arXiv subjects

Giovanna Tinetti

Publications and source records attributed to Giovanna Tinetti.

At least 55 records · Page 3Linked to original sources

arXiv2020

MARVEL, a four-telescope array for high-precision radial-velocity monitoring

Since the first discovery of a planet outside of our Solar System in 1995, exoplanet research has shifted from detecting to characterizing worlds around other stars. The TESS (NASA, launched 2019) and PLATO mission (ESA, planned launch 2026) will find and constrain the size of thousands of exoplanets around bright stars all over the sky. Radial velocity measurements are needed to characterize the orbit and mass, and complete the picture of densities and composition of the exoplanet systems found. The Ariel mission (ESA, planned launch 2028) will characterize exoplanet atmospheres with infrared spectroscopy. Characterization of stellar activity using optical spectroscopy from the ground is key to retrieve the spectral footprint of the planetary atmosphere in Ariel's spectra. To enable the scientific harvest of the TESS, PLATO and Ariel space missions, we plan to install MARVEL as an extension of the existing Mercator Telescope at the Roque De Los Muchachos Observatory on La Palma (SPAIN). MARVEL consists of an array of four 80 cm telescopes linked through optical fibers to a single high-resolution echelle spectrograph, optimized for extreme-precision radial velocity measurements. It can observe the radial velocities of four different stars simultaneously or, alternatively, combine the flux from four telescopes pointing to a single faint target in one spectrum. MARVEL is constructed by a KU Leuven (Belgium) led collaboration, with contributions from the UK, Austria, Australia, Sweden, Denmark and Spain. In this paper, we present the MARVEL instrument with special focus on the optical design and expected performance of the spectrograph, and report on the status of the project.

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arXiv2020

ExoClock Project: An open platform for monitoring the ephemerides of Ariel targets with contributions from the public

The Ariel mission will observe spectroscopically around 1000 exoplanets to further characterise their atmospheres. For the mission to be as efficient as possible, a good knowledge of the planets' ephemerides is needed before its launch in 2028. While ephemerides for some planets are being refined on a per-case basis, an organised effort to collectively verify or update them when necessary does not exist. In this study, we introduce the ExoClock project, an open, integrated and interactive platform with the purpose of producing a confirmed list of ephemerides for the planets that will be observed by Ariel. The project has been developed in a manner to make the best use of all available resources: observations reported in the literature, observations from space instruments and, mainly, observations from ground-based telescopes, including both professional and amateur observatories. To facilitate inexperienced observers and at the same time achieve homogeneity in the results, we created data collection and validation protocols, educational material and easy to use interfaces, open to everyone. ExoClock was launched in September 2019 and now counts over 140 participants from more than 15 countries around the world. In this release, we report the results of observations obtained until the 15h of April 2020 for 119 Ariel candidate targets. In total, 632 observations were used to either verify or update the ephemerides of 83 planets. Additionally, we developed the Exoplanet Characterisation Catalogue (ECC), a catalogue built in a consistent way to assist the ephemeris refinement process. So far, the collaborative open framework of the ExoClock project has proven to be highly efficient in coordinating scientific efforts involving diverse audiences. Therefore, we believe that it is a paradigm that can be applied in the future for other research purposes, too.

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arXiv2020

Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b

Atmospheric characterisation of temperate, rocky planets is the holy grail of exoplanet studies. These worlds are at the limits of our capabilities with current instrumentation in transmission spectroscopy and challenge our state-of-the-art statistical techniques. Here we present the transmission spectrum of the temperate Super-Earth LHS 1140b using the Hubble Space Telescope (HST). The Wide Field Camera 3 (WFC3) G141 grism data of this habitable zone (T$_{\rm{eq}}$ = 235 K) Super-Earth (R = 1.7 $R_\oplus$), shows tentative evidence of water. However, the signal-to-noise ratio, and thus the significance of the detection, is low and stellar contamination models can cause modulation over the spectral band probed. We attempt to correct for contamination using these models and find that, while many still lead to evidence for water, some could provide reasonable fits to the data without the need for molecular absorption although most of these cause also features in the visible ground-based data which are nonphysical. Future observations with the James Webb Space Telescope (JWST) would be capable of confirming, or refuting, this atmospheric detection.

