arXiv subjects

Kevin France

Publications and source records attributed to Kevin France.

At least 127 records · Page 7Linked to original sources

arXiv2019

X-ray Studies of Exoplanets: A 2020 Decadal Survey White Paper

Over the last two decades, the discovery of exoplanets has fundamentally changed our perception of the universe and humanity's place within it. Recent work indicates that a solar system's X-ray and high energy particle environment is of fundamental importance to the formation and development of the atmospheres of close-in planets such as hot Jupiters, and Earth-like planets around M stars. X-ray imaging and spectroscopy provide powerful and unique windows into the high energy flux that an exoplanet experiences, and X-ray photons also serve as proxies for potentially transfigurative coronal mass ejections. Finally, if the host star is a bright enough X-ray source, transit measurements akin to those in the optical and infrared are possible and allow for direct characterization of the upper atmospheres of exoplanets. In this brief white paper, we discuss contributions to the study of exoplanets and their environs which can be made by X-ray data of increasingly high quality that are achievable in the next 10--15 years.

astro-ph.EP↗

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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arXiv2019

Understanding Exoplanet Atmospheres with UV Observations II: The Far UV and Atmospheric Escape

Much of the focus of exoplanet atmosphere analysis in the coming decade will be at infrared wavelengths, with the planned launches of the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope (WFIRST). However, without being placed in the context of broader wavelength coverage, especially in the optical and ultraviolet, infrared observations produce an incomplete picture of exoplanet atmospheres. Scattering information encoded in blue optical and near-UV (NUV) observations can help determine whether muted spectral features observed in the infrared are due to a hazy/cloudy atmosphere, or a clear atmosphere with a higher mean molecular weight. Farther into the UV, observations can identify atmospheric escape and mass loss from exoplanet atmospheres, providing a greater understanding of the atmospheric evolution of exoplanets, along with composition information from above the cloud deck. In this white paper we focus on the science case for exoplanet observations in the far-UV (FUV); an accompanying white paper led by Jessie Christiansen will focus on the science case in the near-UV (NUV) and blue/optical.

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arXiv2019

The fastest components in stellar jets

Young stars accrete mass from a circumstellar disk, but at the same time disk and star eject outflows and jets. These outflows have an onion-like structure where the innermost and fastest layers are surrounded by increasingly lower velocity components. The outer layers are probably photo-evaporative and magnetocentrifugally launched disk winds, but the nature of the inner winds is still uncertain. Since the fastest components carry only a small fraction of the mass, they are best observed at high-energies (X-ray and UV) as the slower, more massive components do not reach plasma temperatures sufficient for relevant X-ray or UV emission. Outflows are the most likely way in which a star or its disk can shed angular momentum and allow accretion to proceed; thus we cannot understand the accretion and the rotation rate of young stars if we cannot solve the origin of the inner jet components. Stellar jets share characteristics with their counterparts in more massive astrophysical objects, such as stellar mass black holes and AGN, with the added benefit that young stars are found at much closer distances and thus scales not accessible in other types of objects can be resolved. To understand the origin and impact of the inner jets, sub-arcsecond imaging and spectroscopy in the UV and X-rays is required, together with theory and modelling to interpret existing and future observations.

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arXiv2019

Understanding Exoplanet Atmospheres with UV Observations I: NUV and Blue/Optical

Much of the focus of exoplanet atmosphere analysis in the coming decade will be atinfrared wavelengths, with the planned launches of the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope (WFIRST). However, without being placed in the context of broader wavelength coverage, especially in the optical and ultraviolet, infrared observations produce an incomplete picture of exoplanet atmospheres. Scattering information encoded in blue optical and near-UV observations can help determine whether muted spectral features observed in the infrared are due to a hazy/cloudy atmosphere, or a clear atmosphere with a higher mean molecular weight. UV observations can identify atmospheric escape and mass loss from exoplanet atmospheres, providing a greater understanding of the atmospheric evolution of exoplanets, along with composition information from above the cloud deck. In this white paper we focus on the science case for exoplanet observations in the near-UV; an accompanying white paper led by Eric Lopez will focus on the science case in the far-UV.

