Extreme-ultraviolet- and X-Ray-driven Photochemistry of Gaseous Exoplanets

The interaction of exoplanets with their host stars causes a vast diversity in bulk and atmospheric compositions and physical and chemical conditions. Stellar radiation, especially at the shorter wavelengths, drives the chemistry in the upper atmospheric layers of close orbiting gaseous giants, providing drastic departures from equilibrium. In this study, we aim at unfolding the effects caused by photons in different spectral bands on the atmospheric chemistry. This task is particularly difficult because the characteristics of chemical evolution emerge from many feedbacks on a wide range of timescales, and because of the existing correlations among different portions of the stellar spectrum. In describing the chemistry, we have placed particular emphasis on the molecular synthesis induced by X-rays. The weak X-ray photoabsorption cross sections of the atmospheric constituents boost the gas ionization to pressures inaccessible to vacuum and extreme-ultraviolet photons. Although X-rays interact preferentially with metals, they produce a secondary electron cascade able to ionize efficiently hydrogen- and helium-bearing species, giving rise to a distinctive chemistry.

Minimal Data Fidelity for Stellar Feature and Companion Detection

Technological advances in instrumentation have led to an exponential increase in exoplanet detection and scrutiny of stellar features such as spots and faculae. While the spots and faculae enable us to understand the stellar dynamics, exoplanets provide us with a glimpse into stellar evolution. While the ubiquity of noise (e.g., telluric, instrumental, or photonic) is unavoidable, combining this with increased spectrographic resolution compounds technological challenges. To account for these noise sources and resolution issues, we use a temporal multifractal framework to study data from the Spot Oscillation And Planet 2.0 tool, which simulates a stellar spectrum in the presence of a spot, a facula or a planet. Given these controlled simulations, we vary the resolution as well as the signal-to-noise ratio (S/N) to obtain a lower limit on the resolution and S/N required to robustly detect features. We show that a spot and a facula with a 1% coverage of the stellar disk can be robustly detected for a S/N (per pixel) of 35 and 60, respectively, for any spectral resolution above 20,000, while a planet with a radial velocity of 10 m s−1 can be detected for a S/N (per pixel) of 600. Rather than viewing noise as an impediment, our approach uses noise as a source of information.

The impact of time-dependent stellar activity on the atmospheric chemistry and observability of exoplanets

<p class="p1">Observations of exoplanets used to characterize the chemistry and dynamics of atmospheres have developed considerably throughout the years. Nonetheless, it remains a difficult task to give a full and detailed description using solely observations. With future space missions such as JWST and ARIEL, both expected to be launched within this decade, it becomes even more crucial to be able to fully explain and predict the underlying chemistry and physics involved. In this research, we focus on modeling star-planet interactions by using synthetic flare spectra to predict chemical tracers for future missions. We make use of a chemical kinetics code that includes synthetic time-dependent stellar spectra and thermal atmospheric escape to simulate the atmospheres of known exoplanets. Using a radiative transfer model we then retrieve emission spectra. This ongoing study is focused on various known planetary systems of which the stellar spectrum has been obtained by the (mega-)MUSCLES collaboration. Preliminary results on these systems show that stellar flares and thermal escape can have a significant effect on the chemistry in atmospheres. </p>

A Full Characterisation of Stars and Planets through High Resolution Spectroscopy and 3D Simulations.

