Alkaline Exospheres of Exoplanet Systems: Evaporative Transmission Spectra

Hydrostatic equilibrium is an excellent approximation for the dense layers of planetary atmospheres where it has been canonically used to interpret transmission spectra of exoplanets. Here we exploit the ability of high-resolution spectrographs to probe tenuous layers of sodium and potassium gas due to their formidable absorption cross-sections. We present an atmosphere-exosphere degeneracy between optically thick and optically thin mediums, raising the question of whether hydrostatic equilibrium is appropriate for Na I lines observed at exoplanets. To this end we simulate three non-hydrostatic, evaporative, density profiles: (i) escaping, (ii) exomoon, and (iii) torus to examine their imprint on an alkaline exosphere in transmission. By analyzing an evaporative curve of growth we find that equivalent widths of $W_{\mathrm{Na D2}} \sim 1- 10$ mA are naturally driven by evaporation rates $\sim 10^3 - 10^5$ kg/s of pure atomic Na. To break the degeneracy between atmospheric and exospheric absorption, we suggest that if the line ratio is $\mathrm{D2/D1} \gtrsim 1.2$ the gas is optically thin on average and roughly indicating a non-hydrostatic structure of the atmosphere/exosphere. We show this is the case for Na I observations at hot Jupiters WASP-49b and HD189733b and also simulate their K I spectra. Lastly, motivated by the slew of metal detections at ultra-hot Jupiters, we suggest a toroidal atmosphere at WASP-76b and WASP-121b is consistent with the Na I data at present.Comment: 23 pages, 21 figures, accepted by MNRA

On the variation in stellar alpha-enhancements of star-forming galaxies in the EAGLE simulation

The ratio of $\alpha$-elements to iron in galaxies holds valuable information about the star-formation history since their enrichment occurs on different timescales. The fossil record of stars in galaxies has mostly been excavated for passive galaxies, since the light of star-forming galaxies is dominated by young stars which have much weaker atmospheric absorption features. Here we use the cosmological EAGLE simulation to investigate the origin of variations in $\alpha$-enhancement among star-forming galaxies at $z=0$. The definition of $\alpha$-enhancement in a composite stellar population is ambiguous. We elucidate two definitions - termed 'mean' and 'galactic' $\alpha$-enhancement - in more detail. While a star-forming galaxy has a high 'mean' $\alpha$-enhancement when its stars formed rapidly, a galaxy with a large 'galactic' $\alpha$-enhancement generally had a delayed star formation history. We find that absorption-line strengths of Mg and Fe correlate with variations in $\alpha$-enhancement. These correlations are strongest for the 'galactic' $\alpha$-enhancement. However, we show that these are mostly caused by other effects which are cross-correlated with $\alpha$-enhancement, such as variations in the light-weighted age. This severely complicates the retrieval of $\alpha$-enhancements in star-forming galaxies. The ambiguity is not severe for passive galaxies and we confirm that spectral variations in these galaxies are caused by measurable variations in $\alpha$-enhancements. We suggest that this more complex coupling between $\alpha$-enhancement and star formation histories can guide the interpretation of new observations of star-forming galaxies.Comment: 16 pages, 12 figures, 4 tables. Submitted to MNRAS. Comments/hints for missing citations are very welcome

The atomic-to-molecular hydrogen transition in the TNG50 simulation:Using realistic UV fields to create spatially resolved H i maps

Cold gas in galaxies provides a crucial test to evaluate the realism of cosmological hydrodynamical simulations. To extract the atomic and molecular hydrogen properties of the simulated galaxy population, post-processing methods taking the local UV field into account are required. We improve upon previous studies by calculating realistic UV fields with the dust radiative transfer code SKIRT to model the atomic-to-molecular transition in TNG50, the highest-resolution run of the IllustrisTNG suite. Comparing integrated quantities such as the H i mass function, we study to what detail the UV field needs to be modelled in order to calculate realistic cold gas properties. We then evaluate new, spatially resolved comparisons for cold gas in galaxies by exploring synthetic maps of atomic hydrogen at redshift zero and compare them to 21-cm observations of local galaxies from the WHISP survey. In terms of non-parametric morphologies, we find that TNG50 H i maps are less concentrated than their WHISP counterparts (median ΔC ≈ 0.3), due in part to central H i deficits related to the ejective character of supermassive black hole feedback in TNG. In terms of the H i column density distribution function, we find discrepancies between WHISP and IllustrisTNG that depend on the total H i abundance in these data sets as well as the post-processing method. To fully exploit the synergy between cosmological simulations and upcoming deep H i/H2 data, we advocate the use of accurate methods to estimate the UV radiation field and to generate mock maps.</p

