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Register a new userThree-dimensional hydrodynamic simulations of the upper atmosphere of $pi$ Men c: comparison with Ly$alpha$ transit observations
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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.
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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.
Strongly irradiated exoplanets develop extended atmospheres that can be utilized to probe the deeper planet layers. This connection is particularly useful in the study of small exoplanets, whose bulk atmospheres are challenging to characterize directly. Here, we report the 3.4{sigma} detection of C II ions during a single transit of the super-Earth {pi} Men c in front of its Sun-like host star. The transit depth and Doppler velocities are consistent with the ions filling the planets Roche lobe and moving preferentially away from the star, an indication that they are escaping the planet. We argue that {pi} Men c possesses a thick atmosphere with abundant heavy volatiles ($>=$ 50{%} by mass of atmosphere) but that needs not be carbon rich. Our reasoning relies upon cumulative evidence from the reported C II detection, the nondetection of H I atoms in a past transit, modeling of the planets interior, and the assumption that the atmosphere, having survived the most active phases of its Sun-like host star, will survive another 0.2-2 Gyr. Depending on the current mass of atmosphere, {pi} Men c may still transition into a bare rocky core. Our findings confirm the hypothesized compositional diversity of small exoplanets, and represent a milestone toward understanding the planets formation and evolution paths through the investigation of their extended atmospheres.
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.
The bright star $pi$ Men was chosen as the first target for a radial velocity follow-up to test the performance of ESPRESSO, the new high-resolution spectrograph at the ESOs Very-Large Telescope (VLT). The star hosts a multi-planet system (a transiting 4 M$_oplus$ planet at $sim$0.07 au, and a sub-stellar companion on a $sim$2100-day eccentric orbit) which is particularly appealing for a precise multi-technique characterization. With the new ESPRESSO observations, that cover a time span of 200 days, we aim to improve the precision and accuracy of the planet parameters and search for additional low-mass companions. We also take advantage of new photometric transits of $pi$ Men c observed by TESS over a time span that overlaps with that of the ESPRESSO follow-up campaign. We analyse the enlarged spectroscopic and photometric datasets and compare the results to those in the literature. We further characterize the system by means of absolute astrometry with Hipparcos and Gaia. We used the spectra of ESPRESSO for an independent determination of the stellar fundamental parameters. We present a precise characterization of the planetary system around $pi$ Men. The ESPRESSO radial velocities alone (with typical uncertainty of 10 cm/s) allow for a precise retrieval of the Doppler signal induced by $pi$ Men c. The residuals show an RMS of 1.2 m/s, and we can exclude companions with a minimum mass less than $sim$2 M$_oplus$ within the orbit of $pi$ Men c). We improve the ephemeris of $pi$ Men c using 18 additional TESS transits, and in combination with the astrometric measurements, we determine the inclination of the orbital plane of $pi$ Men b with high precision ($i_{b}=45.8^{+1.4}_{-1.1}$ deg). This leads to the precise measurement of its absolute mass $m_{b}=14.1^{+0.5}_{-0.4}$ M$_{Jup}$, and shows that the planetary orbital planes are highly misaligned.
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.
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