arXiv subjects

Nikolai V. Erkaev

Publications and source records attributed to Nikolai V. Erkaev.

4 recordsLinked to original sources

arXiv2023

Precise photoionisation treatment and hydrodynamic effects in atmospheric modelling of warm and hot Neptunes

Observational breakthroughs in the exoplanet field of the last decade motivated the development of numerous theoretical models describing atmospheres and mass loss, which is believed to be one of the main drivers of planetary evolution. We aim to outline for which types of close-in planets in the Neptune-mass range the accurate treatment of photoionisation effects is most relevant concerning atmospheric escape and the parameters relevant for interpreting observations. We developed the CHAIN (Cloudy e Hydro Ancora INsieme) model combining 1D hydrodynamic upper atmosphere model with the non-LTE photoionisation and radiative transfer code Cloudy accounting for photochemistry, detailed atomic level populations, and chemical reactions for all elements up to zinc. We apply CHAIN to model the upper atmospheres of a range of Neptune-like planets with masses between 1 and 50 M$_{\oplus}$, varying also the orbital parameters. For the majority of warm and hot Neptunes, we find slower and denser outflows, with lower ion fractions, compared to the predictions of the hydrodynamic model alone. Furthermore, we find significantly different temperature profiles between CHAIN and the hydrodynamic model alone, though the peak values are similar for similar atmospheric compositions. The mass-loss rates predicted by CHAIN are higher for hot, strongly irradiated planets and lower for more moderate planets. All differences between the two models are strongly correlated with the amount of high-energy irradiation. Finally, we find that the hydrodynamic effects impact significantly ionisation and heating. The impact of the precise photoionisation treatment provided by Cloudy strongly depends on the system parameters. This suggests that some of the simplifications typically employed in hydrodynamic modelling might lead to systematic errors when studying planetary atmospheres, even at a population-wide level.

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↗

arXiv2001

Rate of steady-state reconnection in an incompressible plasma

The reconnection rate is obtained for the simplest case of 2D symmetric reconnection in an incompressible plasma. In the short note (Erkaev et al., Phys. Rev. Lett.,84, 1455 (2000)), the reconnection rate is found by matching the outer Petschek solution and the inner diffusion region solution. Here the details of the numerical simulation of the diffusion region are presented and the asymptotic procedure which is used for deriving the reconnection rate is described. The reconnection rate is obtained as a decreasing function of the diffusion region length. For a sufficiently large diffusion region scale, the reconnection rate becomes close to that obtained in the Sweet-Parker solution with the inverse square root dependence on the magnetic Reynolds number, determined for the global size of the current sheet. On the other hand, for a small diffusion region length scale, the reconnection rate turns out to be very similar to that obtained in the Petschek model with a logarithmic dependence on the magnetic Reynolds number. This means that the Petschek regime seems to be possible only in the case of a strongly localized conductivity corresponding to a small scale of the diffusion region.

physics.plasm-ph↗

arXiv1999

Reconnection rate for the steady-state Petschek model

Reconnection rate is found for the canonical simplest case of steady-state two-dimensional symmetric reconnection in an incompressible plasma by matching of outer Petschek solution and internal diffusion region solution. The reconnection rate obtained naturally incorporates both Sweet-Parker and Petschek regimes, the latter seems to be possible only for the case with strongly localized resistivity.

physics.plasm-ph↗