Photoevaporation of Water Dominated Exoplanet Atmospheres

2020 ◽  
Author(s):  
Laura Harbach ◽  
James Owen ◽  
Subhanjoy Mohanty
Keyword(s):  
Mass Loss ◽  
Planetary Atmospheres ◽  
Loss Mechanism ◽  
Hydrodynamic Simulations ◽  
X Ray ◽  
Atmospheric Observations ◽  
Low Mass ◽  
The Subject ◽  
Complex Chemistry ◽  
Atmospheric Mass

<p>The atmospheres of close-in, low-mass exoplanets are extremely vulnerable to the effects of stellar UV to X-ray radiation. Photoevaporation can significantly ablate planetary atmospheres or even strip them entirely, potentially rendering a planet inhabitable. Existing hydrodynamical studies of this important atmospheric mass loss mechanism have mainly considered hydrogen/helium dominated atmospheres. Currently, the effect of more complex chemistry on photoevaporative mass loss has only been the subject of a limited number of studies (e.g. Bolmont et al. 2017). In the era of more advanced exoplanet atmospheric observations, it is more important than ever to determine what, if any atmosphere, these planets may have been able to retain. Here, I present preliminary results of hydrodynamic simulations, showing how the atmosphere of a low-mass planet undergoing photoevaporation is affected by the inclusion of water.</p>

Implications of stellar activity for exoplanetary atmospheres

2010 ◽  
Vol 9 (4) ◽  
pp. 239-243 ◽  
Author(s):  
P. Odert ◽  
M. Leitzinger ◽  
A. Hanslmeier ◽  
H. Lammer ◽  
M.L. Khodachenko ◽  
...  
Keyword(s):  
Extreme Ultraviolet ◽  
Planetary Atmospheres ◽  
Young Star ◽  
Young Stars ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
X Ray ◽  
History Of ◽  
Low Mass ◽  
Exoplanetary Atmospheres

AbstractStellar X-ray and extreme ultraviolet (XUV) radiation is an important driver of the escape of planetary atmospheres. Young stars emit high XUV fluxes that decrease as they age. Since the XUV emission of a young star can be orders of magnitude higher compared to an older one, this evolution has to be taken into account when studying the mass-loss history of a planet. The temporal decrease of activity is closely related to the operating magnetic dynamo, which depends on rotation and convection in Sun-like stars. Using a sample of nearby M dwarfs, we study the relations between age, rotation and activity and discuss the influence on planets orbiting these low-mass stars.

scholarly journals Sculpting the valley in the radius distribution of small exoplanets as a by-product of planet formation: the core-powered mass-loss mechanism

2019 ◽  
Vol 487 (1) ◽  
pp. 24-33 ◽  
Author(s):  
Akash Gupta ◽  
Hilke E Schlichting
Keyword(s):  
Mass Loss ◽  
Thermal Evolution ◽  
Relative Magnitude ◽  
Occurrence Rate ◽  
Loss Mechanism ◽  
Water Ice ◽  
The Core ◽  
Planetary Cores ◽  
Short Period ◽  
Atmospheric Mass

ABSTRACT Recent observations revealed a bimodal radius distribution of small, short-period exoplanets with a paucity in their occurrence, a radius ‘valley’, around 1.5–2.0 R⊕. In this work, we investigate the effect of a planet’s own cooling luminosity on its thermal evolution and atmospheric mass loss (core-powered mass-loss) and determine its observational consequences for the radius distribution of small, close-in exoplanets. Using simple analytical descriptions and numerical simulations, we demonstrate that planetary evolution based on the core-powered mass-loss mechanism alone (i.e. without any photoevaporation) can produce the observed valley in the radius distribution. Our results match the valley’s location, shape and slope in planet radius–orbital period parameter space, and the relative magnitudes of the planet occurrence rate above and below the valley. We find that the slope of the valley is, to first order, dictated by the atmospheric mass-loss time-scale at the Bondi radius and given by d logRp/d logP ≃ 1/(3(1 − β)) that evaluates to −0.11 for β ≃ 4, where Mc/M⊕ = (Rc/R⊕)β(ρc∗/ρ⊕)β/3 is the mass–radius relation of the core. This choice for β yields good agreement with observations and attests to the significance of internal compression for massive planetary cores. We further find that the location of the valley scales as $\rho _{\rm c*}^{-4/9}$ and that the observed planet population must have predominantly rocky cores with typical water–ice fractions of less than ${\sim } 20{{\, \rm per\, cent}}$. Furthermore, we show that the relative magnitude of the planet occurrence rate above and below the valley is sensitive to the details of the planet-mass distribution but that the location of the valley is not.

scholarly journals Atmospheric mass-loss from high-velocity giant impacts

2019 ◽  
Vol 486 (2) ◽  
pp. 2780-2789 ◽  
Author(s):  
Almog Yalinewich ◽  
Hilke Schlichting
Keyword(s):  
Mass Loss ◽  
High Velocity ◽  
Surface Velocity ◽  
Shock Propagation ◽  
Hydrodynamic Simulations ◽  
Giant Impacts ◽  
Self Similar ◽  
The Impact ◽  
Atmospheric Mass ◽  
Impact Shock Wave