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arXiv2019

EUV influences on exoplanet atmospheric stability and evolution

The planetary effective surface temperature alone is insufficient to characterize exoplanet atmospheres and their stability or evolution. Considering the star-planet system as a whole is necessary, and a critical component of the system is the photoionizing stellar extreme ultraviolet emission (EUV; 100-912 Å). EUV photons drive atmospheric mass loss through thermal and nonthermal processes, and an accurate accounting of the EUV energy deposition in a planet's energy budget is essential, especially for terrestrial habitable zone planets and close-in gaseous planets. Direct EUV observations of exoplanet host stars would require a new, dedicated observatory. Archival observations from the $\textit{EUVE}$ satellite, models, and theory alone are insufficient to accurately characterize EUV spectra of the majority of exoplanet host stars, especially for low-mass stars.

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arXiv2019

EUV observations of cool dwarf stars

The EUV (100-912 Å) is a spectral region notoriously difficult to observe due to attenuation by neutral hydrogen gas in the interstellar medium. Despite this, hundreds to thousands of nearby stars of different spectral types and magnetic activity levels are accessible in the EUV range. The EUV probes interesting and complicated regions in the stellar atmosphere like the lower corona and transition region that are inaccessible from other spectral regions. In this white paper we describe how direct EUV observations, which require a dedicated grazing-incidence observatory, cannot yet be accurately substituted with models and theory. Exploring EUV emission from cool dwarf stars in the time domain can make a major contribution to understanding stellar outer atmospheres and magnetism, and offers the clearest path toward detecting coronal mass ejections on stars other than the Sun.

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arXiv2019

Colorado Ultraviolet Transit Experiment Data Simulator

The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat carrying on-board a low-resolution (R~2000--3000), near-ultraviolet (2500--3300 Å) spectrograph. It has a rectangular primary Cassegrain telescope to maximize the collecting area. CUTE, which is planned for launch in Spring 2020, is designed to monitor transiting extra-solar planets orbiting bright, nearby stars aiming at improving our understanding of planet atmospheric escape and star-planet interaction processes. We present here the CUTE data simulator, which we complemented with a basic data reduction pipeline. This pipeline will be then updated once the final CUTE data reduction pipeline is developed. We show here the application of the simulator to the HD209458 system and a first estimate of the precision on the measurement of the transit depth as a function of temperature and magnitude of the host star. We also present estimates of the effect of spacecraft jitter on the final spectral resolution. The simulator has been developed considering also scalability and adaptability to other missions carrying on-board a long-slit spectrograph. The data simulator will be used to inform the CUTE target selection, choose the spacecraft and instrument settings for each observation, and construct synthetic CUTE wavelength-dependent transit light curves on which to develop the CUTE data reduction pipeline.

astro-ph.IM↗

arXiv2018

The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs

M dwarf stars are known for their vigorous flaring. This flaring could impact the climate of orbiting planets, making it important to characterize M dwarf flares at the short wavelengths that drive atmospheric chemistry and escape. We conducted a far-ultraviolet flare survey of 6 M dwarfs from the recent MUSCLES (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) observations, as well as 4 highly-active M dwarfs with archival data. When comparing absolute flare energies, we found the active-M-star flares to be about 10$\times$ more energetic than inactive-M-star flares. However, when flare energies were normalized by the star's quiescent flux, the active and inactive samples exhibited identical flare distributions, with a power-law index of -$0.76^{+0.1}_{-0.09}$ (cumulative distribution). The rate and distribution of flares are such that they could dominate the FUV energy budget of M dwarfs, assuming the same distribution holds to flares as energetic as those cataloged by Kepler and ground-based surveys. We used the observed events to create an idealized model flare with realistic spectral and temporal energy budgets to be used in photochemical simulations of exoplanet atmospheres. Applied to our own simulation of direct photolysis by photons alone (no particles), we find the most energetic observed flares have little effect on an Earth-like atmosphere, photolyzing $\sim$0.01% of the total O$_3$ column. The observations were too limited temporally (73 h cumulative exposure) to catch rare, highly energetic flares. Those that the power-law fit predicts occur monthly would photolyze $\sim$1% of the O$_3$ column and those it predicts occur yearly would photolyze the full O$_3$ column. Whether such energetic flares occur at the rate predicted is an open question.

