arXiv subjects

David Charbonneau

Publications and source records attributed to David Charbonneau.

At least 145 records · Page 8Linked to original sources

arXiv2018

TESS Discovery of a Transiting Super-Earth in the $π$ Mensae System

We report the detection of a transiting planet around $π$ Mensae (HD 39091), using data from the Transiting Exoplanet Survey Satellite (TESS). The solar-type host star is unusually bright (V=5.7) and was already known to host a Jovian planet on a highly eccentric, 5.7-year orbit. The newly discovered planet has a size of $2.04\pm 0.05$ $R_\oplus$ and an orbital period of 6.27 days. Radial-velocity data from the HARPS and AAT/UCLES archives also displays a 6.27-day periodicity, confirming the existence of the planet and leading to a mass determination of $4.82\pm 0.85$ $M_\oplus$. The star's proximity and brightness will facilitate further investigations, such as atmospheric spectroscopy, asteroseismology, the Rossiter--McLaughlin effect, astrometry, and direct imaging.

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arXiv2018

TESS Discovery of an ultra-short-period planet around the nearby M dwarf LHS 3844

Data from the newly-commissioned \textit{Transiting Exoplanet Survey Satellite} (TESS) has revealed a "hot Earth" around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of $1.32\pm 0.02$ $R_\oplus$ and orbits the star every 11 hours. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough ($I=11.9$, $K=9.1$) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy.

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arXiv2018

A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization

A key legacy of the recently launched TESS mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. JWST, although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as ARIEL require the discovery and confirmation of several hundred additional sub-Jovian size planets (R_p < 10 R_Earth) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based ELTs will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al. (2015), we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of $\sim300$ high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.

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arXiv2018

Four new eclipsing mid M-dwarf systems from the New Luyten Two Tenths catalog

Using data from the MEarth-North and MEarth-South transit surveys, we present the detection of eclipses in four mid M-dwarf systems: LP 107-25, LP 261-75, LP 796-24, and LP 991-15. Combining the MEarth photometry with spectroscopic follow-up observations, we show that LP 107-25 and LP 796-24 are short-period (1.388 and 0.523 day, respectively) eclipsing binaries in triple-lined systems with substantial third light contamination from distant companions. LP 261-75 is a short-period (1.882 day) single-lined system consisting of a mid M-dwarf eclipsed by a probable brown dwarf secondary, with another distant visual brown dwarf companion. LP 991-15 is a long-period (29.3 day) double-lined eclipsing binary on an eccentric orbit with a geometry which produces only primary eclipses. A spectroscopic orbit is given for LP 991-15, and initial orbits for LP 107-25 and LP 261-75.

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arXiv2018

New rotation period measurements for M dwarfs in the southern hemisphere: an abundance of slowly rotating, fully convective stars

Stellar rotation periods are valuable both for constraining models of angular momentum loss and for under- standing how magnetic features impact inferences of exoplanet parameters. Building on our previous work in the northern hemisphere, we have used long-term, ground-based photometric monitoring from the MEarth Observatory to measure 234 rotation periods for nearby, southern hemisphere M dwarfs. Notable examples include the exoplanet hosts GJ 1132, LHS 1140, and Proxima Centauri. We find excellent agreement between our data and K2 photometry for the overlapping subset. Amongst the sample of stars with the highest quality datasets, we recover periods in 66%; as the length of the dataset increases, our recovery rate approaches 100%. The longest rotation periods we detect are around 140 days, which we suggest represent the periods that are reached when M dwarfs are as old as the local thick disk (about 9 Gyr).