The TNG50-SKIRT Atlas:Wavelength dependence of the effective radius

Galaxy sizes correlate with many other important properties of galaxies, and the cosmic evolution of galaxy sizes is an important observational diagnostic for constraining galaxy evolution models. The effective radius is probably the most widely used indicator of galaxy size. We used the TNG50-SKIRT Atlas to investigate the wavelength dependence of the effective radius of galaxies at optical and near-infrared (NIR) wavelengths. We find that, on average, the effective radius in every band exceeds the stellar mass effective radius, and that this excess systematically decreases with increasing wavelength. The optical g-band (NIR Ks-band) effective radius is on average 58% (13%) larger than the stellar mass effective radius. Effective radii measured from dust-obscured images are systematically larger than those measured from dust-free images, although the effect is limited (8.7% in the g-band, 2.1% in the Ks-band). We find that stellar population gradients are the dominant factor (about 80%) in driving the wavelength dependence of the effective radius, and that differential dust attenuation is a secondary factor (20%). Comparing our results to recent observational data, we find offsets in the absolute values of the median effective radii, up to 50% for the population of blue galaxies. We find better agreement in the slope of the wavelength dependence of the effective radius, with red galaxies having a slightly steeper slope than green-blue galaxies. Comparing our effective radii with those of galaxies from the Siena Galaxy Atlas in separate bins in z-band absolute magnitude and g-z colour, we find excellent agreement for the reddest galaxies, but again significant offsets for the blue populations: up to 70% for galaxies around Mz=-21.5. This difference in median effective radius for the bluer galaxies is most probably due to intrinsic differences in the morphological structure of observed and TNG50 simulated galaxies. Finally, we find that the median effective radius in any broadband filter increases systematically with decreasing u-r colour and with increasing galaxy stellar mass, total SFR, sSFR, and dust-to-stellar-mass ratio. For the slope of the wavelength dependence of Re, however, there does not seem to be a systematic, monotonic correlation with any of these global properties.</p

The TNG50-SKIRT Atlas:Post-processing methodology and first data release

Galaxy morphology is a powerful diagnostic to assess the realism of cosmological hydrodynamical simulations. Determining the morphology of simulated galaxies requires the generation of synthetic images through 3D radiative transfer post-processing that properly accounts for different stellar populations and interstellar dust attenuation. We use the SKIRT code to generate the TNG50-SKIRT Atlas, a synthetic UV to near-infrared broadband image atlas for a complete stellar-mass selected sample of 1154 galaxies extracted from the TNG50 cosmological simulation at z = 0. The images have a high spatial resolution (100 pc) and a wide field of view (160 kpc). In addition to the dust-obscured images, we also release dust-free images and physical parameter property maps with matching characteristics. As a sanity check and preview application we discuss the UVJ diagram of the galaxy sample. We investigate the effect of dust attenuation on the UVJ diagram and find that it affects both the star-forming and the quiescent galaxy populations. The quiescent galaxy region is polluted by younger and star-forming highly inclined galaxies, while dust attenuation induces a separation in inclination of the star-forming galaxy population, with low-inclination galaxies remaining at the blue side of the diagram and high-inclination galaxies systematically moving towards the red side. This image atlas can be used for a variety of other applications, including galaxy morphology studies and the investigation of local scaling relations. We publicly release the images and parameter maps, and we invite the community to use them.</p

Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets

International audienceExtrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the solar system, Io, we describe how sodium (Na I) and potassium (K I) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal 10 p 11. This tidal energy is also focused into the satellite driving an ∼10 5±2 higher mass-loss rate than Io's supply to Jupiter's Na exosphere based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are, on average, more than sufficient to provide the ∼10 10±1 Na cm −2 required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended (∼1000 R J) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP-49b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles