Five Intermediate-Period Planets from the N2K Sample
to appear in ApJ
Five Intermediate-Period Planets from the N2K Sample 1,2
Debra A. Fischer3, Steven S. Vogt4, Geoffrey W. Marcy5, R. Paul Butler6, Bun’ei Sato7,
Gregory W. Henry8, Sarah Robinson4, Gregory Laughlin4, Shigeru Ida7, Eri Toyota9
Masashi Omiya10 Peter Driscoll11, Genya Takeda12, Jason T. Wright5, John A. Johnson5
fischer@stars.sfsu.edu
ABSTRACT
We report the detection of five Jovian mass planets orbiting high metallicity
stars. Four of these stars were first observed as part of the N2K program and
exhibited low RMS velocity scatter after three consecutive observations. How-
ever, follow-up observations over the last three years now reveal the presence of
1Based on observations obtained at the W. M. Keck Observatory, which is operated by the University of
California and the California Institute of Technology. Keck time has been granted by NOAO and NASA.
2Based on observations obtained at the Subaru Telescope, which is operated by the National Astronomical
Observatory of Japan
3Department of Physics & Astronomy, San Francisco State University, San Francisco, CA 94132; fis-
cher@stars.sfsu.edu
4UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064
5Department of Astronomy, University of California, Berkeley, CA USA 94720
6Department of Terrestrial Magnetism, Carnegie Institute of Washington DC, 5241 Broad Branch Rd.
NW, Washington DC, USA 20015-1305
7Tokyo Institute of Technology, 2-12-1 Okayama, Meguro-ku, Tokyo 152-8550, Japan
8Center of Excellence in Information Systems, Tennessee State University, 3500 John A. Merritt Blvd.,
Box 9501, Nashville, TN 37209
9Department of Earth and Planetary Sciences, Graduate School of Science, Kobe University, 1-1 Rokkodai,
Nada, Kobe 657-8501, Japan
10Department of Physics, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan
11Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218
12Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL
60208
http://arxiv.org/abs/0704.1191v2
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longer period planets with orbital periods ranging from 21 days to a few years.
HD 11506 is a G0V star with a planet of M sin i = 4.74 MJUP in a 3.85 year
orbit. HD 17156 is a G0V star with a 3.12 MJUP planet in a 21.2 day orbit. The
eccentricity of this orbit is 0.67, one of the highest known for a planet with a
relatively short period. The orbital period for this planet places it in a region of
parameter space where relatively few planets have been detected. HD 125612 is a
G3V star with a planet of M sin i = 3.5 MJUP in a 1.4 year orbit. HD 170469 is a
G5IV star with a planet of M sin i = 0.67 MJUP in a 3.13 year orbit. HD 231701
is an F8V star with planet of 1.08 MJUP in a 142 day orbit. All of these stars
have supersolar metallicity. Three of the five stars were observed photometrically
but showed no evidence of brightness variability. A transit search conducted for
HD 17156 was negative but covered only 25% of the search space and so is not
conclusive.
Subject headings: planetary systems – stars: individual (HD 11506, HD 17156,
HD 125612, HD 170469, HD 231701)
1. Introduction
Ongoing Doppler surveys of stars closer than 150 pc have detected more than 200
exoplanets (Butler et al. 2006, Wright et al. 2007). This ensemble of exoplanets exhibit a
diverse range of statistical characteristics (Marcy et al. 2005). Notably, the mass distribution
of exoplanets falls exponentially for masses greater than one Jupiter-mass. In addition, there
is a statistical pile-up of planets in orbits of just a few days, a paucity of planets with periods
between 10 - 100 days, and a rising number of gas giant planets found at separations greater
than 1 AU. Orbital eccentricities span a surprising range from 0 - 0.93, although 92% of
planet eccentricities are less than 0.6 and even for planets with periods longer than five days
(i.e., not tidally circularized) the median exoplanet eccentricity is 0.26.
It has also been shown that planet formation is tied to the chemical composition of the
host star. There is a few percent probability of finding a gas giant planet around a solar
metallicity star, while planet occurrence rises dramatically to ∼ 25% for stars with three
times the heavy metal composition of the Sun (Santos et al. 2005, Fischer & Valenti 2005).
Ida & Lin (2004) have accounted for this metallicity correlation within the context of core
accretion.
The statistical characteristics of exoplanets serve as tracers of planet formation and mi-
gration histories. The planet-metallicity correlation indicates initial high metallicity in the
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protoplanetary disk which in turn may be correlated with a higher surface density of solid
particles in the midplane of the disk that enhances core accretion. Orbital eccentricities and
the proximity of gas giant plants to their host stars are remnant signatures of gravitational
interactions that drive orbital migration. The architecture of multi-planet systems, some-
times locked in resonances, adds to our understanding of evolution of the protoplanetary
disk.
The N2K program (Fischer et al. 2005) is a survey of metal-rich stars, designed to
identify short period planets. These planets are geometrically endowed with a higher transit
probability; transit events provide a rare opportunity to derive information about the planet
density, internal structure and the atmosphere (Burrows et al. 2007, Charbonneau 2006, Sato
et al. 2005). Because short-period planets can be flagged with just a few observations, the
N2K program only requires three Doppler measurements to screen each star. However, an
increased occurrence of planets is correlated with high host star metallicity at all detected
separations. Therefore, additional observations were obtained for the highest metallicity stars
to check for longer period planets. This extended program has detected six new intermediate
period planets: HD 5319 and HD 75898 (Robinson et al. 2007), and four of the five planets
presented here: HD 11506, HD 17156, HD 125612, HD 231701.
2. HD 11506
2.1. Stellar Characteristics
HD 11506 is classified as a G0 star with V=7.51. The Hipparcos catalog (ESA 1997)
lists B-V = 0.607 with a parallax of 18.58 milliarcseconds, corresponding to a distance of 53.8
pc. The distance and apparent magnitude set the absolute visual magnitude as MV =3.85
and the stellar bolometric luminosity as 2.29 L⊙ , including a bolometric correction of -0.033
(VandenBerg & Clem 2003) based on effective temperature, surface gravity and metallicity
of the star. High resolution spectroscopic analysis described in Valenti & Fischer (2005)
yields Teff = 6058 ± 51K, log g = 4.32 ± 0.08, v sin i = 5.0 ± 0.5 km s
−1 , and [Fe/H]= 0.31
± 0.03 dex. HD 11506 is about 0.65 magnitudes above the main sequence. We expect the
star to be located about 0.3 magnitudes above the main sequence because of high stellar
metallicity, so this star appears to be slightly evolved by a few tenths of a magnitude, and
is likely just beginning to transition onto the subgiant branch.
