arXiv subjects

H. R. Wakeford

Publications and source records attributed to H. R. Wakeford.

At least 19 recordsLinked to original sources

arXiv2024

MANTA-Ray: Supercharging Speeds for Calculating the Optical Properties of Fractal Aggregates in the Long-Wavelength Limit

Correctly modelling the absorptive properties of dust and haze particles is of great importance for determining the abundance of solid matter within protoplanetary disks and planetary atmospheres. Rigorous analyses such as the discrete dipole approximation (DDA) can be used to obtain accurate absorption cross-sections, but these require significant computing time and are often impractical to use in models. A simple analytical equation exists for spherical particles in the long-wavelength limit (where the wavelength is much larger than the size of the dust particle), but we demonstrate that this can significantly underestimate the absorption. This effect is found to depend strongly on refractive index, with values of m = 1 + 11i corresponding to an underestimate in absorption by a factor of 1,000. Here we present MANTA-Ray (Modified Absorption of Non-spherical Tiny Aggregates in the RAYleigh regime): a simple model that can calculate absorption efficiencies within 10-20% of the values predicted by DDA, but 10^(13) times faster. MANTA-Ray is very versatile and works for any wavelength and particle size in the long wavelength regime. It is also very flexible with regards to particle shape, and can correctly model structures ranging from long linear chains to tight compact clusters, composed of any material with refractive index 1+0.01i < m < 11+11i. The packaged model is provided as publicly-available code for use by the astrophysical community.

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arXiv2024

The HUSTLE Program: The UV to Near-Infrared HST WFC3/UVIS G280 Transmission Spectrum of WASP-127b

Ultraviolet wavelengths offer unique insights into aerosols in exoplanetary atmospheres. However, only a handful of exoplanets have been observed in the ultraviolet to date. Here, we present the ultraviolet-visible transmission spectrum of the inflated hot Jupiter WASP-127b. We observed one transit of WASP-127b with WFC3/UVIS G280 as part of the Hubble Ultraviolet-optical Survey of Transiting Legacy Exoplanets (HUSTLE), obtaining a transmission spectrum from 200-800 nm. Our reductions yielded a broad-band transit depth precision of 91 ppm and a median precision of 240 ppm across 59 spectral channels. Our observations are suggestive of a high-altitude cloud layer with forward modeling showing they are composed of sub-micron particles and retrievals indicating a high opacity patchy cloud. While our UVIS/G280 data only offers weak evidence for Na, adding archival HST WFC3/IR and STIS observations raises the overall Na detection significance to 4.1-sigma. Our work demonstrates the capabilities of HST WFC3/UVIS G280 observations to probe the aerosols and atmospheric composition of transiting hot Jupiters with comparable precision to HST STIS.

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arXiv2024

A Benchmark JWST Near-Infrared Spectrum for the Exoplanet WASP-39b

Observing exoplanets through transmission spectroscopy supplies detailed information on their atmospheric composition, physics, and chemistry. Prior to JWST, these observations were limited to a narrow wavelength range across the near-ultraviolet to near-infrared, alongside broadband photometry at longer wavelengths. To understand more complex properties of exoplanet atmospheres, improved wavelength coverage and resolution are necessary to robustly quantify the influence of a broader range of absorbing molecular species. Here we present a combined analysis of JWST transmission spectroscopy across four different instrumental modes spanning 0.5-5.2 micron using Early Release Science observations of the Saturn-mass exoplanet WASP-39b. Our uniform analysis constrains the orbital and stellar parameters within sub-percent precision, including matching the precision obtained by the most precise asteroseismology measurements of stellar density to-date, and further confirms the presence of Na, K, H$_2$O, CO, CO$_2$, and SO$_2$ atmospheric absorbers. Through this process, we also improve the agreement between the transmission spectra of all modes, except for the NIRSpec PRISM, which is affected by partial saturation of the detector. This work provides strong evidence that uniform light curve analysis is an important aspect to ensuring reliability when comparing the high-precision transmission spectra provided by JWST.

