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Raphaëlle D. Haywood

Publications and source records attributed to Raphaëlle D. Haywood.

At least 19 recordsLinked to original sources
arXiv2024

Characterizing Solar Center-to-Limb Radial-Velocity Variability with SDO

Stellar photospheric inhomogeneities are a significant source of noise which currently precludes the discovery of Earth-mass planets orbiting Sun-like stars with the radial-velocity (RV) method. To complement several previous studies which have used ground- and spaced-based facilities to characterize the RV of the Sun, we here characterize the center-to-limb variability (CLV) of solar RVs arising from various solar-surface inhomogeneities observed by SDO/HMI and SDO/AIA. By using various SDO observables to classify pixels and calculate line-of-sight velocities as a function of pixel classification and limb angle, we show that each identified feature type, including the umbrae and penumbrae of sunspots, quiet-Sun magnetoconvective cells, magnetic network, and plage, exhibit distinct and complex CLV signatures, including a notable limb-angle dependence in the observed suppression of convective blueshift for magnetically active regions. We discuss the observed distributions of velocities by identified region type and limb angle, offer interpretations of the physical phenomena that shape these distributions, and emphasize the need to understand the RV signatures of these regions as astrophysical signals, rather than simple (un)correlated noise processes.

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arXiv2023

Masses, Revised Radii, and a Third Planet Candidate in the "Inverted" Planetary System Around TOI-1266

Is the population of close-in planets orbiting M dwarfs sculpted by thermally driven escape or is it a direct outcome of the planet formation process? A number of recent empirical results strongly suggest the latter. However, the unique architecture of the TOI-1266 system presents a challenge to models of planet formation and atmospheric escape given its seemingly "inverted" architecture of a large sub-Neptune ($P_b=10.9$ days, $R_{p,b}=2.62\pm 0.11\, \mathrm{R}_{\oplus}$) orbiting interior to that of the system's smaller planet ($P_c=18.8$ days, $R_{p,c}=2.13\pm 0.12\, \mathrm{R}_{\oplus}$). Here we present revised planetary radii based on new TESS and diffuser-assisted ground-based transit observations, and characterize both planetary masses using a set of 145 radial velocity measurements from HARPS-N ($M_{p,b}=4.23\pm 0.69\, \mathrm{M}_{\oplus}, M_{p,c}=2.88\pm 0.80\, \mathrm{M}_{\oplus}$). Our analysis also reveals a third planet candidate ($P_d=32.3$ days, $M_{p,d}\sin{i} = 4.59^{+0.96}_{-0.94}\, \mathrm{M}_{\oplus}$), which if real, would form a chain of near 5:3 period ratios, although the system is likely not in a mean motion resonance. Our results indicate that TOI-1266 b and c are among the lowest density sub-Neptunes around M dwarfs and likely exhibit distinct bulk compositions of a gas-enveloped terrestrial ($X_{\mathrm{env},b}=5.5\pm 0.7$%) and a water-rich world (WMF$_c=59\pm 14$%), which is supported by hydrodynamic escape models. If distinct bulk compositions are confirmed through atmospheric characterization, the system's unique architecture would represent an interesting test case of inside-out sub-Neptune formation at pebble traps.

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arXiv2023

TOI-1695 b: A Water World Orbiting an Early M Dwarf in the Planet Radius Valley

Characterizing the bulk compositions of transiting exoplanets within the M dwarf radius valley offers a unique means to establish whether the radius valley emerges from an atmospheric mass loss process or is imprinted by planet formation itself. We present the confirmation of such a planet orbiting an early M dwarf ($T_{\rm mag} = 11.0294 \pm 0.0074, M_s = 0.513 \pm 0.012\ M_\odot, R_s = 0.515 \pm 0.015\ R_\odot, T_{\rm eff} =3690\pm 50 K$): TOI-1695 b ($P = 3.13$ days, $R_p = 1.90^{+0.16}_{-0.14}\ R_\oplus$). TOI-1695 b's radius and orbital period situate the planet between model predictions from thermally-driven mass loss versus gas depleted formation, offering an important test case for radius valley emergence models around early M dwarfs. We confirm the planetary nature of TOI-1695 b based on five sectors of TESS data and a suite of follow-up observations including 49 precise radial velocity measurements taken with the HARPS-N spectrograph. We measure a planetary mass of $6.36 \pm 1.00\ M_\oplus$, which reveals that TOI-1695 b is inconsistent with a purely terrestrial composition of iron and magnesium silicate, and instead is likely a water-rich planet. Our finding that TOI-1695 b is not terrestrial is inconsistent with the planetary system being sculpted by thermally driven mass loss. We present a statistical analysis of seven well-characterized planets within the M dwarf radius valley demonstrating that a thermally-driven mass loss scenario is unlikely to explain this population.

