arXiv subjects

Ignas Snellen

Publications and source records attributed to Ignas Snellen.

At least 37 records · Page 2Linked to original sources

arXiv2022

Spatially-resolving the terminator: Variation of Fe, temperature and winds in WASP-76 b across planetary limbs and orbital phase

Exoplanet atmospheres are inherently three-dimensional systems in which thermal/chemical variation and winds can strongly influence spectra. Recently, the ultra-hot Jupiter WASP-76 b has shown evidence for condensation and asymmetric Fe absorption with time. However, it is currently unclear whether these asymmetries are driven by chemical or thermal differences between the two limbs, as precise constraints on variation in these have remained elusive due to the challenges of modelling these dynamics in a Bayesian framework. To address this we develop a new model, HyDRA-2D, capable of simultaneously retrieving morning and evening terminators with day-night winds. We explore variations in Fe, temperature profile, winds and opacity deck with limb and orbital phase using VLT/ESPRESSO observations of WASP-76 b. We find Fe is more prominent on the evening for the last quarter of the transit, with $\log(X_\mathrm{Fe}) = {-4.03}^{+0.28}_{-0.31}$, but the morning shows a lower abundance with a wider uncertainty, $\log(X_\mathrm{Fe}) = {-4.59}^{+0.85}_{-1.0}$, driven by degeneracy with the opacity deck and the stronger evening signal. We constrain 0.1 mbar temperatures ranging from $2950^{+111}_{-156}$ K to $2615^{+266}_{-275}$ K, with a trend of higher temperatures for the more irradiated atmospheric regions. We also constrain a day-night wind speed of $9.8^{+1.2}_{-1.1}$ km/s for the last quarter, higher than $5.9^{+1.5}_{-1.1}$ km/s for the first, in line with general circulation models. We find our new spatially- and phase-resolved treatment is statistically favoured by 4.9$σ$ over traditional 1D-retrievals, and thus demonstrate the power of such modelling for robust constraints with current and future facilities.

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arXiv2022

Water observed in the atmosphere of τ Bootis Ab with CARMENES/CAHA

Characterising the atmospheres of hot Jupiters is important in understanding the formation and migration of these exotic planets. However, there are still many open questions about the chemical and physical properties of these atmospheres. Here, we confirm the detection of water vapour in thermal emission from the non-transiting hot Jupiter τ Bootis Ab with the high resolution NIR CARMENES spectrograph. Combining over 17 h of observations (560 spectra) and using a Bayesian cross-correlation to log-likelihood approach, we measure a systemic velocity of $V_{sys} = -11.51^{+0.59}_{-0.60}$ km/s and a radial velocity semi-amplitude of $K_{P} = 106.21^{+1.76}_{-1.71}$ km/s for the planet, which results in an absolute mass of $M_{P} = 6.24^{+0.17}_{-0.18} M_{J}$ and an orbital inclination of $41.6^{+1.0}_{-0.9}$ degrees. Our retrieved $V_{sys}$ shows a significant shift (+5 km/s) from the literature value, which could be caused by an inaccurate time of periastron. Within the explored model grid, we measure a preference for solar water abundance (VMR = $10^{-3}$) and no evidence for additional minor species in the atmosphere. Given the extensive orbital coverage of the data, we searched for a phase dependency in the water signal but found no strong evidence of variation with orbital phase. This detection is at odds with recent observations from SPIRou/CFHT and their tight upper limit on water abundance. We recommend further observations of the atmosphere τ Bootis Ab to try and resolve these discrepancies.