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arXiv2023

JWST-TST Proper Motions: I. High-Precision NIRISS Calibration and Large Magellanic Cloud Kinematics

We develop and disseminate effective point-spread functions and geometric-distortion solutions for high-precision astrometry and photometry with the JWST NIRISS instrument. We correct field dependencies and detector effects, and assess the quality and the temporal stability of the calibrations. As a scientific application and validation, we study the proper motion (PM) kinematics of stars in the JWST calibration field near the Large Magellanic Cloud (LMC) center, comparing to a first-epoch Hubble Space Telescope (HST) archival catalog with a 16-yr baseline. For stars with G~20, the median PM uncertainty is ~13 $μ$as yr$^{-1}$ (3.1 km s$^{-1}$), better than Gaia DR3 typically achieves for its very best-measured stars. We kinematically detect the known star cluster OGLE-CL LMC 407, measure its absolute PM for the first time, and show how this differs from other LMC populations. The inferred cluster dispersion sets an upper limit of 24 $μ$as yr$^{-1}$ (5.6 km s$^{-1}$) on systematic uncertainties. Red-giant-branch stars have a velocity dispersion of 33.8 $\pm$ 0.6 km s$^{-1}$, while younger blue populations have a narrower velocity distribution, but with a significant kinematical substructure. We discuss how this relates to the larger velocity dispersions inferred from Gaia DR3. These results establish JWST as capable of state-of-the-art astrometry, building on the extensive legacy of HST. This is the first paper in a series by our JWST Telescope Scientist Team (TST), in which we will use Guaranteed Time Observations to study the PM kinematics of various stellar systems in the Local Group.

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arXiv2023

MPS-ATLAS library of stellar model atmospheres and spectra

Stellar spectra contain a large amount of information about the conditions in stellar atmospheres. However, extracting this information is challenging and demands comprehensive numerical modelling. Here, we present stellar spectra synthesized using the recently developed state-of-the-art MPS-ATLAS code on a fine grid of stellar fundamental parameters. These calculations have been extensively validated against solar and stellar observations and can be used for various astrophysical applications. The spectra are available at the Max Planck Digital Library (MPDL, https://edmond.mpdl.mpg.de/dataset.xhtml?persistentId=doi:10.17617/3.NJ56TR) and have been also incorporated into the ExoTiC-LD python package (https://github.com/Exo-TiC/ExoTiC-LD/) which returns stellar limb darkening coefficients used for the software package Exoplanet Timeseries Characterisation (ExoTic).

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arXiv2023

A JWST transmission spectrum of a nearby Earth-sized exoplanet

The critical first step in the search for life on exoplanets over the next decade is to determine whether rocky planets transiting small M-dwarf stars possess atmospheres and, if so, what processes sculpt them over time. Because of its broad wavelength coverage and improved resolution compared to previous methods, spectroscopy with JWST offers a new capability to detect and characterize the atmospheres of Earth-sized, M-dwarf planets. Here we use JWST to independently validate the discovery of LHS 475b, a warm (586 K), 0.99 Earth-radius exoplanet, interior to the habitable zone, and report a precise 2.9-5.3 um transmission spectrum. With two transit observations, we rule out primordial hydrogen-dominated and cloudless pure methane atmospheres. Thus far, the featureless transmission spectrum remains consistent with a planet that has a high-altitude cloud deck (similar to Venus), a tenuous atmosphere (similar to Mars), or no appreciable atmosphere at all (akin to Mercury). There are no signs of stellar contamination due to spots or faculae. Our observations demonstrate that JWST has the requisite sensitivity to constrain the secondary atmospheres of terrestrial exoplanets with absorption features <50 ppm, and that our current atmospheric constraints speak to the nature of the planet itself, rather than instrumental limits.

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arXiv2022

Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM

Transmission spectroscopy of exoplanets has revealed signatures of water vapor, aerosols, and alkali metals in a few dozen exoplanet atmospheres. However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations' relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species$-$in particular the primary carbon-bearing molecules. Here we report a broad-wavelength 0.5-5.5 $μ$m atmospheric transmission spectrum of WASP-39 b, a 1200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with JWST NIRSpec's PRISM mode as part of the JWST Transiting Exoplanet Community Early Release Science Team program. We robustly detect multiple chemical species at high significance, including Na (19$σ$), H$_2$O (33$σ$), CO$_2$ (28$σ$), and CO (7$σ$). The non-detection of CH$_4$, combined with a strong CO$_2$ feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4$μ$m is best explained by SO$_2$ (2.7$σ$), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST's sensitivity to a rich diversity of exoplanet compositions and chemical processes.

