Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | A22 | |
Number of page(s) | 10 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/200913698 | |
Published online | 25 February 2010 |
Absolute dimensions of solar-type eclipsing binaries
III. EW Orionis: stellar
evolutionary models tested by a G0 V system
,![[*]](/icons/foot_motif.png)
J. V. Clausen1 - H. Bruntt2,3 - E. H. Olsen1 - B. E. Helt1 - A. Claret4
1 - Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30,
2100 Copenhagen Ø, Denmark
2 - Sydney Institute for Astronomy, School of Physics, University of
Sydney, NSW 2006, Australia
3 - Observatoire de Paris, LESIA, 5 Place Jules Janssen, 95195 Meudon,
France
4 - Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080
Granada, Spain
Received 19 November 2009 / Accepted 14 December 2009
Abstract
Context. Recent studies of inactive and active
solar-type binaries suggest that chromospheric activity, and its effect
on envelope convection, is likely to cause significant radius and
temperature discrepancies. Accurate mass, radius, and abundance
determinations from additional solar-type binaries exhibiting various
levels of activity are needed for a better insight into the structure
and evolution of these stars.
Aims. We aim to determine absolute dimensions and
abundances for the G0 V detached eclipsing
binary EW Ori, and to perform a detailed comparison
with results from recent stellar evolutionary models.
Methods.
light curves and
standard photometry were obtained with the Strömgren Automatic
Telescope, published radial velocity observations from the CORAVEL
spectrometer were reanalysed, and high-resolution spectra were observed
at the FEROS spectrograph; all are/were ESO, La Silla
facilities. State-of-the-art methods were applied for the photometric
and spectroscopic analyses.
Results. Masses and radii that are precise to 0.9%
and 0.5%, respectively, have been established for both components
of EW Ori. The 1.12
secondary component reveals weak Ca II H
and K emission and is probably mildly active;
no signs of activity are seen for the 1.17
primary. We derive an
abundance
of +0.05
0.09
and similar abundances for Si, Ca, Sc, Ti, Cr, and Ni. Yonsai-Yale and
Granada solar-scaled evolutionary models for the observed metal
abundance reproduce the components fairly well at an age of
2 Gyr.
Perfect agreement is, however, obtained at an age of
2.3 Gyr for a combination of a) a slight
downwards adjustment of the envelope mixing length parameter for the
secondary, as seen for other active solar-type stars; and
b) a slightly lower helium content than prescribed by the Y-Z relations
adopted for the standard model grids. The orbit is eccentric (
e =
0.0758
0.0020),
and apsidal motion with a 62% relativistic contribution has
been detected. The apsidal motion period is
U
= 16 300
3900 yr, and the inferred mean central density concentration
coefficient, log(k2) =
-1.66
0.30, agrees marginally with model predictions. The measured rotational
velocities, 9.0
0.7 (primary) and 8.8
0.6 (secondary) km s-1,
are in agreement with both the synchronous velocities and the
theoretically predicted pseudo-synchronous velocities. Finally, the
distance (175
7 pc),
age, and center-of mass velocity (6 km s-1)
exclude suggested membership of the open cluster Collinder 70.
Conclusions. EW Ori now belongs to the
small group of solar-type eclipsing binaries with well-established
astrophysical properties.
Key words: stars: evolution - stars: fundamental parameters - stars: abundances - binaries: eclipsing - techniques: photometric - techniques: spectroscopic
1 Introduction
Recent studies of inactive and active solar-type binaries suggest that chromospheric activity, and its effect on envelope convection, is likely to cause significant radius and temperature discrepancies, which can be removed by adjusting the model mixing length parameter downwards (Clausen et al. 2009, hereafter CBC09, and references herein). In a study of the F8/G0 V eclipsing binary V636 Cen, CBC09 compared the properties of 11 solar-type binaries (at least one component in the 0.80-1.10

We are presently undertaking analyses of several eclipsing binaries with solar-type components; see CBC09. In order to make critical tests of stellar evolutionary models, abundance determinations are in general included. In this paper we present results for the well-known system EW Ori based on new observations.
