arXiv subjects

A. García Muñoz

Publications and source records attributed to A. García Muñoz.

At least 19 recordsLinked to original sources

arXiv2025

Time-resolved X-ray spectra of Proxima Centauri as seen by XMM-Newton

Stellar soft X-ray ([1, 100] Angstrom) and Extreme Ultraviolet (also EUV, [100, 920] Angstrom; jointly, XUV) radiation affects the evolution and chemistry of exoplanet atmospheres. It is however uncertain to what extent the radiation's short-term variability contributes to these effects. We are interested in what this variability might imply for planets around M dwarf stars, and focus on Proxima Centauri (Prox Cen) for three reasons: it is an active M dwarf with high levels of variability; it hosts a likely terrestrial exoplanet within its habitable zone (HZ) that will be a prime target for future direct imaging; its proximity has led to extensive observations. We set out to produce time-resolved XUV spectra of Prox Cen that will serve as input to atmospheric models, and to characterize the intrinsic variability of the star. We analyzed the entire dataset of archival XMM-Newton observations for Prox Cen. To derive the time-resolved X-ray spectra, we implemented a new pile-up correction, a new adaptive time-binning algorithm, and a time-dependent plasma model selection. The estimated EUV spectrum is based on a published template, that we scale with proposed relationships between X-ray and EUV fluxes. We produced spectra of Prox Cen from 1 to 920 Angstrom over ~260 ks of observations with unprecedented time resolution. The instantaneous X-ray flux of Prox Cen varies between about 20 times and one-fifth of the average value over the available baseline, with significant differences between wavelengths. We further quantify how variability affects the estimated average flux when a limited number of snapshots (each typically of 30 ks exposure) are available, as is common in X-ray surveys. Future investigations of the atmospheres of Prox Cen b should fold in the time variability and uncertainties described here.

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arXiv2025

Long-term X-ray variability of the multiple-planet host L 98-59: Hints of an activity cycle

High-energy irradiation in X-rays and UV (XUV) can transform the planetary atmospheres through photoevaporation and photochemistry. This is more crucial for M stars, whose habitable zones for Earth-like planets are located within a few percent of an AU. Transiting exoplanets around M dwarfs offer the opportunity to study their characteristics and habitability conditions. L 98-59 is an M3 dwarf hosting six Earth-like planets, with two of them in the habitable zone of the star. X-ray observations made in 2020 and 2021 detected significant flares above a quiescent luminosity of 4-10 x 10^26 erg/s. We present the results from two short XMM-Newton observations of L 98-59, which are part of a monitoring survey to detect long-term X-ray variability and activity cycles. In October 2024 the X-ray quiescent luminosity of the star was about 5.9 x 10^25 erg/s, and it was about 6.3 x 10^26 erg/s in February 2025. We speculate that in late 2024 the star had a minimum of activity; in 2021 the star was near a maximum of an activity cycle, and in 2025 it was at the middle of the cycle. We suggest a coarse estimate of the period of about 2 years and a peak-to-peak amplitude of about 10, which is the highest among the stars with a known X-ray cycle other than the Sun. We also infer that even the outer planet in the habitable zone, L 98-59f, is exposed to an X-ray dose between 100 and 1600 times the X-ray irradiation of the Earth in the XMM band.

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arXiv2025

Detecting and characterising the magnetic field of exoplanets

Magnetic fields play a crucial role in planetary evolution and habitability. While the intrinsic magnetic fields of solar system planets are relatively well understood, the magnetic properties of exoplanets remain largely unconstrained, despite their potential ubiquity. Detecting exoplanetary magnetic fields is essential to advancing our understanding of planetary habitability beyond the solar system. This paper focuses on two promising spectropolarimetric techniques for detecting magnetic fields in hot exoplanets: direct detection through polarization signatures in the He I 1083 nm triplet and indirect detection via star-planet magnetic interactions manifesting as stellar hot spots. The direct method is particularly suited to close-in gas giants, leveraging the Hanle and Zeeman effects to detect low-amplitude magnetic fields. The indirect method can apply to both giant and low-mass planets by identifying magnetic connectivity-induced features in the stellar atmosphere. Although the interpretation of current detections remain tentative, upcoming high-resolution spectropolarimetric capabilities in the UV and near-infrared, particularly with future missions like HWO, promise to enable definitive measurements of exoplanetary magnetic fields. These advancements will open new avenues for probing the magnetic environments of exoplanets and their implications for atmospheric retention and habitability.