The stellar radius is calculated to be 1.3 R⊙ using L = 4πR
2σT 4. We have also run
a fine grid of evolutionary tracks (described in Takeda et al. 2007), tuned to the uniform
spectroscopic analysis of Valenti & Fischer (2005) and based on the Yale Stellar Evolution
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Code. This analysis provides posterior probability distributions for stellar mass, radius,
gravity and ages. Based on these evolutionary tracks, we derive a stellar mass of 1.19 M⊙,
a radius of 1.38 R⊙, and an age of 5.4 Gyr. As a measure of the formal uncertainties, the
lower and upper 95% credibility intervals from a Bayesian posterior probability distribution
are provided in parentheses in Table 1 for these values.
The Ca II H & K lines (Figure 1) show that HD 11506 is chromospherically inactive.
We measure SHK , the core emission in the Ca II H & K lines relative to the continuum,
for all of our stars. Based on nineteen Keck observations, we measure an average SHK =
0.156 for HD 11506. The ratio of flux from SHK to the bolometric stellar flux is designated
as logR′HK and gives the best diagnostic of chromospheric activity. The average logR
−4.99 for this star and we derive an activity-based rotational period (Noyes et al. 1984),
PROT = 12.6d, and an activity-based age of 5.4 Gyr, in excellent agreement with the age
from evolutionary tracks. The activity, spectral type and evolutionary stage of the star allow
us to estimate an additional source of astrophysical noise, or stellar jitter for the velocities
of each of the stars on our Doppler survey (Wright 2005).
We monitored the brightness of HD 11506 with the T10 0.8 m automatic photometric
telescope (APT) at Fairborn Observatory (Henry 1999, Eaton, Henry & Fekel 2003). The
T10 APT measures the brightness of program stars relative to nearby constant comparison
stars with a typical precision of 0.0015–0.0020 mag for a single measurement. For HD 11506,
we obtained 102 b and y measurements spanning 451 days between 2004 October and 2006
January. The standard deviation of a single observation from the mean was 0.0023 mag, our
upper limit to possible photometric variability in HD 11506. A periodogram analysis found
no significant periodicity between 1 and 225 days, so our photometry confirms the star’s low
chromospheric activity. The stellar parameters are summarized in Table 1.
2.2. Doppler Observations and Keplerian Fit
Doppler observations were made at the Keck telescope using HIRES (Vogt et al. 1994)
with an iodine cell to model the instrumental profile and to provide the wavelength scale
(Butler et al. 1996). An exposure meter maintains a constant signal-to-noise ratio of about
200 in our spectra, yielding a mean radial velocity precision of 2.75 m s−1 for HD 11506.
We obtained a total of 26 Doppler measurements. The observation dates, radial velocities
and measurement uncertainties for the radial velocities are listed in Table 2 and plotted in
Figure 2.
In addition to velocity errors arising from our measurement uncertainties (including
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photon shot noise), the star itself can have pulsations, cool spots or granular convective flows
that contribute non-dynamical velocity noise. These astrophysical sources of noise are termed
jitter and we empirically estimate stellar jitter based on the spectral type and chromospheric
activity of the star, following Wright (2005). For purposes of fitting a Keplerian model, the
stellar jitter is added in quadrature to the formal instrumental errors, however the estimated
jitter is never included in the measurement uncertainties for the tabulated radial velocity
sets.
The periodogram of the radial velocities shows a strong, broad peak in the power spec-
trum at about 1270 days with an associated false alarm probability (FAP) < 0.0001. The
FAP associated with the periodogram tests whether scrambled velocities yield power that
exceeds the observed, unscrambled velocities. A high FAP suggests that the signal is not
significant or could have been caused by a window function in the data. Using a Monte Carlo
simulation, one thousand data sets of noise were generated by randomly drawing (with re-
placement) sets of actual stellar velocities. The fraction of trials with maximum periodogram
power that exceeds the observed value from the initial unscrambled data set defines the FAP
(Cumming 2004).
For each of the radial velocity data sets in this paper, a Levenberg-Marquardt fitting
algorithm was used to model the radial velocities with a theoretical Keplerian orbital curve.
There are six orbital parameters derived in the fit: orbital period (P), time of periastron
passage (TP ), eccentricity (e), the orientation of the orbit (or line of apsides) (ω), the semi-
velocity amplitude (K) and the residual center of mass radial velocity (after subtracting a
median radial velocity).
Uncertainties in the orbital parameters are determined with a bootstrap Monte Carlo
analysis. First, a best-fit Keplerian model is obtained. Then, for each of 100 trials, the
theoretical best fit is subtracted from the observed radial velocities. The residual velocities
are then scrambled (with replacement) and added back to the theoretical best fit velocities
and a new trial Keplerian fit is then obtained. The standard deviation of each orbital
parameter for the 100 Monte Carlo trials was adopted as the parameter uncertainty.
The best fit Keplerian model gives an orbital period of 1405 ± 45 d, semi-velocity
amplitude of 80 ± 3 m s−1and orbital eccentricity of 0.3 ± 0.1. The RMS to this fit is
10.8 m s−1. Based on the chromospheric activity of this star, we estimated a jitter of 2
m s−1 (Wright 2005). When this jitter is added in quadrature with the error bars listed
in Table 2,
χ2ν = 3.2. While the large amplitude Doppler variation is clear, the
χ2ν fit
is worse than usual, suggesting that our velocity errors may be underestimated or that
additional low amplitude dynamical velocities are present. A periodogram of the residual
velocities to the single Keplerian fit show several peaks with similar power. For example, we
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can fit a second planet with a period of 170 days with a significant reduction in the residual
velocity RMS and an improvement in
χ2ν , however this is not yet a unique double planet
fit; additional data are required to better evaluate the possible second signal.