Table 1: Photometric data for EW Ori and the comparison stars.
2 EW Ori
EW Ori (HD 287727, mV =
9.90, Sp. type G0V, P = 6
94),
is a well detached, double-lined eclipsing binary
with 1.17 and 1.12
main-sequence components in a slightly eccentric (e = 0.0758) orbit.
The eclipsing nature of EW Ori was discovered by Hoffmeister (1930), and Lause (1937) established the
first ephemeris. Much later, EW Ori was found to be
double-lined (Lacy 1984).
Several times of minima, back to 1937, have been published,
as well as photometric indices. Popper et al. (1986) obtained
photoelectric (V,R)
light curves and spectrographic material, and they determined absolute
dimensions. Improved spectroscopic elements, based on CORAVEL radial
velocities, were later published by Imbert (2002).
EW Ori has been included in samples of eclipsing binaries used for tests of stellar evolutionary models (Pols et al. 1997; Lastennet & Valls-Gabaud 2002), determination of the helium-to-metal enrichment ratio (Ribas et al. 2000), and as a (possible) test of general relativity through apsidal motion (Giménez 1985; Wolf et al. 1997,2010). It is listed as a possible member of the open cluster Collinder 70 by Sahade & Davila (1963) and Giménez & Clausen (1996).
Although EW Ori is already well-studied, there is
still room for significant improvements. In this paper we
present analyses of new light curves,
leading to much more accurate radii, we derive chemical abundances from
high-resolution spectra, and we perform a detailed comparison with
current stellar evolutionary models. We refer to the more massive,
larger component as the primary (p)
component, which, for the ephemeris we adopt (Eq. (1)),
is eclipsed at phase 0.0.
3 Photometry
Below, we present the new photometric material for EW Ori and refer to Clausen et al. (2001; hereafter CHO01) for further details on observation and reduction procedures, and determination of times of minima.
![]() |
Figure 1: y light curve and b-y and u-b colour curves (instrumental system) for EW Ori. |
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3.1 Light curves
The differential
light curves of EW Ori were observed at the Strömgren
Automatic Telescope (SAT) at ESO, La Silla and its 6-channel
photometer
on 70 nights between
November 2000 and December 2002
(JD 2 451 854-2 452 609).
They contain 624 points per band with most phases covered at
least twice. The observations were done through an 18 arcsec
diameter circular diaphragm at airmasses between 1.2
and 2.0. HD 34658 (HR 1746,
NSV 1922), HD 34745, and
HD 35638 - all within a few degrees from
EW Ori on the sky - were used as comparison stars and
were all found to be constant within a few mmag; see
Table 1.
The light curves are calculated relative to HD 34745, but all
comparison star observations were used, shifting them first to the same
light level. The average accuracy per point is about 6 mmag (
)
and 7 mmag (u).
As seen from Fig. 1, EW Ori is well detached with y eclipse depths of about 0.8 and 0.6 mag, respectively. The secondary eclipse is total (duration of totality about 10 min) and occurs at phase 0.5305. The light curves (Table 11) will only be available in electronic form.
3.2 Standard photometry
Standard
indices for EW Ori and the three comparison stars, observed
and derived as described by CHO01, are presented in Table 1. As seen,
the indices are based on many observations and their precision is high.
For comparison, we have included published photometry from other
sources. In general, the agreement is good, but individual
differences larger than the quoted errors occur; we have used the new
results for the analysis of EW Ori.
3.3 Times of minima, ephemeris, and apsidal motion
Table 2:
Times of primary (P) and secondary (S) minima of EW Ori
determined from the
observations.
Three times of the primary minimum and four of the secondary
have been determined from the light curve
observations; see Table 2.
A complete list of earlier times of minima was kindly provided
by Kreiner and has been included in the ephemeris and apsidal motion
analyses; see Kreiner et al. (2001) and Kreiner
(2004)
.