<p>The study of exoplanets atmospheres is one of the most intriguing challenges in the exoplanet field nowadays and the High Resolution Spectroscopy (HRS) has recently emerged as one of the leading methods for detecting atomic and molecular species in their atmospheres (e.g., Birkby, 2018). While this technique is particularly robust against contaminant absorption in the Earth’s atmosphere, the non-stationary stellar spectrum, in the form of either Doppler shift or distortion of the line profile during planetary transits, creates a non-negligible source of noise that can alter or even prevent detection. In the last years, it has become computationally possible to simulate the stellar surface convection that, in the end, allows to correctly reproduce asymmetric and blue-shifted spectral lines due to the granulation pattern of the stellar disk, which is a very important source of uncertainties (Chiavassa & Brogi, 2019). In the context of HRS and on the planet hand side, only recently multidimensional models have been used to detect the weak planet signal in the spectrum (e.g., Flowers et al. 2019).</p> <p>However, these numerical simulations have been computed independently for star and planet so far, while acquired spectra are the result of the natural coupling at each phase along the planet orbit. A next step forward is needed: coupling stellar and planetary 3D models dynamics during the transit.</p> <p>I will present the unprecedented precise synthetic spectra obtained with the upgraded 3D radiative transfer code Optim3D (Chiavassa et al. 2009). Optim3D takes as inputs the state-of-art 3D RHD stellar simulations (Stagger code, Nordlund et al. 2009, Magic et al. 2013) and the 3D Global Climate Models (SPARC/MITgcms, Showman et al. 2009, Parmentier et al. 2021) for stars and planets respectively, coupling them at any phase along the planet orbit. I will show the impact of this new approach on the detection of molecules by cross-correlating our spectra with HRS observations (e.g., Snellen et al 2010 and Brogi et al. 2016). This approach is particularly advantageous for those molecular species that are present in both the atmospheres and form in the same region of the spectrum, resulting in mixed and overlapped spectral lines (e.g. CO and H2O, crucial to constrain the C/O ratio). Moreover, the use of 3D models provides us with information about the dynamics processes at play, such as stellar convection and planetary winds.</p> <p> </p>

Interior heating and outgassing of Proxima Cen b: Identification of critical parameters

<p>Since the discovery of a potentially low-mass exoplanet around our nearest neighbour star Proxima Centauri, several works have investigated the likelihood of a shielding atmosphere and therefore the potential surface habitability of Proxima Cen b. However, outgassing processes are influenced by several different (unknown) factors such as the actual planet mass, mantle and core composition, and different heating mechanisms in the interior.<br>We aim to identify the critical parameters that influence the mantle and surface evolution of the planet over time, as well as to potentially constrain the time-dependent input of volatiles from mantle into the atmosphere.</p><p><br>To study the coupled star-planet evolution, we analyse the heating produced in the interior of Proxima Cen b due to induction heating, which strongly varies with both depth and latitude. We calculate different rotation evolutionary tracks for Proxima Centauri and investigate the change in its rotation period and magnetic field strength. Unlike the Sun, Proxima Centauri possesses a very strong magnetic field of at least a few hundred Gauss, which was likely higher in the past. <br>We apply an interior structure model for varying planet masses (derived from the unknown inclination of observation of the Proxima Centauri system) and iron weight fractions, i.e. different core sizes, in the range of observed Fe-Mg variations in the stellar spectrum. <br>We use a mantle convection model to study the thermal evolution and outgassing efficiency of Proxima Cen b. For unknown planetary parameters such as initial conditions we chose randomly selected values. We take into account heating in the interior due to variable radioactive heat sources and latitute- and radius-dependent induction heating, and compare the heating efficiency to tidal heating.</p><p><br>Our results show that induction heating may have been significant in the past, leading to local temperature increases of several hundreds of Kelvin (see Fig. 1). This early heating leads to an earlier depletion of the interior and volatile outgassing compared to if the planet would not have been subject to induction heating. We show that induction heating has an impact comparable to tidal heating when assuming latest estimates on its eccentricity. We furthermore find that the planet mass (linked to the planetary orbital inclination) has a first-order influence on the efficiency of outgassing from the interior.</p><p> </p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.53bcd48f2cff56572630161/sdaolpUECMynit/12UGE&app=m&a=0&c=314fe555893c77417d52bf9a6bd3825f&ct=x&pn=gnp.elif&d=1" alt="" width="307" height="339"> </p><p>Fig 1: Local induction heating and resulting temperature variations compared to a simulation without induction heating after 1 Gyr of thermal evolution for an example rocky planet of 1.8 Earth masses with an iron content of 20 wt-%.</p>