Using the stellar mass of 1.19 M⊙ derived from evolutionary tracks, we findM sin i= 4.74
MJUP and a semi-major axis of 2.48 AU. At the distance of this star, this physical separation
corresponds to an angular separation of α = 0.′′04. The Keplerian orbital solution is listed
in Table 3 and the best-fit Keplerian model is plotted in Figure 2.
3. HD 17156
3.1. Stellar Characteristics
HD 17156 is a listed as a G5 star in the SIMBAD database and the Hipparcos catalog.
However, this spectral type seems at odds with other data for the star. The visual magnitude
is V = 8.17, B− V = 0.59, and the Hipparcos parallax (ESA 1997) is 12.78 milliarcseconds,
corresponding to a distance of 78.24 pc. The bolometric correction -0.039 (VandenBerg &
Clem 2003) and absolute visual magnitude, MV=3.70, imply a bolometric stellar luminosity
of 2.6 L⊙ . Spectroscopic analysis yields Teff = 6079 ± 56K, log g = 4.29 ± 0.06, v sin i = 2.6
± 0.5 km s−1 , and [Fe/H]= 0.24 ± 0.03. The B−V color and the effective temperature are
independent measurements that are consistent with each other. Together with the absolute
magnitude and position on the H-R diagram, the spectral type for this star is more likely to
be G0 and the star is just beginning to evolve off the main sequence.
The stellar mass, from evolutionary models described by Takeda et al. (2007), is 1.2
M⊙, and the age is 5.7 Gyr. The stellar radius from evolutionary models is 1.47 R⊙, and
agrees with the value we derive using the observed luminosity and the Stefan-Boltzmann
relation.
The absence of Ca II H & K emission (Figure 1) demonstrates low chromospheric ac-
tivity. Taking the average of 25 observations, we measure SHK = 0.15 and logR
HK = -5.04
and derive a rotational period, PROT = 12.8 d, with an estimated stellar age of 6.4± 2Gyr,
which compares favorably with the age derived above from stellar evolution tracks.
We obtained 241 photometric measurements with the T12 APT spanning 179 days be-
tween 2006 September and 2007 March. The standard deviation of the observations from
their mean was 0.0024 mag, the upper limit to photometric variability in the star. Peri-
odogram analysis revealed no significant periodicity between 1 and 100 days. In particular,
a least-squares sine fit of the observations on the 21.22-day radial velocity period resulted
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in a photometric amplitude of only 0.00039± 0.00023 mag, providing further evidence that
the radial velocity variations in HD 17156 are not due to chromospheric activity. The stellar
characteristics, including our assessment of photometric variability, are summarized in Table
3.2. Doppler Observations and Keplerian Fit
We initially obtained eight Doppler observations of HD 17156 using the High Dispersion
Spectrometer (Noguchi et al. 2002) at the Subaru Telescope in 2004 and 2005. For the
first observing runs, the iodine absorption cell was located behind the entrance slit of the
spectrometer (Kambe et al. 2002, Sato et al. 2002, Sato et al. 2005). The box holding the I2
cell included a window with a lens to maintain constant focal length inside the spectrometer.
This eliminated the need to adjust the collimator position when moving the I2 cell in and
out of the light path (i.e., when taking program and template observations). However, the
lens introduced a different wavelength dispersion for program observations relative to the
template observation. Modeling of those early data is still ongoing, however, standard stars,
known to have constant radial velocities show RMS scatter greater than 15 m s−1, with
larger run-to-run velocity offsets for Doppler observations obtained with that setup.
The Subaru N2K program was awarded ten nights of “intensive” time in summer 2006
and in December 2006. Before the intensive time allocation, the iodine cell was moved
in front of the slit, eliminating the change in wavelength dispersion between template and
program observations. With this new setup, the RMS scatter decreased, ranging from 4 - 12
m s−1 in a set of four RV standard stars.
HD 17156 had exhibited large radial velocity variations in 2004 - 2005 at Subaru. Follow-
up observations at Keck confirmed velocity variations, so the star was observed on nine
consecutive nights at Subaru from 8 December to 16 December, 2006. Setup StdI2b was
used to cover the wavelength region of 3500–6100 Å with a mosaic of two CCDs. The slit
width of 0′′.6 was used to give a reciprocal resolution (λ/∆λ) of 60000. We obtained a
typical signal-to-noise ratio of S/N ∼ 150 pixel−1 at 5500 Å with exposure times of about
120 seconds. Because of the larger systematic errors for observations taken before summer
2006 (with the iodine cell behind the slit), only the nine radial velocities from December
2006 are listed in Table 4. To account for the intrinsic RMS velocity scatter in standard
stars, 5 m s−1 was added in quadrature to the nine Subaru observations in Table 4.
After HD 17156 was flagged as an N2K candidate at Subaru, it was added to the
N2K planet search program at Keck. We obtained 24 radial velocity measurements at the
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Keck Observatory with an average internal velocity precision of 1.6 m s−1. Observation
dates, radial velocities and uncertainties for 33 observations are listed in Table 4. The last
column designates the source of the observations as “K” (Keck Observatory) or “S” (Subaru
Observatory). The periodogram of the radial velocity data shows a strong narrow peak at
21.1 days with a FAP less than 0.0001 (for 10000 Monte Carlo trials).
When combining the Subaru and Keck velocities, we first determined a velocity differ-
ence of about 130 m s−1 between two observations taken at Subaru and Keck on the same
night (JD 2454083.9). With that initial guess, we included a velocity offset as a free pa-
rameter and found that an offset of 116.0 m s−1 produced a minimum
χ2ν . That offset
was added to the Subaru velocities listed in Table 4. The best fit Keplerian model for the
combined Subaru and Keck data sets yields an orbital period of 21.2 ± 0.3 d, semi-velocity
amplitude, K = 275± 15 m s−1, and orbital eccentricity, e = 0.67± 0.08. The RMS to the
fit is 3.97 m s−1. Adding jitter of 3 m s−1 (expected for this star) in quadrature with the
actual single-measurement errors gives
χ2ν = 1.04 for this Keplerian fit.