Separate weighted linear least squares fits to the times of
primary and secondary minima lead to slightly different orbital periods
of
0.00000044 and
0.00000018, respectively, and
nearly identical results are obtained if only photoelectric and
CCD data are used. Whereas this difference indicates a slow
apsidal motion, we have adopted the result from the primary minima for
the analyses of the
light
curves and radial velocities in this paper:
Within uncertainties, the same ephemeris is obtained from the analyses of the

From a weighted least squares analysis, following the
formalism by Giménez & Garcia-Pelayo (1983) and Giménez
& Bastero (1995),
we obtain the apsidal motion parameters presented in Table 3. The
orbital inclination i and
eccentricity e were fixed at the values
derived from the photometric analysis (Table 5).
As seen, a slow but significant motion has been
detected, but the apsidal motion period is still very uncertain. Our
results agree with those published by Wolf et al. (1997); recently,
Wolf et al. (2010)
obtained
0.00004
/cycle.
We note that the sidereal period is equal to the mean of the periods
determined above from primary and secondary eclipses, respectively, and
that the longitude of periastron,
,
derived for the epoch of the
observations is
identical to the result obtained from the light curve analyses
(Sect. 4).
The contribution from general relativity (Giménez 1985,
Eqs. (3), (4)) is 0.00026
/cycle, or
about 62% of the observed rate. Within the rather large
uncertainties, the derived mean central density concentration
coefficient
log(k2) =
-1.66
0.30 marginally agrees with predictions from evolutionary
models, -1.91 (Table 10,
model set #3).
Table 3: Apsidal motion parameters for EW Ori derived from all available times of minima.
4 Photometric elements
Since EW Ori is well detached, the photometric elements have
been determined from JKTEBOP analyses (Southworth et al. 2004a,b) of the light
curves. The underlying Nelson-Davis-Etzel binary model (Nelson
& Davis 1972;
Etzel 1981;
Martynov 1973)
represents the deformed stars as biaxial ellipsoids and applies a
simple bolometric reflection model. We refer to Clausen et al.
(2008, hereafter
CTB08) for details on the binary model and code, and on the general
approach applied. In tables and text, we use the following
symbols:
i orbital inclination; e eccentricity
of orbit;
longitude
of periastron; r relative radius
(in units of the semi-major axis);
;
u linear limb darkening coefficient;
y gravity darkening coefficient; J central
surface brightness; L luminosity;
effective
temperature.
The mass ratio between the components was kept at the spectroscopic value; see Sect. 5. The simple built-in bolometric reflection model was used, linear limb darkening coefficients by Van Hamme (1993) and Claret (2000) were applied, or included as free parameters, and gravity darkening coefficients corresponding to convective atmospheres were adopted.
Solutions for EW Ori are presented in Table 4, and
as seen, the results from the four bands agree well. Changing from Van
Hamme to Claret limb darkening coefficients, which are
0.06-0.10 higher, does not change the orbital and stellar
parameters significantly. Coefficients determined from the light curves
reproduce those by Van Hamme slightly better than those by Claret; they
have formal uncertainties of about .
Including non-linear limb darkening (logarithmic or square-root law)
also has no significant effect on the photometric elements.
Table 4: Photometric solutions for EW Ori from the JKTEBOP code.
The adopted photometric elements listed in Table 5 are the
weighted mean values of the JKTEBOP solutions adopting the
linear limb darkening coefficients by Van Hamme. Realistic errors,
based on 1000 Monte Carlo simulations in each band
and on comparison between the solutions, have been
assigned. The Monte Carlo simulations include random
variations within
of the linear limb darkening coefficients. As seen, the
relative radii have been established to better than 0.5%. Due
to the accurate light curves with about 265 points within
eclipses - coupled to the fact that secondary eclipse is
total - we obtain a reliable photometric determination
of k, and the corresponding luminosity
ratios are exactly identical to those derived directly from the depths
of the total secondary eclipse. Also, e and
are much better constrained than from the radial velocity analyses; see
Sect. 5.
At phase 0.0, about 90% of the y light
from the primary component is eclipsed.
