ABSTRACT
High-energy stellar irradiation can photoevaporate planetary atmospheres, which can be observed in spectroscopic transits of hydrogen lines. For the exoplanet HD189733b, multiple ...observations in the Ly α line have shown that atmospheric evaporation is variable, going from undetected to enhanced evaporation in a 1.5-yr interval. Coincidentally or not, when HD189733b was observed to be evaporating, a stellar flare had just occurred 8 h prior to the observation. This led to the question of whether this temporal variation in evaporation occurred due to the flare, an unseen associated coronal mass ejection (CME), or even the simultaneous effect of both. In this work, we investigate the impact of flares (radiation), winds, and CMEs (particles) on the atmosphere of HD189733b using three-dimensional radiation hydrodynamic simulations that self-consistently include stellar photon heating. We study four cases: first, the quiescent phase including stellar wind; secondly, a flare; thirdly, a CME; and fourthly, a flare that is followed by a CME. Compared to the quiescent case, we find that the flare alone increases the evaporation rate by only 25 per cent, while the CME leads to a factor of 4 increments. We calculate Ly α synthetic transits and find that the flare alone cannot explain the observed high blueshifted velocities seen in the Ly α. The CME, however, leads to an increase in the velocity of escaping atmospheres, enhancing the blueshifted transit depth. While the effects of CMEs show a promising potential, our models are not able to fully explain the blueshifted transit depths, indicating that they might require additional physical mechanisms.
Prominence formation and ejection in cool stars Villarreal D'Angelo, Carolina; Jardine, Moira; See, Victor
Monthly notices of the Royal Astronomical Society. Letters,
03/2018, Letnik:
475, Številka:
1
Journal Article
Recenzirano
Abstract
The observational signatures of prominences have been detected in single and binary G and K type stars for many years now, but recently this has been extended to the M dwarf regime. ...Prominences carry away both mass and angular momentum when they are ejected and the impact of this mass on any orbiting planets may be important for the evolution of exoplanetary atmospheres. By means of the classification used in the massive star community, that involves knowledge of two parameters (the co-rotation and Alfvén radii, rK and rA), we have determined which cool stars could support prominences. From a model of mechanical support, we have determined that the prominence mass mp/M⋆ = (EM/EG)(r⋆/rK)2F where $E_MB_\star ^2r_\star ^3$ and $E_G = GM_\star ^2/r_\star$ are magnetic and gravitational energies and F is a geometric factor. Our calculated masses and ejection frequencies (typically 1016 − 1017 g and 0.4 d, respectively) are consistent with observations and are sufficient to ensure that an exoplanet orbiting in the habitable zone of an M dwarf could suffer frequent impacts.
Slingshot prominence evolution for a solar-like star Villarreal D’Angelo, Carolina; Jardine, Moira; Johnstone, Colin P ...
Monthly notices of the Royal Astronomical Society,
05/2019, Letnik:
485, Številka:
1
Journal Article
Abstract
Atmospheric escape is a fundamental process that affects the structure, composition, and evolution of many planets. The signatures of escape are detectable on close-in, gaseous exoplanets ...orbiting bright stars, owing to the high levels of extreme-ultraviolet irradiation from their parent stars. The Colorado Ultraviolet Transit Experiment (CUTE) is a CubeSat mission designed to take advantage of the near-ultraviolet stellar brightness distribution to conduct a survey of the extended atmospheres of nearby close-in planets. The CUTE payload is a magnifying near-ultraviolet (2479–3306 Å) spectrograph fed by a rectangular Cassegrain telescope (206 mm × 84 mm); the spectrogram is recorded on a back-illuminated, UV-enhanced CCD. The science payload is integrated into a 6U Blue Canyon Technology XB1 bus. CUTE was launched into a polar, low-Earth orbit on 2021 September 27 and has been conducting this transit spectroscopy survey following an on-orbit commissioning period. This paper presents the mission motivation, development path, and demonstrates the potential for small satellites to conduct this type of science by presenting initial on-orbit science observations. The primary science mission is being conducted in 2022–2023, with a publicly available data archive coming online in 2023.
ABSTRACT
Stellar high-energy radiation (X-ray and extreme ultraviolet, XUV) drives atmospheric escape in close-in exoplanets. Given that stellar irradiation depends on the stellar magnetism and that ...stars have magnetic cycles, we investigate how cycles affect the evolution of exoplanetary atmospheric escape. First, we consider a hypothetical HD209458b-like planet orbiting the Sun. For that, we implement the observed solar XUV radiation available over one and a half solar cycles in a 1D hydrodynamic escape model of HD209458b. We find that atmospheric escape rates show a cyclic variation (from 7.6 to 18.5 × 1010 g s−1), almost proportional to the incident stellar radiation. To compare this with observations, we compute spectroscopic transits in two hydrogen lines. We find non-detectable cyclic variations in Ly α transits. Given the temperature sensitiveness of the H α line, its equivalent width has an amplitude of 1.9 mÅ variation over the cycle, which could be detectable in exoplanets such as HD209458b. We demonstrate that the XUV flux is linearly proportional to the magnetic flux during the solar cycle. Secondly, we apply this relation to derive the cyclic evolution of the XUV flux of HD189733 using the star’s available magnetic flux observations from Zeeman Doppler Imaging over nearly a decade. The XUV fluxes are then used to model escape in HD189733b, which shows escape rate varying from 2.8 to 6.5 × 1010 g s−1. Like in the HD209458b case, this introduces variations in Ly α and H α transits, with H α variations more likely to be observable. Finally, we show that a strong stellar flare would enhance significantly Ly α and H α transit depths.