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arXiv2021

Detection of OH in the ultra-hot Jupiter WASP-76b

Ultra-hot Jupiters have dayside temperatures at which most molecules are expected to thermally dissociate. The dissociation of water vapour results in the production of the hydroxyl radical (OH). While OH absorption is easily observed in near-infrared spectra of M dwarfs, which have similar effective temperatures as ultra-hot Jupiters, it is often not considered when studying the atmospheres of ultra-hot Jupiters. We use high-resolution spectroscopic observations of a transit of WASP-76b obtained using CARMENES to study the presence of OH. After validating the OH line list, we generate model transit spectra of WASP-76b with petitRADTRANS. The data are corrected for stellar and telluric contamination and cross-correlated with the model spectra. After combining all cross-correlation functions from the transit, a detection map is constructed. Constraints on the planet properties from the OH absorption are obtained from a Markov chain Monte Carlo analysis. OH is detected in the atmosphere of WASP-76b with a peak signal-to-noise ratio of 6.1. From the retrieval we obtain $K_p=232 \pm 12$ km/s and a blueshift of $-13.2 \pm 1.6$ km/s, which are offset from the expected velocities. Considering the fast spin rotation of the planet, the blueshift is best explained with the signal predominantly originating from the evening terminator and the presence of a strong dayside-to-nightside wind. The increased $K_p$ over its expected value (196.5 km/s) is, however, a bit puzzling. The signal is found to be broad, with a full width at half maximum of $16.8^{+4.6}_{-4.0}$ km/s. The retrieval results in a weak constraint on the mean temperature of 2700-3700 K at the pressure range of the OH signal. We show that OH is readily observable in the transit spectra of ultra-hot Jupiters. Studying this molecule can provide insights into the molecular dissociation processes in the atmospheres of such planets.

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arXiv2021

TOI-1431b/MASCARA-5b: A Highly Irradiated Ultra-Hot Jupiter Orbiting One of the Hottest & Brightest Known Exoplanet Host Stars

We present the discovery of a highly irradiated and moderately inflated ultra-hot Jupiter, TOI-1431b/MASCARA-5b (HD 201033b), first detected by NASA's Transiting Exoplanet Survey Satellite mission (TESS) and the Multi-site All-Sky CAmeRA (MASCARA). The signal was established to be of planetary origin through radial velocity measurements obtained using SONG, SOPHIE, FIES, NRES, and EXPRES, which show a reflex motion of $K=294.1\pm1.1$ m s$^{-1}$. A joint analysis of the TESS and ground-based photometry and radial velocity measurements reveals that TOI-1431b has a mass of $M_{p}=3.12\pm0.18$ $\rm{M_J}$ ($990\pm60$ M$_{\oplus}$), an inflated radius of $R_{p}=1.49\pm0.05$ $\rm{R_J}$ ($16.7\pm0.6$ R$_{\oplus}$), and an orbital period of $P=2.650237\pm0.000003$ d. Analysis of the spectral energy distribution of the host star reveals that the planet orbits a bright ($\mathrm{V}=8.049$ mag) and young ($0.29^{+0.32}_{-0.19}$ Gyr) Am type star with $T_{\rm eff}=7690^{+400}_{-250}$ $\rm{K}$, resulting in a highly irradiated planet with an incident flux of $\langle F \rangle=7.24^{+0.68}_{-0.64}\times$10$^9$ erg s$^{-1}$ cm$^{-2}$ ($5300^{+500}_{-470}\mathrm{S_{\oplus}}$) and an equilibrium temperature of $T_{eq}=2370\pm70$ K. TESS photometry also reveals a secondary eclipse with a depth of $127^{+4}_{-5}$ppm as well as the full phase curve of the planet's thermal emission in the red-optical. This has allowed us to measure the dayside and nightside temperature of its atmosphere as $T_\mathrm{day}=3004\pm64$ K and $T_\mathrm{night}=2583\pm63$ K, the second hottest measured nightside temperature. The planet's low day/night temperature contrast ($\sim$420 K) suggests very efficient heat transport between the dayside and nightside hemispheres.

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arXiv2021

Atmospheric characterization of terrestrial exoplanets in the mid-infrared: biosignatures, habitability & diversity

Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the MIR wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large MIR exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large MIR exoplanet imaging mission will be needed to help answer one of mankind's most fundamental questions: "How unique is our Earth?"

