In early 2003 our team announced the discovery of the second extrasolar transiting planet, around the faint star OGLE-TR-56 (V = 16.6), based on the detection of small changes in the radial velocity of the primary. The star was originally identified as a candidate by the OGLE team from the shallow and periodic dips in its brightness. We present here new precise radial velocity measurements that confirm the variation measured earlier, supporting the conclusion that the companion is indeed a planet. Additional photometric observations are also available, which combined with the spectroscopy yield improved parameters (mass and radius) for the planet

Jha, Saurabh

Konacki, Maciej

Sasselov, Dimitar D.

Torres, Guillermo

English

Caltech Authors - Main

OGLE-TR-56
Guillermo Torres∗, Maciej Konacki†, Dimitar D. Sasselov∗ and
Saurabh Jha∗∗
∗Harvard-Smithsonian Center for Astrophysics
†Caltech, Department of Geological and Planetary Sciences
∗∗University of California–Berkeley, Department of Astronomy
Abstract. In early 2003 our team announced the discovery of the second extrasolar transiting
planet, around the faint star OGLE-TR-56 (V = 16.6), based on the detection of small changes in
the radial velocity of the primary. The star was originally identified as a candidate by the OGLE
team from the shallow and periodic dips in its brightness. We present here new precise radial
velocity measurements that confirm the variation measured earlier, supporting the conclusion that
the companion is indeed a planet. Additional photometric observations are also available, which
combined with the spectroscopy yield improved parameters (mass and radius) for the planet.
INTRODUCTION
One of the most successful searches for transiting extrasolar planets to date has been the
OGLE survey, which has uncovered a total of 137 candidate transiting planets in several
fields toward the bulge of the Galaxy and in the constellation of Carina [1, 2, 3, 4]. All
of these stars show small dips in brightness at the level of a few percent, suggesting the
presence of a low-mass companion in orbit. The stars are typically much fainter than the
usual targets of transit surveys, ranging in brightness from about V = 14 to V = 19. The
fields toward the Galactic center are also quite crowded.
In 2002 we began a systematic spectroscopic follow-up campaign in the bulge sample
to weed out “false positives". Examples of false positives include a stellar (as opposed
to a substellar) companion orbiting a large star (B-A main sequence star, or a giant),
grazing eclipses in a stellar binary, or contamination by the light of a fainter eclipsing
binary along the same line of sight (referred to as a “blend"), so that the otherwise
deep eclipses of that binary are diluted by the other star and look very shallow (transit-
like). As a result of that campaign, we were able to rule out up to 98% of the original
candidates as false positives, and we then focussed on the remaining handful of good
candidates for higher-precision work.
In early 2003 our team announced the discovery of the planet around the star OGLE-
TR-56 (V = 16.6), with a period of only 1.21 days [5]. So far this is the only known
transiting extrasolar planet aside from the well-known case of HD 209458b [6, 7].
We report here additional observations that confirm the original radial velocity varia-
tion detected by [5], and improve the estimate of the mass of the planet. We also report
an updated light curve solution based on new OGLE photometry, yielding an improved
planetary radius.
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FIGURE 1. Radial velocity observations and velocity curve for OGLE-TR-56, as a function of orbital
phase (fixed from the photometry).
SPECTROSCOPIC OBSERVATIONS AND ORBITAL SOLUTION
High-resolution spectra were obtained with the Keck I telescope using the HIRES
instrument (High-Resolution Echelle Spectrometer) on two runs during August 1–3 and
August 11–12 of 2003. This is the same instrumentation used by [5] for the original
observations that led to the planet discovery. The resolving power achieved is λ/∆λ ≈
65,000. The wavelength reference was established using exposures of a Thorium-Argon
lamp before and after each stellar exposure.
Radial velocities were obtained by cross-correlation against a synthetic template for
OGLE-TR-56 with parameters matching those of the star. The uncertainties in the radial
velocities are limited by systematics at the level of about 100 m s−1, as described in
more detail by [8]. Internal errors are smaller, ranging from 70 to 90 m s−1 in most
cases.
