37 research outputs found
Determining the Mass of Kepler-78b With Nonparametric Gaussian Process Estimation
Kepler-78b is a transiting planet that is 1.2 times the radius of Earth and
orbits a young, active K dwarf every 8 hours. The mass of Kepler-78b has been
independently reported by two teams based on radial velocity measurements using
the HIRES and HARPS-N spectrographs. Due to the active nature of the host star,
a stellar activity model is required to distinguish and isolate the planetary
signal in radial velocity data. Whereas previous studies tested parametric
stellar activity models, we modeled this system using nonparametric Gaussian
process (GP) regression. We produced a GP regression of relevant Kepler
photometry. We then use the posterior parameter distribution for our
photometric fit as a prior for our simultaneous GP + Keplerian orbit models of
the radial velocity datasets. We tested three simple kernel functions for our
GP regressions. Based on a Bayesian likelihood analysis, we selected a
quasi-periodic kernel model with GP hyperparameters coupled between the two RV
datasets, giving a Doppler amplitude of 1.86 0.25 m s and
supporting our belief that the correlated noise we are modeling is
astrophysical. The corresponding mass of 1.87 M
is consistent with that measured in previous studies, and more robust due to
our nonparametric signal estimation. Based on our mass and the radius
measurement from transit photometry, Kepler-78b has a bulk density of
6.0 g cm. We estimate that Kepler-78b is 3226% iron
using a two-component rock-iron model. This is consistent with an Earth-like
composition, with uncertainty spanning Moon-like to Mercury-like compositions.Comment: 10 pages, 5 figures, accepted to ApJ 6/16/201
Seeing double with K2: Testing re-inflation with two remarkably similar planets around red giant branch stars
Despite more than 20 years since the discovery of the first gas giant planet
with an anomalously large radius, the mechanism for planet inflation remains
unknown. Here, we report the discovery of EPIC228754001.01, an inflated gas
giant planet found with the NASA K2 Mission, and a revised mass for another
inflated planet, K2-97b. These planets reside on ~9 day orbits around host
stars which recently evolved into red giants. We constrain the irradiation
history of these planets using models constrained by asteroseismology and
Keck/HIRES spectroscopy and radial velocity measurements. We measure planet
radii of 1.31 +\- 0.11 Rjup and and 1.30 +\- 0.07 Rjup, respectively. These
radii are typical for planets receiving the current irradiation, but not the
former, zero age main sequence irradiation of these planets. This suggests that
the current sizes of these planets are directly correlated to their current
irradiation. Our precise constraints of the masses and radii of the stars and
planets in these systems allow us to constrain the planetary heating efficiency
of both systems as 0.03% +0.03%/-0.02%. These results are consistent with a
planet re-inflation scenario, but suggest the efficiency of planet re-inflation
may be lower than previously theorized. Finally, we discuss the agreement
within 10% of stellar masses and radii, and planet masses, radii, and orbital
periods of both systems and speculate that this may be due to selection bias in
searching for planets around evolved stars.Comment: 18 pages, 15 figures, accepted to AJ. Figures 11, 12, and 13 are the
key figures of the pape
Do Close-in Giant Planets Orbiting Evolved Stars Prefer Eccentric Orbits?
The NASA Kepler and K2 Missions have recently revealed a population of transiting giant planets orbiting moderately evolved, low-luminosity red giant branch stars. Here, we present radial velocity (RV) measurements of three of these systems, revealing significantly non-zero orbital eccentricities in each case. Comparing these systems with the known planet population suggests that close-in giant planets around evolved stars tend to have more eccentric orbits than those around main sequence stars. We interpret this as tentative evidence that the orbits of these planets pass through a transient, moderately eccentric phase where they shrink faster than they circularize due to tides raised on evolved host stars. Additional RV measurements of currently known systems, along with new systems discovered by the recently launched NASA Transiting Exoplanet Survey Satellite (TESS) mission, may constrain the timescale and mass dependence of this process
K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star
Strongly irradiated giant planets are observed to have radii larger than thermal evolution models predict. Although these inflated planets have been known for over 15 years, it is unclear whether their inflation is caused by the deposition of energy from the host star or the inhibited cooling of the planet. These processes can be distinguished if the planet becomes highly irradiated only when the host star evolves onto the red giant branch. We report the discovery of K2-97b, a 1.31 ± 0.11 R_J, 1.10 ± 0.11 M_J planet orbiting a 4.20 ± 0.14 R⊙, 1.16 ± 0.12 M⊙ red giant star with an orbital period of 8.4 days. We precisely constrained stellar and planetary parameters by combining asteroseismology, spectroscopy, and granulation noise modeling along with transit and radial velocity measurements. The uncertainty in planet radius is dominated by systematic differences in transit depth, which we measure to be up to 30% between different light-curve reduction methods. Our calculations indicate the incident flux on this planet was 170^(+140)_(-60) times the incident flux on Earth, while the star was on the main sequence. Previous studies suggest that this incident flux is insufficient to delay planetary cooling enough to explain the present planet radius. This system thus provides the first evidence that planets may be inflated directly by incident stellar radiation rather than by delayed loss of heat from formation. Further studies of planets around red giant branch stars will confirm or contradict this hypothesis and may reveal a new class of re-inflated planets
Spinning up the Surface: Evidence for Planetary Engulfment or Unexpected Angular Momentum Transport?