astro-ph.SR↗

arXiv2018

Far-Ultraviolet Activity Levels of F, G, K, and M dwarf Exoplanet Host Stars

We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs with the goals of characterizing their range of FUV activity levels, calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet (EUV) stellar flux, and investigating the potential for FUV emission lines to probe star-planet interactions (SPIs). We build this emission line sample from a combination of new and archival observations with the Hubble Space Telescope-COS and -STIS instruments, targeting the chromospheric and transition region emission lines of Si III, N V, C II, and Si IV. We find that the exoplanet host stars, on average, display factors of 5 - 10 lower UV activity levels compared with the non-planet hosting sample; this is explained by a combination of observational and astrophysical biases in the selection of stars for radial-velocity planet searches. We demonstrate that UV activity-rotation relation in the full F - M star sample is characterized by a power-law decline (with index $α$ ~ -1.1), starting at rotation periods >~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric flux, we present a new analytic relationship to estimate the intrinsic stellar EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of ~2. Finally, we study the correlation between SPI strength and UV activity in the context of a principal component analysis that controls for the sample biases. We find that SPIs are not a statistically significant contributor to the observed UV activity levels.

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arXiv2018

The Transiting Exoplanet Community Early Release Science Program for JWST

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

astro-ph.EP↗

arXiv2018

CUTE Data Simulator and Reduction Pipeline

The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat carrying a low-resolution (R ~3000), near-ultraviolet (255 - 330nm) spectrograph fed by a rectangular primary Cassegrain. CUTE, is planned for launch in spring 2020 and it will monitor transiting extra-solar planets to study atmospheric escape. We present here the CUTE data simulator, which is a versatile tool easily adaptable to any other mission performing single-slit spectroscopy and carrying on-board a CCD detector. We complemented the data simulator with a data reduction pipeline capable of performing a rough reduction of the simulated data. This pipeline will then be updated once the final CUTE data reduction pipeline will be fully developed. We further briefly discuss our plans for the development of a CUTE data reduction pipeline. The data simulator will be used to inform the target selection, improve the preliminary signal-to-noise calculator, test the impact on the data of deviations from the nominal instrument characteristics, identify the best spacecraft orientation for the observation of each target and construct synthetic data to train the science team in the data analysis prior to launch.

astro-ph.IM↗

arXiv2018

Exoplanet Diversity in the Era of Space-based Direct Imaging Missions

This whitepaper discusses the diversity of exoplanets that could be detected by future observations, so that comparative exoplanetology can be performed in the upcoming era of large space-based flagship missions. The primary focus will be on characterizing Earth-like worlds around Sun-like stars. However, we will also be able to characterize companion planets in the system simultaneously. This will not only provide a contextual picture with regards to our Solar system, but also presents a unique opportunity to observe size dependent planetary atmospheres at different orbital distances. We propose a preliminary scheme based on chemical behavior of gases and condensates in a planet's atmosphere that classifies them with respect to planetary radius and incident stellar flux.

