arXiv subjects

Colin P. Johnstone

Publications and source records attributed to Colin P. Johnstone.

17 recordsLinked to original sources

arXiv2026

Self-consistent 1D modelling of Jupiter's upper atmosphere as an exoplanet analogue

Jupiter's upper atmosphere provides a real-world laboratory for validating first-principles models of giant gaseous exoplanets and for constraining the key physical processes that govern them. We extended the 1D first-principles thermo-chemical planetary upper atmosphere model Kompot to simulate hydrogen-rich atmospheres of giant exoplanets and benchmarked it against the archetype giant planet Jupiter. We modelled a longitudinal and latitudinal average thermal and chemical profile of Jupiter's upper atmosphere by solving the equations of energy balance, photochemical kinetics, hydrodynamics, and vertical transport in a 1D radial atmospheric grid. The thermal properties of the upper atmosphere were determined by a balance between heating from solar X-ray and ultraviolet (XUV), and infrared (IR) radiation, thermal conduction, Joule heating, and radiative cooling due to H3+ and CH4. The model results were compared with Jupiter observations from Galileo, JUNO, and other instruments. The simulation results were also used as input in the radiative transfer module of the TauREx code to simulate Jupiter's infrared transmission spectrum. Our model reproduces Jupiter's observed upper-atmospheric thermal structure and observed CH4 volume mixing ratios. The chemical abundances of other gases also exhibit strong agreement with observations and existing photochemical models. Heating and cooling results show that Joule heating is the dominant heating source throughout most of the upper atmosphere. The transmission spectrum indicates the rich presence of CH4 in the near- and mid-IR wavelength ranges. Our physically consistent model framework for Jupiter's upper atmosphere provides a validated baseline for future self-consistent simulations of the thermal and chemical structure of a diverse population of hydrogen-rich exoplanet atmospheres.

astro-ph.EP↗

arXiv2021

Stellar winds and planetary atmospheres

Interactions between the winds of stars and the magnetospheres and atmospheres of planets involve many processes, including the acceleration of particles, heating of upper atmospheres, and a diverse range of atmospheric loss processes. Winds remove angular momentum from their host stars causing rotational spin-down and a decay in magnetic activity, which protects atmospheres from erosion. While wind interactions are strongly influenced by the X-ray and ultraviolet activity of the star and the chemical composition of the atmosphere, the role of planetary magnetic fields is unclear. In this chapter, I review our knowledge of the properties and evolution of stellar activity and winds and discuss the influences of these processes on the long term evolution of planetary atmospheres. I do not consider the large number of important processes taking place at the surfaces of planets that cause exchanges between the atmosphere and the planet's interior.

astro-ph.EP↗

arXiv2020

The active lives of stars: a complete description of rotation and XUV evolution of F, G, K, and M dwarfs

Aims. We study the evolution of rotation and high energy X-ray, extreme ultraviolet (EUV), and Ly-alpha emission for F, G, K, and M dwarfs, with masses between 0.1 and 1.2 Msun, and provide our evolutionary code and a freely available set of evolutionary tracks for use in planetary atmosphere studies. Methods. We develop a physical rotational evolution model constrained by observed rotation distributions in young stellar clusters. Using rotation, X-ray, EUV, and Ly-alpha measurements, we derive empirical relations for the dependences of high energy emission on stellar parameters. Our description of X-ray evolution is validated using measurements of X-ray distributions in young clusters. Results. A star's X-ray, EUV, and Ly-alpha evolution is determined by its mass and initial rotation rate, with initial rotation being less important for lower mass stars. At all ages, solar mass stars are significantly more X-ray luminous than lower mass stars and stars that are born as rapid rotators remain highly active longer than those born as slow rotators. At all evolutionary stages, habitable zone planets receive higher X-ray and EUV fluxes when orbiting lower mass stars due to their longer evolutionary timescales. The rates of flares follow similar evolutionary trends with higher mass stars flaring more often than lower mass stars at all ages, though habitable zone planets are likely influenced by flares more when orbiting lower mass stars. Conclusions. Our results show that single decay-laws are insufficient to describe stellar activity evolution and highlight the need for a more comprehensive description based on the evolution of rotation, including also the effects of short-term variability. Planets at similar orbital distances from their host stars receive significantly more X-ray and EUV energy over their lifetimes when orbiting higher mass stars. (abstract incomplete)

astro-ph.SR↗

arXiv2020

1+1D implicit disk computations

We present an implicit numerical method to solve the time-dependent equations of radiation hydrodynamics (RHD) in axial symmetry assuming hydrostatic equilibrium perpendicular to the equatorial plane (1+1D) of a gaseous disk. The equations are formulated in conservative form on an adaptive grid and the corresponding fluxes are calculated by a spacial second order advection scheme. Self-gravity of the disk is included by solving the Possion equation. We test the resulting numerical method through comparison with a simplified analytical solution as well as through the long term viscous evolution of protoplanetary disk when due to viscosity matter is transported towards the central host star and the disk depletes. The importance of the inner boundary conditions on the structural behaviour of disks is demonstrated with several examples.

