95 research outputs found
Photometry as a proxy for stellar activity in radial velocity analyses
Stellar activity remains a limiting factor in measuring precise planet
parameters from radial velocity spectroscopy, not least in the search for Earth
mass planets orbiting in the habitable zones of Sun-like stars. One approach to
mitigate stellar activity is to use combined analyses of both radial velocity
and time-series photometry. We present an analysis of simultaneous
disk-integrated photometry and radial velocity data of the Sun in order to
determine the useful limits of a combined analysis. We find that simple
periodogram or autocorrelation analysis of solar photometry give the correct
rotation period <50% of the time. We therefore use a Gaussian process to
investigate the time variability of solar photometry and to directly compare
simultaneous photometry with radial velocity data. We find that the
hyperparameter posteriors are relatively stable over 70 years of solar
photometry and the amplitude tracks the solar cycle. We observe good agreement
between the hyperparameter posteriors for the simultaneous photometry and
radial velocity data. Our primary conclusion is a recommendation to include an
additional prior in Gaussian process fits to constrain the evolutionary
timescale to be greater than the recurrence timescale (ie., the rotation
period) to recover more physically plausible and useful results. Our results
indicate that such simultaneous monitoring may be a useful tool in enhancing
the precision of radial velocity surveys.Comment: 10 pages, accepted in A
Long-Period Giant Companions to Three Compact, Multiplanet Systems
Understanding the relationship between long-period giant planets and multiple smaller short-period planets is critical for formulating a complete picture of planet formation. This work characterizes three such systems. We present Kepler-65, a system with an eccentric (e = 0.28 ± 0.07) giant planet companion discovered via radial velocities (RVs) exterior to a compact, multiply transiting system of sub-Neptune planets. We also use precision RVs to improve mass and radius constraints on two other systems with similar architectures, Kepler-25 and Kepler-68. In Kepler-68 we propose a second exterior giant planet candidate. Finally, we consider the implications of these systems for planet formation models, particularly that the moderate eccentricity in Kepler-65\u27s exterior giant planet did not disrupt its inner system
A TESS Dress Rehearsal: Planetary Candidates and Variables from K2 Campaign 17
We produce light curves for all ~34,000 targets observed with K2 in Campaign 17 (C17), identifying 34 planet candidates, 184 eclipsing binaries, and 222 other periodic variables. The forward-facing direction of the C17 field means follow-up can begin immediately now that the campaign has concluded and interesting targets have been identified. The C17 field has a large overlap with C6, so this latest campaign also offers an infrequent opportunity to study a large number of targets already observed in a previous K2 campaign. The timing of the C17 data release, shortly before science operations begin with the Transiting Exoplanet Survey Satellite (TESS), also lets us exercise some of the tools and methods developed for identification and dissemination of planet candidates from TESS. We find excellent agreement between these results and those identified using only K2-based tools. Among our planet candidates are several planet candidates with sizes <4 R⊕ and orbiting stars with Kp ≾ 10 (indicating good RV targets of the sort TESS hopes to find) and a Jupiter-sized single-transit event around a star already hosting a 6 day planet candidate
HAT-P-11: Discovery of a Second Planet and a Clue to Understanding Exoplanet Obliquities
HAT-P-11 is a mid-K dwarf that hosts one of the first Neptune-sized planets
found outside the solar system. The orbit of HAT-P-11b is misaligned with the
star's spin --- one of the few known cases of a misaligned planet orbiting a
star less massive than the Sun. We find an additional planet in the system
based on a decade of precision radial velocity (RV) measurements from
Keck/HIRES. HAT-P-11c is similar to Jupiter in its mass ( ) and orbital period ( year), but has a
much more eccentric orbit (). In our joint modeling of RV and
stellar activity, we found an activity-induced RV signal of 7 m s,
consistent with other active K dwarfs, but significantly smaller than the 31 m
s reflex motion due to HAT-P-11c. We investigated the dynamical coupling
between HAT-P-11b and c as a possible explanation for HAT-P-11b's misaligned
