sdB Pulsating Star V391 Peg's Giant Planet Not Detected

The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data

Authors:

Silvotti et al

Abstract:

V391 Peg (alias HS2201+2610) is a subdwarf B (sdB) pulsating star that shows both p- and g-modes. By studying the arrival times of the p-mode maxima and minima through the O-C method, in a previous article the presence of a planet was inferred with an orbital period of 3.2 yr and a minimum mass of 3.2 M_Jup. Here we present an updated O-C analysis using a larger data set of 1066 hours of photometric time series (~2.5x larger in terms of the number of data points), which covers the period between 1999 and 2012 (compared with 1999-2006 of the previous analysis). Up to the end of 2008, the new O-C diagram of the main pulsation frequency (f1) is compatible with (and improves) the previous two-component solution representing the long-term variation of the pulsation period (parabolic component) and the giant planet (sine wave component). Since 2009, the O-C trend of f1 changes, and the time derivative of the pulsation period (p_dot) passes from positive to negative; the reason of this change of regime is not clear and could be related to nonlinear interactions between different pulsation modes. With the new data, the O-C diagram of the secondary pulsation frequency (f2) continues to show two components (parabola and sine wave), like in the previous analysis. Various solutions are proposed to fit the O-C diagrams of f1 and f2, but in all of them, the sinusoidal components of f1 and f2 differ or at least agree less well than before. The nice agreement found previously was a coincidence due to various small effects that are carefully analysed. Now, with a larger dataset, the presence of a planet is more uncertain and would require confirmation with an independent method. The new data allow us to improve the measurement of p_dot for f1 and f2: using only the data up to the end of 2008, we obtain p_dot_1=(1.34+-0.04)x10**-12 and p_dot_2=(1.62+-0.22)x10**-12

Planetary Systems around Low-mass Stars Unveiled by K2

Planetary Systems around Low-mass Stars Unveiled by K2 

Authors: 

Hirano et al 

Abstract: 

We present the detection and follow-up observations of planetary candidates around low-mass stars observed by the {\it K2} mission. Based on light-curve analysis, adaptive-optics imaging, and optical spectroscopy at low and high resolution (including radial velocity measurements), we validate 16 planets around 12 low-mass stars observed during {\it K2} campaigns 5--10. Among the 16 planets, 12 are newly validated, with orbital periods ranging from 0.96--33 days. For one of the planets (EPIC 220621087.01) we present ground-based transit photometry, allowing us to refine the ephemerides. We also identify EPIC 220187552 as a false positive, based on the multiple stars seen in a high-resolution image and double lines in a high-resolution spectrum. Combining our {\it K2} M-dwarf planets together with the validated or confirmed planets found previously, we investigate the dependence of planet radius Rp on stellar insolation and metallicity [Fe/H]. We confirm that medium-sized planets (Rp=2−5 R⊕) seem to have experienced shrinkage --- plausibly due to photoevaporation --- and we find evidence that the shrinkage occurs at lower insolation for the coolest M dwarfs. Planets larger than ≈3 R⊕ are only found around the most metal-rich M dwarfs, and for the coolest M dwarfs (≲3500 K) there appears to be a correlation between planet size and metallicity.

Average Albedos of Close-in Super-Earths and Super-Neptunes from Statistical Analysis of Long-cadence Kepler Secondary Eclipse Data

Average Albedos of Close-in Super-Earths and Super-Neptunes from Statistical Analysis of Long-cadence Kepler Secondary Eclipse Data
Authors: 

Sheets et al 

Abstract:

We present the results of our work to determine the average albedo for small, close-in planets in the Kepler candidate catalog. We have adapted our method of averaging short-cadence light curves of multiple Kepler planet candidates to long-cadence data, in order to detect an average albedo for the group of candidates. Long-cadence data exist for many more candidates than the short-cadence data, and so we separate the candidates into smaller radius bins than in our previous work: 1–2 ${R}_{\oplus }$, 2–4 ${R}_{\oplus }$, and 4–6 ${R}_{\oplus }$. We find that, on average, all three groups appear darker than suggested by the short-cadence results, but not as dark as many hot Jupiters. The average geometric albedos for the three groups are 0.11 ± 0.06, 0.05 ± 0.04, and 0.23 ± 0.11, respectively, for the case where heat is uniformly distributed about the planet. If heat redistribution is inefficient, the albedos are even lower, since there will be a greater thermal contribution to the total light from the planet. We confirm that newly identified false-positive Kepler Object of Interest (KOI) 1662.01 is indeed an eclipsing binary at twice the period listed in the planet candidate catalog. We also newly identify planet candidate KOI 4351.01 as an eclipsing binary, and we report a secondary eclipse measurement for Kepler-4b (KOI 7.01) of ~7.50 ppm at a phase of ~0.7, indicating that the planet is on an eccentric orbit.

Directly-Imaged Planet HD 131399 Ab may be a False Positive

Evidence that the Directly-Imaged Planet HD 131399 Ab is a Background Star

Authors:

Nielsen et al

Abstract:
We present evidence that the recently discovered, directly-imaged planet HD 131399 Ab is a background star with non-zero proper motion. From new JHK1L' photometry and spectroscopy obtained with the Gemini Planet Imager, VLT/SPHERE, and Keck/NIRC2, and a reanalysis of the discovery data obtained with VLT/SPHERE, we derive colors, spectra, and astrometry for HD 131399 Ab. The broader wavelength coverage and higher data quality allow us to re-investigate its status. Its near-infrared spectral energy distribution excludes spectral types later than L0 and is consistent with a K or M dwarf, which are the most likely candidates for a background object in this direction at the apparent magnitude observed. If it were a physically associated object, the projected velocity of HD 131399 Ab would exceed escape velocity given the mass and distance to HD 131399 A. We show that HD 131399 Ab is also not following the expected track for a stationary background star at infinite distance. Solving for the proper motion and parallax required to explain the relative motion of HD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared to predicted background objects drawn from a galactic model, we find this proper motion to be high, but consistent with the top 4% fastest-moving background stars. From our analysis we conclude that HD 131399 Ab is a background K or M dwarf.

M Dwarfs are Masquerading as Hot Jupiters and Brown Dwarfs

The EBLM Project IV. Spectroscopic orbits of over 100 eclipsing M dwarfs masquerading as transiting hot-Jupiters

Authors:

Amaury et al

Abstract:
We present 2271 radial velocity measurements taken on 118 single-line binary stars, taken over eight years with the CORALIE spectrograph. The binaries consist of F/G/K primaries and M-dwarf secondaries. They were initially discovered photometrically by the WASP planet survey, as their shallow eclipses mimic a hot-Jupiter transit. The observations we present permit a precise characterisation of the binary orbital elements and mass function. With modelling of the primary star this mass function is converted to a mass of the secondary star. In the future, this spectroscopic work will be combined with precise photometric eclipses to draw an empirical mass/radius relation for the bottom of the mass sequence. This has applications in both stellar astrophysics and the growing number of exoplanet surveys around M-dwarfs. In particular, we have discovered 34 systems with a secondary mass below 0.2M⊙, and so we will ultimately double the known number of very low-mass stars with well characterised mass and radii.

We are able to detect eccentricities as small as 0.001 and orbital periods to sub-second precision. Our sample can revisit some earlier work on the tidal evolution of close binaries, extending it to low mass ratios. We find some binaries that are eccentric at orbital periods < 3 days, while our longest circular orbit has a period of 10.4 days.

By collating the EBLM binaries with published WASP planets and brown dwarfs, we derive a mass spectrum with twice the resolution of previous work. We compare the WASP/EBLM sample of tightly-bound orbits with work in the literature on more distant companions up to 10 AU. We note that the brown dwarf desert appears wider, as it carves into the planetary domain for our short-period orbits. This would mean that a significantly reduced abundance of planets begins at ∼3MJup, well before the Deuterium-burning limit.

Could Tabby's Star's Dimming be due to a Ring of Debris in OUR Solar System?