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arXiv2018

Ground-based optical transmission spectroscopy of the small, rocky exoplanet GJ 1132b

Terrestrial Solar System planets either have high mean molecular weight atmospheres, as with Venus, Mars, and Earth, or no atmosphere at all, as with Mercury. We do not have sufficient observational information to know if this is typical of terrestrial planets or a phenomenon unique to the Solar System. The bulk of atmospheric exoplanet studies have focused on hot Jupiters and Neptunes, but recent discoveries of small, rocky exoplanets transiting small, nearby stars provide targets that are amenable to atmospheric study. GJ 1132b has a radius of 1.2 Earth radii and a mass of 1.6 Earth masses, and orbits an M-dwarf 12 parsecs away from the Solar System. We present results from five transits of GJ 1132b taken with the Magellan Clay Telescope and the LDSS3C multi-object spectrograph. We jointly fit our five data sets when determining the best-fit transit parameters both for the white light curve and wavelength-binned light curves. We bin the light curves into 20 nm wavelength bands to construct the transmission spectrum. Our results disfavor a clear, 10x solar metallicity atmosphere at 3.7 sigma confidence and a 10% H2O, 90% H2 atmosphere at 3.5 sigma confidence. Our data are consistent with a featureless spectrum, implying that GJ 1132b has a high mean molecular weight atmosphere or no atmosphere at all, though we do not account for the possible presence of aerosols. This result is in agreement with theoretical work which suggests that a planet of GJ 1132b's mass and insolation should not be able to retain a H2 envelope.

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arXiv2018

Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog With Measured Completeness and Reliability Based on Data Release 25

We present the Kepler Object of Interest (KOI) catalog of transiting exoplanets based on searching four years of Kepler time series photometry (Data Release 25, Q1-Q17). The catalog contains 8054 KOIs of which 4034 are planet candidates with periods between 0.25 and 632 days. Of these candidates, 219 are new and include two in multi-planet systems (KOI-82.06 and KOI-2926.05), and ten high-reliability, terrestrial-size, habitable zone candidates. This catalog was created using a tool called the Robovetter which automatically vets the DR25 Threshold Crossing Events (TCEs, Twicken et al. 2016). The Robovetter also vetted simulated data sets and measured how well it was able to separate TCEs caused by noise from those caused by low signal-to-noise transits. We discusses the Robovetter and the metrics it uses to sort TCEs. For orbital periods less than 100 days the Robovetter completeness (the fraction of simulated transits that are determined to be planet candidates) across all observed stars is greater than 85%. For the same period range, the catalog reliability (the fraction of candidates that are not due to instrumental or stellar noise) is greater than 98%. However, for low signal-to-noise candidates between 200 and 500 days around FGK dwarf stars, the Robovetter is 76.7% complete and the catalog is 50.5% reliable. The KOI catalog, the transit fits and all of the simulated data used to characterize this catalog are available at the NASA Exoplanet Archive.

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arXiv2018

LHS 1610A: A Nearby Mid-M Dwarf with a Companion That is Likely A Brown Dwarf

We present the spectroscopic orbit of LHS 1610A, a newly discovered single-lined spectroscopic binary with a trigonometric distance placing it at 9.9 pm 0.2 pc. We obtained spectra with the TRES instrument on the 1.5m Tillinghast Reflector at the Fred Lawrence Whipple Observatory located on Mt. Hopkins in AZ. We demonstrate the use of the TiO molecular bands at 7065 -- 7165 Angstroms to measure radial velocities and achieve an average estimated velocity uncertainty of 28 m/s. We measure the orbital period to be 10.6 days and calculate a minimum mass of 44.8 pm 3.2 Jupiter masses for the secondary, indicating that it is likely a brown dwarf. We place an upper limit to 3 sigma of 2500 K on the effective temperature of the companion from infrared spectroscopic observations using IGRINS on the 4.3m Discovery Channel Telescope. In addition, we present a new photometric rotation period of 84.3 days for the primary star using data from the MEarth-South Observatory, with which we show that the system does not eclipse.