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arXiv2022

Identifying Exoplanets with Deep Learning. IV. Removing Stellar Activity Signals from Radial Velocity Measurements Using Neural Networks

Exoplanet detection with precise radial velocity (RV) observations is currently limited by spurious RV signals introduced by stellar activity. We show that machine learning techniques such as linear regression and neural networks can effectively remove the activity signals (due to starspots/faculae) from RV observations. Previous efforts focused on carefully filtering out activity signals in time using modeling techniques like Gaussian Process regression (e.g. Haywood et al. 2014). Instead, we systematically remove activity signals using only changes to the average shape of spectral lines, and no information about when the observations were collected. We trained our machine learning models on both simulated data (generated with the SOAP 2.0 software; Dumusque et al. 2014) and observations of the Sun from the HARPS-N Solar Telescope (Dumusque et al. 2015; Phillips et al. 2016; Collier Cameron et al. 2019). We find that these techniques can predict and remove stellar activity from both simulated data (improving RV scatter from 82 cm/s to 3 cm/s) and from more than 600 real observations taken nearly daily over three years with the HARPS-N Solar Telescope (improving the RV scatter from 1.753 m/s to 1.039 m/s, a factor of ~ 1.7 improvement). In the future, these or similar techniques could remove activity signals from observations of stars outside our solar system and eventually help detect habitable-zone Earth-mass exoplanets around Sun-like stars.

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arXiv2021

Radial velocity precision of ESPRESSO through the analysis of the solar twin HIP 11915

Different stellar phenomena affect radial velocities (RVs), causing variations large enough to make it difficult to identify planet signals from the stellar variability. RV variations caused by stellar oscillations and granulation can be reduced through some methods, but the impact of rotationally modulated magnetic activity on RV, due to stellar active regions is harder to correct. New instrumentation promises an improvement in precision of one order of magnitude, from about 1 m/s, to about 10 cm/s. In this context, we report our first results from 24 spectroscopic ESPRESSO/VLT observations of the solar twin star HIP 11915, spread over 60 nights. We used a Gaussian Process approach and found for HIP 11915 a RV residual RMS scatter of about 20 cm s$^{-1}$, representing an upper limit for the performance of ESPRESSO.

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arXiv2021

Extreme Precision Radial Velocity Working Group Final Report

Precise mass measurements of exoplanets discovered by the direct imaging or transit technique are required to determine planet bulk properties and potential habitability. Furthermore, it is generally acknowledged that, for the foreseeable future, the Extreme Precision Radial Velocity (EPRV) measurement technique is the only method potentially capable of detecting and measuring the masses and orbits of habitable-zone Earths orbiting nearby F, G, and K spectral-type stars from the ground. In particular, EPRV measurements with a precision of better than approximately 10 cm/s (with a few cm/s stability over many years) are required. Unfortunately, for nearly a decade, PRV instruments and surveys have been unable to routinely reach RV accuracies of less than roughly 1 m/s. Making EPRV science and technology development a critical component of both NASA and NSF program plans is crucial for reaching the goal of detecting potentially habitable Earthlike planets and supporting potential future exoplanet direct imaging missions such as the Habitable Exoplanet Observatory (HabEx) or the Large Ultraviolet Optical Infrared Surveyor (LUVOIR). In recognition of these facts, the 2018 National Academy of Sciences (NAS) Exoplanet Science Strategy (ESS) report recommended the development of EPRV measurements as a critical step toward the detection and characterization of habitable, Earth-analog planets. In response to the NAS-ESS recommendation, NASA and NSF commissioned the EPRV Working Group to recommend a ground-based program architecture and implementation plan to achieve the goal intended by the NAS. This report documents the activities, findings, and recommendations of the EPRV Working Group.