Correcting the effect of stellar spots on ARIEL transmission spectra

ABSTRACT The goal of this study is to assess the impact of the stellar spots on the extraction of the planetary transmission spectra observed by ARIEL. We develop a method to model the stellar spectrum of a star in the presence of spots by using the out-of-transit observations. It is based on a chi squared minimization procedure of the out-of-transit spectrum on a grid of stellar spectra with different sizes and temperatures of the spots. The approach allows us also to study the temporal evolution of the spots when comparing stellar spectra observed at different epochs. We also present a method to correct the transit depth variations due to non-occulted stellar spots and estimate the error we introduce if we apply the same correction to crossings over the stellar spots. The method is tested on three types of stellar targets that ARIEL will observe in its 4-yr mission lifetime. In all the explored cases, the approach allows us to reliably recover the spot parameters (size and temperature) from out-of-transit observations and, for non-occulted spots, to confidently recover the planetary atmosphere transmission spectrum within the noise level (with average uncertainty of at most $3.3{{\ \rm per\ cent}}$ of the planetary signal). Conversely, we find systematic biases in the inferred planetary spectra due to the occulted spots, with measurable effects for the brightest targets especially for more contrasted spots.

High-resolution spectroscopy of the hot Am star HR 3383

ABSTRACT We present the spectrum analysis of the hot Am star HR 3383 (A1 Vm). Hubble Space Telescope STIS and Nordic Optical Telescope SOFIN data are modelled with synthetic spectra, and abundances are investigated for 78 elements. Most light elements up through oxygen show deficiencies, compared to solar abundances, followed by the general trend of increasing abundance enhancement with atomic number that levels off at a 30-fold enhancement at the lanthanide group and heavier elements. The derived element distribution is generally consistent with what is observed in other hot Am stars. Abundances for HR 3383 are also similar to what is seen for the cooler HgMn stars, with the exception of the platinum-group elements that generally show dramatic enhancements in the HgMn stars. Current theory and calculations are able to predict most observed abundances and abundance trends through the iron group. The large number of derived element abundances in this study provides a constraint for theoretical calculations attempting to explain the heavy element abundances in chemically peculiar stars. This paper includes a comprehensive description of spectral lines useful for an abundance analysis of late B and A type stars, and comments are provided on the atomic data. New data for hyperfine structure components for three lines in Lu iii and a single line in Lu ii are presented, based on laboratory spectra. In addition to the stellar spectrum, lines from the interstellar medium are noted for several of the strongest Fe ii ultraviolet transitions.

The TOI-763 system: sub-Neptunes orbiting a Sun-like star

ABSTRACT We report the discovery of a planetary system orbiting TOI-763(aka CD-39 7945), a V = 10.2, high proper motion G-type dwarf star that was photometrically monitored by the TESS space mission in Sector 10. We obtain and model the stellar spectrum and find an object slightly smaller than the Sun, and somewhat older, but with a similar metallicity. Two planet candidates were found in the light curve to be transiting the star. Combining TESS transit photometry with HARPS high-precision radial velocity (RV) follow-up measurements confirm the planetary nature of these transit signals. We determine masses, radii, and bulk densities of these two planets. A third planet candidate was discovered serendipitously in the RV data. The inner transiting planet, TOI-763 b, has an orbital period of Pb = 5.6 d, a mass of Mb = 9.8 ± 0.8 M⊕, and a radius of Rb = 2.37 ± 0.10 R⊕. The second transiting planet, TOI-763 c, has an orbital period of Pc = 12.3 d, a mass of Mc = 9.3 ± 1.0 M⊕, and a radius of Rc = 2.87 ± 0.11 R⊕. We find the outermost planet candidate to orbit the star with a period of ∼48 d. If confirmed as a planet, it would have a minimum mass of Md = 9.5 ± 1.6 M⊕. We investigated the TESS light curve in order to search for a mono transit by planet d without success. We discuss the importance and implications of this planetary system in terms of the geometrical arrangements of planets orbiting G-type stars.