astro-ph.EP↗

arXiv2018

The Orion fingers: H$_2$ temperatures and excitation in an explosive outflow

We measure H$_2$ temperatures and column densities across the Orion BN/KL explosive outflow from a set of thirteen near-IR H$_2$ rovibrational emission lines observed with the TripleSpec spectrograph on Apache Point Observatory's 3.5-meter telescope. We find that most of the region is well-characterized by a single temperature (~2000-2500 K), which may be influenced by the limited range of upper energy levels (6000-20,000 K) probed by our data set. The H$_2$ column density maps indicate that warm H$_2$ comprises 10$^{-5}$ - 10$^{-3}$ of the total H$_2$ column density near the center of the outflow. Combining column density measurements for co-spatial H$_2$ and CO at T = 2500 K, we measure a CO/H$_2$ fractional abundance of 2$\times$10$^{-3}$, and discuss possible reasons why this value is in excess of the canonical 10$^{-4}$ value, including dust attenuation, incorrect assumptions on co-spatiality of the H$_2$ and CO emission, and chemical processing in an extreme environment. We model the radiative transfer of H$_2$ in this region with UV pumping models to look for signatures of H$_2$ fluorescence from H I Ly$α$ pumping. Dissociative (J-type) shocks and nebular emission from the foreground Orion H II region are considered as possible Ly$α$ sources. From our radiative transfer models, we predict that signatures of Ly$α$ pumping should be detectable in near-IR line ratios given a sufficiently strong source, but such a source is not present in the BN/KL outflow. The data are consistent with shocks as the H$_2$ heating source.

astro-ph.GA↗

arXiv2018

Advanced Environmentally Resistant Lithium Fluoride Mirror Coatings for the Next-Generation of Broadband Space Observatories

Recent advances in the physical vapor deposition (PVD) of protective fluoride films have raised the far-ultraviolet (FUV: 912-1600 Å) reflectivity of aluminum-based mirrors closer to the theoretical limit. The greatest gains, at more than 20%, have come for lithium fluoride-protected aluminum, which has the shortest wavelength cutoff of any conventional overcoat. Despite the success of the NASA FUSE mission, the use of lithium fluoride (LiF)-based optics is rare, as LiF is hygroscopic and requires handling procedures that can drive risk. With NASA now studying two large mission concepts for astronomy, Large UV-Optical-IR Surveyor (LUVOIR) and the Habitable Exoplanet Imaging Mission (HabEx), which mandate throughput down to 1000 Å, the development of LiF-based coatings becomes crucial. This paper discusses steps that are being taken to qualify these new enhanced LiF-protected aluminum (eLiF) mirror coatings for flight. In addition to quantifying the hygroscopic degradation, we have developed a new method of protecting eLiF with an ultrathin (10-20 Å) capping layer of a non-hygroscopic material to increase durability. We report on the performance of eLiF-based optics and assess the steps that need to be taken to qualify such coatings for LUVOIR, HabEx, and other FUV-sensitive space missions.

astro-ph.IM↗

arXiv2018

The Colorado Ultraviolet Transit Experiment (CUTE): A dedicated cubesat mission to study exoplanetary mass loss and magnetic fields

The Colorado Ultraviolet Transit Experiment (CUTE) is a near-UV (2550 - 3300 Angstrom) 6U cubesat mission designed to monitor transiting hot Jupiters to quantify their atmospheric mass loss and magnetic fields. CUTE will probe both atomic (Mg and Fe) and molecular (OH) lines for evidence of enhanced transit absorption, and to search for evidence of early ingress due to bow shocks ahead of the planet's orbital motion. As a dedicated mission, CUTE will observe more than 100 spectroscopic transits of hot Jupiters over a nominal seven month mission. This represents the equivalent of more than 700 orbits of the only other instrument capable of these measurements, the Hubble Space Telescope. CUTE efficiently utilizes the available cubesat volume by means of an innovative optical design to achieve a projected effective area of 28 sq. cm, low instrumental background, and a spectral resolving power of 3000 over the primary science bandpass. These performance characteristics enable CUTE to discern transit depths between 0.1 - 1% in individual spectral absorption lines. We present the CUTE optical and mechanical design, a summary of the science motivation and expected results, and an overview of the projected fabrication, calibration and launch timeline.

