HAT-P-10b: A Light and Moderately Hot Jupiter Transiting A K Dwarf
ABSTRACT We report on the discovery of HAT-P-10b, one of the lowest mass (0.487 ± 0.018 M J) transiting extrasolar planets (TEPs) discovered to date by transit searches. HAT-P-10b orbits the moderately bright V = 11.89 K dwarf GSC 02340-01714, with a period P = 3.7224747 ± 0.0000065 days, transit epoch Tc = 2454759.68683 ± 0.00016 (BJD), and duration 0.1090 ± 0.0008 days. HAT-P-10b has a radius of 1.005+0.032 –0.027 R J yielding a mean density of 0.594 ± 0.052 g cm–3. Comparing these observations with recent theoretical models we find that HAT-P-10b is consistent with a ~4.5 Gyr, almost pure hydrogen and helium gas giant planet with a 10 M ⊕ core. With an equilibrium temperature of T eq = 1020 ± 17 K, HAT-P-10b is one of the coldest TEPs. Curiously, its Safronov number θ = 0.053 ± 0.002 falls close to the dividing line between the two suggested TEP populations.
arXiv:0809.4295v2 [astro-ph] 8 Oct 2008
Draft version October 8, 2008
Preprint typeset using LATEX style emulateapj v. 10/09/06
HAT-P-10b: A LIGHT AND MODERATELY HOT JUPITER TRANSITING A K DWARF†
G.´A. Bakos1,2, A. P´ al1,4, G. Torres1, B. Sip˝ ocz1,4, D. W. Latham1, R. W. Noyes1, G´ eza Kov´ acs3, J. Hartman1,
G. A. Esquerdo1, D. A. Fischer6, J. A. Johnson7, G. W. Marcy5, R. P. Butler8, A. Howard5, D. D. Sasselov1,
G´ abor Kov´ acs1, R. P. Stefanik1, J. L´ az´ ar9, I. Papp9, P. S´ ari9
Draft version October 8, 2008
We report on the discovery of HAT-P-10b, the lowest mass (0.46 ± 0.03MJ) transiting extraso-
lar planet (TEP) discovered to date by transit searches. HAT-P-10b orbits the moderately bright
V=11.89 K dwarf GSC 02340-01714, with a period P = 3.7224690 ± 0.0000067d, transit epoch
Tc = 2454729.90631 ± 0.00030 (BJD) and duration 0.1100 ± 0.0015d. HAT-P-10b has a radius of
cent theoretical models we find that HAT-P-10 is consistent with a ∼ 4.5Gyr, coreless, pure hydrogen
and helium gas giant planet. With an equilibrium temperature of Teq = 1030+26
one of the coldest TEPs. Curiously, its Safronov number θ = 0.047± 0.003 falls close to the dividing
line between the two suggested TEP populations.
Subject headings: planetary systems — stars: individual (HAT-P-10, GSC 02340-01714) techniques:
−0.03RJyielding a mean density of 0.498 ± 0.064gcm−3. Comparing these observations with re-
−19K, HAT-P-10b is
It has become clear in recent years that transiting ex-
trasolar planets (TEPs), especially those around bright
stars, are extremely valuable for understanding the phys-
ical properties of planetary bodies.
self is a periodic event, which — together with high
precision spectroscopic observations and radial veloc-
ity (RV) follow-up – reveals a number of key parame-
ters, notably the relative radius of the planet with re-
spect to the star, and the true mass of the planet with-
out the inclination ambiguity.
nation of the mean density of the planet, and an in-
sight into its basic structural properties. These advan-
tages have been realized early on, and the recent rise in
the detection of TEPs is due to a number of dedicated
transit searches, such as TrES (Brown & Charbonneau
2000; Dunham et al. 2004),
2005), SuperWASP (Pollacco et al. 2006), OGLE (tar-
geting fainter stars; Udalski et al. 2008), and HATNet
(Bakos et al. 2002, 2004). At the time of this writing,
the number of published TEPs with a unique identifi-
cation is ∼ 40, with ∼ 35 of these due to systematic
searches. The properties of known TEPs already span
The transit it-
These allow determi-
XO (McCullough et al.
