Exoplanets and Stellar Physics

Abstract:

Ground- and space-based planet searches employing radial velocity techniques and transit photometry have detected thousands of planet-hosting stars in the Milky Way. With so many planets discovered, the next step toward identifying potentially habitable planets is atmospheric characterization. While the Sun–Earth system provides a good framework for understanding the atmospheric chemistry of Earth-like planets around solar-type stars, the observational and theoretical constraints on the atmospheres of rocky planets in the habitable zones (HZs) around low-mass stars (K and M dwarfs) are relatively few. The chemistry of these atmospheres is controlled by the shape and absolute flux of the stellar spectral energy distribution (SED), however, flux distributions of relatively inactive low-mass stars are poorly understood at present. To address this issue, we have executed a panchromatic (X-ray to mid-IR) study of the SEDs of 11 nearby planet-hosting stars, the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) Treasury Survey. The MUSCLES program consists visible observations from Hubble and ground-based observatories. Infrared and astrophysically inaccessible wavelengths (EUV and Lyα) are reconstructed using stellar model spectra to fill in gaps in the observational data. In this overview and the companion papers describing the MUSCLES survey, we show that energetic radiation (X-ray and ultraviolet) is present from magnetically active stellar atmospheres at all times for stars as late as M6. The emission line luminosities of C iv and Mg ii are strongly correlated with band-integrated luminosities and we present empirical relations that can be used to estimate broadband FUV and XUV (≡X-ray + EUV) fluxes from individual stellar emission line measurements. We find that while the slope of the SED, FUV/NUV, increases by approximately two orders of magnitude form early K to late M dwarfs (≈0.01–1), the absolute FUV and XUV flux levels at their corresponding HZ distances are constant to within factors of a few, spanning the range 10–70 erg cm⁻² s⁻¹ in the HZ. Despite the lack of strong stellar activity indicators in their optical spectra, several of the M dwarfs in our sample show spectacular UV flare emission in their light curves. We present an example with flare/quiescent ultraviolet flux ratios of the order of 100:1 where the transition region energy output during the flare is comparable to the total quiescent luminosity of the star E_flare(UV) ~ 0.3 L*Δt (Δt = 1 s). Finally, we interpret enhanced L(line)/L_Bol ratios for C iv and N v as tentative observational evidence for the interaction of planets with large planetary mass-to-orbital distance ratios (M_plan/a_plan) with the transition regions of their host stars.

Abstract:

We present a study into the capabilities of integrated and spatially resolved integral field spectroscopy of galaxies at z = 2–4 with the future HARMONI spectrograph for the European Extremely Large Telescope (E-ELT) using the simulation pipeline, HSIM. We focus particularly on the instrument's capabilities in stellar absorption line integral field spectroscopy, which will allow us to study the stellar kinematics and stellar population characteristics. Such measurements for star-forming and passive galaxies around the peak star formation era will provide a critical insight into the star formation, quenching and mass assembly history of high-z, and thus present-day galaxies. First, we perform a signal-to-noise study for passive galaxies at a range of stellar masses for z = 2–4, assuming different light profiles; for this population, we estimate that integrated stellar absorption line spectroscopy with HARMONI will be limited to galaxies with M* ≳ 1010.7 M⊙. Secondly, we use HSIM to perform a mock observation of a typical star-forming 1010 M⊙ galaxy at z = 3 generated from the high-resolution cosmological simulation NUTFB. We demonstrate that the input stellar kinematics of the simulated galaxy can be accurately recovered from the integrated spectrum in a 15-h observation, using common analysis tools. Whilst spatially resolved spectroscopy is likely to remain out of reach for this particular galaxy, we estimate HARMONI's performance limits in this regime from our findings. This study demonstrates how instrument simulators such as HSIM can be used to quantify instrument performance and study observational biases on kinematics retrieval; and shows the potential of making observational predictions from cosmological simulation output data.

Abstract:

Bringing global warming to a halt requires that worldwide net emissions of carbon dioxide be brought to essentially zero, and the sooner this occurs, the less warming our descendants for the next thousand years and more will need to adapt to. The widespread fear that the actions needed to bring this about conflict with economic growth is a major impediment to efforts to protect the climate. However, much of this fear is pointless, and the magnitude of the task, while great, is no greater than challenges human ingenuity has surmounted in the past. To light the way forward, there is a need for examining success stories in which nations have greatly reduced their carbon dioxide emissions while simultaneously maintaining vigorous growth in the standard of living. In this article, the example of Sweden is showcased. Through a combination of sensible government infrastructure policies and free-market incentives, Sweden has managed to successfully decarbonize, cutting its per capita emissions by a factor of three since the 1970s, while doubling its pre capita income and providing a wide range of social benefits. This has all be accomplished within a vigorous capitalistic framework which in many ways embodies freemarket principles better than the economy of the United States.