Exoplanet atmospheres

Abstract:

© 2019. The American Astronomical Society. All rights reserved. As part of the Mega-Measurements of the Ultraviolet Spectral Characteristics of Low-Mass Exoplanetary Systems Hubble Space Telescope (HST) Treasury program, we obtained time-series ultraviolet spectroscopy of the M2.5V star, GJ 674. During the far-ultraviolet (FUV) monitoring observations, the target exhibited several small flares and one large flare (E FUV = 10 30.75 erg) that persisted over the entirety of an HST orbit and had an equivalent duration >30,000 s, comparable to the highest relative amplitude event previously recorded in the FUV. The flare spectrum exhibited enhanced line emission from chromospheric, transition region, and coronal transitions and a blue FUV continuum with an unprecedented color temperature of T C ≃ 40,000 -10,000 K. In this Letter, we compare the flare FUV continuum emission with parameterizations of radiative hydrodynamic model atmospheres of M star flares. We find that the observed flare continuum can be reproduced using flare models but only with the ad hoc addition of a hot, dense emitting component. This observation demonstrates that flares with hot FUV continuum temperatures and significant extreme-ultraviolet/FUV energy deposition will continue to be of importance to exoplanet atmospheric chemistry and heating, even as the host M dwarfs age beyond their most active evolutionary phases.

Abstract:

© 2018 Elsevier Inc. Previous investigations have employed more than 100 close observations of Titan by the Cassini orbiter to elucidate connections between the production and distribution of Titan's vast, organic-rich chemical inventory and its atmospheric dynamics. However, as Titan transitions into northern summer, the lack of incoming data from the Cassini orbiter presents a potential barrier to the continued study of seasonal changes in Titan's atmosphere. In our previous work (Thelen et al., 2018), we demonstrated that the Atacama Large Millimeter/submillimeter Array (ALMA) is well suited for measurements of Titan's atmosphere in the stratosphere and lower mesosphere (∼100−500 km) through the use of spatially resolved (beam sizes < 1′′) flux calibration observations of Titan. Here, we derive vertical abundance profiles of four of Titan's trace atmospheric species from the same 3 independent spatial regions across Titan's disk during the same epoch (2012–2015): HCN, HC3N, C3H4, and CH3CN. We find that Titan's minor constituents exhibit large latitudinal variations, with enhanced abundances at high latitudes compared to equatorial measurements; this includes CH3CN, which eluded previous detection by Cassini in the stratosphere, and thus spatially resolved abundance measurements were unattainable. Even over the short 3-year period, vertical profiles and integrated emission maps of these molecules allow us to observe temporal changes in Titan's atmospheric circulation during northern spring. Our derived abundance profiles are comparable to contemporary measurements from Cassini infrared observations, and we find additional evidence for subsidence of enriched air onto Titan's south pole during this time period. Continued observations of Titan with ALMA beyond the summer solstice will enable further study of how Titan's atmospheric composition and dynamics respond to seasonal changes.

Abstract:

© ESO 2019. During their formation and early evolution, rocky planets undergo multiple global melting events due to accretionary collisions with other protoplanets. The detection and characterization of their post-collision afterglows (magma oceans) can yield important clues about the origin and evolution of the solar and extrasolar planet population. Here, we quantitatively assess the observational prospects to detect the radiative signature of forming planets covered by such collision-induced magma oceans in nearby young stellar associations with future direct imaging facilities. We have compared performance estimates for near- and mid-infrared instruments to be installed at ESO's Extremely Large Telescope (ELT), and a potential space-based mission called Large Interferometer for Exoplanets (LIFE). We modelled the frequency and timing of energetic collisions using N-body models of planet formation for different stellar types, and determine the cooling of the resulting magma oceans with an insulating atmosphere. We find that the probability of detecting at least one magma ocean planet depends on the observing duration and the distribution of atmospheric properties among rocky protoplanets. However, the prospects for detection significantly increase for young and close stellar targets, which show the highest frequencies of giant impacts. For intensive reconnaissance with a K band (2.2 μm) ELT filter or a 5.6 μm LIFE filter, the β Pictoris, Columba, TW Hydrae, and Tucana-Horologium associations represent promising candidates for detecting a molten protoplanet. Our results motivate the exploration of magma ocean planets using the ELT and underline the importance of space-based direct imaging facilities to investigate and characterize planet formation and evolution in the solar vicinity. Direct imaging of magma oceans will advance our understanding of the early interior, surface and atmospheric properties of terrestrial worlds.