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Register a new userGlobal 3D hydrodynamic modeling of in-transit Ly{alpha} absorption of GJ436b
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Ildar Shaikhislamov Dr
Publication date
2019
fields
Physics
and research's language is
English
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Using a global 3D, fully self-consistent, multi-fluid hydrodynamic model, we simulate the escaping upper atmosphere of the warm Neptune GJ436b, driven by the stellar XUV radiation impact and gravitational forces and interacting with the stellar wind. Under the typical parameters of XUV flux and stellar wind plasma expected for GJ436, we calculate in-transit absorption in Ly{alpha} and find that it is produced mostly by Energetic Neutral Atoms outside of the planetary Roche lobe, due to the resonant thermal line broadening. At the same time, the influence of radiation pressure has been shown to be insignificant. The modelled absorption is in good agreement with the observations and reveals such features as strong asymmetry between blue and red wings of the absorbed Ly{alpha} line profile, deep transit depth in the high velocity blue part of the line reaching more than 70%, and the timing of early ingress. On the other hand, the model produces significantly deeper and longer egress than in observations, indicating that there might be other processes and factors, still not accounted, that affect the interaction between the planetary escaping material and the stellar wind. At the same time, it is possible that the observational data, collected in different measurement campaigns, are affected by strong variations of the stellar wind parameters between the visits, and therefore, they cannot be reproduced altogether with the single set of model parameters.
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Warm Neptune GJ3470b has been recently observed in 23S-23P transition of metastable helium, yielding absorption of about 1% in Doppler velocity range of [-40; 10] km/s. Along with previous detection of absorption in Ly{alpha} with depth of 20-40% in the blue and red wings of the line, it offers a complex target for simulation and testing of the current models. Obtained results suggest that absorption in both these lines comes from interaction of expanding upper planetary atmosphere with stellar plasma wind, allowing to constrain the stellar plasma parameters and the helium abundance in planet atmosphere.
Aims: We aim at constraining the conditions of the wind and high-energy emission of the host star reproducing the non-detection of Ly$alpha$ planetary absorption. Methods: We model the escaping planetary atmosphere, the stellar wind, and their interaction employing a multi-fluid, three-dimensional hydrodynamic code. We assume a planetary atmosphere composed of hydrogen and helium. We run models varying the stellar high-energy emission and stellar mass-loss rate, further computing for each case the Ly$alpha$ synthetic planetary atmospheric absorption and comparing it with the observations. Results: We find that a non-detection of Ly$alpha$ in absorption employing the stellar high-energy emission estimated from far-ultraviolet and X-ray data requires a stellar wind with a stellar mass-loss rate about six times lower than solar. This result is a consequence of the fact that, for $pi$ Men c, detectable Ly$alpha$ absorption can be caused exclusively by energetic neutral atoms, which become more abundant with increasing the velocity and/or the density of the stellar wind. By considering, instead, that the star has a solar-like wind, the non-detection requires a stellar ionising radiation about four times higher than estimated. This is because, despite the fact that a stronger stellar high-energy emission ionises hydrogen more rapidly, it also increases the upper atmosphere heating and expansion, pushing the interaction region with the stellar wind farther away from the planet, where the planet atmospheric density that remains neutral becomes smaller and the production of energetic neutral atoms less efficient. Conclusions: Comparing the results of our grid of models with what is expected and estimated for the stellar wind and high-energy emission, respectively, we support the idea that the atmosphere of $pi$ Men c is likely not hydrogen-dominated.