orbit, finding that planet-planet Kozai interactions cannot tilt planet b's
orbit due to general relativistic precession; however, nodal precession
operating on million year timescales is a viable mechanism to explain
HAT-P-11b's high obliquity. This leaves open the question of why HAT-P-11c may
have such a tilted orbit. At a distance of 38 pc, the HAT-P-11 system offers
rich opportunities for further exoplanet characterization through astrometry
and direct imaging.Comment: 16 pages, 11 figures, 4 tables. Accepted to A
Planet Candidates from K2 Campaigns 5-8 and Follow-Up Optical Spectroscopy
We present 151 planet candidates orbiting 141 stars from K2 campaigns 5-8
(C5-C8), identified through a systematic search of K2 photometry. In addition,
we identify 16 targets as likely eclipsing binaries, based on their light curve
morphology. We obtained follow-up optical spectra of 105/141 candidate host
stars and 8/16 eclipsing binaries to improve stellar properties and to identify
spectroscopic binaries. Importantly, spectroscopy enables measurements of host
star radii with 10% precision, compared to 40% precision when
only broadband photometry is available. The improved stellar radii enable
improved planet radii. Our curated catalog of planet candidates provides a
starting point for future efforts to confirm and characterize K2 discoveries.Comment: Accepted for publication in the Astronomical Journal; 17 pages, 8
figures, 2 tables, download source for full table
Hot Stars With Kepler Planets Have High Obliquities
It has been known for a decade that hot stars with hot Jupiters tend to have
high obliquities. Less is known about the degree of spin-orbit alignment for
hot stars with other kinds of planets. Here, we re-assess the obliquities of
hot Kepler stars with transiting planets smaller than Neptune, based on
spectroscopic measurements of their projected rotation velocities (vsini). The
basis of the method is that a lower obliquity -- all other things being equal
-- causes sini to be closer to unity and increases the value of vsini. We
sought evidence for this effect using a sample of 150 Kepler stars with
effective temperatures between 5950 and 6550K and a control sample of 101 stars
with matching spectroscopic properties and random orientations. The planet
hosts have systematically higher values of vsini than the control stars, but
not by enough to be compatible with perfect spin-orbit alignment. The mean
value of sini is 0.856 +/- 0.036, which is 4-sigma away from unity (perfect
alignment), and 2-sigma away from pi/4 (random orientations). There is also
evidence that the hottest stars have a broader obliquity distribution: when
modeled separately, the stars cooler than 6250K have = 0.928 +/- 0.042,
while the hotter stars are consistent with random orientations. This is similar
to the pattern previously noted for stars with hot Jupiters. Based on these
results, obliquity excitation for early-G and late-F stars appears to be a
general outcome of star and planet formation, rather than being exclusively
linked to hot Jupiter formation.Comment: AJ, in press [15 pages
Updated Parameters and a New Transmission Spectrum of HD 97658b
Recent years have seen increasing interest in the characterization of sub-Neptune-sized planets because of their prevalence in the Galaxy, contrasted with their absence in our solar system. HD 97658 is one of the brightest stars hosting a planet of this kind, and we present the transmission spectrum of this planet by combining four Hubble Space Telescope transits, 12 Spitzer/IRAC transits, and eight MOST transits of this system. Our transmission spectrum has a higher signal-to-noise ratio than those from previous works, and the result suggests that the slight increase in transit depth from wavelength 1.1–1.7 μm reported in previous works on the transmission spectrum of this planet is likely systematic. Nonetheless, our atmospheric modeling results are inconclusive, as no model provides an excellent match to our data. Nonetheless, we find that atmospheres with high C/O ratios (C/O ≳ 0.8) and metallicities of ≳100× solar metallicity are favored. We combine the mid-transit times from all of the new Spitzer and MOST observations and obtain an updated orbital period of P = 9.489295 ± 0.000005, with a best-fit transit time center at T₀ = 2456361.80690 ± 0.00038 (BJD). No transit timing variations are found in this system. We also present new measurements of the stellar rotation period (34 ± 2 days) and stellar activity cycle (9.6 yr) of the host star HD 97658. Finally, we calculate and rank the Transmission Spectroscopy Metric of all confirmed planets cooler than 1000 K and with sizes between 1 R⊕ and 4 R⊕. We find that at least a third of small planets cooler than 1000 K can be well characterized using James Webb Space Telescope, and of those, HD 97658b is ranked fifth, meaning that it remains a high-priority target for atmospheric characterization