For comparison, Popper et al. (1986) obtained
0.005 and k = 0.955
0.030, an orbital inclination of
,
and a V luminosity
ratio of
0.037. Inside eclipses, only 69 V,R points
from two nights were
available, and to constrain k they adopted
a fixed
,
based on e = 0.068
0.004 from the spectroscopic orbit and
determined
separately from the light curve analysis.
In conclusion, the new photometric elements differ somewhat from those by Popper et al. and are significantly more accurate.
Table 5: Adopted photometric elements for EW Ori.
![]() |
Figure 2:
( |
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5 Spectroscopic elements
Spectroscopic orbits have been derived from re-analyses of the radial
velocities by Popper et al. (1986) and Imbert (2002). We have
used the method of Lehman-Filhés implemented in the SBOP program (Etzel 2004), which is a
modified and expanded version of an earlier code by Wolfe
et al. (1967).
The orbital period P was fixed at the
ephemeris value (Eq. (1)),
and the eccentricity e and longitude of
periastron
at the better constrained results from the photometric analysis
(Table 5).
Equal weights were assigned to the radial velocities of Popper
et al.,
whereas the CORAVEL velocities by Imbert were weighted according to the
inverse square of their internal errors.
The spectroscopic elements are presented in Table 6. The
radial velocities of the components were analysed independently
(SB1 solutions), but we note that SB2 solutions lead
to nearly identical results. Within errors, the semiamplitudes agree
with the published results,
which are, however, based on different orbital periods and treatments
of e and .
For the secondary component, the semiamplitudes of our two solutions
agree well, whereas they differ by almost 1 km s-1
for the primary. We adopt the elements determined from the CORAVEL
velocities, which
are more accurate and have a better phase coverage;
see Fig. 3.
Table 6: Spectroscopic orbital solutions for EW Ori determined from re-analyses of the radial velocity observations by Popper et al. (1986) and Imbert (2002).
![]() |
Figure 3: Adopted spectroscopic orbital solution for EW Ori (solid line: primary; dashed line: secondary) and radial velocities (filled circles: primary; open circles: secondary). The dotted line ( upper panel) represents the center-of-mass velocity of the system. Phase 0.0 corresponds to central primary eclipse. |
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6 Chemical abundances
To determine the chemical composition of EW Ori, we have obtained two high-resolution spectra. They were observed at different phases and opposite line shifts with the FEROS fiber echelle spectrograph at ESO, La Silla January-February 1999; see Table 7. Details on the spectrograph, the reduction of the spectra, and the basic approach followed in the abundance analyses is described by CTB08.
The versatile VWA tool, now extended to analyses of double-lined spectra, was used. We refer to Bruntt et al. (2004,2008) and Bruntt (2009) for detailed descriptions of VWA. It uses the SYNTH software (Valenti & Piskunov 1996) to generate the synthetic spectra. Atmosphere models were interpolated from the recent grid of MARCS model atmospheres (Gustafsson et al. 2008), which adopt the solar composition by Grevesse et al. (2007). Line information was taken from the Vienna Atomic Line Database (VALD; Kupka et al. 1999), but in order to derive abundances relative to the Sun, log(gf) values have been adjusted in such a way that each measured line in the Wallace et al. (1998) solar atlas reproduces the atmospheric abundances by Grevesse et al. (2007). Analyses of a FEROS sky spectrum reproduce these adjustments closely.
The abundance results derived from all useful lines in both
spectra are presented in Table 8. We have
only included lines with measured equivalent widths
above 10 mÅ and below 50 mÅ (primary) and
40 mÅ (secondary). The lines are diluted by a factor of
about 1.8 (primary) and 2.3 (secondary), meaning that
lines with intrinsic strengths
above 90 mÅ are excluded. Comparing the results from the two
spectra, we find no significant
differences. The effective temperatures, surface gravities and
rotational velocities listed in Table 9 were
adopted. Microturbulence velocities were tuned until
Fe I abundances
were independent of line equivalent widths, and the resulting values
are 1.22
0.16 (primary) and 1.45
0.30 (secondary) km s-1.