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arXiv2019

An 11 Earth-Mass, Long-Period Sub-Neptune Orbiting a Sun-like Star

Although several thousands of exoplanets have now been detected and characterized, observational biases have led to a paucity of long-period, low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets. In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b. This planet orbits a Sun-like star (V = 11.27) with M_* = 0.892 +/- (0.051, 0.035) M_sun and R_* = 0.8717 +/- (0.0064, 0.0061) R_sun. Kepler-538b is a 2.215 +/- (0.040, 0.034) R_earth sub-Neptune with a period of P = 81.73778 +/- 0.00013 d. It is the only known planet in the system. We collected radial velocity (RV) observations with HIRES on Keck I and HARPS-N on the TNG. We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross correlation function full width at half maximum (FWHM) observations. By simultaneously modeling Kepler photometry, RV, and FWHM observations, we found a semi-amplitude of K = 1.68 +/- (0.39, 0.38) m s^-1 and a planet mass of M_p = 10.6 +/- (2.5, 2.4) M_earth. Kepler-538b is the smallest planet beyond P = 50 d with an RV mass measurement. The planet likely consists of a significant fraction of ices (dominated by water ice), in addition to rocks/metals, and a small amount of gas. Sophisticated modeling techniques such as those used in this paper, combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime. This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature.

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arXiv2019

Exoplanet Imitators: A test of stellar activity behavior in radial velocity signals

Accurately modeling effects from stellar activity is a key step in detecting radial velocity signals of low-mass and long-period exoplanets. Radial velocities from stellar activity are dominated by magnetic active regions that move in and out of sight as the star rotates, producing signals with timescales related to the stellar rotation period. Methods to characterize radial velocity periodograms assume that peaks from magnetic active regions will typically occur at the stellar rotation period or a related harmonic. However, with surface features unevenly spaced and evolving over time, signals from magnetic activity are not perfectly periodic, and the effectiveness of characterizing them with sine curves is unconfirmed. With a series of simulations, we perform the first test of common assumptions about signals from magnetic active regions in radial velocity periodograms. We simulate radial velocities with quasi-periodic signals that account for evolution and migration of magnetic surface features. As test cases, we apply our analysis to two exoplanet hosts, Kepler-20 and K2-131. Simulating observing schedules and uncertainties of real radial velocity surveys, we find that magnetic active regions commonly produce maximum periodogram peaks at spurious periods unrelated to the stellar rotation period: 81% and 72% of peaks, respectively for K2-131 and Kepler-20. These unexpected peaks can potentially lead to inaccuracies in derived planet masses. We also find that these spurious peaks can sometimes survive multiple seasons of observation, imitating signals typically attributed to exoplanet companions.

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arXiv2019

The effect of stellar activity on the spectroscopic stellar parameters of the young solar twin HIP 36515

Spectroscopic equilibrium allows us to obtain precise stellar parameters in Sun-like stars. It relies on the assumption of the iron excitation and ionization equilibrium. However, several works suggest that magnetic activity may affect chemical abundances of young active stars, calling into question the validity of this widely-used method. We have tested for the first time variations in stellar parameters and chemical abundances for the young solar twin HIP 36515 ($\sim$0.4 Gyr), along its activity cycle. This star has stellar parameters very well established in the literature and we estimated its activity cycle in $\sim$6 years. Using HARPS spectra with high resolving power (115 000) and signal-to-noise ratio ($\sim$270), the stellar parameters of six different epochs in the cycle were estimated. We found that the stellar activity is strongly correlated with the effective temperature, metallicity, and microturbulence velocity. The possibility of changes in the Li I 6707.8 Åline due to flares and star spots was also investigated. Although the core of the line profile shows some variations with the stellar cycle, it is compensated by changes in the effective temperature, resulting in a non variation of the Li abundance.

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arXiv2019

Sensitivity of low-degree solar p modes to active and ephemeral regions: frequency shifts back to the Maunder Minimum

We explore the sensitivity of the frequencies of low-degree solar p-modes to near-surface magnetic flux on different spatial scales and strengths, specifically to active regions with strong magnetic fields and ephemeral regions with weak magnetic fields. We also use model reconstructions from the literature to calculate average frequency offsets back to the end of the Maunder minimum. We find that the p-mode frequencies are at least three times less sensitive (at 95% confidence) to the ephemeral-region field than they are to the active-region field. Frequency shifts between activity cycle minima and maxima are controlled predominantly by the change of active region flux. Frequency shifts at cycle minima (with respect to a magnetically quiet Sun) are determined largely by the ephemeral flux, and are estimated to have been $0.1\,\rm μHz$ or less over the last few minima. We conclude that at epochs of cycle minimum, frequency shifts due to near-surface magnetic activity are negligible compared to the offsets between observed and model frequencies that arise from inaccurate modelling of the near-surface layers (the so-called surface term). The implication is that this will be the case for other Sun-like stars with similar activity, which has implications for asteroseismic modelling of stars.