Adopting a stellar mass of 1.2 M⊙, we derive M sin i = 3.12 MJUP and a semi-major
axis of 0.15 AU. The Keplerian orbital solution is summarized in Table 3. The phase-folded
plot of the Doppler measurements for Keck and Subaru observations are shown in the left
plot of Figure 3 and include 3 m s−1 jitter. Keck observations are represented by diamonds
and the Subaru observations are shown as filled circles.
Because the high eccentricity is unusual, we examined the Keplerian fit for the Keck
data alone, shown in the right plot Figure 3. The Keck data have poor phase coverage
near periastron, and yield a Keplerian fit with lower amplitude and lower eccentricity. The
Subaru observations map periastron passage and help to model the eccentricity of the orbit.
3.3. Transit Search
The 21.22 day period of the companion to HD 17156 is by far the shortest planetary
orbital period in this paper. The orbital semi-major axis of 0.15 AU and the stellar radius of
1.47 R⊙ lead to an a priori transit probability of 7% (Seagroves et al. 2003). Therefore, we
used our 241 brightness measurements to conduct a preliminary transit search. The orbital
parameters in Table 3 constrain the predicted times of transit to about ± 0.3 days, which
is slightly greater than the 0.25-day duration of a central transit. We performed our transit
search, using a technique similar to the one described by Laughlin (2000), over all orbital
phases for periods between 20 and 23 days. The search was negative but was able to cover
effectively only 25% of the period-phase search space corresponding to the uncertainties in
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the orbital parameters. Thus, our photometric data do not preclude the possibility of transits
in HD 17156.
4. HD 125612
4.1. Stellar Characteristics
HD 125612 is a G3V main sequence star with V=8.31, B-V = 0.628, and Hipparcos
parallax (ESA 1997) of 18.93 corresponding to a distance of 52.82 pc and absolute visual
magnitude, MV =4.69. Spectroscopic analysis yields Teff = 5897 ± 40K, log g = 4.45 ± 0.05,
v sin i = 2.1 ± 0.5 km s−1 , and [Fe/H]= 0.24 ± 0.03 dex. The bolometric correction is
-0.061, giving a stellar luminosity of 1.08 L⊙ . The luminosity and Teff imply a stellar radius
of 1.0 R⊙. Within uncertainties, this agrees well with the value of 1.05 R⊙ determined from
stellar evolutionary tracks. We also derive a stellar mass of 1.1 M⊙ from stellar evolution
models and an age of 2.1 Gyr.
Figure 1 shows the Ca H line for HD 125612; the lack of emission indicates low chromo-
spheric activity for this star. Taking the mean of 18 observations, we measure SHK = 0.178
and logR′HK = -4.85, and derive PROT = 10.5 d, and a stellar age of 3.3 ± 2 Gyr (which
compares well with the age of 2.1 Gyr from stellar evolution tracks). Stellar parameters are
summarized in Table 1.
4.2. Doppler Observations and Keplerian Fit
We obtained 19 Keck velocity measurements for HD 125612 with a typical uncertainty
of 2.2 m s−1. Observation dates, radial velocities and instrumental uncertainties in the radial
velocities are listed in Table 5. A periodogram of the velocities shows a strong broad peak
at about 500 days.
The best fit Keplerian model is plotted in Figure 4 and yields a period of 510 ± 14 d,
with semi-velocity amplitude 90.7 ± 8 m s−1, orbital eccentricity, 0.38 ± 0.05, and a linear
trend of 0.037 meters per day. Adopting a stellar mass of 1.1 M⊙, we derive M sin i = 3.5
MJUP and semi-major axis of 1.2 AU (angular separation, α = 0
′′.023). The Keplerian orbital
solution is listed in Table 3 and the RV data are plotted with the best-fit Keplerian model
(solid line) in Figure 4.
The RMS to the Keplerian fit shown in Figure 4 is 10.7 m s−1. The velocity jitter for
this star is expected to be about 2 m s−1. Therefore, the residual RMS is several times
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the typical error bar, consistent with the poor
χ2ν statistic of 3.56. A periodogram of the
residuals to a 510-day planet fit shows power near 3.5 d. However, there are several other
peaks of nearly comparable height, showing that other orbital solutions may give similar
improvements. Thus, while we could fit the residuals with a second Keplerian, the FAP
of the peak does not yet meet our standards of statistical significance, and more data are
required for follow up.
5. HD 170469
5.1. Stellar Characteristics
HD 170469 is a G5 subgiant star with visual magnitude V=8.21, B-V = 0.677, and
Hipparcos parallax (ESA 1997) of 15.39 milliarcseconds, corresponding to a distance of 64.97
pc. The absolute visual magnitude of the star is MV=4.14. The bolometric correction is
-0.072 providing a bolometric stellar luminosity of 1.6 L⊙ and (with Teff ) stellar radius of
1.2 R⊙ calculated from the luminosity. Evolutionary tracks provide a stellar mass estimate
of 1.14 M⊙ and stellar radius of 1.22 R⊙ and age of 6.7 Gyr. Our spectroscopic analysis
gives Teff = 5810 ± 44K, log g = 4.32 ± 0.06, v sin i = 1.7 ± 0.5 km s
−1 , and [Fe/H]= 0.30
± 0.03 dex.
The Ca H & K lines (Figure 1) indicate low chromospheric activity. Taking the mean of
thirteen observations, we measure SHK = 0.145 and logR
HK = -5.06 and derive a rotational
period, PROT = 13.0 d and an activity-calibrated age (Noyes et al. 1984) of 7± 2 Gyr.
We obtained 215 brightness measurements with the T10 APT spanning 630 days be-
tween 2005 March and 2006 November. The standard deviation of the observations was
0.0018 mag, the upper limit to photometric variability in HD 170469. A periodogram anal-
ysis found no significant periodicity between 1 and 315 days, confirming the star’s low chro-
mospheric activity. The stellar characteristics are summarized in Table 6.