Tabetha's Rings

Author:

Katz

Abstract:
Could the dips of "Tabetha's Star" (KIC 8462852) have been caused by matter in our Solar System? The interval between periods of deep dips is nearly twice the orbital period of the Kepler satellite. I consider a clumpy particulate ring in the outer Solar System that grazes the line of sight to the star once per orbit of Kepler. The hypothesis predicts that future dips may be observed from Earth during windows separated by a year, although their detailed structure depends on the distribution of particles along the ring. Dips observed at separated sites will be decorrelated, with correlation lengths ≲1012 cm, and possibly as short as ∼600 m.

A New Yield Simulator for Transiting Planets and False Positives: Application to the Next Generation Transit Survey

A New Yield Simulator for Transiting Planets and False Positives: Application to the Next Generation Transit Survey

Authors:

Günther et al

Abstract:

We present a yield simulator to predict the number and characteristics of planets, false positives and false alarms in transit surveys. The simulator is based on a galactic model and the planet occurrence rates measured by the Kepler mission. It takes into account the observation window function and measured noise levels of the investigated survey. Additionally, it includes vetting criteria to identify false positives. We apply this simulator to the Next Generation Transit Survey (NGTS), a wide-field survey designed to detect transiting Neptune-sized exoplanets. We find that red noise is the main limitation of NGTS up to 14th magnitude, and that its obtained level determines the expected yield. Assuming a red noise level of 1mmag, the simulation predicts the following for a four-year survey: 4 ± 3 Super-Earths, 19 ± 5 Small Neptunes, 16 ± 4 Large Neptunes, 55 ± 8 Saturn-sized planets and 150 ± 10 Jupiter-sized planets, along with 4688 ± 45 eclipsing binaries and 843 ± 75 background eclipsing binaries. We characterize the properties of these objects to enhance the early identification of false positives and discuss follow-up strategies for transiting candidates.

Is Chromospheric Emission a way to Find False Positive hot Jupiters?

Chromospheric emission of planet candidate systems - a way to identify false positives

Authors:

Karoff et al

Abstract:

It has been hypothesized that the presence of closely orbiting giant planets is associated with enhanced chromospheric emission of their host stars. The main cause for such a relation would likely be enhanced dynamo action induced by the planet. We present measurements of chromospheric emission in 234 planet candidate systems from the Kepler mission. This ensemble includes 37 systems with giant planet candidates, which show a clear emission enhancement. The enhancement, however, disappears when systems which are also identified as eclipsing binary candidates are removed from the ensemble. This suggests that a large fraction of the giant planet candidate systems with chromospheric emission stronger than the Sun are not giant planet system, but false positives. Such false-positive systems could be tidally interacting binaries with strong chromospheric emission. This hypotesis is supported by an analysis of 188 eclipsing binary candidates that show increasing chromospheric emission as function of decreasing orbital period.

Are the NY Vir Circumbinary Exoplanets False Positives?

Observing NY Vir and the quest for circumbinary planets

Authors:

Pulley et al

Abstract:

We add 9 new observations of NY Vir and identify four others from AASVO database. Our results indicste that the one and two exo-planet predictions made by earlier authors do match these new results.

Host Star HD 189733 may Interferring With Studies of an Atmosphere

The origin of the excess transit absorption in the HD 189733 system: planet or star?

Authors:

Barnes et al

Abstract:

We have detected excess absorption in the emission cores of Ca II H & K during transits of HD 189733b for the first time. Using observations of three transits we investigate the origin of the absorption, which is also seen in H{\alpha} and the Na I D lines. Applying differential spectrophotometry methods to the Ca II H and Ca II K lines combined, using respective passband widths of Δλ = 0.4 & 0.6 \AA\ yields excess absorption of td = 0.0074 ± 0.0044 (1.7σ; Transit 1) and 0.0214 +/- 0.0022 (9.8σ; Transit 2). Similarly, we detect excess H{\alpha} absorption in a passband of width Δλ = 0.7 \AA, with td = 0.0084 ± 0.0016 (5.2σ) and 0.0121 ± 0.0012 (9.9σ). For both lines, Transit 2 is thus significantly deeper. Combining all three transits for the Na I D lines yields excess absorption of td = 0.0041 ± 0.0006 (6.5σ). By considering the time series observations of each line, we find that the excess apparent absorption is best recovered in the stellar reference frame. These findings lead us to postulate that the main contribution to the excess transit absorption in the differential light curves arises because the normalising continuum bands form in the photosphere, whereas the line cores contain a chromospheric component. We can not rule out that part of the excess absorption signature arises from the planetary atmosphere, but we present evidence which casts doubt on recent claims to have detected wind motions in the planet's atmosphere in these data.

