Investigation of transit-selected exoplanet candidates from the MACHO
  survey
Astronomy & Astrophysics manuscript no. macho c© ESO 2018
November 8, 2018
Investigation of transit-selected exoplanet candidates from the
MACHO survey⋆
S. D. Hügelmeyer1, S. Dreizler1, D. Homeier1, and A. Reiners1,2
1 Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
2 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
Received <date> / Accepted <date>
ABSTRACT
Context. Planets outside our solar system transiting their host star, i. e. those with an orbital inclination near 90◦, are of special
interest to derive physical properties of extrasolar planets. With the knowledge of the host star’s physical parameters, the planetary
radius can be determined. Combined with spectroscopic observations the mass and therefore the density can be derived from Doppler-
measurements. Depending on the brightness of the host star, additional information, e. g. about the spin-orbit alignment between the
host star and planetary orbit, can be obtained.
Aims. The last few years have witnessed a growing success of transit surveys. Among other surveys, the MACHO project provided
nine potential transiting planets, several of them with relatively bright parent stars. The photometric signature of a transit event is,
however, insufficient to confirm the planetary nature of the faint companion. The aim of this paper therefore is a determination of the
spectroscopic parameters of the host stars as well as a dynamical mass determination through Doppler-measurements.
Methods. We have obtained follow-up high-resolution spectra for five stars selected from the MACHO sample, which are consistent
with transits of low-luminosity objects. Radial velocities have been determined by means of cross-correlation with model spectra.
The MACHO light curves have been compared to simulations based on the physical parameters of the system derived from the radial
velocities and spectral analyses.
Results. We show that all transit light curves of the exoplanet candidates analysed in this work can be explained by eclipses of stellar
objects, hence none of the five transiting objects is a planet.
Key words. Stars: planetary systems - Eclipses - Techniques: radial velocities
1. Introduction
After the first detections of planets outside our solar system
(Wolszczan & Frail 1992; Mayor & Queloz 1995), an inten-
sive search with various methods began (see Schneider 2002,
for an overview) resulting in currently more than 200 planets
(http://exoplanet.eu/). Most of these exoplanet detections have
been performed via the radial velocity (RV) method where the
“wobble” of the parent star caused by the planet is measured by
spectral line shifts. Since these effects are very small for low-
mass planets in orbits of tens to hundreds of days, the determi-
nation of orbital period, phase, inclination, eccentricity, and RV
amplitude demands RV accuracies of a few meters per second
(Marcy et al. 2000).
Meanwhile, alternative methods for planet detections have
also been successfully applied. The first four microlensing plan-
ets have been detected (Bond et al. 2004; Udalski et al. 2005;
Beaulieu et al. 2006; Gould et al. 2006), possible first direct im-
ages of extra-solar planets were published (Chauvin et al. 2004,
2005a,b; Neuhäuser et al. 2005; Biller et al. 2006), and the num-
ber of detections due to transit searches is steadily increasing
(McCullough et al. 2006; Bakos et al. 2007; O’Donovan et al.
2006; Collier Cameron et al. 2007).
The transit method is of special interest, since it permits
the derivation of additional physical parameters of the planet,
e. g. the radius can be measured either indirectly via the ra-
⋆ Based on observations made with ESO Telescopes at the La Silla
or Paranal Observatories under programme ID 075.C-0526(A)
dius of the host star or directly via detection of the sec-
ondary eclipse as observed with the Spitzer Space Telescope
(Charbonneau et al. 2005; Deming et al. 2005). If combined
with a radial velocity variation measurement, the mass and mean
density can be determined, revealing constraints for the plane-
tary structure. Furthermore, transiting systems allow us to inves-
tigate the atmospheres of the planets (Charbonneau et al. 2002;
Vidal-Madjar et al. 2004) as well as the spin-orbit-alignment be-
tween the rotational axis of the host star and the planetary orbit
(Wolf et al. 2007; Gaudi & Winn 2007; Winn et al. 2006).