5.2. Doppler Observations and Keplerian Fit
We obtained 35 Keck velocities for HD 170469 with a mean velocity precision of 1.6
m s−1. Observation dates, radial velocities and instrumental uncertainties in the radial
velocities are listed in Table 7. A periodogram of the velocities yields very strong power at
about 1100 days with a FAP less than 0.0001.
The best fit Keplerian model gives an orbital period of 1145 ± 18 d, semi-velocity
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amplitude of 12.0 ± 1.9 m s−1 and orbital eccentricity, 0.11 ± 0.08. The RMS to the fit
is 4.18 m s−1 with
χ2ν = 1.59, including the estimated astrophysical jitter of 2.0 m s
Adopting a stellar mass of 1.14 M⊙, we derive M sin i = 0.67 MJUP and a semi-major axis of
2 AU (α = 0.′′03). The Keplerian orbital parameters are listed in Table 8 and the RV data
are plotted with the best-fit Keplerian model (solid line) in Figure 5.
6. HD 231701
6.1. Stellar Characteristics
HD 231701 is an F8V star with V = 8.97, B − V = 0.539, and Hipparcos parallax
(ESA 1997) of 9.22 milliarcseconds, corresponding to a distance of 108.4 pc. The absolute
visual magnitude is MV =3.79, so this star is beginning to evolve onto the subgiant branch.
Spectroscopic analysis yields Teff = 6208 ± 44K, log g = 4.33 ± 0.06, v sin i = 4.0 ± 0.5
km s−1 , and [Fe/H]= 0.07 ± 0.03 dex. The bolometric correction is -0.037 and bolometric
luminosity is 2.4 L⊙ . The luminosity and effective temperature yield a stellar radius of 1.36
R⊙ . Modeling the stellar evolutionary tracks, we derive a stellar mass of 1.14 M⊙, radius
of 1.35 R⊙, and age of 4.9 Gyr.
The Ca H&K lines (Figure 1) show that the star has low chromospheric activity. We
measure SHK = 0.159 and logR
HK = -5.0 and derive a rotational period, PROT = 12.2 d
and a stellar age of 5.6± 2 Gyr. Stellar parameters are listed in Table 6.
6.2. Doppler Observations and Keplerian Fit
We obtained 17 Keck observations of HD 231701 with mean internal errors of 3.2 m s−1.
Observation dates, radial velocities and measurement uncertainties in the radial velocities
are listed in Table 9. The periodogram of this data set has a FAP of 0.006 for a period near
140 days.
The best fit Keplerian model has an orbital period of 141.6 ± 2.8 d, with semi-velocity
amplitude 39 ± 3.5 m s−1 and orbital eccentricity, 0.1 ± 0.06. The RMS to this fit is 5.9
m s−1. The expected astrophysical jitter for this star is 2.2 m s−1. Adding this jitter in
quadrature with the error bars listed in Table 9 yields
χ2ν = 1.46 for this Keplerian fit.
Adopting the stellar mass of 1.14 M⊙, we derive M sin i = 1.08 MJUP and semi-major axis
of 0.53 AU. The Keplerian orbital solution is listed in Table 8 and the phased RV data is
plotted with the best-fit Keplerian model (solid line) in Figure 6.
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6.3. Discussion
Here, we present the detection of 5 new exoplanets detected with Doppler observa-
tions. For each of the Keplerian models, we also carried out a Markov Chain Monte Carlo
(MCMC) analysis to better estimate the orbital parameters and their uncertainties following
the algorithm described by Ford (2003). Unlike the Levenberg-Marquardt algorithm that we
generally use to determine a best fit Keplerian orbit, the MCMC analysis provides the full
posterior probability density distribution for each parameter. This approach is particularly
useful for data sets where the Levenberg-Marquardt algorithm can minimize
χ2ν with a
model that fits a sparse data set. The MCMC algorithm explores a wider range of param-
eter space because it is not driven solely by
χ2ν minimization. However, MCMC does not
explore an exhaustive range of parameter space. For example, solutions with very different
orbital periods might be missed. For each of the models presented here, we began with the
input parameters found with Levenberg Marquardt fitting and confirmed that the orbital
elements were recovered with strongly peaked probability distributions using MCMC.
HD 170469 is a star on the regular planet search at Keck that has a planet of M sin i =
0.66 MJUP in a ∼3 yr orbit with eccentricity 0.23. The host star is metal-rich with [Fe/H]
= 0.3. The remaining four exoplanets were initially part of the N2K program at Keck. The
N2K program targets metal-rich stars for rapid identification of short-period planets. The
first three radial velocity measurements for the stars presented here had RMS scatter less
than 5 m s−1(except HD 17156, with initial RMS scatter of 34 m s−1 ), so these stars were
not candidates for short period planets. However, a follow-up program to obtain Doppler
observations on N2K-vetted high metallicity stars with low chromospheric activity and low
RMS velocity scatter has detected the presence of these longer period planets.
HD 11506 b is a fairly massive planet, with M sin i = 4.74 MJUP and a semi-major axis
of 2.5 AU. This could well constitute the outer edge of a habitable zone location for putative
rocky moons orbiting the planet, depending on atmospheric properties of any moons. The
host star has a luminosity that is 2.3 times that of the Sun. The eccentricity of this system
is 0.3, so the temperature at the top of the planet atmosphere would change by about 50K
between apastron and periastron.
HD 17156 b has a mass of M sin i = 3.12 MJUP and an orbital period of 21.2 days,
placing it in the so-called period valley between 10 and 100 days (Udry et al. 2003), where
a relatively small fraction of exoplanets have been detected. We derive a substantial orbital
eccentricity of 0.67 for HD 17156 b. At this proximity to the subgiant host star, the planet
moves between 0.05 and 0.25 AU, experiencing temperature changes of a few hundred degrees
between periastron and apastron. It is possible that these thermal changes could be observed
with sensitive IR flux measurements from space, even though the planet is not known to
– 13 –
transit its host star.
The distribution of orbital eccentricities for known exoplanets is shown in Figure 7. An
upper envelope in the distribution of eccentricities rises steeply from periods of a few days
to reach the maximum observed eccentricities (for HD 80606 and HD 20782) at periods of
100 - 1000 days. Although an orbital eccentricity of 0.67 seems remarkable for HD 17156 b,
given its orbital period of just 21.2 days, the eccentricity still falls along the upper edge of
the observed eccentricity distribution.