physics.comp-ph↗

arXiv2019

Hydrodynamic escape of water vapor atmospheres near very active stars

When exposed to the high energy X-ray and ultraviolet radiation of a very active star, water vapor in the upper atmospheres of planets can be photodissociated and rapidly lost to space. In this paper, I study the chemical, thermal, and hydrodynamic processes in the upper atmospheres of terrestrial planets, concentrating on water vapor dominated atmospheres orbiting in the habitable zones of active stars. I consider different stellar activity levels and find very high levels of atmospheric escape in all cases, with the outflowing gas being dominated by atomic hydrogen and oxygen in both their neutral and ion forms. In the lower activity cases, I find that the accumulation of O$_2$ and increases in the D/H ratios in the atmospheres due to mass fractionation are possible, but in the higher activity cases no mass fractionation takes place. Connecting these results to stellar activity evolution tracks for solar mass stars, I show that huge amounts of water vapor can be lost, and both the losses and the amount of O$_2$ that can be accumulated in the atmosphere depend sensitively on the star's initial rotation rate. For an Earth-mass planet in the habitable zone of a low-mass M-dwarf, my results suggest that the accumulation of atmospheric O$_2$ is unlikely unless water loss can take place after the star's most active phase.

astro-ph.EP↗

arXiv2019

Detecting volcanically produced tori along orbits of exoplanets using UV spectroscopy

We suggest to use the Hubble Space Telescople (HST) follow-up observations of the TESS targets for detecting possible plasma tori along the orbits of exoplanets orbiting M dwarfs. The source of the torus could be planetary volcanic activity due to tidal or electromagnetic induction heating. Fast losses to space for planets orbiting these active stars can lead to the lost material forming a torus along the planetary orbit, similar to the Io plasma torus. We show that such torus would be potentially detectable by the HST in the UV.

astro-ph.EP↗

arXiv2019

Close-in sub-Neptunes reveal the past rotation history of their host stars: atmospheric evolution of planets in the HD3167 and K2-32 planetary systems

Planet atmospheric escape induced by high-energy stellar irradiation is a key phenomenon shaping the structure and evolution of planetary atmospheres. Therefore, the present-day properties of a planetary atmosphere are intimately connected with the amount of stellar flux received by a planet during its lifetime, thus with the evolutionary path of its host star. Using a recently developed analytic approximation based on hydrodynamic simulations for atmospheric escape rates, we track within a Bayesian framework the evolution of a planet as a function of stellar flux evolution history, constrained by the measured planetary radius, with the other system parameters as priors. We find that the ideal objects for this type of study are close-in sub-Neptune-like planets, as they are highly affected by atmospheric escape, and yet retain a significant fraction of their primordial hydrogen-dominated atmospheres. Furthermore, we apply this analysis to the HD3167 and K2-32 planetary systems. For HD3167, we find that the most probable irradiation level at 150 Myr was between 40 and 130 times solar, corresponding to a rotation period of 1.78^{+2.69}_{-1.23} days. For K2-32, we find a surprisingly low irradiation level ranging between half and four times solar at 150 Myr. Finally, we show that for multi-planet systems, our framework enables one to constrain poorly known properties of individual planets.

astro-ph.EP↗

arXiv2019

Exoplanet host-star properties: the active environment of exoplanets

The primary objectives of the ExoplANETS-A project are to: establish new knowledge on exoplanet atmospheres; establish new insight on influence of the host star on the planet atmosphere; disseminate knowledge, using online, web-based platforms. The project, funded under the EU's Horizon-2020 programme, started in January 2018 and has a duration ~3 years. We present an overview of the project, the activities concerning the host stars and some early results on the host stars.

astro-ph.EP↗

arXiv2019

Slingshot prominence evolution for a solar-like star

Although the present-day Sun rotates too slowly to exhibit centrifugally-supported 'slingshot prominences', at some time during its past it may have formed these clouds of cool gas and ejected them into the interplanetary medium. We determine the time period for this behaviour by using a rotation evolution code to derive the properties of the formation and ejection of slingshot prominences during the lifetime of a star similar to our Sun. The mass, mass loss rate and rate of ejection of these prominences are calculated using the analytical expression derived in our previous work. We find that for stars with an initial rotation rate larger than $4.6\, Ω_\odot$, about half of all solar mass stars, slingshot prominences will be present even after the star reaches the main sequence phase. In a fast rotator, this means that prominences can form until the star reaches $\sim 800$ Myr old. Our results also indicate that the mass and lifetime of this type of prominence have maximum values when the star reaches the ZAMS at an age of $\sim 40$ Myr for a solar mass star.