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arXiv2020

The future of astronomy with small satellites

The number of small satellites has grown dramatically in the past decade from tens of satellites per year in the mid-2010s to a projection of tens of thousands in orbit by the mid-2020s. This presents both problems and opportunities for observational astronomy. Small satellites offer complementary cost-effective capabilities to both ground-based astronomy and larger space missions. Compared to ground-based astronomy, these advantages are not just in the accessibility of wavelength ranges where the Earth's atmosphere is opaque, but also in stable, high precision photometry, long-term monitoring and improved areal coverage. Astronomy has a long history of new observational parameter spaces leading to major discoveries. Here we discuss the potential for small satellites to explore new parameter spaces in astrophysics, drawing on examples from current and proposed missions, and spanning a wide range of science goals from binary stars, exoplanets and solar system science to the early Universe and fundamental physics.

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arXiv2020

Lessons Learned from the 1st ARIEL Machine Learning Challenge: Correcting Transiting Exoplanet Light Curves for Stellar Spots

The last decade has witnessed a rapid growth of the field of exoplanet discovery and characterisation. However, several big challenges remain, many of which could be addressed using machine learning methodology. For instance, the most prolific method for detecting exoplanets and inferring several of their characteristics, transit photometry, is very sensitive to the presence of stellar spots. The current practice in the literature is to identify the effects of spots visually and correct for them manually or discard the affected data. This paper explores a first step towards fully automating the efficient and precise derivation of transit depths from transit light curves in the presence of stellar spots. The methods and results we present were obtained in the context of the 1st Machine Learning Challenge organized for the European Space Agency's upcoming Ariel mission. We first present the problem, the simulated Ariel-like data and outline the Challenge while identifying best practices for organizing similar challenges in the future. Finally, we present the solutions obtained by the top-5 winning teams, provide their code and discuss their implications. Successful solutions either construct highly non-linear (w.r.t. the raw data) models with minimal preprocessing -deep neural networks and ensemble methods- or amount to obtaining meaningful statistics from the light curves, constructing linear models on which yields comparably good predictive performance.

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arXiv2020

Integrating light-curve and atmospheric modelling of transiting exoplanets

Spectral retrieval techniques are currently our best tool to interpret the observed exoplanet atmospheric data. Said techniques retrieve the optimal atmospheric components and parameters by identifying the best fit to an observed transmission/emission spectrum. Over the past decade, our understanding of remote worlds in our galaxy has flourished thanks to the use of increasingly sophisticated spectral retrieval techniques and the collective effort of the community working on exoplanet atmospheric models. A new generation of instruments in space and from the ground is expected to deliver higher quality data in the next decade, it is therefore paramount to upgrade current models and improve their reliability, completeness and numerical speed with which they can be run. In this paper, we address the issue of reliability of the results provided by retrieval models in the presence of systematics of unknown origin. More specifically, we demonstrate that if we fit directly individual light-curves at different wavelengths (L-retrieval), instead of fitting transit or eclipse depths, as it is currently done (S-retrieval), the results obtained are more robust against astrophysical and instrumental noise. This new approach is tested, in particular, when discrepant simulated observations from HST/WFC3 and Spitzer/IRAC are combined. We find that while S-retrievals converge to an incorrect solution without any warning, L-retrievals are able to identify potential discrepancies between the data-sets.

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arXiv2020

On The Compatibility of Ground-based and Space-based Data: WASP-96 b, An Example

The study of exoplanetary atmospheres relies on detecting minute changes in the transit depth at different wavelengths. To date, a number of ground and space based instruments have been used to obtain transmission spectra of exoplanets in different spectral band. One common practice is to combine observations from different instruments in order to achieve a broader wavelength coverage. We present here two inconsistent observations of WASP-96 b, one by Hubble Space Telescope (HST) and the other by the Very Large Telescope (VLT). We present two key findings in our investigation: 1.) a strong water signature is detected via the HST WFC3 observations. 2.) A notable offset in transit depth (> 1100 ppm) can be seen when the ground-based and space-based observations are combined together. The discrepancy raises the question of whether observations from different instruments could indeed be combined together. We attempt to align the observations by including an additional parameter in our retrieval studies but are unable to definitively ascertain that the aligned observations are indeed compatible. The case of WASP-96 b signals that compatibility of instruments should not be assumed. While wavelength overlaps between instruments can help, it should be noted that combining datasets remains a risky business. The difficulty in combining observations also strengthens the need for next generation instruments which will possess broader spectral coverage.

