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

Abstract

Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planet’s 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 from that on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope ( HST ) observations are unavailable, we have assembled a sample of 15 early to mid-M dwarfs observed by HST and compared their nonsimultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca ii K line at 3933 Å, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H i Ly α . 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 HST and Chandra as part ofmore » 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 ≥10{sup −3} W m{sup −2} and possible proton fluxes ∼10{sup 2}–10{sup 3} pfu, approximately four orders of magnitude more frequently than modern-day Earth.« less

Authors:
; ; ;  [1];  [2];  [3];  [4]; ;  [5]; ;  [6];  [7];  [8];  [9];  [10];  [11];  [12];  [13];
  1. Laboratory for Atmospheric and Space Physics, University of Colorado, 600 UCB, Boulder, CO 80309 (United States)
  2. Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309 (United States)
  3. European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk (Netherlands)
  4. Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 (United States)
  5. Department of Astrophysical and Planetary Sciences, University of Colorado, 2000 Colorado Ave., Boulder, CO 80305 (United States)
  6. Dpto. de Física, Facultad de Ciencias Exactas y Naturales (FCEN), Universidad de Buenos Aires (UBA), Buenos Aires (Argentina)
  7. Department of Astronomy/McDonald Observatory, C1400, University of Texas at Austin, Austin, TX 78712 (United States)
  8. Astronomy Department, Box 351580, University of Washington, Seattle, WA 98195 (United States)
  9. JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309 (United States)
  10. Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459 (United States)
  11. Observatoire de la Cote d’Azur, Boulevard de l’Observatoire, CS 34229 F-06304 NICE Cedex 4 (France)
  12. Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02138 (United States)
  13. School of Earth and Environmental Sciences, University of St. Andrews, Irvine Building, North Street, St. Andrews, KY16 9AL (United Kingdom)
Publication Date:
OSTI Identifier:
22663450
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 843; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; APPROXIMATIONS; COMPARATIVE EVALUATIONS; DWARF STARS; EMISSION; ENERGY SPECTRA; LYMAN LINES; MASS; PARTITION; PHOTOCHEMISTRY; PLANETS; PROTONS; SOFT X RADIATION; STELLAR FLARES; SUN; SURFACES; ULTRAVIOLET RADIATION