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arXiv2024

Ohmic heating in the upper atmosphere of hot exoplanets The influence of a time-varying magnetic field

Exoplanets on close-in orbit are subject to intense X-ray and ultraviolet (XUV) irradiation from their star. Their atmosphere therefore heats up, sometimes to the point where it thermally escapes from the gravitational potential of the planet. Nonetheless, XUV is not the only source of heating in such atmospheres. Indeed, close-in exoplanets are embedded in a medium (the stellar wind) with strong magnetic fields that can significantly vary along the orbit. The variations of this magnetic field can induce currents in the upper atmosphere, which dissipate and locally heat it up through Ohmic heating. The aim of this work is to quantify Ohmic heating in the upper atmosphere of hot exoplanets due to an external time-varying magnetic field, and to compare it to the XUV heating. Ohmic heating depends strongly on the conductivity properties of the upper atmosphere. A 1D formalism is developed to assess the level and the localization of Ohmic heating depending on the conductivity profile, and applied to the specific cases of Trappist-1 b and $π$ Men c. Ohmic heating can reach values up to 10$^{-3}$ erg s$^{-1}$ cm$^{-3}$ in the upper atmospheres of hot exoplanets. It is expected to be stronger the closer the planet is and the lower the central star mass is, as these conditions maximize the strength of the ambient magnetic field around the planet. We confirm that Ohmic heating can play an important role in setting the thermal budget of the upper atmosphere of hot exoplanets, and can even surpass the XUV heating in the most favorable cases. When it is strong, a corollary is that the upper atmosphere screens efficiently time-varying external magnetic fields, preventing them to penetrate deeper in the atmosphere or inside the planet itself. We find that both Trappist-1b and $π$ Men c are likely subject to intense Ohmic heating.

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arXiv2024

Large Interferometer For Exoplanets (LIFE). XIV. Finding terrestrial protoplanets in the galactic neighborhood

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $μ$m is important for decreasing the detection threshold and discriminating emission temperatures.

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arXiv2024

NLTE modelling of water-rich exoplanet atmospheres. Cooling and heating rates

The hydrogen and water molecules respond very differently to the collisional-radiative processes taking place in planetary atmospheres. Naturally, the question arises whether H2O-rich atmospheres are more (or less) resilient to long-term mass loss than H2-dominated ones if they radiate away the incident stellar energy more (or less) efficiently. If confirmed, the finding would have implications on our understanding of the evolution of sub-Neptune exoplanets. As a key step towards answering this question, we present a non-local thermodynamic equilibrium (NLTE) model of H2O for the atmospheric region where the gas accelerates to escape the planet and conditions relevant to close-in sub-Neptunes. Our exploratory calculations for isothermal gas composed of H2, H2O and e- reveal that: 1) In the pressure region ~1e-2 - 1e-4 dyn cm-2 where the stellar extreme-ultraviolet (XUV) photons are typically deposited in the atmosphere, H2O is in rotational LTE but vibrational NLTE. Vibrational LTE is facilitated by high H2O abundances and fractional ionizations, and we report critical densities for the LTE-NLTE transition; 2) Vibrational cooling may locally dominate over rotational cooling, partly because of the comparatively small opacities of ro-vibrational lines; 3) Even low H2O abundances notably enhance the cooling, foreseeably offsetting some of the stellar heating; 4) Heating due to the deposition of stellar infrared (IR) photons is significant at pressures >=0.1 dyn cm-2. We estimate the contribution of H2O excitation to the internal energy of the gas and speculate on the photodissociation from the excited vibrational states. Ultimately, our findings motivate the consideration of NLTE in the mass loss rate calculations of H2O-rich atmospheres.