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arXiv2020

Alfnoor: A Retrieval Simulation of the Ariel Target List

In this work, we present Alfnoor, a dedicated tool optimised for population studies of exoplanet atmospheres. Alfnoor combines the latest version of the retrieval algorithm TauREx 3, with the instrument noise simulator ArielRad and enables the simultaneous retrieval analysis of a large sample of exo-atmospheres. We applied this tool to the Ariel list of planetary candidates and focus on hydrogen dominated, cloudy atmospheres observed in transit with the Tier-2 mode (medium Ariel resolution). As a first experiment, we randomised the abundances - ranging from 10$^{-7}$ to 10$^{-2}$ - of the trace gases, which include H$_2$O, CH$_4$, CO, CO$_2$ and NH$_3$. This exercise allowed to estimate the detection limits for Ariel Tier-2 and Tier-3 modes when clouds are present. In a second experiment, we imposed an arbitrary trend between a chemical species and the effective temperature of the planet. A last experiment was run requiring molecular abundances being dictated by equilibrium chemistry at a certain temperature. Our results demonstrate the ability of Ariel Tier-2 and Tier-3 surveys to reveal trends between the chemistry and associated planetary parameters. Future work will focus on eclipse data, on atmospheres heavier than hydrogen and will be applied also to other observatories.

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arXiv2020

Towards a more complex description of chemical profiles in exoplanets retrievals: A 2-layer parameterisation

State of the art spectral retrieval models of exoplanet atmospheres assume constant chemical profiles with altitude. This assumption is justified by the information content of current datasets which do not allow, in most cases, for the molecular abundances as a function of pressure to be constrained. In the context of the next generation of telescopes, a more accurate description of chemical profiles may become crucial to interpret observations and gain new insights into atmospheric physics. We explore here the possibility of retrieving pressure-dependent chemical profiles from transit spectra, without injecting any priors from theoretical chemical models in our retrievals. The "2-layer" parameterisation presented here allows for the independent extraction of molecular abundances above and below a certain atmospheric pressure. By simulating various cases, we demonstrate that this evolution from constant chemical abundances is justified by the information content of spectra provided by future space instruments. Comparisons with traditional retrieval models show that assumptions made on chemical profiles may significantly impact retrieved parameters, such as the atmospheric temperature, and justify the attention we give here to this issue. We find that the 2-layer retrieval accurately captures discontinuities in the vertical chemical profiles, which could be caused by disequilibrium processes -- such as photo-chemistry -- or the presence of clouds/hazes. The 2-layer retrieval could also help to constrain the composition of clouds and hazes by exploring the correlation between the chemical changes in the gaseous phase and the pressure at which the condensed phase occurs. The 2-layer retrieval presented here therefore represents an important step forward in our ability to constrain theoretical chemical models and cloud/haze composition from the analysis of future observations.

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arXiv2020

Impact of planetary mass uncertainties on exoplanet atmospheric retrievals

In current models used to interpret exoplanet atmospheric observations, the planet mass is treated as a prior and is estimated independently with external methods, such as RV or TTV techniques. This approach is necessary as available spectroscopic data do not have sufficient wavelength coverage and/or SNR to infer the planetary mass. We examine here the impact of mass uncertainties on spectral retrieval analyses for a host of atmospheric scenarios. Our approach is both analytical and numerical: we first use simple approximations to extract analytically the influence of each parameter to the wavelength-dependent transit depth. We then adopt a fully Bayesian retrieval model to quantify the propagation of the mass uncertainty onto other atmospheric parameters. We found that for clear-sky, gaseous atmospheres the posterior distributions are the same when the mass is known or retrieved. The retrieved mass is very accurate, with a precision of more than 10%, provided the wavelength coverage and S/N are adequate. When opaque clouds are included in the simulations, the uncertainties in the retrieved mass increase, especially for high altitude clouds. However atmospheric parameters such as the temperature and trace-gas abundances are unaffected by the knowledge of the mass. Secondary atmospheres are more challenging due to the higher degree of freedom for the atmospheric main component, which is unknown. For broad wavelength range and adequate SNR, the mass can still be retrieved accurately and precisely if clouds are not present, and so are all the other atmospheric/planetary parameters. When clouds are added, we find that the mass uncertainties may impact substantially the retrieval of the mean molecular weight: an independent characterisation of the mass would therefore be helpful to capture/confirm the main atmospheric constituent.