1Harvard-Smithsonian Center for Astrophysics, Cambridge,
3Konkoly Observatory, Budapest, Hungary
4Department of Astronomy, E¨ otv¨ os Lor´ and University, Bu-
5Department of Astronomy, University of California, Berkeley,
6Department of Physics and Astronomy, San Francisco State
University, San Francisco, CA
7Institute for Astronomy, University of Hawaii, Honolulu, HI
96822; NSF Postdoctoral Fellow
8Department of Terrestrial Magnetism, Carnegie Institute of
9Hungarian Astronomical Association, Budapest, Hungary
†Based in part on observations obtained at the W. M. Keck Ob-
servatory, which is operated by the University of California and the
California Institute of Technology. Keck time has been granted by
NOAO (A285Hr) and NASA (N128Hr).
a wide range, from the hot Neptune GJ436b with a
mass of Mp= 0.072MJ(Butler et al. 2004; Gillon et al.
2007) to XO-3 with Mp = 11.79Mp (Johns-Krull et al.
2008), from short period orbits like OGLE-TR-56b with
P = 1.2days (Udalski et al. 2002; Konacki et al. 2003) to
P = 21.2days of HD 17506b (Barbieri et al. 2007). Al-
though most of these planets have circular orbits, some
planets with significant eccentricities, such as HAT-P-
2b (Bakos et al. 2007a), have also been reported. TEPs
have been discovered in a wide range of environments,
from orbiting M dwarfs (GJ436b) up to mid F-dwarfs,
such as HAT-P-7b (P´ al et al. 2008a).
Theoretical investigations have been thriving during
this vigorous discovery era, some focusing on the radius
of these planets (Burrows et al. 2007; Liu et al. 2008;
Chabrier et al. 2004; Fortney et al. 2007), and others on
the atmospheres (e.g. Burrows et al. 2006; Fortney et al.
2007), to mention two of the key observable prop-
erties of transiting planets.
ory with observations, it is also essential to use accu-
rate observational values, along with proper error es-
timates.The recent compilation of TEP parameters
by Torres, Winn & Holman (2008) represents a step for-
ward in this sense. It was also noted throughout these
works that a much larger sample is required for better
understanding of the underlying physics, i.e. more plan-
ets are needed to populate the mass–radius (or other)
parameter space, to improve the statistical significance
of correlations between planetary and stellar parameters,
and to reveal any previously undetected correlations that
may shed light on the physical processes governing the
formation and evolution of TEPs.
The HATNet survey has been a major contributor to
TEP discoveries. Operational since 2003, it has cov-
ered approximately 7% of the Northern sky, searching
for TEPs around bright stars (8 ? I ? 12mag). HAT-
Net operates six wide field instruments:
Fred Lawrence Whipple Observatory (FLWO) in Ari-
zona, and two on the roof of the Submillimeter Array
Hangar (SMA) of SAO. Since 2006, HATNet has an-
When confronting the-
four at the
2Bakos et al.
nounced and published 9 TEPs. In this work we report
on the tenth such discovery.11.
2. PHOTOMETRIC DETECTION
-0.4-0.2 0 0.2 0.4
Fig. 1.— The unbinned light curve of HAT-P-10 including all
2870 instrumental I-band measurements obtained with the HAT-
10 telescope of HATNet (see text for details), and folded with the
period of P = 3.7224690 days (which is the result the fit described
in § 4).
The transits of HAT-P-10b were detected with one of
the HATNet telescopes, HAT-10, located at FLWO. The
region around GSC 02340-01714, a field internally la-
beled as G213, was observed on a nightly basis between
2005 October 3 and 2006 March 14, whenever weather
conditions permitted. We gathered 2870 exposures of 5
minutes at a 5.5-minute cadence. Each image contained
approximately 29,000 stars down to I ∼ 14.0. For the
brightest stars in the field we achieved a per-image pho-
tometric precision of 4mmag.
The calibration of the HATNet frames was done uti-
lizing standard procedures. The calibrated frames were
then subject to star detection and astrometry, as de-
scribed in P´ al & Bakos (2006).
try was performed on each image at the stellar cen-
troids derived from the 2MASS catalog (Cutri et al.