The calibration by Edvardsson et al. (1993) predicts
1.48
0.31 km s-1 (primary) and
1.30
0.31 (secondary). For the adopted effective temperatures we see no
dependency of the abundance on excitation potential, which, however,
occurs if they are changed by more than about
K.
As seen, a robust
is obtained, with identical results from Fe I
and Fe II lines
of both components. Changing the model temperatures by
K
modifies
from the Fe I lines
by about
dex
whereas almost no effect is seen for Fe II lines.
If 0.25 km s-1 higher
microturbulence velocities are adopted,
decreases
by about 0.06 dex for both neutral and ionized lines. Taking
these contributions to the uncertainties into account,
we adopt
=
+0.05
0.09 for EW Ori.
We also find relative abundances close to +0.05 dex
for the other ions listed in Table 8,
including the -elements
Si, Ca, and Ti.
As a supplement to the spectroscopic abundance analyses, we
have derived metal abundances from the de-reddened indices
for the individual components (Table 9) and
the calibration by Holmberg et al. (2007). The
results are:
=
0.03
0.12 (primary) and
=
-0.05
0.12 (secondary). Within errors they agree with those from the
spectroscopic analysis; the quoted
errors
include the uncertainties of the photometric indices and the published
spread of the calibration.
Table 7: Log of the FEROS observations of EW Ori.
Table 8:
Abundances (
)
for the primary and secondary
components of EW Ori determined from the two
FEROS spectra.
7 Absolute dimensions
Absolute dimensions for EW Ori are presented in Table 9, as calculated from the photometric and spectroscopic elements given in Tables 5 and 6. As seen, masses and radii accurate to 0.9% and 0.5%, respectively, have been established for the binary components. For the radii, this is a clear improvement compared to the 1-3% different results listed in the new review on masses and radii by Torres et al. (2009), which are based on the work by Popper et al. (1986) and Imbert (2002).
Table 9: Astrophysical data for EW Ori.
The V magnitudes and
indices for the components included in Table 9 were
calculated from the combined magnitudes and indices of the system
outside eclipses (Table 1) and the
luminosity ratios between the components (Table 5). The V magnitude
and the
indices
obtained for the primary component agree very well with those measured
during the total part of central secondary eclipse (Table 1).
The E(b-y)
interstellar reddening, also given in Table 9, was
determined from the calibration by Olsen (1988), using the
standard
photometry for the combined light outside eclipses. For
comparison, Popper et al. (1986)
estimated E(b-y) =
0.010
0.009, equivalent to the reddening listed by Torres et al. (2009), E(B-V) =
0.014
0.012. The model by Hakkila et al. (1997) yields a
negative reddening in
the direction of and at the distance of EW Ori,
whereas the maps by Burstein & Heiles (1982) and Schlegel
et al. (1998)
give high total E(B-V) reddenings
of 0.09 and 0.16, respectively. Knude (private
communication) finds that EW Ori is located in or in front of
a tiny cloud in the outskirts of the
Orion OB Ia association.
From the individual indices and the calibration by Holmberg
et al. (2007),
we derive effective temperatures of 6070
95 and 5870
95 K for the primary and secondary
components, respectively, assuming the final
abundance.
The temperature uncertainties include those of the
indices,
E(b-y),
,
and the calibration itself. Temperatures based on the calibrations by
Alonso et al. (1996)
and Ramírez & Meléndez (2005)
are slightly lower but agree within errors. 2MASS photometry
of the combined light at phase 0.58 and the
calibration
by Masana et al. (2006)
gives an ``average'' temperature of 6100 K compared to
about 6000 K obtained from the combined
indices.
Finally, the empirical flux scale by Popper (1980) and the y flux
ratio between the components (Table 5) yield a
well-established temperature difference between the components
of 170
30 K (excluding possible errors of the scale itself).
Consequently, we assign temperatures of 6070 and
5900 K. They are about 100 K higher than adopted by
Popper et al. (1986)
and Torres et al. (2009).