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arXiv2020

KELT-11 b: Abundances of water and constraints on carbon-bearing molecules from the Hubble transmission spectrum

In the past decade, the analysis of exoplanet atmospheric spectra has revealed the presence of water vapour in almost all the planets observed, with the exception of a fraction of overcast planets. Indeed, water vapour presents a large absorption signature in the wavelength coverage of the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3), which is the main space-based observatory for atmospheric studies of exoplanets, making its detection very robust. However, while carbon-bearing species such as methane, carbon monoxide and carbon dioxide are also predicted from current chemical models, their direct detection and abundance characterisation has remained a challenge. Here we analyse the transmission spectrum of the puffy, clear hot-Jupiter KELT-11 b from the HST WFC3 camera. We find that the spectrum is consistent with the presence of water vapor and an additional absorption at longer wavelengths than 1.5um, which could well be explained by a mix of carbon bearing molecules. CO2, when included is systematically detected. One of the main difficulties to constrain the abundance of those molecules is their weak signatures across the HST WFC3 wavelength coverage, particularly when compared to those of water. Through a comprehensive retrieval analysis, we attempt to explain the main degeneracies present in this dataset and explore some of the recurrent challenges that are occurring in retrieval studies (e.g: the impact of model selection, the use of free vs self-consistent chemistry and the combination of instrument observations). Our results make this planet an exceptional example of chemical laboratory where to test current physical and chemical models of hot-Jupiters' atmospheres.

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arXiv2020

WASP-117 b: an eccentric hot-Saturn as a future complex chemistry laboratory

We present spectral analysis of the transiting Saturn-mass planet WASP-117b, observed with the G141 grism of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. We reduce and fit the extracted spectrum from the raw transmission data using the open-source software Iraclis before performing a fully Bayesian retrieval using the publicly available analysis suite TauREx 3.0. We detect water vapour alongside a layer of fully opaque cloud, retrieving a terminator temperature of 833 K. In order to quantify the statistical significance of this detection, we employ the Atmospheric Detectability Index (ADI), deriving a value of 2.30, which provides positive but not strong evidence against the flatline model. Due to the eccentric orbit of WASP-117b, it is likely that chemical and mixing timescales oscillate throughout orbit due to the changing temperature, possibly allowing warmer chemistry to remain visible as the planet begins transit, despite the proximity of its point of ingress to apastron. We present simulated spectra of the planet as would be observed by the future space missions Ariel and JWST and show that, despite not being able to probe such chemistry with current HST data, these observatories should make it possible in the not too distant future.

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arXiv2020

An Updated Study of Potential Targets for Ariel

Ariel has been selected as ESA's M4 mission for launch in 2028 and is designed for the characterisation of a large and diverse population of exoplanetary atmospheres to provide insights into planetary formation and evolution within our Galaxy. Here we present a study of Ariel's capability to observe currently-known exoplanets and predicted TESS discoveries. We use the Ariel Radiometric model (ArielRad) to simulate the instrument performance and find that ~2000 of these planets have atmospheric signals which could be characterised by Ariel. This list of potential planets contains a diverse range of planetary and stellar parameters. From these we select an example Mission Reference Sample (MRS), comprised of 1000 diverse planets to be completed within the primary mission life, which is consistent with previous studies. We also explore the mission capability to perform an in-depth survey into the atmospheres of smaller planets, which may be enriched or secondary. Earth-sized planets and Super-Earths with atmospheres heavier than H/He will be more challenging to observe spectroscopically. However, by studying the time required to observe ~110 Earth-sized/Super-Earths, we find that Ariel could have substantial capability for providing in-depth observations of smaller planets. Trade-offs between the number and type of planets observed will form a key part of the selection process and this list of planets will continually evolve with new exoplanet discoveries replacing predicted detections. The Ariel target list will be constantly updated and the MRS re-selected to ensure maximum diversity in the population of planets studied during the primary mission life.

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arXiv2020

The 2020 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres

The ExoMol database (www.exomol.com) provides molecular data for spectroscopic studies of hot atmospheres. While the data is intended for studies of exoplanets and other astronomical bodies, the dataset is widely applicable. The basic form of the database is extensive line lists; these are supplemented with partition functions, state lifetimes, cooling functions, Landé g-factors, temperature-dependent cross sections, opacities, pressure broadening parameters, $k$-coefficients and dipoles. This paper presents the latest release of the database which has been expanded to consider 80 molecules and 190 isotopologues totaling over 700 billion transitions. While the spectroscopic data is concentrated at infrared and visible wavelengths, ultraviolet transitions are being increasingly considered in response to requests from observers. The core of the database comes from the ExoMol project which primarily uses theoretical methods, albeit usually fine-tuned to reproduce laboratory spectra, to generate very extensive line lists for studies of hot bodies. The data has recently been supplemented by line lists deriving from direct laboratory observations, albeit usually with the use of ab initio transition intensities. A major push in the new release is towards accurate characterisation of transition frequencies for use in high resolution studies of exoplanets and other bodies.

