Predicted bulk compositions and geodynamical properties of terrestrial exoplanets in the Solar neighbourhood

<p>Our knowledge of the physical, chemical, and mechanical (i.e., rheological) properties of terrestrial planets is based almost entirely on our Solar System. Terrestrial exoplanets, however, show a startling diversity compared to our local experience. This observation challenges our understanding of terrestrial planet formation and of the thermal and mechanical behaviour of such worlds, some of which are vastly different from our own. To better understand the range and consequences of exoplanetary diversity, we integrate results from astrophysical models and observations, geodynamical simulations, and petrological experiments. Terrestrial exoplanet modelling requires plausible constraints to be placed on bulk planet compositions; bulk composition modulates interior properties, including core size, mantle mineralogy, and mantle melting behaviour. This may in turn affect the interaction between the planet’s interior and atmosphere, and thereby impact its potential to host a biosphere. Bulk composition may leave a signature on the mass and composition of the atmosphere, which could be detected in the future.</p><p>Here, we constrain exoplanetary diversity in terms of bulk planet composition, based on observations of stellar abundances in the Solar neighbourhood. We apply the devolatilization/fractionation trend between a planet and its host star [Wang+, 2019], to stellar abundances from the Hypatia catalogue [Hinkel+, 2014]. After applying a simplified model of rock-metal differentiation, we predict bulk planet and bulk silicate compositions of hypothetical exoplanets in the habitable zones of nearby stars. We further select 20 end-member compositions that span the full range of hypothetical bulk compositions based on our analysis.</p><p>With the compositions of these 20 end-members and by assuming Earth-like planetary masses and radii, we infer mineralogy and density profiles, as well as physical properties (e.g., viscosity) of the mantle using thermodynamic model Perple_X [Connolly, 2005]. These profiles and physical properties are prescribed in geodynamical models of exoplanet mantle evolution. We use convection code StagYY [Tackley, 2008] to model mantle convection and surface tectonic behaviour in a 2D spherical annulus geometry. We find that mantle viscosity increases with decreasing Mg:Si ratio of mantle rocks, with strong effects on planetary cooling and the likelihood of plate tectonics. In turn, the propensity of plate tectonics regulates the heat and chemical exchange between mantle and crust, affecting surface conditions and, by extension, atmospheric composition. This establishes a link between interior composition and surface conditions, and shows the importance of studying this aspect of planetary diversity. We recommend our 20 suggested end-members of terrestrial exoplanet compositions for subsequent modelling work.</p>

Did Sgr cause the vertical waves in the solar neighbourhood?

ABSTRACT The vertical distribution of stars in the solar neighbourhood is not in equilibrium but contains a wave signature in both density and velocity space originating from a perturbation. With the discovery of the phase-space spiral in Gaia data release (DR) 2, determining the origin of this perturbation has become even more urgent. We develop and test a fast method for calculating the perturbation from a passing satellite on the vertical component of a part of a disc galaxy. This fast method allows us to test a large variety of possible perturbations to the vertical disc very quickly. We apply our method to the range of possible perturbations to the solar neighbourhood stemming from the recent passage of the Sagittarius dwarf galaxy (Sgr), varying its mass, mass profile, and present-day position within their observational uncertainties, and its orbit within different realistic models for the Milky Way’s gravitational potential. We find that we are unable to reproduce the observed asymmetry in the vertical number counts and its concomitant breathing mode in velocity space for any plausible combination of Sgr and Milky Way properties. In all cases, either the amplitude or the perturbation wavelength of the number-count asymmetry and of the oscillations in the mean vertical velocity produced by the passage of Sgr are in large disagreement with the observations from Gaia DR2. We conclude that Sgr cannot have caused the observed oscillations in the vertical disc or the Gaia phase-space spiral.

Co-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2

Since thin disc stars are younger than thick disc stars on average, the thin disc is predicted by some models to start forming after the thick disc had formed, around 10 Gyr ago. Accordingly, no significant old thin disc population should exist. Using 6-D coordinates from Gaia-DR2 and age estimates from Sanders & Das (2018), we select ∼24000 old stars (${\tau > 10{\, \rm{Gyr}}}$, with uncertainties $\lesssim 15\%$) within $2{\, \rm{kpc}}$ from the Sun (full sample). A cross-match with APOGEE-DR16 (∼1000 stars) reveals comparable fractions of old chemically defined thin/thick disc stars. We show that the full sample pericenter radius (rper) distribution has three peaks, one associated with the stellar halo and the other two having contributions from the thin/thick discs. Using a high-resolution N-body+SPH simulation, we demonstrate that one peak, at ${r_\rm{per}}\approx 7.1{\, \rm{kpc}}$, is produced by stars from both discs which were born in the inner Galaxy and migrated to the Solar Neighbourhood. In the Solar Neighbourhood, ∼1/2 (∼1/3) of the old thin (thick) disc stars are classified as migrators. Our results suggest that thin/thick discs are affected differently by radial migration inasmuch as they have different eccentricity distributions, regardless of vertical scale heights. We interpret the existence of a significant old thin disc population as evidence for an early co-formation of thin/thick discs, arguing that clump instabilities in the early disc offer a compelling explanation for the observed trends.

Star formation history of the solar neighbourhood as told by Gaia

ABSTRACT The Gaia data release 2 (DR2) catalogue is the best source of stellar astrometric and photometric data available today. The history of the Milky Way galaxy is written in stone in this data set. Parallaxes and photometry tell us where the stars are today, when were they formed, and with what chemical content, that is, their star formation history (SFH). We develop a Bayesian hierarchical model suited to reconstruct the SFH of a resolved stellar population. We study the stars brighter than $G\, =\, 15$ within 100 pc of the Sun in Gaia DR2 and derive an SFH of the solar neighbourhood in agreement with previous determinations and improving upon them because we detect chemical enrichment. Our results show a maximum of star formation activity about 10 Gyr ago, producing large numbers of stars with slightly below solar metallicity (Z = 0.014), followed by a decrease in star formation up to a minimum level occurring around 8 Gyr ago. After a quiet period, star formation rises to a maximum at about 5 Gyr ago, forming stars of solar metallicity (Z = 0.017). Finally, star formation has been decreasing until the present, forming stars of Z = 0.03 at a residual level. We test the effects introduced in the inferred SFH by ignoring the presence of unresolved binary stars in the sample, reducing the apparent limiting magnitude, and modifying the stellar initial mass function.

Birth sites of young stellar associations and recent star formation in a flocculent corrugated disc

ABSTRACT With backwards orbit integration, we estimate birth locations of young stellar associations and moving groups identified in the solar neighbourhood that are younger than 70 Myr. The birth locations of most of these stellar associations are at a smaller galactocentric radius than the Sun, implying that their stars moved radially outwards after birth. Exceptions to this rule are the Argus and Octans associations, which formed outside the Sun’s galactocentric radius. Variations in birth heights of the stellar associations suggest that they were born in a filamentary and corrugated disc of molecular clouds, similar to that inferred from the current filamentary molecular cloud distribution and dust extinction maps. Multiple spiral arm features with different but near corotation pattern speeds and at different heights could account for the stellar association birth sites. We find that the young stellar associations are located in between peaks in the radial/tangential (UV) stellar velocity distribution for stars in the solar neighbourhood. This would be expected if they were born in a spiral arm, which perturbs stellar orbits that cross it. In contrast, stellar associations seem to be located near peaks in the vertical phase-space distribution, suggesting that the gas in which stellar associations are born moves vertically together with the low-velocity dispersion disc stars.