Mariela C. Vieytes

High stellar FUV/NUV ratio and oxygen contents in the atmospheres of potentially habitable planets

Recent observations of several planet-hosting M dwarfs show that most have FUV/NUV flux ratios 1000 times greater than that of the Sun. Here we show that the atmospheric oxygen contents (O2 and O3) of potentially habitable planets in this type of UV environment could be 2–3 orders of magnitude greater than those of their counterparts around Sun-like stars as a result of decreased photolysis of O3, H2O2, and HO2. Thus detectable levels of atmospheric oxygen, in combination with the existence of H2O and CO2, may not be the most promising biosignatures on planets around stars with high FUV/NUV ratios such as the observed M dwarfs.

The Muscles Treasury Survey. I. Motivation and Overview

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 Lyalpha) 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 (is equivalent to 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 (approximately equal 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 per (sq cm) s 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(sub flare)(UV) approximately 0.3 L(sub *) delta (t) (delta t = 1 s). Finally, we interpret enhanced L(line)/L(sub 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(sub plan)/A(sub plan)) with the transition regions of their host stars.

Chromospheric changes in K stars with activity

We study the differences in chromospheric structure induced in K stars by stellar activity, to expand our previous work for G stars, including the Sun as a star. We selected six stars of spectral type K with 0.82 <B − V< 0.90, including the widely studied Epsilon Eridani and a variety of magnetic activity levels. We computed chromospheric models for the stars in the sample, in most cases in two different moments of activity. The models were constructed to obtain the best possible match with the Ca II K and the Hβ observed profiles. We also computed in detail the net radiative losses for each model to constrain the heating mechanism that can maintain the structure in the atmosphere. We find a strong correlation between these losses and SCa II, the index generally used as a proxy for activity, as we found for G stars.

Improving the Ni I atomic model for solar and stellar atmospheric models

Neutral nickel (Ni I) is abundant in the solar atmosphere and is one of the important elements that contribute to the emission and absorption of radiation in the spectral range between 1900 and 3900 A. Previously, the Solar Radiation Physical Modeling (SRPM) models of the solar atmosphere only considered a few levels of this species. Here, we improve the Ni I atomic model by taking into account 61 levels and 490 spectral lines. We compute the populations of these levels in full NLTE using the SRPM code and compare the resulting emerging spectrum with observations. The present atomic model significantly improves the calculation of the solar spectral irradiance at near-UV wavelengths, which is important for Earth atmospheric studies, and particularly for ozone chemistry.

Semi-Empirical Modeling of the Photosphere, Chromosphere, Transition Region, and Corona of the M-Dwarf Host Star GJ 832

Stellar radiation from X-rays to the visible provides the energy that controls the photochemistry and mass loss from exoplanet atmospheres. The important extreme ultraviolet (EUV) region (10--91.2~nm) is inaccessible and should be computed from a reliable stellar model. It is essential to understand the formation regions and physical processes responsible for the various stellar emission features in order to predict how the spectral energy distribution varies with age and activity levels. We compute a state-of-the-art semi-empirical atmospheric model and the emergent high-resolution synthetic spectrum of the moderately active M2~V star GJ~832 as the first of a series of models for stars with different activity levels. Using non-LTE radiative transfer techniques and including many molecular lines, we construct a one-dimensional simple model for the physical structure of the stars chromosphere, chromosphere-corona transition region, and corona. The synthesized spectrum for this model fits the continuum and lines across the UV to optical spectrum. Particular emphasis is given to the emission lines at wavelengths shorter than 300~nm observed with {\em HST}, which have important effects on the photochemistry in the exoplanet atmospheres. The FUV line ratios indicate that the transition region of GJ~832 is more biased to hotter material than that of the quiet Sun. The excellent agreement of our computed EUV luminosity with that obtained by two other techniques indicates that our model predicts reliable EUV emission from GJ~832. We find that unobserved EUV flux of GJ~832, which heats the outer atmospheres of exoplanets and drives their mass loss, is comparable to the active Sun.

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planets potential habitability, particularly for M dwarfs as they are prime targets for current and near-term exoplanet characterization efforts and atmospheric models predict that their UV radiation can produce photochemistry on habitable zone planets different than on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope observations are unavailable, we have assembled a sample of fifteen early-to-mid M dwarfs observed by Hubble, and compared their non-simultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca II K line at 3933 Angstroms, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H I Lyman alpha. In addition, we address another potential driver of habitability: energetic particle fluxes associated with flares. We present a new technique for estimating soft X-ray and >10 MeV proton flux during far-UV emission line flares (Si IV and He II) by assuming solar-like energy partitions. We analyze several flares from the M4 dwarf GJ 876 observed with Hubble and Chandra as part of the MUSCLES Treasury Survey and find that habitable zone planets orbiting GJ 876 are impacted by large Carrington-like flares with peak soft X-ray fluxes >1e-3 W m-2 and possible proton fluxes ~100-1000 pfu, approximately four orders of magnitude more frequently than modern-day Earth.

12 years of stellar activity observations in Argentina

We present an observational program we started in 1999, to systematically obtain mid-resolution spectra of late-type stars, to study in particular chromospheric activity. In particular, we found cyclic activity in four dM stars, including Prox-Cen. We directly derived the conversion factor that translates the known S index to flux in the Ca II cores, and extend its calibration to a wider spectral range. We investigated the relation between the activity measurements in the calcium and hydrogen lines, and found that the usual correlation observed is the product of the dependence of each flux on stellar color, and it is not always preserved when simultaneous observations of a particular star are considered. We also used our observations to model the chromospheres of stars of different spectral types and activity levels, and found that the integrated chromospheric radiative losses, normalized to the surface luminosity, show a unique trend for G and K dwarfs when plotted against the S index.

The Ni I lines in the solar spectrum

The stratosphere is the region where the ozone chemistry is important for the balance of energy, and radiation in the near UV plays a fundamental role in the creation and destruction of ozone. However, the radiation in this range of wavelength has not been very well modeled. One of the most important elements, according to its abundance in the solar atmosphere, that contribute to the emission and absorption of radiation in the spectral range between 1900 and 3900 ˚ is neutral nickel (Ni I). In this work we improve the atomic model of this element, taking into account 490 lines over the spectrum. We solve these lines in NLTE using the Solar Radiation Physical Modeling (SRPM) program and compare the results with observation of the quiet sun spectrum.