2003) and the individual astrometrical solutions. The
resulting light curves were decorrelated against trends
using the External Parameter Decorrelation technique
(EPD, see Bakos et al. 2007b) and the Trend Filter-
ing Algorithm (TFA, see Kov´ acs et al. 2005).
light curves were searched for periodic box-like sig-
nals using the Box Least Squares method (BLS, see
Kov´ acs et al. 2002).We detected a significant signal
in the light curve of GSC 02340-01714 (also known as
2MASS 03092855+3040249; α = 03h09m28.55s, δ =
+30d40m24.9s; J2000), with a depth of ∼ 15mmag, and
a period of P = 3.7225days. The dip had a relative du-
ration (first to last contact) of q ≈ 0.027, equivalent to a
total duration of Pq ≈ 2.5 hours (see Fig. 1).
3. FOLLOW-UP OBSERVATIONS
3.1. Reconnaissance Spectroscopy
All HATNet candidates are subject to thorough inves-
tigation before using more precious time on large tele-
scopes, such as Keck I, to observe them. One of the im-
portant tools for establishing whether the transit-feature
in the light curve of a candidate is due to astrophysical
phenomena other than a planet transiting a star is the
11After submission of this paper, it was realized that HAT-P-
10b and WASP-11b refer to the same object, independently discov-
ered, with WASP-11b submitted 7 days earlier to A&A. The two
discovery groups agreed on calling it in future papers as WASP-
11b/HAT-P-10b, with separate entries on www.exoplanet.eu
CfA Digital Speedometer (DS; Latham 1992), mounted
on the FLWO 1.5 m telescope.
High-resolution spectra with low signal-to-noise ratio
from this facility have been used routinely in the past to
derive radial velocities with moderate precision (roughly
1kms−1) and to classify the effective temperature and
surface gravity of the host star, to weed out false alarms,
such as F dwarfs orbited by M dwarfs, grazing eclipsing
binaries, triple and quadruple star systems, or giant stars
where the transit signal is either false, or comes from a
nearby, blended eclipsing binary.
The RV measurements of HAT-P-10 showed an rms
residual of 0.43kms−1, consistent with no detectable
RV variation. Atmospheric parameters for the star, in-
cluding the effective temperature Teff = 5000K, sur-
face gravity logg = 4.5, and projected rotational ve-
locity v sini = 1.5kms−1, were derived as described by
Torres et al. (2002). The effective temperature and sur-
face gravity corresponds to an early K dwarf.
3.2. High resolution, high S/N spectroscopy
Given the significant detection by HATNet, and the
positive DS results that exclude the usual suspects, we
proceeded with the follow-up of this candidate by obtain-
ing high-resolution and high S/N spectra to characterize
the radial velocity variations and to determine the stellar
parameters with higher precision. We obtained 6 expo-
sures with an iodine cell, plus one iodine-free template,
using the HIRES instrument (Vogt et al. 1994) on the
Keck I telescope located on Mauna Kea, Hawaii. The
observations were made on the nights of 2008 March 21-
22, July 27 and on three nights between September 13
and 17. The small RV variations based on the March
2007 run made this target a firm planet candidate, but
more observations were required to derive an orbit, and
to check spectral bisector variations (see § 4.2).
Fig. 2.— (Top) Radial-velocity measurements from Keck for
HAT-P-10, along with an orbital fit, shown as a function of or-
bital phase, using our best fit period (see § 4). The center-of-mass
velocity has been subtracted. (Middle) Phased residuals after sub-
tracting the orbital fit (also see § 4). The rms variation of the
residuals is about 4.2ms−1. (Bottom) Bisector spans (BS) for 5 of
the 6 Keck spectra plus the single template spectrum (§ 4.2). The
mean value has been subtracted. Note the different vertical scale
of the panels.
The width of the spectrometer slit used on HIRES was
Relative radial velocity
measurements of HAT-P-10
0.′′86, resulting in a resolving power of λ/∆λ ≈ 55,000,
with a wavelength coverage of ∼ 3800− 8000˚ A. The io-
dine gas absorption cell was used to superimpose a dense
forest of I2lines on the stellar spectrum and establish an
accurate wavelength fiducial (see Marcy & Butler 1992).