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arXiv2024

A model of scattered thermal radiation for Venus from 3 to 5 $μ$ m

Thermal radiation becomes a prominent feature in the continuum spectrum of Venus longwards of $\sim$3 $μ$m. The emission is traceable to the upper cloud and haze layers in the planet's mesosphere. Venus' thermal radiation spectrum is punctuated by CO$_2$ bands of various strengths probing into different atmospheric depths. It is thus possible to invert measured spectra of thermal radiation to infer atmospheric temperature profiles and offer some insight into the cloud and haze structure. In practice, the retrieval becomes complicated by the fact that the outgoing radiation is multiply scattered by the ubiquitous aerosol particles before leaving the atmosphere. We numerically investigate the radiative transfer problem of thermal radiation from the Venus night side between 3 and 5 $μ$m with a purpose-built model of Venus' mesosphere. Special emphasis is laid on the significance of scattering. The simulations explore the space of model parameters, which includes the atmospheric temperature, cloud opacity, and the aerosols' size and chemical composition. We confirm that aerosol scattering must be taken into account in a prospective temperature retrieval, which means an additional complication to the already ill-posed retrieval problem. We briefly touch upon the degeneracy in the spectrum's shape associated with parameterization of the Venus clouds. Reasonable perturbations in the chemical composition and size of aerosols do not significantly impact the model simulations. Although the experiments are specific to the technical characteristics of the Visual and Infrared Thermal Imaging Spectrometer on the Venus Express spacecraft, the conclusions are generally valid.

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arXiv2024

Potential Vorticity of the South Polar Vortex of Venus

Venus' atmosphere shows highly variable warm vortices over both of the planet's poles. The nature of the mechanism behind their formation and properties is still unknown. Potential vorticity is a conserved quantity when advective processes dominate over friction and diabatic heating, and is a quantity frequently used to model balanced flows. As a step toward understanding the vortices' dynamics, we present maps of Ertel's potential vorticity (EPV) at Venus' south polar region. We analyze three configurations of the South Polar Vortex at the upper cloud level ($P\sim 240 mbar$; $z\sim 58 km$), based on our previous analyses of cloud motions and thermal structure from data acquired by the VIRTIS instrument onboard Venus Express. Additionally, we tentatively estimate EPV at the lower cloud level ($P\sim 2200 mbar$; $z\sim 43 km$), based on our previous wind measurements and on static stability data from Pioneer Venus and the VIRA model. Values of EPV are on the order of $10^{-6}$ and $10^{-8} K m^2 kg^{-1} s^{-1}$ at the upper and lower cloud levels, respectively, being 3 times larger than the estimated errors. The morphology observed in EPV maps is mainly determined by the structures of the vertical component of the relative vorticity. This is in contrast to the vortex's morphology observed in 3.8 or 5 $μm$ images which are related to the thermal structure of the atmosphere at the cloud top. Some of the EPV maps point to a weak ringed structure in the upper cloud while a more homogenous EPV field is found in the lower cloud.

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arXiv2024

Instantaneous Three-dimensional Thermal Structure of the South Polar Vortex of Venus

The Venus thermal radiation spectrum exhibits the signature of $CO_2$ absorption bands. By means of inversion techniques, those bands enable the retrieval of atmospheric temperature profiles. We have analyzed VIRTIS-M-IR night-side data obtaining high-resolution thermal maps of Venus south polar region between 55 and 85 km altitudes for three dynamical configurations of the vortex. The cold collar is clearly distinguishable at $\sim 62$ km altitude level, and it is more than 15 K colder than the pole on average. The South Polar Vortex appears as a vertically extended hot region close to the pole and squeezed by the cold collar between altitudes 55 and 67 km but spreading equatorward at about 74 km. Both the instantaneous temperature maps and their zonal averages show that the top altitude limit of the thermal signature of the vortex is at $\sim 80$ km altitude, at least on the night-side of the planet. The upper part of the atmosphere (67 - 85 km) is more homogeneous and has long-scale horizontal temperature differences of about 25 K over horizontal distances of $\sim 2,000$ km. The lower part (55 - 67 km) shows more fine-scale structure, creating the vortex' morphology, with thermal differences of up to about 50 K over $\sim 500$ km horizontal distances. We also study the vertical stability of different atmospheric layers within the 55 - 85 km altitude range for the three vortex configurations. It is always positive, but the cold collar is the most vertically stable structure at polar latitudes, while the vortex and sub-polar latitudes show lower stability values. Furthermore, the hot filaments present within the vortex exhibit lower stability values than their surroundings. The layer between 62 and 67 km resulted to be the most stable. These results are in good agreement with conclusions from previous radio occultation analyses.