HD 125612 b has M sin i = 3.5 MJUP with a semi-major axis of 1.2 AU. This planet
has an eccentricity of 0.38. The planet is carried from 0.47 AU at periastron, where the
temperature at the top of the atmosphere is about 300 K, to about 2.1 AU where the
temperature falls below the freezing point of water to about 200 K. A single planet model
does not appear to adequately describe the velocities of HD 125612 because the RMS to that
fit is 10.7 m s−1, yet the star is chromospherically quiet and slowly rotating, with v sin i = 2
km s−1. This star may well have an additional planet orbiting in a relatively short period.
Velocity variations in HD 231701 have been modeled as a planet with M sin i = 1.08
MJUP with a semi-major axis of 0.53 AU and orbital eccentricity of about 0.1. The MCMC
probability distributions for HD 231701 are consistent with this Keplerian model, but al-
low for eccentricity solutions that extend to zero. This analysis alerts us that more RV
measurements should be taken to better constrain the orbital eccentricity of this system.
We have now obtained three or more Doppler observations for 423 stars at Keck Ob-
servatory as part of the N2K program. Spectral synthesis modeling has been carried out for
all of these stars, and we plot the percentage of stars with detected planets in each 0.1 dex
metallicity bin in Figure 8. Superimposed on this plot is the planet detectability curve from
Fischer & Valenti (2005). A planet probability can be assigned based on the stellar metal-
licity. Integrating planet probabilities we expect 27± 5 exoplanets with masses greater than
1 MJUP and orbital periods shorter than 4 years. Fourteen, or about half of the expected
planets in the sample have now been detected.
We gratefully acknowledge the dedication and support of the Keck Observatory staff,
in particular Grant Hill for support with HIRES. We thank Akito Tajitsu and Tae-Soo Pyo
for their expertise and support of the Subaru HDS observations. DAF acknowledges sup-
port from NASA grant NNG05G164G and from Research Corporation. SSV acknowledges
support from NSF AST-0307493. BS is supported by Grants-in-Aid for Scientific Research
(No. 17740106) from the Japan Society for the Promotion of Science (JSPS). We thank
the Michelson Science Center for travel support through the KPDA program. We thank
the NASA and UC Telescope assignment committees for generous allocations of telescope
– 14 –
time. The authors extend thanks to those of Hawaiian ancestry on whose sacred mountain
of Mauna Kea we are privileged to be guests. Without their kind hospitality, the Keck
observations presented here would not have been possible. This research has made use of
the SIMBAD database, operated at CDS, Strasbourg, France, and of NASA’s Astrophysics
Data System Bibliographic Services and is made possible by the generous support of Sun
Microsystems, NASA, and the NSF.
– 15 –
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Burrows, A., Hubeny, I., Budaj, J., Hubbard, W. 2007, ApJ, in press
Charbonneau, D., Brown, T. M., Burrows, A., Laughlin, G. 2006, in Protostars and Planets
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Wright, J. T., Marcy, G. W., Fischer, D. A., Butler, R. P., Vogt, S. S., Tinney, C. G., Jones,
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This preprint was prepared with the AAS LATEX macros v5.2.
– 17 –
Fig. 1.— The Ca H line for HD 11506 and 17156 are plotted in the left panel, with the
same wavelength segment of the Sun shown for comparison. HD 125612, HD 170469 and
HD 231701 are plotted in the right panel. All of these stars have low chromospheric activity
based on our measurement of line core emission relative to the continuum.
– 18 –
2004.0 2005.0 2006.0 2007.0 2008.0
 Time  (Years)
Keck Obs
 P = 3.85 yr
 K = 80.3 ms
 e = 0.30
 RMS = 10.8 ms
Fig. 2.— Radial velocities for HD 11506. The velocity error bars have been augmented by
adding 2 m s−1 in quadrature to the single measurement precision listed in Table 2. This
gives
χ2ν = 3.2 for the Keplerian fit. With a stellar mass of 1.19 M⊙, we derive a planet
mass of M sin i = 4.74 MJUP and semi-major axis, arel = 2.48 AU.
– 19 –
0.0 0.5 1.0
 Orbital Phase
 P = 21.22 day
 K = 275. m s
 e = 0.67
 RMS = 3.98 m s
0.0 0.5 1.0
 Orbital Phase
 P = 21.22 day
 K = 147. m s
 e = 0.43
 RMS = 3.67 m s
Fig. 3.— (left) Radial velocities for HD 17156 from Keck Observatory (diamonds) and
Subaru Observatory (filled circles) have 3 m s−1 added in quadrature to the uncertainties
listed in Table 4 to account for expected photospheric jitter. Adopting a stellar mass of 1.2
M⊙ we derive a planet mass, M sin i = 3.12 MJUP and semi-major axis for the orbit, arel =
0.15 AU. (right) The plot on the right shows the Keck velocities only. Although the phase
coverage misses periastron the Keck velocites alone confirm high eccentricity in HD 17156 b.
– 20 –
2004.5 2005.0 2005.5 2006.0 2006.5 2007.0 2007.5
 Time  (Years)
Keck Obs
 P = 1.39 yr
 K = 90.7 ms
 e = 0.38
 RMS = 10.7 ms
Fig. 4.— Radial velocities for HD 125612. The velocity measurements have 2 m s−1 added
to their error bars listed in Table 5 to account for the level of astrophysical noise (jitter) we
expect from the star. With the assumed stellar mass of 1.1 M⊙, we derive a planet mass,
M sin i = 3.5 MJUP and semi-major axis of 1.2 AU. This Keplerian model still has a high
RMS and
χ2ν of 3.56, suggesting the possible presence of an additional planet.
– 21 –
2002 2004 2006 2008
 Time  (Years)
 P = 3.13 yr
 K = 12.0 ms
 e = 0.11
 RMS = 4.18 ms
Fig. 5.— Keck radial velocities for HD 170469 include 2.0 m s−1 added in quadrature with
the internal error bars listed in Table 7. With the added jitter, the Keplerian fit has
χ2ν =
1.59. The assumed stellar mass of 1.14 M⊙ yields a planet mass of M sin i = 0.67 MJUP and
semi-major axis of about 2 AU.
