arXiv subjects

Yamila Miguel

Publications and source records attributed to Yamila Miguel.

At least 91 records · Page 5Linked to original sources

arXiv2019

Jupiter formed as a pebble pile around the N$_2$ ice line

The region around the H$_2$O ice line, due to its higher surface density, seems to be the ideal location to form planets. The core of Jupiter, as well as the cores of close in gas giants are thus thought to form in this region of the disk. Actually constraining the formation location of individual planets has proven to be difficult, however. We aim to use the Nitrogen abundance in Jupiter, which is around 4 times solar, in combination with \textit{Juno} constraints on the total mass of heavy elements in Jupiter, to narrow down its formation scenario. Different pathways of enrichment of Jupiter's atmosphere, such as the accretion of enriched gas, pebbles or planetesimals are considered and their implications for the oxygen abundance of Jupiter is discussed. The super solar Nitrogen abundance in Jupiter necessitates the accretion of extra N$_2$ from the proto-solar nebula. The only location of the disk that this can happen is outside, or just inside the N$_2$ ice line. These constraints favor a pebble accretion origin of Jupiter, both from the composition as well as from a planet formation perspective. We predict that Jupiter's oxygen abundance is between 3.6 and 4.5 times solar.

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arXiv2019

ESA Voyage 2050 White Paper: Detecting life outside our solar system with a large high-contrast-imaging mission

In this white paper, we recommend the European Space Agency plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestrial biological activity. We provide an overview of relevant European expertise, and advocate ESA to start a technology development program towards detecting life outside the Solar system.

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arXiv2019

Saturn's deep atmospheric flows revealed by the Cassini Grand Finale gravity measurements

How deep do Saturn's zonal winds penetrate below the cloud-level has been a decades-long question, with important implications not only for the atmospheric dynamics, but also for the interior density structure, composition, magnetic field and core mass. The Cassini Grand Finale gravity experiment enables answering this question for the first time, with the premise that the planet's gravity harmonics are affected not only by the rigid body density structure but also by its flow field. Using a wide range of rigid body interior models and an adjoint based thermal wind balance, we calculate the optimal flow structure below the cloud-level and its depth. We find that with a wind profile, largely consistent with the observed winds, when extended to a depth of around 8,800 km, all the gravity harmonics measured by Cassini are explained. This solution is in agreement with considerations of angular momentum conservation, and is consistent with magnetohydrodynamics constraints.

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arXiv2019

Transitional disk archeology from exoplanet population synthesis

Increasingly better observations of resolved protoplanetary disks show a wide range of conditions in which planets can be formed. Many transitional disks show gaps in their radial density structure, which are usually interpreted as signatures of planets. It has also been suggested that observed inhomogeneities in transitional disks are indicative of dust traps which may help the process of planet formation. However, it is yet to be seen if the configuration of fully evolved exoplanetary systems can yield information about the later stages of their primordial disks. We use synthetic exoplanet population data from Monte Carlo simulations of systems forming under different density perturbation conditions, which are based on current observations of transitional disks. The simulations use a core instability, oligarchic growth, dust trap analytical model that has been benchmarked against exoplanetary populations.

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arXiv2018

Observability of molecular species in a nitrogen dominated atmosphere for 55 Cancri e

One of the key goals of exoplanet science is the atmospheric characterisation of super-Earths. Atmospheric abundances provide insight on the formation and evolution of those planets and help to put our own rocky planets in context. Observations on 55 Cancri e point towards a N-dominated atmosphere. In this paper we explore this possibility, showing which will be the most abundant gases and observable species in emission and transmission spectroscopy of such an atmosphere. We use analytical arguments and observed parameters to estimate the possible thermal profile of the atmosphere and test three different extreme possibilities. The chemistry is calculated using equilibrium calculations and adopting Titan's elemental abundances as a potential N-dominated atmospheric composition. We also test the effect of different N/O ratios in the atmosphere. Emission and transmission spectra are computed and showed with a resolution relevant to future missions suitable to observe super-Earths (e.g. JWST, ARIEL). We find that even though N$_2$ is the most abundant molecule in the atmosphere followed by H$_2$ and CO, the transmission spectra shows strong features of NH3 and HCN, and CO and HCN dominate emission spectra. We also show that a decrease in the N/O ratio leads to stronger H2O, CO and CO2 and weaker NH3 and HCN features. A larger N/O is also more consistent with observations. Our exploration of a N-atmosphere for 55 Cancri e serve as a guide to understand such atmospheres and provide a reference for future observations.