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arXiv2023

Heating and ionization by non-thermal electrons in the upper atmospheres of water-rich exoplanets

Context. The long-term evolution of an atmosphere and the remote detectability of its chemical constituents are susceptible to how the atmospheric gas responds to stellar irradiation. The response remains poorly characterized for water and its dissociation products, however, this knowledge is relevant to our understanding of hypothetical water-rich exoplanets. Aims: Our work investigates the effect of photoelectrons, namely, the non-thermal electrons produced by photoionizing stellar radiation on the heating and ionization of extended atmospheres dominated by the dissociation products of water. Methods: We used a Monte Carlo model and up-to-date collision cross sections to simulate the slowing down of photoelectrons in O-H mixtures for a range of fractional ionizations and photoelectron energies. Results: We find that that the fraction of energy of a photoelectron that goes into heating is similar in a pure H gas and in O-H mixtures, except for very low fractional ionizations, whereby the O atom remains an efficient sink of energy. The O-H mixtures will go on to produce more electrons because the O atom is particularly susceptible to ionization. We quantified all that information and present it in a way that can be easily incorporated into photochemical-hydrodynamical models. Conclusions: Neglecting the role of photoelectrons in models of water-rich atmospheres will result in overestimations of the atmospheric heating and, foreseeably, the mass-loss rates as well. It will also underestimate the rate at which the atmospheric gas becomes ionized, which may have implications for the detection of extended atmospheres with Lyman-α transmission spectroscopy. Our simulations for the small exoplanets π Men c and TRAPPIST-1 b reveal that they respond very differently to irradiation from their host stars, with water remaining in molecular form at lower pressures in the latter case.

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arXiv2023

The Hubble PanCET program: The near-ultraviolet transmission spectrum of WASP-79b

We present Hubble Space Telescope (HST) transit observations of the Hot-Jupiter WASP-79b acquired with the Space Telescope Imaging Spectrograph (STIS) in the near ultraviolet (NUV). Two transit observations, part of the PanCET program, are used to obtain the transmission spectra of the planet between 2280 and 3070Å. We correct for systematic effects in the raw data using the jitter engineering parameters and polynomial modelling to fit the white light curves of the two transits. We observe an increase in the planet-to-star radius ratio at short wavelengths, but no spectrally resolved absorption lines. The difference between the radius ratios at 2400 and 3000Å reaches $0.0191\pm0.0042$ ($\sim$4.5$-σ$). Although the NUV transmission spectrum does not show evidence of hydrodynamical escape, the strong atmospheric features are likely due to species at very high altitudes. We performed a 1D simulation of the temperature and composition of WASP-79b using Exo-REM. The temperature pressure profile crosses condensation curves of radiatively active clouds, particularly MnS, Mg$_2$SiO$_4$, Fe, and Al$_2$O$_3$. Still, none of these species produces the level of observed absorption at short wavelengths and can explain the observed increase in the planet's radius. WASP-79b's transit depth reaches 23 scale height, making it one of the largest spectral features observed in an exoplanet at this temperature ($\sim$1700 K). The comparison of WASP-79b's transmission spectrum with three warmer hot Jupiters shows a similar level of absorption to WASP-178b and WASP-121b between 0.2 and 0.3$μ$m, while HAT-P-41b's spectrum is flat. The features could be explained by SiO absorption.

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arXiv2022

Detection of Paschen $β$ absorption in the atmosphere of KELT-9 b: A new window into the atmospheres of ultra-hot Jupiters

Hydrogen and helium transmission signals trace the upper atmospheres of hot gas-giant exoplanets, where the incoming stellar extreme ultraviolet and X-ray fluxes are deposited. Further, for the hottest stars, the near-ultraviolet excitation of hydrogen in the Balmer continuum may play a dominant role in controlling the atmospheric temperature and driving photoevaporation. KELT-9 b is the archetypal example of such an environment as it is the hottest gas-giant exoplanet known to date (T$_{eq}$ $\sim$ 4500 K) and orbits an A0V-type star. Studies of the upper atmosphere and escaping gas of this ultra-hot Jupiter have targeted the absorption in the Balmer series of hydrogen (n$_1$ = 2 $\rightarrow$ n$_2$ $>$ 2). Unfortunately, the lowermost metastable helium state that causes the triplet absorption at 108.3 nm is not sufficiently populated for detection. Here, we present evidence of hydrogen absorption in the Paschen series in the transmission spectrum of KELT-9 b observed with CARMENES. Specifically, we focus on the strongest line covered by its NIR channel, Paschen-$β$ at 1282.16 nm (n$_1$ = 3 $\rightarrow$ n$_2$ = 5). The observed absorption shows a contrast of (0.53 $^{+0.12}_{-0.13}$)%, a blueshift of $-$14.8 $^{+3.5}_{-3.2}$ km/s, and a FWHM of 31.9$^{+11.8}_{-8.3}$ km/s. The observed blueshift in the absorption feature could be explained by day-to-night circulation within the gravitationally bound atmosphere or, alternatively, by Paschen-$β$ absorption originating in a tail of escaping gas moving toward the observer as a result of extreme atmospheric evaporation. This detection opens a new window for investigating the atmospheres of ultra-hot Jupiters, providing additional constraints of their temperature structure, mass-loss rates, and dynamics for future modeling of their scorching atmospheres.