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arXiv2019

High-Energy Photon and Particle Effects onExoplanet Atmospheres and Habitability

It is now recognized that energetic stellar photon and particle radiation evaporates and erodes planetary atmospheres and controls upper atmospheric chemistry. Key exoplanet host stars will be too faint at X-ray wavelengths for accurate characterization using existing generation and future slated X-ray telescopes. Observation of stellar coronal mass ejections and winds are also beyond current instrumentation. In line with theCommittee on an Exoplanet Science Strategy recognition that holistic observational approaches are needed, we point out here that a full understanding of exoplanet atmospheres, their evolution and determination of habitability requires a powerful high-resolution X-ray imaging and spectroscopic observatory. This is the only capability that can: (1) characterize by proxy the crucial, difficult to observe, EUV stellar flux, its history and its variations for planet hosting stars; (2) observe the stellar wind; (3) detect the subtle Doppler signatures of coronal mass ejections.

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arXiv2018

A second terrestrial planet orbiting the nearby M dwarf LHS 1140

LHS 1140 is a nearby mid-M dwarf known to host a temperate rocky super-Earth (LHS 1140 b) on a 24.737-day orbit. Based on photometric observations by MEarth and Spitzer as well as Doppler spectroscopy from HARPS, we report the discovery of an additional transiting rocky companion (LHS 1140 c) with a mass of $1.81\pm0.39~{\rm M_{Earth}}$ and a radius of $1.282\pm0.024~{\rm R_{Earth}}$ on a tighter, 3.77795-day orbit. We also obtain more precise estimates of the mass and radius of LHS 1140 b to be $6.98\pm0.89~{\rm M_{Earth}}$ and $1.727\pm0.032~{\rm R_{Earth}}$. The mean densities of planets b and c are $7.5\pm1.0~\rm{g/cm^3}$ and $4.7\pm1.1~\rm{g/cm^3}$, respectively, both consistent with the Earth's ratio of iron to magnesium silicate. The orbital eccentricities of LHS 1140 b and c are consistent with circular orbits and constrained to be below 0.06 and 0.31, respectively, with 90% confidence. Because the orbits of the two planets are co-planar and because we know from previous analyses of Kepler data that compact systems of small planets orbiting M dwarfs are commonplace, a search for more transiting planets in the LHS 1140 system could be fruitful. LHS 1140 c is one of the few known nearby terrestrial planets whose atmosphere could be studied with the upcoming James Webb Space Telescope.

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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

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

A Kepler Study of Starspot Lifetimes with Respect to Light Curve Amplitude and Spectral Type

Wide-field high precision photometric surveys such as Kepler have produced reams of data suitable for investigating stellar magnetic activity of cooler stars. Starspot activity produces quasi-sinusoidal light curves whose phase and amplitude vary as active regions grow and decay over time. Here we investigate, firstly, whether there is a correlation between the size of starspots - assumed to be related to the amplitude of the sinusoid - and their decay timescale and, secondly, whether any such correlation depends on the stellar effective temperature. To determine this, we computed the autocorrelation functions of the light curves of samples of stars from Kepler and fitted them with apodised periodic functions. The light curve amplitudes, representing spot size were measured from the root-mean-squared scatter of the normalised light curves. We used a Monte Carlo Markov Chain to measure the periods and decay timescales of the light curves. The results show a correlation between the decay time of starspots and their inferred size. The decay time also depends strongly on the temperature of the star. Cooler stars have spots that last much longer, in particular for stars with longer rotational periods. This is consistent with current theories of diffusive mechanisms causing starspot decay. We also find that the Sun is not unusually quiet for its spectral type - stars with solar-type rotation periods and temperatures tend to have (comparatively) smaller starspots than stars with mid-G or later spectral types.

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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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