Citation Formats

Youngblood, Allison, France, Kevin, Loyd, R. O. Parke, Mason, James P., Brown, Alexander, Schneider, P. Christian, Tilley, Matt A., Berta-Thompson, Zachory K., Kowalski, Adam, Buccino, Andrea, Mauas, Pablo J. D., Froning, Cynthia S., Hawley, Suzanne L., Linsky, Jeffrey, Redfield, Seth, Miguel, Yamila, Newton, Elisabeth R., Rugheimer, Sarah, E-mail: allison.youngblood@colorado.edu, and and others. The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA76DD.
Youngblood, Allison, France, Kevin, Loyd, R. O. Parke, Mason, James P., Brown, Alexander, Schneider, P. Christian, Tilley, Matt A., Berta-Thompson, Zachory K., Kowalski, Adam, Buccino, Andrea, Mauas, Pablo J. D., Froning, Cynthia S., Hawley, Suzanne L., Linsky, Jeffrey, Redfield, Seth, Miguel, Yamila, Newton, Elisabeth R., Rugheimer, Sarah, E-mail: allison.youngblood@colorado.edu, & and others. The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs. United States. doi:10.3847/1538-4357/AA76DD.
Youngblood, Allison, France, Kevin, Loyd, R. O. Parke, Mason, James P., Brown, Alexander, Schneider, P. Christian, Tilley, Matt A., Berta-Thompson, Zachory K., Kowalski, Adam, Buccino, Andrea, Mauas, Pablo J. D., Froning, Cynthia S., Hawley, Suzanne L., Linsky, Jeffrey, Redfield, Seth, Miguel, Yamila, Newton, Elisabeth R., Rugheimer, Sarah, E-mail: allison.youngblood@colorado.edu, and and others. Sat . "The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs". United States. doi:10.3847/1538-4357/AA76DD.
@article{osti_22663450,
title = {The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs},
author = {Youngblood, Allison and France, Kevin and Loyd, R. O. Parke and Mason, James P. and Brown, Alexander and Schneider, P. Christian and Tilley, Matt A. and Berta-Thompson, Zachory K. and Kowalski, Adam and Buccino, Andrea and Mauas, Pablo J. D. and Froning, Cynthia S. and Hawley, Suzanne L. and Linsky, Jeffrey and Redfield, Seth and Miguel, Yamila and Newton, Elisabeth R. and Rugheimer, Sarah, E-mail: allison.youngblood@colorado.edu and and others},
abstractNote = {Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planet’s 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 from that on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope ( HST ) observations are unavailable, we have assembled a sample of 15 early to mid-M dwarfs observed by HST and compared their nonsimultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca ii K line at 3933 Å, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H i Ly α . 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 HST 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 ≥10{sup −3} W m{sup −2} and possible proton fluxes ∼10{sup 2}–10{sup 3} pfu, approximately four orders of magnitude more frequently than modern-day Earth.},
doi = {10.3847/1538-4357/AA76DD},
journal = {Astrophysical Journal},
number = 1,
volume = 843,
place = {United States},
year = {Sat Jul 01 00:00:00 EDT 2017},
month = {Sat Jul 01 00:00:00 EDT 2017}
}
  • The ultraviolet (UV) spectral energy distributions (SEDs) of low-mass (K- and M-type) stars play a critical role in the heating and chemistry of exoplanet atmospheres, but are not observationally well-constrained. Direct observations of the intrinsic flux of the Ly α line (the dominant source of UV photons from low-mass stars) are challenging, as interstellar H i absorbs the entire line core for even the closest stars. To address the existing gap in empirical constraints on the UV flux of K and M dwarfs, the MUSCLES Hubble Space Telescope Treasury Survey has obtained UV observations of 11 nearby M and Kmore » dwarfs hosting exoplanets. This paper presents the Ly α and extreme-UV spectral reconstructions for the MUSCLES targets. Most targets are optically inactive, but all exhibit significant UV activity. We use a Markov Chain Monte Carlo technique to correct the observed Ly α profiles for interstellar absorption, and we employ empirical relations to compute the extreme-UV SED from the intrinsic Ly α flux in ∼100 Å bins from 100–1170 Å. The reconstructed Ly α profiles have 300 km s{sup −1} broad cores, while >1% of the total intrinsic Ly α flux is measured in extended wings between 300 and 1200 km s{sup −1}. The Ly α surface flux positively correlates with the Mg ii surface flux and negatively correlates with the stellar rotation period. Stars with larger Ly α surface flux also tend to have larger surface flux in ions formed at higher temperatures, but these correlations remain statistically insignificant in our sample of 11 stars. We also present H i column density measurements for 10 new sightlines through the local interstellar medium.« less
  • A star's ultraviolet (UV) emission can greatly affect the atmospheric chemistry and physical properties of closely orbiting planets with the potential for severe mass loss. In particular, the Lyα emission line at 1216 Å, which dominates the far-ultraviolet (FUV) spectrum, is a major source of photodissociation of important atmospheric molecules such as water and methane. The intrinsic flux of Lyα, however, cannot be directly measured due to the absorption of neutral hydrogen in the interstellar medium and contamination by geocoronal emission. To date, reconstruction of the intrinsic Lyα line based on Hubble Space Telescope spectra has been accomplished for 46more » FGKM nearby stars, 28 of which have also been observed by the Galaxy Evolution Explorer (GALEX). Our investigation provides a correlation between published intrinsic Lyα and GALEX far- and near-ultraviolet (NUV) chromospheric fluxes for K and M stars. The negative correlations between the ratio of the Lyα to the GALEX fluxes reveal how the relative strength of Lyα compared to the broadband fluxes weakens as the FUV and NUV excess flux increase. We also correlate GALEX fluxes with the strong NUV Mg II h+k spectral emission lines formed at lower chromospheric temperatures than Lyα. The reported correlations provide estimates of intrinsic Lyα and Mg II fluxes for the thousands of K and M stars in the archived GALEX all-sky surveys. These will constrain new stellar upper atmosphere models for cool stars and provide realistic inputs to models describing exoplanetary photochemistry and atmospheric evolution in the absence of UV spectroscopy.« less
  • 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 spectralmore » 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{sup −2} s{sup −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{sub flare}(UV) ∼ 0.3 L{sub *}Δt (Δt = 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.« less
  • CaZrF 6 has recently been shown to combine strong negative thermal expansion (NTE) over a very wide temperature range (at least 10–1000 K) with optical transparency from mid-IR into the UV range. Variable-temperature and high-pressure diffraction has been used to determine how the replacement of calcium by magnesium and zirconium by niobium(IV) modifies the phase behavior and physical properties of the compound. Similar to CaZrF 6, CaNbF 6 retains a cubic ReO 3-type structure down to 10 K and displays NTE up until at least 900 K. It undergoes a reconstructive phase transition upon compression to ~400 MPa at room temperature and pressure-induced amorphization above ~4 GPa. Prior to the first transition, it displays very strong pressure-induced softening. MgZrF 6 adopts a cubic ( Fmmore » $$\bar{3}$$m) structure at 300 K and undergoes a symmetry-lowering phase transition involving octahedral tilts at ~100 K. Immediately above this transition, it shows modest NTE. Its’ thermal expansion increases upon heating, crossing through zero at ~500 K. Unlike CaZrF 6 and CaNbF 6, it undergoes an octahedral tilting transition upon compression (~370 MPa) prior to a reconstructive transition at ~1 GPa. Cubic MgZrF 6 displays both pressure-induced softening and stiffening upon heating. MgNbF 6 is cubic ( Fm$$\bar{3}$$m) at room temperature, but it undergoes a symmetry-lowering octahedral tilting transition at ~280 K. It does not display NTE within the investigated temperature range (100–950 K). Furthermore the replacement of Zr(IV) by Nb(IV) leads to minor changes in phase behavior and properties, the replacement of the calcium by the smaller and more polarizing magnesium leads to large changes in both phase behavior and thermal expansion.« less
  • Observed far-UV to visible flux ratios of all DA white dwarfs hotter than about 25,000 K are compared with ratios predicted by models in order to derive effective temperatures for the stars. All suitable IUE spectra of seven hot DA white dwarfs for which accurate temperatures based on hydrogen line profiles were available are used to derive a flux correction to the 1980 IUE absolute calibration. Application of the corrections to the IUE spectra results in derived temperature which are consistent with the line profile temperatures. 70 refs.