MOST did not Detect Proxima Centauri b Transits

No Conclusive Evidence for Transits of Proxima b in MOST photometry

Authors:

Kipping et al

Abstract:

The analysis of Proxima Centauri's radial velocities recently led Anglada-Escud\'e et al. (2016) to claim the presence of a low mass planet orbiting the Sun's nearest star once every 11.2 days. Although the a-priori probability that Proxima b transits its parent star is just 1.5%, the potential impact of such a discovery would be considerable. Independent of recent radial velocity efforts, we observed Proxima Centauri for 12.5 days in 2014 and 31 days in 2015 with the MOST space telescope. We report here that we cannot make a compelling case that Proxima b transits in our precise photometric time series. Imposing an informative prior on the period and phase, we do detect a candidate signal with the expected depth. However, perturbing the phase prior across 100 evenly spaced intervals reveals one strong false-positive and one weaker instance. We estimate a false-positive rate of at least a few percent and a much higher false-negative rate of 20-40%, likely caused by the very high flare rate of Proxima Centauri. Comparing our candidate signal to HATSouth ground-based photometry reveals that the signal is somewhat, but not conclusively, disfavored (1-2 sigmas) leading us to argue that the signal is most likely spurious. We expect that infrared photometric follow-up could more conclusively test the existence of this candidate signal, owing to the suppression of flare activity and the impressive infrared brightness of the parent star.

Effects of disc asymmetries on astrometric measurements - Can they mimic planets?

Effects of disc asymmetries on astrometric measurements - Can they mimic planets?

Authors:

Kral et al

Abstract:

Astrometry covers a parameter space that cannot be reached by RV or transit methods to detect terrestrial planets on wide orbits. In addition, high accuracy astrometric measurements are necessary to measure the inclination of the planet's orbits. Here we investigate the principles of an artefact of the astrometric approach. Namely, the displacement of the photo-centre due to inhomogeneities in a dust disc around the parent star. Indeed, theory and observations show that circumstellar discs can present strong asymmetries. We model the pseudo-astrometric signal caused by these inhomogeneities, asking whether a dust clump in a disc can mimic the astrometric signal of an Earth-like planet. We show that these inhomogeneities cannot be neglected when using astrometry to find terrestrial planets. We provide the parameter space for which these inhomogeneities can affect the astrometric signals but still not be detected by mid-IR observations. We find that a small cross section of dust corresponding to a cometary mass object is enough to mimic the astrometric signal of an Earth-like planet. Astrometric observations of protoplanetary discs to search for planets can also be affected by the presence of inhomogeneities. Some further tests are given to confirm whether an observation is a real planet astrometric signal or an impostor. Eventually, we also study the case where the cross section of dust is high enough to provide a detectable IR-excess and to have a measurable photometric displacement by actual instruments such as Gaia, IRAC or GRAVITY. We suggest a new method, which consists in using astrometry to quantify asymmetries (clumpiness) in inner debris discs that cannot be otherwise resolved.

Sorting Through Radial Velocity Noise to Detect Exoplanets Around M Dwarfs

A Goldilocks principle for modeling radial velocity noise

Authors:

Feng et al

Abstract:

The doppler measurements of stars are diluted and distorted by stellar activity noise. Different choices of noise models and statistical methods have led to much controversy in the confirmation of exoplanet candidates obtained through analysing radial velocity data. To quantify the limitation of various models and methods, we compare different noise models and signal detection criteria for various simulated and real data sets in the Bayesian framework. According to our analyses, the white noise model tend to interpret noise as signal, leading to false positives. On the other hand, the red noise models are likely to interprete signal as noise, resulting in false negatives. We find that the Bayesian information criterion combined with a Bayes factor threshold of 150 can efficiently rule out false positives and confirm true detections. We further propose a Goldilocks principle aimed at modeling radial velocity noise to avoid too many false positives and too many false negatives. We propose that the noise model with RHK-dependent jitter is used in combination with the moving average model to detect planetary signals for M dwarfs. Our work may also shed light on the noise modeling for hotter stars, and provide a valid approach for finding similar principles in other disciplines.

