Proyecto de Investigación: SPICE DUNE 947634
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947634
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The Hubble PanCET program: long-term chromospheric evolution and flaring activity of the M dwarf host GJ 3470
(EDP Sciences, 2021-06-08) Bourrier, V.; Dos Santos, L. A.; Sanz Forcada, J.; García Muñoz, Antonio; Henry, G. W.; Lavvas, P.; Lecavelier, A.; López Morales, M.; Mikal Evans, T.; Sing, D. K.; Wakeford, H. R.; Ehrenreich, D.; European Research Council (ERC)
Neptune-size exoplanets seem particularly sensitive to atmospheric evaporation, making it essential to characterize the stellar high-energy radiation that drives this mechanism. This is particularly important with M dwarfs, which emit a large and variable fraction of their luminosity in the ultraviolet and can display strong flaring behavior. The warm Neptune GJ 3470b, hosted by an M2 dwarf, was found to harbor a giant exosphere of neutral hydrogen thanks to three transits observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). Here we report on three additional transit observations from the Panchromatic Comparative Exoplanet Treasury program, obtained with the HST Cosmic Origin Spectrograph. These data confirm the absorption signature from GJ 3470b’s exosphere in the stellar Lyman-α line and demonstrate its stability over time. No planetary signatures are detected in other stellar lines, setting a 3σ limit on GJ 3470b’s far-ultraviolet (FUV) radius at 1.3 times its Roche lobe radius. We detect three flares from GJ 3470. They show different spectral energy distributions but peak consistently in the Si III line, which traces intermediate-temperature layers in the transition region. These layers appear to play a particular role in GJ 3470’s activity as emission lines that form at lower or higher temperatures than Si III evolved differently over the long term. Based on the measured emission lines, we derive synthetic X-ray and extreme-ultraviolet (X+EUV, or XUV) spectra for the six observed quiescent phases, covering one year, as well as for the three flaring episodes. Our results suggest that most of GJ 3470’s quiescent high-energy emission comes from the EUV domain, with flares amplifying the FUV emission more strongly. The neutral hydrogen photoionization lifetimes and mass loss derived for GJ 3470b show little variation over the epochs, in agreement with the stability of the exosphere. Simulations informed by our XUV spectra are required to understand the atmospheric structure and evolution of GJ 3470b and the role played by evaporation in the formation of the hot-Neptune desert.
HST PanCET program: non-detection of atmospheric escape in the warm Saturn-sized planet WASP-29 b
(EDP Sciences, 2021-05-07) Dos Santos, L. A.; Bourrier, V.; Ehrenreich, D.; Sanz Forcada, J.; López Morales, M.; Sing, D. K.; García Muñoz, Antonio; Henry, G. W.; Lavvas, P.; Lecavelier des Etangs, A.; Mikal Evans, T.; Vidal Madjar, A.; Wakeford, H. R.; Centre National D'Etudes Spatiales (CNES); European Research Council (ERC); Agencia Estatal de Investigación (AEI); Dos Santos, L. A. [0000-0002-2248-3838]; Sanz Forcada, J. [0000-0002-1600-7835]; López Morales, M. [0000-0003-3204-8183]; Sing, D. K. [0000-0001-6050-7645]; García Muñoz, A. [0000-0003-1756-4825]; Henry, G. W. [0000-0003-4155-8513]; Lecavelier des Etangs, A. [0000-0002-5637-5253]; Mikal Evans, T. [0000-0001-5442-1300]
Short-period gas giant exoplanets are susceptible to intense atmospheric escape due to their large scale heights and strong high-energy irradiation. This process is thought to occur ubiquitously, but to date we have only detected direct evidence of atmospheric escape in hot Jupiters and warm Neptunes. The latter planets are particularly more sensitive to escape-driven evolution as a result of their lower gravities with respect to Jupiter-sized planets. But the paucity of cases for intermediate, Saturn-sized exoplanets at varying levels of irradiation precludes a detailed understanding of the underlying physics in atmospheric escape of hot gas giants. Aiming to address this issue, our objectives here are to assess the high-energy environment of the warm (Teq = 970 K) Saturn WASP-29 b and search for signatures of atmospheric escape. We used far-ultraviolet observations from the Hubble Space Telescope to analyze the flux time series of H I, C II, Si III, Si IV, and N V during the transit of WASP-29 b. At 88 pc, a large portion of the Lyman-α core of the K4V-type host WASP-29 is attenuated by interstellar medium absorption, limiting our ability to probe the escape of H at velocities between −84 and +35 km s−1. At 3σ confidence, we rule out any in-transit absorption of H I larger than 92% in the Lyman-α blue wing and 19% in the red wing. We found an in-transit flux decrease of 39%−11%+12% in the ground-state C II emission line at 1334.5 Å. But due to this signal being significantly present in only one visit, it is difficult to attribute a planetary or stellar origin to the ground-state C II signal. We place 3σ absorption upper limits of 40, 49, and 24% on Si III, Si IV, and for excited-state C II at 1335.7 Å, respectively. Low activity levels and the faint X-ray luminosity suggest that WASP-29 is an old, inactive star. Nonetheless, an energy-limited approximation combined with the reconstructed EUV spectrum of the host suggests that the planet is losing its atmosphere at a relatively large rate of 4 × 109 g s−1. The non-detection at Lyman-α could be partly explained by a low fraction of escaping neutral hydrogen, or by the state of fast radiative blow-out we infer from the reconstructed Lyman-α line.
