The massive binary δ Ori and the problem of the spectroscopic

The massive binary δ Ori and the problem of the
spectroscopic detection of its weak secondary
2
ˇ
P. Harmanec , P. Mayer , M. Slechta
1,∗
1
1
Astronomical Institute of the Charles University, Faculty of Mathematics and Physics, V Holeˇsoviˇck´ach 2, CZ–180 00 Praha 8, Czech Republic
2
Astronomical Institute of the Academy of Sciences, CZ-251 65 Ondˇrejov, Czech Republic
∗e-mail: [email protected]
Introduction
The massive O9.5 II spectroscopic and eclipsing binary δ Ori A (HD 36486, HR 1852) was studied many times over more than a century. It has an orbital period of 5.73 d and slightly eccentric
orbit with an apsidal period of 234 yrs (Harvey et al. 1987, Mayer et al. 2010). Harvin et al. (2002) carried out a tomographic separation of the UV and optical spectra and concluded that the
binary is composed from O9.5 II and B0.5 III components having unexpectedly low masses of 11.2 and 5.6 M⊙. However, Mayer et al. (2010) pointed out that the second system of spectral
lines belongs to a speckle-interferometric tertiary Ab, similarly hot as the primary, suggested a mass ratio M2/M1 ∼ 0.4 and concluded that the system has probably normal masses. They
were unable, however, to detect the lines of the weak secondary.
A new attempt to detect the secondary
Discussion of the results
Having a rich collection of 67 additional Ondˇrejov red CCD spectra at our disposal,
we made a new attempt at the detection of spectral lines of the secondary. The most
powerful technique is the spectra disentangling, realized by the computer program
KOREL (Hadrava 2004 and references therein). There is a problem, however. In the
case of δ Ori, the spectra are dominated by the strong sets of spectral lines, the
primary and an almost stationary tertiary, so the total sum of squares of residuals is
basically determined by them and any contribution from the secondary is burried in
the noise. We attempted a new approach to the problem. We fixed the orbital solution
for the eclipsing pair from the combined light curve and radial-velocity curve solution
by Mayer et al. (2010; the last column of their Table 3) and adopted the orbit of the
visual companion from Table 6 of Mason et al. (2009). Using our most numerous set
of 281 He I 6678 ˚
A line profiles from the years 1993 – 2013, we disentangled only the
spectra of the primary and tertiary. Since KOREL allows to obtain the residual spectra
in the rest frame of the system, we simply added 1.0 to the flux values of these and
used them to another KOREL solution, in which only the secondary was disentangled.
The map of the dependence of the sum of squares of residuals for KOREL solutions with
all three components considered, and for only the secondary in the residual spectra, is
in Fig. 1. It is seen that only the map for the residual spectra gives a clear minimum
of the sum of squares of residuals, identifying the most probable value of K2.
When we allowed for a free convergency of K2 in the residual spectra, the best value
was 273.29 km s−1. The solution for all three components gave K1 = 109.02 km s−1,
K2 = 273.37 km s−1, and q = 0.3988. The tiny line profile of the secondary for both
solutions is shown in Fig. 2, while the line profiles of the primary and tertiary are shown
in Fig. 3. Note that the line profile of the secondary is found in both solutions but
the solution for the residual spectra was vital to the identification of the true binary
mass ratio. Our result confirms the model of the system put forward by Mayer et al.
(2010).
There can be a problem, however, with the long orbit of 201 yrs derived by Mason et al.
(2009). Only a small part of it is so far covered by observations, so a true determination
of the stellar properties will require further systematic observations. Since the spectrum
of the tertiary is close to that of the primary, one would expect M3 ∼ 15 − 20 M⊙.
For i = 74◦, for instance, our solution gives M1 = 26.4 M⊙ and M2 = 10.5 M⊙, and
a semimajor axis of the long orbit along = 27523 − 28380 R⊙. Adopting along = 0.′′26
after Mason et al., it implies a parallax of 492-508 pc. All photometric determinations
of the distance to the Orion cluster agree on a parallax slightly over 400 pc, while the
Hipparcos parallax (which we suspect cannot be correct) is 189 – 242 pc (van Leeuwen
2007). Clearly, a continuation of the speckle-interferometric observations of the outer
orbit is highly desirable.
Figure 1: A comparison of the dependence of the sum of squares of residuals on the semiamplitude of the secondary component K2 for the KOREL
solution for all three stars (red line) and for a solution carried out for the
secondary only in the residual spectra after disentangling the primary and
tertiary only (blue line).
Figure 2: A comparison of the disentangled profile of the He I 6678 ˚
A line
of the secondary (normalized to the joint continuum of the system) from
the residual spectra after removal of the primary and tertiary (red line)
and from a KOREL solution for all three components (blue line).
Figure 3: Disentangled He I 6678 ˚
A line profiles of the primary (red
line) and tertiary (green line) normalized to the joint continuum of the
system. The primary was shifted for 0.1 in the relative flux for clarity.
The He II 6683 ˚
A line is seen in the red wing of the primary profile.
Acknowledgements
ˇ
We acknowledge the use of the computer program KOREL written by Dr. P. Hadrava. Some of the new spectra were obtained by our colleagues Drs. D. Korˇc´akov´a, J. Kub´at, P. Skoda,
V. Votruba, M. Wolf and P. Zasche and by Ms. L. Kotkov´a and Ms. J. Nemravov´a. The research of PH and PM was supported by the grant P209/10/0715 of the Czech Science Foundation
and from the research program MSM0021620860.
References
Hadrava, P. 2004 Publ. Astron. Inst. Acad. Sci. Czech Republic, 89, 15
Harvey, A.S., Stickland, D.J., Howarth, I.D., Zuiderwijk, E.J. 1987, Observatory, 107, 205
Harvin, J.A., Gies, D.R., Bagnuolo, W.G., Jr. 2002, ApJ, 565, 1216
ˇ
Mayer, P., Harmanec, P., Wolf, M., Boˇzi´c, H., Slechta,
M. 2010, A&A, 520, A89
van Leeuwen, F. 2007, A&A, 474, 653

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