Relative RVs in the Solar System barycentric frame were
derived as described by Butler et al. (1996), incorporat-
ing full modeling of the spatial and temporal variations
of the instrumental profile. The final RV data and their
errors are listed in Table 1. The folded data, with our
best fit (see § 4) superimposed, are plotted in Fig. 2.
3.3. Photometric follow-up observations
-0.2 -0.15-0.1-0.05 0 0.05 0.1
z-band magnitude - 10.80
Time from transit center (days)
Unbinned instrumental Sloan z-band transit light
curve acquired with KeplerCam at the FLWO 1.2 m telescope on
2008 September 19 MST; superimposed is the best-fit transit model
light curve. Below we show the residuals from the fit.
We observed a complete transit event of HAT-P-10 on
the night of 2008 September 19/20 MST with the Ke-
plerCam CCD on the FLWO 1.2 m telescope. Altogether
278 frames were acquired with a cadence of 65 seconds
in Sloan z band. The reduction of the images was per-
formed as follows. After bias and flat calibration, we
derived an initial second order astrometrical transforma-
tion between the ∼ 110 brightest stars and the 2MASS
catalog, as described in P´ al & Bakos (2006), yielding a
residual of ∼ 0.3 pixels. In order to avoid systematic
errors resulting from the proper motion of the stars, we
generated a new catalog. This catalog was based on the
detected stellar centroids, the coordinates of which were
transformed to the same reference system using the ini-
tial astrometrical solutions, and then averaged out using
3-σ rejection. Using this new catalogue as reference, the
final astrometrical solution was derived for each frame,
yielding a residual of ∼ 0.04 pixel. In the next step, aper-
ture photometry was performed using a series of aper-
tures with radii of 6.0, 7.5, 9.0 and 10.5 pixels.
instrumental magnitude transformation was also done in
two steps: first, all magnitude values were transformed
to the photometric reference frame (selected to be the
sharpest image), using the individual Poisson noise error
estimations as weights. In the second step, the mag-
nitude fit was repeated using the mean individual light
curve magnitudes as reference and the rms of these light
curves as weights. In both of the magnitude transfor-
mations, we excluded from the fit the target star itself
and the 3-σ outliers. We performed EPD against trends
simultaneously with the light curve modeling (for more
details, see § 4). From the series of apertures we chose
the one with a radius of 9.0 pixels, yielding the smallest
fit rms. This aperture falls in the middle of the aperture
series, confirming the plausible selection for the aper-
tures. The final light curve is shown in the upper plot of
Fig. 3, superimposed with our best fit transit light curve
model (see also § 4).
In this section we describe briefly our analysis yielding
the orbital, planetary and stellar parameters of the HAT-
4.1. Planetary, orbital and stellar parameters
First, using the template spectrum obtained by the
Keck/HIRES instrument, we derived the stellar atmo-
spheric parameters.We used the SME package of
Valenti & Piskunov (1996), which yielded the following
values with conservative errors: Teff = 4980 ± 60K,
logg⋆ = 4.5 ± 0.1(cgs), [Fe/H] = 0.13 ± 0.08, and
v sini = 0.5 ± 0.2kms−1.
In modeling both the HATNet and the follow-up tran-
sit light curves, we used the quadratic limb darkening
formalism of Mandel & Agol (2002). The limb darkening
coefficients used for the above stellar atmospheric param-
eters by interpolating in the tables provided by Claret
(2004). The coefficients we derived for I and z photo-
metric passbands were γ1(I)= 0.3806, γ2(I)= 0.2535,
γ1(z)= 0.3214, and γ2(z)= 0.2693.
Following this, a joint fit was done using all of the
available data, including the HATNet light curve, the
follow-up light curve and the radial velocity measure-
ments. Throughout the analysis, we refer to the transit
event observed on 2008 September 19/20 as Ntr= 0.