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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.

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arXiv2018

A better characterization of the chemical composition of exoplanets atmospheres with ARIEL

Since the discovery of the first extrasolar planet more than twenty years ago, we have discovered nearly four thousand planets orbiting stars other than the Sun. Current observational instruments (on board the Hubble Space Telescope, Spitzer, and on ground-based facilities) have allowed the scientific community to obtain important information on the physical and chemical properties of these planets. However, for a more in-depth characterisation of these worlds, more powerful telescopes are needed. Thanks to the high sensitivity of their instruments, the next generation of space observatories (e.g. JWST, ARIEL) will provide observations of unprecedented quality, allowing us to extract far more information than what was previously possible. Such high quality observations will provide constraints on theoretical models of exoplanet atmospheres and lead to a greater understanding of their physics and chemistry. Important modelling efforts have been carried out during the past few years, showing that numerous parameters and processes (such as the elemental abundances, temperature, mixing, etc.) are likely to affect the atmospheric composition of exoplanets and subsequently the observable spectra. In this manuscript, we review the different parameters that can influence the molecular composition of exoplanet atmospheres. We show that the high-precision of ARIEL observations will improve our view and characterisation of exoplanet atmospheres. We also consider future developments that are necessary to improve atmospheric models, driven by the need to interpret the available observations.

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arXiv2017

Modeling synthetic spectra for transiting extrasolar giant planets: detectability of H$_2$S and PH$_3$ with JWST

JWST$'$s large aperture and wide wavelength coverage will enable it to collect the highest quality transit spectra observed so far. For exoplanetary atmospheres we expect to retrieve the abundance of the most abundant molecules, such as H$_2$O, CO, and CH$_4$. Other molecules, such as H$_2$S and PH$_3$, have been observed in Jupiter and Saturn but their chemistry and detectability in strongly irradiated planets is highly unknown. In this paper, we make the first effort to study their spectral features in solar composition atmospheres, and evaluate their detectability with JWST. We model the chemistry of phosphorus and sulfur in solar composition atmospheres. Our model includes the effect of vertical transport. Photochemistry effects are not included in our calculations. Using the abundance profiles, we model the JWST transmission and emission spectra for K=6.8 G-type star and for planets with cloud-free solar composition atmospheres. We find PH$_3$ is detectable at 3 sigma from transmission spectra of the simulated atmosphere with $T_{\rm eq}$ $<$ 500K using the NIRCam LW grism F444W mode with a total observing time of 28.8 hrs. H$_2$S is detectable at 3 sigma in the transmission and emission spectra for the simulated planet with $T_{\rm eq}$ $>$ 1500K using the NIRCam LW grism F322W2 mode with a total observing time of 24.0 hrs. Our results specifically highlight the importance of including H$_2$S for future abundances retrieval with JWST. The presence of clouds and hazes challenges the detections of PH$_3$ and H$_2$S, but H$_2$S features are still expected to be present in the emission spectra.

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arXiv2017

The MUSCLES Treasury Survey IV: Scaling Relations for Ultraviolet, Ca II K, and Energetic Particle Fluxes from M Dwarfs

Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planet's potential habitability, particularly for M dwarfs as they are prime targets for current and near-term exoplanet characterization efforts and atmospheric models predict that their UV radiation can produce photochemistry on habitable zone planets different than on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope observations are unavailable, we have assembled a sample of fifteen early-to-mid M dwarfs observed by Hubble, and compared their non-simultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca II K line at 3933 Angstroms, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H I Lyman alpha. In addition, we address another potential driver of habitability: energetic particle fluxes associated with flares. We present a new technique for estimating soft X-ray and >10 MeV proton flux during far-UV emission line flares (Si IV and He II) by assuming solar-like energy partitions. We analyze several flares from the M4 dwarf GJ 876 observed with Hubble and Chandra as part of the MUSCLES Treasury Survey and find that habitable zone planets orbiting GJ 876 are impacted by large Carrington-like flares with peak soft X-ray fluxes >1e-3 W m-2 and possible proton fluxes ~100-1000 pfu, approximately four orders of magnitude more frequently than modern-day Earth.