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arXiv2021

The 13O-rich atmosphere of a young accreting super-Jupiter

Isotope abundance ratios play an important role in astronomy and planetary sciences, providing insights in the origin and evolution of the Solar System, interstellar chemistry, and stellar nucleosynthesis. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (~89) in the Solar System, but do vary on galactic scales with 12C/13C~68 in the current local interstellar medium. In molecular clouds and protoplanetary disks, 12CO/13CO isotopologue ratios can be altered by ice and gas partitioning, low-temperature isotopic ion exchange reactions, and isotope-selective photodissociation. Here we report on the detection of 13CO in the atmosphere of the young, accreting giant planet TYC 8998-760-1 b at a statistical significance of >6 sigma. Marginalizing over the planet's atmospheric temperature structure, chemical composition, and spectral calibration uncertainties, suggests a 12CO/13CO ratio of 31 [+17,-10] (90% confidence), a significant enrichment in 13C with respect to the terrestrial standard and the local interstellar value. Since the current location of TYC 8998 b at >160 au is far beyond the CO snowline, we postulate that it accreted a significant fraction of its carbon from ices enriched in 13C through fractionation. Future isotopologue measurements in exoplanet atmospheres can provide unique constraints on where, when and how planets are formed.

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arXiv2020

Original use of MUSE's laser tomography adaptive optics to directly image young accreting exoplanets

We present recent results obtained with the VLT/MUSE Integral Field Spectrograph fed by the 4LGSF and its laser tomography adaptive optics module GALACSI. While this so-called narrow-field mode of MUSE was not designed to perform directly imaging of exoplanets and outflows, we show that it can be a game changer to detect and characterize young exoplanets with a prominent emission lines (i.e Hα, tracer of accretion), at moderate contrasts. These performances are achieved thanks to the combo of a near-diffraction limited PSF and a medium resolution spectrograph and a cross-correlation approach in post-processing . We discuss this in the context of ground and space, infrared and visible wavelengths, preparing for missions like JWST and WFIRST in great synergy and as pathfinder for future ELT/GSMT (Extremely Large and/or Giant Segmented Mirror Telescopes) instruments.

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arXiv2019

Gravity-Darkening Analysis of Misaligned Hot Jupiter MASCARA-4 b

MASCARA-4 b is a hot Jupiter in a highly-misaligned orbit around a rapidly-rotating A3V star that was observed for 54 days by the Transiting Exoplanet Survey Satellite (\tess). We perform two analyses of MASCARA-4 b using a stellar gravity-darkened model. First, we measure MASCARA-4 b's misaligned orbital configuration by modeling its \tess~photometric light curve. We take advantage of the asymmetry in MASCARA-4 b's transit due to its host star's gravity-darkened surface to measure MASCARA-4 b's true spin-orbit angle to be $104^{\circ+7^\circ}_{-13^\circ}$. We also detect a $\sim4σ$ secondary eclipse at $0.491\pm0.007$ orbital phase, proving that the orbit is slightly eccentric. Second, we model MASCARA-4 b's insolation including gravity-darkening and find that the planet's received XUV flux varies by $4$\% throughout its orbit. MASCARA-4 b's short-period, polar orbit suggests that the planet likely underwent dramatic orbital evolution to end up in its present-day configuration and that it receives a varying stellar irradiance that perpetually forces the planet out of thermal equilibrium. These findings make MASCARA-4 b an excellent target for follow-up characterization to better understand orbital evolution and current-day of planets around high-mass stars.