– 22 –
0.0 0.5 1.0
 Orbital Phase
 P = 141.6 day
 K = 39.0 m s
 e = 0.10
 RMS = 5.90 m s
Fig. 6.— Phase-folded radial velocities for HD 231701 include stellar jitter of 2.2 m s−1added
to the errors listed in Table 9, giving
χ2ν = 1.46. The stellar mass of 1.14 M⊙ implies a
planet mass of M sin i = 1.08 MJUP and orbital radius of 0.53 AU.
– 23 –
Fig. 7.— Orbital eccentricity distribution for exoplanets. A rising envelope defines the
distribution for planets with periods between 2 and 100 days. The distribution peaks for
periods between 100 - 1000 days. The arrow points to the dot representing HD 17156 b.
With an orbital period of 21 days and eccentricity of 0.67, HD 17156 b still fits within the
envelope of this eccentricity distribution.
– 24 –
Fig. 8.— Stars with at least 4 Doppler observations that were observed at Keck as part of
the N2K program were been binned according to metallicity. In each 0.1 dex metallicity bin,
the percentage of stars with detected planets is plotted and Poisson error bars are shown.
Superimposed on this histogram is the curve of planet occurrence as a function of metallicity
from Fischer & Valenti (2005). Most stars on the N2K program have only 4-5 observations,
however the metallicity correlation is still emerging and is supported in this sample.
– 25 –
Table 1. Stellar Parameters
Parameter HD11506 HD17156 HD125612
V 7.51 8.17 8.31
MV 3.85 3.70 4.69
B-V 0.607 0.590 0.628
Spectral Type G0V G0V G3V
Distance (pc) 53.82 78.24 52.82
Lbol/L⊙ 2.29 2.6 1.08
[Fe/H] 0.31 (0.03) 0.24 (0.03) 0.24 (0.03)
Teff (K) 6058 (51) 6079 (56) 5897 (40)
v sin i (km s−1) 5.0 (0.50) 2.6 (0.50) 2.1 (0.50)
log g 4.32 (0.08) 4.29 (0.06) 4.45 (0.05)
MSTAR (M⊙ )
a (1.1) 1.19 (1.29) (1.1) 1.2 (1.3) (1.04) 1.1 (1.17)
RSTAR (R⊙ )
a (1.25) 1.38 (1.53) (1.3) 1.47 (1.6) (0.99) 1.05 (1.13)
Age (Gyr)a (3.9) 5.4 (7.0) (3.8) 5.7 (7.0) (0.16) 2.1 (5.6)
SHK 0.156 0.15 0.178
logR′HK -4.99 -5.04 -4.85
PROT (d) 12.6 d 12.8 d 10.5 d
σphot (mag) 0.0023 0.0024 · · ·
aStellar masses, radii and ages are derived from evolutionary tracks
– 26 –
Table 2. Radial Velocities for HD 11506
JD RV Uncertainties
-2440000. (m s−1 ) (m s−1 )
13014.73505 -6.67 2.94
13015.73893 -6.58 2.89
13016.74089 -18.73 2.98
13191.12201 -43.89 3.56
13207.10116 -71.30 3.17
13208.08401 -56.92 3.41
13368.83778 -80.87 2.26
13369.75897 -80.35 2.18
13370.73242 -81.24 2.17
13397.73009 -75.66 2.36
13750.73807 59.16 2.67
13775.72853 71.94 2.77
13776.70435 74.90 2.57
13777.72528 76.27 2.93
13778.71858 78.70 2.73
13779.74737 71.93 2.77
13926.12744 73.68 2.66
13933.09065 75.37 2.62
13959.13935 80.47 2.46
13961.12421 81.24 2.73
13981.98256 74.91 3.12
14023.97438 44.32 3.07
14083.84327 26.82 2.44
14085.92115 38.66 2.54
14129.74332 56.29 2.48
14286.11838 28.66 3.01
– 27 –
– 28 –
Table 3. Orbital Parameters
Parameter HD 11506 HD 17156 HD 125612
P (d) 1405 (45) 21.2 (0.3) 510 (14)
Tp (JD) 13603 (102) 13738.529 (0.5) 13228.3 (12)
ω (deg) 262 (19) 121 (11) 21 (9)
ecc 0.3 (0.1) 0.67 (0.08) 0.38 (0.05)
K1 (m s
−1) 80 (3) 275 (15) 90.7 (8)
dv/dt (m s−1per day) · · · · · · 0.037
arel (AU) 2.48 0.15 1.2
a1 sin i (AU) 0.0099 0.00039 0.0039
f1(m) (M⊙) 6.53e-08 1.83e-08 3.09e-08
M sin i (MJup) 4.74 3.12 3.5
Nobs 26 33 19
RMS (m s−1 ) 10.8 3.97 10.7
Jitter (m s−1 ) 2 3 2
Reduced
χ2ν 3.2 1.17 3.56
FAP (periodogram) < 0.0001 < 0.0001 0.0003
– 29 –
Table 4. Radial Velocities for HD 17156
JD RV Uncertainties Observatory
-2440000. (m s−1 ) (m s−1 )
13746.75596 88.49 1.70 K
13748.79814 138.15 1.73 K
13749.79476 151.35 1.67 K
13750.80160 169.65 1.76 K
13775.77821 235.17 1.83 K
13776.80791 253.80 1.81 K
13779.82897 239.14 1.64 K
13959.13219 97.33 1.55 K
13962.07028 152.64 1.51 K
13963.10604 165.48 1.68 K
13964.13118 194.69 1.70 K
13982.03231 132.55 1.20 K
13983.08575 146.35 1.70 K
13983.99480 166.11 1.32 K
13985.00847 187.22 1.57 K
14023.95206 114.39 1.77 K
14047.95773 166.01 1.74 K
14078.01162 -116.60 5.14 S
14078.92501 -261.56 5.18 S