OGLE-2013-BLG-0723: The Circum Brown Dwarf Orbiting Venus Might be a False Positive

A New Non-Planetary Interpretation of the Microlensing Event OGLE-2013-BLG-0723

Authors:

Han et al

Abstract:

Recently, the discovery of a Venus-mass planet orbiting a brown-dwarf host in a binary system was reported from the analysis of the microlensing event OGLE-2013-BLG-0723. We reanalyze the event considering the possibility of other interpretations. From this, we find a new solution where the lens is composed of 2 bodies in contrast to the 3-body solution of the previous analysis. The new solution better explains the observed light curve than the previous solution with Δχ2∼202, suggesting that the new solution is a correct model for the event. From the estimation of the physical parameters based on the new interpretation, we find that the lens system is composed of two low-mass stars with ∼0.2 M⊙ and ∼0.1 M⊙ and located at a distance ∼3 kpc. The fact that the physical parameters correspond to those of the most common lens population located at a distance with a large lensing probability further supports the likelihood of the new interpretation. Considering that two dramatically different solutions can approximately explain the observed light curve, the event suggests the need of carefully testing all possible lens-system geometries.

Radial Velocity Planet Detection Biases at the Stellar Rotational Period

Radial Velocity Planet Detection Biases at the Stellar Rotational Period

Authors:

Venderburg et al

Abstract:

Future generations of precise radial velocity (RV) surveys aim to achieve sensitivity sufficient to detect Earth mass planets orbiting in their stars’ habitable zones. A major obstacle to this goal is astrophysical radial velocity noise caused by active areas moving across the stellar limb as a star rotates. In this paper, we quantify how stellar activity impacts exoplanet detection with radial velocities as a function of orbital and stellar rotational periods. We perform data-driven simulations of how stellar rotation affects planet detectability and compile and present relations for the typical timescale and amplitude of stellar radial velocity noise as a function of stellar mass. We show that the characteristic timescales of quasi-periodic radial velocity jitter from stellar rotational modulations coincides with the orbital period of habitable zone exoplanets around early M-dwarfs. These coincident periods underscore the importance of monitoring the targets of RV habitable zone planet surveys through simultaneous photometric measurements for determining rotation periods and activity signals, and mitigating activity signals using spectroscopic indicators and/or RV measurements at different wavelengths.

HD 219134 has a 12 Year Solar Acitivity Cycle

A 12-Year Activity Cycle for HD 219134

Authors:

Johnson et al

Abstract:

The nearby (6.5 pc) star HD 219134 was recently shown by Motalebi et al. (2015) and Vogt et al. (2015) to host several planets, the innermost of which is transiting. We present twenty-seven years of radial velocity observations of this star from the McDonald Observatory Planet Search program, and nineteen years of stellar activity data. We detect a long-period activity cycle measured in the Ca II SHK index, with a period of 4230±100 days (11.7 years), very similar to the 11-year Solar activity cycle. Although the period of the Saturn-mass planet HD 219134 h is close to half that of the activity cycle, we argue that it is not an artifact due to stellar activity. We also find a significant periodicity in the SHK data due to stellar rotation with a period of 22.8 days. This is identical to the period of planet f identified by Vogt et al. (2015), suggesting that this radial velocity signal might be caused by rotational modulation of stellar activity rather than a planet. Analysis of our radial velocities allows us to detect the long-period planet HD 219134 h and the transiting super-Earth HD 219134 b. Finally, we use our long time baseline to constrain the presence of longer-period planets in the system, excluding to 1σ objects with Msini greater than 0.36MJ at 12 years (corresponding to the orbital period of Jupiter) and Msini greater than 0.72MJ at a period of 16.4 years (assuming a circular orbit for an outer companion).