The projected rotational velocities listed in Table 9 were
determined from broadening function analyses (e.g. Kaluzny
et al. 2006)
of several orders of the two FEROS spectra (Table 7). Within errors
they agree with the synchronous and the pseudo-synchronous values (Hut 1981,
Eq. (42)). The turbulent dissipation and radiative damping
formalism of Zahn (1977,1989) predicts
synchronization time scales of 3.4
108 yr (primary) and 3.6
108 yr (secondary), and a time scale
for circularization of 7.4
109 yr, compared to the predicted age
of EW Ori of about 2
109 yr (Sect. 9).
The distance to EW Ori was calculated from the
``classical'' relation (see e.g. CTB08), adopting the
solar values and bolometric corrections given in Table 9, and
AV/E(b-y)
= 4.28 (Crawford & Mandwewala 1976).
As seen, identical values are obtained for the two components,
and the distance has been established to 4%, accounting for
all error sources and including the use of other BC scales
(e.g. Code et al. 1976;
Bessell et al. 1998;
Girardi et al. 2002).
The empirical K surface
brightness - relation
by Kervella et al. (2004)
leads to nearly identical and perhaps
even more precise distances (about
pc); see Southworth
et al. (2005)
for details.
As mentioned in Sect. 2, EW Ori has been considered as a possible member of Collinder 70. According to Kharchenko et al. (2005) the distance to this open cluster is 391 pc, its radial velocity is +19.49 km s-1, and its age is 5.1 Gyr. This rules out that EW Ori is a member; see Tables 9, 6, and Sect. 9.
8 Stellar activity
Popper et al. (1986) reported weak evidence for intrinsic variability in their V,R light curves. We see no clear signs of periodic and/or yearly variations, e.g. due to spots, in the


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9 Discussion
In the following, we compare the absolute dimensions obtained for EW Ori with properties of recent theoretical stellar evolutionary models. A detailed comparison with other similar, well-studied eclipsing binaries will be included in a forthcoming paper.![]() |
Figure 4:
EW Ori compared with Y2
models for |
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Table 10: Information on the Claret models and ages inferred from the observed radii; see Figs. 7 and 8.
Figures 4-6 illustrate
the results from comparisons with the
Yonsei-Yale (Y2)
evolutionary tracks and isochrones by Demarque et al. (2004). The mixing length
parameter in convective envelopes is calibrated using the Sun, and is
held fixed at
.
The enrichment law Y
= 0.23 + 2Z is adopted, together with the solar
mixture by Grevesse et al. (1996),
leading to (X, Y, Z)
=
(0.71564, 0.26624, 0.01812). Only models for
= 0.0
have been considered. We refer to CTB08 for a brief description of
other aspects of their up-to-date input physics.
![]() |
Figure 5:
EW Ori compared with Y2 models
for |
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As seen from Fig. 4,
models for the observed masses and abundance,
=
+0.05, equivalent to (X, Y, Z) =
(0.70955, 0.27030, 0.02015), are hotter than
observed. Also, the well-established temperature difference between the
components, 170
30 K, is slightly larger than between the
corresponding models, although this is partly covered by the
uncertainty in the track positions coming from the small
0.9% mass errors. The uncertainty of
is
dex,
and models for
=
+0.14, equivalent to (X, Y, Z) =
(0.69695, 0.27870, 0.02435), fit the components
better. However, for EW Ori, the correlation between
and
is such,
that a 0.09 dex higher metal abundance corresponds to
150 K higher temperatures (see Sect. 6), meaning that
=
+0.14 models actually tend to become a bit too cool. The best
match is obtained for the combination of 75 K higher
temperatures, e.g. coming from a 0.01 mag higher interstellar
reddening, and a 0.04 dex higher
.
Turning to the scale-independent masses and radii,
Fig. 5
shows that the models predict nearly identical ages, close to
2 Gyr for the components, perhaps with a slight tendency of a
higher value for the secondary. From the observed masses and
luminosities, the models predict identical but less precise ages of
about 1 Gyr (
= +0.05)
and 2 Gyr (
= +0.14);
see Fig. 6.