14079.91371 -164.10 5.23 S
14080.98093 -89.57 5.13 S
14081.89406 -44.08 5.15 S
14082.86071 2.39 5.14 S
14083.88445 37.62 5.16 S
14083.90314 32.76 1.33 K
14084.82860 63.22 1.63 K
14085.82560 86.67 5.20 S
14085.86537 84.28 1.56 K
14086.87960 99.01 5.20 S
– 30 –
Table 4—Continued
JD RV Uncertainties Observatory
-2440000. (m s−1 ) (m s−1 )
14129.92513 113.30 1.43 K
14130.73019 133.66 1.32 K
14131.85485 151.11 1.77 K
14138.76720 261.56 1.37 K
– 31 –
Table 5. Radial Velocities for HD 125612
JD RV Uncertainties
-2440000. (m s−1 ) (m s−1 )
13190.83262 52.80 2.85
13197.83363 61.40 2.66
13198.85557 60.49 2.59
13199.83792 44.35 2.70
13604.75480 -60.07 2.04
13754.12824 45.68 1.83
13776.15380 17.80 2.03
13777.11940 21.71 2.16
13838.01492 -77.76 2.35
13841.94894 -86.49 2.52
13927.79007 -90.28 2.01
13961.75080 -79.07 2.01
13962.74127 -80.81 1.83
13981.72862 -83.16 1.90
13983.74037 -81.92 2.11
13984.72815 -80.88 2.03
14130.13392 -22.62 2.18
14139.12383 1.31 1.94
14251.82778 90.28 2.39
– 32 –
Table 6. Stellar Parameters
Parameter HD 170469 HD 231701
V 8.21 8.97
MV 4.14 3.79
B-V 0.677 0.539
Spectral Type G5IV F8V
Distance (pc) 64.97 108.4
Lbol/L⊙ 1.6 2.4
[Fe/H] 0.30 (0.03) 0.07 (0.03)
Teff (K) 5810 (44) 6208 (44)
v sin i (km s−1) 1.7 (0.5) 4 (0.50)
log g 4.32 (0.06) 4.33 (0.06)
MSTAR (M⊙ )
a (1.05) 1.14 (1.16) (1.08) 1.14 (1.22)
RSTAR (R⊙ )
a (1.15) 1.22 (1.3) (1.16) 1.35 (1.55)
Age (Gyr)a (5.0) 6.7 (7.8) (3.5) 4.9 (6.2)
SHK 0.145 0.159
logR′HK -5.06 -5.00
PROT (d) 13.0 d 12.2 d
σphot (mag) 0.0018 · · ·
aStellar masses, radii and ages are derived from evolu-
tionary tracks
– 33 –
Table 7. Radial Velocities for HD 170469
JD RV Uncertainties
-2440000. (m s−1 ) (m s−1 )
11705.96808 6.04 1.51
11793.81330 -0.05 1.39
12008.04881 -10.21 1.50
12099.03294 -14.05 1.59
12162.76894 -15.48 1.42
12364.13287 -7.07 1.67
12390.12499 -2.46 1.63
12391.12567 2.80 1.76
12445.93867 -12.12 1.72
12515.82777 14.53 1.96
12535.75539 3.45 1.53
12536.74191 0.06 1.50
12537.82520 1.72 1.50
12538.74254 0.83 1.30
12539.75501 4.11 1.51
12572.69435 6.59 1.60
12573.69333 5.76 1.34
12574.70725 10.11 1.45
12575.69822 2.35 1.40
12778.04455 15.48 1.96
12804.05044 7.80 1.56
12848.92274 5.06 2.35
13180.90825 -14.03 1.55
13181.89752 -12.91 1.57
13548.99248 -2.83 1.54
13603.80234 4.11 1.50
13842.01212 9.79 1.59
13932.96654 14.41 1.49
13960.91798 10.12 1.42
– 34 –
Table 7—Continued
JD RV Uncertainties
-2440000. (m s−1 ) (m s−1 )
13961.83115 11.29 1.54
13981.82421 7.85 1.27
13982.77494 5.33 1.26
13983.76067 5.96 1.33
13984.83377 7.38 1.22
14250.01196 -4.92 1.57
– 35 –
Table 8. Orbital Parameters
Parameter HD 170469 HD 231701
P (d) 1145 (18) 141.6 (2.8)
Tp (JD) 11669.0 (21) 13180.0 (4.2)
ω (deg) 34 (19) 46 (24)
ecc 0.11 (0.08) 0.10 (0.08)
K1 (m s
−1 ) 12.0 (1.9) 64 (8)
dv/dt (m s−1per day) · · · · · ·
arel (AU) 2.1 0.53
a1 sin i (AU) 0.00126 0.0005
f1(m) (M⊙) 2.03e-10 8.6e-10
M sin i (MJup) 0.67 1.08
Nobs 35 17
RMS (m s−1 ) 4.18 5.90
Jitter (m s−1 ) 2.0 2.22
Reduced
χ2ν 1.59 1.46
FAP (periodogram) < 0.0001 0.006
– 36 –
Table 9. Radial Velocities for HD 231701
JD RV Uncertainties
-2440000. (m s−1 ) (m s−1 )
13190.98047 1.98 3.41
13198.03808 -2.44 4.00
13199.02309 -11.19 3.74
13199.95689 -16.16 3.62
13603.87134 40.18 3.32
13928.04498 -38.57 2.90
13931.08095 -32.84 2.93
13932.02123 -36.55 3.38
13961.87814 -31.29 3.34
13981.76292 5.66 3.18
13983.77988 5.49 3.11
14023.71873 25.79 3.53
14083.69519 -39.18 2.43
14085.70009 -37.48 2.83
14217.13510 -40.18 2.99
14250.07410 -15.89 2.90
14286.00169 31.17 2.86
	Introduction
	HD 11506
	Stellar Characteristics
	Doppler Observations and Keplerian Fit
	HD 17156
	Stellar Characteristics
	Doppler Observations and Keplerian Fit
	Transit Search
	HD 125612
	Stellar Characteristics
	Doppler Observations and Keplerian Fit
	HD 170469
	Stellar Characteristics
	Doppler Observations and Keplerian Fit
	HD 231701
	Stellar Characteristics
	Doppler Observations and Keplerian Fit
	Discussion