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arXiv2022

Identification of carbon dioxide in an exoplanet atmosphere

Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (i.e., elements heavier than helium, also called "metallicity"), and thus formation processes of the primary atmospheres of hot gas giants. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2 but have not yielded definitive detections due to the lack of unambiguous spectroscopic identification. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science Program (ERS). The data used in this study span 3.0 to 5.5 μm in wavelength and show a prominent CO2 absorption feature at 4.3 μm (26σ significance). The overall spectrum is well matched by one-dimensional, 10x solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide, and hydrogen sulfide in addition to CO2, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 μm that is not reproduced by these models.

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arXiv2022

Large Interferometer For Exoplanets (LIFE): I. Improved exoplanet detection yield estimates for a large mid-infrared space-interferometer mission

One of the long-term goals of exoplanet science is the atmospheric characterization of dozens of small exoplanets in order to understand their diversity and search for habitable worlds and potential biosignatures. Achieving this goal requires a space mission of sufficient scale. We seek to quantify the exoplanet detection performance of a space-based mid-infrared nulling interferometer that measures the thermal emission of exoplanets. For this, we have developed an instrument simulator that considers all major astrophysical noise sources and coupled it with Monte Carlo simulations of a synthetic exoplanet population around main-sequence stars within 20 pc. This allows us to quantify the number (and types) of exoplanets that our mission concept could detect over a certain time period. Two different scenarios to distribute the observing time among the stellar targets are discussed and different apertures sizes and wavelength ranges are considered. Within a 2.5-year initial search phase, an interferometer consisting of four 2 m apertures with a total instrument throughput of 5% covering a wavelength range between 4 and 18.5 $μ$m could detect up to ~550 exoplanets with radii between 0.5 and 6 R$_\oplus$ with an integrated SNR$\ge$7. At least ~160 of the detected exoplanets have radii $\le$1.5 R$_\oplus$. Depending on the observing scenario, ~25-45 rocky exoplanets (objects with radii between 0.5 and 1.5 $_{\oplus}$) orbiting within the empirical habitable zone (eHZ) of their host stars are among the detections. With an aperture size of 3.5 m, the total number of detections can increase to up to ~770, including ~60-80 rocky, eHZ planets. With 1 m aperture size, the maximum detection yield is ~315 exoplanets, including $\le$20 rocky, eHZ planets. In terms of predicted detection yield, such a mission can compete with large single-aperture reflected light missions. (abridged)

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arXiv2022

The high energy spectrum of Proxima Centauri simultaneously observed at X-ray and FUV wavelengths

The M dwarf Proxima~Centauri is known to be magnetically active and it hosts a likely Earth-like planet in its habitable zone. Understanding the characteristics of stellar radiation by understanding the properties of the emitting plasma is of paramount importance for a proper assessment of the conditions on Proxima~Centauri~b and exoplanets around M dwarfs in general. We determine the temperature structure of the coronal and transition region plasma of Proxima Centauri from simultaneous X-ray and far-ultraviolet (FUV) observations. The differential emission measure distribution (DEM) was constructed for flaring and quiescent periods by analysing optically thin X-ray and FUV emission lines. Four X-ray observations of Proxima Centauri were conducted by the LETGS instrument on board of the Chandra X-ray Observatory and four FUV observations were carried out using the Hubble STIS spectrograph. From the X-ray light curves, we determined a variation of the quiescent count rate by a factor of two within 20\% of the stellar rotation period. To obtain the DEM, 18 optically thin emission lines were analysed (12 X-ray and six FUV). The flare fluxes differ from the quiescence fluxes by factors of 4-20 (FUV) and 1-30 (X-ray). The temperature structure of the stellar corona and transition region was determined for both the quiescence and flaring state by fitting the DEM(T) with Chebyshev polynomials for a temperature range $\log$T = 4.25 - 8. Compared to quiescence, the emission measure increases during flares for temperatures below 0.3\,MK (FUV dominated region) and beyond 3.6\,MK (X-ray dominated region). The reconstructed DEM shape provides acceptable line flux predictions compared to the measured values. We provide synthetic spectra at 1-1700 Å, which may be considered as representative for the high-energy irradiation of Proxima~Cen~b during quiescent and flare periods.