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arXiv2017

Metal enrichment leads to low atmospheric C/O ratios in transiting giant exoplanets

We predict the carbon-to-oxygen (C/O) ratios in the hydrogen-helium envelope and atmospheres of a sample of nearly 50 relatively cool ($T_{\mathrm eq}<$ 1000 K) transiting gas giant planets. The method involves planetary envelope metallicity estimates that use the structure models of Thorngren et al. (2016) and the disk and planetary accretion model of Öberg et al. (2011). We find that nearly all of these planets are strongly metal-enriched which, coupled with the fact that solid material is the main deliverer of metals in the protoplanetary disk, implies that the substellar C/O ratios of their accreted solid material dominate compared to the enhanced C/O ratio of their accreted gaseous component. We predict that these planets will have atmospheres that are typically reduced in their C/O compared to parent star values independent of the assessed formation locations, with C/O $<1$ a nearly universal outcome within the framework of the model. We expect water vapor absorption features to be ubiquitous in the atmospheres of these planets, and by extension, other gas giants.

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arXiv2016

Jupiter internal structure: the effect of different equations of state

Heavy elements, even though its smaller constituent, are crucial to understand Jupiter formation history. Interior models are used to determine the amount of heavy elements in Jupiter interior, nevertheless this range is still subject to degeneracies due to uncertainties in the equations of state. Prior to Juno mission data arrival, we present Jupiter optimized calculations exploring the effect of different model parameters in the determination of Jupiter's core and heavy element's mass. We perform comparisons between equations of state published recently. The interior model of Jupiter is calculated from the equations of hydrostatic equilibrium, mass and energy conservation, and energy transport. The mass of the core and heavy elements is adjusted to match Jupiter's observational constrains radius and gravitational moments. We show that the determination of Jupiter interior structure is tied to the estimation of its gravitational moments and the accuracy of equations of state of hydrogen, helium and heavy elements. The location of the region where Helium rain occurs as well as its timescale are important to determine the distribution of heavy elements and helium in the interior of Jupiter. We show that differences find when modeling Jupiter's interior with recent EOS are more likely due to differences in the internal energy and entropy calculation. The consequent changes in the thermal profile lead to different estimations of the mass of the core and heavy elements, explaining differences in recently published Jupiter interior models. Our results help clarify differences find in Jupiter interior models and will help the interpretation of upcoming Juno data.

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arXiv2016

The MUSCLES Treasury Survey II: Intrinsic Lyman Alpha and Extreme Ultraviolet Spectra of K and M Dwarfs with Exoplanets

The ultraviolet (UV) spectral energy distributions of low-mass (K- and M-type) stars play a critical role in the heating and chemistry of exoplanet atmospheres, but are not observationally well-constrained. Direct observations of the intrinsic flux of the Lyman alpha line (the dominant source of UV photons from low-mass stars) are challenging, as interstellar HI absorbs the entire line core for even the closest stars. To address the existing gap in empirical constraints on the UV flux of K and M dwarfs, the MUSCLES HST Treasury Survey has obtained UV observations of 11 nearby M and K dwarfs hosting exoplanets. This paper presents the Lyman alpha and extreme-UV spectral reconstructions for the MUSCLES targets. Most targets are optically inactive, but all exhibit significant UV activity. We use a Markov Chain Monte Carlo technique to correct the observed Lyman alpha profiles for interstellar absorption, and we employ empirical relations to compute the extreme-UV spectral energy distribution from the intrinsic Lyman alpha flux in ~100 Å bins from 100-1170 Å. The reconstructed Lyman alpha profiles have 300 km/s broad cores, while >1% of the total intrinsic Lyman alpha flux is measured in extended wings between 300 km/s to 1200 km/s. The Lyman alpha surface flux positively correlates with the MgII surface flux and negatively correlates with the stellar rotation period. Stars with larger Lyman alpha surface flux also tend to have larger surface flux in ions formed at higher temperatures, but these correlations remain statistically insignificant in our sample of 11 stars. We also present HI column density measurements for 10 new sightlines through the local interstellar medium.