astro-ph.SR↗

arXiv2018

Overcoming the limitations of the energy-limited approximation for planet atmospheric escape

Studies of planetary atmospheric composition, variability, and evolution require appropriate theoretical and numerical tools to estimate key atmospheric parameters, among which the mass-loss rate is often the most important. In evolutionary studies, it is common to use the energy-limited formula, which is attractive for its simplicity but ignores important physical effects and can be inaccurate in many cases. To overcome this problem, we consider a recently developed grid of about 7000 one-dimensional upper-atmosphere hydrodynamic models computed for a wide range of planets with hydrogen-dominated atmospheres from which we extract the mass-loss rates. The grid boundaries are [1:39] MEARTH in planetary mass, [1:10] REARTH in planetary radius, [300:2000] K in equilibrium temperature, [0.4:1.3] MSUN in host star's mass, [0.002:1.3] au in orbital separation, and about [10^{26}:5*10^{30}] erg/s in stellar X-ray and extreme ultraviolet luminosity. We then derive an analytical expression for the atmospheric mass-loss rates based on a fit to the values obtained from the grid. The expression provides the mass-loss rates as a function of planetary mass, planetary radius, orbital separation, and incident stellar high-energy flux. We show that this expression is a significant improvement to the energy-limited approximation for a wide range of planets. The analytical expression presented here enables significantly more accurate planetary evolution computations without increasing computing time.

astro-ph.EP↗

arXiv2018

Interaction of infalling solid bodies with primordial atmospheres of disk-embedded planets

Planets that form early enough to be embedded in the circumstellar gas disk accumulate thick atmospheres of nebular gas. Models of these atmospheres need to specify the surface luminosity (i.e. energy loss rate) of the planet. This luminosity is usually associated with a continuous inflow of solid bodies, where the gravitational energy released from these bodies is the source of energy. However, if these bodies release energy in the atmosphere instead of at the surface, this assumption might not be justified. Our aim is to explore the interactions of infalling planetesimals with primordial atmospheres at an embedded phase of evolution. We investigate effects of atmospheric interaction on the planetesimals (mass loss) and the atmosphere (heating/cooling). We used atmospheric parameters from a snapshot of time-dependent evolution simulations for embedded atmospheres and simulated purely radial, infall events of siliceous planetesimals in a 1D, explicit code. We implemented energy transfer between friction, radiation transfer by the atmosphere and the body and thermal ablation; this gives us the possibility to examine the effects on the planetesimals and the atmosphere. We find that a significant amount of gravitational energy is indeed dissipated into the atmosphere, especially for larger planetary cores, which consequently cannot contribute to the atmospheric planetary luminosity. Furthermore, we examine that planetesimal infall events for cores, $M_\mathrm{C} > 2$M$_{\oplus}$, which actually result in a local cooling of the atmosphere; this is totally in contradiction with the classical model.

astro-ph.EP↗

arXiv2018

The Upper Atmospheres of Terrestrial Planets: Carbon Dioxide Cooling and the Earth's Thermospheric Evolution

Context: The thermal and chemical structures of the upper atmospheres of planets crucially influence losses to space and must be understood to constrain the effects of losses on atmospheric evolution. Aims: We develop a 1D first-principles hydrodynamic atmosphere model that calculates atmospheric thermal and chemical structures for arbitrary planetary parameters, chemical compositions, and stellar inputs. We apply the model to study the reaction of the Earth's upper atmosphere to large changes in the CO$_2$ abundance and to changes in the input solar XUV field due to the Sun's activity evolution from 3~Gyr in the past to 2.5~Gyr in the future. Methods: For the thermal atmosphere structure, we consider heating from the absorption of stellar X-ray, UV, and IR radiation, heating from exothermic chemical reactions, electron heating from collisions with non-thermal photoelectrons, Joule heating, cooling from IR emission by several species, thermal conduction, and energy exchanges between the neutral, ion, and electron gases. For the chemical structure, we consider $\sim$500 chemical reactions, including 56 photoreactions, eddy and molecular diffusion, and advection. In addition, we calculate the atmospheric structure by solving the hydrodynamic equations. To solve the equations in our model, we develop the Kompot code and provide detailed descriptions of the numerical methods used in the appendices. Results: We verify our model by calculating the structures of the upper atmospheres of the modern Earth and Venus. By varying the CO$_2$ abundances at the lower boundary (65~km) of our Earth model, we show that the atmospheric thermal structure is significantly altered. [Abstract Truncated]