ABSTRACT
The two planetary systems, TOI-942 and TOI-421, share many similar characteristics, apart from their ages (50 Myr and 9 Gyr). Each of the stars hosts two sub-Neptune-like planets at similar ...orbits and in similar mass ranges. In this paper, we aim to investigate whether the similarity of the host stars and configuration of the planetary systems can be taken as proof that the two systems were formed and evolved in a similar way. In paper I of this series, we performed a comparative study of these two systems using three-dimensional (3D) modelling of atmospheric escape and its interaction with the stellar wind, for the four planets. We demonstrated that though the strong wind of the young star has a crucial effect on observable signatures, its effect on the atmospheric mass loss is minor in the evolutionary context. Here, we use atmosphere evolution models to track the evolution of planets in the younger system TOI-942 and also to constrain the past of the TOI-421 system. We demonstrate that despite all the similarities, the two planetary systems are on two very different evolutionary pathways. The inner planet in the younger system, TOI-942, will likely lose all of its atmosphere and become a super-Earth-like planet, while the outer planet will become a typical sub-Neptune. Concerning the older system, TOI-421, our evolution modelling suggests that they must have started their evolution with very substantial envelopes, which can be a hint of formation beyond the snow line.
ABSTRACT
Atmospheric escape in exoplanets has traditionally been observed using hydrogen Lyman-α and Hα transmission spectroscopy, but more recent detections have utilized the metastable helium ...triplet at 1083 nm. Since this feature is accessible from the ground, it offers new possibilities for studying atmospheric escape. Our goal is to understand how the observability of escaping helium evolves during the lifetime of a highly irradiated gas giant. We extend our previous work on 1D self-consistent hydrodynamic escape from hydrogen-only atmospheres as a function of planetary evolution to the first evolution-focused study of escaping hydrogen–helium atmospheres. Additionally, using these novel models we perform helium triplet transmission spectroscopy. We adapt our previous hydrodynamic escape model to now account for both hydrogen and helium heating and cooling processes and simultaneously solve for the population of helium in the triplet state. To account for the planetary evolution, we utilize evolving predictions of planetary radii for a close-in 0.3 MJup gas giant and its received stellar flux in X-ray, hard and soft extreme-ultraviolet (UV), and mid-UV wavelength bins assuming a K-dwarf stellar host. We find that the helium triplet signature diminishes with evolution. Our models suggest that young (≲ 150 Myr), close-in gas giants (∼1 to 2 RJup) should produce helium 1083 nm transit absorptions of $\sim 4~{{\ \rm per\ cent}}$ or $\sim 7~{{\ \rm per\ cent}}$, for a slow- or fast-rotating K dwarf, respectively, assuming a 2 per cent helium abundance.
We perform an analysis of ∼80 000 photometric measurements for the following 10 stars hosting transiting planets: WASP-2, -4, -5, -52, Kelt-1, CoRoT-2, XO-2, TrES-1, HD 189733, GJ 436. Our analysis ...includes mainly transit light curves from the Exoplanet Transit Database, public photometry from the literature, and some proprietary photometry privately supplied by other authors. Half of these light curves were obtained by amateurs. From this photometry we derive 306 transit timing measurements, as well as improved planetary transit parameters. Additionally, for 6 of these 10 stars we present a set of radial velocity measurements obtained from the spectra stored in the HARPS, HARPS-N and SOPHIE archives using the HARPS–TERRA pipeline. Our analysis of these transit timing and radial velocity data did not reveal significant hints of additional orbiting bodies in almost all of the cases. In the WASP-4 case, we found hints of marginally significant TTV signals having amplitude 10–20 s, although their parameters are model dependent and uncertain, while radial velocities did not reveal statistically significant Doppler signals.
ABSTRACT
The GJ 436 planetary system is an extraordinary system. The Neptune-sized planet that orbits the M3 dwarf revealed in the Ly α line an extended neutral hydrogen atmosphere. This material ...fills a comet-like tail that obscures the stellar disc for more than 10 h after the planetary transit. Here, we carry out a series of 3D radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. With these models, we seek to reproduce the ${\sim}56{{\ \rm per\ cent}}$ absorption found in Ly α transits, simultaneously with the lack of absorption in H α transit. Varying the stellar wind strength and the EUV stellar luminosity, we search for a set of parameters that best fit the observational data. Based on Ly α observations, we found a stellar wind velocity at the position of the planet to be around 250–460 km s−1 with a temperature of 3–4 × 105 K. The stellar and planetary mass-loss rates are found to be 2 × 10−15 M⊙ yr−1 and ∼6–10 × 109 g s−1, respectively, for a stellar EUV luminosity of 0.8–1.6 × 1027 erg s−1. For the parameters explored in our simulations, none of our models present any significant absorption in the H α line in agreement with the observations.
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