Drake & Cook (2004, hereafter DC) published a list of
nine restrictively selected, transiting planet candidates from the
MACHO project (Alcock et al. 1992). Only transit light curves
with no indication of gravitational distortion and only those with
clear U-shaped transit events were considered. De-reddened
colours as well as light curve fitting provide a good estimate
of the companion radius. Only companions below 3 Jupiter radii
were selected.
Based on high-resolution spectra, the orbital velocities of
five potential host stars of exoplanet candidates have been
measured. We analysed the RV measurements together with
MACHO transit light curves in order to determine the system
parameters complemented by a spectral analysis.
The paper is organized as follows: In the next section, we
shortly describe the spectroscopic observations and the spectral
analysis as well as the Doppler-measurements. In section 3, we
describe the results of the individual systems and summarise in
section 4.
http://arxiv.org/abs/0704.1366v1
http://exoplanet.eu/
2 S. D. Hügelmeyer et al.: Investigation of transit-selected exoplanet candidates
Table 1. Orbital elements, rotation velocities, and stellar parameters for all five analysed systems. Components c and d of MACHO
118.18272.189 and component b of MACHO 120.22041.3265 are not visible in the spectra. PMACHO is taken from Drake & Cook
(2004) and P denotes the period derived using the light curves and RV measurements. K is the semi-amplitude of the RV variations,
V0 the system velocity, and i the orbital inclination. In case of systems with elliptical orbits, e is the eccentricity and ω the longitude
of the periastron. Furthermore, the mass MRV is given in cases where the RV amplitude of two components is known. Then T RVeff
and R are calculated for zero- and terminal-age main sequence models. T SAeff is the effective temperature derived from the spectral
analyses. In cases where just T SAeff from the spectral analyses is known, M
SA and R are derived masses and radii from evolution
models. All values in this table relate to the assumption of zero-age main sequence stars.
MACHO ID P PMACHO K V0 i e ω M
RV T RVeff T
eff M
[days] [days] [km s−1] [km s−1] [◦] [◦] [M⊙] [K] [K] [M⊙] [R⊙]
118.18272.189 a – 1.9673 0.00 −25.51± 0.03 – – – – – 5800 – –
b – – 0.00 +05.46± 0.03 – – – – – 5800 – –
c 3.9346 – – – (90.0) – – 0.41 3730 – – 0.38
d 3.9346 – – – – – 0.41 3730 – – 0.38
118.18407.57 a 4.7972 2.3986 78.84± 0.10 −20.48± 0.07 84.0 – – 1.27 6430 6200 – 1.23
b – – 0.00 −08.39± 0.03 – – – – 6600 – –
c 4.7972 2.3986 89.68± 0.09 −20.48± 0.06 – – 1.11 5980 6200 – 1.04
118.18793.469 a 4.0744 2.0372 75.81± 0.18 −56.30± 0.11 85.6 0.041 89.94 0.90 5140 5400 – 0.81
b 4.0744 2.0372 83.67± 0.25 −56.30± 0.14 0.82 5070 5400 – 0.76
120.22041.3265 a 5.4083 5.4083 22.18± 0.06 −24.00± 0.04 89.8 0.108 19.98 – – 6200 1.19 1.15
b 5.4083 5.4083 114.90 −24.00± 0.04 – – (3340) (0.23) 0.28
402.47800.723 a 8.5496 4.2748 75.91± 0.04 +00.40± 0.04 85.8 – – 1.26 6400 6400 – 1.22
b – – 0.00 −26.40± 0.04 – – – – 5800 – –
c 8.5496 4.2748 68.09± 0.07 +00.40± 0.07 – – 1.40 6820 6400 – 1.37
2. Observations and analyses
In period 75 we secured three spectra for each of the five
brightest candidates. We used ESO’s Fibre-fed Extended Range
Échelle Spectrograph (FEROS) mounted on the 2.2 m telescope
at La Silla, Chile. The spectrograph provides a spectral resolu-
tion of R ∼ 48 000 and covers a wavelength range from 3500 Å
to 9200 Å. The instrumental specifications list a RMS velocity
error of ∼ 25 m s−1. This is sufficient to detect faint low-mass
star companions and distinguish them from sub-stellar compan-
ions, which was the primary aim of the observations. The sec-
ondary aim is to use the spectra for a spectral analysis in order
to derive the stellar parameters of the host stars.