ABSTRACT Using moving mesh hydrodynamic simulations, we determine the shock propagation and resulting ground velocities for a planet hit by a high-velocity impactor. We use our results to determine the atmospheric mass-loss caused by the resulting ground motion due to the impact shock wave. We find that there are two distinct shock propagation regimes. In the limit in which the impactor is significantly smaller than the target (Ri << Rt), the solutions are self-similar and the shock velocity at a fixed point on the target scale as $m_{\rm i}^{2/3}$, where mi is the mass of the impactor. In addition, the ground velocities follow a universal profile given by vg/vi = (14.2x2 − 25.3x + 11.3)/(x2 − 2.5x + 1.9) + 2ln Ri/Rt, where x = sin (θ/2), θ is the latitude on the target measured from the impact site, and vg and vi are the ground velocity and impact velocity, respectively. In contrast, in the limit in which the impactor is comparable to the size of the target (Ri ∼ Rt), we find that shock velocities decline with the mass of the impactor significantly more weakly than $m_{\rm i}^{2/3}$. We use the resulting surface velocity profiles to calculate the atmospheric mass-loss for a large range of impactor masses and impact velocities and apply them to the Kepler-36 system and the Moon forming impact. Finally, we present and generalize our results in terms of the vg/vi and the impactor to target size ratio (Ri/Rt) such that they can easily be applied to other collision scenarios.

scholarly journals The Influences of Stellar Activity on Planetary Atmospheres

2016 ◽  
Vol 12 (S328) ◽  
pp. 168-179
Author(s):  
Colin P. Johnstone
Keyword(s):  
Mass Loss ◽  
Planetary Atmospheres ◽  
Time Dependent ◽  
Stellar Winds ◽  
Additional Mass ◽  
Stellar Activity ◽  
X Ray

AbstractOn evolutionary timescales, the atmospheres of planets evolve due to interactions with the planet's surface and with the planet's host star. Stellar X-ray and EUV (=’XUV’) radiation is absorbed high in the atmosphere, driving photochemistry, heating the gas, and causing atmospheric expansion and mass loss. Atmospheres can interact strongly with the stellar winds, leading to additional mass loss. In this review, I summarise some of the ways in which stellar output can influence the atmospheres of planets. I will discuss the importance of simultaneously understanding the evolution of the star's output and the time dependent properties of the planet's atmosphere.

scholarly journals X-rays From Centrifugal Magnetospheres in Massive Stars

2014 ◽  
Vol 9 (S307) ◽  
pp. 449-450 ◽  
Author(s):  
Christopher Bard ◽  
Richard Townsend
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Massive Stars ◽  
Mass Loss Rate ◽  
X Rays ◽  
Stellar Rotation ◽  
Hydrodynamic Simulations ◽  
X Ray ◽  
Ob Stars ◽  
Magnetically Confined

AbstractIn the subset of massive OB stars with strong global magnetic fields, X-rays arise from magnetically confined wind shocks (Babel & Montmerle 1997). However, it is not yet clear what the effect of stellar rotation and mass-loss rate is on these wind shocks and resulting X-rays. Here, we present results from a grid of Arbitrary Rigid-Field Hydrodynamic simulations (ARFHD) of a B-star centrifugal magnetosphere with an eye towards quantifying the effect of stellar rotation and mass-loss rate on the level of X-ray emission. The results are also compared to a generalized XADM model for X-rays in dynamical magnetospheres (ud-Doula et al. 2014).

scholarly journals Photoevaporation versus core-powered mass-loss: model comparison with the 3D radius gap

2021 ◽  
Author(s):  
James G Rogers ◽  
Akash Gupta ◽  
James E Owen ◽  
Hilke E Schlichting
Keyword(s):  
Mass Loss ◽  
Model Comparison ◽  
Hypothesis Test ◽  
Space Mission ◽  
Data Sets ◽  
X Ray ◽  
Wide Range ◽  
Cent Level ◽  
Loss Models ◽  
Atmospheric Mass

Abstract The EUV/X-ray photoevaporation and core-powered mass-loss models are both capable of reproducing the bimodality in the sizes of small, close-in exoplanets observed by the Kepler space mission, often referred to as the ‘radius gap’. However, it is unclear which of these two mechanisms dominates the atmospheric mass-loss which is likely sculpting the radius gap. In this work, we propose a new method of differentiating between the two models, which relies on analysing the radius gap in 3D parameter space. Using models for both mechanisms, and by performing synthetic transit surveys we predict the size and characteristics of a survey capable of discriminating between the two models. We find that a survey of ≳ 5000 planets, with a wide range in stellar mass and measurement uncertainties at a $\lesssim 5{{\ \rm per\ cent}}$ level is sufficient. Our methodology is robust against moderate false positive contamination of $\lesssim 10{{\ \rm per\ cent}}$. We perform our analysis on two surveys (which do not satisfy our requirements): the California Kepler Survey and the Gaia-Kepler Survey and find, unsurprisingly, that both data-sets are consistent with either model. We propose a hypothesis test to be performed on future surveys which can robustly ascertain which of the two mechanisms formed the radius gap, provided one dominates over the other.