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arXiv2018

An ultra-short period rocky super-Earth with a secondary eclipse and a Neptune-like companion around K2-141

Ultra-short period (USP) planets are a class of low mass planets with periods shorter than one day. Their origin is still unknown, with photo-evaporation of mini-Neptunes and in-situ formation being the most credited hypotheses. Formation scenarios differ radically in the predicted composition of USP planets, it is therefore extremely important to increase the still limited sample of USP planets with precise and accurate mass and density measurements. We report here the characterization of an USP planet with a period of 0.28 days around K2-141 (EPIC 246393474), and the validation of an outer planet with a period of 7.7 days in a grazing transit configuration. We derived the radii of the planets from the K2 light curve and used high-precision radial velocities gathered with the HARPS-N spectrograph for mass measurements. For K2-141b we thus inferred a radius of $1.51\pm0.05~R_\oplus$ and a mass of $5.08\pm0.41~M_\oplus$, consistent with a rocky composition and lack of a thick atmosphere. K2-141c is likely a Neptune-like planet, although due to the grazing transits and the non-detection in the RV dataset, we were not able to put a strong constraint on its density. We also report the detection of secondary eclipses and phase curve variations for K2-141b. The phase variation can be modeled either by a planet with a geometric albedo of $0.30 \pm 0.06$ in the Kepler bandpass, or by thermal emission from the surface of the planet at $\sim$3000K. Only follow-up observations at longer wavelengths will allow us to distinguish between these two scenarios.

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arXiv2017

The Science Case for an Extended Spitzer Mission

Although the final observations of the Spitzer Warm Mission are currently scheduled for March 2019, it can continue operations through the end of the decade with no loss of photometric precision. As we will show, there is a strong science case for extending the current Warm Mission to December 2020. Spitzer has already made major impacts in the fields of exoplanets (including microlensing events), characterizing near Earth objects, enhancing our knowledge of nearby stars and brown dwarfs, understanding the properties and structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to study galaxy birth and evolution. By extending Spitzer through 2020, it can continue to make ground-breaking discoveries in those fields, and provide crucial support to the NASA flagship missions JWST and WFIRST, as well as the upcoming TESS mission, and it will complement ground-based observations by LSST and the new large telescopes of the next decade. This scientific program addresses NASA's Science Mission Directive's objectives in astrophysics, which include discovering how the universe works, exploring how it began and evolved, and searching for life on planets around other stars.

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arXiv2017

Precise Masses in the WASP-47 System

We present precise radial velocity observations of WASP-47, a star known to host a hot Jupiter, a distant Jovian companion, and, uniquely, two additional transiting planets in short-period orbits: a super-Earth in a ~19 hour orbit, and a Neptune in a ~9 day orbit. We analyze our observations from the HARPS-N spectrograph along with previously published data to measure the most precise planet masses yet for this system. When combined with new stellar parameters and reanalyzed transit photometry, our mass measurements place strong constraints on the compositions of the two small planets. We find unlike most other ultra-short-period planets, the inner planet, WASP-47 e, has a mass (6.83 +/- 0.66 Me) and radius (1.810 +/- 0.027 Re) inconsistent with an Earth-like composition. Instead, WASP-47 e likely has a volatile-rich envelope surrounding an Earth-like core and mantle. We also perform a dynamical analysis to constrain the orbital inclination of WASP-47 c, the outer Jovian planet. This planet likely orbits close to the plane of the inner three planets, suggesting a quiet dynamical history for the system. Our dynamical constraints also imply that WASP-47 c is much more likely to transit than a geometric calculation would suggest. We calculate a transit probability for WASP-47 c of about 10%, more than an order of magnitude larger than the geometric transit probability of 0.6%.

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arXiv2017

Three's Company: An additional non-transiting super-Earth in the bright HD 3167 system, and masses for all three planets