We adjusted the following parameters: Tc,−290, the
time of first transit center in the HATNet campaign;
Tc,0, the time of the transit center on September 19/20;
K, the radial velocity semi-amplitude; k = ecosω and
h = esinω, the Lagrangian orbital elements related to
the eccentricity and argument of periastron; p ≡ Rp/R⋆,
the fractional planetary radius; b2, the square of the
impact parameter; the quantity ζ/R⋆, which is related
to the transit duration Tdur as (ζ/R⋆)−1= Tdur/2;
and M0 and M1, the out-of-transit instrumental mag-
nitudes of the HATNet and FLWO/KeplerCam light
curves.As noted by Bakos et al. (2007b), the quan-
tity ζ/R⋆shows only a small correlation with the other
light curve parameters (Rp/R⋆, b2), which makes it a
good parameter to use. For eccentric orbits, this quan-
tity is related to the normalized semi-major axis a/R⋆as
ζ/R⋆= (2π/P)(a/R⋆)√1 − e2(1 − b2)−1/2(1 + h)−1. To
find the best fit values and the uncertainties, we utilized
4Bakos et al.
Stellar parameters for HAT-P-10
v sini (kms−1)
logg⋆ (cgs) ...
L⋆ (L⊙) ......
Age (Gyr) ....
Distance (pc) .
text.bY2+LC+SME = Yale-Yonsei isochrones
(Yi et al. 2001), light curve parameters, and
4980 ± 60
0.13 ± 0.08
0.5 ± 0.2
0.82 ± 0.03
4.54 ± 0.03
6.12 ± 0.12
11.2 ± 4.1
Valenti & Piskunov
the method of Markov Chain Monte-Carlo (MCMC; Ford
2006) which provides the a posteriori distribution of the
The values and uncertainties of the k and h orbital
elements were found to be consistent with zero within 1-
σ, namely k = 0.04±0.11 and h = 0.11±0.12. Therefore
we conclude that the observations are consistent with a
circular planetary orbit, and we repeated the fit by fixing
the eccentricity to zero.
The results for the simultaneous fit are reported in
Table 3, except for the auxiliary parameters Tc,−290 =
2453650.39029±0.00195 (BJD), Tc,0= 2454729.90631±
0.00030 (BJD) that are used to derive the epoch and the
period as shown in P´ al et al. (2008a). The RV jitter is
the additional velocity uncertainty that should be added
quadratically to the nominal errors (estimated from the
Poisson-noise) in order to have a reduced χ2of unity
(this is the quadratic sum of the residuals, divided by the
degrees of freedom of the RV fit, i.e. 4). Our final value
for the jitter is 4.2ms−1, and the error-bars on Fig. 2
(top and middle panel) have been inflated accordingly.
The results of the joint fit, together with the initial
results from spectroscopy enable us to refine the param-
eters of the star. As described by Sozzetti et al. (2007)
and Torres, Winn & Holman (2008), a/R⋆is a better lu-
minosity indicator than the spectroscopic value of logg⋆
since stellar surface gravity has only a subtle effect on
the line profiles. Therefore, we used the values of Teff
and [Fe/H] from the initial SME analysis, together with
the distribution of a/R⋆to estimate the stellar properties
from comparison with the Yonsei-Yale (Y2) stellar evo-
lution models by Yi et al. (2001) and Demarque et al.
(2004). Using the relation between a/R⋆and ζ/R⋆, we
derive the a posteriori distribution for the former one,
and used the derived stellar density as an input for the
stellar evolution models in order to have an a posteri-
ori distribution for the stellar parameters (see P´ al et al.
2008a,c, for more details). Since the mass and radius
(and their respective distributions) of the star are known,
it is straightforward to obtain the surface gravity and
its uncertainty together. The derived surface gravity
is logg⋆ = 4.54 ± 0.03. Since the surface gravity from
the initial SME analysis agrees well with the one derived
above, we accept the latter as final value (listed, together
Orbital and planetary parameters
Light curve parameters
P (days) ....................
E (BJD) ....................
T12= T34 (days)a..........
b ≡ acosi/R⋆ ...............
i (deg) ......................
3.7224690 ± 0.0000067
2454729.90631 ± 0.00030
0.1100 ± 0.0015
0.0136 ± 0.0014
0.092 ± 0.062
20.72 ± 0.14
0.1332 ± 0.0013
88.5 ± 0.6
K (ms−1) ..................