astro-ph.EP↗

arXiv2016

An Overabundance of Low-density Neptune-like Planets

We present a uniform analysis of the atmospheric escape rate of Neptune-like planets with estimated radius and mass (restricted to $M_{\rm p}<30\,M_{\oplus}$). For each planet we compute the restricted Jeans escape parameter, $Λ$, for a hydrogen atom evaluated at the planetary mass, radius, and equilibrium temperature. Values of $Λ\lesssim20$ suggest extremely high mass-loss rates. We identify 27 planets (out of 167) that are simultaneously consistent with hydrogen-dominated atmospheres and are expected to exhibit extreme mass-loss rates. We further estimate the mass-loss rates ($L_{\rm hy}$) of these planets with tailored atmospheric hydrodynamic models. We compare $L_{\rm hy}$ to the energy-limited (maximum-possible high-energy driven) mass-loss rates. We confirm that 25 planets (15\% of the sample) exhibit extremely high mass-loss rates ($L_{\rm hy}>0.1\,M_{\oplus}{\rm Gyr}^{-1}$), well in excess of the energy-limited mass-loss rates. This constitutes a contradiction, since the hydrogen envelopes cannot be retained given the high mass-loss rates. We hypothesize that these planets are not truly under such high mass-loss rates. Instead, either hydrodynamic models overestimate the mass-loss rates, transit-timing-variation measurements underestimate the planetary masses, optical transit observations overestimate the planetary radii (due to high-altitude clouds), or Neptunes have consistently higher albedos than Jupiter planets. We conclude that at least one of these established estimations/techniques is consistently producing biased values for Neptune planets. Such an important fraction of exoplanets with misinterpreted parameters can significantly bias our view of populations studies, like the observed mass--radius distribution of exoplanets for example.

astro-ph.EP↗

arXiv2015

Short-period stellar activity cycles with Kepler photometry

We study the short-periodic component of stellar activity with a cycle periods Pcyc up to 1000 days using the Kepler mission photometry of fast-rotating (rotational periods from 1 to 4 days) stars with spectra of M4V to F3V. Applying the originally developed two non-spectral methods, we measured the effective period of stellar cycles in 462 objects. The obtained results are in accordance with previous measurements by Vida et al. (2014), do not seem to result from a beating effect. The performed measurements of Pcyc cluster in a specific branch which covers the previously unstudied region in the Saar-Brandenburg (1999) diagram, and connects the branch of inactive stars with the area populated by super-active objects. It is shown that the formation of the discovered branch is due to the alpha-quenching effect, which saturates the magnetic dynamo and decreases the cycle periods with the increase of inverted Rossby number. This finding is important in the context of the discussion on catastrophic quenching and other heuristic approximations of the non-linear alpha-effect.

astro-ph.SR↗

arXiv2015

The Coronal Temperatures of Low-Mass Main-Sequence Stars

Aims. We study the X-ray emission of low-mass main-sequence stars to derive a reliable general scaling law between coronal temperature and the level of X-ray activity. Methods. We collect ROSAT measurements of hardness ratios and X-ray luminosities for a large sample of stars to derive which stellar X-ray emission parameter is most closely correlated with coronal temperature. We calculate average coronal temperatures for a sample of 24 low-mass main-sequence stars with measured emission measure distributions (EMDs) collected from the literature. These EMDs are based on high-resolution X-ray spectra measured by XMM-Newton and Chandra. Results. We confirm that there is one universal scaling relation between coronal average temperature and surface X-ray flux, Fx, that applies to all low-mass main-sequence stars. We find that coronal temperature is related to Fx by Tcor=0.11 Fx^0.26, where Tcor is in MK and Fx is in erg/s/cm^2.

astro-ph.SR↗

arXiv2015

The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star

Aims. We aim to describe the pre-main sequence and main-sequence evolution of X-ray and extreme-ultaviolet radiation of a solar mass star based on its rotational evolution starting with a realistic range of initial rotation rates. Methods. We derive evolutionary tracks of X-ray radiation based on a rotational evolution model for solar mass stars and the rotation-activity relation. We compare these tracks to X-ray luminosity distributions of stars in clusters with different ages. Results. We find agreement between the evolutionary tracks derived from rotation and the X-ray luminosity distributions from observations. Depending on the initial rotation rate, a star might remain at the X-ray saturation level for very different time periods, approximately from 10 Myr to 300 Myr for slow and fast rotators, respectively. Conclusions. Rotational evolution with a spread of initial conditions leads to a particularly wide distribution of possible X-ray luminosities in the age range of 20 to 500 Myrs, before rotational convergence and therefore X-ray luminosity convergence sets in. This age range is crucial for the evolution of young planetary atmospheres and may thus lead to very different planetary evolution histories.

astro-ph.SR↗

arXiv2013

Constraining Stellar Winds of Young Sun-like Stars

As part of the project Pathways to Habitability (http://path.univie.ac.at/), we study the properties of the stellar winds of low-mass and Sun-like stars, and their influences on the atmospheres of potentially habitable planets. For this purpose, we combine mapping of stellar magnetic fields with magnetohydrodynamic wind models.

astro-ph.SR↗