings from two photographic magnitudes $\sigma \sim 0.04$ days. 
2. Observed minimum. 
3. Observations from three minima used to
construct a complete minimum. The times of minima are derived from
Lohsen's ephemeris. 
4. Two isolated observations very close to minimum.
5. Timing derived from a major part of an ingress, $\sigma \sim 0.01$ days. 6. 
Isolated visual observation during the eclipse. 
7. Timing derived from the faintest visual observations of six consecutive 
minima.
8. Timing derived from a major part of an egress, $\sigma \sim 0.01$ days. 
9. Timing derived from part of an egress, $\sigma \sim 0.01$ days.
10. Time of minimum derived by Sowell \& Hall from a major part of an
egress. 
11. Not used in the solution.
}

\IBVSfig{8cm}{4809-f1.eps}{The light curve of $\theta^{1}$ Ori A
around primary minimum showing the normal points derived from Lohsen (1976) and 
the solution with $T_{2} = 9\,000$~K from Table 2 over plotted.}

\IBVSfig{8cm}{4809-f2.eps}{The modelled light curve around
secondary minimum showing the decreasing visibility of the eclipse with
decreasing temperature of the secondary component.}

To determine an accurate ephemeris the times of minimum have been derived
from all the published eclipse observations but in practice this is not
simple as the eclipse is composed of fragments. Observations
around the deepest part of minimum do not necessarily provide the best
timing as the light curve is rather flat, with $\Delta V < 0.1$ mag over
4 hours, and the observations are invariably noisier than at other times.
Only Lohsen (1976) and Vitrichenko (1998) have sufficient observations to
claim complete coverage of the eclipse and even these have some
significant gaps. The fragments of light curve have been phased with
respect to the adopted shape of the minimum from Lohsen (1976) and the
times of primary minimum are given in Table 1. The derived ephemeris is
$$
{\rm JD_{MinI}} = 2442752.022\mathbin{(\pm} 0.004) + 65.43280\mathbin{(\pm} 
0.00008)\times E.
$$

The {\sc light2} code (Hill {\rm et~al.}\ 1989) has been used to model the
system with a series of evenly sampled normal points, where there are
sufficient observations, derived from the light curve of Lohsen (1976)
(see Figure~1). In the {\sc light2} solutions the temperature of the
primary has been fixed at 26\,000~K, which is appropriate for a B1V star.
Not surprisingly, given the lack of a secondary eclipse, the temperature
of the secondary component is essentially undefined and its radius is
also poorly determined (see Table 2). However, this result conceals two
types of solution. At lower temperatures, $T_{2} < 12\,000$~K, the radius
of the primary is larger than the secondary but at higher temperatures
the relative sizes of the components is reversed. The details are given
in Table 2. Two solutions with the temperature of the secondary fixed at
$T_{2} = 6\,000$~K and 9\,000~K are essentially identical. 

Assuming a mass appropriate for a B1V star, $M_{1} =
12$\,\Msun, and taking $K = 33$~\kms\ from the orbital solution, gives
$R_{1} = 6.6$\,\Rsun, $M_{2} = 3.0$\,\Msun, and $R_{2} = 5.2$\,\Rsun. The
radius of the primary is consistent with the radius of a B1V star, and,
by definition, so are the mass and temperature. However, the
secondary mass of 3.0\,\Msun\ corresponds to a main-sequence star of
spectral type near A0, but the radius of 5.2\,\Rsun\ is more appropriate for an 
early--mid B-type star. Clearly the secondary is not a main-sequence
star and from evolutionary arguments Stickland \& Lloyd conclude that it
is most likely a pre-main-sequence star. For the solutions with a higher
temperature secondary, the radius of the primary is too small for a B1V
star, implying that it is either of later spectral type, so cooler and
less massive, or some physically unrealistic object. The radius of the
secondary is too large, by a factor of two, for a mid B-type star, so
this component must either be of earlier spectral type, so hotter and more 
massive, or evolved. Therefore, it does not seem possible to construct a 
realistic model with the hotter, $T_2 = 15\,000$~K, secondary.