HD 3167 is a bright (V = 8.9), nearby K0 star observed by the NASA K2 mission (EPIC 220383386), hosting two small, short-period transiting planets. Here we present the results of a multi-site, multi-instrument radial velocity campaign to characterize the HD 3167 system. The masses of the transiting planets are 5.02+/-0.38 MEarth for HD 3167 b, a hot super-Earth with a likely rocky composition (rho_b = 5.60+2.15-1.43 g/cm^3), and 9.80+1.30-1.24 MEarth for HD 3167 c, a warm sub-Neptune with a likely substantial volatile complement (rho_c = 1.97+0.94-0.59 g/cm^3). We explore the possibility of atmospheric composition analysis and determine that planet c is amenable to transmission spectroscopy measurements, and planet b is a potential thermal emission target. We detect a third, non-transiting planet, HD 3167 d, with a period of 8.509+/-0.045 d (between planets b and c) and a minimum mass of 6.90+/-0.71 MEarth. We are able to constrain the mutual inclination of planet d with planets b and c: we rule out mutual inclinations below 1.3 degrees as we do not observe transits of planet d. From 1.3-40 degrees, there are viewing geometries invoking special nodal configurations which result in planet d not transiting some fraction of the time. From 40-60 degrees, Kozai-Lidov oscillations increase the system's instability, but it can remain stable for up to 100Myr. Above 60 degrees, the system is unstable. HD 3167 promises to be a fruitful system for further study and a preview of the many exciting systems expected from the upcoming NASA TESS mission.

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arXiv2017

A temperate rocky super-Earth transiting a nearby cool star

M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy [1]. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars [2,3]. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away [4]. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf [5], but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away [6,7], but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone [8]. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth [9,10]. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future [2,3].

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arXiv2017

The Kepler-19 system: a thick-envelope super-Earth with two Neptune-mass companions characterized using Radial Velocities and Transit Timing Variations

We report a detailed characterization of the Kepler-19 system. This star was previously known to host a transiting planet with a period of 9.29 days, a radius of 2.2 R$_\oplus$ and an upper limit on the mass of 20 M$_\oplus$. The presence of a second, non-transiting planet was inferred from the transit time variations (TTVs) of Kepler-19b, over 8 quarters of Kepler photometry, although neither mass nor period could be determined. By combining new TTVs measurements from all the Kepler quarters and 91 high-precision radial velocities obtained with the HARPS-N spectrograph, we measured through dynamical simulations a mass of $8.4 \pm 1.6$ M$_\oplus$ for Kepler-19b. From the same data, assuming system coplanarity, we determined an orbital period of 28.7 days and a mass of $13.1 \pm 2.7$ M$_\oplus$ for Kepler-19c and discovered a Neptune-like planet with a mass of $20.3 \pm 3.4$ M$_\oplus$ on a 63 days orbit. By comparing dynamical simulations with non-interacting Keplerian orbits, we concluded that neglecting interactions between planets may lead to systematic errors that could hamper the precision in the orbital parameters when the dataset spans several years. With a density of $4.32 \pm 0.87$ g cm$^{-3}$ ($0.78 \pm 0.16$ $ρ_\oplus$) Kepler-19b belongs to the group of planets with a rocky core and a significant fraction of volatiles, in opposition to low-density planets characterized by transit-time variations only and the increasing number of rocky planets with Earth-like density. Kepler-19 joins the small number of systems that reconcile transit timing variation and radial velocity measurements.

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arXiv2017

The Transiting Exoplanet Survey Satellite: Simulations of planet detections and astrophysical false positives

The Transiting Exoplanet Survey Satellite (TESS) is a NASA-sponsored Explorer mission that will perform a wide-field survey for planets that transit bright host stars. Here, we predict the properties of the transiting planets that TESS will detect along with the eclipsing binary stars that produce false-positive photometric signals. The predictions are based on Monte Carlo simulations of the nearby population of stars, occurrence rates of planets derived from Kepler, and models for the photometric performance and sky coverage of the TESS cameras. We expect that TESS will find approximately 1700 transiting planets from 200,000 pre-selected target stars. This includes 556 planets smaller than twice the size of Earth, of which 419 are hosted by M dwarf stars and 137 are hosted by FGK dwarfs. Approximately 130 of the $R < 2~R_\oplus$ planets will have host stars brighter than K = 9. Approximately 48 of the planets with $R < 2~R_\oplus$ lie within or near the habitable zone ($0.2 < S/S_\oplus < 2$), and between 2-7 such planets have host stars brighter than K = 9. We also expect approximately 1100 detections of planets with radii 2-4 R_Earth, and 67 planets larger than $4~R_\oplus$. Additional planets larger than $2~R_\oplus$ can be detected around stars that are not among the pre-selected target stars, because TESS will also deliver full-frame images at a 30-minute cadence. The planet detections are accompanied by over one thousand astrophysical false positives. We discuss how TESS data and ground-based observations can be used to distinguish the false positives from genuine planets. We also discuss the prospects for follow-up observations to measure the masses and atmospheres of the TESS planets.