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arXiv2016

The MUSCLES Treasury Survey I: Motivation and Overview

Ground- and space-based planet searches employing radial velocity techniques and transit photometry have detected thousands of planet-hosting stars in the Milky Way. The chemistry of these atmospheres is controlled by the shape and absolute flux of the stellar spectral energy distribution, however, flux distributions of relatively inactive low-mass stars are poorly known at present. To better understand exoplanets orbiting low-mass stars, we have executed a panchromatic (X-ray to mid-IR) study of the spectral energy distributions of 11 nearby planet hosting stars, the {\it Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems} (MUSCLES) Treasury Survey. The MUSCLES program consists of contemporaneous observations at X-ray, UV, and optical wavelengths. We show that energetic radiation (X-ray and ultraviolet) is present from magnetically active stellar atmospheres at all times for stars as late as M5. Emission line luminosities of \ion{C}{4} and \ion{Mg}{2} are strongly correlated with band-integrated luminosities. We find that while the slope of the spectral energy distribution, FUV/NUV, increases by approximately two orders of magnitude form early K to late M dwarfs ($\approx$~0.01~to~1), the absolute FUV and XUV flux levels at their corresponding habitable zone distances are constant to within factors of a few, spanning the range 10~--~70 erg cm$^{-2}$ s$^{-1}$ in the habitable zone. Despite the lack of strong stellar activity indicators in their optical spectra, several of the M dwarfs in our sample show spectacular flare emission in their UV light curves. Finally, we interpret enhanced $L(line)$/$L_{Bol}$ ratios for \ion{C}{4} and \ion{N}{5} as tentative observational evidence for the interaction of planets with large planetary mass-to-orbital distance ratios ($M_{plan}$/$a_{plan}$) with the transition regions of their host stars.

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arXiv2015

A semi-analytical model for exploring Galilean satellites formation from a massive disk

A better knowledge of Jovian satellites' origins will bring light on the environment that surrounded Jupiter during its formation and can help us to understand the characteristics of this unique satellite system. We developed a semi-analytical model to investigate Jupiter's regular satellite formation and present the results of our population synthesis calculations. We performed simulations adopting a massive, static, low-viscosity circumplanetary disk model, in agreement with a current study of magnetorotational instability in a circum-planetary disk. We find that the high gas density leads to very rapid migration of satellitesimals due to gas drag and type II migration of satellites in a faster disk-dominated mode. A large concentration of solids, large building blocks and longer type II migration time-scales favor formation and survival of large satellites. However, bodies as massive as Ganymede and those located far away from Jupiter, such as Callisto, are difficult to form with this scenario.

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arXiv2015

Characterizing the Habitable Zones of Exoplanetary Systems with a Large Ultraviolet/Visible/Near-IR Space Observatory

Understanding the surface and atmospheric conditions of Earth-size, rocky planets in the habitable zones (HZs) of low-mass stars is currently one of the greatest astronomical endeavors. Knowledge of the planetary effective surface temperature alone is insufficient to accurately interpret biosignature gases when they are observed in the coming decades. The UV stellar spectrum drives and regulates the upper atmospheric heating and chemistry on Earth-like planets, is critical to the definition and interpretation of biosignature gases, and may even produce false-positives in our search for biologic activity. This white paper briefly describes the scientific motivation for panchromatic observations of exoplanetary systems as a whole (star and planet), argues that a future NASA UV/Vis/near-IR space observatory is well-suited to carry out this work, and describes technology development goals that can be achieved in the next decade to support the development of a UV/Vis/near-IR flagship mission in the 2020s.