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arXiv2022

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope IV. Capabilities and predicted performance for exoplanet characterization

The Near-Inrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) is a very versatile instrument, offering multiobject and integral field spectroscopy with varying spectral resolution ($\sim$30 to $\sim$3000) over a wide wavelength range from 0.6 to 5.3 micron, enabling scientists to study many science themes ranging from the first galaxies to bodies in our own Solar System. In addition to its integral field unit and support for multiobject spectroscopy, NIRSpec features several fixed slits and a wide aperture specifically designed to enable high precision time-series and transit as well as eclipse observations of exoplanets. In this paper we present its capabilities regarding time-series observations, in general, and transit and eclipse spectroscopy of exoplanets in particular. Due to JWST's large collecting area and NIRSpec's excellent throughput, spectral coverage, and detector performance, this mode will allow scientists to characterize the atmosphere of exoplanets with unprecedented sensitivity.

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arXiv2021

H$α$ and He I absorption in HAT-P-32 b observed with CARMENES -- Detection of Roche lobe overflow and mass loss

We analyze two high-resolution spectral transit time series of the hot Jupiter HAT-P-32 b obtained with the CARMENES spectrograph. Our new XMM-Newton X-ray observations of the system show that the fast-rotating F-type host star exhibits a high X-ray luminosity of 2.3e29~erg/s (5-100 A), corresponding to a flux of 6.9e4 erg/cm**2/s at the planetary orbit, which results in an energy-limited escape estimate of about 1e13 g/s for the planetary mass-loss rate. The spectral time series show significant, time-dependent absorption in the Halpha and He I triplet lines with maximum depths of about 3.3% and 5.3%. The mid-transit absorption signals in the Halpha and He I lines are consistent with results from one-dimensional hydrodynamic modeling, which also yields mass-loss rates on the order of 1e13 g/s. We observe an early ingress of a redshifted component of the transmission signal, which extends into a redshifted absorption component, persisting until about the middle of the optical transit. While a super-rotating wind can explain redshifted ingress absorption, we find that an up-orbit stream, transporting planetary mass in the direction of the star, also provides a plausible explanation for the pre-transit signal. This makes HAT-P-32 a benchmark system for exploring atmospheric dynamics via transmission spectroscopy.

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arXiv2021

The changing face of AU Mic b: stellar spots, spin-orbit commensurability, and Transit Timing Variations as seen by CHEOPS and TESS

AU Mic is a young planetary system with a resolved debris disc showing signs of planet formation and two transiting warm Neptunes near mean-motion resonances. Here we analyse three transits of AU Mic b observed with the CHaracterising ExOPlanet Satellite (CHEOPS), supplemented with sector 1 and 27 Transiting Exoplanet Survey Satellite (TESS) photometry, and the All-Sky Automated Survey (ASAS) from the ground. The refined orbital period of AU Mic b is 8.462995 \pm 0.000003 d, whereas the stellar rotational period is P_{rot}=4.8367 \pm 0.0006 d. The two periods indicate a 7:4 spin--orbit commensurability at a precision of 0.1%. Therefore, all transits are observed in front of one of the four possible stellar central longitudes. This is strongly supported by the observation that the same complex star-spot pattern is seen in the second and third CHEOPS visits that were separated by four orbits (and seven stellar rotations). Using a bootstrap analysis we find that flares and star spots reduce the accuracy of transit parameters by up to 10% in the planet-to-star radius ratio and the accuracy on transit time by 3-4 minutes. Nevertheless, occulted stellar spot features independently confirm the presence of transit timing variations (TTVs) with an amplitude of at least 4 minutes. We find that the outer companion, AU Mic c may cause the observed TTVs.

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