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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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arXiv2018

The orbital eccentricity of small planet systems

We determine the orbital eccentricities of individual small Kepler planets, through a combination of asteroseismology and transit light-curve analysis. We are able to constrain the eccentricities of 51 systems with a single transiting planet, which supplement our previous measurements of 66 planets in multi-planet systems. Through a Bayesian hierarchical analysis, we find evidence that systems with only one detected transiting planet have a different eccentricity distribution than systems with multiple detected transiting planets. The eccentricity distribution of the single-transiting systems is well described by the positive half of a zero-mean Gaussian distribution with a dispersion $σ_e = 0.32 \pm 0.06$, while the multiple-transit systems are consistent with $σ_e = 0.083^{+0.015}_{-0.020}$. A mixture model suggests a fraction of $0.76^{+0.21}_{-0.12}$ of single-transiting systems have a moderate eccentricity, represented by a Rayleigh distribution that peaks at $0.26^{+0.04}_{-0.06}$. This finding may reflect differences in the formation pathways of systems with different numbers of transiting planets. We investigate the possibility that eccentricities are "self-excited" in closely packed planetary systems, as well as the influence of long-period giant companion planets. We find that both mechanisms can qualitatively explain the observations. We do not find any evidence for a correlation between eccentricity and stellar metallicity, as has been seen for giant planets. Neither do we find any evidence that orbital eccentricity is linked to the detection of a companion star. Along with this paper we make available all of the parameters and uncertainties in the eccentricity distributions, as well as the properties of individual systems, for use in future studies.

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arXiv2018

The mass of the young planet Pictoris b through the astrometric motion of its host star

The young massive Jupiters discovered with high-contrast imaging provide a unique opportunity to study the formation and early evolution of gas giant planets. A key question is to what extent gravitational energy from accreted gas contributes to the internal energy of a newly formed planet. This has led to a range of formation scenarios from 'cold' to 'hot' start models. For a planet of a given mass, these initial conditions govern its subsequent evolution in luminosity and radius. Except for upper limits from radial velocity studies, disk modelling, and dynamical instability arguments, no mass measurements of young planets are yet available to distinguish between these different models. Here we report on the detection of the astrometric motion of Beta Pictoris, the 21 Myr-old host star of an archetypical directly-imaged gas giant planet, around the system's centre of mass. Subtracting the highly accurate Hipparcos-Gaia proper motion from the internal 3-yr Hipparcos astrometric data reveals the reflex motion of the star, giving a model-independent planet mass of M=11+-2 MJup. This is consistent with scenarios in which the planet is formed in a high-entropy state as assumed by hot start models. The ongoing data collection by Gaia will in the near future lead to mass measurements of other young gas giants and form a great asset to further constrain early evolution scenarios.

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arXiv2018

An asteroseismic view of the radius valley: stripped cores, not born rocky

Various theoretical models treating the effect of stellar irradiation on planetary envelopes predict the presence of a radius valley: i.e. a bimodal distribution of planet radii, with super-Earths and sub-Neptune planets separated by a valley at around $\approx 2~R_\oplus$. Such a valley was observed recently, owing to an improvement in the precision of stellar, and therefore planetary radii. Here we investigate the presence, location and shape of such a valley using a small sample with highly accurate stellar parameters determined from asteroseismology, which includes 117 planets with a median uncertainty on the radius of 3.3%. We detect a clear bimodal distribution, with super-Earths ($\approx 1.5~R_\oplus$) and sub-Neptunes ($\approx 2.5~R_\oplus$) separated by a deficiency around $2~R_\oplus$. We furthermore characterize the slope of the valley as a power law $R \propto P^γ$ with $γ= {-0.09^{+0.02}_{-0.04}}$. A negative slope is consistent with models of photo-evaporation, but not with the late formation of rocky planets in a gas-poor environment, which would lead to a slope of opposite sign. The exact location of the gap further points to planet cores consisting of a significant fraction of rocky material.