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arXiv2014

The effect of Lyman $α$ radiation on mini-Neptune atmospheres around M stars: application to GJ 436b

Mini-Neptunes orbiting M stars are a growing population of known exoplanets. Some of them are located very close to their host star, receiving large amounts of UV radiation. Many M stars emit strong chromospheric emission in the H I Lyman $α$ line (Ly$α$) at 1215.67 Å, the brightest far-UV emission line. We show that the effect of incoming Ly$α$ flux can significantly change the photochemistry of mini-Neptunes' atmospheres. We use GJ 436b as an example, considering different metallicities for its atmospheric composition. For solar composition, H$_2$O-mixing ratios show the largest change because of Ly$α$ radiation. H$_2$O absorbs most of this radiation, thereby shielding CH$_4$, whose dissociation is driven mainly by radiation at other far-UV wavelengths ($\sim1300$ Å). H$_2$O photolysis also affects other species in the atmosphere, including H, H$_2$, CO$_2$, CO, OH and O. For an atmosphere with high metallicity, H$_2$O- and CO$_2$-mixing ratios show the biggest change, thereby shielding CH$_4$. Direct measurements of the UV flux of the host stars are important for understanding the photochemistry in exoplanets' atmospheres. This is crucial, especially in the region between 1 and 10$^{-6}$ bars, which is the part of the atmosphere that generates most of the observable spectral features.

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arXiv2013

Exploring atmospheres of hot mini-Neptunes and extrasolar giant planets orbiting different stars with application to HD 97658b, WASP-12b, CoRoT-2b, XO-1b and HD 189733b

We calculated an atmospheric grid for hot mini-Neptune and giant exoplanets, that links astrophysical observable parameters- orbital distance and stellar type- with the chemical atmospheric species expected. The grid can be applied to current and future observations to characterize exoplanet atmospheres and serves as a reference to interpret atmospheric retrieval analysis results. To build the grid, we developed a 1D code for calculating the atmospheric thermal structure and link it to a photochemical model that includes disequilibrium chemistry (molecular diffusion, vertical mixing and photochemistry). We compare thermal profiles and atmospheric composition of planets at different semimajor axis (0.01$\leq$a$\leq$0.1AU) orbiting F, G, K and M stars. Temperature and UV flux affect chemical species in the atmosphere. We explore which effects are due to temperature and which due to stellar characteristics, showing the species most affected in each case. CH$_4$ and H$_2$O are the most sensitive to UV flux, H displaces H$_2$ as the most abundant gas in the upper atmosphere for planets receiving a high UV flux. CH$_4$ is more abundant for cooler planets. We explore vertical mixing, to inform degeneracies on our models and in the resulting spectral observables. For lower pressures observable species like H$_2$O or CO$_2$ can indicate the efficiency of vertical mixing, with larger mixing ratios for a stronger mixing. By establishing the grid, testing the sensitivity of the results and comparing our model to published results, our paper provides a tool to estimate what observations could yield. We apply our model to WASP-12b, CoRoT-2b, XO-1b, HD189733b and HD97658b.

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arXiv2011

The Diversity of Planetary Systems Architectures: Contrasting Theory with Observations

We develop a semi-analytical model for computing planetary system formation with the aim of explaining the observed diversity of planetary systems architectures and relate this primordial diversity with the initial properties of the disc where they were born. We adopt different initial conditions based on recent results in protoplanetary discs observations, to generate a variety of planetary systems and analyze them statistically. We explore the relevance of the mass and size of the disc, its metallicity, the mass of the central star and the time-scale of gaseous disc dissipation, in defining the architecture of the planetary system. We also test different values of some key parameters of our model, to find out which factors best reproduce the diverse sample of observed planetary systems. According to this, we predict which systems are the most common in the solar neighbourhood. Our results show that planetary systems with only terrestrial planets are the most common, being the only planetary systems formed when considering low metallicity discs and which also represent the best environment for the developing of rocky, potentially habitable planets. We also found that planetary systems like our own are not rare in the solar neighbourhood, being its formation favoured in massive discs where there is not a large accumulation of solids in the inner region of the disc. Regarding the planetary systems that harbor hot and warm Jupiter planets, we found that this systems are born in very massive, metal-rich discs. Also a fast migration rate is required in order to form these systems. According to our results, most of the hot and warm Jupiter systems are composed by only one giant planet, which is also a tendency of the current observational data.

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