\begin{center}
\normalsize
{\bf Table 2.} Solutions to the light curve with $T_{1} = 26\,000$~K.
\vskip 2mm
\begin{tabular}{cccccc} \hline 
$T_2$ (K) & $R_1/a$ & $R_2/a$ & $i$ (deg) & $R_1$ (\Rsun) & $R_2$ (\Rsun) \\ 
\hline
$7\,300 \pm 14\,500$ & $0.0307 \pm 0.0005$ & $0.0240 \pm 0.0035$ & $89.6 \pm 
0.3$
& 6.6 & 5.2 \\
~6\,000 & $0.0308 \pm 0.0003$ & $0.0239 \pm 0.0011$ & $89.6 \pm 0.3$ & 6.7 & 
5.2 \\
~9\,000 & $0.0307 \pm 0.0003$ & $0.0244 \pm 0.0008$ & $89.7 \pm 0.3$ & 6.6 & 
5.3 \\
15\,000 & $0.0244 \pm 0.0006$ & $0.0302 \pm 0.0014$ & $89.6 \pm 0.2$ & 5.3 & 
6.5 \\
\hline
\end{tabular}
\end{center}

The light curves around secondary eclipse for the three fixed-temperature 
solutions in Table~2 are shown in Figure~2 and it is clear that observations of 
this eclipse would largely resolve the issue. The high-temperature secondary 
solution could easily be eliminated and with careful observation it should at 
least be possible to place a reliable upper limit on the depth of the eclipse, 
and with it, the temperature of
the secondary. An ephemeris for the current observing season is given in
Table 3 and observations of both primary and secondary minimum, covering the 
period of uncertainty, are encouraged.

\begin{center}
\normalsize
{\bf Table 3.} Ephemeris of $\theta^{1}$ Ori A for the current
observing season.
\vskip 2mm
\begin{tabular}{lcrll} \hline
 JD & & & Date UT \\ \hline
     2451520.017 & 1999 & 12 & ~~7.517 & MinI \\
     $2451528.6 \pm 1.0$ & 1999 & 12 & 16.1 & MinII \\
     2451585.450 & 2000 &  2 & 10.950 & MinI \\
     $2451594.0 \pm 1.0$ & 2000 &  2 & 19.5 & MinII \\
     2451650.883 & 2000 &  4 & 16.383 & MinI \\
     $2451659.4 \pm 1.0$ & 2000 &  4 & 24.9 & MinII \\
\hline
\end{tabular}
\end{center}

\references

Agerer F., Huebscher J., 1997, {\it IBVS}, No.\,4472

Baldwin M., 1976, {\it IAU Circ.}, No.\,3004

Bossi M., {\rm et~al.}, 1989, {\it A\&A}, {\bf 222}, 117

Caton D.B., Fallon F.W., Wilson R.E., 1977, {\it PASP}, {\bf 89}, 530

Franz O.G., 1977, {\it IBVS}, No.\,1274

Hill G., Fisher W.A., Holmgren D., 1989, {\it A\&A}, {\bf 211}, 81

Lohsen E., 1975, {\it IBVS}, No.\,988

Lohsen E., 1976, {\it IBVS}, No.\,1129

Mattei J.A., 1977, {\it JRAS Can.}, {\bf 71}, 475

Sowell J.R., Hall D.S., 1982, {\it IBVS}, No.\,2076

Stickland D.J., Lloyd C., 2000, {\it The Observatory}, {\bf 200}, in press

Strand K.Aa., 1975, {\it IBVS}, No.\,1025

Vitrichenko E.A., 1998, {\it Astr.~Lett.}, {\bf 24}, 611

Vitrichenko E.A., 1999, {\it Astr.~Lett.}, {\bf 25}, 179

Vitrichenko E.A., Klochkova V.G., Plachinda S.I., 1998, {\it Astr.~Lett.}, {\bf 
24}, 296

Walker M.F., 1976, {\it IBVS}, No.\,1148

Walker M.F., 1977, {\it IBVS}, No.\,1238

Zakirov M.M., 1979, {\it Perem.\ Zvezdy}, {\bf 21}, 223

\end{document}