Identifying False Alarms in the Kepler Planet Candidate Catalog

Identifying False Alarms in the Kepler Planet Candidate Catalog

Authors:

Mullally et al

Abstract:

We present a new automated method to identify instrumental features masquerading as small, long period planets in the \kepler\ planet candidate catalog. These systematics, mistakenly identified as planet transits, can have a strong impact on occurrence rate calculations because they cluster in a region of parameter space where Kepler's sensitivity to planets is poor. We compare individual transit-like events to a variety of models of real transits and systematic events, and use a Bayesian Information Criterion to evaluate the likelihood that each event is real. We describe our technique and test its performance on simulated data. Results from this technique are incorporated in the \kepler\ Q1-17 DR24 planet candidate catalog of \citet{Coughlin15}.

PTFO 8-8695b is Probably a False Positive

A reappraisal of parameters for the putative planet PTFO 8-8695b and its potentially precessing parent star

Author:

Howarth

Abstract:

Published photometry of fading events in the PTFO 8-8695 system is modelled using improved treatments of stellar geometry, surface intensities, and, particularly, gravity darkening, with a view to testing the planetary-transit hypothesis. Variability in the morphology of fading events can be reproduced by adopting convective-envelope gravity darkening, but near-critical stellar rotation is required. This leads to inconsistencies with spectroscopic observations; the model also predicts substantial photometric variability associated with stellar precession, contrary to observations. Furthermore, the empirical ratio of orbital to rotational angular momenta is at odds with physically plausible values. An exoplanet transiting a precessing, gravity-darkened star may not be the correct explanation of periodic fading events in this system.

KIC 8462852 did NOT Fade During the Last 100 Years (hint: calibration is IMPORTANT!)

KIC 8462852 did likely not fade during the last 100 years

Authors:

Hippke et al

Abstract:

A recent analysis found a "completely unprecedented" dimming of 0.165±0.013 magnitudes per century in the F3 main sequence star KIC8462852. This star is interesting, as it shows episodes of day-long dips with up to 20% dimming of unknown origin. We re-analyze the same Harvard archival Johnson B photometry and find comparable dimmings, and structural breaks, for 18 of 28 checked F-dwards (64%) in the Kepler field of view. We conclude that the Harvard plates photometry suffers from imperfect long-term (1890--1989) calibration. The most likely explanation for the century-long dimming of KIC8462852 is thus a data artefact, and it is probably not of astrophysical origin.

Enigmatic and Ephemeral M Dwarf System KOI 6705 Might be a Very Strange False Positive

The Enigmatic and Ephemeral M Dwarf System KOI 6705: Cheshire Cat or Wild Goose?

Authors:

Gaidos et al

Abstract:

We confirm a 0.995 d periodic planetary transit-like signal, KOI 6705.01, in the Kepler lightcurve of the star KIC 6423922. Optical and infrared spectra show that this star is a mid M-type dwarf with an effective temperature =3327±60K, metallicity [Fe/H] =−0.08±0.10, radius =0.31±0.03R⊙, and mass =0.28±0.05M⊙. The star is ≈70 pc away and its space motion, rotation period, and lack of Hα emission indicate it is an older member of the "thin disk" population. On the other hand, the star exhibits excess infrared emission suggesting a dust disk more typical of a very young star. If the KOI 6705.01 signal is produced by a planet, the transit depth of 60 ppm means its radius is only 0.26+0.034−0.029R⊕, or about the size of the Moon. However, the duration (≳3~hr) and time variation of KOI 6705.01 are anomalous: the signal was undetected in the first two years of the mission and increased through the latter two years. These characteristics require implausible orbits and material properties for any planet and rule out such an explanation, although a dust cloud is possible. We excluded several false positive scenarios including background stars, scattered light from stars that are nearby on the sky, and electronic cross-talk between detector readout channels. We find the most likely explanation to be that KOI 6705.01 is a false positive created by charge transfer inefficiency in a detector column on which KIC 6423922 and a 1.99 d eclipsing binary both happened to fall.