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arXiv2018

SUPERSHARP - Segmented Unfolding Primary for Exoplanet Research via Spectroscopic High Angular Resolution Photography

We propose to search for biosignatures in the spectra of reflected light from about 100 Earth-sized planets that are already known to be orbiting in their habitable zones (HZ). For a sample of G and K type hosts, most of these planets will be between 25 and 50 milli-arcsec (mas) from their host star and 1 billion to 10 billion times fainter. To separate the planet's image from that of its host star at the wavelength (763nm) of the oxygen biosignature we need a telescope with an aperture of 16 metres. Furthermore, the intensity of the light from the host star at the position in the image of the exoplanet must be suppressed otherwise the exoplanet will be lost in the glare. This presents huge technical challenges. The Earth's atmosphere is turbulent which makes it impossible to achieve the required contrast from the ground at 763nm. The telescope therefore needs to be in space and to fit the telescope in the rocket fairing it must be a factor of 4 or more times smaller when folded than when operational. To obtain spectroscopy of the planet's biosignature at 763nm we need to use an integral field spectrometer (IFS) with a field of view (FOV) of 1000 x 1000 milli-arcsec (mas) and a spectral resolution of 100. This is a device that simultaneously takes many pictures of the exoplanet each at a slightly different wavelength which are then recorded as a data cube with two spatial dimensions and one wavelength dimension. In every data cube wavelength slice, the background light from the host star at the location of the planet image must be minimised. This is achieved via a coronagraph which blocks the light from the host star and active/adaptive optics techniques which continuously maintain very high accuracy optical alignment to make the images as sharp as possible. These are the technical challenges to be addressed in a design study.

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arXiv2016

HARPS3 for a Roboticized Isaac Newton Telescope

We present a description of a new instrument development, HARPS3, planned to be installed on an upgraded and roboticized Isaac Newton Telescope by end-2018. HARPS3 will be a high resolution (R = 115,000) echelle spectrograph with a wavelength range from 380-690 nm. It is being built as part of the Terra Hunting Experiment - a future 10 year radial velocity measurement programme to discover Earth-like exoplanets. The instrument design is based on the successful HARPS spectrograph on the 3.6m ESO telescope and HARPS-N on the TNG telescope. The main changes to the design in HARPS3 will be: a customised fibre adapter at the Cassegrain focus providing a stabilised beam feed and on-sky fibre diameter ~ 1.4 arcsec, the implementation of a new continuous flow cryostat to keep the CCD temperature very stable, detailed characterisation of the HARPS3 CCD to map the effective pixel positions and thus provide an improved accuracy wavelength solution, an optimised integrated polarimeter and the instrument integrated into a robotic operation. The robotic operation will optimise our programme which requires our target stars to be measured on a nightly basis. We present an overview of the entire project, including a description of our anticipated robotic operation.

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arXiv2015

Combining high-dispersion spectroscopy (HDS) with high contrast imaging (HCI): Probing rocky planets around our nearest neighbors

Aims: In this work, we discuss a way to combine High Dispersion Spectroscopy and High Contrast Imaging (HDS+HCI). For a planet located at a resolvable angular distance from its host star, the starlight can be reduced up to several orders of magnitude using adaptive optics and/or coronography. In addition, the remaining starlight can be filtered out using high-dispersion spectroscopy, utilizing the significantly different (or Doppler shifted) high-dispersion spectra of the planet and star. In this way, HDS+HCI can in principle reach contrast limits of ~1e-5 x 1e-5, although in practice this will be limited by photon noise and/or sky-background. Methods: We present simulations of HDS+HCI observations with the E-ELT, both probing thermal emission from a planet at infrared wavelengths, and starlight reflected off a planet atmosphere at optical wavelengths. For the infrared simulations we use the baseline parameters of the E-ELT and METIS instrument, with the latter combining extreme adaptive optics with an R=100,000 IFS. We include realistic models of the adaptive optics performance and atmospheric transmission and emission. For the optical simulation we also assume R=100,000 IFS with adaptive optics capabilities at the E-ELT. Results: One night of HDS+HCI observations with the E-ELT at 4.8 um (d_lambda = 0.07 um) can detect a planet orbiting alpha Cen A with a radius of R=1.5 R_earth and a twin-Earth thermal spectrum of T_eq=300 K at a signal-to-noise (S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected light from an Earth-size planet in the habitable zone of Proxima Centauri can be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI observations have shown the potential of this technique by determining the spin-rotation of the young massive exoplanet beta Pictoris b. [abridged]

