Estimating the Ultraviolet Emission of M Dwarfs with Exoplanets from Ca ii and Hα

Abstract

M dwarf stars are excellent candidates around which to search for exoplanets, including temperate, Earth-sized planets. To evaluate the photochemistry of the planetary atmosphere, it is essential to characterize the UV spectral energy distribution of the planet’s host star. This wavelength regime is important because molecules in the planetary atmosphere such as oxygen and ozone have highly wavelength-dependent absorption cross sections that peak in the UV (900–3200 Å). We seek to provide a broadly applicable method of estimating the UV emission of an M dwarf, without direct UV data, by identifying a relationship between noncontemporaneous optical and UV observations. Our work uses the largest sample of M dwarf star far- and near-UV observations yet assembled. We evaluate three commonly observed optical chromospheric activity indices—Hα equivalent widths and log10 L Hα /L bol, and the Mount Wilson Ca ii H&K S and indices—using optical spectra from the HARPS, UVES, and HIRES archives and new HIRES spectra. Archival and new Hubble Space Telescope COS and STIS spectra are used to measure line fluxes for the brightest chromospheric and transition region emission lines between 1200 and 2800 Å. Our results show a correlation between UV emission-line luminosity normalized to the stellar bolometric luminosity and Ca ii with standard deviations of 0.31–0.61 dex (factors of ∼2–4) about the best-fit lines. We also find correlations between normalized UV line luminosity and Hα log10 L Hα /L bol and the S index. These relationships allow one to estimate the average UV emission from M0 to M9 dwarfs when UV data are not available.

Effects of UV stellar spectral uncertainty on the chemistry of terrestrial atmospheres

“…We revisit this idea in depth in Section 5. In the meantime, we conclude that the Melbourne et al (2020) UV reconstructions may not adequate for modeling the photochemistry in hydrocarbon haze-producing atmospheres.…”

“…1. We have used the MUSCLES Treasury survey Mdwarf spectra (France et al 2016) coupled with UV reconstructions from Melbourne et al (2020) to generate photochemical and transmission spectrum models of terrestrial atmospheres at Earthlike instellation, with and without hazes.…”

“…Youngblood et al (2017) determined a scaling relation between the equivalent width of the Ca II K line and various UV emission lines using the UV spectra from the MUSCLES survey. Melbourne et al (2020) extended this work to also consider the full set of available HST UV spectra of M dwarfs (a factor of ∼ 7 increase in sample size) and found that the R HK index -the Ca II H & K line core intensity index defined in Rutten (1984) -was the best predictor of UV emission line strength of the observable proxies that they considered. The advantage of using optical proxies is that, in principle, a star's UV spectrum can be approximated in the absence of observed UV data using information readily accessible to ground-based observatories.…”

“…This work aims to close the loop on that question with respect to photochemical modeling in particular. We do this by directly comparing the outputs of photochemical models run using observed stellar spectra vs. those run using the Melbourne et al (2020) reconstructions of the same UV spectra. We then examine the degree to which the transmission spectra of each of the modeled exoplanets are altered by the use of the UV reconstructions, and we comment on implications for interpreting observations from JWST.…”

“…Table 2 provides a list of stars used in this work as well as properties relevant to each star's UV spectrum reconstruction and transmission spectrum calculations. Next, we regenerate each of our photochemical models using reconstructed MUSCLES UV spectra obtained by applying the UV line scaling relations described in Melbourne et al (2020). Specifically, these scaling relations estimate a given line luminosity using the following equation:…”