The observations of the five targets have been performed
between August 19 and September 16, 2005. For each object
we have obtained three spectra with exposure times between
2400 s and 3500 s, depending on the brightness of the object.
The signal-to-noise ratio is ∼ 10.
The data were reduced using the FEROS Data Reduction
System (DRS). The échelle spectra were bias and flat field cor-
rected and wavelength calibrated. The latter calibration was ad-
ditionally quality checked by cross-correlating the observation
with a sky line spectrum. The spectra were then corrected by ap-
plying relative wavelength shifts. Barycentric and Earth rotation
velocity influences to the wavelengths are accounted for auto-
matically by the DRS.
For the determination of the radial velocities we used the
extracted FEROS spectra and synthetic spectra of main se-
quence model stars calculated from LTE model atmospheres us-
ing PHOENIX (Hauschildt et al. 1999) version 14.2. Both spectra
were normalised and relative fluxes were interpolated on a log-
arithmic wavelength scale. A cross-correlation (CC) between a
model with Teff = 5600 K and observation was performed be-
tween 5000 Å and 7000 Å. The CC was implemented using a
grid with 200 steps of ∆ log
λ/[Å]
= 2.2 · 10−6 in each di-
rection. This method turned out to be robust against the use of
different model spectra. We could identify up to three spectro-
scopic components in our data. Each of the peaks in the CC was
then fitted by a Gaussian and the position of the maximum of the
fit gives the radial velocity. The errors of the RV measurements
were calculated from the standard deviation of the Gaussian plus
the accuracy limit of FEROS of 25 m s−1. These RV errors are
in the range between 50 and 350 m s−1.
The CC function was also used to determine the projected
rotation velocities of the stars. We therefore applied a solar spec-
trum as template convolved with rotational profiles following
Gray (2005). This method allows to derive stellar radii in bi-
naries, assuming a synchronised orbit. In this analysis, the deter-
mined rotational velocity v sin i is of the order of the uncertainty
in most cases, which, due to the low signal-to-noise ratio, is
about 5 km s−1. These derived radii are consistent with the ones
obtained from main sequence models (see Table 1). Additional
constraints for the radii of the binary components visible in the
spectra can therefore not be derived.
In order to spectroscopically identify the components of the
analysed systems, we again used the PHOENIXmodel grid which
ranges from 4000 K to 6800 K in Teff and from −1.5 to 0.0 in
relative solar metallicity [Fe/H]. It should be noted that this is
not sufficient for a detailed abundance determination, which was
not the aim of this work. The surface gravity is kept constant at
log g = 4.5. Knowing the RV of the individual components of a
system, the models were gauss-folded to the resolution of the ob-
servation and shifted to their position in the observed spectrum.
Depending on the number of spectral components, all possible
combinations of model spectra were tested for each observed
spectrum. A χ2-test was used to identify the best fitting models.
Given the low signal-to-noise ratio of the spectra, we estimate
an uncertainty of about 400 K in effective temperature.
In cases where we know the RV amplitudes for two compo-
nents (MACHO 118.1407.57, 118.18793.469, 402.47800.723),
S. D. Hügelmeyer et al.: Investigation of transit-selected exoplanet candidates 3
M sin i is known for both components. Assuming i = 90◦ for
the first iteration, we determined radii and effective tempera-
tures (T RVeff in Table 1) from interpolation of the Geneva model
tracks (Schaerer et al. 1993) assuming zero-age main sequence
(ZAMS) or terminal-age main sequence (TAMS) stars. We then
applied the eclipsing binary simulation software Nightfall1
with the derived stellar and orbital parameters from the previous
step as input and calculated a best model fit to the R-band light
curve and radial velocity measurements simultaneously. We used
the third light contribution and the inclination as free parameters
and calculated χ2-values for the light curve fits assuming ZAMS
and TAMS stars. In a second iteration, we repeated the fit with
the now known inclination (see Fig. 3). For these three systems
the so derived effective temperature can be compared to the one
of the spectral analyses (T SAeff in Table 1). Deviations are within
our estimated uncertainties and show the overall consistency of
our main-sequence solution.