We use 3D hydrodynamics simulations followed by synthetic line profile calculations to examine the effect increasing the strength of the stellar wind has on observed Ly-$alpha$ transits of a Hot Jupiter (HJ) and a Warm Neptune (WN). We find that increasing the stellar wind mass-loss rate from 0 (no wind) to 100 times the solar mass-loss rate value causes reduced atmospheric escape in both planets (a reduction of 65% and 40% for the HJ and WN, respectively, compared to the no wind case). For weaker stellar winds (lower ram pressure), the reduction in planetary escape rate is very small. However, as the stellar wind becomes stronger, the interaction happens deeper in the planetary atmosphere and, once this interaction occurs below the sonic surface of the planetary outflow, further reduction in evaporation rates is seen. We classify these regimes in terms of the geometry of the planetary sonic surface. Closed refers to scenarios where the sonic surface is undisturbed, while open refers to those where the surface is disrupted. We find that the change in stellar wind strength affects the Ly-$alpha$ transit in a non-linear way. Although little change is seen in planetary escape rates ($simeq 5.5times 10^{11}$g/s) in the closed to partially open regimes, the Ly-$alpha$ absorption (sum of the blue [-300, -40 km/s] & red [40, 300 km/s] wings) changes from 21% to 6% as the stellar wind mass-loss rate is increased in the HJ set of simulations. For the WN simulations, escape rates of $simeq 6.5times 10^{10}$g/s can cause transit absorptions that vary from 8.8% to 3.7%, depending on the stellar wind strength. We conclude that the same atmospheric escape rate can produce a range of absorptions depending on the stellar wind and that neglecting this in the interpretation of Ly-$alpha$ transits can lead to underestimation of planetary escape rates.
Strong atmospheric escape has been detected in several close-in exoplanets. As these planets consist mostly of hydrogen, observations in hydrogen lines, such as Ly-alpha and H-alpha, are powerful diagnostics of escape. Here, we simulate the evolution of atmospheric escape of close-in giant planets and calculate their associated Ly-alpha and H-alpha transits. We use a one-dimensional hydrodynamic escape model to compute physical properties of the atmosphere and a ray-tracing technique to simulate spectroscopic transits. We consider giant (0.3 and 1M_jup) planets orbiting a solar-like star at 0.045au, evolving from 10 to 5000 Myr. We find that younger giants show higher rates of escape, owing to a favourable combination of higher irradiation fluxes and weaker gravities. Less massive planets show higher escape rates (1e10 -- 1e13 g/s) than those more massive (1e9 -- 1e12 g/s) over their evolution. We estimate that the 1-M_jup planet would lose at most 1% of its initial mass due to escape, while the 0.3-M_jup planet, could lose up to 20%. This supports the idea that the Neptunian desert has been formed due to significant mass loss in low-gravity planets. At younger ages, we find that the mid-transit Ly-alpha line is saturated at line centre, while H-alpha exhibits transit depths of at most 3 -- 4% in excess of their geometric transit. While at older ages, Ly-alpha absorption is still significant (and possibly saturated for the lower mass planet), the H-alpha absorption nearly disappears. This is because the extended atmosphere of neutral hydrogen becomes predominantly in the ground state after ~1.2 Gyr.
Lyman $alpha$ observations of the transiting exoplanet HD 209458b enable the study of exoplanets exospheres exposed to stellar EUV fluxes, as well as the interacting stellar wind properties. In this study we present 3D hydrodynamical models for the stellar-planetary wind interaction including radiation pressure and charge exchange, together with photoionization, recombination and collisional ionization processes. Our models explore the contribution of the radiation pressure and charge exchange on the Ly$alpha$ absorption profile in a hydrodynamical framework, and for a single set of stellar wind parameters appropriate for HD 209458. We find that most of the absorption is produced by the material from the planet, with a secondary contribution of neutralized stellar ions by charge exchange. At the same time, the hydrodynamic shock heats up the planetary material, resulting in a broad thermal profile. Meanwhile, the radiation pressure yielded a small velocity shift of the absorbing material. While neither charge exchange nor radiation pressure provide enough neutrals at the velocity needed to explain the observations at $-100~mathrm{km~s^{-1}}$ individually, we find that the two effects combined with the broad thermal profile are able to explain the observations.
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