Comparisons with solar-scaled (VRSS) Victoria-Regina models (VandenBerg
et al. 2006)
lead to nearly identical results.
![]() |
Figure 6:
EW Ori compared with Y2
models for |
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In conclusion, solar-scaled models provide fairly acceptable fits to
the observed properties of EW Ori within their uncertainties.
It is, however, of interest to see if specific model
tunings can lead to perfect reproduction of EW Ori.
To that end, we have calculated dedicated models for the
observed
using the Granada code by Claret (2004),
which assumes an enrichment law of
Y
= 0.24 + 2.0Z together with the solar mixture by
Grevesse & Sauval (1998).
The observed
=
+0.05 then corresponds to (X, Y, Z) =
(0.70, 0.28, 0.02). The envelope mixing length
parameter needed to reproduce the Sun is
,
and the adopted amount of core overshooting is
= 0.20
(in units of the pressure scale height)
.
Table 10
lists the few models we have investigated, and they are compared to
EW Ori in Figs. 7
and 8.
The model set #1, which is closest to the Y2 models
for = +0.05, show the
same temperature and age differences as discussed above, although the
track shapes are somewhat different. By decreasing
by 0.14 (primary) and
0.18 (secondary), the models
(set #2) match EW Ori perfectly at an age of
1.45 Gyr. Alternatively, keeping the solar
for the primary and decreasing
it slightly by 0.08 for the
secondary (model set #3) also gives a good fit,
provided the helium content is lowered to
,
i.e. close to the Y2 value.
The predicted age is 2.3 Gyr. For the secondary, which
exhibits signs of activity at a mild level (see Sect. 3.1),
a lower
- and thereby larger
model radius and lower model temperature - is consistent with
findings for other active solar-type
binary components (CBC09). On the other hand, for the primary there is
no observational background for a lower
,
and besides, 2D radiation hydrodynamics calculations (Ludwig
et al. 1999)
predict mixing length parameters close to the solar value for inactive
stars with temperatures and surface gravities like those of the
EW Ori components. So, the model
set #3 is our preferred fit.
Indications of a need for a slight downwards revision of the helium content, compared to the Y-Z relations adopted for the model grids, has been seen in a few other cases (e.g. VZ Hya, CTB08; V1130 Tau, Clausen et al. 2010). It is, however, still too early to drawn any firm conclusions. We will return to this issue in forthcoming papers on analyses of several new solar-type binaries; see the list in CBC09.
10 Summary and conclusions
From state-of-the-art observations and analyses, precise (0.5-0.9%)
absolute dimensions have been established for the components of the
totally eclipsing
G0 V system EW Ori.
A detailed spectroscopic analysis yields an iron abundance
relative to the Sun of =
+0.05
0.09 and similar relative abundances for Si, Ca, Sc, Cr,
and Ni.
The 1.12
secondary component reveals weak Ca II H
and K emission and is probably mildly active; we see no signs
of activity for the 1.17
primary. Apsidal motion (
U =
16 300
3900 yr)
with a 62% relativistic contribution has been
detected for the eccentric orbit (
e =
0.0758
0.0020), and the inferred mean central density concentration
coefficient, log(k2) =
-1.66
0.30, agrees marginally with model predictions. The measured rotational
velocities, 9.0
0.7 (primary) and 8.8
0.6 (secondary) km s-1,
are in agreement with both synchronous rotation and the theoretically
predicted pseudo-synchronous velocities.
![]() |
Figure 7:
EW Ori compared to Claret models for the observed masses and
|
Open with DEXTER |
![]() |
Figure 8:
EW Ori compared to Claret models for the observed masses and
|
Open with DEXTER |
Stellar models with solar-scaled envelope mixing length parameters
reproduce the observed properties of EW Ori fairly well at an
age of 2 Gyr.