In this paper, we report the potential detection of a nonmonotonic radial
rotation profile in a low-mass lower-luminosity giant star. For most low- and
intermediate-mass stars, the rotation on the main sequence seems to be close to
rigid. As these stars evolve into giants, the core contracts and the envelope
expands, which should suggest a radial rotation profile with a fast core and a
slower envelope and surface. KIC 9267654, however, seems to show a surface
rotation rate that is faster than its bulk envelope rotation rate, in conflict
with this simple angular momentum conservation argument. We improve the
spectroscopic surface constraint, show that the pulsation frequencies are
consistent with the previously published core and envelope rotation rates, and
demonstrate that the star does not show strong chemical peculiarities. We
discuss the evidence against any tidally interacting stellar companion.
Finally, we discuss the possible origin of this unusual rotation profile,
including the potential ingestion of a giant planet or unusual angular momentum
transport by tidal inertial waves triggered by a close substellar companion,
and encourage further observational and theoretical efforts.Comment: 18 pages, 9 figures, submitted to AAS Journal
TESS Giants Transiting Giants V -- Two hot Jupiters orbiting red-giant hosts
In this work we present the discovery and confirmation of two hot Jupiters
orbiting red-giant stars, TOI-4377 b and TOI-4551 b, observed by TESS in the
southern ecliptic hemisphere and later followed-up with radial-velocity (RV)
observations. For TOI-4377 b we report a mass of $0.957^{+0.089}_{-0.087} \
M_\mathrm{J}1.348 \pm 0.081 \ R_\mathrm{J}1.36 \ \mathrm{M}_\odot3.52 \
\mathrm{R}_\odot4.3781.49 \pm 0.13 \ M_\mathrm{J}1.058^{+0.110}_{-0.062} \ R_\mathrm{J}1.31 \ \mathrm{M}_\odot3.55 \
\mathrm{R}_\odot9.9561.91 \pm 0.48\%2.19 \pm 0.45\%$ for
TOI-4377 b and TOI-4551 b respectively. These values are in line with the known
population of hot Jupiters, including hot Jupiters orbiting main sequence
hosts, which suggests that the radii of our planets have reinflated in step
with their parent star's brightening as they evolved into the
post-main-sequence. Finally, we evaluate the potential to observe orbital decay
in both systems.Comment: 14 pages with 8 figures and 6 tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
The masses of retired A stars with asteroseismology::Kepler and K2 observations of exoplanet hosts
We investigate the masses of "retired A stars" using asteroseismic detections
on seven low-luminosity red-giant and sub-giant stars observed by the NASA
Kepler and K2 Missions. Our aim is to explore whether masses derived from
spectroscopy and isochrone fitting may have been systematically overestimated.
Our targets have all previously been subject to long term radial velocity
observations to detect orbiting bodies, and satisfy the criteria used by
Johnson et al. (2006) to select survey stars that may have had A-type (or early
F-type) main-sequence progenitors. The sample actually spans a somewhat wider
range in mass, from up to . Whilst for five of the seven stars the reported discovery mass from
spectroscopy exceeds the mass estimated using asteroseismology, there is no
strong evidence for a significant, systematic bias across the sample. Moreover,
comparisons with other masses from the literature show that the absolute scale
of any differences is highly sensitive to the chosen reference literature mass,
with the scatter between different literature masses significantly larger than
reported error bars. We find that any mass difference can be explained through
use of differing constraints during the recovery process. We also conclude that
underestimated uncertainties on the input parameters can significantly bias the
recovered stellar masses, which may have contributed to the controversy on the
mass scale for retired A stars.Comment: Accepted MNRAS, 14 pages, 7 Figures, 3 Table
Three temperate Neptunes orbiting nearby stars
We present the discovery of three modestly irradiated, roughly Neptune-mass planets orbiting three nearby Solartype stars. HD 42618 b has a minimum mass of 15.4±2.4 M⊕, a semimajor axis of 0.55 au, an equilibrium temperature of 337 K, and is the first planet discovered to orbit the solar analogue host star, HD 42618. We also discover new planets orbiting the known exoplanet host stars HD 164922 and HD 143761 (p CrB). The new planet orbiting HD 164922 has a minimum mass of 12.9±1.6 M⊕ and orbits interior to the previously known Jovian mass planet orbiting at 2.1 au. HD 164922 c has a semimajor axis of 0.34 au and an equilibrium temperature of 418 K. HD 143761 c orbits with a semimajor axis of 0.44 au, has a minimum mass of 25±2 M⊕, and is the warmest of the three new planets with an equilibrium temperature of 445 K. It orbits exterior to the previously known warm Jupiter in the system. A transit search using space-based CoRoT data and ground-based photometry from the Automated Photometric Telescopes (APTs) at Fairborn Observatory failed to detect any transits, but the precise, high-cadence APT photometry helped to disentangle planetary-reflex motion from stellar activity. These planets were discovered as part of an ongoing radial velocity survey of bright, nearby, chromospherically inactive stars using the Automated Planet Finder (APF) telescope at Lick Observatory. The high-cadence APF data combined with nearly two decades of radial velocity data from Keck Observatory and gives unprecedented sensitivity to both short-period low-mass, and long-period intermediate-mass planets