γ (kms−1) ..................
69.1 ± 3.5
35.5 ± 3.0
Mp (MJ) ....................
Rp (RJ) .....................
ρp (g cm−3) .................
a (AU) ......................
loggp (cgs) ..................
Teq (K) .....................
aT14: total transit duration, time between first and last con-
tact; T12 = T34: ingress/egress time, time between first and
second, or third and fourth contact.
0.460 ± 0.028
0.498 ± 0.064
0.047 ± 0.003
with other parameters in Table 2).
The stellar evolution modeling also yields the abso-
lute magnitudes and colors in various photometric pass-
bands. The derived V − I color of the star is (V −
I)YY = 0.917 ± 0.019, slightly smaller than the color
from the TASS catalog (Droege et al. 2006), namely
(V −I)TASS= 1.09±0.11. Since this excess is most likely
due to interstellar reddening, we used the 2MASS J mag-
nitude to estimate the distance. The observed J band
magnitude of HAT-P-10 is J = 10.015± 0.020 while the
stellar modeling gives MJ= 4.530±0.087, which lead to
a distance modulus for the star of J−MJ= 5.484±0.089,
corresponding to a distance of d = 125+6
The planetary parameters and their uncertainties can
be derived by direct combination of the a posteriori dis-
tributions of the light curve, radial velocity and stellar
parameters (see also P´ al et al. 2008a). We find that the
mass of the planet is Mp = 0.460 ± 0.028MJ, the ra-
dius is Rp = 1.045+0.050
0.498±0.064gcm−3. The final planetary parameters are
summarized at the bottom of Table 3.
−0.033RJ, and its density is ρp =
4.2. Excluding blend scenarios
Following Torres et al. (2007), we explored the possi-
bility that the measured radial velocities are not real,
but are instead caused by distortions in the spectral line
profiles due to contamination from a nearby unresolved
eclipsing binary. A bisector analysis based on the Keck
spectra was done as described in earlier HATNet detec-
tion papers (see §5 in Bakos et al. (2007a)).
The first spectrum in Table 1 is contaminated, as it was
taken at high airmass, through cloud-cover and in strong
800 1000 1200 1400 1600 1800 2000 2200 2400
Fig. 4.— (Left): Mass–radius diagram of published and uniquely identified TEPs. HAT-P-10b is shown as a large filled circle on the left.
Overlaid are Baraffe et al. (2003) zero insolation planetary isochrones for ages of 0.5Gyr (upper, solid line) and 5Gyr (lower dashed-dotted
line), respectively, as well as isodensity lines for 0.4, 0.7, 1.0, 1.33, 5.5 and 11.9gcm−3(dashed lines). (Right): Equilibrium temperature
versus Safronov number (Hansen & Barman 2007).
moon-light. While the RV does not seem to be affected,
the bisector span is unreliable, thus we omitted it from
the analysis. We detect no bisector variation in excess of
the measurement uncertainties (see Fig. 2 bottom panel).
We have also tested the significance of the correlation
between the radial velocity and the bisector variations,
and these appear to be negligible. Therefore, we conclude
that the velocity variations are real, and that the star is
orbited by a close-in giant planet.
It is interesting to compare the properties of HAT-
P-10b with the other known TEPs so as to place it in
a broader context.This planet falls at the low-mass
end of the current distribution, as shown in Fig. 4 (left
panel), where we overplot Baraffe et al. (2003) planetary
isochrones, which indicate that the radius of HAT-P-10 is
broadly consistent with these models. As we noted ear-
lier, HAT-P-10 is formally the smallest mass TEP discov-
ered by transit searches. The even smaller HD 149026b
(Sato et al. 2005) and GJ436b (Butler et al. 2004) were
discovered by RV searches, and their transits were found
We compared our mass and radius to theoretical esti-
mates of Liu et al. (2008) for a 0.5MJ body at various
orbital distances from a G2V star. We note that the
equivalent semi-major axis (with the same incident flux)
of HAT-P-10b around a solar type star is arel= 0.076.
