Estimating the Ultraviolet Emission of M dwarfs with Exoplanets from Ca II and Hα

The M dwarf sample studied in this work spans a range of ∼1000 K in effective temperature, with an assumed uncertainty of ±50 K on each value, and ∼3 dex in activity levels as represented by the log R0 HK values (see Section 2.4). The relative radius of each star is demonstrated by the size of the points, with a range of 0.11 to 1.07 R . Young, early M dwarfs are included in this sample that have radii around 1 R (see Table 2 for further clarification). This plot shows the 45 stars (of 69 total targets) for which an R0 HK value is reported in this work.

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 non-contemporaneous 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 LHα/Lbol, and the Mount Wilson Ca II H&K S and R′HK 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 1200-2800 Å. Our results show a correlation between UV emission line luminosity normalized to the stellar bolometric luminosity and Ca II R′HK 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 LHα/Lbol 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.

Katherine Melbourne, Allison Youngblood, Kevin France, C. S. Froning, J. Sebastian Pineda, Evgenya L. Shkolnik, David J. Wilson, Brian E. Wood, Sarbani Basu, Aki Roberge, Joshua E. Schlieder, P. Wilson Cauley, R. O. Parke Loyd, Elisabeth R. Newton, Adam Schneider, Nicole Arulanantham, Zachory Berta-Thompson, Alexander Brown, Andrea P. Buccino, Eliza Kempton, Jeffrey L. Linsky, Sarah E. Logsdon, Pablo Mauas, Isabella Pagano, Sarah Peacock, Seth Redfield, Sarah Rugheimer, P. Christian Schneider, D. J. Teal, Feng Tian, Dennis Tilipman, Mariela Vieytes

Comments: 34 pages, 12 figures, 5 tables (one machine readable table available online). Accepted to AAS Journals
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2009.07869 [astro-ph.SR] (or arXiv:2009.07869v1 [astro-ph.SR] for this version)
Submission history
From: Katherine Melbourne
[v1] Wed, 16 Sep 2020 18:01:23 UTC (10,661 KB)

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