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arXiv2020

Original Research By Young Twinkle Students (ORBYTS): Ephemeris Refinement of Transiting Exoplanets II

We report follow-up observations of four transiting exoplanets, TRES-2b, HAT-P-22b, HAT-P-36b and XO-2b, as part of the Original Research By Young Twinkle Students (ORBYTS) programme. These observations were taken using the Las Cumbres Observatory Global Telescope Network's (LCOGT) robotic 0.4 m telescopes and were analysed using the HOlomon Photometric Software (HOPS). Such observations are key for ensuring accurate transit times for upcoming telescopes, such as the James Webb Space Telescope (JWST), Twinkle and Ariel, which may seek to characterise the atmospheres of these planets. The data have been uploaded to ExoClock and a significant portion of this work has been completed by secondary school students in London.

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arXiv2020

Original Research By Young Twinkle Students (ORBYTS): Ephemeris Refinement of Transiting Exoplanets

We report follow-up observations of transiting exoplanets that have either large uncertainties (>10 minutes) in their transit times or have not been observed for over three years. A fully robotic ground-based telescope network, observations from citizen astronomers and data from TESS have been used to study eight planets, refining their ephemeris and orbital data. Such follow-up observations are key for ensuring accurate transit times for upcoming ground and space-based telescopes which may seek to characterise the atmospheres of these planets. We find deviations from the expected transit time for all planets, with transits occurring outside the 1 sigma uncertainties for seven planets. Using the newly acquired observations, we subsequently refine their periods and reduce the current predicted ephemeris uncertainties to 0.28 - 4.01 minutes. A significant portion of this work has been completed by students at two high schools in London as part of the Original Research By Young Twinkle Students (ORBYTS) programme.

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arXiv2020

Ions in the Thermosphere of Exoplanets: Observable Constraints Revealed by Innovative Laboratory Experiments

With the upcoming launch of space telescopes dedicated to the study of exoplanets, the \textit{Atmospheric Remote-Sensing Infrared Exoplanet Large-survey} (ARIEL) and the \textit{James Webb Space Telescope} (JWST), a new era is opening in the exoplanetary atmospheric explorations. However, especially in relatively cold planets around later-type stars, photochemical hazes and clouds may mask the composition of the lower part of the atmosphere, making it difficult to detect any chemical species in the troposphere or to understand whether there is a surface or not. This issue is particularly exacerbated if the goal is to study the habitability of said exoplanets and to search for biosignatures.\par This work combines innovative laboratory experiments, chemical modeling and simulated observations at ARIEL and JWST resolutions. We focus on the signatures of molecular ions that can be found in upper atmospheres above cloud decks. Our results suggest that H$_3^+$ along with H$_3$O$^+$ could be detected in the observational spectra of sub-Neptunes based on realistic mixing ratio assumption. This new parametric set may help to distinguish super-Earths with a thin atmosphere from H$_2$-dominated sub-Neptunes, to address the critical question whether a low-gravity planet around a low-mass active star is able to retain its volatile components. These ions may also constitute potential tracers to certain molecules of interest such as H$_2$O or O$_2$ to probe the habitability of exoplanets. Their detection will be an enthralling challenge for the future JWST and ARIEL telescopes.

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arXiv2020

Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence

In this paper we propose a new mechanism that could explain the survival of hydrogen atmospheres on some hot super-Earths. We argue that on close-orbiting tidally-locked super-Earths the tidal forces with the orbital and rotational centrifugal forces can partially confine the atmosphere on the nightside. Assuming a super terran body with an atmosphere dominated by volcanic species and a large hydrogen component, the heavier molecules can be shown to be confined within latitudes of $\lesssim 80^{\circ}$ whilst the volatile hydrogen is not. Because of this disparity the hydrogen has to slowly diffuse out into the dayside where XUV irradiation destroys it. For this mechanism to take effect it is necessary for the exoplanet to become tidally locked before losing the totality of its hydrogen envelop. Consequently, for super-Earths with this proposed configuration it is possible to solve the tidal-locking and mass-loss timescales in order to constrain their formation `birth' masses. Our model predicts that 55 Cancri e formed with a day-length between approximately $17-18.5$ hours and an initial mass less than $\rm \sim12 M_{\oplus}$ hence allowing it to become tidally locked before the complete destruction of its atmosphere. For comparison, CoRoT-7b, an exoplanet with very similar properties to 55 Cancri e but lacking an atmosphere, formed with a day-length significantly different from $\sim 20.5$ hours whilst also having an initial mass smaller than $\rm \sim9 M_{\oplus}$

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