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arXiv2022

HST/WFC3 transmission spectroscopy of the cold rocky planet TRAPPIST-1h

TRAPPIST-1 is a nearby ultra-cool dwarf star transited by seven rocky planets. We observed three transits of its outermost planet, TRAPPIST-1h, using the G141 grism of the Wide Field Camera 3 instrument aboard the Hubble Space Telescope to place constraints on its potentially cold atmosphere. In order to deal with the effect of stellar contamination, we model TRAPPIST-1 active regions as portions of a cooler and a hotter photosphere, and generate multi-temperature models that we compare to the out-of-transit spectrum of the star. Using the inferred spot parameters, we produce corrected transmission spectra for planet h under five transit configurations and compare these data to planetary atmospheric transmission models using the forward model CHIMERA. Our analysis reveals that TRAPPIST-1h is unlikely to host an aerosol-free H/He-dominated atmosphere. While the current data precision limits the constraints we can put on the planetary atmosphere, we find that the likeliest scenario is that of a flat, featureless transmission spectrum in the WFC3/G141 bandpass due to a high mean molecular weight atmosphere (>1000x solar), no atmosphere, or an opaque aerosol layer, all in absence of stellar contamination. This work outlines the limitations of modeling active photospheric regions with theoretical stellar spectra, and those brought by our lack of knowledge of the photospheric structure of ultracool dwarf stars. Further characterization of the planetary atmosphere of TRAPPIST-1h would require higher precision measurements over wider wavelengths, which will be possible with the James Webb Space Telescope.

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arXiv2022

A Comprehensive Analysis of WASP-17b's Transmission Spectrum from Space-Based Observations

Due to its 1770 K equilibrium temperature, WASP-17b, a 1.99 $R_\mathrm{Jup}$, 0.486 $M_\mathrm{Jup}$ exoplanet, sits at the critical juncture between hot and ultra-hot Jupiters. We present its 0.3-5 $μm$ transmission spectrum, with newly obtained with Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) measurements, and, taking advantage of improved analysis techniques, reanalysed HST Space Telescope Imaging Spectrograph (STIS) and Spitzer Space Telescope Infrared Array Camera (IRAC) observations. We achieve a median precision of 132 ppm, with a mean of 272 ppm across the whole spectrum. We additionally make use of Transiting Exoplanet Survey Satellite (TESS) and ground-based transit observations to refine the orbital period of WASP-17b. To interpret the observed atmosphere, we make use of free and equilibrium chemistry retrievals using the POSEIDON and ATMO retrieval codes respectively. We detect absorption due to H$_2$O at $>7 σ$, and find evidence of absorption due to CO$_2$ at $>3 σ$. We see no evidence of previously detected Na and K absorption. Across an extensive suite of retrieval configurations, we find the data favours a bimodal solution with high or low metallicity modes as a result of poor constraints in the optical, and demonstrate the importance of using multiple statistics for model selection. Future James Webb Space Telescope (JWST) GTO observations, combined with the presented transmission spectrum, will enable precise constraints on WASP-17b's atmosphere.

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arXiv2022

The Hubble PanCET program: Emission spectrum of hot Jupiter HAT-P-41b

We present the most complete emission spectrum for inflated hot Jupiter HAT-P-41b combining new HST WFC/G141 spectrum from the Hubble Panchromatic Comparative Exoplanet Treasury (PanCET) program with archival Spitzer eclipse observations. We found a near blackbody-like emission spectrum which is best fitted with an isothermal temperature-pressure (TP) profile that agrees well with the dayside heat redistribution scenario assuming zero Bond albedo. The non-inverted TP profile is consistent with the non-detection of NUV/optical absorbers in the transit spectra. We do not find any evidence for significant H$^-$ opacity nor a metal-rich atmosphere. HAT-P-41b is an ideal target that sits in the transitioning parameter space between hot and ultra-hot Jupiters, and future JWST observations will help us to better constrain the thermal structure and chemical composition.

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arXiv2021

The Hubble PanCET program: Transit and Eclipse Spectroscopy of the Strongly Irradiated Giant Exoplanet WASP-76b

Ultra-hot Jupiters with equilibrium temperature greater than 2000K are uniquely interesting targets as they provide us crucial insights into how atmospheres behave under extreme conditions. This class of giant planets receives intense radiation from their host star and usually has strongly irradiated and highly inflated atmospheres. At such high temperature, cloud formation is expected to be suppressed and thermal dissociation of water vapor could occur. We observed the ultra-hot Jupiter WASP-76b with 7 transits and 5 eclipses using the Hubble Space Telescope (HST) and $Spitzer$ for a comprehensive study of its atmospheric chemical and physical processes. We detect TiO and H$_2$O absorption in the optical and near-infrared transit spectrum. Additional absorption by a number of neutral and ionized heavy metals like Fe, Ni, Ti, and SiO help explain the short wavelength transit spectrum. The secondary eclipse spectrum shows muted water feature but a strong CO emission feature in Spitzer's 4.5 $μ$m band indicating an inverted temperature pressure profile. We analyzed both the transit and emission spectrum with a combination of self-consistent PHOENIX models and retrieval models (ATMO $\&$ PLATON). Both spectra are well fitted by the self-consistent PHOENIX forward atmosphere model in chemical and radiative equilibrium at solar metallicity, adding to the growing evidence that both TiO/VO and NUV heavy metals opacity are prominent NUV-optical opacity sources in the stratospheres of ultra-hot Jupiters.

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arXiv2021

HST PanCET program: Non-detection of atmospheric escape in the warm Saturn-sized planet WASP-29 b

(Abridged) Short-period gas giant exoplanets are susceptible to intense atmospheric escape due to their large scale heights and strong high-energy irradiation. This process is thought to occur ubiquitously, but to date we have only detected direct evidence of atmospheric escape in hot Jupiters and warm Neptunes. The paucity of cases for intermediate, Saturn-sized exoplanets at varying levels of irradiation precludes a detailed understanding of the underlying physics in atmospheric escape of hot gas giants. Our objectives here are to assess the high-energy environment of the warm ($T_\mathrm{eq} = 970$ K) Saturn WASP-29 b and search for signatures of atmospheric escape. We used far-ultraviolet (FUV) observations from the Hubble Space Telescope to analyze the flux time series of H I, C II, Si III, Si IV, and N V during the transit of WASP-29 b. At 3$σ$ confidence, we rule out any in-transit absorption of H Ilarger than 92% in the Lyman-$α$ blue wing and 19% in the red wing. We found an in-transit flux decrease of $39\%^{+12\%}_{-11\%}$ in the ground-state C II emission line at 133.45 nm. But due to this signal being significantly present in only one visit, it is difficult to attribute a planetary or stellar origin for the ground-state C II signal. We place 3$σ$ absorption upper limits of 40%, 49% and 24% for Si III, Si IV, and for excited-state C II at 133.57 nm, respectively. Low activity levels and the faint X-ray luminosity suggest that WASP-29 is an old, inactive star. An energy-limited approximation combined with the reconstructed EUV spectrum of the host suggests that the planet is losing its atmosphere at a rate of $4 \times 10^9$ g s$^{-1}$. The non-detection at Lyman-$α$ could be partly explained by a low fraction of escaping neutral hydrogen, or by the state of fast radiative blow-out we infer from the reconstructed stellar Lyman-$α$ line.

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arXiv2021

The Hubble PanCET program: Long-term chromospheric evolution and flaring activity of the M dwarf host GJ 3470

Neptune-size exoplanets seem particularly sensitive to atmospheric evaporation, making it essential to characterize the stellar high-energy radiation that drives this mechanism. This is particularly important with M dwarfs, which emit a large and variable fraction of their luminosity in the UV and can display strong flaring behavior. The warm Neptune GJ3470b, hosted by an M2 dwarf, was found to harbor a giant hydrogen exosphere thanks to 3 transits observed with the HST/STIS. Here we report on 3 additional transit observations from the PanCET program, obtained with the HST/COS. These data confirm the absorption signature from GJ3470b's exosphere in the stellar Ly-alpha line and demonstrate its stability over time. No planetary signatures are detected in other lines, setting a 3sigma limit on GJ3470b's FUV radius at 1.3x its Roche lobe radius. We detect 3 flares from GJ3470. They show different spectral energy distributions but peak consistently in the Si III line, which traces intermediate-temperature layers in the transition region. These layers appear to play a particular role in GJ3470's activity as emission lines that form at lower or higher temperatures than Si III evolved differently over the long term. Based on the measured emission lines, we derive synthetic XUV spectra for the 6 observed quiescent phases, covering one year, as well as for the 3 flaring episodes. Our results suggest that most of GJ3470's quiescent high-energy emission comes from the EUV domain, with flares amplifying the FUV emission more strongly. The hydrogen photoionization lifetimes and mass loss derived for GJ3470b show little variation over the epochs, in agreement with the stability of the exosphere. Simulations informed by our XUV spectra are required to understand the atmospheric structure and evolution of GJ3470b and the role played by evaporation in the formation of the hot-Neptune desert.

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arXiv2020

The Exoplanet Perspective on Future Ice Giant Exploration

Exoplanets number in their thousands, and the number is ever increasing with the advent of new surveys and improved instrumentation. One of the most surprising things we have learnt from these discoveries is not that small-rocky planets in their stars habitable zones are likely common, but that the most typical size of exoplanet is that not seen in our solar system - radii between that of Neptune and the Earth dubbed mini-Neptunes and super-Earths. In fact, a transiting exoplanet is four times as likely to be in this size regime than that of any giant planet in our solar system. Investigations into the atmospheres of giant hydrogen/helium dominated exoplanets has pushed down to Neptune and mini-Neptune sized worlds revealing molecular absorption from water, scattering and opacity from clouds, and measurements of atmospheric abundances. However, unlike measurements of Jupiter, or even Saturn sized worlds, the smaller giants lack a ground truth on what to expect or interpret from their measurements. How did these sized worlds form and evolve and was it different from their larger counterparts? What is their internal composition and how does that impact their atmosphere? What informs the energy budget of these distant worlds? In this we discuss what characteristics we can measure for exoplanets, and why a mission to the ice giants in our solar system is the logical next step for understanding exoplanets.

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arXiv2020

Into the UV: A precise transmission spectrum of HAT-P-41b using Hubble's WFC3/UVIS G280 grism

The ultraviolet-visible wavelength range holds critical spectral diagnostics for the chemistry and physics at work in planetary atmospheres. To date, exoplanet time-series atmospheric characterization studies have relied on several combinations of modes on Hubble's STIS/COS instruments to access this wavelength regime. Here for the first time, we apply the Hubble WFC3/UVIS G280 grism mode to obtain exoplanet spectroscopy from 200-800 nm in a single observation. We test the G280 grism mode on the hot Jupiter HAT-P-41b over two consecutive transits to determine its viability for exoplanet atmospheric characterization. We obtain a broadband transit depth precision of 29-33ppm and a precision of on average 200ppm in 10nm spectroscopic bins. Spectral information from the G280 grism can be extracted from both the positive and negative first order spectra, resulting in a 60% increase in the measurable flux. Additionally, the first HST orbit can be fully utilized in the time-series analysis. We present detailed extraction and reduction methods for use by future investigations with this mode, testing multiple techniques. We find the results fully consistent with STIS measurements of HAT-P-41b from 310-800 nm, with the G280 results representing a more observationally efficient and precise spectrum. We fit HAT-P-41b's transmission spectrum with a forward model at Teq=2091K, high metallicity, and significant scattering and cloud opacity. With these first of their kind observations, we demonstrate that WFC3/UVIS G280 is a powerful new tool to obtain UV-optical spectra of exoplanet atmospheres, adding to the UV legacy of Hubble and complementing future observations with the James Webb Space Telescope.

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arXiv2019

Exoplanet Atmosphere Forecast: Observers Should Expect Spectroscopic Transmission Features to be Muted to 33%

To ensure robust constraints are placed on exoplanet atmospheric transmission spectra, future observations need to obtain high signal-to-noise ratio (SNR) measurements assuming smaller amplitude molecular signatures than those of clear solar metallicity atmospheres. Analyzing 37 exoplanet transmission spectra we find clear solar molecular features are measured in <7% of cases.

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arXiv2018

Hubble PanCET: An extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470 b

GJ 3470b is a warm Neptune transiting an M-dwarf star at the edge of the evaporation desert. It offers the possibility of investigating how low-mass, close-in exoplanets evolve under the irradiation from their host stars. We observed three transits of GJ 3470b in the Lyman-alpha line with the Hubble Space Telescope (HST) as part of the Panchromatic Comparative Exoplanet Treasury (PanCET) program. Absorption signatures are detected with similar properties in all three independent epochs, with absorption depths of 35+-7% in the blue wing of the line, and 23+-5% in the red wing. The repeatability of these signatures, their phasing with the planet transit, and the radial velocity of the absorbing gas allow us to conclude that there is an extended upper atmosphere of neutral hydrogen around GJ 3470 b. We determine from our observations the stellar radiation pressure and XUV irradiation from GJ 3470 and use them to perform numerical simulations of the upper atmosphere of GJ 3470b with the EVaporating Exoplanets (EVE) code. The unusual redshifted signature can be explained by the damping wings of dense layers of neutral hydrogen that extend beyond the Roche lobe and are elongated in the direction of the planet motion. This structure could correspond to a shocked layer of planetary material formed by the collision of the expanding thermosphere with the wind of the star. The blueshifted signature is well explained by neutral hydrogen atoms escaping at rates of about 1e10 g s-1 that are blown away from the star by its strong radiation pressure and are quickly photoionized, resulting in a smaller exosphere than that of the warm Neptune GJ 436b. The stronger escape from GJ 3470b, however, may have led to the loss of about 4-35% of its current mass over its 2 Gyr lifetime.

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arXiv2018

Helium in the eroding atmosphere of an exoplanet

Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres. Searches for helium, however, have hitherto been unsuccessful. Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near- infrared transmission spectrum of the warm gas giant WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 +/- 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 10^10 to 3 x 10^11 grams per second (0.1-4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.

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