In the other two cases (MACHO 120.22041.3265 and
MACHO 118.18272.189), the effective temperature from the
spectral analysis was used to derive masses and radii of each
components, again assuming ZAMS and TAMS stars. In the
light curve simulations for the MACHO R-band photometry we
varied the inclination and the radius R2 of the potential transiting
object, assuming ZAMS and TAMS primary stars.
3. Results
We will present results for the five targeted MACHO objects for
which we found an orbital solution that explains the detected
transits and the measured radial velocities. In Fig. 1 we show fit-
ted light curves to the photometric MACHO data (bottom pan-
els in the plots) and RV curve fits to the Doppler-measurements
(asterisks in the top panel of the plots). The dashed lines are
for circular orbits and the solid lines show a best fit ellipti-
cal orbit. Fig. 2 again shows Nightfall light curve solutions
to the photometric data as well as the RV fits to the Doppler-
measurements. Here circular orbits reproduce the observations
best. All stars were assumed to be on the ZAMS for the fits in
Figs. 1 and 2. χ2-contour plots for both ZAMS and TAMS stars
are depicted in Fig. 3. The inclination of the orbital plane and
the third light contribution (bottom three plots) and the radius
R2 (top two plots) of the potential transiting objects were treated
to vary. A list of the orbital parameters PMACHO (period given by
Drake & Cook 2004), the derived period P in our analyses, the
RV amplitude K, the system velocity V0, the orbital inclination
i, and in cases of systems with elliptical orbits, e the eccentric-
ity and ω the longitude of the periastron as well as the mass,
effective temperature, and radius is shown in Table 1.
MACHO 120.22041.3265
MACHO 120.22041.3265 is the only system in our sample with
just one component visible in the spectra. Spectral analysis
yields Teff = 6200 K and indicates a low metallicity ([Fe/H] =
−1.0). The fit of a sinusoidal to the RVs folded to a period of
5.4083 d (DC, dashed curve in Fig. 1) differs from the RV mea-
surement at a phase of 0.87 by ∼ 10 km s−1. A better fit is pro-
vided by an elliptical orbit with an eccentricity of e = 0.108,
a longitude of periastron of ω = 19.98, and an orbital semi-
amplitude of K = 22.18 km s−1. For such a system the radius
and mass of the secondary is R2 = 0.3 R⊙ and M2 = 0.23 M⊙
for a ZAMS and R2 = 0.5 R⊙ and M2 = 0.25 M⊙ for a TAMS
1 http://www.hs.uni-hamburg.de/DE/Ins/Per/Wichmann/Nightfall.html
Fig. 1. Radial velocity and light curve fits for systems with ellip-
tical orbits. The dashed lines show best-fit sinusoidals while the
solid lines show best-fit eccentric orbits. Component a is plotted
in black, component b in grey. The system velocity for the circu-
lar orbit is shown by the thin line, and for the elliptical orbit by
the thick dotted line. The solutions shown are calculated assum-
ing ZAMS stars. The error bars for the RV measurements are of
the size of the symbols.
primary (see Fig. 3), clearly indicating an M dwarf companion.
With these parameters, the system is very similar to OGLE-TR-
78 (Pont et al. 2005).
We used equation (6.2) of Zahn (1977) to calculate an esti-
mate for the circularisation time of the system. Due to the low
mass ratio q = M2/M1, we find a circularisation time of the order
of the Hubble time even for this close binary system.