It is also noteworthy that when a detailed comparison
is done, the effects of the environment on the planetary
properties are not as simple as scaling the integrated stel-
lar flux, since the detailed spectrum of the star (e.g. UV
flux) may also be important.
Based on the models presented by Liu et al. (2008), for
˙Eh/˙Eins = 10−6the equilibrium radius of HAT-P-10b
would be ∼1.1RJ, where ˙Ehis the energy per unit time
due to orbital tidal heating or similar internal heating,
and˙Einsis the energy received via insolation. For larger
values of ˙ Eh/˙Einsthe expected radius is larger, and for
smaller values it asympotically converges to 1.1RJ. This
makes us conclude that a small core of approx. 20M⊕
is required so that the model values match the observed
1.05RJradius of HAT-P-10b. This would be also consis-
tent with the core-mass—stellar metallicity relation pro-
posed by Burrows et al. (2007)
When comparing with models of Fortney et al. (2008),
we obtain similar results. The current mass, radius and
insolation of HAT-P-10b are consistent with a 500Myr
model with a 25M⊕core mass, or a 4.5Gyr coreless pure
hydrogen and helium model. It is noted that low-mass,
core-free planets are hard to model, thus our current find-
ing will hopefully provide a further constraint for theo-
The radiation that HAT-P-10b receives from its host
star is ∼ 2.56 · 108ergs−1cm−2. With the definitions of
Fortney et al. (2008), HAT-P-10b belongs to the pL class
of planets. There is only one transiting planet that has
a lower mean incident flux: HD 17156b (Barbieri et al.
2007), but this planet orbits on a highly eccentric orbit,
with incident flux increasing to over 109ergs−1cm−2at
The other planet with a similarly low incident flux is
OGLE-TR-111b (Udalski et al. 2002; Pont et al. 2004),
orbiting an I = 15.55mag star with Teff= 5040K (San-
tos, 2006). HAT-P-10b appears to be a near-by analog of
OGLE-TR-111b in many respects, since their parameters
are very similar (parentheses show those of OGLE-TR-
111); the period is 3.7225d (4.01d), the stellar mass is
0.82M⊙ (0.85M⊙), the stellar radius is 0.81(0.83R⊙),
the luminosity is 0.36 (0.4), the metallicity is 0.13±0.08
(0.19±0.07), and the planetary radius is 1.05MJ(1.05).
Interestingly, even the impact parameter of their tran-
sits is similar. There is a slight difference in their
masses, with HAT-P-10b being smaller (0.46 ± 0.03MJ
vs. 0.55±0.1MJ). One crucial difference between the two
systems is that HAT-P-10 is 10 times closer to us, being
−5pc vs. 1500pc for OGLE-TR-111, and is more
than 4 magnitudes brighter, thus enabling more detailed
follow-up in the near future.
Another interesting observational fact is that the Θ =
0.047±0.003 Safronov number of HAT-P-10b falls fairly
close to the dividing line between the proposed Class
I and Class II planets (Hansen & Barman 2007).
the low end of the equilibrium temperature range of
the plot (excluding GJ436b), HAT-P-10b seems to be
at a point where the two distributions overlap (see
Fig. 4, right panel). Finally, we note that HAT-P-10b
strengthens the orbital period vs. surface gravity rela-
tion (Southworth, Wheatley, & Sams 2007), falling al-
most exactly on the linear fit between these two quanti-
6Bakos et al.
ties (Torres, Winn & Holman 2008).
HATNet operations have been funded by NASA grants
NNG04GN74G, NNX08AF23G and SAO IR&D grants.
Work of G.´A.B. and J. Johnson were supported by the
Postdoctoral Fellowship of the NSF Astronomy and As-
trophysics Program(AST-0702843and AST-0702821, re-
spectively). We acknowledge partial support also from
the Kepler Mission under NASA Cooperative Agree-
ment NCC2-1390 (D.W.L., PI). G.K. thanks the Hun-
garian Scientific Research Foundation (OTKA) for sup-
port through grant K-60750. This research has made use
of Keck telescope time granted through NOAO (program
A285Hr) and NASA (N128Hr).
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