We demonstrate, however, that perfect agreement can be obtained
by a) a slight downwards adjustment of the envelope mixing
length parameter for the secondary, as seen for other active solar-type
stars; and b) a slightly lower helium
content than prescribed by the Y-Z relations
adopted for the various standard model grids.
This study is part of a larger project on solar-type eclipsing binaries; see e.g. CBC09.
It is a great pleasure to thank the many colleagues and students, who have shown interest in our project and have participated in the extensive (semi)automatic observations of EW Ori at the SAT: Gwillerm Berard, Vanessa Doublier, Mathias P. Egholm, Anders Johansen, Erling Johnsen, Helene Jørgensen, Raslan Leguet, Gilbert Mahoux, and John D. Pritchard. Excellent technical support was received from the staffs of Copenhagen University and ESO, La Silla. We thank J. M. Kreiner for providing a complete list of published times of eclipses for EW Ori and J. Southworth for access to his JKTEBOP code. G. Torres and J. Knude kindly made independent interstellar reddening information available. The projects ``Stellar structure and evolution - new challenges from ground and space observations'' and ``Stars: Central engines of the evolution of the Universe'', carried out at Copenhagen University and Aarhus University, are supported by the Danish National Science Research Council. The following internet-based resources were used in research for this paper: the NASA Astrophysics Data System; the SIMBAD database and the VizieR service operated by CDS, Strasbourg, France; the ariv scientific paper preprint service operated by Cornell University; the VALD database made available through the Institute of Astronomy, Vienna, Austria; the MARCS stellar model atmosphere library. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/ California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
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Footnotes
- ... system
- Based on observations carried out at the Strömgren Automatic Telescope (SAT) and the 1.5 m telescope (62.L-0284) at ESO, La Silla.
- ...
- Table 11 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A22
- ...2004)
- http://www.as.ap.krakow.pl/ephem
- ... coefficient
- See e.g. Giménez (1985) for the definition of k2 and for references to the ``classical'' papers.
- ... SBOP
- Spectroscopic Binary Orbit Program, http://mintaka.sdsu.edu/faculty/etzel/
- ... Rossby
- Defined as the ratio of the rotation period to the convective turnover time.
- ... component
- Popper et al. (1986) saw no emission, probably due to inadequate resolution.
- ...2004)
- http://www.astro.yale.edu/demarque/yystar.html
- ... height)
- Models without core overshooting are very similar at the age of EW Ori.
All Tables
Table 1: Photometric data for EW Ori and the comparison stars.
Table 2:
Times of primary (P) and secondary (S) minima of EW Ori
determined from the
observations.
Table 3: Apsidal motion parameters for EW Ori derived from all available times of minima.
Table 4: Photometric solutions for EW Ori from the JKTEBOP code.
Table 5: Adopted photometric elements for EW Ori.
Table 6: Spectroscopic orbital solutions for EW Ori determined from re-analyses of the radial velocity observations by Popper et al. (1986) and Imbert (2002).
Table 7: Log of the FEROS observations of EW Ori.
Table 8:
Abundances (
)
for the primary and secondary
components of EW Ori determined from the two
FEROS spectra.
Table 9: Astrophysical data for EW Ori.
Table 10: Information on the Claret models and ages inferred from the observed radii; see Figs. 7 and 8.
All Figures
![]() |
Figure 1: y light curve and b-y and u-b colour curves (instrumental system) for EW Ori. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
( |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Adopted spectroscopic orbital solution for EW Ori (solid line: primary; dashed line: secondary) and radial velocities (filled circles: primary; open circles: secondary). The dotted line ( upper panel) represents the center-of-mass velocity of the system. Phase 0.0 corresponds to central primary eclipse. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
EW Ori compared with Y2
models for |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
EW Ori compared with Y2 models
for |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
EW Ori compared with Y2
models for |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
EW Ori compared to Claret models for the observed masses and
|
Open with DEXTER | |
In the text |
![]() |
Figure 8:
EW Ori compared to Claret models for the observed masses and
|
Open with DEXTER | |
In the text |
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