scholarly journals EUV irradiation of exoplanet atmospheres occurs on Gyr time-scales

2020 ◽  
Vol 501 (1) ◽  
pp. L28-L32
Author(s):  
George W King ◽  
Peter J Wheatley
Keyword(s):  
Mass Loss ◽  
Time Scales ◽  
High Energy ◽  
Planetary Atmospheres ◽  
X Rays ◽  
X Ray ◽  
Atmospheric Escape ◽  
Euv Emission ◽  
Energy Emission ◽  
High Energy Emission

ABSTRACT Exoplanet atmospheres are known to be vulnerable to mass-loss through irradiation by stellar X-ray and extreme-ultraviolet (EUV) emission. We investigate how this high-energy irradiation varies with time by combining an empirical relation describing stellar X-ray emission with a second relation describing the ratio of solar X-ray to EUV emission. In contrast to assumptions commonly made when modelling atmospheric escape, we find that the decline in stellar EUV emission is much slower than in X-rays, and that the total EUV irradiation of planetary atmospheres is dominated by emission after the saturated phase of high-energy emission (which lasts around 100 Myr after the formation of the star). The EUV spectrum also becomes much softer during this slow decline. Furthermore, we find that the total combined X-ray and EUV emission of stars occurs mostly after the saturated phase. Our results suggest that models of atmospheric escape that focus on the saturated phase of high-energy emission are oversimplified, and when considering the evolution of planetary atmospheres it is necessary to follow EUV-driven escape on Gyr time-scales. This may make it more difficult to use stellar age to separate the effects of photoevaporation and core-powered mass-loss when considering the origin of the planet radius valley.

Rigid Field Hydrodynamic simulations of the magnetosphere of σ Orionis E

2010 ◽  
Vol 6 (S272) ◽  
pp. 194-195
Author(s):  
Nicholas R. Hill ◽  
Richard H. D. Townsend ◽  
David H. Cohen ◽  
Marc Gagné
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Late Type ◽  
Hydrodynamic Simulations ◽  
X Ray ◽  
Upper Limit ◽  
Magnetically Confined

AbstractWe present Rigid Field Hydrodynamic simulations of the magnetosphere of σ Ori E. We find that the X-ray emission from the star's magnetically confined wind shocks is very sensitive to the assumed mass-loss rate. To compare the simulations against the measured X-ray emission, we first disentangle the star from its recently discovered late-type companion using Chandra HRC-I observations. This then allows us to place an upper limit on the mass-loss rate of the primary, which we find to be significantly smaller than previously imagined.

scholarly journals Signatures of the core-powered mass-loss mechanism in the exoplanet population: dependence on stellar properties and observational predictions

2020 ◽  
Vol 493 (1) ◽  
pp. 792-806 ◽  
Author(s):  
Akash Gupta ◽  
Hilke E Schlichting
Keyword(s):  
Mass Loss ◽  
Massive Stars ◽  
Stellar Mass ◽  
Loss Mechanism ◽  
Data Set ◽  
First Order ◽  
Bolometric Luminosity ◽  
The Core ◽  
Stellar Properties ◽  
Atmospheric Mass

ABSTRACT Recent studies have shown that atmospheric mass-loss powered by the cooling luminosity of a planet’s core can explain the observed radius valley separating super-Earths and sub-Neptunes, even without photoevaporation. In this work, we investigate the dependence of this core-powered mass-loss mechanism on stellar mass (M*), metallicity (Z*), and age (τ*). Without making any changes to the underlying planet population, we find that the core-powered mass-loss model yields a shift in the radius valley to larger planet sizes around more massive stars with a slope given by dlog Rp/dlog M* ≃ 0.35, in agreement with observations. To first order, this slope is driven by the dependence of core-powered mass-loss on the bolometric luminosity of the host star and is given by dlog Rp/dlog M* ≃ (3α − 2)/36 ≃ 0.33, where (L*/L⊙) = (M*/M⊙)α is the stellar mass–luminosity relation and α ≃ 4.6 for the CKS data set. We therefore find, in contrast to photoevaporation models, no evidence for a linear correlation between planet and stellar mass, but cannot rule it out either. In addition, we show that the location of the radius valley is, to first order, independent of stellar age and metallicity. Since core-powered mass-loss proceeds over Gyr time-scales, the abundance of super-Earths relative to sub-Neptunes increases with age but decreases with stellar metallicity. Finally, due to the dependence of the envelope’s cooling time-scale on metallicity, we find that the radii of sub-Neptunes increase with metallicity and decrease with age with slopes given by dlog Rp/dlog Z* ≃ 0.1 and dlog Rp/dlog τ* ≃ −0.1, respectively. We conclude with a series of observational tests that can differentiate between core-powered mass-loss and photoevaporation models.

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