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arXiv2017

Two massive rocky planets transiting a K-dwarf 6.5 parsecs away

HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several low-mass planets were recently discovered. The Spitzer space telescope detected a transit of the innermost of these planets, HD 219134 b, whose mass and radius (4.5 MEarth and 1.6 REarth respectively) are consistent with a rocky composition. Here, we report new highprecision time-series photometry of the star acquired with Spitzer revealing that the second innermost planet of the system, HD 219134 c, is also transiting. A global analysis of the Spitzer transit light curves and the most up-to-date HARPS-N velocity data set yields mass and radius estimations of 4.74+-0.19 MEarth and 1.602+-0.055 REarth for HD 219134 b, and of 4.36+-0.22 MEarth and 1.511+-0.047 REarth for HD 219134 c. These values suggest rocky compositions for both planets. Thanks to the proximity and the small size of their host star (0.778+-0.005 Rsun), these two transiting exoplanets - the nearest to the Earth to date - are well-suited for a detailed characterization (precision of a few percent on mass and radius, constraints on the atmospheric properties...) that could give important constraints on the nature and formation mechanism of the ubiquitous short-period planets of a few Earth masses.

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arXiv2017

Characterizing K2 Candidate Planetary Systems Orbiting Low-Mass Stars I: Classifying Low-mass Host Stars Observed During Campaigns 1-7

We present near-infrared spectra for 144 candidate planetary systems identified during Campaigns 1-7 of the NASA K2 Mission. The goal of the survey was to characterize planets orbiting low-mass stars, but our IRTF/SpeX and Palomar/TripleSpec spectroscopic observations revealed that 49% of our targets were actually giant stars or hotter dwarfs reddened by interstellar extinction. For the 72 stars with spectra consistent with classification as cool dwarfs (spectral types K3 - M4), we refined their stellar properties by applying empirical relations based on stars with interferometric radius measurements. Although our revised temperatures are generally consistent with those reported in the Ecliptic Plane Input Catalog (EPIC), our revised stellar radii are typically 0.13 solar radii (39%) larger than the EPIC values, which were based on model isochrones that have been shown to underestimate the radii of cool dwarfs. Our improved stellar characterizations will enable more efficient prioritization of K2 targets for follow-up studies.

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arXiv2016

A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100 Hour Spitzer Campaign

We present the results of a search for additional bodies in the GJ 1132 system through two methods: photometric transits and transit timing variations of the known planet. We collected 21 transit observations of GJ 1132b with the MEarth-South array since 2015. We obtained 100 near-continuous hours of observations with the $Spitzer$ Space Telescope, including two transits of GJ 1132b and spanning 60\% of the orbital phase of the maximum period at which bodies coplanar with GJ 1132b would pass in front of the star. We exclude transits of additional Mars-sized bodies, such as a second planet or a moon, with a confidence of 99.7\%. When we combine the mass estimate of the star (obtained from its parallax and apparent $K_s$ band magnitude) with the stellar density inferred from our high-cadence $Spitzer$ light curve (assuming zero eccentricity), we measure the stellar radius of GJ 1132 to be $0.2105^{+0.0102}_{-0.0085} R_\odot$, and we refine the radius measurement of GJ 1132b to $1.130 \pm 0.056 R_\oplus$. Combined with HARPS RV measurements, we determine the density of GJ 1132b to be $6.2 \pm 2.0$\ g cm$^{-3}$, with the mass determination dominating this uncertainty. We refine the ephemeris of the system and find no evidence for transit timing variations, which would be expected if there was a second planet near an orbital resonance with GJ 1132b.

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