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arXiv2015

The EChO science case

The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse planet population. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? What causes the exceptional diversity observed as compared to the Solar System? EChO (Exoplanet Characterisation Observatory) has been designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large and diverse planet sample within its four-year mission lifetime. EChO can target the atmospheres of super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300K-3000K) of F to M-type host stars. Over the next ten years, several new ground- and space-based transit surveys will come on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current rapid rate of discovery would allow the target list to be further optimised in the years prior to EChO's launch and enable the atmospheric characterisation of hundreds of planets. Placing the satellite at L2 provides a cold and stable thermal environment, as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. A 1m class telescope is sufficiently large to achieve the necessary spectro-photometric precision. The spectral coverage (0.5-11 micron, goal 16 micron) and SNR to be achieved by EChO, thanks to its high stability and dedicated design, would enable a very accurate measurement of the atmospheric composition and structure of hundreds of exoplanets.

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arXiv2015

Evidence against a strong thermal inversion in HD 209458 b from high-dispersion spectroscopy

Broadband secondary-eclipse measurements of hot Jupiters have indicated the existence of atmospheric thermal inversions, but their presence is difficult to determine from broadband measurements because of degeneracies between molecular abundances and temperature structure. We apply high-resolution (R = 100 000) infrared spectroscopy to probe the temperature-pressure profile of HD 209458 b. This bright, transiting hot-Jupiter has long been considered the gold standard for a hot Jupiter with an inversion layer, but this has been challenged in recent publications. We observed the thermal dayside emission of HD 209458 b with CRIRES / VLT during three nights, targeting the carbon monoxide band at 2.3 microns. Thermal inversions give rise to emission features, which means that detecting emission lines in the planetary spectrum, as opposed to absorption lines, would be direct evidence of a region in which the temperature increases with altitude. We do not detect any significant absorption or emission of CO in the dayside spectrum of HD 209458 b, although cross-correlation with template spectra either with CO absorption lines or with weak emission at the core of the lines show a low-significance correlation signal with a signal-to-noise ratio of 3 - 3.5. Models with strong CO emission lines show a weak anti-correlation with similar or lower significance levels. Furthermore, we found no evidence of absorption or emission from H2O at these wavelengths. The non-detection of CO in the dayside spectrum of HD 209458 b is interesting in light of a previous CO detection in the transmission spectrum. That there is no signal indicates that HD 209458 b either has a nearly isothermal atmosphere or that the signal is heavily muted. Assuming a clear atmosphere, we can rule out a full-disc dayside inversion layer in the pressure range 1 bar to 1 mbar.

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arXiv2014

The fast spin-rotation of a young extrasolar planet

The spin-rotation of a planet arises from the accretion of angular momentum during its formation, but the details of this process are still unclear. In the solar system, the equatorial rotation velocities and spin angular momentum of the planets show a clear trend with mass, except for Mercury and Venus which have significantly spun down since their formation due to tidal interactions. Here we report on near-infrared spectroscopic observations at R=100,000 of the young extra-solar gas giant beta Pictoris b. The absorption signal from carbon monoxide in the planet's thermal spectrum is found to be blueshifted with respect to the velocity of the parent star by (-15+-1.7) km/sec, consistent with a circular orbit. The combined line profile exhibits a rotational broadening of 25+-3 km/sec, meaning that Beta Pictoris b spins significantly faster than any planet in the solar system, in line with the extrapolation of the known trend in spin velocity with planet mass.

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