Effects of Mg/Si on Exoplanetary Refractory Oxygen Budget

Effects of Mg/Si on Exoplanetary Refractory Oxygen Budget

Authors:

Unterborn et al

Abstract:

Solar photospheric abundances of refractory elements mirror the Earth's to within ~10 mol% when normalized to the dominant terrestrial planet-forming elements Mg, Si and Fe. This allows for the adoption of Solar composition as an order-of-magnitude proxy for Earth's. It is not known, however, the degree to which this mirroring of stellar and terrestrial planet abundances holds true for other star-planet systems without determination of the composition of initial planetesimals via condensation sequence calculations and post condensation processes. We present the open-source Arbitrary Composition Condensation Sequence calculator (ArCCoS) to assess how the elemental composition of a parent star affects that of the planet-building material, including the extent of oxidation within the planetesimals. We demonstrate the utility of ArCCoS by showing how variations in stellar refractory elements Mg and Si affect the condensation of oxygen, which directly lead to the resulting core to silicate ratio in exoplanets. This, thereby, removes significant degeneracy in the interpretation of the structures of exoplanets as well as providing observational tests for the validity of this model.

The Mineral Clouds of HD 189733b

Dynamic mineral clouds on HD 189733b I. 3D RHD with kinetic, non-equilibrium cloud formation

Authors:

Lee et al

Abstract:

3D modelling of cloud formation in atmospheres of extrasolar planets coupled to the atmospheric radiative, hydrodynamic and thermo-chemical properties has long been an open challenge. We present a 3D radiative-hydrodynamic (RHD) atmosphere model of HD 189733b fully coupled to a kinetic, microphysical mineral cloud formation model. We include the feedback effects of cloud advection and settling, gas phase element advection and depletion/replenishment and include the radiative effects of cloud and gas opacity. The 3D Navier-Stokes equations are solved consistently with a two-stream radiative transfer scheme coupled with the cloud moment conservation equations. We model the cloud particles as a mix of mineral materials which change in size and composition as they travel through atmospheric thermo-chemical environments. The local cloud properties such as number density, grain size and material composition are time-dependently calculated. Gas phase element depletion as a result of cloud formation are calculated and uncondensed elements are advected across the globe. Mean cloud particle sizes are typically sub-micron (0.01-0.5 {\mu}m) at pressures less than 1 bar with hotter equatorial regions containing the smallest grains. Denser cloud structures occur near terminator regions and deeper (∼ 1 bar) atmospheric layers. A thick, greyer opaque cloud layer is found to be at 100 mbar - 10 bar which contains micron sized or larger cloud particles. The cloud material composition differs across the globe, with various minerals dominating dependent on local thermo-chemical conditions. Silicate materials such as MgSiO3[s] are found to be abundant at mid-high latitudes. Elements involved in the cloud formation can be depleted by several orders of magnitude, with the exception of Oxygen which is depleted by a maximum of 30%.

A Near-Infrared Spectroscopic Survey of 886 Nearby M Dwarfs

A Near-Infrared Spectroscopic Survey of 886 Nearby M Dwarfs

Authors:

Terrien et al

Abstract:

We present a catalog of near-infrared (NIR) spectra and associated measurements for 886 nearby M dwarfs. The spectra were obtained with the NASA-Infrared Telescope Facility SpeX Spectrograph during a two-year observing campaign; they have high signal-to-noise ratios (SNR greater than 100−150), span 0.8-2.4 μm and have R∼2000. Our catalog of measured values contains useful Teff and composition-sensitive features, empirical stellar parameter measurements, and kinematic, photometric, and astrometric properties compiled from the literature. We focus on measures of M dwarf abundances ([Fe/H] and [M/H]), capitalizing on the precision of recently published empirical NIR spectroscopic calibrations. We explore systematic differences between different abundance calibrations, and to other similar M dwarf catalogs. We confirm that the M dwarf abundances we measure show the expected inverse dependence with kinematic, activity, and color-based age indicators. Finally, we provide updated [Fe/H] and [M/H] for 16 M dwarf planet hosts. This catalog represents the largest published compilation of NIR spectra and associated parameters for M dwarfs. It provides a rich and uniform resource for the nearby M dwarfs, and will be especially valuable for measuring Habitable Zone locations and comparative abundances of the M dwarf planet hosts that will be uncovered by upcoming exoplanet surveys.

Ancient Habitable Zone Terrestrial Exoplanets Were VERY Different Than Earth

Which type of planets do we expect to observe in the Habitable Zone?

Authors:

Adibekyan et al

Abstract:

We used a sample of super-Earth-like planets detected by the Doppler spectroscopy and transit techniques to explore the dependence of orbital parameters of the planets on the metallicity of their host stars. We confirm the previous results that super-Earths orbiting around metal-rich stars are not observed to be as distant from their host stars as we observe their metal-poor counterparts to be. The orbits of these super-Earths with metal-rich hosts usually do not reach into the Habitable Zone (HZ), keeping them very hot and inhabitable. We found that most of the known planets in the HZ are orbiting their GK-type hosts which are metal-poor. The metal-poor nature of planets in the HZ suggests a high Mg abundance relative to Si and high Si abundance relative to Fe. These results lead us to speculate that HZ planets might be more frequent in the ancient Galaxy and had compositions different from that of our Earth.

Stars With High Mg/Si Ratios Have low Mass Exoplanets

From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio

Authors:

Abidekyan et al

Abstract:

The main goal of this work is to study element ratios that are important for the formation of planets of different masses. We study potential correlations between the existence of planetary companions and the relative elemental abundances of their host stars. We use a large sample of FGK-type dwarf stars for which precise Mg, Si, and Fe abundances have been derived using HARPS high-resolution and high-quality data. A first analysis of the data suggests that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet hosts present [Mg/Si] that is lower than field stars. However, we found that the [Mg/Si] ratio significantly depends on metallicity through Galactic chemical evolution. After removing the Galactic evolution trend only the difference in the [Mg/Si] elemental ratio between low-mass planet hosts and non-hosts was present in a significant way. These results suggests that low-mass planets are more prevalent around stars with high [Mg/Si]. Our results demonstrate the importance of Galactic chemical evolution and indicate that it may play an important role in the planetary internal structure and composition.

Elemental Ratios in Stars vs ExoPlanets

Elemental ratios in stars vs planets

Authors:

Thiabaud et al

Abstract:

Context.

The chemical composition of planets is an important constraint for planet formation and subsequent differentiation. While theoretical studies try to derive the compositions of planets from planet formation models in order to link the composition and formation process of planets, other studies assume that the elemental ratios in the formed planet and in the host star are the same.

Aims.

Using a chemical model combined with a planet formation model, we aim to link the composition of stars with solar mass and luminosity with the composition of the hosted planets. For this purpose, we study the three most important elemental ratios that control the internal structure of a planet: Fe/Si, Mg/Si, and C/O.

Methods.

A set of 18 different observed stellar compositions was used to cover a wide range of these elemental ratios. The Gibbs energy minimization assumption was used to derive the composition of planets, taking stellar abundances as proxies for nebular abundances, and to generate planets in a self-consistent planet formation model. We computed the elemental ratios Fe/Si, Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed in different protoplanetary discs, and compared them to stellar abundances.

Results.

We show that the elemental ratios Mg/Si and Fe/Si in planets are essentially identical to those in the star. Some deviations are shown for planets that formed in specific regions of the disc, but the relationship remains valid within the ranges encompassed in our study. The C/O ratio shows only a very weak dependence on the stellar value.