astro-ph.IM↗

arXiv2017

The LUVOIR Ultraviolet Multi-Object Spectrograph (LUMOS): Instrument Definition and Design

The Large Ultraviolet / Optical / Infrared Surveyor (LUVOIR) is one of four large mission concepts currently undergoing community study for consideration by the 2020 Astronomy and Astrophysics Decadal Survey. The LUVOIR Ultraviolet Multi-Object Spectrograph, LUMOS, is being designed to support all of the UV science requirements of LUVOIR, from exoplanet host star characterization to tomography of circumgalactic halos to water plumes on outer solar system satellites. LUMOS offers point source and multi-object spectroscopy across the UV bandpass, with multiple resolution modes to support different science goals. The instrument will provide low (R = 8,000-18,000) and medium (R = 30,000-65,000) resolution modes across the far-ultraviolet (FUV: 100-200 nm) and near-ultraviolet (NUV: 200-400 nm) windows, and a very low resolution mode (R = 500) for spectroscopic investigations of extremely faint objects in the FUV. Imaging spectroscopy will be accomplished over a 3 x 1.6 arcminute field-of-view by employing holographically-ruled diffraction gratings to control optical aberrations, microshutter arrays (MSA), advanced optical coatings for high-throughput in the FUV, and next generation large-format photon-counting detectors. The spectroscopic capabilities of LUMOS are augmented by an FUV imaging channel (100-200nm, 13 milliarcsecond angular resolution, 2 x 2 arcminute field-of-view) that will employ a complement of narrow and medium-band filters. We present an overview of LUMOS' observing modes and estimated performance curves for effective area, spectral resolution, and imaging performance. Example "LUMOS 100-hour Highlights" observing programs are presented to demonstrate the potential power of LUVOIR's ultraviolet spectroscopic capabilities.

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arXiv2017

Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks: I. Non-thermal Populations

The environment around protoplanetary disks (PPDs) regulates processes which drive the chemical and structural evolution of circumstellar material. We perform a detailed empirical survey of warm molecular hydrogen (H$_2$) absorption observed against H I-Ly$α$ (Ly$α$: $λ$ 1215.67 Å) emission profiles for 22 PPDs, using archival Hubble Space Telescope (HST) ultraviolet (UV) spectra from the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) to identify H$_2$ absorption signatures and quantify the column densities of H$_2$ ground states in each sightline. We compare thermal equilibrium models of H$_2$ to the observed H$_2$ rovibrational level distributions. We find that, for the majority of targets, there is a clear deviation in high energy states (T$_{exc}$ $\gtrsim$ 20,000 K) away from thermal equilibrium populations (T(H$_2$) $\gtrsim$ 3500 K). We create a metric to estimate the total column density of non-thermal H$_2$ (N(H$_2$)$_{nLTE}$) and find that the total column densities of thermal (N(H$_2$)) and N(H$_2$)$_{nLTE}$ correlate for transition disks and targets with detectable C IV-pumped H$_2$ fluorescence. We compare N(H$_2$) and N(H$_2$)$_{nLTE}$ to circumstellar observables and find that N(H$_2$)$_{nLTE}$ correlates with X-ray and FUV luminosities, but no correlations are observed with the luminosities of discrete emission features (e.g., Ly$α$, C IV). Additionally, N(H$_2$) and N(H$_2$)$_{nLTE}$ are too low to account for the H$_2$ fluorescence observed in PPDs, so we speculate that this H$_2$ may instead be associated with a diffuse, hot, atomic halo surrounding the planet-forming disk. We create a simple photon-pumping model for each target to test this hypothesis and find that Ly$α$ efficiently pumps H$_2$ levels with T$_{exc}$ $\geq$ 10,000 K out of thermal equilibrium.

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