Because the object transits the parent star every 1.21 days, the ephemeris is well
known from the light curve analysis (see below). We have adopted this ephemeris for
the spectroscopic solution, and so the only parameters that need to be adjusted are a
radial velocity offset and the velocity semi-amplitude (K), since the orbit is assumed to
be circular.
Our radial velocity curve is shown in Figure 1, where the circles represent the 8 new
measurements (August 2003) and the triangles are based on the original 3 spectra by [5],
re-reduced for uniformity.
The new measurements confirm the variation detected originally, although with a
larger amplitude than the first estimate, which was based on only 3 observations (with
two free parameters). The significance of the determination is now much greater, as
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FIGURE 2. Bisector span used as a proxy for line asymmetry for each of our spectra of OGLE-TR-56,
as a function of orbital phase. Over-plotted for reference is the velocity curve from Fig. 1, which shows
that there is no correlation of the asymmetries with phase.
can be seen visually in the figure, and the errors are better characterized because of
the increased number of observations. The measured K amplitude is 265± 38 m s−1,
and the RMS residual from the fit is 114 m s−1. The minimum mass for the planet is
Mp sin i = 1.33± 0.21× 10−3 M× (M∗+ Mp)2/3, where M∗ is the mass of the parent
star and i is the inclination angle of the orbit. The absolute mass of the planet is derived
below.
TESTING FOR BLENDS: THE SPECTRAL LINE BISECTORS
Spurious radial velocity measurements can result if the spectral lines of the main star are
blended with those of another fainter object, which can introduce slight asymmetries in
the line profiles. Changes in the asymmetry correlating with orbital phase are a sign that
the velocity variations are not real, and thus that there is no planet.
We have tested this for OGLE-TR-56 by computing the spectral line bisectors [e.g.,
9] for each of our spectra directly from our cross-correlation functions, which are
representative of the average spectral line profile. The difference in velocity between the
bisectors at two different levels in the correlation function (‘high’ and ‘low’) is defined
as the “bisector span", and is used as a measure of the asymmetry of the mean spectral
line.
Figure 2 shows the bisector span as a function of orbital phase. For reference we
display also the orbital solution from Figure 1. There is no significant correlation with
phase, either positive or negative. This means that the velocity variations measured above
are real, and thus confirms that we have a planet.
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TABLE 1. Orbital and physical parameters for OGLE-TR-56b.
Parameter Value
Orbital period (days) . . . . . . . . . . . . . . . . . 1.2119189 ± 0.0000059
Transit epoch (HJD−2,400,000) . . . . . . . 52075.1046 ± 0.0017
Center-of-mass velocity (km s−1) . . . . . . −48.317 ± 0.045
Eccentricity (fixed) . . . . . . . . . . . . . . . . . . . 0
Velocity semi-amplitude (m s−1) . . . . . . . 265 ± 38
Inclination angle (deg) . . . . . . . . . . . . . . . . 81.0 ± 2.2
Stellar mass (M) (adopted) . . . . . . . . . . 1.04 ± 0.05
Stellar radius (R) (adopted) . . . . . . . . . . 1.10 ± 0.10
Limb darkening coefficient (I band) . . . . 0.56 ± 0.06
Planet mass (MJup) . . . . . . . . . . . . . . . . . . 1.45 ± 0.23
Planet radius (RJup) . . . . . . . . . . . . . . . . . 1.23 ± 0.16
Planet density (g cm−3) . . . . . . . . . . . . . . 1.0 ± 0.3
Semi-major axis (AU) . . . . . . . . . . . . . . . . 0.0225 ± 0.0004
RESULTS AND CONCLUSIONS
Additional photometric transits have been detected by the OGLE team in 2003 (for a
total of 13), and refinements in the analysis have removed small systematic errors in
the original photometry. We have carried out a new light curve solution resulting in an
improvement in the photometric parameters.
The results for OGLE-TR-56 are given in Table 1. The planet is twice as massive as
HD 209458b, but has approximately the same size and orbits at only half the distance to
its parent star. This makes it particularly interesting from the theoretical point of view.
Several other good transit candidates from the lists released by the OGLE team are
currently being followed up spectroscopically with Keck. With the measurement and
analysis techniques we have developed, we expect to be able to discriminate with a high
degree of confidence between true planets and false positives.
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