60 Validated Planets from K2 Campaigns 5-8
We present a uniform analysis of 155 candidates from the second year of
NASA's mission (Campaigns 5-8), yielding 60 statistically validated
planets spanning a range of properties, with median values of = 2.5
, = 7.1 d, = 811 K, and = 11.3 mag. The
sample includes 24 planets in 11 multi-planetary systems, as well as 18 false
positives, and 77 remaining planet candidates. Of particular interest are 18
planets smaller than 2 , five orbiting stars brighter than = 10
mag, and a system of four small planets orbiting the solar-type star EPIC
212157262. We compute planetary transit parameters and false positive
probabilities using a robust statistical framework and present a complete
analysis incorporating the results of an intensive campaign of high resolution
imaging and spectroscopic observations. This work brings the yield to over
360 planets, and by extrapolation we expect that will have discovered
600 planets before the expected depletion of its on-board fuel in late
2018.Comment: 33 pages, 13 figures, 5 tables, accepted for publication in A
The K2-3 system revisited: testing photoevaporation and core-powered mass loss with three small planets spanning the radius valley
Multi-planet systems orbiting M dwarfs provide valuable tests of theories of
small planet formation and evolution. K2-3 is an early M dwarf hosting three
small exoplanets (1.5-2.0 Earth radii) at distances of 0.07-0.20 AU. We measure
the high-energy spectrum of K2-3 with HST/COS and XMM-Newton, and use
empirically-driven estimates of Ly-alpha and extreme ultraviolet flux. We use
EXOFASTv2 to jointly fit radial velocity, transit, and SED data. This
constrains the K2-3 planet radii to 4% uncertainty and the masses of K2-3b and
c to 13% and 30%, respectively; K2-3d is not detected in RV measurements. K2-3b
and c are consistent with rocky cores surrounded by solar composition envelopes
(mass fractions of 0.36% and 0.07%), H2O envelopes (55% and 16%), or a mixture
of both. However, based on the high-energy output and estimated age of K2-3, it
is unlikely that K2-3b and c retain solar composition atmospheres. We pass the
planet parameters and high-energy stellar spectrum to atmospheric models.
Dialing the high-energy spectrum up and down by a factor of 10 produces
significant changes in trace molecule abundances, but not at a level detectable
with transmission spectroscopy. Though the K2-3 planets span the small planet
radius valley, the observed system architecture cannot be readily explained by
photoevaporation or core-powered mass loss. We instead propose 1) the K2-3
planets are all volatile-rich, with K2-3d having a lower density than typical
of super-Earths, and/or 2) the K2-3 planet architecture results from more
stochastic processes such as planet formation, planet migration, and impact
erosion.Comment: 15 pages, 7 figure, published in AJ, HLSPs at
https://archive.stsci.edu/hlsp/mstarpanspe
A TESS Dress Rehearsal: Planetary Candidates and Variables from K2 Campaign 17
We produce light curves for all ~34,000 targets observed with K2 in Campaign 17 (C17), identifying 34 planet candidates, 184 eclipsing binaries, and 222 other periodic variables. The forward-facing direction of the C17 field means follow-up can begin immediately now that the campaign has concluded and interesting targets have been identified. The C17 field has a large overlap with C6, so this latest campaign also offers an infrequent opportunity to study a large number of targets already observed in a previous K2 campaign. The timing of the C17 data release, shortly before science operations begin with the Transiting Exoplanet Survey Satellite (TESS), also lets us exercise some of the tools and methods developed for identification and dissemination of planet candidates from TESS. We find excellent agreement between these results and those identified using only K2-based tools. Among our planet candidates are several planet candidates with sizes <4 R⊕ and orbiting stars with Kp ≾ 10 (indicating good RV targets of the sort TESS hopes to find) and a Jupiter-sized single-transit event around a star already hosting a 6 day planet candidate
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