MACHO 118.18793.469
Two spectral components could be identified, each with Teff =
5400 K and a highly subsolar ([Fe/H] = −1.5) metallicity. For
the light curve and RV fits with Nightfall we used the RV
amplitudes of the two stars to derive masses by the procedure
described in the previous section. A reasonable fit to the RV
measurements folded to twice the period of DC can be achieved
with sinusoidals (dashed curves in Fig. 1), i. e. assuming a cir-
cular orbit for the two components. However, an improved fit
can be achieved by fitting the light curve and radial veloci-
ties in Nightfall at the same time to an elliptical orbit (solid
4 S. D. Hügelmeyer et al.: Investigation of transit-selected exoplanet candidates
Fig. 2. Radial velocity and light curve fits for systems with cir-
cular orbits. Component a is plotted in black, component b in
grey, and component c in a lighter grey. The solutions shown
are calculated assuming ZAMS stars. The error bars for the RV
measurements are of the size of the symbols.
curves in Fig. 1). The best fit is achieved with a small eccen-
tricity of e = 0.041 and a periastron longitude of ω = 89.94◦.
By varying the third light and the inclination, we construct the
χ2-map shown in Fig. 3. As suggested by the spectral analysis of
MACHO 118.18793.469, the lowest χ2-value is found for a third
light of zero. The inclination is 85.6◦ for the ZAMS and 82.2◦
for the TAMS. The low depths of the transit is therefore due to
a grazing eclipse. This is also supported by the V-shape of the
best-fit model.
The best-fit light curve model shows different transit depths.
This is an indicator for two transits in one orbital period caused
by two stars of slightly different size.
MACHO 118.18407.57
Three components are visible in the CCs of the three spectra,
one of which shows RV variations below 1 km s−1. Therefore,
this component is a third component, either in a wider orbit or
physically unrelated to the other two. Component a and c show
RV changes of over 100 km s−1. They can be well fitted with
sinusoidals of twice the period given by DC, i. e. 4.7972 d. If
the photometric data are phased accordingly, we then see both
transits where the transit depths are reduced due to third light of
component b.
For the light curve simulation we once more used the RV
amplitudes of a and c to get the masses and varied the incli-
nation and third light coming from component b. The effective
temperatures and radii of the components are interpolated from
the Geneva evolution tracks assuming young stars on the ZAMS
and older stars on the TAMS. The contribution of component b
meets the expectations from the spectral analyses (Teff = 6200 K
for components a and c and Teff = 6600 K for component b, also
see Fig. 3). The inclination of the system is 84◦ assuming that
the stars are on the ZAMS and 79.5◦ for the TAMS. The system
shows different transit depths, as MACHO 118.18793.469 does.
MACHO 402.47800.723
The second brightest object in the sample shows three compo-
nents in the spectra. Components a and c are best fitted by a
model with Teff = 6400 K, b has Teff = 5800 K. As in the case
of MACHO 118.18407.57, the masses are derived from the ra-
dial velocities. The RV measurements of a and c are well fitted
assuming a circular orbit with twice the period of DC. The third
component only shows small RV variations and therefore seems
to have a larger period than the other two. The fractional third
light contribution for this component is ∼ 1/3 and an inclination
of the eclipsing system is 85.8◦ assuming stars on the ZAMS and
82.3◦ for the TAMS (see Fig. 3). We again see transit depth dif-
ferences. Due to the high signal-to-noise ratio of the light curve,
these are quite obvious and amount to ∆R = 0.015. This observa-
tion is also expected from the orbital semi-amplitude differences.
MACHO 118.18272.189
Each of the three FEROS spectra displays two components. The
spectral analysis reveals that both components have a similar ef-
fective temperature (Teff = 5800 K) and a subsolar metallicity
of [Fe/H] = −0.5. The cross-correlation shows that component
b has a constant RV of ∼ 5.46 km s−1 within the above men-
tioned statistical errors. Component a shows RV variations of
∼ 3.5 km s−1. Folding the RV measurements to the orbital pe-
riod given by DC, one sees that the two components visible in
the spectrum cannot be responsible for the transit in the light
curve since one RV point is very close to the transit. However,
here the two components should almost have the same RV. This
is clearly not present in the data. The same is the case if we dou-
ble the period (see Fig. 2). Thus, we exclude the scenario that
the two visible components are responsible for the transit.
S. D. Hügelmeyer et al.: Investigation of transit-selected exoplanet candidates 5
In another plausible scenario, we treat component b as third
light and assume that component a is eclipsed by a low mass
object not visible in the spectra. However, if we fit a sinusoidal
to the RV points, in our solution the star would move away from
the observer after the transit while it should do the opposite. We
can therefore discard this scenario.
One could argue that the variations in RV measured for com-
ponent a is just caused by systematic errors and that the eclipse
visible in the light curve is caused by a planet orbiting a with-
out causing any noticeable RV changes. We have fitted this sce-
nario taking the light from component b into account and found
a radius of R2 = 0.25 R⊙ assuming that a is on the ZAMS and
R2 = 0.35 R⊙ for a being on the TAMS (see Fig. 3). These val-
ues, however, seem unrealisticly high for planets and we can re-
ject the 3-body scenario.
Finally, one scenario that can explain both the transit light
curve and the measured RVs is a four body system consisting
of the two G stars which are visible in the spectra and two M
dwarfs invisible in the spectra. Here the two faint components or-
bit each other in twice the period from DC and eclipse each other
twice. We assume an inclination of 90◦ and two low-mass stars
of equal size. The effective temperature of the eclipsing bodies
was derived from the transit depth of the MACHO R-band light
curve using blackbody fluxes for all four components. The tran-
sit depth is reduced by the light of components a and b. The
RV variations of component a can in this scenario be explained
by the reflex motion of a to the orbit of the binary M star system
with a much larger period. We therefore do not observe a correla-
tion between the transits and the RV. This scenario is underlined
by the fact that the two RV measurements in Fig. 2 at periods of
∼ 1.0 and ∼ 1.3, which have approximately the same RVs, are
from two spectra only taken one day apart, while the third RV
value comes from a spectrum 26 days later. Component b would
be in a very wide orbit or physically unrelated to the other three
stars.
4. Summary
For none of the five analysed MACHO-candidates a planetary or
brown dwarf companion could be identified. We therefore con-
firm the speculation of DC that due to the depths of the transits in
the photometric data the objects would be low-mass stars rather
than sub-stellar objects. From the five candidates, we found
one grazing eclipse of two nearly identical G stars (MACHO
118.18793.469), two blends of deep eclipses of G stars with a
significant third light contribution (MACHO 118.18407.57 and
MACHO 402.47800.723), one binary star with a G type primary
and an M dwarf secondary (MACHO 120.22041.3265) and one
rather complicated, blended system with four stars, of which
each two are nearly identical (G and M type). With this work
we could show that also for deep transit surveys for extrasolar
planets, follow-up observations to weed out false positives are
efficiently possible with moderate effort.
After all, our results once again underline the need for spec-
troscopic follow-up of transit planet candidates as already shown
by Bouchy et al. (2005) and Pont et al. (2005) for the OGLE sur-
vey and Torres et al. (2004) in the case of a blend scenario.
Acknowledgements. We would like to thank the referee for very useful com-
ments.
S.D.H. gratefully acknowledges the support of the German-Israeli Foundation
for Scientific Research and Development grant I-788-108.7/2003.
A.R. has received research funding from the European Commission’s Sixth
Framework Programme as an Outgoing International Fellow (MOIF-CT-2004-
002544).
This paper utilizes public domain data obtained by the MACHO Project, jointly
funded by the US Department of Energy through the University of California,
Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48,
by the National Science Foundation through the Center for Particle Astrophysics
of the University of California under cooperative agreement AST-8809616, and
by the Mount Stromlo and Siding Spring Observatory, part of the Australian
National University.
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6 S. D. Hügelmeyer et al.: Investigation of transit-selected exoplanet candidates
Fig. 3. χ2-contour plots for all analysed systems. In the left column we assume that the stars are on the zero-age main sequence and
in the right column on the terminal-age main sequence. The bottom three plots show the χ2-contours for third light and inclination
as fitted parameters. For the top two the radius of the eclipsing component and the inclination have been varied. The crosses mark
best-fit values.
	Introduction
	Observations and analyses
	Results
	Summary
